High latitude thermospheric circulation during the Energy Budget Campaign

Abstract

The thermospheric wind data obtained during 16 nights of observation from Esrange during the Energy Budget Campaign (EBC), using a Fabry-Perot interferometer (FPI) to observe the OI 630 emission line, are assessed with the aid of a global theoretical dynamical model of the thermosphere. The systematic behaviour of thermospheric winds, as seen from Kiruna, northern Sweden, is of a poleward day-time wind, which evolves into a nearly ubiquitous equatorward wind between about 1600 UT and 0300 UT. The times of the changes between poleward and equatorward winds are influenced by the nature and intensity of the geomagnetic activity on any given night. The magnitude of the night-time equatorward wind is related to auroral and magnetic activity, but not directly to indices such as K p . Equatorward winds of 400 m s −1 or more occurred at some time on about 50% of the nights reported here, with a highest wind of above 600 m s −1 (11 Nov. 1981). Zonal winds respond more quickly and to a larger extent to magnetic activity than do meridional winds. Under very quiet magnetic conditions, the zonal wind may flow eastward (anti-sunward) until 0200 UT and then reverse to westward until the end of observations at dawn (0700 UT), following the general pattern of winds at middle latitudes due to the solar-driven diurnal pressure gradients. With increasing activity, a period of evening westward winds occurs, usually with only a small meridional component. These westward winds are due to ion drag acceleration in the afternoon/evening sector of the auroral oval, with entrainment of the gas in the region of rapid westward ion motion for periods of many hours due to an approximate balance between Coriolis and inertial acceleration terms. The amplitude of the westward winds increases and their time of onset becomes earlier as the level of geomagnetic activity increases. After about 2000–2100 UT (magnetic midnight at Kiruna) on geomagnetically disturbed nights, there is a sudden onset of strong eastward and equatorward winds, with the onset occurring earlier with increasing activity. For the same ground-level magnetic disturbance at Kiruna, westward (evening) winds exceed eastward (morning) winds by a factor of 2. Westward windsof 500 ms −1 and eastward winds of 300 ms −1 are equally frequent. However, since the eastward winds are usually associated with an approximately equal southward meridional component, the total wind magnitude and energy does not differ too much between the two regimes. Even under very disturbed conditions, the wind usually returns poleward or anti-sunward before 0600 UT. This behaviour has to be interpreted in the context of global circulation, due to an ubiquitous low-latitude solar EUV energy source and a complex high latitude energy and momentum source dependent on geomagnetic activity, but it is also modulated by UT and season, due to the variable solar illumination of the geomagnetic polar cap. At 1200 UT the auroral oval is far poleward of northern Scandinavia (1000 km) except during extremely disturbed conditions, whereas between 1800 and 2200 UT the auroral oval is usually overhead and may be well equatorward under very disturbed conditions. With increasing magnetic activity there is a general equatorward expansion of the auroral oval, so that the time at which the locality comes under the influence of the auroral oval shifts successively to earlier UT and local time. During disturbed periods the cross-polar cap electric potential usually increases, driving ions to higher velocities. The enhancement of ionospheric electron density which occurs virtually throughout the entire auroral oval at the time of geomagnetic disturbances increases both the conductivity, and thus the ionospheric current (auroral electrojet), and also the effectiveness of ion drag in the regions of enhanced convective ion velocities. At magnetic midnight the auroral oval is usually close to Kiruna, however, due to continuity, the thermospheric circulation regime which is observed is dominated by fast anti-sunward winds flowing over the polar cap. This is the reason for the earlier change to eastward and equatorward winds with increasing activity. Intense local heating and ion drag acceleration occurs in the region about magnetic midnight during discrete magnetospheric substorms, which causes marked modulation of the thermospheric wind. There were three occasions during the 16 nights reported here when the local input during a substorm was sufficient to halt or temporarily reverse a pre-existing equatorward wind of 300 ms −1.