Multi-spacecraft study of the January 21st 2005 ICME

FOULLON, C.; OWEN C.J.; DASSO S.; GREEN L.M.; DANDOUREAS I.; ELLIOTT H.; FAZAKERLEY A.N.; BOGDANOVA Y.V.; CROOKER N.U.

SOLAR PHYSICS

Año: 2007 vol. 244 p. 139 - 165

0038-0938

We examine the near-Earth Interplanetary Coronal Mass Ejection (ICME) apparentlyrelated to the intense Solar Energetic Particle (SEP) event of 20 January 2005. Ourpurpose is to contribute to the understanding of the macroscopic structure, evolution anddynamics of the solar corona and heliosphere. Using Cluster, ACE and Wind data in thesolar wind, and Geotail data in the magnetosheath, we perform a multi-spacecraft analysisof the ICME-driven shock, post-shock magnetic discontinuities and ejecta. Traversals bythe well-separated near-Earth spacecraft provide a coherent picture of the ICME geometry.Following the shock, the ICME sequence starts with a hot pileup, i.e., a sheath, followedby a fast ejecta characterised by a non-compressive density enhancement (NCDE), which iscaused essentially by an enrichment in helium. The plasma and magnetic observations of theejecta are consistent with the outskirts of a structure in strong expansion, consisting of nestedmagnetic loops still connected to the Sun.Within the leading edge of the ejecta, we establishthe presence of a tilted current sheet substructure. An analysis of the observations suggeststhat the tilted current sheet is draped within the overlying cloud canopy, ahead of a magneticcloud-like structure. The flux rope interpretation of this structure near L1, confirmedby observations of the corresponding magnetic cloud, provided by Ulysses at 5.3 AU andaway from the Sun – Earth line, indicates that the bulk of the cloud is in the northwest sectoras seen from the Earth, with its axis nearly perpendicular to the ecliptic. This is consistentwith the primary direction of travel of the fast halo CME observed at the Sun. Moreover, theNCDE and helium enrichment are consistent with the position near the streamer belt of theflaring active region NOAA 10720 associated with the CME. However, differences betweeninterplanetary and solar observations indicate a large rotation of the erupting filament andoverlying arcade, which can be attributed to the flux rope being subject to the helical kinkinstability.