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Analysis of the radio contacts between the Canary Islands and South America in the 50 MHz band in March 2010

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MainTechnical ArticlesAnalysis of the radio contacts between the Canary Islands and South America in the 50 MHz band in March 2010

Este artículo también está disponible en español (this article is also available in Spanish).

Abstract:

Analysis of the DX events in the 50 MHz band, between the Canary Islands, Brazil, Uruguay and Argentina, occured from 17th to 22nd March 2010 and the 1st April 2010. The radio links were of the transequatorial type, in the VHF band, with several peculiarities such as the wide angle of the radio path and the geomagnetic North-South direction, the lack of simmetry with respect to the geomagnetic equator or the poor conditions normally expected during the solar cycle minimum. Several hyphotesis are studied, such as the nE modes, the nF modes, complex modes or eTEP, through the analysis of ionospheric conditions, the approximate geometry of the link and the power budget. The analysis has not been finished yet. If you want to provide your comments or additional data, please use this form.

 

Table of contents.

1. Introduction.

2. Equatorial ionosphere characteristics.

2.1. Equatorial anomaly.

2.2. FAI and turbulences.

3. Technical information on the radio contacts.

3.1. Radio contact logs.

3.2. Statistical analysis.

3.3. Methodology used in the calculations.

4. Analysis of the ionospheric conditions.

4.1. Space weather conditions.

4.2. Total electron content (TEC).

4.3. Ionograms.

4.3.1.Tucumán ionosonde data.

4.3.2. Ascension Island ionosonde data.

4.3.3. Fortaleza ionosonde data.

4.3.4. Sao Luis ionosonde data.

4.3.5. Cachoeira Paulista ionosonde data.

4.4. Ionospheric scintillation maps.

5. Propagation modes hyphotesis.

5.1. One-hop hyphotesis.

5.2. 2F hyphotesis.

5.3. Complex modes hyphotesis.

5.4. eTEP hyphotesis.

5.5. 2E/3E/4E hyphotesis.

6. Conclusions.

7. References.

8. Glossary.

9. Acknowledgements.

 


1. Introduction.

This is an study about the radio contacts made by stations of the Amateur Radio Service between the Canary Islands (Spain) and several countries in South America, in the 50 MHz band, between the days 17 and 21 March 2010.The peculiarity of those contacts is due to:

Using technical and scientific data available to the public, several hyphotesis about the possible propagation modes used are raised. For each case, the following aspects will be considered:

The first section is a summary of the available technical information about the radio links. An analysis about the ionospheric conditions during the days under study follows.

Later, several hyphotesis about the possible propagation modes are raised and a summary of the conclusions is offered. We will consider the following:

At the end of the article you will find a section about the references used and a glossary.

A great part of the opinions and data about those contacts have been provided by several spanish amateur radio operators, such as EA3EPH, EA5EF, EC7DND, EA8BPX, EA1DDO, EA8DD, EA6DX, EA6VQ and EA1FBF, via the Spanish Amateur Radio Union (URE) online forum.

Due to the fact that those contacts may be of some scientific interest on the matter of radio propagation studies, technical report have been sent to IPS Radio and Space Services, an institute managed by the Australian Government with a wide expertise on radio communications, and to the Ebro Observatory, in Spain.

 


2. Equatorial ionosphere characteristics.

The ionosphere is a non homogeneus medium presenting great differences with the latitude. We can distinguish three regions:

The equatorial ionosphere presents also other characteristics of interest, such as anomalies and irregularities, which will be described in the following sections.

 


2.1. Equatorial anomaly.

In the equatorial ionosphere, due to the fact that the Sun radiates in an orthogonal fashion over the Equator, it should be expected that the ionization maximums occur in this area. However, measurements show that those maximums occur at magnetic latitudes between 10º and 20º, phenomenon known as equatorial anomaly or Appleton anomaly. Let's remember also that the magnetic Equator and the geographic Equator have different locations.

The figure 2.1 shows an image of the total electron contect (TEC) of the ionosphere, the day 04/01/2010 at 17:45 GMT, obtained from the model Earth Space 4-D global 4-D ionosphere provided by Space Environment Technologies. The figure shows also the geomagnetic Equator position (orange line) and the radio path between the Canary Islands and Brazil (white line). The areas with warmest colors indicate an higher electron content. Pay attention to the fact that the maximums occur at both sides of the geomagnetic Equator.

 

Fig 2.1. Equatorial anomaly (ES4D global ionosphere, Space Environment Technologies).

 

The regions with higher TEC will have also the highest values of foF2 (critical frequency of the F2 layer).

 


2.2. Turbulences and FAI.

Another characteristic of the equatorial ionosphere is the generation, around sunset, of turbulences shaped as bubbles, known as equatorial plasma bubbles (EPB), whose density of ionization is low compared to the surrounding areas [Ref.Harrison].

Those bubbles can be between 40 km and 350 km in diameter and are originated at the bottom part of the ionosphere, raising at speeds of 125-350 km/s to altitudes up to 1500 km. As the bubbles rise, they are aligned with the geomagnetic field (parallel to the Earth's surface in this region, as stated before), following the magnetic North-South lines. This is the reason they are called also field aligned irregularities (FAI), and can spread along the magnetic field line several thousand kilometers [Ref.Maruyama].

On the other hand, being composed of moving electrons, the bubbles are also affected by equatorial electrojet (EEJ), which drifts them eastwards at speeds between 25-125 m/s.

The figure 2.2 shows an orientative sketch of the evolution of this kind of ionospheric turbulences with time.

 

Fig 2.2. Formation of field aligned irregularities (FAI).

 

The figure 2.3 is an image taken by the NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) spacecraft, where we can see the formation of EPBs in the equatorial area, aligned with the geomagnetic field.

 

Fig 2.3. Example of EPBs observed by the TIMED NASA's spacecraft.

The existing gradient of density of ionization between the inner part and the outer part of the bubbles causes the radioelectric waves travelling through them to be subject to refraction, scattering, diffraction and amplitude and phase variations, phenomenon known as scintillation, which may affect the satellite communication systems and also the HF/VHF radio links.

Those gradients may involve the formation of tubes or ducts which conduct the radio waves, allowing i.e. the propagation of VHF radio waves at long distances between both Earth's hemispheres (TEP, transequatorial propagation), as shown in the figure 2.4.

 

Fig 2.4. Formation of propagation ducts by the FAIs (E.R.Young et al, 1984).

 

The occurrence of those irregularities can be monitored using several methods, among them:

 


3. Technical information on the radio contacts.

In this section, the available technical information about the radio contacts is shown: dates, hours, stations contacted and their location. We discuss the azimuth-distance distribution, taking into account the locations of all the stations involved.

The methodology to calculate the theoretical fading margins is explained, considering also the technical data to calculate the power budgets. This data will be employed later for the validation of each hyphotesis.

 


3.1. Radio contact logs.

Several amateur radio stations in the Canary Islands (Spain) have reported to contact amateur radio stations in South America, especially in the north-east area of Brazil, using the 50 MHz band, between the days 18 and 21 March, 2010 and the 1st April, 2010.

The figure 3.1. shows the representation in Google Earth of the locations of all the radio stations contacted. The kmz file is available for download: "EA8-PY_DX_50_MHz_Mar_2010.kmz".

 

Fig 3.1. Locations of all the stations involved in the contacts.
Click on each image to see a larger version

As we will see, in all the contacts registered the reported signal levels in the Canary Islands were high and the EA8 operators said that those signals kept at a steady level during the QSOs. Operators said also that they didn't notice neither signal fading nor frequency shifts during the contacts.

In the analysis, it is necessary to take into account the lack of information about the distribution of the population of amateur radio operators in the territory, especially in the theoretical areas of radio coverage. In example, when evaluating the multiple hop modes, it is possible that in some of the areas where radio coverage should exist, there aren't any contacts registered due to the fact that there aren't any amateur radio operators in the area or even that they weren't active at the hour and frequency of the band openings.

In some cases, the theoretical coverage areas for some of the hops lay in the Atlantic Ocean, where the probablity of having operative amateur radio stations is extremely low.

