Dynamics of helium abundance inside and around ICME

The interplanetary manifestations of coronal mass ejections (ICME) typically characterized by an increased relative abundance of doubly ionized helium, which can be several time higher comparative to the slow solar wind streams and exceed 10%. At the same time the helium abundance can dynamically change as a result of local processes in solar wind plasma. However, the details of helium abundance dynamic have not yet been sufficiently studied.

The study is devoted to the changing of the helium abundance inside the ICME including both its types - Magnetic Clouds (MC) and EJECTA, and also in the vicinity of these large scale structures. We consider the helium abundance dynamic, and its relation with other solar wind parameters both at large (>10⁶ km) and medium (10⁴-10⁵ km) scales, based on the statistic and correlation analysis of plasma and magnetic field parameters [Yermolaev et al., 2020; Khokhlachev et al., 2021]. Hourly average data from OMNI-2 database and 3 second WIND spacecraft measurements are used for the analysis of corresponding scales. Selection of the intervals related to ICME events (MC, EJECTA and SHEATH regions) was produced with the help of the catalog of large-scale events of Space Research Institute (http://iki.rssi.ru/pub/omni/catalog/ [Yermolaev et al., 2009]). It is shown that on large scales the helium abundance generally increases with a decrease in the plasma parameter β in the ICME. This relation can be predominantly explained by the strong positive correlation of helium abundance with magnetic pressure, while correlation with thermal pressure is ambiguous: weak negative for Magnetic Clouds and weak positive for EJECTA. This confirms the previously proposed hypothesis of the ion current flow enriched by helium ions inside the ICME [Yermolaev et al., 2020]. On medium scales, the trend of anticorrelation of the helium abundance with plasma β-parameter in the ICME is observed also. However, the dependences of the helium abundance on the plasma and magnetic field parameters can dynamically change at smaller scales. For example, several local medium-scale structures with a negative correlation of helium abundance and magnetic field magnitude can be observed on the background of positive correlation at large-scale structures.

References.

Yermolaev Y.I. et al., Catalog of large-scale solar wind phenomena during 1976–2000, Cosmic Res., 2009, vol. 47, no. 2, pp. 81–94

Yermolaev Y.I. et al., Dynamics of large-scale solar-wind streams obtained by the double superposed epoch analysis. 4. Helium abundance, Journal of Geophysical Research, 2020, 125 (7) https://doi.org/10.1029/2020JA027878

Khokhlachev A. A. et al., Variations of Protons and Doubly Ionized Helium Ions in the Solar Wind, Cosmic Research, 2021, vol. 59, no. 6, pp. 415-426
https://doi.org/10.1134/S0010952521060022