V arc interplanetary coronal mass ejections observed with the Solar Mass Ejection Imager

J. Geophys. Res. 112, A09103, 2007

V arc interplanetary coronal mass ejections observed with the Solar Mass Ejection Imager

S.W. Kahler
Space Vehicles Directorate, Air Force Research Laboratory, Hanscom Air Force Base, MA

D.F. Webb
Institute for Scientific Research, Boston College, Chestnut Hill, MA

Abstract

Since February 2003, the Solar Mass Ejection Imager (SMEI) has been observing interplanetary coronal mass ejections (ICMEs) at solar elongation angles ε > 20°. The ICMEs generally appear as loops or arcs in the sky, but five show distinct outward concave shapes that we call V arcs. We expect to observe some V arcs, formed by trailing edges of ICME flux ropes or by leading ICME edges sheared by solar wind (SW) speed gradients at the heliospheric current sheet. We characterize the properties of these V arcs and compare them with average properties of all SMEI ICMEs. The typical V arc speeds argue against a slow MHD shock interpretation for their structures. We estimate the V arc solar source locations and their opening angle dynamics as tests for SW shearing. The first test contradicts but the second supports the SW shearing explanation. The implications of the small number of V arcs observed with SMEI is considered. The point P approximation used to determine the V arc locations and inferred solar source regions is critically examined in Appendix A.