 

3.1.1. EA8BDD's log.

In the table 3.1 is a list of the contacts made by the spanish station EA8DD, placed in the southern area of the Tenerife Island:

 

Date Hour UTC Freq (MHz) Station RS City Country Locator Dist (km) Azimuth (deg)
18/03/2010 22:53 50.115 PR7AR 57 Paraíba Brasil HI23GD 4357 210,5
19/03/2010 21:50 50.110 PP5XX 59 Itapoá Brasil GG53QW 6811 212,6
19/03/2010 21:54 50.110 PY1ZV 59 Río Janeiro Brasil GG87KO 6219 209,7
19/03/2010 21:57 50.110 PT7TT 59 Aracau Brasil GI97WC 4236 220,1
19/03/2010 21:59 50.110 PT7ZAP 59 Fortaleza Brasil HI06RG 4219 217,2
19/03/2010 22:06 50.110 PP1CZ 59 Vitoria Brasil GG99UQ 5896 208,0
19/03/2010 22:19 50.110 PY2MAJ 59 Sao Paulo Brasil GG66RJ 6487 212,2
19/03/2010 22:22 50.110 PY2REK 59 Bal Suarao Brasil GG65PU 6545 212,1
19/03/2010 22:32 50.110 PY5EW 59 Londrina Brasil GG46IP 6698 216,6
20/03/2010 22:34 50.115 PY7XAF 55 Itamaracá Brasil HI22ME 4425 209,0
21/03/2010 22:31 50.110 PY0FF 59 Fdo.Noronha Brasil HI36TD 3918 208,0
01/04/2010 18:56 50.110 CX5CR 59 Montevideo Uruguay FF82HH 8516 216,0
01/04/2010 18:58 50.110 LU5CAB 59 Buenos Aires Argentina GF05SJ 8028 214,9

Table 3.1. Stations contacted by EA8DD.

 

The table 3.2 shows more information about those radio contacts:

 

Date LST EA8 Station Locator Lat GEO Long GEO Lat MAG Long MAG LST LST Diff
18/03/2010 22:53 PR7AR HI23GD -6,85417 -35,45833 1,17 36,46 20:23 1:15
19/03/2010 21:50 PP5XX GG53QW -26,06250 -48,62500 -16,86 22,07 18:27 2:09
19/03/2010 21:54 PY1ZV GG87KO -22,39583 -43,12500 -13,60 27,55 18:53 1:47
19/03/2010 21:57 PT7TT GI97WC -2,89583 -40,12500 5,56 32,22 19:08 1:35
19/03/2010 21:59 PT7ZAP HI06RG -3,72917 -38,54167 4,58 32,72 19:17 1:28
19/03/2010 22:06 PP1CZ GG99UQ -20,31250 -40,29167 -11,76 30,44 19:17 1:35
19/03/2010 22:19 PY2MAJ GG66RJ -23,60417 -46,54167 -14,55 24,22 19:05 2:00
19/03/2010 22:22 PY2REK GG65PU -24,14583 -46,70833 -15,07 24,02 19:07 2:01
19/03/2010 22:32 PY5EW GG46IP -23,35417 -51,29167 -13,99 19,75 18:59 2:19
20/03/2010 22:34 PY7XAF HI22ME -7,81250 -34,87500 0,15 36,94 20:07 1:13
21/03/2010 22:31 PY0FF HI36TD -3,85417 -32,37500 3,82 39,84 20:14 1:03
01/04/2010 18:56 CX5CR FF82HH -37,68750 -63,37500 -27,81 7,91 14:38 3:08
01/04/2010 18:58 LU5CAB GF05SJ -34,60417 -58,45833 -24,88 12,46 15:00 2:48

Table 3.2. Local solar time and geomagnetic coordinates of the stations contacted by EA8DD.

The equipment used by EA8DD consists on a radio transceiver using 100 W of RF power and a 4 element directive antenna, model Hy-Gain VB-64DX, with a gain of 10,35 dBi (fig.3.2). The radio contacts were made pointing the antenna with an azimuth of 220º-230º. The receiver has a sensitivity of 0,25 uV (-119 dBm).

 

Fig 3.2. Antennas of the EA8DD amateur radio station (courtesy EA8DD).

 

3.1.2. EA8BPX's log.

In the table 3.3 is a list of the contacts made by the spanish station EA8BPX, placed in the northern area of the Tenerife Island:

 

Date Hour UTC Freq (MHz) Station RS City Country Locator Dist (km) Azimuth (deg)
17/03/2010 22:33 6 m LU1FVE 55 Castelar Argentina FF88XJ 7995 219,2
17/03/2010 22:34 6 m PP5XX 59 Itapoá Brazil GG53QW 6849 212,6
17/03/2010 22:53 6 m PY0FF 57 Fernando de Noronha Brazil HI36TD 3958 207,9
17/03/2010 22:57 6 m PY2MAJ 54 Sao Paulo Brazil GG66RJ 6525 212,2
17/03/2010 23:05 6 m PY2GH 59 Santo André Brazil GG66RI 6529 212,2
18/03/2010 22:49 6 m PR7AR 55 Paraíba Brazil HI23GD 4397 210,4
19/03/2010 21:47 6 m PT7ZAP 55 Fortaleza Brazil HI06RG 4256 217,0
19/03/2010 22:05 6 m PP1CZ 59 Vitoria/ES Brazil GG99UQ 5936 208,0
19/03/2010 22:45 6 m PU4CQQ 59 Divinopolis Brazil GG79NU 6123 212,7
19/03/2010 22:55 6 m PY2ZXU 57 Santos Brazil GG66TB 6549 211,8
20/03/2010 18:39 6 m LU8EMH 57 Pehuajo Argentina FF94BE 8328 216,6
20/03/2010 18:54 6 m LU8EEM 55 Lincoln Argentina FF95GD 8229 216,7
20/03/2010 18:57 6 m LU5FCI 55 Santo Tomé Argentina FF98OI 7933 218,4
20/03/2010 19:14 6 m 9Z4BM 55 San Fernando Trinidad and Tobago FK90GG 5079 255,9
20/03/2010 22:27 6 m PU2CQQ 58 Sao Paulo Brazil GG66RK  6521 212,2
20/03/2010 22:27 6 m PY7XAF 59 Ilha de Itamaracá Brazil HI22NE 4465 208,9
24/03/2010 19:17 6 m KP4EIT 53 Ciales (Puerto Rico) USA FK68SI 5207 268,9
24/03/2010 20:08 6 m TN5SM 55 Desconocida Congo JI75PR 4919 131,8
24/03/2010 20:11 6 m 5N7M 59 Abuja Nigeria JJ39SB 3293 125,9
25/03/2010 19:52 6 m PP5XX 59 Itapoá Brazil GG53QW 6849 212,6
01/04/2010 17:25 6 m LU1DMA 59 Buenos Aires Argentina GF05PH 8083 215,0
01/04/2010 17:30 6 m LU6DLB 52 Buenos Aires Argentina GF05PH 8083 215,0
01/04/2010 17:34 6 m LU8EMH 55 Buenos Aires Argentina FF94BE 8328 216,3
01/04/2010 17:41 6 m CX5CR 51 Montevideo Uruguay FF82HH 8552 216,0
01/04/2010 17:41 6 m LW3EX 53 Buenos Aires Argentina GF05RL 8062 215,0
01/04/2010 18:31 6 m LU5CAB 57 Buenos Aires Argentina GF05SJ 8065 214,9
01/04/2010 18:32 6 m LU8ADX 53 Buenos Aires Argentina GF05HJ 8108 215,5

Table 3.3. Stations contacted by EA8BPX.

