Properties of Interplanetary Coronal Mass Ejections at One AU During 1995 – 2004 |
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| Authors: | L Jian C T Russell J G Luhmann R M Skoug |
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| Institution: | (1) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles E. Young Dr. East, 6862 Slichter, Los Angeles, CA, 90095, U.S.A.;(2) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles E. Young Dr. East, 6869 Slichter, Los Angeles, CA, 90095, U.S.A.;(3) Space Sciences Laboratory, University of California, Berkeley, CA, 94720, U.S.A.;(4) Los Alamos National Laboratory, Los Alamos, NM, 87545, U.S.A. |
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| Abstract: | We present a comprehensive survey of 230 interplanetary CMEs (ICMEs) during 1995 – 2004 using Wind and ACE in situ observations near one AU, and examine the solar-cycle variation of the occurrence rate, shock association rate, scale size,
velocity change, and other properties of ICMEs. The ICME occurrence rate increases (from 5 in 1996 to 40 in 2001) with solar
activity; and 66% of all ICMEs occurred with shock(s). A compound parameter, the total pressure perpendicular to the magnetic
field (Pt), i.e., the sum of magnetic and perpendicular plasma thermal pressures, assists us in effectively distinguishing ICMEs from other
solar-wind structures such as stream interactions, and in quantifying the interaction strength. We interpret the characteristic
signatures of the Pt temporal variation in terms of the inferred distance perpendicular to the flow to the center of the obstacle. Group 1 includes
events that appear to be traversed near the ICME center, showing an apparent enhanced central Pt; Group 3 represents ICMEs passed far away from the center, displaying a rapid rise and then gradual decay in Pt; and Group 2 includes events with intermediate signatures. About 36% of 198 classifiable ICMEs are Group 1 events, consistent
with the conventional wisdom that at one AU a magnetic cloud is found during crossings of only ~1/3 of ICMEs. Our set of Group
1 ICMEs and the set of magnetic clouds from other researchers have significant overlap and a similar solar-cycle dependence.
The rough decline of the Group 1 fraction as solar activity increases, is consistent with rough increases of scale size, shock
percentage, and peak Pt. These results call into question the need to have different mechanisms to create differently appearing ICMEs. Rather it
is possible that all ICMEs have a central flux rope that is traversed about 33% of the time, but in the majority of cases
is missed by the spacecraft.
Electronic Supplementary Material Supplementary material is available for this article at |
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