Temporal Changes in the Rigidity Spectrum of Forbush Decreases Based on Neutron Monitor Data

Abstract

The Forbush decrease (Fd) of the Galactic cosmic ray (GCR) intensity and disturbances in the Earth's magnetic field generally take place simultaneously and are caused by the same phenomenon, namely a coronal mass ejection (CME) or a shock wave created after violent processes in the solar atmosphere. The magnetic cut-off rigidity of the Earth's magnetic field changes because of the disturbances, leading to additional changes in the GCR intensity observed by neutron monitors and muon telescopes. Therefore, one may expect distortion in the temporal changes in the power-law exponent of the rigidity spectrum calculated from neutron monitor data without correcting for the changes in the cut-off rigidity of the Earth's magnetic field. We compare temporal changes in the rigidity spectrum of Fds calculated from neutron monitor data corrected and uncorrected for the geomagnetic disturbances. We show some differences in the power-law exponent of the rigidity spectrum of Fds, particularly during large disturbances of the cut-off rigidity of the Earth's magnetic field. However, the general features of the temporal changes in the rigidity spectrum of Fds remain valid as they were found in our previous study. Namely, at the initial phase of the Fd, the rigidity spectrum is relatively soft and it gradually becomes hard up to the time of the minimum level of the GCR intensity. Then during the recovery phase of the Fd, the rigidity spectrum gradually becomes soft. This confirms that the structural changes of the interplan- etary magnetic field turbulence in the range of frequencies of 10 −6 -10 −5 Hz are generally responsible for the time variations in the rigidity spectrum we found during the Fds.

The influence of different turbulence models on the diffusion coefficients of energetic particles

“…In the context of interstellar shocks, such as supernova remnant shocks, usually simple diffusion models are employed but it was shown recently that the form of the diffusion coefficient can have a strong impact on the cosmic ray spectrum [see Ferrand et al , ]. Analytical forms of the diffusion coefficients are also required for studies of solar modulation and space weather [see, e.g., Alania et al , ; Engelbrecht and Burger , ; Manuel et al , ; Potgieter et al , ; Zhao et al , ; Engelbrecht and Burger , ]. To understand the propagation of cosmic rays in different astrophysical environments, the knowledge of transport parameters is also required [see, e.g., Shalchi and Büsching , ; Thornbury and Drury , ].…”

The Implicit Contribution of Slab Modes to the Perpendicular Diffusion Coefficient of Particles Interacting With Two-Component Turbulence

“…• For solar modulation studies (see, e.g., Alania et al 2013, Engelbrecht & Burger 2013, Manuel et al 2014);…”

Turbulence-based model of the Forbush decrease

“…A dependence of the Fd amplitude on the rigidity of GCR particles is one of its essential characteristics, called the rigidity spectrum of the Fd. The rigidity dependence of the GCR intensity in different phases of the Fd was presented in [4,5,6,7,8,9,10] and references therein. In these papers it has been shown that a rigidity spectrum δ D(R)/D(R) ∝ R −γ of the vast majority of the Fds gradually hardens during the decreasing and minimum phases of the Fd and gradually softens in the recovery phase of the Fd.…”

“…In these papers it has been shown that a rigidity spectrum δ D(R)/D(R) ∝ R −γ of the vast majority of the Fds gradually hardens during the decreasing and minimum phases of the Fd and gradually softens in the recovery phase of the Fd. Moreover, the time profiles of the Fds rigidity spectrum are associated with the changes of the power spectral density (PSD) of the interplanetary magnetic field (IMF) turbulence (PSD = P( f f 0 ) −ν , where P is power, f is frequency and f 0 is normalization frequency) [4,5,6,7,8,9,10]. Precisely, changes in the exponent γ of the power law rigidity R spectrum are explicitly determined by the changes of the exponent ν of the PSD in the range of frequency f ∈ [10 −6 , 10 −5 ] Hz of the IMF turbulence, to which neutron monitors and ground muon telescopes respond.…”