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\rdetectors have found precursor anisotropies before the arrival of an\rinterplanetary shock and subsequent Forbush decrease, possibly providing\radvance warning of space weather effects on shock impact at the Earth's\rmagnetosphere. Surprisingly, muon detectors observe precursors with a\rgreater lead time than neutron monitors. Here, we explain both loss cone\rand shock reflection precursors in a common mathematical framework and\rperform time-dependent numerical simulations of cosmic-ray transport\rnear an oblique, planar shock. We examine parameters of loss cone\rprecursors as a function of the shock-magnetic field angle and q, the\rspectral index of magnetic turbulence. More energetic particles\rcorrespond to a lower value of q and a higher value of λ, the\rinterplanetary scattering mean free path. We conclude that loss cones\rshould typically be detectable 4 hr prior to shock arrival at neutron\rmonitor energies (~10 GeV) and 15 hr prior to shock arrival at muon\rdetector energies (~30 GeV). In addition, the angular width of the loss\rcone provides a potential method of forecasting the shock-field angle,\ras the predicted width is substantially larger for quasi-parallel shocks\rthan for quasi-perpendicular shocks, leading to a better indication of\rthe shock arrival time.