Abstract
A usual event, called anisotropic cosmic-ray enhancement (ACRE), was observed as a small increase ({leq},5%) in the count rates of polar neutron monitors during 12 – 19 UT on 07 June 2015. The enhancement was highly anisotropic, as detected only by neutron monitors with asymptotic directions in the southwest quadrant in geocentric solar ecliptic (GSE) coordinates. The estimated rigidity of the corresponding particles is {leq},1 GV. No associated detectable increase was found in the space-borne data from the Geostationary Operational Environmental Satellite (GOES), the Energetic and Relativistic Nuclei and Electron (ERNE) on board the Solar and Heliospheric Observatory (SOHO), or the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instruments, whose sensitivity was not sufficient to detect the event. No solar energetic particles were present during that time interval. The heliospheric conditions were slightly disturbed, so that the interplanetary magnetic field strength gradually increased during the event, followed by an increase of the solar wind speed after the event. It is proposed that the event was related to a crossing of the boundary layer between two regions with different heliospheric parameters, with a strong gradient of low-rigidity ({<},1 GV) particles. It was apparently similar to another cosmic-ray enhancement (e.g., on 22 June 2015) that is thought to have been caused by the local anisotropy of Forbush decreases, with the difference that in our case, the interplanetary disturbance was not observed at Earth, but passed by southward for this event.
Highlights
Variations in galactic cosmic-ray (GCR) flux near Earth are driven by solar variability on different timescales (Vainio et al, 2009)
In addition to GCR variability, sporadic enhancements of neutron monitors (NMs) count rates are caused by solar energetic particle (SEP) events when a bulk of particles can be accelerated during solar eruptive events, flares, and/or coronal mass ejections (CMEs) to energies of hundreds of MeV up to several GeV (e.g. Desai and Giacalone, 2016; Klein and Dalla, 2017)
In contrast to the event on 22 June 2015 mentioned above, it was not accompanied by a Forbush decrease or other heliospheric disturbances recorded on Earth and only affected the lowest detectable energies below 1 GeV. This was a moderate increase in NM count rate that was not associated with a SEP flux, it was very anisotropic and lasted for several hours
Summary
Variations in galactic cosmic-ray (GCR) flux near Earth are driven by solar variability on different timescales (Vainio et al, 2009). Cosmic-ray flux was enhanced on 22 June 2015, which was observed at high energies (several tens of GeV) by the global muon detector network during the active phase of a strong Forbush decrease and was speculated to be caused by a local anisotropic effect near the heliospheric current sheet in highly disturbed conditions (Munakata et al, 2018). In contrast to the event on 22 June 2015 mentioned above, it was not accompanied by a Forbush decrease or other heliospheric disturbances recorded on Earth and only affected the lowest detectable energies below 1 GeV This was a moderate increase in NM count rate that was not associated with a SEP flux, it was very anisotropic and lasted for several hours. We present a detailed analysis of the event as recorded by the worldwide NM network and space-borne instruments, considering the corresponding interplanetary conditions, and conclude that it was caused by focused scattering of GCRs on an interplanetary disturbance that passed Earth by
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