Cosmic ray modulation by solar wind disturbances^⋆

Hvar Observatory, Faculty of Geodesy, Kačićeva 26, 10000 Zagreb, Croatia
e-mail: m.dumbovic@geof.hr

Received: 26 October 2010
Accepted: 30 March 2011

Abstract

Aims. We perform a systematic statistical study of the relationship between characteristics of solar wind disturbances, caused by interplanetary coronal mass ejections and corotating interaction regions, and properties of Forbush decreases (FDs). Since the mechanism of FDs is still being researched, this analysis should provide a firm empirical basis for physical interpretations of the FD phenomenon.

Methods. The analysis is based on the ground-based neutron monitor data and the solar wind data recorded by the Advanced Composition Explorer, where the disturbances were identified as increases in proton speed, magnetic field, and magnetic field fluctuations. We focus on the relative timing of FDs, as well as on the correlations between various FD and solar wind parameters, paying special attention to the statistical significance of the results.

Results. It was found that the onset, the minimum, and the end of FDs are delayed after the onset, the maximum, and the end of the magnetic field enhancement. The t-test shows that at the 95% significance level the average lags have to be longer than 3, 7, and 26 h, respectively. FD magnitude (| FD|) is correlated with the magnetic field strength (B), magnetic field fluctuations (δB), and speed (v), as well as with combined parameters, Bt_B, Bv, vt_B, and Bvt_B, where t_B is the duration of the magnetic field disturbance. In the |FD|(B) dependence, a “branching” effect was observed, i.e., two different trends exist. The analysis of the FD duration and recovery period reveals a correlation with the duration of the magnetic field enhancement. The strongest correlations are obtained for the dependence on combined solar wind parameters of the product of the FD duration and magnitude, implying that combined parameters are in fact true variables themselves, rather than just a product of variables.

Conclusions. From the time lags we estimate that “the penetration depth” in the disturbance, at which FD onset becomes recognizable, is on the order of 100 Larmor radii and is comparable to a typical shock-sheath dimension. The results for the FD time profile indicate “shadow effect” of the solar wind disturbance before and after it passes the observer. The importance of reduced parallel diffusion during the passage of the disturbance is discussed, along with the influence of terrestrial effects on the observed “branching effect”.