Sep 6, 2007 - side of the SAID channel is aligned with the plasmapause and its outer edge ... generators [e.g., Anderson et al., 1993; Galperin, 2002]. The former places ..... [20] Acknowledgments. We thank Bill Burke of BC and Fred Rich of.
GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L24101, doi:10.1029/2007GL031925, 2007
SAID: Plasmaspheric short circuit of substorm injections E. V. Mishin1 and P. A. Puhl-Quinn2 Received 6 September 2007; accepted 20 September 2007; published 18 December 2007.
[1] We present magnetically-conjugate Cluster/DMSP observations of subauroral ion drifts (SAID) after the onset of a weak substorm on 18 March 2002 that confirm and expand on the previous Cluster/DMSP results. The outer side of the SAID channel is aligned with the plasmapause and its outer edge marks the dispersionless boundary of the electron precipitation. The SAID’s inner edge co-locates with a sharp decrease in the flux of the injected ions whose minimum energy increases with the electric potential. Downward ionospheric FACs flow within the channel, mainly near its outer side, concentrating near the density maxima. The SAID peak co-locates either with that in the density or lies on the outer wall of the trough. The overall SAID features are consistent with a short circuiting of the substorm injection front over the plasmasphere and subsequent formation of a turbulent overlap region. Citation: Mishin, E. V., and P. A. Puhl-Quinn (2007), SAID: Plasmaspheric short circuit of substorm injections, Geophys. Res. Lett., 34, L24101, doi:10.1029/2007GL031925.
1. Introduction [2] Enhanced electric fields (Es) develop in the perturbed subauroral ionosphere. Latitudinally-narrow (1°), poleward-directed fields (>30 mV/m) produce supersonic (>1 km/s) jets of westward convection, termed SAID and conventionally described in terms of voltage and current generators [e.g., Anderson et al., 1993; Galperin, 2002]. The former places SAID between the inner boundaries of ion and electron convection (Alfve´n layers). The current generator explores closure of the Region 2 downward FACs (j#k) through a low-conductive ionosphere resulting in locallyenhanced E(I) s . Hereafter, the superscript I (M) denotes ionospheric (magnetospheric) magnitudes. (I) [3] SAID are observed with widely-varying j#(I) k /Es #(I) ratios, and the strength of jk can range anywhere from 0 to some mA m2 [e.g., Rich et al., 1980; Karlsson et al., 1998]. De Keyser et al. [1998] suggested that SAID appear near the plasmapause (PP) mainly due to the thermoelectric effect at the interface between the cold and injected hot plasma. Their model gives the SAID’s width Ds rh (the hot ion gyroradius), which is too narrow. De Keyser [1999] described SAID as the rotational discontinuity near the PP, presuming that its thickness and inward speed are 50 rh and 10 km/s, respectively, and that the cold plasma is pushed aside by the front and precipitates.
1 Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA. 2 Space Science Center, University of New Hampshire, Durham, New Hampshire, USA.
Copyright 2007 by the American Geophysical Union. 0094-8276/07/2007GL031925
[4] SAID features in the near-equatorial magnetosphere were virtually unknown until recently, except for two events from ISEE-1 [Maynard et al., 1980]. Puhl-Quinn et al. [2007] (hereinafter referred to as PQ07) presented first magnetically-conjugate observations of a SAID event near the magnetic equator and in the ionosphere from Cluster (’4.3RE) and DMSP F13-16 (altitudes ’850 km), respectively. After the substorm onset on 8 April 2004, the four ’ 25 mV/m, which maps Cluster spacecraft measured E(M) s ’ 320 mV/m [PQ07]. Taking advantage of the high into E(mI) s degree of magnetic and temporal conjugacy, PQ07 found that (1) the outer side of the SAID channel is aligned with the PP, (2) the SAID’s outer edge marks the dispersionless boundary concenof the electron precipitation, (3) a small-scale j#(I) k trates near the SAID’s outer edge, and (4) the SAID’s core lies at the outer side of the ionospheric density trough, which is absent at Cluster altitude. Hereafter, ‘outer/inner’ in the ionosphere indicates ‘poleward/equatorward’. [5] This paper presents magnetically-conjugate ClusterDMSP observations of a substorm SAID event on 18 March 2002 that confirm and expand on the PQ07 results. Vallat et al. [2005] already used the Cluster observations during the inner-magnetospheric pass of 18 March 2002 to study currents in the ring current region. Although they noted the presence of small-scale downward FACs near the plasmapause, the consequences of that for SAID studies were not pursued.
2. Observations [6] The IMAGE/FUV observations on 18 March 2002 showed the onset of a substorm near L = 6.8 and 22.9 MLT at 09:48 UT [Frey et al., 2004]. The SAID structure in the southern hemisphere was encountered by DMSP F14 and Cluster 25 min later at 63.5° and 63.2° ILat (L ’ 4.9), respectively. As in PQ07, the northern portion of the Cluster pass documented almost identical features. Figure 1 shows the southern SAID channel from (left) Cluster and (right) F14. Shown are (a) the C4/CIS-CODIF energy-time spectrogram for 0.03–40-keV H+ ion fluxes, (b) the radial/outward component of the EFW electric field, E(M) s , (c) the deviation from IGRF-90 (B0) of the azimuthal/eastward component of the FGM magnetic field, DBE, and (d) the EFW probe-tospacecraft potential, fp and fluxes of (e) downcoming 0.03– 30-keV electrons and (f) ions, (g) the southward electric component, (h) the eastward magnetic component DBE averaged over 2-s, and (i) the total ion density ni. Hereafter, the vertical dashed/dash-dotted line indicates the SAID’s outer/inner edge. [7] By comparing the channel positions during subsequent crossings by C1, C4, C2, and C3, we derive a radial speed of
