Solar Energetic Particle Spectrum on 13 December 2006 Determined by IceTop
The IceTop air shower array now under construction at the South Pole as the surface component of the IceCube neutrino telescope (Achterberg et al. 2006) detected an unusual near-solar-minimum Ground Level Enhancement (GLE) after a solar flare on 13 December 2006. Beginning at 0220 UT, the 4B class flare occurred at solar coordinates S06 W24, accompanied by strong (X3.4) X-ray emission and type II and IV radio bursts. The LASCO coronagraph on the SOHO spacecraft observed a halo CME launch from the Sun at {approx} 0225 UT with speed estimated to be {approx} 1770 km/s. We have begun (Bieber et al. 2007) a comprehensive analysis of the propagation of solar energetic particles in this event. However the focus of this Letter is the new and unique ability of IceTop to derive the energy spectrum of these particles in the multi-GeV regime from a single detector with a well defined viewing direction. When completed, IceTop will have approximately 500 square meters of ice Cherenkov collecting area arranged in an array of 80 stations on a 125 m triangular grid to detect air showers from one PeV to one EeV. Each station consists of two, two meter diameter tanks filled with ice to a depth of 90 cm. Tanks are instrumented with two Digital Optical Modules (DOM) operated at different gain settings to provide appropriate dynamic range to cover both large and small air showers. Each DOM contains a 10 inch photomultiplier and an advanced readout system capable of digitizing the full waveform. For historical reasons, the two discriminator counting rates recorded in each DOM are termed SPE (Single Photo Electron), and MPE (Multi Photo Electron). In the present analysis the SPE threshold corresponds approximately to 20 photoelectrons (PE), and the MPE threshold to 100 PE. Due to the high altitude (2835m) and the nearly zero geomagnetic cutoff at the South Pole, secondary particle spectra at the detector retain a significant amount of information on the spectra of the primary particles. In a thin, ionization detector these secondary particles either would not interact, or would produce virtually indistinguishable signals. This is not the case in the thick Ice-Top detector, where a traversing muon produces 130 PE and the typical electron only 15 PE. Signal amplitude therefore carries information about the composition and spectra of the incident particles, albeit integrated over broad regions of the spectrum. In particular, differences in counting rates of discriminators at different thresholds allow us to infer the particle spectrum incident at the top of the atmosphere.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Nuclear Science Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 962940
- Report Number(s):
- LBNL-2087E; PRLTAO; TRN: US0902976
- Journal Information:
- Physical Review Letter, Journal Name: Physical Review Letter; ISSN 0031-9007
- Country of Publication:
- United States
- Language:
- English
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