Interpretation of an unusual high latitude density decrease in terms of thermospheric density cells

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Abstract
The discovery of a cellular structure in the high latitude thermospheric (120–350 km) neutral density provides a new framework in which to interpret high latitude density data. During geomagnetically active times, 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 terms of the cells. In general the high latitude neutral density is expected to increase during magnetically active conditions. On the contrary, the SETA‐1 satellite measured a 40% density decrease near 200 km during the geomagnetic storm of April 3, 1979. In the past, such ‘anomalous’ observations have often been attributed to the inseparability of density and winds in accelerometer measurements. We first show that the depletion cannot be caused by winds, and then show that the depletion is a natural consequence of the cellular structure inherent in the mass density at 200 km.