Abstract
Abstract. During the August–September 2005 burst of solar activity, close to the current solar cycle minimum, a significant number of powerful X-ray flares were recorded, among which was the outstanding X17.0 flare of 7 September 2005. Within a relatively short period (from 22 August to 17 September) two severe magnetic storms were also recorded as well as several Forbush effects. These events are studied in this work, using hourly mean variations of cosmic ray density and anisotropy, derived from data of the neutron monitor network. During these Forbush effects the behavior of high energy cosmic ray characteristics (density and anisotropy) is analyzed together with interplanetary disturbances and their solar sources, and is compared to the variations observed in geomagnetic activity. A big and long lasting (~6 h) cosmic ray pre-decrease (~2%) is defined before the shock arrival on 15 September 2005. The calculated cosmic ray gradients for September 2005 are also discussed.
Highlights
The descending phase of solar cycle 23 evolved into a succession of bright manifestations of solar activity (e.g. Mavromichalaki et al, 2005; Belov et al, 2005; Papaioannou et al, 2009; Eroshenko et al, 2004)
Each disturbance modulated the galactic cosmic rays (GCR) creating a series of significant Forbush effect (FE) starting from 24 August
The characteristic changes in a behavior of CR vector anisotropy may be used for an advance warning before the arrival of the interplanetary shock
Summary
The descending phase of solar cycle 23 evolved into a succession of bright manifestations of solar activity (e.g. Mavromichalaki et al, 2005; Belov et al, 2005; Papaioannou et al, 2009; Eroshenko et al, 2004). In less than one month (from 22 August to 17 September) 30 M-class and 11 X-class solar X-ray flares (SF) were recorded Out of these 41 events, the largest one was the X17.0 flare, occurring on 7 September. The proton fluxes from such remote eastern flares are not observable near Earth (Belov, 2008; Eroshenko et al, 2004). The proton flux for >10 MeV particles increased up to 1000 pfu on 10 September (http://spidr.ngdc.noaa.gov; http://www.sec.noaa.gov). Such great proton fluxes originating from eastern flares (even when considering all flares with >45◦ E longitudes) have never before been observed near Earth. An unusually effective acceleration of solar particles may apparently be assumed in this case, probably including particles with energies >1 GeV, even though such high energies did not reach Earth, because of the remote eastern source location
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