Fig.1: asymptotic directions determined during the first polar pass that registered the 2012 May 17 event.
Also shown are the asymptotic directions of the NM that registered the primary GLE beam [3].

THE USE OF THE EARTH AS A MAGNETIC SPECTROMETER
Dr. Alessandro Bruno - INFN and University of Bari, Italy

The PAMELA space experiment is providing first direct observations of SEPs with energies from about 80 MeV to several GeV in near-Earth orbit,bridging the low energy measurements by in-situspacecrafts and the GLE data by the worldwide network of NMs. Its unique observational capabilitiesinclude not only the possibility of measuring the flux energetic spectrum and composition, but also its angular distribution, thus investigating possibleanisotropies associated to SEP events [1].Cosmic Ray cutoff rigidities and asymptotic arrival directions are commonly evaluated by simulations accounting for the effect of the geomagnetic field on the particle transport. Using spacecraft ephemeris data (position, orientation, time), and the particle rigidity and direction provided by the PAMELA tracking system, trajectories of all detected protons are reconstructed by means of a tracing program based on numerical integration methods, and implementing the IGRF-11 and the TS07D [2] models for the description of internal and external geomagnetic sources, respectively. Solar wind and IMF parameters are obtained from the high-resolution Omniweb database. Each trajectory is back propagated from the measurement location with no constraint limiting the total path-length or tracing time, and the corresponding asymptotic arrival direction is evaluated with respect to the IMF direction.Since the PAMELA aperture is 20 deg, the observable pitch-angle range is quite small (a few deg) except in regions close to the geomagnetic cutoff (discarded from the analysis). However, because it is a moving platform, it sweeps through pitch angle space allowing one to construct a pitch angle distribution of the SEPs.Consequently, a quite large pitch-angle range is covered during the whole polar pass. Fig.1 reports PAMELA's vertical asymptotic directions of view (0.39-2.5 GV) during the first polar pass (0158 - 0220 UT) that registered the May 17, 2012 event [3], for different values of particle rigidity (color code). The spacecraft position is indicated by the grey curve. The contour curves represent values of constant pitch angle with respect to the IMF direction, denoted with crosses. In this case the IMF direction is almost perpendicular to the sunward direction. As PAMELA is moving (eastward) and changing its orientation along the orbit, observed asymptotic directions rapidly vary performing a (clockwise) loop over the region above Brazil. PAMELA data can be combined with data from NMs and other space-based detectors, in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy.
[1] A. Bruno et al. (2015), Proc. 34th Intl. Cosmic Ray Conf., PoS(ICRC2015)085.
[2] N. A. TsyganenkoM. I. Sitnov( 2007), J. Geophys. Res., 112, A06225.
[3] O. Adrian et al. (2015), ApJ 801 L3.