Takao Kuwabara

High-energy cosmic ray modulation associated with interplanetary shocks observed by the GMDN

Interplanetary shocks are caused both by interplanetary counterparts of coronal mass ejections (ICMEs) and by co-rotating interaction regions (CIRs) propagating in the interplanetary medium. CIRs are formed by the interaction between high-speed and slow solar wind streams. When the interplanetary disturbance propagates faster than the magnetosonic wave speed, in the solar wind frame, a shock wave is formed. Shocks frequently produce decreases of cosmic rays observed both by neutron monitors and muon detectors located at the Earth’s surface. In this work, we analyze this kind of modulation of high-energy cosmic rays (> 50 GeV) observed by the Global Muon Detector Network (GMDN). After correcting both the atmospheric temperature and pressure effects, we calculated the isotropic intensity and the anisotropy vector. From a list of 38 interplanetary shocks identified in 2015 using interplanetary magnetic field and plasma parameters, we performed a superposed epoch analysis grouping the events by type and orientation of shocks. We found that the cosmic ray isotropic intensity is higher when it is associated to fast forward shocks when compared to fast reverse shocks. We also identified some differences in the anisotropy vector when comparing different types of shocks or shocks that are quasi-perpendicular with the remaining ones.

Average features of the interplanetary shock observed with the Global Muon Detector Network (GMDN)

From three-dimensional spatial density gradient of galactic cosmic rays (GCRs) observed with the nGlobal Muon Detector Network (GMDN), we derive average features of the GCR depleted region nbehind the IP (interplanetary) shock. We identify 207 IP-shocks that passed the earth based on the ngeomagnetic storm sudden commencements (SSCs) and extract 50 events that are associated with nsolar coronal mass ejections (CMEs) in a period between 2006 and 2014. From the first order nGCR anisotropy corrected for the solar wind convection and Compton-Getting effect arising from nthe earth’s orbital motion, we deduce the density gradient on an hourly basis for each event. We nthen derive the average temporal variation of the density gradient by superposing its variations nat the SSC onset timing. We confirm that the density gradient components are clearly enhanced nafter the shock passage, indicating the existence of GCR depleted region behind the shock which ncauses the Forbush Decrease in the cosmic ray intensity. The enhancement of the radial gradient nshows longer duration when the earth has encountered the western flank of the shock, implying nan asymmetric shielding effect of the shock on the GCRs. The longitudinal gradient, on the other nhand, shows that the GCR density minimum is located around the longitudinal center behind the nshock, which can be ascribed to the centered ejecta driving IP-shock.