Latitude survey investigation of galactic cosmic ray solar modulation during 1994-2007

• Main Authors: Nuntiyakul, W, Evenson, P, Ruffalo, D, Saiz, A, Bieber, JW, Clem, J, Pyle, R, Duldig, ML, Humble, JE
• Format: Conference Object
• Language: English
• Published: Scuola Internazionale Superiore di Studi Avanzati (S I S S A),International School of Advanced Studies 2015
• Subjects: Physical Sciences; Astronomical and Space Sciences; High Energy Astrophysics; Cosmic Rays; Antarctic; Antarc*; Australian Antarctic Division
• Online Access: http://ecite.utas.edu.au/110302

The Galactic cosmic ray spectrum exhibits subtle variations over the 22 yr solar magnetic cyclein addition to the more dramatic variations over the 11 yr sunspot cycle. Neutron monitors arelarge ground-based detectors that provide accurate measurements of variations in the cosmic rayflux at the top of the atmosphere above the detector. At any given location the magnetic field ofthe Earth excludes particles below a well-defined rigidity (momentum per unit charge) known asthe cutoff rigidity, which can be accurately calculated using detailed models of the geomagneticfield. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself servesas a magnet spectrometer. By repeating such latitude surveys with identical equipment a sensitivemeasurement of changes in the spectrum can be made. In this work, we analyze data from the1994 through 2007 series of latitude surveys conducted by the Bartol Research Institute, the Universityof Tasmania, and the Australian Antarctic Division. We confirm the curious crossover"in spectra measured near solar minima during epochs of opposite solar magnetic polarity, andshow that it is related directly to a sudden change in the spectral behavior of solar modulation atthe time of the polarity reversal, as revealed from contemporaneous variations in the survey dataand a fixed station. We suggest that the spectral change and crossover result from the interactionof effects due to gradient/curvature drifts with a systematic change in the interplanetary diffusioncoefficient caused by turbulent magnetic helicity.