27-day variation of the GCR intensity based on corrected and uncorrected for geomagnetic disturbances data of neutron monitors

TL;DR

This study analyzes 27-day variations in galactic cosmic ray intensity during 2005-2008 using neutron monitor data, comparing corrected and uncorrected data for geomagnetic disturbances, revealing insights into rigidity spectrum and solar activity effects.

Contribution

It provides a comparative analysis of GCR 27-day variations using corrected and uncorrected neutron monitor data, highlighting differences in rigidity spectrum and temporal changes.

Findings

Amplitudes of 27-day variations are similar across different cut-off rigidities.

Uncorrected data show a soft rigidity spectrum, corrected data show a hard spectrum.

Temporal softening of the spectrum occurs approaching solar minimum.

Abstract

We study 27-day variations of the galactic cosmic ray (GCR) intensity for 2005- 2008 period of the solar cycle #23. We use neutron monitors (NMs) data corrected and uncorrected for geomagnetic disturbances. Besides the limited time intervals when the 27-day variations are clearly established, always exist some feeble 27-day variations in the GCR 5 intensity related to the constantly present weak heliolongitudinal asymmetry in the heliosphere. We calculate the amplitudes of the 27-day variation of the GCR intensity based on the NMs data corrected and uncorrected for geomagnetic disturbances. We show that these amplitudes do not differ for NMs with cut-off rigidities smaller than 4-5 GV comparing with NMs of higher cut-off rigidities. Rigidity spectrum of the 27-day variation of the GCR intensity found in the uncorrected data is soft while it is hard in the case of the corrected data. For both cases exists definite tendency of softening the temporal changes of the 27-day variation's rigidity spectrum in period of 2005 to 2008 approaching the minimum of solar activity. We believe that a study of the 27-day variation of the GCR intensity based on the data uncorrected for geomagnetic disturbances should be carried out by NMs with cut-off rigidities smaller than 4-5 GV.