Comparison of magnetic field observations of an average magnetic cloud with a simple force free model: the importance of field compression and expansionReport as inadecuate




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1 NASA 2 Goddard Earth and Science Technology Center 3 River Hill High School

Abstract : We investigate the ability of the cylindrically symmetric force-free magnetic cloud MC fitting model of Lepping et al. 1990 to faithfully reproduce actual magnetic field observations by examining two quantities: 1 a difference angle, called ?, i.e., the angle between the direction of the observed magnetic field Bobs and the derived force free model field Bmod and 2 the difference in magnitudes between the observed and modeled fields, i.e., ?B=|Bobs|?|Bmod|, and a normalized ?B i.e., ?B- is also examined, all for a judiciously chosen set of 50 WIND interplanetary MCs, based on quality considerations. These three quantities are developed as a percent of MC duration and averaged over this set of MCs to obtain average profiles. It is found that, although is small less than 8° and approximately constant all throughout the MC. The field intensity enhancements are due mainly to interaction of the MC with the surrounding solar wind plasma causing field compression at front and rear. For example, for a typical MC, ?B- is: 0.21±0.27 very early in the MC, ?0.11±0.10 at the center and ?0.085±0.12 averaged over the full -central region,- i.e., for 30% to 80% of duration, and 0.05±0.29 very late in the MC, showing a double sign change as we travel from front to center to back, in the MC. When individual MCs are examined we find that over 80% of them possess field enhancements within several to many hours of the front boundary, but only about 30% show such enhancements at their rear portions. The enhancement of the MC-s front field is also due to MC expansion, but this is usually a lesser effect compared to compression. It is expected that this compression is manifested as significant distortion to the MC-s cross-section from the ideal circle, first suggested by Crooker et al. 1990, into a more elliptical-oval shape, as some global MC studies seem to confirm e.g., Riley and Crooker, 2004 and apparently also as confirmed for local studies of MCs e.g., Hidalgo et al., 2002; Nieves-Chinchilla et al., 2005.





Author: R. P. Lepping - T. W. Narock - H. Chen -

Source: https://hal.archives-ouvertes.fr/



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