Abstract
Abstract. Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure (Pdyn) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and Pdyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.
Highlights
Shock leading edge aTlh.,e2s0e0fl9u)c.tIunaatidodnistiaonnd, BthoeriorvcsokryrealantdioFnuwnsithenth(e20u0p2st)rdeaemosnoslatrawteidndthcaot nthdeiteionnhsanhcaevde ubpesetnresatmudIieMdFinttuernbsuilveenlcyediunecrteoasthees tihmepomrtoamnceentoufmPct5ranwsafveresfrinomthetheenemrgaegtnicetpoasrhteicalteh tirnatnosptohret manadgenneetorgspizhaetrieo,nroefseuleticntgronins tmo orerelatsitvirisritincgleovfeltshe(e.mg.a,gBnaektoe-r before mid−sheath after pre−shock post−shock pre−LE post−LE
We found that the IMF and Pdyn ULF power have opposite profiles in the mid-sheath region (Fig. 3 and Table 3); the IMF ULF power is strongly enhanced during the first half of the mid-sheath, while the Pdyn ULF power is more enhanced during the latter half of the mid-sheath, in particular for the strongest events
We found that the strongest peaks in the ULF power are concentrated just after the shock and in the vicinity of the ICME leading edge
Summary
Pdyn ULF power shows a broad enhancement starting from the latter part of the midsheath for fast ICMEs, while for slow ICMEs the levels stay relatively smooth, except the upper quartile curve showing a sharp peak at the leading edge. The Pdyn power shows strong peak in the post-shock region, and the power gets enhanced from the latter part of the mid-sheath through the pre- and post-LE regions, and for strongest events the www.ann-geophys.net/31/1559/2013/. The main difference between events in the different categories was that for fast ICMEs, the upper quartile curve showed a clear increase in the power towards the shock, while for low-speed ICMEs the profile stayed relatively flat until the shock was reached. The average Pdyn power calculated from the upper quartile curve is 5.8 times higher in the pre-shock region than in the before region
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