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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

GOES Event List – 10 and 100 MeV NIPR: November 2016

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

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

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

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

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

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

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