Yearly variations in the low-latitude topside ionosphere

Abstract

Abstract. Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the lowlatitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no signi®cant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the eects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is dierent for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive.

Studying the variability in the diurnal and seasonal variations in GPS total electron content over Nigeria

“…Scherliess and Fejer (1999) had earlier inferred that daytime E×B drift velocities are larger in the equinoctial months and winter months than in the summer months, and this could result in semi-annual variation. Olatunji (1967), Bailey et al (2000), and Liu et al (2006Liu et al ( , 2009 found that this semi-annual variation is related to the variation in the noon solar zenith angle, which is an important factor in ionization. Wu et al (2004), Rama Rao et al (2006a, and Lee et al (2010) attributed the semi-annual variation to a combined effect of solar zenith angle and geomagnetic field geometry.…”

“…Rabiu (2004) observed semi-annual variation with equinoctial maxima in ranges of H , D, and Z components of the geomagnetic field and suggested the cause may be due to one or more of three models commonly referred to as axial, equinoctial, and Russell-McPherron mechanisms (for example Clua de Gonzalez, et al, 1993Gonzalez, et al, , 2001Russell and McPherron, 1973;Legrand and Simon, 1989;Simon and Legrand, 1989;Crooker and Siscoe, 1986;Orlando, et al, 1993; and references therein). Olatunji (1967), Scherliess and Fejer (1999), Bailey et al (2000), and Liu et al (2006Liu et al ( , 2009 suggested that daytime E×B drift velocities are larger in the equinoctial months (February, March, April, August, September, and October) and winter months (November, December, and January) than in the summer months (May, June, and July) and this could result in semi-annual variation. Olatunji (1967), Bailey et al (2000), and Liu et al (2006Liu et al ( , 2009) related this semi-annual variation to the variation in the noon solar zenith angle, which is an important factor for the production of ionization.…”

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

“…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.…”

Electron Density Comparison Between IRI 2007 and DEMETER Satellite Data in Solar Minimum Year

“…3, reveals that when the solar zenith angle is inclined to the southern hemisphere (in Northern Hemisphere Winter), the N e was overestimated in the southern hemisphere and underestimated in the northern middle latitudes, and vice versa. The dynamic process at middle latitude between the thermosphere and ionosphere should consider additional factors such as neutral gas density of (N 2 ) (O), and global thermospheric circulation effects (Bailey et al 2000).…”