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We investigate the ability of the cylindrically symmetric\udforce-free magnetic cloud (MC) fitting model of Lepping et al. (1990) to\udfaithfully reproduce actual magnetic field observations by examining two\udquantities: (1) a difference angle, called β, i.e., the angle between the direction\udof the observed magnetic field (Bobs) and the derived force\udfree model field (Bmod) and (2) the difference in magnitudes between the observed and\udmodeled fields, i.e., ΔB(=|Bobs|−|Bmod|),\udand a normalized ΔB (i.e., ΔB/<B>) is also examined, all for a judiciously chosen set of 50 WIND\udinterplanetary MCs, based on quality considerations. These three quantities\udare developed as a percent of MC duration and averaged over this set of MCs\udto obtain average profiles. It is found that, although <ΔB> and\udits normalize version are significantly enhanced (from a broad central\udaverage value) early in an average MC (and to a lesser extent also late in\udthe MC), the angle <β> is small (less than 8°) and\udapproximately constant all throughout the MC. The field intensity enhancements are due mainly to\udinteraction of the MC with the surrounding solar wind plasma causing field\udcompression at front and rear. For example, for a typical MC, ΔB/<B> is:\ud0.21±0.27 very early in the MC, −0.11±0.10 at the center\ud(and −0.085±0.12 averaged over the full "central region," i.e.,\udfor 30% to 80% of duration), and 0.05±0.29 very late in the\udMC, showing a double sign change as we travel from front to center to back,\udin the MC. When individual MCs are examined we find that over 80% of them\udpossess field enhancements within several to many hours of the front\udboundary, but only about 30% show such enhancements at their rear\udportions. The enhancement of the MC's front field is also due to MC\udexpansion, but this is usually a lesser effect compared to compression. It\udis expected that this compression is manifested as significant distortion to\udthe MC's cross-section from the ideal circle, first suggested by Crooker et\udal. (1990), into a more elliptical/oval shape, as some global MC studies\udseem to confirm (e.g., Riley and Crooker, 2004) and apparently also as\udconfirmed for local studies of MCs (e.g., Hidalgo et al., 2002;\udNieves-Chinchilla et al., 2005)

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