Transient Large-Scale Anisotropy in TeV Cosmic Rays due to an Interplanetary Coronal Mass Ejection

TL;DR

This study reports the first observation of transient large-scale anisotropy in TeV cosmic rays linked to an interplanetary coronal mass ejection, revealing energy-dependent anisotropy variations and opening new avenues for space weather research.

Contribution

It provides the first evidence of time-varying large-scale anisotropy in TeV cosmic rays associated with solar storm events, using data from LHAASO.

Findings

Enhanced anisotropy observed during specific hours on Nov 4, 2021

Anisotropy gradient varies with energy as E^{-0.5}

Dipole anisotropy of at least 1% at energies ≤1.0 TeV

Abstract

Large- or medium-scale cosmic ray anisotropy at TeV energies has not previously been confirmed to vary with time. Transient anisotropy changes have been observed below 150 GeV, especially near the passage of an interplanetary shock and coronal mass ejection containing a magnetic flux rope ejected by a solar storm, which can trigger a geomagnetic storm with practical consequences. In such events, cosmic rays provide remote sensing of the magnetic field properties. Here we report the observation of transient large-scale anisotropy in TeV cosmic ray ions using data from the Large High Altitude Air Shower Observatory (LHAASO). We analyze hourly skymaps of the transient cosmic ray intensity excess or deficit, the gradient of which indicates the direction and magnitude of transient large-scale anisotropy across the field of view. We observe enhanced anisotropy above typical hourly fluctuations with $>$5$\sigma$ significance during some hours of November 4, 2021, in separate data sets for four primary cosmic ray energy ranges of median energy from $E$=0.7 to 3.1 TeV. The gradient varies with energy as $E^{\gamma}$, where $\gamma\approx-0.5$. At a median energy $\leq$1.0 TeV, this gradient corresponds to a dipole anisotropy of at least 1\%, or possibly a weaker anisotropy of higher order. This new type of observation opens the opportunity to study interplanetary magnetic structures using air shower arrays around the world, complementing existing in situ and remote measurements of plasma properties.