CAU Kiel April 20, 2010

CAU KielApril 20, 2010 Solar Physics with the IceTop Air Shower Array Paul Evenson University of Delaware Department of Physics and Astronomy

The First Extraterrestrial Event Detected by IceCube IceTop and Spaceship Earth Observations of the Solar Flare Dec 13, 2006 X3-Class Solar Flare (SOHO) Dec 14, 2006 photograph of auroras near Madison, WI

0.9 m Diffusely reflecting liner 2 m IceTop Detectors • Blocks of clear ice produced in tanks at the Pole • Cherenkov radiation measured by standard IceCube photon detectors • DESY Zeuthen is one production site for these “Digital Optical Modules” or DOM • Two tanks separated by 10 meters form a station

Large showers with E ~ 100-1000 PeV will clarify transition from galactic to extra-galactic cosmic rays. Showers triggering 4 stations give ~300 TeV threshold for EAS array Small showers (2-10 TeV) associated with the dominant muon background in the deep detector are detected as 2-tank coincidences at a station.

Low Energy (1 GeV) “Showers” • Particles with energy as low as 1 GeV produce secondaries that survive to the surface • Rarely does a single detector see more than one secondary from a primary • Large detectors can have high enough counting rates to make statistically significant measurements of the primary flux • Conventional detectors count muons or neutrons • Energy spectra are determined from detectors located at different geomagnetic cutoffs • Flux anisotropy is a source of error in the spectral measurement

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

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

Particle Response Functions (Arbitrary Normalization) • IceTop tank particle response functions change with selection of the threshold • We are now working with FLUKA calculated response functions

IceTop Event Overview • A lot of this structure is due to pressure variations • Much is due to cosmic ray variability • The flare event and the “Forbush Decrease” at the end of day 347 are clear • The blast of plasma that produces the decrease is what triggers the anomalous auroral activity

Solar Particle Spectrum Determination (I) • Excess count rate (averaged over approximately one hour near the peak of the event) as a function of pre-event counting rate. • Each point represents one discriminator in one DOM. • By using the response function for each DOM we fit a power law (in momentum) to the data • The lines show this fit and the one sigma (systematic) errors

Solar Particle Spectrum Determination (II) • IceTop proton spectrum (heavy blue line with one sigmaerror band). • Black line is the assumed background cosmic-ray proton spectrum • Points are maximum proton fluxes from GOES spacecraft data.

Putting the IceTop Observation in Context Spaceship Earth Neutron Monitor Array Spaceship Earth is a network of neutron monitors strategically deployed to provide precise, real-time, 3-dimensional measurements of the angular distribution of solar cosmic rays: • 12 Neutron Monitors on 4 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)

Cosmic Ray Detectors at High Latitude Trajectories are shown for vertically incident primaries corresponding to the 10-, 20-, … 90-percentile rigidities of a typical solar spectrum

Circles denote station geographical locations. Average asymptotic direction (squares) and range (lines) are separated from station geographical locations. Station Location is Carefully Chosen STATION CODES IN: Inuvik, Canada FS: Fort Smith, Canada PE: Peawanuck, Canada NA: Nain, Canada BA: Barentsburg, Norway MA: Mawson, Antarctica AP: Apatity, Russia NO: Norilsk, Russia TB: Tixie Bay, Russia CS: Cape Schmidt, Russia TH: Thule, Greenland MC: McMurdo, Antarctica

Determination of the Pitch Angle Distribution Individual station data fitted to an angular distribution of the form f(μ) = c0 + c1 exp(b μ), with μ cosine of pitch angle, and c0, c1, and b free parameters. The symmetry axis from which pitch angles are measured was also a free parameter.

13 December 2006 Event Animation

Neutron Monitor Response Calculated from IceTop Spectrum • Good agreement (with understanding of viewing direction) • Continuous determination of precise spectrum • All information on anisotropy comes from the monitor network

IceTop and PAMELA Credit: M. Casolino

Towards Precision Spectral Information • We are reconfiguring IceTop to provide uniform coverage from 500 to 10,000 Hz • Up to 2000 Hz each DOM will generate a rate histogram • Above 2000 Hz the SPE discriminators will be used, set to a range of thresholds • For larger events this will enable us to go far beyond the simple power spectrum analysis • Exact spectral shape is diagnostic of particle acceleration mechanisms.

Just Completed: • Assemble an IceTop tank in a freezer container in Uppsala • Fill with water and freeze • Load on Oden • Take data on 2009-10 voyage to McMurdo and back

Example of a Latitude Survey • Left: Course plot with geomagnetic cutoff contours in units of GV • Right: Counting rate of two detectors and geomagnetic cutoff as a function of time

Determining a Response Function • Counting rate of a 3NM64 is plotted against cutoff. • To deal with the scatter of the points we fit a “Dorman Function” as indicated. • This function has an analytic derivative as shown. This derivative is exactly the response function for this particular detector. • In the collaboration involving Uppsala we will derive a whole sequence of response functions for different thresholds, but each will be determined by a method similar to this

Preparing the Ice Block

(Neutron Monitor Latitude Surveys On Polar Star And Polar Sea)

Neutrons on Oden Voyage

Summary • IceTop, the surface component of the IceCube Neutrino Observatory at the South Pole, is an air shower detector aimed primarily at studying PeV and above cosmic rays. • In this mode the 160 (planned) detectors are operated in coincidence. • Individual detectors are sensitive to the secondary products of particles with energy as low as one GeV, typical of energetic solar particle events. • Information on the spectrum of particles at a few GeV can be extracted from IceTop, • We are working to get a better calibration of these detectors as part of a collaboration including Uppsala University.

CAU Kiel April 20, 2010