F. Pitout

Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster

The immediate effect of the rotation of the interplanetary magnetic field (IMF) from southward to northward on cusp precipitation has been rarely observed by a polar orbiting satellite in the past. The four Cluster spacecraft observed such an event on 23 September 2004 as they were crossing the polar cusp within 2–16 min from each other. Between the first three and the last spacecraft crossing the cusp, the IMF rotated from southward to northward with a dominant By (GSM) component. For the first time we can examine the changes in the particle precipitation immediately after such IMF change. The first two spacecraft observed typical IMF-southward ion dispersion, while the last one observed both an IMF-southward-like dispersion in the boundary layer and an IMF-northward dispersion in the cusp. After the IMF turning, the cusp is shown to have grown in size in both the poleward and equatorward directions. A three-dimensional magnetohydrodynamic simulation is used to determine the locations of the sources of the ions and the topology of the magnetic field during the event.

Temporal evolution of a staircase ion signature observed by Cluster in the mid-altitude polar cusp

[1]xa0We use the Cluster string of pearls configuration to investigate temporal variations of ion precipitation in the mid-altitude polar cusp. On 7 Aug. 2004, Cluster 4 was moving poleward through the Northern cusp, followed by Cluster 1, Cluster 2, and finally Cluster 3. The Wind spacecraft detected a Southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF-Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion dispersion around 1 keV on the equatorward side of the cusp. C1, entering the cusp around 1 minute later, did not observe the high energy step anymore but a partial dispersion with a low energy cut-off reaching 100 eV. About 9 min later, C3 entered the cusp and observed a full ion dispersion from a few keV down to around 50 eV. The open-closed boundary, identified by electron precipitation, was initially moving equatorward at a rate of −0.43° ILAT/minute at the beginning of the event and then slowed down to −0.16° ILAT/minute, suggesting the erosion of the dayside magnetosphere under IMF Southward. This event is explained by the onset of dayside reconnection when the IMF turned southward; the step being the first signature of the reconnection that would then evolve as a full dispersion as reconnection goes on. We observed 1–3 keV ions near the open-closed boundary on the three spacecraft crossings that suggests a continuous reconnection during about 9 minutes.

Response of the mid-altitude cusp to rapid rotations of the IMF

[1]xa0On 12 August 2003, the four Cluster spacecraft crossed the mid-altitude cusp one after the other a minute or two apart. Shortly after the cusp crossing, two of the Cluster observed three structures poleward of the cusp that appeared and grew in successive satellite passes. In these structures, high fluxes of low-energy magnetosheath-like ions and electrons are observed. The analysis of particle and magnetic field data reveals that it is the cusp region that moved back and forth over the spacecraft. We show that the cusp reacts extremely fast to rotations of the interplanetary magnetic field (IMF) and that each of the three northward turnings of the IMF is accompanied by a poleward displacement of the cusp. The latitudinal component of the cusp velocity at ∼5 Re altitude is estimated to be of the order of 30 km/s.

TC1 and Cluster observation of an FTE on 4 January 2005: A close conjunction

Observations of a Flux Transfer Event (FTE) signature at the dayside magnetopause are reported, which was consecutively observed on 4 January 2005 by both the Double Star/TC1 spacecraft and the Cluster quartet, while the spacecraft were traversing through the northern-dusk magnetopause. The event occurred as a magnetosheath FTE first at the Cluster spacecraft at about 07:13 UT on 4 January 2005 and crossed each of the others within 2 minutes. The spatial separations between the Cluster spacecraft were of the order of 200 km. The TC1 signature occurred about 108s after Cluster. All findings including magnetic fluxes, orientations and hot ion velocity distributions strongly suggest that Cluster and TC1 encountered the magnetosheath branch of the same flux tube at two different positions along its length and this is borne out by computation of the expected time delay. Four-spacecraft timing is used to obtain the velocity of FTE.

Electron structure of the magnetopause boundary layer: Cluster/Double star observations

We present a comparison of two events, monitored by the Double Star and Cluster spacecraft at separate locations on the dayside magnetopause, which exhibit distinct properties at high and low latitudes in the magnetopause boundary layer during the occurrence of low-latitude reconnection. On 6 April 2004, the four Cluster and TC-1 spacecraft were on near-coincident, outbound transits of the dawnside magnetosphere at north and south midlatitudes, respectively. The observations show a series of oppositely directed flux transfer events (FTEs), fed by a low-latitude reconnection line located between the spacecraft. Although both spacecraft locations were nearly equidistant from the active reconnection region, the associated magnetopause boundary layer was maintained at TC-1 but not at Cluster. We suggest an asymmetric north and south extent of the LLBL so as to be more extensive at TC-1, where the local magnetic shear across the magnetopause is small. On 4 January 2005, the Cluster and TC-1 spacecraft all repeatedly traverse the northern, duskside magnetopause almost simultaneously, before and after a strong reversal in the IMF from northward to southward during a period of turbulent solar wind. Open flux tubes are observed within minutes of the southward turning, arising from the sudden formation of a nearby subsolar reconnection line. Before the IMF change, a complex and energized boundary layer, largely absent at the lower latitudes of TC-1, and containing an energetic (>40 keV) electron population of locally trapped and field-aligned distributions, is present at the high-latitude Cluster locations. Following reconnection onset after the IMF reversal, the boundary layer is seen to extend to TC-1, and the electron distribution, which depends on position through the boundary layer, develops as an energetic, field-aligned (bistreaming) distribution. The analysis is utilizes an extended electron distribution for energies ranging from a few to 400 keV and by reordering the transition through the magnetopause to the electron distribution.