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. Energy Spectrum With Neutron Monitors
Neutron monitors and muon detectors are comparatively insensitive to the particle spectrum. They are mostly “yes/no”.
Different configurations of neutron detectors have somewhat different energy response and can be used to measure the energy spectrum.
In the past, this has mostly been done with neutron monitors at different altitude.
At South Pole, we use physically different configurations
NIPR: November 2016

4. 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

5. ENERGY SPECTRUM: POLAR BARE METHOD
Since “Polar Bares” responds to lower energy particles, the 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

6. New Type: Cherenkov Detectors such as Auger, HAWC and IceTop
Primarily detect electromagnetic component
Cherenkov radiation measured by standard photon detectors
Auger, HAWC: Liquid water tanks
IceTop: Blocks of clear ice produced in tanks at the Pole
2 m
0.9 m
Diffusely reflecting liner
NIPR: November 2016

7. Why IceTop Works as a GeV Particle Spectrometer
As discussed earlier, neutron monitors (and muon detectors) are comparatively insensitive to the particle spectrum. They are mostly “yes/no”.
Cherenkov detectors are thick (90 g/cm2 or more) so the light output is a function of both the species and energy of incoming particles.
In addition, IceTop has individual waveform recording, and extensive real time processing, that allow the return of pulse height spectra even at the kilohertz counting rate inherent to the detector.
NIPR: November 2016

8. Secondary Particle Spectra
At the South Pole, the high altitude and low geomagnetic cutoff together allow the spectra of secondary particles to “remember” much information about the primary spectrum.
NIPR: November 2016

9. Response Functions
Even simple count rates from IceTop, above different discriminator thresholds, yield multiple response functions simultaneously, whereas the neutron monitor has only a single response function.
NIPR: November 2016

10. Solar Particle Spectrum Published in Ap J Letters
Here we show the excess count rate as a function of pre-event counting rate.
Each point represents one discriminator in one optical detector.
By using the response function for each detector we fit a power law (in momentum) to the data.
The thin lines show one sigma (systematic) errors
NIPR: November 2016

11. 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

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

13. IceTop and PAMELA
Credit: M. Casolino
NIPR: November 2016

14. Future 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 current work is intended to characterize the present solar cycle in terms of “detections”, with possible spectral analysis as a future topic.
NIPR: November 2016

15. Objective of Current Work
Determine the extent to which multi – GeV particles are present in solar particle events. Specifically, see if IceTop can 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

16. 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

17. GOES Event List – 10 and 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. 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

20. 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

21. Reminder: Change vs Base (with example of 2014 Jan 6)
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.
The problem is to select which properties of this pattern best separate solar particle signatures from fluctuations in the background galactic cosmic ray flux.
NIPR: November 2016

22. 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

23. Preliminary Conclusions: IceTop Spectral Parameter vs GOES Intensity
Red points are negative values – indicative of fluctuations. All of the “known GLE” are seen There are more blue points than red, but other detections remain unclear
NIPR: November 2016

24. Preliminary Conclusions
We are still working on the best method to separate solar particles from background fluctuations. The “known GLE” are always detected. There appear to be GeV particles in other events on a statistical basis. Confirmation in any individual event remains elusive.
NIPR: November 2016