The Instantaneous Response of Geomagnetic Field, near-Earth IMF and Cosmic-Ray Intensity to Solar Flares

Abstract

We show using superposed epoch analysis (SEA) that the most energetic protons (greater than 60 MeV) in near-Earth IMF have a peak almost immediately (less than a day) after peak in solar flare index (SFI), while protons greater than 10 MeV peak one day after the SFI and protons greater than 1 MeV two days after the SFI. The geomagnetic indices AU, -AL, PC, Ap and -Dst peak after two to three days in SEAs after the peak in SFI. The auroral electrojet indices AU and -AL, however, have only low peaks. Especially, the response of the eastward electrojet, AU, to SFI is negligible compared to other geomagnetic indices. The SEAs of the SFI and cosmic ray counts (CR) show that the deepest decline in the CR intensity follows also with 2-3 day lag the maximum of the SFI for the Solar Cycles 20-24. The depth of the declines are related to the SFI strength of each cycle, i.e, the average decline is about 5% for the Cycles 21 and 22, but only 3% for the Cycle 24. The strongest Cycle 19, however, differs from the other cycles such that it has double-peaked decline and lasts longer than the decline of the other cycles. The double superposed epoch analyses show that the response of IMF Bv2, which is about two days, and CR to SFI are quite simultaneous, but sometimes Bv2 may peak somewhat earlier than the decline existing in CR.