The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis

Abstract

Abstract. We report on the observation of three high-altitude cusp crossings by the Cluster spacecraft under steady northward IMF conditions. The focus of this study is on the exterior cusp and its boundaries. At the poleward edge of the cusp, large downward jets are present; they are characterized by a dawn-dusk component of the convection velocity opposite to the IMF B y direction and a gradual evolution (velocity filter effect) corresponding to an injection site located at the high-latitude magnetopause tailward of the cusp, with subsequent sunward convection. As one moves from the poleward edge into the exterior cusp proper, the plasma gradually becomes stagnant as the result of the mirroring and scattering of the aforementioned plasma flows. The existence of such a stagnant region (Stagnant Exterior Cusp: SEC) is found in all events studied here even when the IMF B y is large and the clock angle is ∼90 • . The SEC-magnetosheath boundary appears as a spatial structure that has a normal component of the magnetic field pointing inward, in accordance with a probable connection between the region and the magnetosheath (with northward field). This boundary generally has a deHoffmann-Teller velocity that is slow and oriented sunward and downward, compatible with a discontinuity propagating from a location near the high-latitude magnetopause. Although the tangential stress balance is not always satisfied, the SEC-magnetosheath boundary is possibly a rotational discontinuity. Just outside this boundary, there exists a clear sub-Alfvénic plasma depletion layer (PDL). These results are all consistent with the existence of a nearly steady reconnection site at the high-latitude magnetopause tailward of the cusp. We suggest that the stability of the external discontinuity (and of the whole region) is maintained by the presence of the sub-Alfvénic PDL. However, examinationCorrespondence to: B. Lavraud (lavraud@lanl.gov) of the electron data shows the presence of heated electrons propagating parallel to the magnetic field (upward) just outside of the SEC-magnetosheath boundary. This appears inconsistent with their source being the northern lobe reconnection site. Finally, the definition of the magnetopause at high latitudes is revisited. To define the SEC-magnetosheath boundary as the magnetopause would lead to the misnaming of the "exterior cusp".

3-D mesoscale MHD simulations of magnetospheric cusp-like configurations: cusp diamagnetic cavities and boundary structure

“…4). These flows are qualitatively consistent with reports in the literature of field aligned flows along the inner boundary for northward IMF (Lavraud et al, 2004bDunlop et al, 2005;Nykyri et al, 2011a), though the results presented herein indicate significantly smaller flows. It should be noted, however, that the density asymmetry (between the magnetosphere and the magnetosheath) is rather small in this configuration, characterized by a change of only a factor of ∼1.5.…”

“…These inner boundaries also exhibit significant fieldaligned flows. These flows as seen in the simulation results, though qualitatively consistent with observations (Lavraud et al, 2004bDunlop et al, 2005;Nykyri et al, 2011) are relatively small. It was previously noted that this appears to be the result of the low density asymmetry (between the magnetosphere and the magnetosheath) in this configuration.…”

“…The normal components of the magnetic field and velocity across the transition indicate an open boundary across which plasma is transported from the magnetosheath to the CDC. Similar open boundaries have been reported in case studies by Cargill et al (2004) under southward IMF conditions and by Lavraud et al (2002Lavraud et al ( , 2004b Fig. 13.…”

“…The inner boundaries appear in the simulation as gradual transitions which exhibit field-aligned flows emanating from the side of the cusp where reconnection is occurring: the dayside plasma sheet boundary for southward IMF and the lobe boundary for northward IMF. The gradual appearance of the inner boundaries further supports the suggestion that it is appropriate to view the high-altitude cusp region containing the CDC and SEC as an extension of the magnetosphere (Russell, 2000;Lavraud et al, 2004b), and not as a detached structure as implied by the term "exterior". Though the above features are reasonably well in agreement with observations, there remain a few aspects of the model results which disagree with some observational reports.…”

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

“…In such a case, the plasma flow inside the cusp is very slow (Woch and Lundin, 1992;Lavraud et al, 2004) without the plasma mantle (Sckopke et al, 1976;Yamauchi and Lundin, 1993), and therefore we cannot simply apply our model to the cusp flow. However, the solar wind flow may still exist in the magnetosphere, through the prenoon boundary and postnoon of the cusp, flowing toward the low-latitude boundary layer: LLBL (Mitchell et al, 1987;Raeder et al, 1997).…”