Loss Cone Precursors to Forbush Decreases and Advance Warning of Space Weather Effects

Abstract

Ground-based observations of cosmic rays by neutron monitors and muon detectors have found precursor anisotropies before the arrival of an interplanetary shock and subsequent Forbush decrease, possibly providing advance warning of space weather effects on shock impact at the Earth's magnetosphere. Surprisingly, muon detectors observe precursors with a greater lead time than neutron monitors. Here, we explain both loss cone and shock reflection precursors in a common mathematical framework and perform time-dependent numerical simulations of cosmic-ray transport near an oblique, planar shock. We examine parameters of loss cone precursors as a function of the shock-magnetic field angle and q, the spectral index of magnetic turbulence. More energetic particles correspond to a lower value of q and a higher value of λ, the interplanetary scattering mean free path. We conclude that loss cones should typically be detectable 4 hr prior to shock arrival at neutron monitor energies (~10 GeV) and 15 hr prior to shock arrival at muon detector energies (~30 GeV). In addition, the angular width of the loss cone provides a potential method of forecasting the shock-field angle, as the predicted width is substantially larger for quasi-parallel shocks than for quasi-perpendicular shocks, leading to a better indication of the shock arrival time.