the thermosphere ( >= 110 km), the different species drift relative to each other. Collisions between different species lead to Joule dissipation of the motion. Assuming the thermosphere to be static (not necessarily true since heating would induce motions, but these motional effects are not of much consequence), the Joule heating rate for ions and neutrals Qj (ergs/cm3 sec) is given in Gaussian units by (Cowling, 1957; Rees and Walker, 1968) [[spi:math]] For purposes of illustration, we shall consider a fixed neutral atmosphere as defined by the [[spi:math]] = 1000*K Jacchia model (1971). The electron density Ne is taken from the model of Chiu (1975). It should be noted that Ne varies strongly in space, time and solar conditions. Figure 14 shows a 42 [[spi:math]] (46) In (46), B is the geomagnetic field strength, [[spi:math]] is the ion-neutral collision frequency, [[spi:math]] is the ion-neutral reduced mass mimn/(mi + mn), [[spi:math]] is the ion gyrofrequency and c is the speed of light. The ion-neutral collision frequency can be approximated by (Chapman, 1956) (47) where mH is the hydrogen atomic mass. The major variations of Qj are summarized by [[spi:math]] (48) as is indicated on Figure 13, which also summarizes the Joule heating scenario.
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