 

The table 3.4 shows more information about those radio contacts:

Date LST EA8 Station Locator Lat GEO Long GEO Lat MAG Long MAG LST LST Diff
17/03/2010 21:18 LU1FVE FF88XJ -31,60417 -62,04167 -21,76 9,33 18:16 3:02
17/03/2010 21:19 PP5XX GG53QW -26,06250 -48,62500 -16,86 22,07 19:11 2:08
17/03/2010 21:38 PY0FF HI36TD -3,85417 -32,37500 3,82 39,84 20:35 1:03
17/03/2010 21:42 PY2MAJ GG66RJ -23,60417 -46,54167 -14,55 24,22 19:42 2:00
17/03/2010 21:50 PY2GH GG66RI -23,64583 -46,54167 -14,59 24,21 19:50 2:00
18/03/2010 21:35 PR7AR HI23GD -6,85417 -35,45833 1,17 36,46 20:19 1:16
19/03/2010 20:33 PT7ZAP HI06RG -3,72917 -38,54167 4,58 33,72 19:05 1:28
19/03/2010 20:51 PP1CZ GG99UQ -20,31250 -40,29167 -11,76 30,44 19:16 1:35
19/03/2010 21:31 PU4CQQ GG79NU -20,14583 -44,87500 -11,22 26,08 19:37 1:54
19/03/2010 21:41 PY2ZXU GG66TB -23,93750 -46,37500 -14,89 24,35 19:41 2:00
20/03/2010 17:25 LU8EMH FF94BE -35,81250 -61,87500 -25,97 9,32 14:24 3:01
20/03/2010 17:40 LU8EEM FF95GD -34,85417 -61,45833 -25,03 9,73 14:40 3:00
20/03/2010 17:43 LU5FCI FF98OI -31,64583 -60,79167 -21,84 10,47 14:46 2:57
20/03/2010 18:00 9Z4BM FK90GG 10,27083 -61,45833 20,07 11,28 15:00 3:00
20/03/2010 21:13 PU2CQQ GG66RK  -23,56250 -46,54167 -14,50 24,22 19:13 2:00
20/03/2010 21:13 PY7XAF HI22NE -7,81250 -34,87500 0,15 36,94 20:00 1:13
24/03/2010 18:04 KP4EIT FK68SI 18,35417 -66,45834 28,29 6,21 14:44 3:20
24/03/2010 18:55 TN5SM JI75PR -4,27083 15,29167 -3,77 86,81 21:02 2:07
24/03/2010 18:58 5N7M JJ39SB 9,06250 7,54167 10,69 81,48 20:34 1:36
25/03/2010 18:40 PP5XX GG53QW -26,06250 -48,62500 -16,86 22,07 16:31 2:09
01/04/2010 16:15 LU1DMA GF05PH -34,68750 -58,70833 -24,95 12,23 13:26 2:49
01/04/2010 16:20 LU6DLB GF05PH -34,68750 -58,70833 -24,95 12,23 13:31 2:49
01/04/2010 16:24 LU8EMH FF94BE -35,81250 -61,87500 -25,97 9,32 13:22 3:02
01/04/2010 16:31 CX5CR FF82HH -37,68750 -63,37500 -27,81 7,91 13:23 3:08
01/04/2010 16:31 LW3EX GF05RL -34,52083 -58,54167 -24,79 12,39 13:42 2:49
01/04/2010 17:21 LU5CAB GF05SJ -34,60417 -58,45833 -24,88 12,46 14:33 2:48
01/04/2010 17:22 LU8ADX GF05HJ -34,60417 -59,37500 -24,84 11,63 14:30 2:52

Table 3.4. Local solar time and geomagnetic coordinates of the stations contacted by EA8BPX.

 


3.2. Statistical analysis.

In this section a statistical analysis is applied to the logs, paying attention to three parameters: azimuth and distance from the Canary Islands and hourly distribution.

 

3.2.1. Azimuth-distance distribution.

The figure 3.3 shows the azimuth-distance distribution of the radio contacts made by EA8DD.

 

Fig 3.3. Azimuth-distance distribution of the radio contacts from the Canary Islands (EA8DD).

 

The figure 3.4 shows the azimuth-distance distribution of the radio contacts made by EA8BPX.

 

Fig 3.4. Azimuth-distance distribution of the radio contacts from the Canary Islands (EA8BPX).

 

Finally, the figure 3.5 shows the global azimuth-distance distribution of all the radio contacts, considering both stations.

 

Fig 3.5. Global azimuth-distance distribution of all the radio contacts from the Canary Islands.

 

We can see 4 big groups, which can be described as follows:

The groups QSO 4200, QSO 6400 and QSO 8200 are within azimuths of about 212º-216º from the Canary Islands. The stations which belong to the group "Others" are within entirely different azimuths.

 

3.2.2. Hourly distribution.

The figure 3.6 shows the hourly distribution of all the radio contacts.

 

Fig 3.6. Distribución horaria de todos los contactos desde las Islas Canarias.

 

We can see the time windows shown in the table 3.3.

 

Table 3.3. Time windows of the contacts.

An explanation of each column follows:

We can see that the duration of each aperture was from 5 minutes at minimum to 1 hour and 33 minutes at maximum.

The window 1 took place the day 03/17/2010 between 22:33-23:05 UTC (32 minutes). Stations of the groups QSO 4200, QSO 6400 and QSO 8200 were contacted. It is the only window with contacts of these three groups.

The window 2 took place the day 03/18/2010 between 22:49-22:53 UTC, lasting barely 4 minutes. There were contacts with stations of the group QSO 4200 only.

The window 3 took place the day 03/19/2010 between 21:47-22:55 UTC (1 hour and 8 minutes). It's the window with more contacts, but only with stations of the groups QSO 4200 and QSO 6400.

The window 4 took place the day 03/20/2010 between 18:39-19:14 UTC, lasting 35 minutes. It was the first opening of the two ocurred this day. Oddly, only stations of the group QSO 8200 were contacted, without traces of the other groups nearest to the Canary Islands.

The window 5 took place the same day 03/20/2010 between 22:27-22:34 UTC (7 minutes). There were contacts with stations of the groups QSO 4200 and QSO 4600.

The window 6 took place the day 03/21/2010 between 22:31-22:36 UTC, lasting barely 5 minutes and with only one contact with a station of the group QSO 4200.

The window 7 took place 3 days later, on the 03/24/2010, between 19:17-20:11 UTC (54 minutes). During this window, there weren't contacts with stations of the three big groups, but with Puerto Rico, Congo and Nigeria instead, placed at azimuths absolutely different of the main one.

The window 8 took place the day 03/25/2010, between 19:52-19:57 UTC, lasting 5 minutes only, with only one contact with a station of the group QSO 6400.

And finally, the window 9 took place several days later, on the 04/01/2010, between 17:25-18:58 UTC, being the longest opening with 1 hour and 33 minutes. There were 9 contacts, all of them with stations of the group QSO 8200. As in the window 4, there weren't traces of the other groups nearest to the Canary Islands

Please note that the vernal equinox occured on 20/03/2010 at 16:31 UTC.

 


3.3. Methodology used in the calculations.

Currently there is no available technical data about the equipment used by the stations in Brazil, so we will assume that they used identical antenna gain, RF power and receiver sensitivity to the used by the EA8DD station.

The analysis will take into account several aspects such as the geometry of the links, the power budgets and the computation of the maximum usable frequency from critical frequency values measured by ionosondes.

In the case of the simple propagation modes, the geometrical calculations are based on trigonometry, using the approach of straight rays not affected by refraction nor scattering [Ref.Pellejero].

The power budget calculations are based on the computation of free spaces losses determined by the transmission equation, making also the following theoretical approaches about the ionospheric propagation losses [Ref.Pellejero]:

Finally, in some cases the maximum usable frequency of the different layers of the ionosphere is extrapolated from the critical frequency values measured by ionosondes, following the method also described in [Ref.Pellejero].

 


4. Analysis of the ionospheric conditions.

We have gathered scientific data from several ionospheric sounding stations around the Equator, with the goal of analyzing the ionization levels in the different layers of the ionosphere, during the days and hours of the contacts. I have collected data from the stations indicated in the table 4.1, being necessary to emphasize that in some cases the distance between the sounding station and the radio path is quite high and this fact could detract from the validity of the data for the propagation analysis of the radio links under study. In spite of this fact, those data can be valuable to get an idea of the general conditions of ionization in similar latitudes.

 

Ionosonde Country URSI code Latitude Longitude
Tucumán Argentina TUJ2O 26,9ºS 65,4ºW
Fortaleza Brazil FZA0M 3,8ºS 38ºW
Sao Luis Brazil SAA0K 2,6ºS 44,2ºW
Cachoeira Paulista Brazil CAJ2M 23,20ºS 45,8ºW
Isla Ascensión UK AS00Q 7,95ºS 14,4ºW
Kwajalein Marshall Islands KJ609 9ºN 167,2ºE

Table 4.1. Ionospheric sounding stations checked

 

The ionospheric sounding stations checked make vertical soundings every 10 or 15 minutes in order to measure several parameters of the ionosphere, such as the presence of the different layers, their critical frequencies or they thickness.

As we may expect, only the first samples of each day (until 20:40 UTC maximum) show the presence of the E layer.

