Forbush decreases and turbulence levels at coronal mass ejection fronts

Abstract

Aims. We seek to estimate the average level of MHD turbulence near coronal mass ejection (CME) fronts as they propagate from the Sun to the Earth. Methods. We examined the cosmic ray data from the GRAPES-3 tracking muon telescope at Ooty, together with the data from other sources for three closely observed Forbush decrease events. Each of these event is associated with frontside halo coronal mass ejections (CMEs) and near-Earth magnetic clouds. The associated Forbush decreases are therefore expected to have significant contributions from the cosmic-ray depressions inside the CMEs/ejecta. In each case, we estimate the magnitude of the Forbush decrease using a simple model for the diffusion of high-energy protons through the largely closed field lines enclosing the CME as it expands and propagates from the Sun to the Earth. The diffusion of high-energy protons is inhibited by the smooth, large-scale magnetic field enclosing the CME and aided by the turbulent fluctuations near the CME front. We use estimates of the cross-field diffusion coefficient D ⊥ derived from the published results of extensive Monte Carlo simulations of cosmic rays propagating through turbulent magnetic fields. We then compare our estimates with the magnitudes of the observed Forbush decreases. Results. Our method helps constrain the ratio of energy density in the turbulent magnetic fields to that in the mean magnetic fields near the CME fronts. This ratio is found to be ∼2% for the 2001 April 11 Forbush decrease event, ∼6% for the 2003 November 20 Forbush decrease event and ∼249% for the much more energetic event of 2003 October 29.

How are Forbush decreases related to interplanetary magnetic field enhancements?

“…We used a low-pass filter to remove oscillations having frequency >1 d −1 . This filter was explained in Subramanian et al (2009). The measured variation of the muon flux in percent for the 24 November 2001 event is shown in Fig.…”

“…The cutoff rigidity due to the geomagnetic field at Ooty is 17 GV along the vertical direction and varies from 14 to 42 GV across the 2.2 sr. field of view of the muon telescope. Details of the muon telescope are given in Hayashi et al (2005), Nonaka et al (2006), and Subramanian et al (2009).…”

High-rigidity Forbush decreases: due to CMEs or shocks?

“…The GRAPES-3 muon telescope records the flux of muons in nine independent directions (labeled NW, N, NE, W, V, E, SW, S and SE), and the geomagnetic cutoff rigidity over this field of view varies from 12 to 42 GV. The details of this telescope are discussed in Hayashi et al (2005), Nonaka et al (2006), and Subramanian et al (2009). Thus, the GRAPES-3 telescope observes the cosmic ray muon flux in nine independent directions with varying cutoff rigidities simultaneously.…”

Estimation of index of power law rigidity spectrum of cosmic rays using effective rigidity of multidirectional muon detector

“…This argument can be used, for example, to define identical channels to increase the statistical significance of certain measurements (Alaniya et al 1975). Similar considerations for a detector with many solid angle bins may be used for regrouping data recorded in fine solid angle bins into data in coarse bins (Subramanian et al 2009). Fig.…”