Energetic-particle-flux decreases related to magnetic cloud passages as observed by the Helios 1 and 2 spacecraft

Abstract

It has been observed that a magnetic cloud (MC) can affect the propagation conditions of solar energetic particles and low-energy cosmic rays. This effect is commonly observed as a decrease in the energetic-particle fluxes, which are partially excluded from the interior of the cloud. The twin spacecraft Helios 1 and Helios 2 explored the inner heliosphere between 0.29 AU and 1 AU from the mid 1970s to early 1980s. The E6 Experiment onboard Helios is the energetic-particle detector able to measure electrons, protons and alphas in the range of 300 keV/n to >50 MeV/n. It has been shown previously that, in absence of strong solar-particle events, the single detector rates of the E6 anti-coincidence and saphire Cherenkov detectors are sensitive to cosmic rays with rigidities above GV. Because their statistical precision is in the order of hundreds of counts per second, both detectors are very well suited for studying the short-term decreases observed in their count rates during magnetic cloud passages. A total of 35 magnetic clouds have been identified at the Helios locations. Nineteen of them were free of solar energetic-particle contamination. This subset led us to investigate the effect of magnetic clouds on the galactic cosmic ray (GCR) flux. The depth of the decreases are studied in terms of the solar wind and magnetic field properties of the magnetic cloud. We found dependences with the MC magnetic field strength, magnetic rigidity and with the MC time of flight, with the latter supporting the idea of magnetically closed MCs, i.e. with the two legs rooted in the Sun. We also studied MC properties and found evidence of MC expansion during its journey through the inner heliosphere.

An Analytical Diffusion–Expansion Model for Forbush Decreases Caused by Flux Ropes

“…Kühl et al 2015). The models based on perpendicular diffusion (Cane et al 1995;Munakata et al 2006;Quenby et al 2008;Subramanian et al 2009) are in a good qualitative agreement with the observation, reflecting some of the observational characteristics of FDs caused by ejecta such as symmetric shape constrained to the spatial extent of the flux rope and relation of the FD amplitude to the magnetic field strength, the FR size and ICME transit time (Cane 1993;Belov 2009;Dumbović et al 2011Dumbović et al , 2012Blanco et al 2013;Badruddin and Kumar 2016;Masías-Meza et al 2016), although there are evidences that simple diffusion FD models cannot fully explain observations (Richardson and Cane 2011).…”

“…The calculated temporal variation of the FD magnitude depends on the expansion case (Figure 7b), due to different D E , as is expected from the analysis presented in Figure 3. To check whether the time evolution of the FD magnitude for the 2014 May 30 CME is quantitatively realistic, we compare it to the results of the statistical study by Blanco et al (2013). In their statistical study, Blanco et al (2013) used spacecraft measurements from a Saphire Cherenkov detector onboard Helios 1 and Helios 2, that is mainly sensitive to ∼ 1 GV particles, comparable to the response of the SOHO/EPHIN (Kühl et al 2015).…”

“…The diffusion approach is generally considered valid when the gyroradius is smaller than the size of the FR (e.g. Subramanian et al 2009), andBlanco et al (2013) showed that for FRs detected by Helios spacecraft typical rigidity of a GCR with gyroradius corresponding to the size of FR is of the order of ∼ 100 GV. Therefore, the diffusion approach should be valid to explain FR associated FDs detected in a variety of instruments corresponding to different mean energies: muon telescopes at ∼ 50 GV (e.g.…”

Properties of Solar Wind Structures at Mercury's Orbit

“…According to Burlaga et al (1981), we focused our attention to B Z variations as reports in Table 4. So, we first selected data from Bothmer and Schwenn (1998), Blanco et al (2013), and Cane et al (1997) (Cliver et al, 1996). In fact, the relative percentage of each class depends on the phase and amplitude of solar activity cycle.…”

“…, namely, enhanced |B|, almost planar rotation of B components, and low T. According toBurlaga et al (1981), we focused our attention to B Z variations as reports in Table4. So, we first selected data from Bothmer and Schwenn (1998),Blanco et al (2013), andCane et al (1997) catalogs for H1 or H2 located between 0.29 and 0.47 AU. In addition to these, other events having high |B| and B Z variations have been included.…”