Comparison of the variations of solar indices, interplanetary plasma parameters, and cosmic ray neutron monitor intensities during 1991–2001

Abstract

For the 11‐year interval 1991–2001, the monthly values of three solar indices, interplanetary plasma density N, wind speed V, and total magnetic field B, and cosmic ray neutron monitor intensities at six locations were subjected to spectral analysis. Solar indices showed similar spectra between themselves and so did the cosmic rays between themselves, but the periodicities of solar indices (∼12, 20, 40 months) were diferent from the periodicities of cosmic rays (∼17, 24, 60 months), while interplanetary N, V had ∼13, 17, 28 months, and B had ∼17 and 40 months. Thus solar periodicities do not invariably pervade into the interplanetary space or affect the cosmic rays. On the other hand, interplanetary plasma parameters and cosmic rays develop their own periodicities, unlike each other and unlike solar indices. During two intervals, each of 132 daily values, 11 October 1997–19 February 1998 (Event II) and 20 February–1 July 1998 (Event III), a spectral analysis of the daily values of solar indices showed a strong 27‐day signal, but it was seen only in interplanetary N. Cosmic rays did not show the 27‐day signal. Instead, significant signals were seen in cosmic rays at ∼13.5 and 20 days, seen in solar indices also but much smaller than the 27‐day signal.