SEP Temporal Fluctuations Related to Extreme Solar Flare Events Detected by SOHO/CELIAS/SEM

Abstract

The SOHO CELIAS/SEM measurements of the solar Extreme Ultraviolet (EUV) irradiance in the He II 30.4 nm first-order channel (26–34 nm) are highly sensitive to impacts of Solar Energetic Particles (SEP). A model of the SEM response to a quasi-isotropic SEP fluence allowed us to determine both the range of proton incident energies and the SEM sensitivity to the SEP flux, which has a maximum around 12 MeV. We propose to use high-cadence (15 s) SEM first-order count-rates to analyze the temporal fluctuations of the SEP flux arriving from extreme solar flare events. A comparison of these temporal fluctuations for the July 14, 2000, October 28, November 2, November 4, 2003, and January 20, 2005 events shows that the most intense high-frequency RMS variations of the first-order count-rates at the time when SEPs started to arrive, were associated with the January 20 event. These high-frequency variations are produced by packets of SEPs. Two (plus and minus) first-order SEM detectors with the distance between them of about 62 mm allow us to analyze the spatial coincidences and temporal distribution of SEP related signals at 1 AU. The largest temporal separation of the SEP packets is observed at the time when the packets start to arrive. RMS fluctuations (variances) for all analyzed events do not follow the photon noise ( √ N) in assumption of the Poisson or normal distribution but correlate (R = 0.999) with the speed at which the SEP flux grows. If the found correlation is confirmed on a larger statistical data base, it may allow the prediction of the SEP flux growth profile by analyzing the RMS amplitudes for the initial phase of the SEP impact.