Cusp field‐aligned currents and ion outflows

On September 24 and 25, 1998, the Polar spacecraft observed intense outflows of terrestrial ions in association with the passage of an interplanetary shock and coronal mass ejection. The orbit of the Fast Auroral Snapshot (FAST) Explorer was in the noon-midnight meridian during this ion outflow event, and FAST passed through the day side cusp region at ∼4000 km altitude every 2.2 hours. FAST was therefore able to monitor the ion outflows subsequently observed by Polar. We show that while the outflows were more intense after the shock passage, the overall particle and field signatures within the cusp region were qualitatively similar both before and after the shock passage. FAST observations show that the cusp particle precipitation marks the lower latitude leg of a pair of field-aligned currents and further, that both field-aligned current sheets appear to be on open field lines. Moreover, the polarity of the cusp currents is controlled by the polarity of the interplanetary magnetic field (IMF) y-component, such that the magnetic field perturbation associated with the pair of cusp currents is in the same direction as the IMF By. This is a consequence of the reconnection of cusp-region field lines at the magnetopause, with the flux transport resulting in electromagnetic energy being transmitted along field lines to the ionosphere as Poynting flux. We show that this Poynting flux can be as high as 120 mW m−2 (120 ergs cm−2 s−1) at FAST altitudes (∼500 mW m−2 at ionospheric altitudes), presumably because of the strong IMF By (∼40 nT), and is the dominant energy input to the cusp-region ionosphere. Furthermore, we find that the peak ion outflow flux is correlated with the peak downward Poynting flux, although only a few passes through the cusp centered around the time of the shock passage were used to determine this correlation. The energy carried by Poynting flux is dissipated as heat within the ionosphere, through Joule dissipation. The heating will tend to increase the ionospheric scale height, allowing greater access of ionospheric ions to the altitudes where transverse ion heating via ELF waves can occur. Thus electromagnetic energy supplied by the transport of reconnected magnetic flux is the essential first step in a multistep process that enhances the outflow of ionospheric plasma in the dayside cusp.

[1]  R. Greenwald,et al.  Direct determination of IMF BY‐related cusp current systems, using SuperDARN radar and multiple ground magnetometer data: A link to theory on cusp current origin , 1999 .

[2]  T. Moore,et al.  Ionospheric mass ejection in response to a CME , 1999 .

[3]  T. Moore,et al.  Sudden compression of the outer magnetosphere associated with an ionospheric mass ejection , 1999 .

[4]  R. Elphic,et al.  Observations of polar cap arcs on FAST , 1999 .

[5]  M. André,et al.  A statistical study of ion energization mechanisms in the auroral region , 1998 .

[6]  J. G. Watzin,et al.  The Fast Auroral SnapshoT (FAST) Mission , 1998 .

[7]  Nikolai A. Tsyganenko,et al.  Effects of the solar wind conditions on the global magnetospheric configuration as deduced from data-based field models , 1996 .

[8]  Eugene N. Parker,et al.  The alternative paradigm for magnetospheric physics , 1996 .

[9]  M. Smith,et al.  Low and middle altitude cusp particle signatures for general magnetopause reconnection rate variations: 1. Theory , 1994 .

[10]  J. Cladis Parallel acceleration and transport of ions from polar ionosphere to plasma sheet , 1986 .

[11]  Lou‐Chuang Lee,et al.  On the origin of the cusp field-aligned currents , 1985 .

[12]  D. Klumpar,et al.  Direct evidence for two-stage (bimodal) acceleration of ionospheric ions , 1984 .

[13]  Wolfgang Baumjohann,et al.  Ionospheric and Birkeland current distributions for northward interplanetary magnetic field: Inferred polar convection , 1984 .

[14]  Stanley W. H. Cowley,et al.  Magnetospheric asymmetries associated with the y-component of the IMF , 1981 .

[15]  Nancy U. Crooker,et al.  Dayside merging and cusp geometry , 1979 .

[16]  J. Wilhjelm,et al.  The relationship between ionospheric and field‐aligned currents in the dayside cusp , 1978 .