The longitudinal asymmetry of the interplanetary perturbation producing Forbush decreases

Abstract

It is well known that type-IV solar flares occurring between 0° and 40° East of the central meridian can produce the largest Forbush decreases observed at the Earth,i.e. inside the Forbush decrease modulated region the maximum cosmic-ray modulation is observed in the West (∼20° W) of the flare’s meridian plane. A flare-produced perturbation asymmetric in longitude is required in order to explain this asymmetry. The analysis of the two-step Forbush decreases shows that the first step of the decrease is symmetric with respect to the flare’s meridian plane and it is due to the shock wave which is observed ahead of the interplanetary perturbation; on the contrary the second step is strongly asymmetric, the maximum amplitude being observed at ∼30° W of the flare’s meridian plane, and it seems to be due to the magnetic perturbation following the shock; this perturbation is an irregular and compressed magnetic field, the maximum magnetic-field magnitude is found to be placed West of the flare’s meridian plane. The total Forbush decrease amplitude is correlated (correlation coefficient 0.96) with a perturbation parameter which accounts for the «strength» of the shock and of the following magnetic perturbation. The first step of the decrease is correlated (correlation coefficient 0.88) with the shock «strength».