Two classes of cosmic ray decrease

Abstract

From an analysis of the time variations during 1968–1971 of the fast neutron flux in the upper atmosphere (mean energy of response to primaries, 1–2 GeV per nucleon) versus those of ground‐based neutron monitors we have identified two classes of transient intensity decrease on the basis of differences in their spectral responses, time histories, and flare associations. Type I events are found to be classic Forbush decreases, sharp declines accompanying a geomagnetic storm sudden commencement, following by 1–3 days a large optical flare with radio noise and energetic particle production, whereas type II events are more symmetric in their time histories and are therefore not associated with a particular flare. There are also differences in the spectral responses of the two types. During a type I decrease the flux change of the lower‐rigidity cosmic ray particles lags the flux change of the high‐rigidity particles both in the decline and in the recovery, tracing out a hysteresis loop. During a type II event, if there is any hysteresis at all, the lower‐rigidity primaries tend to ‘overrecover’ in comparison to the higher‐rigidity primaries. Intercomparison of neutron monitor data for median response rigidities from 10 to 30 GV reveals that the spectral response in type II events is softer on the average for low‐rigidity (< 10 GV) primaries and harder for high‐rigidity (> 10 GV) primaries than that in type I events. Comparison of intermixed sequences of type I and II events with recurrences of active regions reveals an identifiable but complex evolutionary relation between decrease occurrence and active region development. Long‐lived (of the order of days) low‐energy (<1 MeV) proton events occur during all but one of the type II events identified, supporting an association with solar active region transits. We interpret type II events as either a subsequent evolution of type I (Forbush decrease) events or a quasi‐stationary ‘corotating’ spatial structure loosely associated with an active region. Therefore both type I and type II decreases occur in intermixed recurrence series at intervals of 20–30 days.

Hysteresis of Primary Cosmic Rays Associated with Forbush Decreases

“…High rigidity particles are removed faster in the early phase of the decrease and they recover faster. A similar observation has been reported by Verschell et al (1973) for type I Forbush decreases, but the effect reported in the present paper is much more general.…”

“…in April 1967. The plot of the equilibrium intensities, mentioned in point (3), indicates a constant spectral index except during May-September 1965 (Verschell et al 1973).…”

Corotating high‐speed solar‐wind streams and recurrent cosmic ray modulation

“…Recurrent cosmic ray modulation due to CIRs and high‐speed streams have been studied using ground‐based neutron monitors and spacecraft data [e.g., Vershell et al , 1975; Parker , 1976; Shah et al , 1978; Iucci et al , 1979; Duggal et al , 1981; Venkatesan et al , 1982; Tiwari et al , 1983; Burlaga et al , 1984; Mishra et al , 1990; Yadav et al , 1994; Shrivastava and Shukla , 1994; Kunow et al , 1995; Richardson et al , 1996, 1999; Badruddin , 1997; Zhang , 1997; Paizis et al , 1999; Alania et al , 2001; Reames and Ng , 2001; Gil et al , 2005, Gupta and Badruddin , 2005; Singh and Badruddin , 2005; Venkatesan and Badruddin , 1990; Simpson , 1998; McKibben et al , 1999; Richardson , 2004]. However, such depressions in cosmic ray intensity have been associated with enhanced convection by high‐speed solar wind [ Newkirk and Fisk , 1985; Iucci et al , 1979; Richardson et al , 1996], diffusion in enhanced/compressed field region [ Burlaga et al , 1984; Kota and Jokipii , 1991], and/or particle drifts in large‐scale heliospheric magnetic field [ Kota and Jokipii , 1991, 2001; Burger and Hitge , 2004].…”

Enhanced isotropic cosmic ray intensity waves during cosmic ray storms

“…On the other hand, although no definitive physical explanation has yet been proposed, many workers have demonstrated that the CMP of active regions are associated with cosmic ray modulations [Simpson et al, 1952;Meyer and Simpson, 1954;Venkatesan, 1959;Antonucci et al, 1971;Ballif and Jones, 1969;Verschell et al, 1975;Pomerantz and Duggal, 1975]. Figure 6 reveals that a large number of active regions dominated the sun during the period covered by the waves.…”