Latitude Survey Investigation of Galactic Cosmic Ray Solar Modulation During 1994-2007

Abstract

The Galactic cosmic ray spectrum exhibits subtle variations over the 22 yr solar magnetic cycle in addition to the more dramatic variations over the 11 yr sunspot cycle. Neutron monitors are large ground-based detectors that provide accurate measurements of variations in the cosmic ray flux at the top of the atmosphere above the detector. At any given location the magnetic field of the Earth excludes particles below a well-defined rigidity (momentum per unit charge) known as the cutoff rigidity, which can be accurately calculated using detailed models of the geomagnetic field. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself serves as a magnet spectrometer. By repeating such latitude surveys with identical equipment, a sensitive measurement of changes in the spectrum can be made. In this work, we analyze data from the 1994 through 2007 series of latitude surveys conducted by the Bartol Research Institute, the University of Tasmania, and the Australian Antarctic Division. We confirm the curious "crossover" in spectra measured near solar minima during epochs of opposite solar magnetic polarity, and show that it is directly related to a sudden change in the spectral behavior of solar modulation at the time of the polarity reversal, as revealed from contemporaneous variations in the survey data and a fixed station. We suggest that the spectral change and crossover result from the interaction of effects due to gradient/curvature drifts with a systematic change in the interplanetary diffusion coefficient caused by turbulent magnetic helicity.

Cosmic Ray Modulation Observed by the Princess Sirindhorn Neutron Monitor at High Rigidity Cutoff

“…Note that the period of high tilt angle during the previous solar cycle (during 2000) was much shorter than during the present solar cycle (during 2010-2014). We also confirm the results of [19,21] that the slope of the linear relationship between NMs at different cutoffs was higher during the time of negative solar magnetic polarity (Figure 3(b)). This Figure also shows that the slopes were very similar during the periods of rising and falling GCR flux at Doi Inthanon during the qA < 0 period.…”

“…Note that these time periods correspond to relatively lower tilt angles, roughly speaking when the tilt angle was below about 50 • . Such a linear relationship was also found for a ship-borne NM and the McMurdo NM during 1994-2007 [19], with the exception of the year 2000, which again was when the tilt angle was high. Note that the period of high tilt angle during the previous solar cycle (during 2000) was much shorter than during the present solar cycle (during 2010-2014).…”

“…During the period of low solar activity, the modulation at high and low cutoffs are linearly related as found by [19]. The slope of the linear relationship depends mainly on the solar polarity.…”

“…This Figure also shows that the slopes were very similar during the periods of rising and falling GCR flux at Doi Inthanon during the qA < 0 period. at such times the two stations' count rates have a linear relationship, and the slope depends on solar magnetic polarity as found by [19,21]. For higher tilt angle, from mid-2010 to late 2014, there is no longer a linear relationship, as DI has a distinct pattern of solar modulation from that observed at low cutoff.…”

An Unusual Pattern of Cosmic-Ray Modulation During Solar Cycles 23 and 24

“…For example, from comparing the HCS tilt angles with the NM counts of one ground-based station (Newark), Cliver, Richardson, and Ling (2013) first suggested (already in 2011) that the drift of charged GCR was not a dominant factor in the minimum of cycle 24, contrary to expectations. Recent studies found an association between the 2009 anomalous peak and higher values of the diffusion coefficient (e.g., Alania, Modzelewska, and Wawrzynczak, 2014;Nuntiyakul et al, 2014). Other studies also found evidence of the remission of the drift process in the solar modulation during the last minimum and suggested that changes in the solar wind turbulence might produce a diffusion-dominated solar modulation period (e.g., Starodubtsev and Grigoryev, 2011;Potgieter et al, 2014).…”