Hourly measurement of cosmic ray anisotropy by LHAASO WCDA at ∼1 TeV: Effects of an interplanetary flux rope during 2021 November

Abstract

Interplanetary coronal mass ejections (ICMEs) are known to affect the intensity and anisotropy of Galactic cosmic rays of energies up to ∼100 GeV, but effects at higher energies have rarely been reported. Here we analyze data from the Large High Altitude Air Shower Observatory (LHAASO), and in particular its Water Cherenkov Detector Array (WCDA), which has been in full operation since March 2021. WCDA is sensitive to primary cosmic rays above about 100 GeV with a wide field of view (FOV), from which we select events within 45 degrees of the zenith. We normalize cosmic ray skymaps relative to a monthly average. Then for each hour of data, we express the cosmic ray anisotropy as the gradient of the excess cosmic ray rate over the FOV. For the ICME passage of 2021 Nov 4-5, a strong anisotropy was recently reported in data from muon detectors and neutron monitors. We present evidence for an enhanced anisotropy in LHAASO-WCDA data during that time period, for primary cosmic ray energy ranges both below and above 1 TeV.