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Over-expansion of a coronal mass ejection generates sub-Alfvénic plasma conditions in the solar wind at Earth
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SYSNO ASEP 0552241 Document Type J - Journal Article R&D Document Type Journal Article Subsidiary J Článek ve WOS Title Over-expansion of a coronal mass ejection generates sub-Alfvénic plasma conditions in the solar wind at Earth Author(s) Chané, E. (BE)
Schmieder, B. (FR)
Dasso, S. (AR)
Verbeke, C. (BE)
Grison, Benjamin (UFA-U) RID, ORCID
Démoulin, P. (FR)
Poedts, S. (BE)Number of authors 7 Article number A149 Source Title Astronomy & Astrophysics. - : EDP Sciences - ISSN 0004-6361
Roč. 647, Mar 26 (2021)Number of pages 12 s. Language eng - English Country FR - France Keywords coronal mass ejections (CMEs) ; infrared ; solar wind ; magnetohydrodynamics (MHD) Subject RIV BL - Plasma and Gas Discharge Physics OECD category Fluids and plasma physics (including surface physics) R&D Projects GA18-05285S GA ČR - Czech Science Foundation (CSF) Method of publishing Open access Institutional support UFA-U - RVO:68378289 UT WOS 000636920700001 EID SCOPUS 85103480384 DOI 10.1051/0004-6361/202039867 Annotation Context. From May 24–25, 2002, four spacecraft located in the solar wind at about 1 astronomical unit (au) measured plasma densities
one to two orders of magnitude lower than usual. The density was so low that the flow became sub-Alfvénic for four hours, and the
Alfvén Mach number was as low as 0.4. Consequently, the Earth lost its bow shock, and two long Alfvén wings were generated.
Aims. This is one of the lowest density events ever recorded in the solar wind at 1 au, and the least documented one. Our goal is to
understand what caused the very low density.
Methods. Large Angle and Spectrometric Coronagraph (LASCO) and in situ data were used to identify whether something unusual
occurred that could have generated such low densities
Results. The very low density was recorded inside a large interplanetary coronal mass ejection (ICME), which displayed a long,
linearly declining velocity profile, typical of expanding ICMEs. We deduce a normalised radial expansion rate of 1.6. Such a strong
expansion, occurring over a long period of time, implies a radial size expansion growing with the distance from the Sun to the power
1.6. This can explain a two-orders-of-magnitude drop in plasma density. Data from LASCO and the Advanced Composition Explorer
show that this over-expanding ICME was travelling in the wake of a previous ICME.
Conclusions. The very low densities measured in the solar wind in May 2002 were caused by the over-expansion of a large ICME.
This over-expansion was made possible because the ICME was travelling in a low-density and high-velocity environment present in
the wake of another ICME coming from a nearby region on the Sun and ejected only three hours previously. Such conditions are very
unusual, which explains why such very low densities are almost never observed.Workplace Institute of Atmospheric Physics Contact Kateřina Adamovičová, adamovicova@ufa.cas.cz, Tel.: 272 016 012 ; Kateřina Potužníková, kaca@ufa.cas.cz, Tel.: 272 016 019 Year of Publishing 2022 Electronic address https://hal-obspm.ccsd.cnrs.fr/obspm-03368166/file/chane21_overexpansion_CME_23May02_final.pdf
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