We have also collected data about total electron content (TEC) measurements and ionospheric scintillation maps, provided to the public by several agencies devoted to the study of space weather.

 

 


4.1. Space weather conditions.

The table 4.2 shows the most significative space weather parameters during the days of the contacts (source: Joint USAF/NOAA Solar and Geophysical Activity Summary).

 

Day
SFI
SSN
Ap
Kp max
Geomag. Act.
03/17/2010 87 28 7 4 Quiet
03/18/2010 86 27 5 3 Quiet
03/19/2010 84 24 4 2 Quiet
03/20/2010 84 25 7 3 Quiet
03/21/2010 85 25 2 1 Quiet
03/24/2010 84 27 3 2 Quiet
03/25/2010 88 25 5 2 Quiet
04/01/2010 79 25 12 4 (*)

Tabla 4.2.
Space weather conditions during the days of the contacts

 

(*) The geomagnetic field has been predominantly at unsettled levels with an isolated period of major storming at high lattitudes (12:00-15:00Z).

An explanation of each parameter follows:

The conditions are typical of day of the minimum of the solar cycle, without relevant geomagnetic activity.

 


4.2. Contenido total de electrones (TEC).

The figure 4.1 shows a capture of the total electron content (TEC) provided by the Space Weather Prediction Center (SWPC) of the north-american agency NOAA, corresponding to the 22:50 UTC hours of 03/18/2010. A line has been superimposed, showing the trajectory of the radio links under study.

 

Fig 4.1. TEC registered by NOAA (United States).

 

There is no available data for the other dates. In the figure, in the area of the radio path, the equatorial anomaly is clear: TEC both sides of the Equator is higher than in the Equator itself.

 


4.3. Ionograms.

Ionograms are the graphic result of the measurements made by ionospheric sounding stations, also known as ionosondes, which made steady soundings of the ionosphere by means of transmitting radio waves at different frequencies, following perpendicular or oblique paths from the Earth's surface. The detection of the reflected waves and the measurement of their travelling time allow to calculate several parameters of the ionosphere, such as the presence of its different layers, their thickness and the critical or cut frequency of each layer.

The ionosondes are also able to detect ionospheric irregularities, which show as range spread (spread-F) in the ionograms [Ref.Reinisch].

We have gathered data from the ionosondes at Tucumán (Argentina), Ascension Island (UK), Fortaleza (Brazil) and Sao Luis (Brazil), being those of interest due to their proximity to the radio paths under study (fig.4.2). We have also checked data from other ionosondes near the Equator, such as Kwajalein (Marshall Islands).

 

Fig 4.2. Ionospheric sounding stations closest to the radio path.

 

For each case, we have collected the data in the interval 03/17/2010-03/22/2010, between 20:00 UTC and 23:45 UTC, analyzing the following parameters:

In order to try to identify characteristic patterns of those parameters during the days of the contacts, a wider time margin has been analyzed also, between 03/05/2010-04/05/2010.

Knowing the critical frequency of each layer of the ionosphere (fo), given as a measured value by the ionosonde, we
can determine the maximum usable frequency (MUF) for oblique incidence using the secant law [Ref.Pellejero].

The fig.4.3 shows the minimum fo necessary at a particular altitude to allow a MUF of 50 MHz, working with the antennas pointing to the horizon and using the hyphotesis of straight rays.

 

Fig 4.3. Minimum fo required in each layer to allow oblique reflection with MUF = 50 MHz.

 

In order to allow oblique reflections at 50 MHz in the E layer, the sporadic E layer or the F2 layer, the critical frequencies
measured by the ionosonde must be:

We must consider that those values are related to pure ionospheric reflection. It is common that, before reaching the reflection condition, radio waves are refracted and as a result the rays are bent. This occurs at frequencies even lower than those listed.

 


4.3.1. Tucumán ionosonde.

This section shows the data collected by the ionospheric sounding station located at Tucumán, Argentina (URSI code TUJ2O), coordinates 26,9ºS / 65,4ºW. The measurements were done between the days 17 and 22 march 2010, in the time window between 20:00 UTC and 23:45 UTC.

In the tables, the empty cells show sounding failures or the absence of data. The values in red match the dates and hours of the radio contacts.

The table 4.3 shows the critical frequency of the F2 layer (foF2) values, measured at Tucumán.

 

Table 4.3. Measurements of foF2 at Tucumán (Argentina).

 

Those measurements are plotted in the figure 4.4.

 

Fig 4.4. Plot of the foF2 values registered at Tucumán (Argentina).

 

In the figure 4.5, for each indicated date, the red dotted line show the foF2 values measured by the ionosonde and the white dotted line show the foF2 monthly mean expected value.

 


Fig 4.5. foF2 deviation from the monthly mean in Tucumán (Argentina).

 

We can see that during the days of the radio contacts, from 17-20 hours UTC to more than 23 hours UTC, the foF2 values were up to 3 MHz higher that the monthly mean. That is, there were exceptional ionization conditions.

The table 4.4 shows the prediction of the MUF for a range of 3000 km, using reflection at the F2 layer, known as MUF(3000)F2, from the Tucumán station for the same time intervals.

 

Table 4.4. MUF(3000)F2 predicted at Tucumán (Argentina).

 

The prediction is plotted in the figure 4.6.

 

Fig 4.6. Plot of the MUF(3000)F2 prediction at Tucumán (Argentina).

 

It can be seen that MUF(3000)F2 was over 50 MHz for several hours, the days 17, 19, 20 and 22.

It is necessary to take into account that in the ionograms offered by this station, it is difficult to characterize range spread conditions, which for sure took place as shown by other nearby ionosondes. During range spread episodes, the measurements of the ionosonde may have siginificative errors.

 


4.3.2. Ascension Island ionosonde.

This section shows the data collected by the ionospheric sounding station located at Ascension Island, UK (URSI code AS00Q), coordinates 7,95ºS / 14,4ºW. The measurements were done between the days 17 and 22 march 2010, in the time window between 20:00 UTC and 23:45 UTC.

The figure 4.7 shows the plot of the measurements of the cut frequencies corresponding to the F2 layer (foF2, red plot), the E layer (foE, blue plot) and the sporadic E layer (foEs, yellow plot).

 

Fig 4.7. Plot of the foF2, foE and foEs values registered at Ascension Island (UK).
Click on the image to see a larger version

 

The figure 4.8 shows the plot of the measurements of the minimum altitudes corresponding to the F2 layer (h'F2, red plot), the E layer (h'E, blue plot) and the sporadic E layer (h'Es, yellow plot).

 

Fig 4.8. Plot of the h'F2, h'E and h'Es values registered at Ascension Island (UK).
Click on the image to see a larger version

The figure 4.9 shows the plot of the measurements of range spread in the F layer (QF, pink plot) and in the E layer (QE, blue plot).

 

Fig.4.9. Plot of the QF and QE values registered at Ascension Island (UK).
Click on the image to see a larger version

The figure 4.10 shows an example of range spread, corresponding to the day 03/19/2010 at 22:30 UTC, the hour of one of the radio contacts.

 

Fig 4.10. Range spread or spread-F observed at Ascension Island.

 

The figure 4.11 shows the prediction of the MUF for a range of 3000 km, using reflection at the F2 layer, known as MUF(3000)F2, from the Ascension Island station for the same time intervals.

 

Fig 4.11. Plot of the MUF(3000)F2 prediction at Ascension Island.

 

It can be seen that MUF(3000)F2 was always below 39 MHz. Both foF2 and MUF(3000)F2 plots follow a descending trend with time, due to the effects of electron recombination in the ionosfere at dusk. The effects of range spread seem not to affect much to the measurements and the predictions.

 


4.3.3. Fortaleza ionosonde.

This section shows the data collected by the ionospheric sounding station located at Fortaleza, Brazil (URSI code FZA0M), coordinates 3,8ºS / 38ºW. There isn't available data from the day 17.

The figure 4.12 shows the plot of the measurements of the cut frequencies corresponding to the F2 layer (foF2, red plot), the E layer (foE, blue plot) and the sporadic E layer (foEs, yellow plot), registered at Fortaleza between the days 03/05/2010 and 04/06/2010.

 

Fig 4.12. Plot of the foF2, foE and foEs values registered at Fortaleza (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.13 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.13. Plot of the foF2, foE and foEs values registered at Fortaleza (Brazil) - Detail.
Click on the image to see a larger version

 

We can see that during the time intervals where the contacts were established, the Es layer was not present. However, we may stress that this layer could be hidden in the ionograms during the range spread espisodes.

