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

• Main Authors: R. P. Lepping, T. W. Narock, H. Chen
• Format: Article
• Language: English
• Published: Copernicus Publications 2007-01-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/25/2641/2007/angeo-25-2641-2007.pdf

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 <i>difference angle</i>, called β, i.e., the angle between the direction of the observed magnetic field (<i>B_obs</i>) and the derived force free model field (<i>B_mod</i>) and (2) the <i>difference in magnitudes<i> between the observed and modeled fields, i.e., &Delta;<i>B</i>(=|<i>B</i>_obs|&minus;|</i>B</i>_mod|), and a <i>normalized</i> &Delta;<i>B</i> (i.e., &Delta;<i>B</i>/&lt;<i>B</i>&gt;) 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 <&Delta;<i>B</i>> and its normalize version are significantly enhanced (from a broad central average value) early in an average MC (and to a lesser extent also late in the MC), the angle <&beta;> is small (less than 8&deg;) and <i>approximately constant</i> all throughout the MC. The field intensity enhancements are due mainly to interaction of the MC with the surrounding solar wind plasma causing field <i>compression</i> at front and rear. For example, for a typical MC, &Delta;<i>B</i>/<<i>B</i>> is: 0.21&plusmn;0.27 very early in the MC, &minus;0.11&plusmn;0.10 at the center (and &minus;0.085&plusmn;0.12 averaged over the full "central region," i.e., for 30% to 80% of duration), and 0.05&plusmn;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).