Solar activity dependence of the topside ionosphere at low latitudes

Abstract

[1] We investigated the solar activity dependence of the topside ionosphere with ROCSAT-1 observations. The distribution of the plasma density at 600 km altitude shows features with considerable local time, season, and solar activity differences. In the daytime, plasma density peaks around the dip equator. This peak is more distinct in equinoxes and weaker in May-July, and it enhances with solar activity in all seasons. The seasonal behavior of this peak is primarily controlled by the seasonal variations of neutral density and E Â B vertical drift. The enhancement of the peak with solar activity is related to the effect of E Â B vertical drift. Around sunset, double peaks are found in the latitudinal distribution of plasma density in solar maximum equinoxes and December solstice, which are mainly attributed to the effects of strong prereversal enhancement (PRE) vertical drift. Moreover, the plasma density at 600 km altitude strongly depends on the solar proxy P = (F 107 + F 107A )/2. At higher altitudes, e.g., 800 km, the amplification trend prevails in the solar activity variations of plasma density. In contrast, the plasma density at 600 km altitude presents three kinds of patterns (linear, amplification, and saturation), which has not been reported. Saturation effect is found at equinox sunset around the dip equator. This saturation effect is attributed to the increase in the PRE vertical drift with solar activity. Solar activity effects of ROCSAT-1 plasma density are argued to be the combined effects induced by the changes in the peak height, the scale height, and the peak electron density, respectively. Among these factors, the rise of the F 2 peak is more important for the equatorial plasma density at 600 km altitude.

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

“…The increased atomic composition may affect NmF2 via dynamic processes even [O]/[N 2 ] is invariant between ME and SE. Chen et al (2009) indicated that the increased neutral density (without [O]/[N 2 ] changing) results in higher electron density at low latitudes. Thus the equinoctial difference of neutral density also affects the equinoctial asymmetry of the electron density.…”

“…Solar zenith is equal at March equinox (ME) and September equinox (SE), but the ionosphere presents some differences between the two equinoxes under equivalent solar activity conditions (e.g. Bailey et al, 2000;Balan et al, 1997Balan et al, , 1998Chen et al, 2009;Essex, 1977;Kawamura et al, 2002), namely ionospheric equinoctial asymmetry. This asymmetry may originate from the equinoctial differences in neutral winds, thermospheric composition and density, electric fields, and so on.…”

Further study on the solar activity variation of daytime NmF2

“…Many works showed the nonlinearity appearing in the variations of the ionospheric electron density with solar indices [e.g., Balan et al, 1994aBalan et al, , 1996Belehaki et al, 2001;Chen et al, 2008Chen et al, , 2009Gupta and Singh, 2000;Kouris et al, 1998;Liu et al, 2003Liu et al, , 2004aLiu et al, , 2006Liu et al, , 2007Liu and Chen, 2009;Ma et al, 2009;Park et al, 2008;Richards, 2001;Sethi et al, 2002;Su et al, 1999]. An important nonlinear character is the saturation effect that was found in the variations of N m F 2 (maximum electron density of the F 2 layer) or TEC (total electron content) with solar indices; namely, N m F 2 (TEC) increases linearly with solar indices with a higher rate at lower solar activity levels, whereas the increase rate decreases, or even N m F 2 (TEC) does not obviously increase, at higher solar activity levels.…”

How does ionospheric TEC vary if solar EUV irradiance continuously decreases?

“…Under the normal solar cycle condition, TEC peaks at the dip equator during equinoxes and at a low-latitude band of the summer hemisphere during solstices. At nighttime, with the vanishing of the equatorial fountain effect, the EIA structure disappears except at solar maxima when the pre-reversal enhancement of the upward plasma drift induces a strong EIA at sunset, which can last to nighttime (e.g., Chen et al 2009). There is no EIA structure after sunset at lower solar activity levels as the pre-reversal enhancement of the upward plasma drift is positively correlated with the solar activity level (e.g., Fejer et al 1991).…”