The figure 4.14 shows the plot of the measurements of the minimum altitudes corresponding to the F2 layer (h'F2, red plot), the E layer (h'E, blue plot) and the sporadic E layer (h'Es, yellow plot), registered at Fortaleza between the days 03/05/2010 and 04/06/2010.

 

Fig 4.14. Plot of the h'F2, h'E and h'ES values registered at Fortaleza (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.15 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.15. Plot of the h'F2, h'E and h'ES values registered at Fortaleza (Brazil) - Detail.
Click on the image to see a larger version

 

The figure 4.16 shows the plot of the measurements of range spread in the F layer (QF, pink plot) and in the E layer (QE, blue plot).

 

Fig.4.16. Plot of the QF and QE values registered at Fortaleza (Brasil) .
Click on the image to see a larger version

The figure 4.17 shows an example of range spread, corresponding to the day 03/19/2010 at 22:30 UTC, the hour of one of the radio contacts.

 

Fig 4.17. Range spread or spread-F observed at Fortaleza (Brazil).

 

The figure 4.18 shows the plot of the MUF(3000)F2 prediction at Fortaleza between the days 03/05/2010 and 04/06/2010.

 

Fig 4.18. MUF(3000)F2 predicted at Fortaleza (Brasil) - General.
Click on the image to see a larger version

 

The figure 4.19 shows a detail of this prediction between the days 03/17/2010 and 03/22/2010.

 

Fig 4.19. MUF(3000)F2 predicted at Fortaleza (Brazil) - Detail.
Click on the image to see a larger version

 

To summarize, the table 4.5 shows the maximum values of all the parameters analysed during the time windows of the radio contacts (table 3.3).

 

Table 4.5. Values of interest at Fortaleza (Brazil) during the time windows of the contacts

 

It can be seen that MUF(3000)F2 was below 38 MHz during all the windows. We can see also some differences between the data from this ionosonde (latitude 3,8ºS) and those from the ionosonde in the Ascension Island (7,95ºS):

 


4.3.4. Sao Luis ionosonde.

This section shows the data collected by the ionospheric sounding station located at Sao Luis, Brazil (URSI code SAA0K), coordinates 2,6ºS / 44,2ºW.

The figure 4.20 shows the plot of the measurements of the cut frequencies corresponding to the F2 layer (foF2, red plot), the E layer (foE, blue plot) and the sporadic E layer (foEs, yellow plot), registered at Sao Luis between the days 03/05/2010 and 04/06/2010.

 

Fig 4.20. Plot of the foF2, foE and foEs values registered at Sao Luis (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.21 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.21. Plot of the foF2, foE and foEs values registered at Sao Luis (Brazil) - Detail.
Click on the image to see a larger version

 

The figure 4.22 shows the plot of the measurements of the minimum altitudes corresponding to the F2 layer (h'F2, red plot), the E layer (h'E, blue plot) and the sporadic E layer (h'Es, yellow plot), registered at Fortaleza between the days 03/05/2010 and 04/06/2010.

 

Fig 4.22. Plot of the h'F2, h'E and h'Es values registered at Sao Luis (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.23 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.23. Plot of the h'F2, h'E and h'Es values registered at Sao Luis (Brazil) - Detail.
Click on the image to see a larger version

 

We can see the presence of the sporadic layer Es during almost all the hours of the time interval under study for the day 18th and only at the beginning of the interval the days 19th and 20th. It is necessary to take into account that the ionosonde detected spread-F during this interval the days 19th and 20th, so those measurements could be subject to errors. The maximum foEs observed is 5,850 MHz, indicating high levels of ionization but seemingly not enough to reflect radio waves of 50 MHz with oblique incidence. The Es layer was present during only one of the contacts, the day 18th at 22:50 UTC.

The figure 4.24 shows the plot of the measurements of range spread in the F layer (QF, pink plot) and in the E layer (QE, blue plot).

 

Fig.4.24. Plot of the QF and QE values registered at Sao Luis (Brasil) .
Click on the image to see a larger version

The figure 4.25 shows an example of range spread, corresponding to the day 03/19/2010 at 22:30 UTC, the hour of one of the radio contacts.

 

Fig 4.25. Range spread or spread-F observed at Sao Luis (Brazil).

 

In this case, we can see that range spread took place during the most time of all the analyzed cases. Every day from 22:00 UTC the foF2 measurements suffer from high variations, most probably due to autoscaling errors as a consequence of range spread.

The figure 4.26 shows the prediction of the MUF for a range of 3000 km, using reflection at the F2 layer, known as MUF(3000)F2, from the Sao Luis station for the same time intervals.

 

Fig 4.26. MUF(3000)F2 predicted at Sao Luis (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.27 shows a detail of this prediction between the days 03/17/2010 and 03/22/2010.

 

Fig 4.27. MUF(3000)F2 predicted at Sao Luis (Brazil) - Detail.
Click on the image to see a larger version

 

To summarize, the table 4.6 shows the maximum values of all the parameters analysed during the time windows of the radio contacts (table 3.3).

 

Table 4.6. Values of interest at Sao Luis (Brazil) during the time windows of the contacts

 

The MUF(3000)F2 predicted was always below 29 MHz during all the openings.

 


4.3.5. Cachoeira Paulista ionosonde.

This section shows the data collected by the ionospheric sounding station located at Cachoeira Paulista, Brazil (URSI code CAJ2M), coordinates 23,20ºS / 45,8ºW.

The figure 4.28 shows the plot of the measurements of the cut frequencies corresponding to the F2 layer (foF2, red plot), the E layer (foE, blue plot) and the sporadic E layer (foEs, yellow plot), registered at Cachoeira Paulista between the days 03/05/2010 and 04/06/2010.

 

Fig 4.28. Plot of the foF2, foE and foEs values registered at Cachoeira Paulista (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.29 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.29. Plot of the foF2, foE and foEs values registered at Cachoeira Paulista (Brazil) - Detail.
Click on the image to see a larger version

The figure 4.30 shows the plot of the measurements of the minimum altitudes corresponding to the F2 layer (h'F2, red plot), the E layer (h'E, blue plot) and the sporadic E layer (h'Es, yellow plot), registered at Cachoeira Paulista between the days 03/05/2010 and 04/06/2010.

Fig 4.30. Plot of the h'F2, h'E and h'Es values registered at Cachoeira Paulista (Brazil) - General.
Click on the image to see a larger version

 

The figure 4.31 is a detail of those measurements between the days 03/17/2010 and 03/22/2010.

 

Fig 4.31. Plot of the h'F2, h'E and h'Es values registered at Cachoeira Paulista (Brazil) - Detail.
Click on the image to see a larger version

 

The figure 4.32 shows the plot of the measurements of range spread in the F layer (QF, pink plot) and in the E layer (QE, blue plot).

 

Fig.4.32. Plot of the QF and QE values registered at Cachoeira Paulista (Brasil) .
Click on the image to see a larger version

We can see that there wasn't any range spread event during the hours of the contacts in the examined window.

The figure 4.33 shows the plot of the MUF(3000)F2 prediction between the days 03/05/2010 and 04/06/2010.


Fig 4.33. MUF(3000)F2 predicted at Cachoeira Paulista (Brasil) - General.
Click on the image to see a larger version

 

The figure 4.34 shows a detail of this prediction between the days 03/17/2010 and 03/22/2010.

 

Fig 4.34. MUF(3000)F2 predicted at Cachoeira Paulista (Brasil) - Detail.
Click on the image to see a larger version

 

To summarize, the table 4.7 shows the maximum values of all the parameters analysed during the time windows of the radio contacts (table 3.3).

 

Table 4.7. Values of interest at Cachoeira Paulista (Brazil) during the time windows of the contacts

 

The MUF(3000)F2 predicted was always below 49 MHz during all the openings.

 


4.4. Ionospheric scintillation maps.

The European Space Agency (ESA) manages the Ionospheric Scintillations Monitoring Service, as a part of its Space Weather European Network (SWENET).

Scintillation is the phenomenon suffered by radio waves travelling through regions of the ionosphere affected by irregularities which deplete about 10% the density of ionization, causing diffraction and scattering. This may affect the satellite communication systems and due to this is steadily monitored, employing terrestrial networks such as the based on the analysis of the GPS constellation signals.

