Spectral Analysis of Forbush Decreases Using a New Yield Function
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Spectral Analysis of Forbush Decreases Using a New Yield Function M. Livada1 · H. Mavromichalaki1
Received: 14 April 2020 / Accepted: 30 July 2020 © Springer Nature B.V. 2020
Abstract The Forbush decreases of the cosmic ray intensity observed on 24 December 2014 and on 8 September 2017 were chosen for cosmic ray spectral analysis. At first an analytical study of the solar and geomagnetic parameters of these events was carried out due to the fact that both are typical cosmic ray events. Hourly cosmic ray data of the neutron monitor stations obtained from the high-resolution neutron monitor database were used for calculating the cosmic ray density and anisotropy variations. Following the method of the coupling coefficients, the galactic cosmic ray spectral index was calculated using the technique of Wawrzynczak and Alania (Adv. Space Res. 45, 622, 2010). A newly presented yield function by Mishev, Usoskin, and Kovaltsov (J. Geophys. Res. 118, 2783, 2013) including a geometrical correction factor, already used in the spectral analysis of the cosmic ray ground level enhancements, was applied for the first time to the case of Forbush decreases. A comparison of these results during the events is performed by using two other coupling functions: the function presented in the work of Clem and Dorman (Space Sci. Rev. 93, 335, 2000) and the one in the work of Belov and Struminsky (Proc. 25th Int. Cosmic Ray Conf. 1, 201, 1997). The latter includes an extension for neutron monitor stations with rigidity 1 GV < R < 2.78 GV. The obtained spectral index and the calculated cosmic ray intensity in the heliosphere during the two Forbush decreases after the coupling by these three functions are presented and discussed. Keywords Galactic cosmic rays · Neutron monitors · Spectral analysis · Forbush decreases · Ground level enhancements
1. Introduction Sharp reductions of the cosmic rays, observed followed by a gradual recovery lasting for 7 – 10 days, are defined as Forbush decreases (Fds). They were first noted by Forbush (1937)
B H. Mavromichalaki
[email protected] M. Livada [email protected]
1
Faculty of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece
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M. Livada, H. Mavromichalaki
and Hess and Demmelmair (1937). These events are related to large solar flares and coronal mass ejections (CMEs) (Burlaga, 1995; Cane, 2000). An important feature of Fds when detected by neutron monitors is that their biggest decrease of the cosmic ray intensity in each station, defined as the amplitude of the Fd (%), is anticorrelated to the geomagnetic rigidity R (GV) of the galactic cosmic ray (GCR) particles (Smart et al., 2006). It is shown by Cane (2000) that it can be expressed by a power law R −γ , where γ is the spectral index, that varies from ≈0.4 to 1.2. In fact, the spectral index values vary from ≈0.5 to 2 according to the method of Wawrzynczak and Alania (2010). In the study of the galactic cosmic ray intensity, Dorman (1963) introduced the use of coupling functions with differen
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