A technique for using incoherent scatter to estimate F-region zonal winds during Joule heating

Abstract

The zonal neutral wind speed in the F-region is difficult to measure when the thermosphere is sunlit; this paper presents a simple, readily applied technique employing incoherent scatter measurements during Joule heating events. At the F-layer peak (where neutral and ionic masses are equal), equal amounts of energy are imparted by the ionospheric electric field during ion-neutral collisions to the ion and neutral gases; because there are far fewer ions than neutrals, the ion temperature rapidly rises, thereby increasing the rate that heat is transferred from ion to neutral gases through ion-neutral collisions. The key to the technique is this result: At the sunlit F-layer peak, this heat transfer (which depends linearly upon the ion-neutral temperature difference) is so rapid that it approximately equals one-half the input Joule power, which in turn depends directly upon the square of the relative Hall ion drift speed. The ion temperature is measured, but during Joule heating the neutral temperature cannot be found from incoherent scatter measurements; however, by finding limits for the neutral temperature, limits on the ion-neutral heat transfer are readily found. Because the total input Joule power is twice the increase in ion-neutral heat transfer, these limiting values provide limits on the relative Hall speed. The zonal wind is the dominant wind component in the relative Hall speed (and the meridional wind speed is measurable), so the method yields upper and lower limits for the zonal wind during Joule heating. The upper neutral temperature limit is derived by assuming that the thermosphere expands only vertically; to ensure that an upper limit is obtained, all loss terms except adiabatic expansion at constant pressure are dropped. The lower neutral temperature limit is obtained by assuming that the Joule heating does not increase the initial neutral temperature. In the two cases studied to date these nearly opposite assumptions about the energy balance of the thermosphere lead to zonal wind limits that are not greatly different and that thereby provide useful estimates. It is concluded from these events that the neutral wind at the sunlit F-layer peak responds so rapidly to ion drag that the effective electric field averages no more than one-half the applied magnetospheric electric field during the time when E > 15 mV/ m and is increasing. This result suggests that the ion drift and neutral wind velocities in the sunlit polar cap are nearly parallel and of comparable magnitude.