Cosmic ray detector response to transient solar modulation: Forbush decreases

Abstract

[1] The observed modulation of galactic cosmic ray (GCR) intensity, on time and spatial scales, contains information regarding their transport in the heliosphere. One way to extract crucial information from the data is to study the rigidity dependence of modulation. A methodology is described to study the rigidity dependence of the short-term variations in galactic cosmic ray intensity Earth, in terms of the median rigidity of detector response (R m ) to cosmic ray spectrum. We define R m as the rigidity below which lies 50% of detector counting rate. For neutron monitors (NMs), it is easily calculated from the latitude survey obtained at sea level and higher elevations. We compute R m values for some NM sites; solar cycle dependence for them is shown to be small. Their practical utility is demonstrated in a study of transient cosmic ray solar modulation (Forbush decreases (FDs)), using data published in the literature. We plot the rigidity dependence of the amplitudes of three large Forbush decreases that occurred during the declining phase of three solar cycles (19, 20, and 21). We show that the Forbush decrease amplitude varies inversely with R m over a large range (1 to 300 GV) of GCR rigidities. Thereby, we rule out the existence of a ''transition rigidity'' suggested by Jokipii even for a short-term modulation (large Forbush decreases). We speculate that the force field model may be applicable to large Forbush decreases.

Mean energy of response to galactic cosmic ray spectrum: IMP 8 and Climax neutron monitor

“…So, over solar cycle 21 the mean energy of CL/NM changes by about 1.5 GeV ($ 16%). We obtained similar results for solar cycle variations of the median rigidity of response for CL/NM using coupling functions derived by Nagashima et al (1989); the reader may refer to Ahluwalia and Fikani (2007). The modulation function also undergoes very similar variations.…”

“…We also suspected that the median rigidity (energy) of response for the penetrating proton channel on IMP 8 is an underestimate (Ahluwalia et al, 2006). These impressions were reinforced in our recent study of the transient solar modulations (Forbush decreases) for three solar cycles (Ahluwalia and Fikani, 2007). In both studies we showed that the radial diffusion coefficient (k) varies inversely with the median rigidity of detector response (Rm) for positive (A > 0) as well as negative (A < 0) solar cycles.…”

“…Afterwards it reaches a high value of 5.2 GeV for BRN 2021 (6/6/1981) and starts to decrease, reaching a value of 3.9 GeV for BRN 2034 (5/23/1982). It increases sharply afterwards to about 4.8 GeV for BRN 2039BRN (10/5/1982; in between a large Forbush decrease (FD) occurred on 14 July 1982 (Ahluwalia and Fikani, 2007). Fillius and Axford (1985) note that the FD reduced GCR intensity to the lowest level for cycle 21, three years away from sunspot maximum, well past the epoch of solar polar field reversal in 1980 (Ahluwalia, 1994).…”

Modulation of galactic cosmic rays during the unusual solar minimum between cycles 23 and 24

“…Therefore, for different NMs we can compare the computed M(E eff ) with observational data of the NM count rates. Note that the effective energy is quite different from the median rigidity below which lies 50% of detector counting rate [Ahluwalia and Fikani, 2007], widely used for transient cosmic ray solar modulation studies [Ahluwalia et al, 2014]. Table 2 shows the local geomagnetic cutoff rigidity P c and the corresponding effective energy E eff of NMs used in our work.…”

Theoretical and Experimental Studies of the Rigidity Spectrum of the 27-Day Variation of the Galactic Cosmic Ray Intensity in Different Epochs of Solar Activity

“…This requirement is acceptably satisfied by Kiel and Rome neutron monitors which we use in our analysis; for Kiel neutron monitor R m = 2.29 GV and for Rome neutron monitor R m = 6.32 GV. Ahluwalia and Fikani (2007) demonstrated that the median rigidity of the response of a detector can be used as an alternative to the cut-off rigidity. The median rigidity for the Kiel neutron monitor is 17 GV and for Rome it is 23 GV.…”