At every hour the radio contacts where made (see table 3.3), in the Cape Verde Islans area the ESA network detected high levels of scintillation (red spots in the map). This could be a good indicator of the existence of ionospheric irregularities in the area, such as the described in section 2.2.

The figure 4.35 shows the scintillation map of 03/18/2010 at 22:00 UTC.

 

Fig 4.35. Scintillation map for the day 03/18/2010 22:00 UTC (ESA/SWENET).

 

The figure 4.36 shows the scintillation map of 03/19/2010 at 22:00 UTC.

 

Fig 4.36. Scintillation map for the day 03/19/2010 22:00 UTC (ESA/SWENET).

 

The figure 4.37 shows the scintillation map of 03/20/2010 at 22:00 UTC.

 

Fig 4.37. Scintillation map for the day 03/20/2010 22:00 UTC (ESA/SWENET).

 

The figure 4.38 shows the scintillation map of 03/21/2010 at 22:00 UTC.

 

Fig 4.38. Scintillation map for the day 03/21/2010 22:00 UTC (ESA/SWENET).

 

 


5. Propagation modes hyphotesis.

From the data analyzed, we can have some conclusions about the ionization levels existing in the area, which may provide an idea of the theoretical MUF between the Canary Islands and Brazil using one or two hops in the F2 layer (1F and 2F modes).

Other possibility is based on the use of the E layer or the sporadic Es layer, through one hop (1E and 1Es modes) or several hops (nE and nEs modes).

Complex modes could also have occurred, combining hops in the F and E layers, such as 1F1E or 1E1F modes.

Finally, some other typical transequatorial modes may have occurred, such as chordal modes in the F layer (FF mode) or duct conduction due to equatorial plasma bubbles (EPB) or a path between the E and F layers.

Determining which propagation mode made this contacts possible is extremelly difficult, mostly if we don't have data about the propagation delay, which could be very helpful to calculate the possible radio paths. In this section we offer different hyphotesis based on the available data.

 


5.1. One-hop hyphotesis.

This hyphotesis is based on the possibility of the radio link established using only one hop in one of the layers of the ionosphere, by means of reflection or scattering.

M.Dolukhanov devotes a section of his book "Propagation of Radio Waves" [Ref.Dolukhanov] to the propagation of metric waves through scatter from the ionosphere, mode discovered by Baily in 1951. According to Dolukhanov, scattering may take place in the D layer or in the lower part of the E layer at night, providing radio ranges up to 2000 km.

 

5.1.1. Geometric analysis using straight rays.

In order to check whether this mode of propagation was possible using only one hop through the E layer (E mode) or through a possible sporadic Es layer (Es mode), we can use the geometrical analysis shown in the figure 5.1.

 

Fig 5.1. Geometry of the link using only one hop.

 

A = Location of the transmitter at Tenerife (Canary Islands).

S = Projection of the reflection/scattering point in the ionosphere over the Earth's surface.

dE = Distance from the transmitter to the reflection/scattering point in the ionosphere.

HE = Altitude of the reflection/scattering point.

RT = Earth's radius = 6370 km

dA = Arc length between points A and S.

The other half of the radio link would be completely simmetrical to the one shown in the figure.

If we take into account that the transmitter's antenna is pointing to the horizon, following the tangent to the Earth's surface, and that the altitude where reflection/scattering occurs in the E layer is HE = 150 km, we can conclude that this reflection/scattering would have taken place at a point located about 200 km north of the Cape Verde archipelago, whose coordinates are 17,25ºN, 23,52ºW, as shown in the figure 5.2.

 

Fig 5.2. Theoretical reflection point at an altitude of 150 km.

 

This point is 1370 km away from Tenerife (Canary Islands) and 3166 km away from Paraíba (Brazil). Direct reflection or scattering towards Paraíba would be impossible because the path would be under the terrestrial tangent at Paraíba, that is, hidden by the Earth's curvature, excluding extraordinary cases of tropospheric refraction. So there is a very low probability for the E or Es mode to occur between both points.

Following the calculations shown in the figure 5.1, we can determine from which altitude the reflected/scattered ray would reach Paraíba thanks to line of sight conditions, being HE = 425 km, that is, a possible F mode. The reflection/scattering would take place just in the middle point of the path between Tenerife and Paraíba, a point over the Atlantic with coordinates 9,73ºN, 27,16ºW (fig.5.3). The F mode of propagation of radio waves up to 50 MHz has been described by several authors [Ref.Morgan], indicating that diurnal peaks of ionization, low latitude points of reflection and years of maximum sunspot activity are most favorable for such transmission.

 

Fig 5.3. Theoretical reflection point at an altitude of 425 km.

 

5.1.2. Analysis using BeamFinder.

It seems more logical that a reflection in the F layer takes place at an altitude of about HE = 300 km. Considering this geometry, the area where the contacted stations are placed would be out of the expected coverage area for one hop, as shown in the map of the figure 5.4, made with the BeamFinder software [Ref.BeamFinder].

 

Fig 5.4. Coverage areas in the 1F and 2F modes (courtesy Volker Grassmann, DF5AI).

 

5.1.3. Ionospheric parabolic model.

In 1990, a study [Ref.Rappaport] tried to justify that some radio links of the Amateur Radio Service in the 50 MHz band with great circle distances between 4.200-6.500 km and up to 10.000 km may have been due to propagation using one or two hops in the F2 layer of the ionosphere, describing the possible geometries with the ionospheric parabolic model. In the fig.5 the results are shown for a maximum free electron density of 3E+12 and different values of the parabolic semi-thickness (150 km, 200 km and 250 km).


Fig 5.x. Great circle path distances in 50 MHz as a function of the takeoff angle
(Rappaport et Al. model).


Each plot corresponds to a different solution reached using numeric models. The vertical axis is the great circle path distance and the horizontal axis is the antenna's takeoff angle. For the specific parameters evaluated, we can see that using takeoff angles between 1º and 3º, distances corresponding to our groups QSO 4200, QSO 6400 and QSO 8200 are possible. However, it is necessary to remark the lack of data from ionosondes in the estimated ionospheric reflection areas, which would allow to extrapolate the data from the graphs to our study.

 

5.1.4. Scattering from regular ionization areas.

Another possibility of propagation related to the F layer is the scattering from regular ionization [Ref.Morgan]. During the fifties, radio amateur communication in the range 50-54 MHz inequatorial regions has been observed in the postsunset period in equinoctial months. Distances between path terminals range from 2,000-7,500 km. The signals
are often garbled and weak and appear to result from F-region scattering.

 

5.1.5. Power budget.

From the power budget point of view, having HE = 425 km the radio path length would be 4645 km. Considering free space losses only, the fading margin would be MF = 47,78 dB, which seems enough to permit this mode.

 

5.1.6. Ionospheric conditions.

If we consider the case where HE = 425 km, this phenomenom is not likely to occur at such altitude because it is commonly registered at lower altitudes. Besides, although the link is geometrically possible, the MUF(3000)F2 predictions from the ionosondes studied don't lead to MUF values higher than 50 MHz between the Canary Islands and Brazil.

The geometry of the link could be even different, not following the straight line between Tenerife and Paraiba. During the time window of the radio contacts, the region of the ionosphere at longitudes near both points is more ionized on the West than in the East, being the eastward part more elevated and less dense at the bottomside, as an effect of recombination. The general perspective would be an F layer tilted towards the West, which would reflect radio waves towards the East, as discovered in some studies of transequatorial propagation in the HF band [Ref.Maruyama]. That is, another possible trajectory would be the reflection in a point of the ionosphere located over the Atlantic with a longitude West of Paraíba, although we would need to take into account the gain loss of the antennas in this azimuth.

 

5.1.7. Discussion.

Aspects for the one-hop hyphotesis:

Aspects against the one-hop hyphotesis:

 


5.2. 2F hyphotesis.

This hyphotesis is based on the possibility of the radio link established using two hops in the F layer of the ionosphere.

 

5.2.1. Geometric analysis.

The figure 5.5 shows a sketch of the hyphotesis based on the radio link using two reflections in the F2 layer, mode also known as 2F.

 

Fig 5.5. Sketch of a 2F radio link.

 

Using the diagram shown in the figure 5.6 we can calculate the lenght of the radio path.

 

Fig 5.6. Geometry of the link using two hops.

