Ionospheric variations over Indian low latitudes close to the equator and comparison with IRI-2012
In this paper, we analyze daytime observations of the critical frequencies of the F2 (<i>fo</i>F2) and F3 (<i>fo</i>F3) layers based on ionosonde observations made from Indian low latitudes close to the magnetic equator and study their local time, seasonal, planetary-scale...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-08-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/33/997/2015/angeo-33-997-2015.pdf |
| Summary: | In this paper, we analyze daytime observations of the critical frequencies
of the F2 (<i>fo</i>F2) and F3 (<i>fo</i>F3) layers based on ionosonde observations made from
Indian low latitudes close to the magnetic equator and study their local
time, seasonal, planetary-scale variations (including the solar rotation effect),
and solar activity dependence. Given the occurrence of the F3 layer, which has remarkable local time, seasonal and solar activity dependences, variations in <i>fo</i>F2 have been evaluated.
Local time
variations in <i>fo</i>F2 and <i>fo</i>F3 show noon "bite-out" in all seasons and in all
solar activity conditions, which are attributed to vertically upward plasma
transport by the zonal electric field and meridional neutral wind. Comparison of
observed <i>fo</i>F2 with those of the IRI-2012 model clearly shows that the model
values are always higher than observed values and the largest difference is
observed during noontime owing to the noon bite-out phenomenon. Peak frequency of the
F layer (<i>fo</i>F2 / <i>fo</i>F3), however, is found to have better agreement with
IRI-2012 model. Seasonal variations of <i>fo</i>F2 and <i>fo</i>F3 show stronger asymmetry
at the solstices than at the equinoxes. The strong asymmetry at the solstice is
attributed to the asymmetry in the meridional neutral wind with a
secondary contribution from <b>E</b> × <b>B</b> drifts, and the relatively
weak asymmetry observed at the equinox is attributed to the asymmetry in
<b>E</b> × <b>B</b> drifts. Variations in <i>fo</i>F2 and <i>fo</i>F3
with solar flux clearly show the saturation effect when <i>F</i><sub>10.7</sub> exceeds
~ 120 sfu, which is different from that of the mid-latitudes.
Irrespective of solar flux, both <i>fo</i>F2 and <i>fo</i>F3 in summer, however, are found
to be remarkably lower than those observed in other seasons. Variations in
<i>fo</i>F2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in <i>fo</i>F2 are found to be maximum in low solar activity
(LSA), moderate in medium solar activity (MSA) and minimum in high solar
activity (HSA), while the amplitudes of ~ 27-day variations in
<i>F</i><sub>10.7</sub> are minimum in LSA, moderate in MSA and maximum in HSA. These
results are presented and discussed in light of current observational
and model-based knowledge on the variations of low-latitude <i>fo</i>F2 and <i>fo</i>F3. |
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| ISSN: | 0992-7689 1432-0576 |