First results from the RAPID imaging energetic particle spectrometer on board Cluster

B Wilken,K Aarsnes,F Gliem,I Sandahl,C H Perry,J B Blake,D N Baker,F Søraas,Z Y Pu,B Nikutowski,U Mall,M Carter,J Woch,G D Reeves,G Kettmann,S Livi,H Borg,R D Belian,E T Sarris,J F Fennell,P W Daly,A Korth,T A Fritz,R H W Friedel ,К Мурсула ,J L Roeder ,K Kecskeméty ,M Grandé ,Jörg Büchner ,S Mckenna‐Lawlor ,Q Zong

doi:10.5194/angeo-19-1355-2001

Abstract

Abstract. The advanced energetic particle spectrometer RAPID on board Cluster can provide a complete description of the relevant particle parameters velocity, V , and atomic mass, A, over an energy range from 30 keV up to 1.5 MeV. We present the first measurements taken by RAPID during the commissioning and the early operating phases. The orbit on 14 January 2001, when Cluster was travelling from a perigee near dawn northward across the pole towards an apogee in the solar wind, is used to demonstrate the capabilities of RAPID in investigating a wide variety of particle populations. RAPID, with its unique capability of measuring the complete angular distribution of energetic particles, allows for the simultaneous measurements of local density gradients, as reflected in the anisotropies of 90° particles and the remote sensing of changes in the distant field line topology, as manifested in the variations of loss cone properties. A detailed discussion of angle-angle plots shows considerable differences in the structure of the boundaries between the open and closed field lines on the nightside fraction of the pass and the magnetopause crossing. The 3 March 2001 encounter of Cluster with an FTE just outside the magnetosphere is used to show the first structural plasma investigations of an FTE by energetic multi-spacecraft observations.Key words. Magnetospheric physics (energetic particles, trapped; magnetopause, cusp and boundary layers; magnetosheath)

Highlights

The advanced energetic particle spectrometer RAPID on board Cluster can provide a complete description of the relevant particle parameters velocity, V, and atomic mass, A, over an energy range from 30 keV up to 1.5 MeV
The basic magnetospheric features visited by Cluster throughout this tour on 14 January 2001 are schematically depicted in Fig. 5, as seen from a tail position. It appears that Cluster at 04:15 UT barely scratched the northern edge of the nightside region with closed field lines, which corresponds to the northern edge of the plasma sheet; by 04:45 UT, Cluster has reached open field lines emanating from the polar cap
While we are moving towards the magnetopause, whose crossing we identify by inspection of the magnetic field data and the disappearance of energetic particles in frame 15:16 UT (Fig. 7), we note that in frame 14:54 UT, the 90◦ flux starts to be strongly reduced and the flux in one of the loss cones reaches a background count level

Summary

Observations on 14 January 2001

This trajectory took the four spacecraft from an initial region characterized by the presence of trapped particles (located about 04:15 to 04:45 UT), as indicated by the azimuthal and polar distributions of proton count rates. It appears that Cluster at 04:15 UT barely scratched the northern edge of the nightside region with closed field lines, which corresponds to the northern edge of the plasma sheet; by 04:45 UT, Cluster has reached open field lines emanating from the polar cap

Entry into the trapped region

Crossing the trapped region and penetrating the magnetopause

Observations on 3 March 2001

Summary and conclusions