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Ground Level Enhancement of May 17, 2012 Observed at South Pole

Ground Level Enhancement of May 17, 2012 Observed at South Pole. Paul Evenson University of Delaware. OBSERVATION OF COSMIC RAYS WITH GROUND-BASED DETECTORS.

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Ground Level Enhancement of May 17, 2012 Observed at South Pole

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  1. Ground Level Enhancement of May 17, 2012 Observed at South Pole Paul Evenson University of Delaware February 7, 2010

  2. OBSERVATION OF COSMIC RAYSWITH GROUND-BASED DETECTORS • Ground-based detectors measure byproducts of the interaction of primary cosmic rays (predominantly protons and helium nuclei) with Earth’s atmosphere • Two common types: • Neutron Monitor Typical energy of primary: ~1 GeV for solar cosmic rays, ~10 GeV for Galactic cosmic rays • Muon Detector / Hodoscope Typical energy of primary: ~50 GeV for Galactic cosmic rays (surface muon detector) February 7, 2010

  3. NEUTRON MONITORS • Older type “BP28” – proportional counter filled with BF3: n + 10B → α + 7Li • Modern type – counter filled with 3He: n + 3He → p + 3H • Both types are called “NM64” Neutron Monitor in Nain, Labrador Construction completed November 2000 February 7, 2010

  4. Spaceship Earth Spaceship Earth is a network of neutron monitors strategically deployed to provide precise, real-time, three-dimensional measurements of the angular distribution of solar cosmic rays: Twelve Neutron Monitors on four continents Multi-national participation: Bartol Research Institute, University of Delaware (U.S.A.) IZMIRAN (Russia) Polar Geophysical Inst. (Russia) Inst. Solar-Terrestrial Physics (Russia) Inst. Cosmophysical Research and Aeronomy (Russia) Inst. Cosmophysical Research and Radio Wave Propagation (Russia) Australian Antarctic Division Aurora College (Canada) February 7, 2010

  5. The Role of Pole • Pole “looks” near the equator, like most Spaceship Earth Stations • High altitude permits measurement of particle spectra February 7, 2010

  6. May 17 Event Overview – Neutron Monitor Network February 7, 2010

  7. ENERGY SPECTRUM: POLAR BARE METHOD • South Pole station has both an NM64 and tubes lacking the lead shielding. “Polar Bares” responds to lower particle energy on average. • Bare to NM64 ratio provides information on the particle spectrum. • Beautiful dispersive onset appears as the faster particles arrive first. • Spectrum softens to ~P – 5 (where P is rigidity), which is fairly typical for GLE. • Dip around 06:55 UT may be related to the change in propagation conditions indicated by our transport model February 7, 2010

  8. Photo: James Roth, Dec 8, 2007 IceTop Air Shower Array Two ice blocks per station High and low-gain DOM for dynamic range Waveforms give some m/e discrimination

  9. Why IceTop Works as a GeV Particle Spectrometer • The IceTop detectors are thick (90 g/cm2) so the Cherenkov light output is a function of both the species and energy of incoming particles • Individual waveform recording, and extensive onboard processing, allow the return of pulse height spectra with 10 second time resolution even at the kilohertz counting rate inherent to the detector February 7, 2010

  10. Secondary Particle Spectra • At the South Pole, spectra of secondary particles “remember” a lot of information about the primary spectrum. February 7, 2010

  11. Particle Response Functions (Arbitrary Normalization) • IceTop particle response functions change with counting threshold February 7, 2010

  12. Particle Response Functions (Arbitrary Normalization) • Cherenkov and neutron monitor response functions have qualitative differences February 7, 2010

  13. Element Composition and Spectrum are Separated with Combined IceTop and Neutron Monitor Analysis • Simulated loci of constant value of the indicated ratio, varying spectral index (horizontal) and helium fraction (vertical). Statistical errors (+/- one sigma) are shown by thickened lines. • 20 January 2005 spectrum and galactic composition are assumed • IceTop alone does not resolve composition and spectrum • Adding the information from a Neutron Moderated Detector and a standard NM64 neutron monitor differentiates composition and spectrum February 7, 2010

  14. May 17 Event Overview –South Pole Detectors February 7, 2010

  15. Ice Top Detail of “Pulse” February 7, 2010

  16. Preliminary (Snow Corrected) Analysis Summary “Power law in rigidity” spectrum qualitatively reproduces IceTop data but is inconsistent with other observations February 7, 2010

  17. Conclusions • IceTop is a valuable addition to the ground based network of solar particle observatories • Better understanding of yield functions is needed • Indications are that analysis in terms of simple power law spectra is insufficient. February 7, 2010

  18. Supplementary Slides February 7, 2010

  19. Why are all the stations at high latitude?Reason 1: Uniform energy response • Plot shows neutron monitor response to a simulated (rigidity)-5 solar particle spectrum • Below a geomagnetic cutoff of about 0.6 GV, atmospheric absorption determines the cutoff • All stations have a uniform energy response in this regime February 7, 2010

  20. Why are all the stations at high latitude?Reason 2: Excellent directional sensitivity • Trajectories are shown for vertically incident primaries • Steps correspond to the 10-, 20-, … 90-percentile rigidities of a typical solar spectrum February 7, 2010

  21. Why are all the stations at high latitude?Reason 3: Focusing of obliquely incident primaries • Particles are focused by the converging polar magnetic field • Primaries with widely divergent angles of incidence have similar asymptotic directions • Calculations are made by following time-reversed trajectories February 7, 2010

  22. Snow height dependence ofbase count rate, MPE discriminator

  23. Snow height dependence ofbase count rate, SPE and MPE

  24. Assume base count rate at different threshold exponentially decrease by snow height Then determine parameter one A, and Cth for each threshold, and get corrected rate from

  25. Snow height dependence ofincrease count rate at pulse enhance Thinking how correct snow effect of this…

  26. Snow height dependence ofincrease count rate at broad enhance Thinking how correct snow effect of this…

  27. Before and after snow correction (pulse)for now I just correct base count…

  28. Before and after snow correction (broad)

  29. With fitted curve

  30. GLE spectrum at pulse enhance IceTop 80% energy is from 0.172GeV to 0.946 GeV

  31. GLE spectrum at broad enhance

  32. Band function fitting Use A and alpha from NM spectrum A=1.1e-3, alpha=-4.3 Other parameters are determined from fitting beta=-11. E0=6.4

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