Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

Abstract

Abstract. The ionosonde NmF2 data (covering several solar cycles) and the JPL TEC maps (from 1998 through 2009) were collected to investigate the equinoctial asymmetries in ionospheric electron density and its variation with solar activity. With solar activity increasing, the equinoctial asymmetry of noontime NmF2 increases at middle latitudes but decreases or changes little at low latitudes, while the equinoctial asymmetry of TEC increases at all latitudes. The latitudinal feature of the equinoctial asymmetry at high solar activity is different from that at low solar activity. The increases of NmF2 and TEC with the solar proxy P = (F 10.7 + F 10.7A )/2 also show equinoctial asymmetries that depend on latitudes. The increase rate of NmF2 with P at March equinox (ME) is higher than that at September equinox (SE) at middle latitudes, but the latter is higher than the former at the EIA crest latitudes, and the difference between them is small at the EIA trough latitudes. The phenomenon of higher increase rate at SE than at ME does not appear in TEC. The increase rate of noontime TEC with P at ME is higher than that at SE at all latitudes, and the difference between them peaks at both sides of dip equator. It is mentionable that the equinoctial asymmetries of NmF2 and TEC increase rates present some longitudinal dependence at low latitude. The influences of equinoctial differences in the thermosphere and ionospheric dynamics processes on the equinoctial asymmetry of the electron density were briefly discussed.

Characteristics of low‐latitude TEC during solar cycles 23 and 24 using global ionospheric maps (GIMs) over Indian sector

“…For example, by Essex [] during solar high (2 years), by Titheridge and Buonsanto [] during solar high (3 years), by Zhao et al [] during solar high (6 years), by Chakraborty and Hajra [] during ascending solar phase (4 years), by L. Liu et al [] only during low solar (3 years), and by Chen et al [] during complete cycle 23 (11 years). As shown in Figure 1 of Chen et al [], the equinoctial asymmetry existed predominantly over all latitudes in low as well as high solar activity with peak of this asymmetry found near the crests of EIA and a decreasing trend elsewhere. In contrast to their results, our results evidently show (Figure d) an opposite equinoctial asymmetry over northern LL location during 2011–2013 (3 years) and over equator and southern LL locations only during 2011 of the current solar cycle 24.…”

Statistical characteristics of ionogram spread-F and satellite traces over a Chinese low-latitude station Sanya

“…By examining the sunset time during the two equinoxes, Liu et al (2015) reported that the mean sunset time during spring months is 25 min later than that in autumn months which led to the equinoctial scintillation onset time difference. It is relevant to mention that the phenomenon of equinoctial difference also exists in seasonal variation of ionospheric electron density at low and middle latitudes (Liu et al, 2010;Chen et al, 2012). Ren et al (2011Ren et al ( , 2012 suggested that the equinoctial asymmetry of equatorial vertical plasma drift plays an important role for the equinoctial difference of plasma density.…”

High-latitude thermospheric zonal winds during low solar activity period

“…This period spanning over three years provides enough data for detecting the diurnal variations at the different seasons. In general, solar activity may be different between March equinox and September equinox of a year (Chen et al, 2012), thus the need to separate the equinox seasons. Figure 2 shows the distribution of the wind speeds within the selected latitude band.…”