Large Forbush Decreases and their Solar Sources: Features and Characteristics
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Large Forbush Decreases and their Solar Sources: Features and Characteristics M. Papailiou1 · M. Abunina2 · A. Belov2 · E. Eroshenko2 · V. Yanke2 · H. Mavromichalaki1
Received: 1 November 2019 / Accepted: 7 November 2020 © Springer Nature B.V. 2020
Abstract One of the factors responsible for the wide variety of Forbush decreases is the different solar sources related to them. In this investigation the different features and characteristics of Forbush decreases, with emphasis on large Forbush decreases and their association with solar sources, are examined. Initially, a wider selection of events from the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences Forbush decreases database served as a starting point for this study, which was then narrowed down to a group of large Forbush decreases. According to the helio-longitude of the solar source, the events under study were separated into three subcategories: western (21◦ ≤ helio-longitude ≤ 60◦ ), eastern (−60◦ ≤ helio-longitude ≤ −21◦ ), and central (−20◦ ≤ helio-longitude ≤ 20◦ ). The selected events cover the period 1967 – 2017. The “Global Survey Method” was used for analyzing the aforementioned Forbush decreases, along with data on solar flares, solar-wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. The superimposed epoch method was applied to display the temporal profiles for the selected events. This detailed analysis reveals interesting results concerning the features of cosmic-ray decreases in relation to the helio-longitude of the solar sources. Specifically, Forbush decreases related to central or eastern solar sources are more often observed, have a greater magnitude, and present a slower development than Forbush decreases related to western sources, which are rarer, have a smaller magnitude, and have a shorter lifespan. Nevertheless, regardless of the helio-longitude of the solar source, large Forbush decreases are accompanied by increased geomagnetic activity and increased anisotropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases. Keywords Space weather · Cosmic rays · Forbush decreases · Solar sources
B H. Mavromichalaki
[email protected]
1
Nuclear and Particle Physics Department, Faculty of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece
2
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation Russian Academy of Sciences (IZMIRAN), Troitsk, Moscow, 108840, Russia
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1. Introduction Solar-terrestrial relations define space weather, which is of great practical importance (Kudela et al., 2000; Singer, Heckman, and Hirman, 2001; Schwenn, 2006; Pulkkinen, 2007; Gopalswamy, 2009; Mavromichalaki, 2012), and this is the reason for its intense study. An aspect of this study is galactic cosmic rays interacting with the interplanetary disturbances, even before they arrive at the Earth, and therefore they provide
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