Features of cosmic ray neutron monitor intensity in relation to CMEs and IMF

Abstract

It is well known that Coronal Mass Ejections are vast structures of plasma and magnetic fields that are ex- pelled from the sun into the heliosphere, which is detected by remote sensing and in-situ spacecraft observations. Re- cent work indicates that the open magnetic flux from coronal holes may be a more important driver of cosmic-ray modulation than coronal mass ejections (CMEs), which originate from closed-field regions on the Sun. Influence of four types of CMEs namely Asymmetric 'Full' Halo CMEs, Partial Halo CMEs, Asymmetric and Complex 'Full' Halo CMEs and 'Full' Halo CMEs on cosmic ray neutron monitor intensity have been investigated in this study. The data of two different ground based neutron monitors and CME events observed with instruments onboard and Wind spacecraft have been used in the present analysis. The method of superposed epoch (Chree) analysis has been used to the arrival times of these CMEs. The occurrence frequency of three different types of CMEs used in the present anal- ysis shows complex behavior. However, significant increase in cosmic ray intensity is observed few days after the onset of asymmetric full halo and few days after the onset of full halo CMEs. The fluctuations in cosmic ray intensity are more prior to the onset of both types of the CMEs. However, during Partial Halo CMEs the cosmic ray intensity peaks, 8- 9 days prior to the onset of CMEs and depressed 3 days prior to the onset of CMEs, whereas in case of asymmetric and complex full CMEs, the intensity depressed 2 days prior to the onset of CMEs and enhanced 2 days after the onset of CMEs. The deviations in cosmic ray intensity are more pronounced in case for asymmetric and complex full halo CMEs compared to other CMEs. The cosmic ray intensity shows nearly good anti-correlation with interplanetary magnetic field strength (B) during asymmetric full halo CMEs and partial halo CMEs, whereas it shows poor correlation with B during other CMEs. The interplanetary magnetic field strength B shows significant characteristics before and after the onset of these three types of CMEs.