Depression in cosmic ray intensity influenced by interplanetary disturbances

摘要

Interplanetary manifestations of coronal mass ejections with specific plasma and field properties, called "in interplanetary magnetic clouds,'' have been observed in the heliosphere since the mid terplanetary mid-1960s. Depending on their a as- ssociated features, a set of observed magnetic clouds identified at 1 AU were grouped in four different classes using sociated 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 c clouds "pushed'' by the high louds high-speed streams from behind, forming an interaction region between the two (MIH structure), and (4) shock shock-associated magnetic clouds followed by high high-speed streams (SMH structure). A detailed analysis has been carried out in the p present work to study the o resent on- nset times of cosmic set cosmic-ray decreases occurring during 2005 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 ma mag- gne netic field (Bz), solar wind velocity (V), sunspot number (R) and disturbance storm time index (Dst) associated with ic these events have been studied in the present work. The data (neutron monitor count rate) of different cosmic ray ne neu- utron monitors have be tron been used. It is noted that on the onset of magnetic cloud the cosmic ray intensity, Bz component en of IMF, Disturbance storm time index Dst found to decrease for one day and then all the three components increases gradually, whereas interplanetary magnetic fi field B increases for one day and then decrease sharply. However the s eld so- olar wind velocity found to remain constant and sunspot number (R) increases after one day of the onset of cloud with lar some depression. This southward turning of Bz produces large geomagnet geomagnetic disturbances, which reflects in Dst value. ic Increase in Dst index, sunspot number (R) and Bz after the magnetic cloud event seems to be associated with cosmic ray intensity increase.