Subauroral ion drifts

A magnetospheric substorm involves a reconfiguration of the whole magnetosphere, during which magnetotail plasma is injected into the near-Earth environment. We have developed a model for the inward moving injection front that separates the hot injected plasma and the cold near-Earth plasma. We compute the polarization electric field at this interface. When mapped to the ionosphere, this field is sufficiently strong so as to drive ion flows > 1 km/s in the subauroral ionosphere (subauroral ion drifts or SAID). 

The figure shows how the ionospheric ion drift profile changes as the injected plasma front moves inward from L=10 towards a plasmapause at L=4. 

High altitude electrostatic fields driving subauroral ion drifts
J. Lemaire, M. Roth, and J. De Keyser 
In: R.L. Xu and A.T.Y. Lui (eds.), Magnetospheric Research with Advanced Techniques, p. 61-64, Elsevier Science Publishers, B.V., 1998 

A subauroral ion drift (SAID) layer is characterised by a narrow peak of westward ion drift speed exceeding 1000 m/s. It is confined in less than 1 degree in latitude and located equatorward of the nightside auroral zone but poleward of the plasmapause. We propose a mechanism for the narrow peak electric field driving this phenomenon: an electrostatic potential is produced by thermo-electric charge separation across the front edge of a hot plasma cloud moving inward from the tail and penetrating into the colder background plasma in the plasmatrough and at the plasmapause. Quantitative calculations corroborate this scenario. 

The magnetospheric driver of subauroral ion drifts
J. De Keyser, M. Roth, and J. Lemaire 
Geophys. Res. Lett., 25, 1625-1628, 1998 

Subauroral ion drifts (SAID) are narrow layers of intense westward ionospheric flow observed during substorms. We present numerical simulations showing that the combined effect of thermo-electric and convection electric fields in a magnetospheric current sheet---when mapped down to the ionosphere---can account for the westward direction of the ion drift, the width and intensity of the drift speed peak, and the lifetime of SAID. The model can also explain why SAID occur mainly in the pre-midnight sector. 

Formation and evolution of subauroral ion drifts in the course of a substorm
J. De Keyser 
J. Geophys. Res., 104, 12,339-12,349, 1999 

We propose a physical mechanism that explains how ``polarization jets'' (PJ) or ``subauroral ion drifts'' (SAID) are formed in the course of a substorm and how they evolve. A PJ/SAID is considered to be the ionospheric signature of an inward moving injected plasma front. The flow shear that exists across this interface when it arrives in the vicinity of the plasmapause is responsible for the generation of intense electric fields in the premidnight sector, where PJ/SAID are observed. Quantitative simulation of this mechanism accounts for PJ/SAID width and peak drift velocity. The mechanism explains why PJ/SAID are observed poleward of or in the vicinity of the plasmapause. The inward traveling time of the injected plasma agrees with the delay between substorm onset and the apparition of PJ/SAID; the evolution of the ionospheric signature is consistent with observations as well.

Storm-time energetic particle penetration into the inner magnetosphere as the electromotive force in the subauroral ion drift current circuit
J. De Keyser 
in S. Ohtani (ed.), Magnetospheric Current Systems, Geophysical Monograph Series, AGU, Washington, D.C., 2000. To appear.

Subauroral ion drift layers (SAID) with strong westward ion flows are frequently observed in the ionosphere during substorms. They are interpreted as the ionospheric footprint of a current sheet that separates the cold
magnetospheric plasma from the energetic particles penetrating into the inner magnetosphere during the substorm. We present numerical simulations of this current sheet and the corresponding SAID based on observed injected particle energy spectra. We find an SAID signature that is not very sensitive to the precise form of the spectra.
 
 

Author: J. De Keyser   Curator: J. De Keyser Johan.DeKeyser@oma.be