Response of the magnetotail to changes in the open flux content of the magnetosphere

Abstract

[1] We compare the open flux content of the magnetosphere, quantified by measurements of the size of the northern ionospheric polar cap, with the radius of the magnetotail at X % À25 R E , deduced from observations made by the IMP-8 spacecraft. During an 8-hour period of observation we estimate that the proportion of terrestrial flux that is interconnected with the solar wind varies between 12 and 2.5%. This latter extreme, representing an almost closed magnetosphere, follows the incidence of a solar wind dynamic pressure step, the onset of a large substorm, and a 3-hour period of northward IMF. The deflated and compressed magnetotail is predicted to have a radius as small as 12 R E at this time. The magnetotail does not reinflate to more typical dimensions until some time after a southward turning of the IMF, leading to accumulation of open flux through low latitude reconnection. We compare our observations with estimates of the shape of the magnetopause from an empirical model. We also present a simple model of the varying length of the magnetotail, based on upstream solar wind conditions, and observations of the size of the polar cap.

Modeling the magnetospheric X‐ray emission from solar wind charge exchange with verification from XMM‐Newton observations

“…This is especially true with magnetopause modeling, as the movement of the boundary layer provides a proxy for monitoring the transfer of solar wind energy into the magnetosphere [Milan et al, 2004]. Hence, modeling and testing of the terrestrial charge exchange process is necessary for understanding future imaging studies.…”

A simple model of the flux content of the distant magnetotail

“…In the case of the connected tail, it is not necessary to know the past or present rate of reconnection in the tail, F C , as this is related to points 1 and 2 through equation (1). The size of the polar cap (point 1) can be found from ionospheric observations as discussed by Milan et al [2003Milan et al [ , 2004. The last two, points 2 and 3, can in principle be determined from upstream solar wind and IMF measurements.…”

“…[22] Two previous studies of Milan et al [2003Milan et al [ , 2004 employed observations from Polar UVI, SuperDARN, and DMSP, FAST, and NOAA satellites to determine the variation in the size of the Northern Hemisphere polar cap during 8-hour intervals on 5 June and 26 August 1998. These determinations are used to illustrate the model described above.…”

“…[7] The dayside and nightside reconnection processes are time-dependent and independent of each other, and so the proportion of the terrestrial magnetic field that is interconnected with the IMF, that is ''open,'' is determined by the competition between the opening of flux at the subsolar magnetopause and the closure of flux in the magnetotail [e.g., Coroniti and Kennel, 1972;Siscoe and Huang, 1985;Cowley and Lockwood, 1992;Russell, 2000;Milan et al, 2003Milan et al, , 2004. The amount of open flux F PC which threads the ionospheric polar cap (or equally the whole surface of the ''connected'' magnetosphere) is governed by the following statement of Faraday's Law:…”

“…This model allows the nominal length and flux content of the Dungey magnetotail (hereafter just ''magnetotail'') to be calculated based on a knowledge of the size of the ionospheric polar cap and the past history of the dayside reconnection rate. New techniques have recently been developed which allow the instantaneous size of the terrestrial polar cap to be measured [Milan et al, 2003[Milan et al, , 2004, which in conjunction with the model presented here will allow the instantaneous state of the Earth's magnetotail to be determined on a caseby-case basis.…”