The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia

Abstract

Abstract. Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9°N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.

Latitudinal extension of low-latitude scintillations measured with a network of GPS receivers

“…On many days, GPS scintillations, TEC depletions, and radar plumes were detected even when the southern crest was absent. The main contributor to the asymmetry of the anomaly is the meridional wind that during the December solstice blows from south to north (Walker et al, 1994). Other factors may be an asymmetrical distribution of the O/N2 ratio or latitudinal gradients of the zonal or vertical neutral wind.…”

Electrojet control of ambient ionization near the crest of the equatorial anomaly in the Indian zone

“…The higher "m" values and larger correlation coefficients at high level of significance are also recorded around 19:00-20:00 IST, though the overall EEJ strength around the period (17:00-18:00 IST -before sunset) is observed to be low. The anomaly is fully developed around 13:00-16:00 LT depending on season and solar activity level (Walker et al, 1994). After the development, a decaying trend as well as equatorward movement of the crest follows.…”

“…The latitudes of the anomaly crest and strength of the anomaly vary with day, month, season and solar activity as well as with longitudes and wind systems (Rastogi, 1966;Golton and Walker, 1971;Rush and Richmond, 1973;Huang et al, 1989;Walker et al, 1994;Balan and Bailey, 1995). The vertical E×B drift of plasma over the magnetic equator at the F-layer altitude and subsequent diffusion along the magnetic field lines, known as equatorial fountain, generate the EIA.…”

“…TEC variability due to EEJ was studied by several groups (Walker and Ma, 1972;Sethia et al, 1980;Balan and Iyer, 1983;Rastogi and Klobuchar, 1990;Walker et al, 1994;Rama Rao et al, 2006). From low latitude region TEC is reported to exhibit a positive correlation with EEJ strength while a negative correlation is reflected from the equatorial stations.…”

GPS-TEC Variation during Low to High Solar Activity Period (2010-2014) under the Northern Crest of Indian Equatorial Ionization Anomaly Region

“…in Africa, [8]- [13], in South America, [14] [15] [16] [17] [18] over China [19] [20], Huo et al [21] and Perevalova et al [22] over North America, Zakharenkova et al [23] over Japan; Venkatesh et al [24] [25] [26] over Brazil and many more. Ionospheric TEC variations have been investigated in the Indian region, using this data and other separate TEC measurements at Surat [27], Agra [28] and other different stations, namely Trivandrum, Waltair, Raipur and Delhi [6].…”