Observations of joule and particle heating in the auroral zone

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high-resolution Fabry-Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 A) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F-region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three-dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F-region heights in the high-latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

Nighttime thermospheric winds at high latitudes

Middle and low latitude effects of auroral disturbances from incoherent-scatter

A two-dimensional numerical model is developed to study the momentum coupling between the ionosphere and neutral atmosphere in the vicinity of discrete high-latitude features, such as convection channels and plasma density troughs. Based on generalized magnetohydrodynamic equations the model takes account of global pressure gradients, viscous dissipation, ion drag, the Coriolis force, and electrodynamic drifts. Among the findings of an initial steady state investigation are the following: (1) in convection channels, significant shears and rotations of the thermospheric flow can occur below 200 km if a minimum in the electron density profile is present between the E and F regions; (2) in convection channels, the thermospheric wind decreases with height in the F region owing to the effects of horizontal viscosity; and (3) at low altitudes, the boundaries of convection channels may produce Ekman spirals.

Ion‐neutral momentum coupling near discrete high‐latitude ionospheric features

In contrast with the low-latitude thermosphere, which derives most of its energy from the absorption of solar extreme ultraviolet radiation (EUV), the energy balance of the high-latitude thermosphere is dependent on processes of magnetospheric origin. Particles precipitating from the magnetosphere deposit energy in the lower thermosphere throughout the high-latitude region. Processes such as magnetospheric convection drive bulk motions of charged particles; both energy and momentum are transferred to the neutral atmosphere by collisions between neutral constituents and these charged particles. The present paper presents a review of both observational and theoretical investigations of the response of the neutral upper atmosphere to processes which are unique to high latitudes. It also describes in some detail recent three-dimensional model calculations dealing with the effects of magnetospheric convection on thermospheric dynamics. The three-dimensional calculations, which treat the effects of viscosity, ion drag, the Coriolis force, thermal conduction, and solar EUV heating, as well as Joule and particle precipitation heating, allow an assessment of the relative importance of processes characteristic of high latitudes.

Shih-Chih Chang

Papers

Joule heating and winds due to geomagnetic disturbances