 

A = Location of the transmitter at Tenerife (Canary Islands).

B = Location of the receiver at Paraíba (Brazil).

S = Middle point of the trajectory transmitter/receiver over the Earth's surface.

dA = Arc length between points A and S.

dC = Chord length between points A and S.

RT = Earth's radius = 6370 km.

HF = Altitude of the reflection points.

dF = Distance from the transmitter to the first reflection point in the ionosphere.

The total length of the radio path is 4 x dF. If we consider that ionospheric reflection occurs at the F layer, being HF = 230 km, and that the distance from A to B is 4536 km, the radio path would have a length of 4701 km.

The figure 5.7 shows the projection of the different reflection points over the Earth's surface.

 

Fig 5.7. Theoretical reflection points in the mode 2F.

 

The first reflection in the ionosphere would occur at a latitude of 19º and the second one at a latitude of 1º, practically over the Equator.

If we consider an altitude HF = 300 km, the analysis made with BeamFinder [Ref.BeamFinder] shows that the stations in the northeast of Brazil would be inside the shadow area between the first and the second hops, as we can see in the map shown in the figure 5.4.

 

5.2.2. Power budget.

Considering a radio path length of 4701 km and taking into account free space propagation losses only, the fading margin would be MF = 47,83 dB.

However, the takeoff angle necessary for this geometry would be of 16 degrees, so if the antennas are pointing to the horizon their gain at this angle would be lower, and even lower if the azimuth is incorrect, making this fading margin lower also and perhaps too risky.

 

5.2.3. Ionospheric conditions.

The first approach consisted on the utilisation of a prediction software (W6ELProp) to simulate the MUF between Tenerife and Paraíba, using the parameters of the days under study. The results are shown in the figure 5.8. As can be seen, the calculated MUF was always below 33 MHz and never reached the 50 MHz band.

 

Fig 5.8. MUF between Tenerife-Paraíba calculated with W6ELProp

 

We used also the Proplab map provided by Solar Terrestrial Dispatch, which allows to calculate the approximated values of the MUF for radio links greater than 3000 km. The figure 5.7 shows the map corresponding to 03/21/2010 at 21:55 UTC. The calculated MUF was not higher than 36 MHz in any case.

 

Fig 5.7. Proplab map used to calculate MUFs

 

Due to those two facts, and taking into account also that we are currently almost in a minimum of the solar cycle (SSN=33). the 2F mode hyphotesis was initially discarded.

However, once the Tucuman (Argentina) ionosonde data was available, we saw that the MUF(3000)F2 prediction reached values higher than 50 MHz during the time window where some of the contacts took place (see fig.4.5), reviving this way the 2F mode hyphotesis. It is necessary to take into account that this ionosonde is located in Argentina, not in Brazil. Also, the distance of the radio contacts made was higher than 3000 km, so the MUF Tenerife-Paraíba could have been even higher, supporting this way the 2F theory.

Anyway, in the case of other ionosondes in nearest places to the radio path, the MUF(3000)F2 reached maximums of 40 MHz (Fortaleza) and 36 MHz (Sao Luis) when they weren't affected by spreaf-F, that is, near 20:00 UTC. It should be normal that MUF(3000)F2 values were even lower as time advanced, due to lower photoionozation and higher recombination.

In regards of the reflection points in the ionosphere, the first point is very near the ionization peak due to the equatorial anomaly, but the second one is at the minimum between peaks. Although a wave of 50 MHz could be reflected at the first point, something we can doubt about due to the solar cycle and the hour of the day, it would be much less probable for the second reflection to occur.

 

5.2.4. Discussion.

Aspects for the 2F hyphotesis:

Aspects against the 2F hyphotesis:

 


5.3. Complex modes hyphotesis.

The complex propagation modes consist on multiple refractions or reflections in different layers of the ionosphere. The following modes would have a certain possibility to occur:

 

5.3.1. Ionospheric conditions.

The radio contacts were established in the evening, so the electron recombination in the ionosphere was suppossed to have started and the E layer to start vanishing, something we have seen in the ionograms. Due to this, the 1F1E and 1E1F modes would be discarded.

In regards of the sporadic E layer, the ionosondes nearest to the radio path (Fortaleza and Sao Luis) show the presence of this layer during several hours of the days, although there is time coincidence with only one of the radio contacts established. On the other hand, although the foEs values (critical frequency of the Es layer) are high, with maximums up to 5,850 MHz (figs. 4.12 and 4.20), it is not clear that they would be high enough to permit reflection of radio waves of 50 MHz with oblique incidence.

However, the area where the second ionospheric reflection would take place in a theoretical 1F1Es is somewhat far of the ionosondes, so we must not discard the possibility of a sporadic E layer over the Atlantic with enough density of ionization to permit reflection at 50 MHz.

In regards of the possible 1Es1F mode, we don't have information about the possible development of an sporadic Es layer between the Canary Islands and the Cape Verde Islands. This zone is within the equatorial anomaly, with an high density of ionization during the day, so in case the Es would have existed, its foEs could have been even higher than the measured by the ionosondes at Brazil, although it is impossible to know whether enough to permit the propagation at 50 MHz. Anyway, the second hop would have took place in the F layer, near the Equator, where the density of ionization is much lower and most probably not enough to permit the propagation at this frequency, mostly considering the current situation of solar minimum.

 

5.3.2. Discussion.

Aspects for the complex modes hyphotesis:

Aspects against the complex modes hyphotesis:

 


5.4. eTEP hyphotesis.

Taking into account the hour and date of the contacts, one of the most accurate hyphotesis points to evening transequatorial radio propagation (eTEP). All the contacts were made between 21:50 GMT and 22:53 GMT (GMT was y the local time at the Canary Islands), in or around the date of the spring equinox, which seems to be favourable for TEP to occur.

 

5.4.1. Introduction to the TEP modes.

There are two different modes of transequatorial propagation. The first one is the afternoon TEP (aTEP), which occurs during the afternoon, in conditions of maximum ionization [Ref.Yeh]. As we have seen in the section 2.1, as a consequence of the equatorial anomaly the ionizacion maximum doesn't happen in the region of the ionosphere above the Equator, but at latitudes between 10º and 20º. If the density of ionization is very high (i.e. around the solar cycle maximum), within those ionization peaks refraction of VHF radio waves can occur, as shown in the figure 5.10. If refraction occurs in both peak regions, long distance transequatorial radio links are possible in the indicated bands. This mode is of chordal type and is called FF mode.

 

Fig 5.10. Sketch of a aTEP (FF) radio link

 

The hour of the radio contacts was between 21:50 UTC and 22:52 UTC, so there wasn't aTEP. However, they are within the time window of the other type of transequatorial propagation, called evening TEP (eTEP).

As we have seen in the section 2.2, during the evening, in the equatorial zone and low latitudes, equatorial plasma bubbles (EPB) appear forming field aligned irregularities (FAI): as a result of the dynamic processes of the beginning of electron recombination during the evening, irregular ionization clouds are created, growing with time, moving upwards and vanishing around 03:00 AM. Those clouds are aligned with the geomagnetic field. The gaps in density of ionization between those clouds and the areas less ionized cause refraction and even propagation of radio waves through ducts, increasing their range, as shown in the figure 5.11.

 

Fig 5.11. Sketch of an eTEP radio link

 

Those ducts are aligned following the direction of the magnetic North-South (perpendicular to the magnetic Equator) with a maximum deviation of about 15 degrees. The figure 5.12 shows the number of EPBs registered between 1989 and 2000 [Ref.Burke]. The horizontal axis is the geographic longitude and the vertical axis is the number of the day within the year. The number of observed EPBs is represented with the color code. In our particular case, we are between 234º (Paraiba) and 344º (Canary Islands), between the days 77 (03/17/2010) and 81 (03/21/2010), being the probability of EPB events at medium/high level from a statistical point of view.

 

Fig 5.12. Distribution Day-Longitude for the number of EPBs registered between 1989 and 2000.

 

5.4.2. Geometric analysis.

The peculiarity of the radio contacts under study is that the angle between the radio path and the geomagnetic North-South direction is not within the expected values, as shown in the figure 5.13.

 

Fig 5.13. Angle of the radio path with the geomagnetic N-S direction.

 

In the figure, the radio path is represented by a white line, the geographic coordinate system by a red grid and the geomagnetic coordinate system with a yellow grid (thin lines).

