Large-amplitude Bidirectional Anisotropy of Cosmic-Ray Intensity Observed with Worldwide Networks of Ground-based Neutron Monitors and Muon Detectors in 2021 November

Abstract

We analyze the cosmic-ray variations during a significant Forbush decrease observed with worldwide networks of ground-based neutron monitors and muon detectors during 2021 November 3–5. Utilizing the difference between primary cosmic-ray rigidities monitored by neutron monitors and muon detectors, we deduce the rigidity spectra of the cosmic-ray density (or omnidirectional intensity) and the first- and second-order anisotropies separately for each hour of data. A clear two-step decrease is seen in the cosmic-ray density with the first ∼2% decrease after the interplanetary shock arrival followed by the second ∼5% decrease inside the magnetic flux rope (MFR) at 15 GV. Most strikingly, a large bidirectional streaming along the magnetic field is observed in the MFR with a peak amplitude of ∼5% at 15 GV, which is comparable to the total density decrease inside the MFR. The bidirectional streaming could be explained by adiabatic deceleration and/or focusing in the expanding MFR, which have stronger effects for pitch angles near 90°, or by selective entry of GCRs along a leg of the MFR. The peak anisotropy and density depression in the flux rope both decrease with increasing rigidity. The spectra vary dynamically, indicating that the temporal variations of density and anisotropy appear different in neutron monitor and muon detector data.

Galactic cosmic ray variations associated with February 2022 “Starlink” magnetic storms observed with global networks of neutron monitors and muon detectors

“…Table 1 lists NMs and MDs used in this paper with their characteristics. Our calculations of the median primary rigidity 𝑃 𝑚 , latitude and longitude of the asymptotic viewing direction (𝜆 𝑎𝑠𝑦𝑚 𝑝 and 𝜙 𝑎𝑠𝑦𝑚 𝑝 ), and the response functions used in our calculations are described in [4]. These 20 NMs are selected to maximize the range of viewing directions with less overlapping directions.…”

Cosmic ray variation caused by coronal mass ejections without direct impact on Earth

“…ICMEs without direct impact on Earth Y. Hayashi GMDN data alone [5], 𝛾 𝑛 (𝑡) is treated as a constant, as 𝛾 0 = −1, 𝛾 1 = 0, while Munakata et al [2] treated it as a free parameter. 𝛾 0 is the power-law index of rigidity spectra of the density, while 𝛾 1 , 𝛾 2 are indices of the first-and second-order anisotropy, respectively.…”

“…In this paper, we analyze NM and GMDN data and deduce variations of cosmic ray density and anisotropy, each as a function of time and the rigidity of primary cosmic rays, by performing the best-fit analysis presented in Munakata et al [2], and report preliminary results.…”

Hourly Measurement of Cosmic Ray Anisotropy by LHAASO WCDA at ~1 TeV: Effects of an Interplanetary Flux Rope during 2021 November

“…In further work, we plan to model the "background" gradient expected from the known (measured) sidereal diurnal anisotropy and the calculated Compton-Getting solar diurnal anisotropy, to verify that the quiet-time profile of g can be explained by these effects. Following [15] we transform g to geocentric solar ecliptic (GSE) coordinates, for comparison with other solarheliospheric data (such as the sunward or interplanetary magnetic field direction) and to facilitate comparison with the transient anisotropy observed in lower-energy CR ions [16].…”

“…From an analysis of data from neutron monitors and muon detector worldwide, with a total of 90 asymptotic look directions, [16] analyzed a particularly strong transient anisotropy around the times of passage of an interplanetary shock and ICME on 2021 November 3-5. Most of the 21 neutron monitors were sensitive to a median CR rigidity close to 15 GV, while a typical median In each skymap, directions northward from the zenith are at the top and directions eastward of the zenith are at the right.…”