ICMEs observed by the MESSENGER and Venus Express spacecraft have been catalogued and analysed. The ICMEs were identified by smooth rotations of the B-field direction consistent with a flux rope structure, coinciding with an enhanced field strength. Thirty-five ICMEs were found in the surveyed MESSENGER data (~March 2007 to April 2012), and eighty-four ICMEs in the surveyed Venus Express data (May 2006 to December 2013). Ropes with northward leading edges were four times more common than ropes with southward leading edges, in agreement with a previously established solar cycle dependence. Ropes at low inclinations to the solar equatorial plane were four times more common than high-inclination ropes, possibly an observational effect. In addition, data from MESSENGER, Venus Express, STEREO and ACE were examined for multi-point signatures of the catalogued ICMEs. For spacecraft separations < 15° in heliocentric longitude, the second spacecraft observed the ICME’s rope in 82% of cases; this percentage fell to 49% for 15–30° separations, to 18% for 30–45° separations, and to 12% for 45–60° separations. One ICME, observed by MESSENGER and STEREO-B while the spacecraft were radially aligned, has been analysed in detail. Few such radial observations of ICMEs have been reported previously. Force-free fitting indicates that the ICME’s rope diameter increased with an r_H^0.94 dependence on heliocentric distance, and the axial B-field strength dropped with an r_H^-1.84 dependence, clear indications of expansion. Axial magnetic flux was conserved, suggesting that the rope underwent no significant erosion through reconnection. The change in the rope’s angular width has been estimated via helicity conservation. The rope axis rotated by ~30° between the spacecraft towards the solar equatorial plane. A preliminary study of a larger number of radially-observed ICMEs indicates that rope rotations of this magnitude are not uncommon; such rotations would have important implications for space weather forecasting.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:724139 |
| Date | January 2016 |
| Creators | Good, Simon William |
| Contributors | Forsyth, Robert |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/50710 |
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