Abstract. The plasmaspheric mass density at L •_ 2 was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to ___ 1/4 of the pre-storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as L = 2, which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern.

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Previous work by Chun et al. [1999] has shown that the polar cap (PC) index can be used as a proxy indicator of the integrated Joule heating rate in the Northern Hemisphere. However, knowledge of the spatial distribution of Joule heating is also important. The PC index is a single magnetometer station‐derived index which measures the level of geomagnetic activity in the polar cap. A negative PC index corresponds to a condition related to lobe‐merging convection conditions in the polar cap, near‐zero PC indicates a quiet polar cap, and a positive PC indicates geomagnetically active periods. In this study we developed average patterns of Joule heating as a function of PC using 56 days (∼12,800 individual patterns) of Assimilative Mapping of Ionospheric Electrodynamics data from various case studies. With PC ranging from −3 to 8, we divide the Joule heating patterns into PC bins of 1.0, finding that there is a clear spatial evolution of Joule heating from negative PC to positive PC. When PC is negative, Joule heating on average is constrained to the high‐latitude dayside. As PC goes to zero, Joule heating disappears. When PC increases in the positive direction, Joule heating intensifies throughout the auroral oval, with primary heating occurring along the dawn/dusk flanks. Further analysis reveals that it is primarily changes in the electric potential and not the Pedersen conductance that accounts for Joule heating shifting from the auroral zone to the high‐latitude dayside when PC goes negative. We also find that the cross polar cap potential is linearly proportional to the PC index, consistent with previous studies. Finally, we compare the hemispheric energy budget between Joule heating and electron particle energy. We find that globally, Joule heating is roughly 4 times that of particle energy during geomagnetically active times (positive PC). During quiet times (zero PC), Joule heating and particle energy are equal contributors. When lobe‐merging conditions occur (negative PC), Joule heating again dominates over particle energy except during the winter season, when polar cap conductivity is driven more by particle precipitation.

Abstract. This paper describes the development of a major space storm during November 2-11, 1993. We discuss the history of the contributing high-speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high-speed flow which prolonged the storm and the near-Earth manifestations of the storm. The 8-day storm period was unusually long; the result of a high-speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near-Earth environment was in a state of tumult. A super-dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100% and F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200øK within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was-190 PJ, with 30% of that generated within 24 hours of storm onset.

[1] We evaluate the response of the high-latitude Joule heating to orientation and magnitude of the interplanetary magnetic field (IMF). Approximately 9000 individual Joule heating patterns derived from data assimilation for the northern hemisphere were used to develop averaged and hemispherically integrated Joule power maps for the northern hemisphere north of 40°magnetic latitude. Hemispherically integrated Joule heating increases with IMF magnitude when the IMF is southward, but is relatively unchanged with increased IMF magnitude when the IMF is north. Only a small IMF B y effect is evident in the Joule heating patterns. We quantify the IMF effects with linear fits of integrated Joule heating as a function of IMF magnitude for eight different IMF clock angle bins in the GSM Y-Z plane. During extreme northward IMF conditions, Joule heating is restricted to the high latitude dayside. During extreme southward IMF conditions, Joule heating is located predominantly in the auroral region, with increased heating in the morning sector compared to the evening sector. This additional heating may lead to the increased incidence of gravity waves reported in the morning sector. Our estimate of the increase of hemispherically integrated Joule heating with IMF magnitude during southward IMF periods is 13GW/nT.