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We analyze the cosmic-ray variations during a significant Forbush decreaseobserved with world-wide networks of ground-based neutron monitors and muondetectors during November 3-5, 2021. Utilizing the difference between primarycosmic-ray rigidities monitored by neutron monitors and muon detectors, wededuce the rigidity spectra of the cosmic-ray density (or omnidirectionalintensity) and the first- and second-order anisotropies separately, for eachhour of data. A clear two-step decrease is seen in the cosmic-ray density withthe first $\sim2\%$ decrease after the interplanetary shock arrival followed bythe second $\sim5\%$ decrease inside the magnetic flux rope (MFR) at 15 GV.Most strikingly, a large bidirectional streaming along the magnetic field isobserved in the MFR with a peak amplitude of $\sim5\%$ at 15 GV which iscomparable to the total density decrease inside the MFR. The bidirectionalstreaming could be explained by adiabatic deceleration and/or focusing in theexpanding MFR, which have stronger effects for pitch angles near 90$^\circ$, orby selective entry of GCRs along a leg of the MFR. The peak anisotropy anddensity depression in the flux rope both decrease with increasing rigidity. Thespectra vary dynamically indicating that the temporal variations of density andanisotropy appear different in neutron monitor and muon detector data.

astrophysical journal/the astrophysical journal

Large amplitude bidirectional anisotropy of cosmic-ray intensity  observed with world-wide networks of ground-based neutron monitors and muon  detectors in November, 2021