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Daily evolution of the ionosphere

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MainTechnical ArticlesDaily evolution of the ionosphere

Este artículo también está disponible en español (this paper is also available in spanish).

Abstract:

This article shows a didactic description, taking advantage of scientific data, of the daily evolution of the different layers of the ionosphere, whose knowledge is critical to work properly with HF radio communication systems.

In order to achieve this, I have done an analysis of the parameters measured by the ionospheric sounding station that the INTA (Spanish National Institute of Aerospace Technology) uses in El Arenosillo (Huelva, Spain), during the past March 18th, 2010. The ionosonde measures several parameters each 15 minutes, during the 24 hours of the day, using in this particular case vertical sounding techniques, that is, with an elevation angle of 90 degrees (perpendicular to the Earth's surface).

Those measurements provide lots of information about the ionosphere, such as the existence of the different layers at particular hours, their altitude, their thickness or their critical frequency.

Each layer of the ionosphere has its particular critical or cut frequency (fo), defined as the maximum working frequency that will allow the reflection of a radio wave in this layer, using a takeoff angle of 90 degrees. This way, the sporadic layer Es has a critical frequency known as foEs, and the F2 layer will have its own, known as foF2. Knowledge of the foF2 frequency is of particular importance to work NVIS (Near Vertical Incident Skywave), a mode commonly used in tactical and emergency communications, due to the fact that at each instant of the day it will put a limit to the maximum frequency we can use.

All the data registered by the ionosonde is stored in a SAO file and can be played with the tool SAO Explorer, developed by the Center for Atmospheric Research, University of Massachussetts Lowell. For the particular case of the day under study, I have created a video which shows what is happening in the ionosphere, every 15 minutes, during the 24 hours of the day.

 


1. Interpretation of the information.

In the reference system used, frequency (MHz) is represented in the the abscissa axis, and altitude over the Earth's surface (meters) is represented in the ordinate axis. In the right upper part, in white letters, the date and hour of each measurement are shown. The video goes on showing the measurements taken every 15 minutes. In the upper part, in yellow letters, the tool shows the record of every particular layer of the ionosphere (E, F1, F2) at each hour, and also its critical frequency (foE, foF1, foF2) in MHz. In HF radio communications, reflection at the F2 layer is the most commonly mode used, so the evolution of the foF2 values will be interesting to follow.

In the lower part of the screen, in yellow letters, an indication will appear every time the sporadic layer Es is registered, showing also its critical frequency foEs.

In the central part of the screen, the white curve shows the results of the measurements related to the F2 layer. The point where this curve cuts the perpendicular line marking the foF2 frequency allows to determine, using the ordinate axis on the left, at which altitude the reflection at this layer occurs.

To make easier the interpretation of the data, other parameters measured by the ionosonde are omitted, making reference to them in the chronological analysis only if they can provide information of interest.


2. Chronological analysis.

Now you will find the chronological analysis of all the events observed. All the hours are expressed in GMT. In the day under study (2010/03/18), the hour in Spain, where the ionosonde is located, was GMT+1 (GMT in the Canary Islands), the sunspot number was SSN=24 and the solar flux index SFI=84. There weren't any events related neither to solar nor to geomagnetic storms.

It is recommended that you pause the video at each key hour, so you will be able to see each event carefully.

00:00 During the night, only the F layer is registered, here named as F2, located at an altitude of 292 km. The critical frequency foF2 reached only 3.100 kHz. Until 06:30 in the morning, only the 160 meters band will be usable for NVIS operations.

02:15 1st occurrence of the Es sporadic layer, located at an altitude of 122 km, with a critical frequency foEs of 1.900 kHz. It will last until 02:45 and foEs will change to 2.300 during this period.

04:15 2nd occurrence of the Es sporadic layer, located at an altitude of 90 km, with a critical frequency foEs of 2.000 kHz. It will last15 minutes.

04:55 The dawn arrives, the Sun starts to show in the horizon.

05:00 3rd occurrence of the Es sporadic layer, located at an altitude of 120 km, with a critical frequency foEs of 2.000 kHz. It will last 20 minutes.

06:00 The critical frequency foF2 reaches its minimum of the day at 2.500 kHz. Electron recombination at the ionosphere reaches its maximum level at this point of the night. From this moment, the presence of the Sun in the East will be more and more noted and the foF2 will start to rise. Until this hour, the altitude of the F layer has oscillated between 250 km and 317 km.

06:45 Creation of the E layer at an altitude of 108 km, with a critical frequency foE of 1.420 kHz. The critical frequency of the F2 layer, foF2, has already risen to 4.425 kHz, so the 80 meters band starts to be usable for NVIS operations. This working window will last until 20:00 hours.

