INTERPRETATION OF AN UNUSUAL HIGH-LATITUDE DENSITY DECREASE IN TERMS OF THERMOSPHERIC DENSITY CELLS

The discovery of a cellular structure in the high latitude thermospher ic (120-350 bm) neutral density provides a new framework in which to i nterpret high latitude density data. During geomagnetically active tim es, above about 170 km, the cells consist of density enhancements near noon and midnight and depletions near dawn and dusk. In this paper, a previously unexplained observation is presented and interpreted in te rms of the cells. In general the high latitude neutral density is expe cted to increase during magnetically active conditions. On the contrar y, the SETA-1 satellite measured a 40% density decrease near 200 km du ring the geomagnetic storm of April 3, 1979. In the past, such 'anomal ous' observations have often been attributed to the inseparability of density and winds in accelerometer measurements. We first show that th e depletion cannot be caused by winds, and then show that the depletio n is a natural consequence of the cellular structure inherent in the m ass density at 200 km.