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

TL;DR

This work reports the first observation of transient large-scale anisotropy in TeV cosmic rays, associated with the passage of an ICME on 2021-11-04. Using hourly skymaps from LHAASO's WCDA and KM2A, the authors extract a sky-gradient vector $\mathbf{g}$ and find peak anisotropy of about $g \approx 0.01$ (≈$1\%$) in the $0.7$–$3.1$ TeV range, with a shallow energy dependence $g \propto E^{\gamma}$ and $\gamma \approx -0.5$. The results cannot be explained by simple motional electric-field effects and are instead attributed to enhanced scattering from turbulent magnetic fields in the ICME sheath, offering a new tool to study interplanetary magnetic structures with air-shower arrays. This complements in situ solar wind measurements and demonstrates the potential of high-energy CR anisotropy studies to probe dynamic heliospheric magnetic fields.

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 $>$5$σ$ 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 $E^γ$, where $γ\approx-0.5$. 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.

Figures (11)

• Figure 1: Skymaps of relative CR intensity centered at the zenith and extending to zenith angle 45° (outer circle), for showers with $30 \leq N_{hit} < 40$ in LHAASO/WCDA for each hour in Universal Time (UT) on November 4, 2021. An Interplanetary Coronal Mass Ejection (ICME) impacted Earth at about 12:00 UT. Black arrow represents the best-fit CR gradient vector, with arrow length proportional to gradient magnitude; it points away from deficit areas (blue) and towards areas of enhancement (red). The anisotropy increased strongly a few hours before 1200 UT, the time of ICME arrival, with a maximum gradient magnitude of 1.2% during 10:00-11:00 UT, and continued to be strong for a few hours into ICME passage.
• Figure 2: Gradient of relative anisotropy (arrow with length proportional to gradient magnitude) and LHAASO field of view (FOV, within the curve) in GSE coordinates for each hour from 09:00 UT to 17:00 UT on November 4, 2021 for $30\leq N_{hit}<40$ (median energy 0.7 TeV). Sunward (yellow symbol) and anti-Sunward (red circle) viewing directions are also indicated. Other than 15:00-16:00, when the FOV may have included a direction of minimal flux, the gradient has $\geq4\sigma$ significance for each hour and usually indicates a higher cosmic ray flux from directions closer to the Sunward viewing direction.
• Figure 3: Time series of skymap gradient components and magnitude and their significance for various WCDA energy ranges during November 1-7, 2021. Green lines indicate start and end times of ICME passage. Median primary CR energy is indicated at the top left of each panel. Solid traces indicate normalized significance of the gradient magnitude and components, while the dashed trace indicates the formal significance of the magnitude based on fitting uncertainty alone.
• Figure 4: Observed gradient magnitude $g$ vs. median CR energy during 10:00–11:00 UT on November 4, 2021, with power-law fit to the first four energy ranges.
• Figure 5: Illustration of the magnetic flux rope of an ICME passing Earth on November 4, 2021, which was preceded by an interplanetary shock and a sheath region with intense magnetic fluctuations. Transient anisotropy of TeV CRs across LHAASO's FOV was strongest shortly before arrival of the leading edge of the ICME, with a lower flux in directions from the outer heliosphere. We attribute this to enhanced scattering of CRs along trajectories passing through the sheath region of enhanced magnetic turbulence.