1. Paul Evenson University of Delaware
GeV Cosmic and Solar Energetic Particle Observation from Ground Based Detectors
Paul Evenson
University of Delaware

2. Observation Of Cosmic Rays With 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)
NIPR: November 2016

3. New Type: Cherenkov “Tanks” Such as Auger, HAWC and IceTop
IceTop: Blocks of clear ice produced in tanks at the Pole
Cherenkov radiation measured by standard IceCube photon detectors
Two tanks separated by 10 meters form a station
2 m
0.9 m
Diffusely reflecting liner
NIPR: November 2016

4. Why IceTop Works as a GeV Particle Spectrometer
Neutron monitors are comparatively insensitive to the particle spectrum
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 ten second time resolution even at the kilohertz counting rate inherent to the detector
NIPR: November 2016

5. Secondary Particle Spectra
At the South Pole, spectra of secondary particles “remember” a lot of information about the primary spectrum.
NIPR: November 2016

6. Response Functions
Analog information from IceTop yields multiple response functions simultaneously
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7. Solar Particle Spectrum Published in Ap J Letters
Excess count rate 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 assuming that the composition is the same as galactic cosmic rays
The lines show one sigma (systematic) errors
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8. IceTop and PAMELA
Credit: M. Casolino
NIPR: November 2016

9. Neutron Monitors and IceTop
Good agreement (with understanding of viewing direction)
IceTop determines a precise spectrum
Anisotropy comes entirely from the monitor network
Here we see the failure of the “separability” assumption in one particular analysis of neutron monitor data alone
NIPR: November 2016

10. Neutron Monitors and IceTop
Cherenkov and neutron monitor response functions are similar but have significant differences
NIPR: November 2016

11. ENERGY SPECTRUM: POLAR BARE METHOD
South Pole station has a 3-NM64 and detectors with no lead shielding.
These “Polar Bares” responds to lower particle energy on average.
NIPR: November 2016

12. ENERGY SPECTRUM: POLAR BARE METHOD
“Polar Bares” responds to lower energy particles.
Bare to NM64 ratio provides information on the particle spectrum.
This event shows a dispersive onset 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
Element composition is a source of systematic error in the spectral index
NIPR: November 2016

13. Element Composition and Spectrum from IceTop and the Neutron Monitor
Simulated loci of count rate ratios, varying spectral index (horizontal) and helium fraction (vertical).
Statistical errors (+/- one sigma) are shown by line thickness.
20 January 2005 spectrum
“Galactic” composition
IceTop (black, blue) lines converge in the “interesting” region
Bare/NM64 (red) line crosses at the proper (i.e. simulation input) values
NIPR: November 2016

14. Spacecraft now approach the energy range of ground based observations (deNolfo 2015)
NIPR: November 2016

15. Role of IceTop
AMS-02 has approximately the same collecting power as the South Pole neutron monitor.
It has massively better energy and composition sensitivity, but only “looks” is one, constantly changing, direction at a time.
The duration of one orbit is much longer than the timescale of the evolution of anisotropy in a typical solar event.
The neutron monitor network will remain a vital partner for the life of AMS-02 (if we ever see another large GLE).
IceTop can work with the neutron monitor at the South Pole to understand event systematics.
Is the lack of GLE in the present solar cycle due to the overall size of the event, or is it a spectral effect?
The present study is intended to characterize the present solar cycle in terms of “detections”, with possible spectral analysis as a future topic.
NIPR: November 2016

16. Objective
Determine the extent to which multi – GeV particles are present in solar particle events. Specifically, can IceTop detect any events that are not seen as “classic” GLE (Ground Level Enhancements) Traditionally, the Pole Neutron Monitor qualifies as “ground level”.
NIPR: November 2016

17. A Really Clear Case 17 May 2012
Pink and Blue shading show “off source” and “on source” intervals selected for the event survey
Threshold
(Low to
High)
MPE
Low/High Ratio
Neutron Monitor
Bare to NM64 ratio
GOES
10, 50, 100 MeV
NIPR: November 2016

18. Methodology
Construct a list of SEP (Solar Energetic Particles) seen by GOES (Geostationary Operational Environmental Satellite) in the 100 MeV channel.
Develop a template based on known GLE
Apply the template to the list to get detections or limits
NIPR: November 2016

19. Status
Construct a list of SEP (Solar Energetic Particles) seen by GOES (Geostationary Operational Environmental Satellite) in the 100 MeV channel.
Done
Develop a template based on known GLE
The three known GLE are clearly visible
Apply the template to the list to get detections or limits
Preliminary results are available
NIPR: November 2016

20. GOES Event List – 10 and 100 MeV
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21. Another Case 6 Jan 2014
Pink and Blue shading show “off source” and “on source” intervals selected for the event survey
Threshold
(Low to
High)
MPE
Low/High Ratio
Neutron Monitor
Bare to NM64 ratio
GOES
10, 50, 100 MeV
NIPR: November 2016

22. 2014 Jan 6 Change vs Base
The SPE thresholds in IceTop are set to a range of thresholds.
The lower rate DOMs have higher thresholds
A “soft” solar particle spectrum produces larger increases at lower threshold
NIPR: November 2016

23. Preliminary Results
Red points are negative values – indicative of fluctuations. All of the GLE are seen More blue than red, but other detections are “iffy”
NIPR: November 2016

24. Spectral Variations are Large
Test study using GOES 700 MeV – note how little the 700 MeV predicts the IceTop result. The “known GLE” are also high at 700MeV, but other events are just as high at 700 MeV and not seen at all by IceTop This is possibly different from our findings in the last solar cycle
NIPR: November 2016