We can see that the angle of the radio path with the geomagnetic meridians is 35º at Paraiba and 42º at the Canary Islands, in both cases very far from the theoretical 15 degrees stated for TEP propagation [Ref.TRP] [Ref.Harrison]. This is one of the facts we consider of interest about those radio contacts.

 

5.4.3. Ionospheric conditions.

The observations point to the formation of equatorial plasma bubbles (EPB), as shown in the ionograms of Ascension Island (fig.4.9), Fortaleza (fig.4.16) and Sao Luis (fig.4.24), revealing range spread during almost all the hours of the radio contacts. Those range spread observations indicate the presence of ionospheric irregularities in the southerner part of the radio path, most probably aligned with the geomagnetic field (FAI). There is no data available for the northern part of the radio path. However, the scintillation maps (fig.4.35-38) reveal scintillation south of the Canary Islands during the hours of the radio contacts.

Thus, we can affirm that during the hours of the radio contacts there were ionospheric irregularities probably due to EPB, which may have formed ducts allowing the propagation of 50 MHz radio waves at long distances through the Equator.

However, those ducts are normally aligned or almost aligned with the N-S geomagnetic lines (fig.2.2), so the radio path would not coincide with the orientation of the ducts, unless there were other type of additional phenomenons causing the appearance of ramifications in an oblique way with respect to the N-S direction. Another possibility would be the development of ducts between different EPBs following the trajectory of the radio path. As was indicated in the section 4.1, the geomagnetic field was inactive (Kp=0) and there weren't events of solar radiation storms.

On the other hand, the formation of those ducts is more typical during the solar cycle maximums, and during the radio contacts we were in the solar minimum (SSN=24-28), although it is remarkable that during those days the ionization seemed to be higher than the expected mean values, as seen in the measurements of foF2 and foEs, and also in the predictions of MUF(3000)F2.

Another possibility of propagation consists on scattering with the equatorial plasma bubbles, as suggested in other studies such as the reception of long distance TV signals in Japan in the 47.5-52.2 MHz band [Ref.Takano].

 


5.5. 2E/3E/4E hyphotesis.

The 2E/3E/4E hyphotesis has been raised from an analysis made by DF5AI, using his software BeamFinder [Ref.BeamFinder]. According to this hyphotesis, the radio contacts were established using two, three and four reflections in the E layer of the ionosphere, or even in sporadic E layers.

 

5.5.1. Geometric analysis.

The calculations made for multiple E layer or sporadic E layer hops at an altitude of 105 km, gave the results shown in the figure 5.14.

 

Fig 5.14. Coverage areas for 2E and 3E modes, calculated with BeamFinder (courtesy Volker Grassmann, DF5AI).

 

The dark blue squares represent the coverage areas for two hops (2E) or three hops (3E) in the E layer or the sporadic E layer, from the Canary Islands. The coincidence with the locations of the stations in Brazil is amazing (fig.3.1). If we extrapolate the results to a possible fourth hop (4E), the stations in Uruguay and Argentina would lay within the coverage area.

Regarding the reflections in the Earth's surface, according to the map shown in fig.5.14 we can see the following for the three reflection points used to complete the 4E mode:

The first reflection in the Earth's surface would be in the Atlantic Ocean, southwest of the Cape Verde archipelago, that is, with an excellent electrical conductivity.

The second reflection in the Earth's surface would be in the northeastern coast of Brazil, around the locations of PT7TT, PT7ZAP, PY0FF, PR7AR and PY7XAF (fig.5.15). The white line crossing diagonally is the radio path between Tenerife (Canary Islands) and Buenos Aires (Argentina). We can conclude that there is reflection also in the Atlantic Ocean, that is, with very good electrical conductivity.

 

Fig 5.15. Area of the second reflection in the Earth's surface for the 2E/3E/4E modes.

 

The third reflection in the Earth's surface would be in the inner land of Brazil, around the locations of PP1CZ, PY1ZV, PY2MAJ, PY2REK, PP5XX and PY5EW. The figure 5.16 shows a detail of the area obtained from Google Earth, marking the most significant water masses with blue poligons: Iguaçu River, Itaipu Lake, Parana River, Xavantes Dam, Jurumirim Dam, Barra de Bonita Dam and Promissao Dam. The white line crossing diagonally is the radio path between Tenerife (Canary Islands) and Buenos Aires (Argentina). We can conclude that in this area there are enough water masses to allow the reflection of the radio waves back to the ionosphere.

 

F

Fig 5.16. Area of the third reflection in the Earth's surface for the 3E/4E modes.

 

Regarding the reflections in the ionosphere, they would occur roughly in the regions around the following points:

 

5.5.2. Power budget.

From the point of view of the power budget and using the geometric calculations shown in the fig.5.1, the radio path would have the following lengths (HE = 105 km):

Then we can calculate the fading margin after the losses of propagation only (section 3):

That is, the power budget seems to be enough to allow all the proposed modes.

 

5.5.3. Ionospheric conditions.

As we have seen in section 4, the ionograms taken near the radio path show the presence of the E layer only in the first samples of each day (until 20:40 UTC maximum), and the radio contacts took place later, so we can expect that at those hours the E layer had vanished. Anyway, some of those ionosondes registered sporadic E layers with critical frequencies up to foEs = 5,900 MHz, that is, quite high. So we must not discard the possibility of sporadic E layers developed at least in 4 areas of the radio path to allow the modes 2E/3E/4E, having enough density of ionization to allow operations in the 50 MHz band.

 

5.5.4. Discussion.

Aspects for the 2E/3E/4E modes hyphotesis:

Aspects against the 2E/3E/4E modes hyphotesis:

 


7. References.

[Ref.TRP]
"Transequatorial Radio Propagation".
IPS Radio and Space Services (Australia).

[Ref.Harrison]
"Evening transequatorial VHF propagation".
R. Harrison.

[Ref.Dolukhanov]
"Propagation of Radio Waves".
M. Dolukhanov
Ed.Mir, Moscow (1965).

[Ref.Yeh]
"An Investigation of Motions of the Equatorial Anomaly Crest".
K. C. Yeh, S. J. Franke, E. S. Andreeva, V. E. Kunitsyn (2001).

[Ref.Maruyama]
"Equatorial ionospheric disturbance observed through a transequatorial HF propagation experiment".
T. Maruyama and M. Kawamura.
Annales Geophysicae, 24: 1401-1409 (2006).

[Ref.Burke]
"Seasonal-longitudinal variability of equatorial plasma bubbles".
W. J. Burke, C. Y. Huang, L. C. Gentile, and L. Bauer
Annales Geophysicae, 22: 3089-3098 (2004).

[Ref.Reinisch]
"Multistation digisonde observations of equatorial spread F in South America".
B.W. Reinisch, M. Abdu, I. Batista, G. S. Sales, G. Khmyrov, T. A. Bullett, J. Chau, and V. Rios
Annales Geophysicae, 22: 3145-3153 (2004).

[Ref.BeamFinder]
BeamFinder analysis software: http://www.beamfinder.net/
Dr. V. Grassmann (DF5AI).

[Ref.Pellejero]
"Geometry and operative aspects of the simple modes of ionospheric propagation".
I. Pellejero (EA4FSI)

[Ref.Morgan]
"A Review of VHF Ionospheric Propagation".
Morgan, M.G.
Proceedings of the IRE. Volume: 41 , Issue: 5 (1953). DOI: 10.1109/JRPROC.1953.27439

[Ref.Takano]
"Broadband Observations of Propagation Anomaly of VHF WavesTransmitted from Oversea Broadcasting Stations".
Takano, T.; Sakai, K.; Nagashima, I.; Nakata, H.; Akaike, H.; Ujigawa, S.; Higasa, H.; Shimakura, S.;
IEEE Radio Science Conference, 2004. Proceedings. 2004 Asia-Pacific. DOI: 10.1109/APRASC.2004.1422486

[Ref.Rappaport]
"A Single-hop F2 Propagation Model For Frequencies Above 30 MHz And Path Distances Greater Than 4000 KM".
Rappaport, T.S.; Campbell, R.L.; Pocock, E.;
Geoscience and Remote Sensing Symposium, 1990. IGARSS '90. 'Remote Sensing Science for the Nineties'., 10th Annual Internationa. DOI: 10.1109/IGARSS.1990.688284

 


8. Glossary.

 


9. Acknowledgements.


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