AN EQUINOCTIAL ASYMMETRY IN THE HIGH-LATITUDE THERMOSPHERE AND IONOSPHERE

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Th roughout the solar cycle, average nighttime thermospheric meridional w inds 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 reverse d. Numerical simulations of the thermosphere and ionosphere have not p redicted such asymmetries because they generally assume forcing functi ons that are symmetric about the solstices. The proposed explanation l ies in the annual and diurnal variation in solar wind-magnetosphere co upling caused by changes in the orientation of the geomagnetic pole, a nd hence the magnetosphere, with respect to the average orientation of the IMF (the Russell-McPherron effect). This causes a 12-hour phase d ifference between the times of maximum solar wind-magnetosphere coupli ng at the two equinoxes. In addition, the orientation of the geomagnet ic axis with respect to the average IMF is such that <B-yB-z>>0 for t he March equinox and <B-yB-z><0 for September. This results in a furt her source of asymmetry of forcing of the high-latitude ionosphere as the result of electric fields associated with the four sign combinatio ns of B-y and B-z. Several predictions arise from the explanation give n: for example, a high-latitude station measuring thermospheric neutra l winds in Alaska, 180 degrees in longitude from Kiruna, might be expe cted to see nighttime thermospheric winds that are larger in the autum n than in the spring.