Corotating interaction regions and the 27 day variation of galactic cosmic rays intensity at 1 AU during the cycle 23/24 solar minimum

Abstract

The results show that the CIR forward/reverse shock pairs or compression/rarefaction regions play important roles in the transport of GCR particles and directly control the observed 27 day periodic intensity variations. We find that stream interfaces (SIs) in CIRs and the heliospheric current sheet (HCS) are both closely associated with the GCR depression onset, in agreement with the observations at 1 AU. The HCS is more important when its tilt angle becomes small during the declining phase of the solar minimum, while the passages of SIs control the onset of GCR depressions for larger HCS tilt angles. The mechanism of GCR intensity variation near 1 AU can be explained through an interplay between the effects of particle drift and diffusion. The simulated plasma background and GCR intensity are compared with the observations from spacecraft and a neutron monitor on the ground, to find good qualitative agreement. Evidently, CIRs had a substantial modulational effect on GCR during the recent solar minimum.

Cosmic-Ray Transport in Heliospheric Magnetic Structures. II. Modeling Particle Transport through Corotating Interaction Regions

“…Second, the amplitude of the intensity variation is smaller than in the case without CIRs: while the maximum values at the beginning and the end of the considered time period (when Ulysses was located at higher heliographic latitudes) are about the same in both cases (see the left panel of Figure 7), the minimum levels around day 222 in Figures 4 and 6 (when Ulysses was close to the ecliptic) are significantly (about 10%) higher when CIRs are present. On the one hand, this finding of reduced modulation of GCR protons in the presence of CIRs confirms the analogous result arrived at by Guo & Florinski (2014), who employed a similar, albeit simplified, simulation and nearly the same diffusion setup. On the other hand, here the simulations are extended to higher latitudes, where this effect of the CIRs is, as one should expect, much weaker and even reversed (see below).…”

“…In order to corroborate their findings, the study presented here improves that by Guo & Florinski (2014 in several respects. We employ the solar wind model computed with the CRONOS MHD framework in PaperI.…”

“…Of particular interest beyond the quiet, essentially latitudinally structured solar wind during the Sun's activity minima, characterized by the periodic heliospheric current sheet (HCS), are the (quasi-periodic) corotating interaction regions (CIRs), which form as a result of the interaction of fast with slow solar wind streams and usually persist for several solar rotations (see, e.g., Balogh et al 1999, and references therein). As recently argued by Guo & Florinski (2014) and Guo & Florinski (2016), various questions are still unanswered, such as that of the modulation efficiency of CIRs on short timescales (e.g., Heber et al 1999). By indirectly simulating neutron monitor data, Guo & Florinski (2014) found that CIRs-though occasionally referred to as "modulation barriers" (e.g., Toptygin 1985)-did actually facilitate the access of Galactic CRs (GCRs) significantly during the activity minimum of the solar cycle 23/24: the inclusion of CIRs into the modeling increased the GCR intensities by about 10%.…”

“…As recently argued by Guo & Florinski (2014) and Guo & Florinski (2016), various questions are still unanswered, such as that of the modulation efficiency of CIRs on short timescales (e.g., Heber et al 1999). By indirectly simulating neutron monitor data, Guo & Florinski (2014) found that CIRs-though occasionally referred to as "modulation barriers" (e.g., Toptygin 1985)-did actually facilitate the access of Galactic CRs (GCRs) significantly during the activity minimum of the solar cycle 23/24: the inclusion of CIRs into the modeling increased the GCR intensities by about 10%. In theimprovement of this modeling, Guo & Florinski (2016) added a solar wind turbulence model and repeated the analysis.…”

“…The fact that this period is also covered in the studies by Guo & Florinski (2014) and Guo & Florinski (2016) allows a partial comparison with their findings. In the following subsections, we present and discuss the simulation results for the CR proton transport in two heliospheric configurations, namely in a periodic (the variation in theazimuthal direction is determined merely by the HCS) 3D-structured solar wind without CIRs and in a fully 3D, CIR-structured solar wind.…”