Cluster observations of a cusp diamagnetic cavity: Structure, size, and dynamics

Abstract

[1] We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing and compared them with high-resolution MHD simulations. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as the IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side of the cusp because the reconnection site had changed location. We found evidence of magnetic reconnection both during northward and southward IMF conditions. The Cluster separation was ∼5000 km, enabling for the first time measurements both inside the DMC and surrounding boundaries that allowed us to construct the structure of the DMC and put the observations of ion pitch angle distributions in context of local reconnection topology and gradients of the boundaries. The cavity is characterized by strong magnetic field fluctuations and high-energy particles. At the magnetosheath boundary the high-energy particle fluxes reduced by several orders of magnitude. Throughout the magnetosheath, the high-energy proton fluxes remained low except during brief intervals when sc4 and sc1 dropped back into the cavity due to changes in solar wind dynamic pressure. However, the high-energy O+ fluxes did not drop as much in the magnetosheath and were mostly at 60°-120°pitch angles, indicative of a trapped population in the DMC which is observed in the magnetosheath due to a large gyroradius. Significant fluxes of protons and ionized oxygen were also observed escaping from the diamagnetic cavity antiparallel to the magnetic field in a time scale more consistent with the local DMC source than with a reflected bow shock source.Citation: Nykyri, K., A. Otto, E. Adamson, E. Dougal, and J. Mumme (2011), Cluster observations of a cusp diamagnetic cavity: Structure, size, and dynamics,

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.…”

“…The inner boundaries of the cusp form a funnel shape and are seen as gradual (occurring over the span of 1-2 R E ) transitions in the density and temperature from typical magnetospheric values to those more comparable to the magnetosheath. This gradual nature of the inner lobe/CDC boundary under northward IMF conditions is in agreement observations (Lavraud et al, 2002;Nykyri et al, 2011a) however Lavraud et al (2002) report a much more abrupt boundary between the dayside plasma sheet and CDC for the same IMF. The transition from the lobe is accompanied by increasing (largely fieldaligned) flows on approach to the CDC region (see bottom plot in Fig.…”

“…This accumulation of plasma resulting from the outflow from the reconnection site towards the converging dipolar geomagnetic field generates the diamagnetic cavity. Thus, the cavity develops, offset from the dipolar axis in the direction of the IMF in agreement, for example, with the recent results of Nykyri et al (2011a), who report results from a cluster event in which the position of the CDC moves in accordance with a rotation of the IMF. This behavior is evidenced in the model results as shown in Fig.…”

“…Though some observations agree with this gradual character for the inner transitions, for example Niehof et al (2010); Nykyri et al (2011a), others describe a much more rapid transition (Lavraud et al, 2002). This may be partly due to the dynamic nature of the cusp region, for instance Nykyri et al (2011b) have shown that some of the local fluctuations in plasma parameters within the CDC may be attributed to the reaction of the CDC to solar wind changes on relatively short timescales.…”

“…For example, Trattner et al (2006) identify the high altitude cusp by the presence of inflowing plasma resulting from reconnection as indicated by the time-of-flight effect or velocity filter effect (Rosenbauer et al, 1975;Onsager et al, 1993), while Peterson and Trattner (2011) identify the region by the presence of a mixture of boundary layer and newly reconnected solar wind plasma similar to the criteria used by Newell et al (2004). We focus our identification of the exterior cusp on the typical low magnetic field region or Cusp Diamagnetic Cavity (CDC) similar to ;Fritz et al (2003); Niehof et al (2008); Lavraud et al (2002); Nykyri et al (2011a). These CDC's are characterized by magnetosheath-like plasma density, enhanced thermal pressure and low, fluctuating magnetic field.…”