Depression in cosmic ray intensity influenced by interplanetary disturbances

Abstract

Interplanetary manifestations of coronal mass ejections with specific plasma and field properties, called magnetic clouds,'' have been observed in the heliosphere since the mid-1960s. Depending on their associated features, a set of observed magnetic clouds identified at 1 AU were grouped in four different classes using data over four decades: (1) interplanetary magnetic clouds moving with the ambient solar wind (MC structure), (2) magnetic clouds moving faster than the ambient solar wind and forming a shock/sheath structure of compressed plasma and field ahead of it (SMC structure), (3) magnetic clouds pushed'' by the high-speed streams from behind, forming an interaction region between the two (MIH structure), and (4) shock-associated magnetic clouds followed by high-speed streams (SMH structure). A detailed analysis has been carried out in the present work to study the onset times of cosmic-ray decreases occurring during 2005-06 with respect to the arrival times of interplanetary shocks and magnetic clouds. The interplanetary magnetic field strength (B), north south component of interplanetary magnetic field (Bz), solar wind velocity (V), sunspot number (R) and disturbance storm time index (Dst) associated with these events have been studied in the present work. The data (neutron monitor count rate) of different cosmic ray neutron monitors have been used. It is noted that on the onset of magnetic cloud the cosmic ray intensity, Bz component of IMF, Disturbance storm time index Dst found to decrease for one day and then all the three components increases gradually, whereas interplanetary magnetic field B increases for one day and then decrease sharply. However the solar wind velocity found to remain constant and sunspot number (R) increases after one day of the onset of cloud with some depression. This southward turning of Bz produces large geomagnetic disturbances, which reflects in Dst value. Increase in Dst index, sunspot number (R) and Bz after the magnetic cloud event seems to be associated with cosmic ray intensity increase.