Corotating high‐speed solar‐wind streams and recurrent cosmic ray modulation

Abstract

[1] We studied the solar magnetic cycle dependence of cosmic ray depressions due to the corotating high-speed solar wind streams (CSWS) during different polarity states of the heliosphere. The daily averaged cosmic ray intensity data from Climax, Oulu, and Thule neutron monitors together with simultaneous solar wind plasma and field data were subjected to the superposed epoch analysis with respect to CSWS start time. These analyses were carried out separately in different polarity states of the heliosphere A < 0 and A > 0 during solar minimum as well as during the periods of variable solar activity. Although the average variations in the solar wind velocity, IMF strength, and its variance are almost similar, the amplitudes of CSWS-associated cosmic ray depressions are quite different during different polarity epochs; they are larger during A > 0 than A < 0 periods. Further, correlation analysis between cosmic ray intensity and solar wind velocity during CSWS shows differences in their relationship during A > 0 and A < 0; they are much better during A > 0 than A < 0. Two other solar wind parameters, IMF strength and its variance, do not show a significant relationship with cosmic ray intensity change through the passage of these streams, although the initial depression coincides the enhancement of the two parameters. These results are discussed in the light of existing models of galactic cosmic ray modulation.Citation: Singh, Y. P., and Badruddin (2007), Corotating high-speed solar-wind streams and recurrent cosmic ray modulation,

Cosmic ray modulation by different types of solar wind disturbances

“…Note that all Forbush decrease curves were cut at the end of FD, i.e., at the point where CR count comes to the pre-event level. Figure 4 clearly depicts differences between CIRs, ICMEs and mixed events in both |FD| and B, in agreement with results presented by Badruddin et al (1986); Badruddin (1996); Singh & Badruddin (2007). In ICMEs the magnetic field strength increases sharply and returns to the pre-increase level fairly gradually.…”

“…A two-sample t-test was applied, and no difference was found between the two, showing that a shock does not affect the duration of FD. The same behavior was noticed by Singh & Badruddin (2007), who found that the duration of the recovery phase, and therefore the total duration, was influenced primarily by the high-speed stream, regardless of the association with shock. In addition, we found that for shock-associated events the duration of FD correlates with all SWD parameter products that involve t B , where for events without shock only correlation with Bt B product was found.…”

“…Figures 6a and b show the dependence of FD magnitude on magnetic field enhancement (B) and fluctuations (δB) for shock and no-shock events. Evidently, disturbances associated with shocks cause deeper depressions, as was reported by, e.g., Badruddin et al (1986) and Singh & Badruddin (2007). Shockrelated data points in Fig.…”

“…Furthermore, the depressions tend to be deeper when a shock is formed at the leading edge of the disturbance (e.g. Badruddin et al 1986;Singh & Badruddin 2007), producing a two-step decrease (so-called classical FD, see e.g. Cane 2000).…”

“…Iucci et al 1979;Badruddin et al 1986;Badruddin 1996;Singh & Badruddin 2007), statistical analysis (e.g. Cane et al 1996;Richardson et al 1996;Belov et al 2001;Čalogović et al 2009;Chilingarian & Bostanjyan 2010) and case-studies (e.g.…”