Abstract:
The relationship of Forbush decreases (FDs) observed
in Moscow neutron monitor with the interplanetary
magnetic field (B) and solar wind speed (Vsw) is investigated
in detail for the FDs associated with Interplanetary
Coronal Mass Ejections (ICMEs) during 2001–2004. The
classical two-step FD events are selected, and characteristics
of the first step (mainly associated with shock), as well as
of complete decrease (main phase) and recovery phase, are
studied here. It is observed that the onset of FD occurs generally
after zero to a few hours of shock arrival, indicating
in the post-shock region that mainly sheath and ICME act
as important drivers of FD. A good correlation is observed
between the amplitude of B and associated FD magnitude
observed in the neutron count rate of the main phase. The
duration of the main phase observed in the neutron count
rate also shows good correlation with B. This might indicate
that stronger interplanetary disturbances have a large dimension
of magnetic field structure which causes longer fall time
of FD main phase when they transit across the Earth. It is observed
that Vsw and neutron count rate time profiles show
considerable similarity with each other during complete FD,
especially during the recovery phase of FD. Linear relationship
is observed between time duration/e-folding time of FD
recovery phase and Vsw. These observations indicate that
the FDs are influenced by the inhibited diffusion of cosmic
rays due to the enhanced convection associated with
B A. Bhaskar
ankushbhaskar@gmail.com
1 Indian Institute of Geomagnetism, Kalamboli Highway,
New Panvel, Navi Mumbai 410 218, India
2 The GRAPES-3 Experiment, Cosmic Ray Laboratory,
Raj Bhavan, Ooty 643 001, India
3 Department of Physics, University of Mumbai, Mumbai 400 098,
India
the interplanetary disturbances. We infer that the inhibited
cross-field diffusion of the cosmic rays due to enhanced B
is mainly responsible for the main phase of FD whereas the
expansion of ICME contributes in the early recovery phase
and the gradual variation of Vsw beyond ICME boundaries
contributes to the long duration of FD recovery through reduced
convection–diffusion.
