The Distribution Function of the Average Iron Charge State at 1 AU: From a Bimodal Wind to ICME Identification
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The Distribution Function of the Average Iron Charge State at 1 AU: From a Bimodal Wind to ICME Identification C. Larrodera1
· C. Cid1
Received: 10 July 2020 / Accepted: 22 October 2020 / Published online: 10 November 2020 © Springer Nature B.V. 2020
Abstract We aim to investigate the distribution function of the iron charge state, at 1 AU to check if it corresponds to a bimodal wind. We use data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument on board the Advanced Composition Explorer (ACE) along 20 years. We propose the bi-Gaussian function as the probability distribution function that fits the average iron charge state, QFe , distribution. We study the evolution of the parameters of the bimodal distribution with the solar cycle. We compare the outliers of the sample with the existing catalogs of interplanetary coronal mass ejections (ICMEs) and identify new ICMEs. The QFe at 1 AU shows a bimodal distribution related to the solar cycle. Our results confirm that QFe > 12 is a trustworthy proxy for ICME identification and a reliable signature in the ICME boundary definition. Keywords Sun: heliosphere-solar wind
1. Introduction The existence of the solar wind was first confirmed by in situ spacecraft in the 1960s (Gringauz et al., 1960; Gringauz, 1961; Gringauz, Bezrukikh, and Musatov, 1967). Neugebauer and Snyder (1966) showed for the first time a bimodal wind, based on Mariner 2 observations of recurring streams of high speed plasma. From the mid-1970s onward, new space missions with new instruments have studied the solar wind using different physical magnitudes like proton speed, proton temperature or interplanetary magnetic field among others (Hundhausen, 1972; Rossi, 1991). Nowadays, the solar wind is commonly classified into slow and fast (which form the bulk of the wind) and transient events coming from coronal mass ejections, where material
B C. Larrodera
[email protected]
B C. Cid
[email protected]
1
Space Weather Research Group, Departamento de Física y Matemáticas, Universidad de Alcalá, A-2, km 33,600, Alcalá de Henares, Spain
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from the solar atmosphere is thrown into the interplanetary space (Schwenn, 2006b; Viall and Borovsky, 2020). Each type of bulk wind has a different source. The fast solar wind is considered to come from the open magnetic field that emerges from the Sun through the coronal holes (Banaszkiewicz et al., 1997; Schwenn, 2006a). The source of the slow solar wind is not clear, and many suggestions have been made, e.g. plasma released by reconnection between open and closed magnetic field lines (Lionello et al., 2005) or flow emerging from small equatorial coronal holes (Bale et al., 2019). Nevertheless, Neugebauer et al. (2002), from a study of the sources of the solar wind during the maximum of Solar Cycle 23 (1998 – 2001), conclude that near the solar maximum the characteristics of the fast solar wind are very different from the fast solar wind during the minimum when the polar coronal holes ar
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