Browsing by Autor "John Clem"

Now showing 1 - 2 of 2

GIANT GROUND LEVEL ENHANCEMENT OF RELATIVISTIC SOLAR PROTONS ON 2005 JANUARY 20. I. SPACESHIP EARTH OBSERVATIONS
(IOP Publishing, 2013) J. W. Bieber; John Clem; P. A. Evenson; R. Pyle; A. Sáiz; D. Ruffolo
A ground level enhancement (GLE) is a solar event that accelerates ions (mostly protons) to GeV range energies in such great numbers that ground-based detectors, such as neutron monitors, observe their showers in Earth's atmosphere above the Galactic cosmic ray background. GLEs are of practical interest because an enhanced relativistic ion flux poses a hazard to astronauts, air crews, and aircraft electronics, and provides the earliest direct indication of an impending space radiation storm. The giant GLE of 2005 January 20 was the second largest on record (and largest since 1956), with up to 4200% count rate enhancement at sea level. We analyzed data from the Spaceship Earth network, supplemented to comprise 13 polar neutron monitor stations with distinct asymptotic viewing directions and Polar Bare neutron counters at South Pole, to determine the time evolution of the relativistic proton density, energy spectrum, and three-dimensional directional distribution. We identify two energy-dispersive peaks, indicating two solar injections. The relativistic solar protons were initially strongly beamed, with a peak maximum-to-minimum anisotropy ratio over 1000:1. The directional distribution is characterized by an axis of symmetry, determined independently for each minute of data, whose angle from the magnetic field slowly varied from about 60° to low values and then rose to about 90°. The extremely high relativistic proton flux from certain directions allowed 10 s tracking of count rates, revealing fluctuations of period ≳ 2 minutes with up to 50% fractional changes, which we attribute to fluctuations in the axis of symmetry.
LATITUDE SURVEY INVESTIGATION OF GALACTIC COSMIC RAY SOLAR MODULATION DURING 1994-2007
(IOP Publishing, 2014) Waraporn Nuntiyakul; P. A. Evenson; D. Ruffolo; A. Sáiz; J. W. Bieber; John Clem; R. Pyle; M. L. Duldig; J. E. Humble
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.