September 19, 2013.
Approximately 15 times per 11-year solar cycle, the Sun emits cosmic rays with sufficient energy and intensity to raise radiation levels on Earth’s surface markedly above background levels due to Galactic cosmic rays. These events, termed “ground level enhancements” (GLEs), provide an exceptionally clear picture of particle acceleration on the Sun, first because GLE particles travel near the speed of light and thus enable a precise linkage between the particles and the solar source event, and second because they have large mean free paths, and their time profiles are comparatively undistorted by transport processes in the interplanetary medium. Information gained from the observation and analysis of GLEs is clearly pertinent to the field of heliophysics and is also interesting for traditional astrophysics for the challenge it poses to acceleration models (e.g., Roussev et al. 2004).
GLEs are also of practical interest owing to their significance for space weather forecasting and specification. Solar cosmic rays can damage sensitive electronic components aboard spacecraft, and they pose a major radiation hazard to astronauts (Hu et al. 2009). Because GLE particles travel at nearly the speed of light, they provide the earliest indication of an impending radiation storm in some events (Kuwabara et al. 2006). For radiation exposure to air crews and aircraft electronics, GLEs are the only events of relevance, because the lower energy particles that are a major concern in space do not penetrate to aircraft altitudes (Wilson et al. 2003; Lantos 2006). Air routes through the north polar region have multiplied in recent years, because these routes are the most cost-efficient for flights from North America to the Far East (Hanson & Jensen 2002). Radiation hazard is greatest along these routes, because Earth’s magnetic field provides little or no protection in polar regions. Fortunately, numerous cosmic ray monitoring stations (neutron monitors) in Canada, Russia, and Greenland are well situated to provide alerts and to monitor radiation hazard on polar airline routes.
This research (Bieber et al. 2013) reports neutron monitor observations of the giant GLE of 2005 January 20, an event that was the most intense observed in nearly half a century and the second largest observed since systematic observations began in the 1930s. We analyze neutron monitor data from 11 stations of the Spaceship Earth neutron monitor array (Bieber & Evenson 1995) supplemented by two additional stations. A companion article (A. Sáiz et al. 2013, in preparation) will report how we perform theoretical modeling of the event, determine the interplanetary scattering conditions encountered by the GLE particles, and derive the particle injection function onto the Sun–Earth magnetic field line.
Figure 1. Percentage increases of relativistic solar ions above the Galactic cosmic ray (GCR) background for the giant GLE of 2005 January 20 at six polar, low-altitude neutron monitors. The detected neutrons are secondary cosmic rays generated by nuclear cascades in Earth’s atmosphere. The relativistic primary cosmic rays that initiate the cascades are predominantly protons above 500 MeV. Because the neutron monitors view different directions in the sky, the major difference between the traces indicates an initially strong anisotropy in relativistic solar protons.
Figure 2. Geographic locations and asymptotic viewing directions of the 13 polar neutron monitors considered in this work at 06:53 UT, the time of peak count rate of the giant GLE of 2005 January 20 (a) in geographic coordinates and (b) in geocentric solar ecliptic (GSE) coordinates. Taking into account the bending of particle trajectories in Earth’s magnetic field, each neutron monitor measures the relativistic ion flux from specific asymptotic viewing directions, shown for the median rigidity of 2.14 GV (squares) and for the central 80% of the detector response, 1.05–5.33 GV (lines). The combined data from these stations indicate the directional distribution of solar particles from space as a function of time.
Figure 3. (a) One-minute data of percent increases recorded at the South Pole by a standard neutron monitor and a Polar Bare neutron counter that lacks the usual lead shielding. (b) The ratio of percent increases at 1 minute resolution (solid circles) provides an indication of the spectral index γ for a momentum flux p- γ. When solar particles start to arrive at Earth, γ starts low because more energetic particles arrive first, and then γ increases toward its steady-state value. We have found a second feature of low γ at about 06:55 UT, which we attribute to a second injection of particles from the Sun.
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