An equinoctial asymmetry in the high‐latitude thermosphere and ionosphere

Abstract

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Throughout the solar cycle, average nighttime thermospheric meridional winds are larger in spring than autumn despite similar levels of solar insolation. The average ion velocities are also larger in spring than autumn at solar maximum, but at solar minimum this position is reversed. Numerical simulations of the thermosphere and ionosphere have not predicted such asymmetries because they generally assume forcing functions that are symmetric about the solstices. The proposed explanation lies in the annual and diurnal variation in solar wind‐magnetosphere coupling caused by changes in the orientation of the geomagnetic pole, and hence the magnetosphere, with respect to the average orientation of the IMF (the Russell‐McPherron effect). This causes a 12‐hour phase difference between the times of maximum solar wind‐magnetosphere coupling at the two equinoxes. In addition, the orientation of the geomagnetic axis with respect to the average IMF is such that <By*Bz> > 0 for the March equinox and By*Bz> < 0 for September. This results in a further source of asymmetry of forcing of the high‐latitude ionosphere as the result of electric fields associated with the four sign combinations of By and Bz. Several predictions arise from the explanation given: for example, a high‐latitude station measuring thermospheric neutral winds in Alaska, 180° in longitude from Kiruna, might be expected to see nighttime thermospheric winds that are larger in the autumn than in the spring.

Yearly variations in the low-latitude topside ionosphere

“…3 suggest that, for low latitudes, the behaviour of the equinoctial asymmetry is not signi®cantly dierent to that in the longitude sector that diers by 180°. Thus the mechanism that Aruliah et al (1996bAruliah et al ( , 1997 suggested does not extend to low-latitudes.…”

“…In Aruliah et al (1996bAruliah et al ( , 1997, it has been suggested that, at high latitudes for the same hemisphere, the neutral wind should be higher at one equinox in one longitude sector and at the other equinox in the longitude sector that diers by 180°. However, there is no data at high latitudes to test this suggestion.…”

“…Observations showing the hemisphere dependency and the lack of signi®cant longitudinal variation of the equinoctial asymmetry strongly support this suggested source mechanism for the asymmetry at low latitudes. Aruliah et al (1996bAruliah et al ( , 1997 suggested that, at high latitudes, the equinoctial asymmetry in the nighttime neutral wind arises from the diurnal variation in the cross polar cap potential dierence, which is caused by an annual and diurnal variation in the orientation of the magnetosphere with respect to the interplanetary magnetic ®eld. The source mechanism of the asymmetry suggested by Aruliah et al Fig.…”

“…The existence of an equinoctial asymmetry in the nighttime thermospheric neutral wind at high latitudes has been reported by Aruliah et al (1996a). In Aruliah et al (1996bAruliah et al ( , 1997, it is suggested the asymmetry arises from the diurnal variation in the cross polar cap potential dierence due to the annual and diurnal variation in the orientation of the magnetosphere with respect to the interplanetary magnetic ®eld.…”

Thermospheric temperature and density variations

“…The Fabry-Perot interferometer (FPI), which can detect the Doppler shifted and broadening emissions from the atmospheric species, is one of the most important instruments to observe the neutral temperature and wind in the thermosphere (e.g., Aruliah et al, 1991;Ishii et al, 1999;Shiokawa et al, 2003;Ford et al, 2007). In addition, the radar techniques enable us to obtain information of the thermospheric wind and temperature.…”