Plasmaspheric storm time erosion

Unusually low whistler mode group delay times are observed by VLF Doppler r eceivers at both Faraday, Antarctica, and Dunedin, New Zealand, following m agnetic storms. These are typically caused by plasmaspheric electron concen tration depletions near L=2.4 and not by; changes in the VLF wave propagati on path. Using a data set that is almost continuous since 1986, we find tha t depletions during storms in the solar minimum of 1995 are significantly d eeper than in the minimum of 1986. Event studies at Faraday show that the e lectron concentration depletions caused by storms were about a factor of 2 in 1986 and a factor of 3-4 in 1995, independent of the time of year. Howev er, the depletions observed bg both sites are significantly deeper than tho se observed in 1958 and 1961 using natural whistlers (i.e., factors of 2-4 compared to 1.3). The Sheffield University Plasmasphere Ionosphere Model (S UPIM) has been used to investigate possible causes of the plasmaspheric ele ctron concentration depletions observed in the whistler mode data. Thermosp heric parameters, including a reduction in the concentration of neutral hyd rogen and oxygen at all altitudes, were perturbed by a factor of 10 from th eir normal levels. However, the plasmaspheric depletions produced were only of the order of 10% after 27 hours. It is unlikely therefore that thermosp heric modifications alone art! responsible for the depletions observed in t he data. Additionally, a tube of plasma was moved to higher L shell under t he influence of an equatorial meridional E x B drift velocity of 1000 m s(- 1) and showed levels of depiction of about a factor of 2. Although it is po ssible to generate plasmaspheric concentration depletions using the driftin g tube model, the depletions are smaller than those observed and the outwar d E x B drift velocity needed is a factor of 2 greater than those reported previously at L=2.4. It is therefore unlikely that the tube drifting mechan ism is the principal cause of tl le observed plasmaspheric electron concent ration depletions at L=2.4. Although no mechanism is clearly identified in this study, the ground-based results presented in this paper indicate erosi on levels of similar structure and magnitude to electron concentration prof iles from the ISEE 1 satellite in the aftermath of magnetic disturbances du ring 1983, thus providing a long-term record of plasmaspheric erosion.