Pitch-Angle features in cosmic rays in advance of severe magnetic storms: neutron monitor observations

Bartol Research Institute, University of Delaware, Newark, DE 19716, U.S.A. Abstract. The behavior of cosmic rays in advance of the severe geomagnetic storms of 1978-1982 (from the Gosling list of severe storms) is analyzed on the basis of the cosmic ray anisotropy and pitch angle distribution derived from neutron monitor observations. This analysis aims to reveal cosmic ray predictive effects (loss cone and pre-increase) before arrival of the shock and solar wind disturbed region. Essentially in all cases the changes in cosmic rays, in particular their anisotropy, start well before the beginning of major magneticstorms, which do not always coincide with the time of shock arrival; the severe magnetic storm may start later (up to 20 hours after), depending on the structure, orientation and magnetic topology of solar ejecta. We show examples for which the characteristic changes in CR anisotropy and pitch angle distribution (as a narrow decrease oriented along the magnetic force line) are revealed. These loss cone effects are predictors of strong geomagnetic disturbances. 1 Introduction The angular distribution of high-energy cosmic rays (CR) is usually well described by the sum of the first and second spherical harmonics, with rare exception. Such exceptions (“nonharmonical” distribution) appear sometimes before the great interplanetary disturbances arrive at Earth, when a combination of CR reflection and acceleration at the shock and “loss cone” effect resulting from magnetic connection with the cosmic ray depleted region behind the shock is usually observed. In this case the CR intensity from nearby directions may increase and decrease simultaneously. Examples of such behavior may be found in a set of experimental papers (e.g. Nagashima et al; 1994; Belov et al., 1995; Morishita et. al., 1997; Ruffolo et al., 1999; Bieber et al., 1999). In the recent numerical model by Correspondence to: E. Eroshenko (erosh@izmiran.rssi.ru) Ruffolo (1999) the complex pitch angle distribution of charged particles upstream of a shock is obtained theoretically. A combination of CR narrow predecrease (or loss cone) along the interplanetary magnetic field line and wider preincrease is sometimes observed just in the last hours before the shock, and other times for a long time ahead of the shock. Such an effect lasting more than a day in September 1992 was studied by Munakata et al. (2000) and Belov et al. (2001). Peculiar behavior of high energy CR appears to be a precursor of strong geomagnetic disturbances, and becomes more and more a subject of interest. In the present report we try to identify similar features in CR pitch-angle distributions measured by neutron monitors (NM). The set of severe magnetic storms over the period 1978-1982, collected by Gosling et al. (1990) has been used for this analysis. The same set of 14 events was used already in a similar study of muon telescope data (Munakata et al., 2000). 2 Data and methods Data on the interplanetary magnetic field (IMF) and solar wind (SW) (OMNI Data) as well as geomagnetic (Kp, Ap and Dst indices) and solar data have been used in our studies. The main analysis employed cosmic ray data including hourly CR intensities from the world wide neutron monitor network and hourly values of density and anisotropy of 10 GV cosmic rays derived from the network (about 40 stations) by the global survey method. All these data are collected into a special database oriented towards analysis of complex CR events. It allows selection of desired events, picking out and visible presentation of all relevant data, calculation of the CR pitch angle and time-longitudinal distribution, and other parameters needed for the analysis. For each event we studied 2 days before and 5 days after a storm sudden commencement (SSC). A CR pitch-angle distribution has been found using CR variations