Over-expansion of a coronal mass ejection generates sub-Alfvénic plasma conditions in the solar wind at Earth

Abstract

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.

Near-Sun In Situ and Remote-sensing Observations of a Coronal Mass Ejection and its Effect on the Heliospheric Current Sheet

“…The spacecraft enters CME2 (orange region) at time t 7a , crossing another magnetic inversion line. The region after t 7b is highly sub-Alfvénic and could be caused due to the expansion of CME2 within the wake of CME1 (Chané et al 2021). This interval persists until time t 9 when PSP exits the entire ICMEperturbed plasma.…”

Over-expansion of coronal mass ejections modelled using 3D MHD EUHFORIA simulations

“…Previous studies indicated that several spacecraft have already observed periods of very low density in the solar wind (Lugaz et al, 2016;Chané et al, 2021;Hajra & Tsurutani, 2022). Chané et al (2021) provided an explanation of such a low density observed between May 24-25 2002 by overexpansion of an ICME during its journey to Earth.…”

“…Previous studies indicated that several spacecraft have already observed periods of very low density in the solar wind (Lugaz et al, 2016;Chané et al, 2021;Hajra & Tsurutani, 2022). Chané et al (2021) provided an explanation of such a low density observed between May 24-25 2002 by overexpansion of an ICME during its journey to Earth. Because the ICME was travelling in the wake of another CME ejected just a few hours earlier, i.e., the preceding ICME cleared away the ambient solar wind plasma and the frozen-in magnetic field, the observed over-expansion was made possible.…”

“…Other characteristics of ME1 are atypical for isolated ICMEs as follows. Single ICMEs are typically in expansion with a linear decreasing velocity profile and higher magnetic field strength than the background solar wind (Démoulin & Dasso, 2009b;Regnault et al, 2020;Chané et al, 2021). In case of ICME1, the magnetic field strength in the ME is lower than the much stronger field that is present in front of the sheath.…”

“…In order to initiate the simulations we use two different CME models: the cone model which represents a hydrodynamic pulse and the Linear Force Free (LFF) spheromak model (Verbeke et al, 2019), which includes an intrinsic magnetic field. Furthermore, we derived the expansion rate from the linear profile of the observed velocity profile, assuming a nearly self-similar expansion through the heliosphere, similar as in Chané et al (2021).…”