Galactic Cosmic Ray Modulation near the Heliospheric Current Sheet

Abstract

Galactic cosmic rays (GCRs) are modulated by the heliospheric magnetic field (HMF) both over decadal time scales (due to long-term, global HMF variations), and over time scales of a few hours (associated with solar wind structures such as coronal mass ejections or the heliospheric current sheet, HCS). Due to the close association between the HCS, the streamer belt, and the band of slow solar wind, HCS crossings are often associated with corotating interaction regions where fast solar wind catches up and compresses slow solar wind ahead of it. However, not all HCS crossings are associated with strong compressions. In this study we categorise HCS crossings in two ways: Firstly, using the change in magnetic polarity, as either away-to-toward (AT) or toward-to-away (TA) magnetic field directions relative to the Sun and, secondly, using the strength of the associated solar wind compression, determined from the observed plasma density enhancement. For each category, we use superposed epoch analyses to show differences in both solar wind parameters and GCR flux inferred from neutron monitors. For strong-compression HCS crossings, we observe a peak in neutron counts preceding the HCS crossing, followed by a large drop after the crossing, attributable to the so-called 'snow-plough' effect. For weak-compression HCS crossings, where magnetic field polarity effects are more readily observable, we instead observe that the neutron counts have a tendency to peak in the away magnetic field sector. By splitting the data by the dominant polarity at each solar polar region, we find that the increase in GCR flux prior to the HCS crossing is primarily from strong compressions in cycles with negative north polar fields due to GCR drift effects. Finally, we report on unexpected differences in GCR behaviour between TA weak compressions during opposing polarity cycles.

Distribution of tensor anisotropy of cosmic rays near the neutral current sheet

“…Similar results have been obtained in [Thomas et al, 2014]. Apparently, such behavior of 0 0 R is caused by the magnetic mirror in the region of interaction between fast and slow SW flows, which prevents the penetration of CR particles and leads to a CR intensity decrease during the crossings.…”

“…The same approach is used by other authors (e.g. [Thomas et al, 2014]). Notice that abrupt changes of CR intensity such as Forbush decreases and ground level enhancements are excluded from the analysis to get rid of distortion factors.…”

Lightning as a space‐weather hazard: UK thunderstorm activity modulated by the passage of the heliospheric current sheet

“…Neutron counts show stronger ordering about the HCS in rough agreement with the solar wind structure "sweeping up" GCRs in the inner heliosphere. Neutron counts do show different fluxes in T to A and A to T transitions, in approximate agreement with higher neutron counts in A sectors, as has been reported elsewhere Thomas et al, 2014]. But unlike the R L variations, these differences are most pronounced in the 1-2 days prior to the HCS crossing.…”

“…Times of near‐Earth heliospheric current sheet (HCS) encounters are obtained from the catalog of Thomas et al . []. They manually identified “clean” HCS crossings, in which there is a clear, sharp transition in HMF polarity from a Parker spiral configuration [ Parker , ] pointing toward (T) the Sun to away from the Sun (A) or vice versa.…”

“…Heliospheric magnetic field (HMF) and solar wind parameters are taken from OMNI data set of near-Earth spacecraft observations [King and Papitashvili, 2005]. Times of near-Earth heliospheric current sheet (HCS) encounters are obtained from the catalog of Thomas et al [2014]. They manually identified "clean" HCS crossings, in which there is a clear, sharp transition in HMF polarity from a Parker spiral configuration [Parker, 1958] pointing toward (T) the Sun to away from the Sun (A) or vice versa.…”