On the validity of force-field approximation for galactic cosmic ray spectrum during Forbush decreases using PAMELA data
Abstract
Precise measurements of galactic cosmic rays (GCR) are performed nowadays by space-based instruments, but they are limited in time from short-time campaigns to a half-solar cycle long PAMELA and a few-year long AMS-2 space missions. On the other hand, simple model approximations for the GCR spectrum, modulated by solar magnetic activity, are used in many practical applications. The most common is the so-called force-field model, which is often used in such fields as atmospheric physics, long-term solar studies, etc. The GCR spectrum is approximated by a prescribed form with the single parameter, the modulation potential, whose value can be determined from the ground-based neutron monitor data. In this way, GCR spectrum can be modelled for the last about 60 years. While the validity of the force-field model has been confirmed for regular periods of solar modulation, it was not known if it is still valid during Forbush decreases, when GCR are locally modulated by strong interplanetary transients. Accordingly, this led to essential uncertainty in applying this approach. The recent direct data from PAMELA instrument cover a major Forbush decrease in December 2006, which makes it possible to directly test the validity of the force-field model. We made a detailed study, by fitting the observed daily GCR spectra as measured by PAMELA during the Forbush decrease, separately for protons and alpha-particles, and comparing them with the modulation potential values assessed from the neutron monitor data. We conclude that even during a strong Forbush decrease, the force-field model based on neutron monitor data reasonably approximate the actual GCR spectrum at the daily time scale. This validate the use of the force-field approximation even during periods of greatly disturbed heliospheric conditions.
- Publication:
-
40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E3446U