07:30 4th occurrence of the Es sporadic layer, located at an altitude of 117 km, with a critical frequency foEs of 2.100 kHz. It will last 15 minutes.

08:15 5th occurrence of the Es sporadic layer, located at an altitude of 132 km, with a critical frequency foEs of 3.200 kHz. It will last 45 minutes and foEs will vary from 1.900 kHz to 5.200 kHz during this period.

08:30 The effects of the rising Sun are now remarkable and photoionization starts. The F2 layer critical frequency, foF2, now reaches 5.900 kHz.

09:00 The F layer, uniform until now, starts to be split into two sublayers: F1 at an altitude of 187 km and F2 at an altitude of 242 km.

09:30 6th occurrence of the Es sporadic layer, located at an altitude of 124 km, with a critical frequency foEs of 3.800 kHz. It will last 30 minutes.

10:15 7th occurrence of the Es sporadic layer, located at an altitude of 110 km, with a critical frequency foEs of 2.900 kHz. It will last 45 minutes.

10:45 The F2 sublayer is suddenly located at an altitude of 307 km. Its critical frequency, foF2, has risen up to 7.050 kHz. The F1 sublayer is much lower, at an altitude of 177 km.

11:00 The F2 layer critical frequency, foF2, has reached 7.400 kHz, allowing the 40 meters band to be useable for NVIS operations. This working window will last until 18:45 in the evening.

11:15 8th occurrence of the Es sporadic layer, located at an altitude of 110 km, with a critical frequency foEs of 3.200 kHz. It will last 15 minutes. Now, the F1 and F2 sublayers have merged again into only one F layer, placed at an altitude of 222 km. A quarter of an hour later, they will be split again.

11:45 The Sun reaches its highest elevation point, photoionization is at its maximum and now the F2 layer critical frequency, foF2, reaches also its maximum at 8.625 kHz. The ionosonde registers also the 9th occurrence of the Es sporadic layer, located at an altitude of 107 km, with a critical frequency foEs of 3.200 kHz. It will last 2 hours and foEs will vary from 3.200 kHz to 6.300 kHz during this period.

12:30 The F1 and F2 sublayers merge again into only one F2 layer, at an altitude of 237 km, reaching its foF2 critical frequency 8.000 kHz.

13:00 A sounding problem occurs and the ionosonde does not provide data during 15 minutes. Similar failures will occur at 14:00, 15:15 and 20:15 hours.

13:15 The F layer is split again into the sublayers F1 and F2. The ionosonde registers also the 10th occurrence of the Es sporadic layer, located at an altitude of 100 km, with a critical frequency foEs of 4.000 kHz. It will last 2 hours and a quarter, and foEs will reach 6.300 kHz.

14:45 The F1 and F2 sublayers are merged again, during 15 minutes, into only one F layer at an altitude of 242 km. Minutes later they will split again.

15:45 The F1 and F2 sublayers are merged again, during 30 minutes, into only one F layer at an altitude of 222 km. Minutes later they will split again.

16:45 11th occurrence of the Es sporadic layer, located at an altitude of 135 km, with a critical frequency foEs of 3.800 kHz. It will last 1 hour and 15 minutes.

18:30 When the evening falls, the photoionization is weaker and the process of recombination of free electrons in the ionosphere starts. The electron density in the E layer is so low that the ionosonde is not able to register this layer no more, although it will not disappear completely. The F1 sublayer won't be registered again until the next day, and the critical frequency of the F2 layer, foF2, will start to fall abruptly, although its current value is still 7.600 kHz.

18:45 The F2 layer critical frequency, foF2, has fallen down to about 7.000 kHz, making the 40 meters band unusable for NVIS from this moment.

19:15 Although the grade of ionization of the E layer is very weak, the ionosonde registers the 12th occurrence of the Es sporadic layer, located at an altitude of 120 km, with a critical frequency foEs of 2.700 kHz. It will last 45 minutes.

20:00 The F2 layer critical frequency, foF2, continues falling, reaching now 4.400 kHz. The 80 meters band will be unusable for NVIS operations in the next 15 minutes, until the next day.

20:15 The dusk arrives, the sun starts to hide in the horizon.

20:45 13th occurrence of the Es sporadic layer, located at an altitude of 120 km, with a critical frequency foEs of 2.600 kHz. This sporadic will last until midnight, and foEs will vary from 1.900 kHz to 2.900 kHz during this period

22:45 The F2 layer critical frequency, foF2, continues dropping, reaching now 3.400 kHz.

23:45 Midnight is about to arrive and the effects of electron recombination are quite remarkable now, to the point that foF2 has fallen to 2.700 kHz. A new day will start.

 


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Ismael Pellejero - EA4FSI
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