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[1] This first-principles examination of physics driving the cusp/polar upper thermosphere response to significant input energy impulses discloses previously unappreciated factors essential to thermospheric input-response relationships. The physics essential to coupling of cusp input-response processes is detailed, to make previously unexplained up-to-doubling of air density and drag near 400 km not only understandable but expected, if not inevitable. Presented as a common natural consequence of magnetic reconnection near the magnetopause, this energy-coupling from sun to upper atmosphere is through familiar processes, but by inadequately appreciated linkages. The underlying physics applies more broadly than this. We trace a logic path that should clarify the inputresponse, and lay out a path which if followed should enable most existing time-dependent 3-D global thermospheric models to significantly improve the realism of their representation and prediction of cusp/polar thermosphere disturbances to transient energy sources. We illustrate the concept with a sample model-run incorporating representative data. Citation:

The seasonal variation of nighttime thermospheric winds observed at Kiruna has been found to be significantly dependent on solar activity. Of particular interest is the observation that there is a large difference between the March and September equinox winds, despite similar levels of solar insolation. The September equinox winds are more December solstice‐like. The average March equinox meridional winds are up to 70% larger than for September. The existence of an equinoctial asymmetry has not been predicted by either thermospheric or ionospheric model simulations, which assume that the equinoxes are fundamentally the same, and use forcing functions which are symmetric about the solstices. The average ion velocities measured at EISCAT are larger during the March equinox than the September at solar maximum, while the converse is true at solar minimum. In contrast, the March equinox nighttime thermospheric winds are larger for both solar maximum and solar minimum. Furthermore, the asymmetry is greater at solar maximum.

Abstract. This paper presents the first direct empirical evidence that mesoscale variations in ion velocities must be taken into consideration when calculating Joule heating and relating it to changes in ion temperatures and momentum transfer to the neutral gas. The data come from the first tristatic Fabry-Perot Interferometer (FPI) measurements of the neutral atmosphere co-located with tristatic measurements of the ionosphere made by the European Incoherent Scatter (EISCAT) radar which were carried out during the nights of 27-28 February 2003 and 28 February until 1 March 2003. Tristatic measurements mean that there are no assumptions of uniform wind fields and ion drifts, nor zero vertical winds. The independent, tristatic, thermospheric measurements presented here should provide unambiguous vector wind information, and hence reduce the need to supplement observations with information obtained from models of the neutral atmosphere, or with estimates of neutral parameters derived from ionospheric measurements. These new data can also test the assumptions used in models and in ion-neutral interactions. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia, which is a region well covered by a network of complementary instruments. These provide a larger scale context within which to interpret our observations of mesoscale variations on the scales of tens of kilometres spatially and minutes temporally. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Calculations using the tristatic volume measurements show that the magnitude of the neutral wind dynamoCorrespondence to: A. L. Aruliah (a.aruliah@ucl.ac.uk) contribution was on average 29% of Joule heating during the first night of observation. At times it either enhanced or reduced the effective electric field by up to several tens of percent. The tristatic experiment also presents the first validation of absolute temperature measurements from a common volume observed by independently calibrated FPIs. Comparison of EISCAT ion temperatures at an altitude of 240 km with FPI neutral temperatures show that T i was around 200 K below T n for nearly 3 h on the first night during a period of strong geomagnetic activity. This is inconsistent with energy transfer. Comparison with FPI temperatures from surrounding regions indicate that it could not be accounted for by height variations. Indeed, these first results seem to indicate that the 630-nm emission did not stray too far from 240 km. There were also apparent drops in T e at the same time as the anomalous T i values which are energetically implausible. Incorrect assumptions of composition or nonMaxwellian spectra are likely to be the problem.

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 > 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.