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 interplanetary 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.
Highlights
The short-term depressions in the cosmic ray flux reaching the Earth are called the Forbush decreases (Fds)
The corresponding corrections for the Galactic cosmic ray (GCR) intensity, J, caused by the changes in the geomagnetic cut-off rigidity Rc during these Fds are marked by green triangles in Figure 1 as well
The spectrographic global survey (SGS) method was applied to calculate the changes in the vertical cut-off rigidity caused by geomagnetic disturbances during three large Fds of the 23rd solar cycle
Summary
The short-term depressions in the cosmic ray flux reaching the Earth are called the Forbush decreases (Fds) They are caused by the interplanetary counterpart of coronal mass ejections (CMEs) (and the shocks they drive) and by the corotating interaction regions (CIRs) originating from the Sun. The geomagnetic disturbances and Fds have a common origin in the interplanetary space, namely the Earth’s encounter with a strong interplanetary structure. According to Dungey’s (1961) mechanism, relatively high-energy particles of the solar wind rush towards the Earth but are diverted around the Earth in circular orbits in the equatorial plane These particles form a ring current at several Earth radii and cause large geomagnetic field reduction. There are significant differences in the exact evolution of these indices (Cane, 2000) Both rise with increasing interplanetary magnetic field (IMF) and solar wind velocity V. In general not all CMEs are geoeffective, and many smaller geomagnetic disturbances are not related to CMEs
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