7 research outputs found
Drift effects and the cosmic ray density gradient in a solar rotation period: First observation with the Global Muon Detector Network (GMDN)
We present for the first time hourly variations of the spatial density
gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006.
By inversely solving the transport equation, including diffusion, we deduce the
gradient from the anisotropy that is derived from the observation made by the
Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new
analysis method to the GMDN data is precise and free from atmospheric
temperature effects on the muon count rate recorded by ground based detectors.
We find the derived north-south gradient perpendicular to the ecliptic plane is
oriented toward the Helioshperic Current Sheet (HCS) (i.e. southward in the
toward sector of the Interplanetary Magnetic Field (IMF) and northward in the
away sector). The orientation of the gradient component parallel to the
ecliptic plane remains similar in both sectors with an enhancement of its
magnitude seen after the Earth crosses the HCS. These temporal features are
interpreted in terms of a local maximum of the cosmic ray density at the HCS.
This is consistent with the prediction of the drift model for the epoch.
By comparing the observed gradient with the numerical prediction of a simple
drift model, we conclude that particle drifts in the large-scale magnetic field
play an important role in organizing the density gradient, at least in the
present epoch. We also found that corotating interaction regions did not
have such a notable effect. Observations with the GMDN provide us with a new
tool for investigating cosmic ray transport in the IMF.Comment: 35 pages, 10 figures, submitted to the Astrophysical Journa
Short-term variability of the Sun-Earth system: an overview of progress made during the CAWSES-II period
PRECURSORS OF THE FORBUSH DECREASE ON 2006 DECEMBER 14 OBSERVED WITH THE GLOBAL MUON DETECTOR NETWORK (GMDN)
AVERAGE SPATIAL DISTRIBUTION OF COSMIC RAYS BEHIND THE INTERPLANETARY SHOCK—GLOBAL MUON DETECTOR NETWORK OBSERVATIONS
We analyze the galactic cosmic ray (GCR) density and its spatial gradient in
Forbush Decreases (FDs) observed with the Global Muon Detector Network (GMDN)
and neutron monitors (NMs). By superposing the GCR density and density gradient
observed in FDs following 45 interplanetary shocks (IP-shocks), each associated
with an identified eruption on the sun, we infer the average spatial
distribution of GCRs behind IP-shocks. We find two distinct modulations of GCR
density in FDs, one in the magnetic sheath and the other in the coronal mass
ejection (CME) behind the sheath. The density modulation in the sheath is
dominant in the western flank of the shock, while the modulation in the CME
ejecta stands out in the eastern flank. This east-west asymmetry is more
prominent in GMDN data responding to 60 GV GCRs than in NM data
responding to 10 GV GCRs, because of softer rigidity spectrum of the
modulation in the CME ejecta than in the sheath. The GSE-y component of the
density gradient, shows a negative (positive) enhancement in FDs caused
by the eastern (western) eruptions, while shows a negative (positive)
enhancement in FDs by the northern (southern) eruptions. This implies the GCR
density minimum being located behind the central flank of IP-shock and
propagating radially outward from location of the solar eruption. We also
confirmed the average changing its sign above and below the heliospheric
current sheet, in accord with the prediction of the drift model for the
large-scale GCR transport in the heliosphere.Comment: 54 pages, 11 figures, 1 table, accepted for publication in the
Astrophysical Journa