April 4, Neutron Monitor: The Once and Future CosRay Paul Evenson University of Delaware A-118-S

April 4, Origins of CosRay “Neutron Monitor” is an unfortunate term. “Right Whale” and “Edible Dormouse” also come to mind in this context. It is a detector type, not an investigation One could also call IceCube a “photomultiplier array”

April 4, What We Do: Detect energetic (~1-10 GeV ) particles that propagate through the interplanetary magnetic field Characterize time variations, energy spectra, and composition of particles accelerated on the Sun –Understand the interplanetary magnetic field –Figure out what is happening on the Sun and when it happened Relate all of the above to astrophysical acceleration and propagation

April 4, Solar Magnetic Field Unlike the earth, the magnetic field of the sun has not yet become nearly a dipole at the surface There is a dipole component, but it takes careful measurement to find it in the surface fields The dipole component reverses every eleven years or so

April 4, Solar Corona – Temperature and Magnetic Field Surface motions on the sun are reflected in the motion of the magnetic field This energy is released through “reconnection” high above the surface where it is not possible to radiate it quickly This forms the very hot (10 6 K) region known as the solar corona

April 4, Origin and Structure of the Solar Wind The hot solar corona expands, filling the solar system with plasma This heat engine is very efficient, so the wind is “cold” and “supersonic” The highly conductive plasma carries a magnetic field Magnetic and electric interactions cause it to behave much like a fluid, even though the particles almost never actually collide This is the origin of the term “wind”

April 4, The Interplanetary Magnetic Field is organized into the “Parker Spiral”

April 4, 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)

April 4, Why Crawl When You Can Fly? Spacecraft instruments are elegant examples of design that return fantastically detailed information on particle intensity and spectrum They are almost invariably small and even in principle cannot detect enough high energy particles to be useful for transient events (e.g. solar flares) Although ground based detectors are crude by comparison, they can be made big –Excellent timing –Statistically extracted spectra

April 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)

April 4, SPACESHIP EARTH VIEWING DIRECTIONS Optimized for solar cosmic rays Nine stations view equatorial plane at 40-degree intervals Thule, McMurdo, Barentsburg provide crucial three dimensional perspective Solid symbols denote station geographical locations. Average viewing directions (open squares) and range (lines) are separated from station geographical locations because particles are deflected by Earth's magnetic field. 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

April 4, The Role of Pole Pole “looks” near the equator, like most Spaceship Earth Stations High altitude permits measurement of particle spectra STATION CODES FS: Fort Smith, Canada TH: Thule, Greenland MC: McMurdo, Antarctica NA: Nain, Canada SP: South Pole, Antarctica BA: Barentsburg, Norway MA: Mawson, Antarctica AP: Apatity, Russia NO: Norilsk, Russia TB: Tixie Bay, Russia CS: Cape Schmidt, Russia IN: Inuvik, Canada

April 4, Secondary Particle Spectra South Pole is both at high altitude and low geomagnetic cutoff Spectra of secondary particles “remember” a lot of information about the primary spectrum.

April 4, Neutron Monitors 100 MeV hadrons interact with 208 Pb to produce multiple low energy “evaporation” neutrons Neutrons “thermalize” in polyethylene Detected by fission proportional counters –BF 3 “ BP-28 ” n + 10 B → α + 7 Li – 3 He: n + 3 He → p + 3 H Both types are called “ NM64 ” Neutron Monitor in Nain, Labrador Construction completed November 2000

April 4, 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 South Pole station has both a 3-NM64 an array of detectors lacking the lead shielding. “Polar Bares” responds to lower particle energy on average. Bare to NM64 ratio provides information on the particle spectrum. ENERGY SPECTRUM: POLAR BARE METHOD

April 4, Neutron Monitors and IceTop Cherenkov “tanks” and neutron monitor response functions are similar but have significant differences Analog information from IceTop yields multiple response functions simultaneously

April 4, 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

April 4, 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 Composition is a source of systematic error in spectra from neutron or Cherenkov detectors separately

April 4, Future Work (Tylka) Double power law spectra (in rigidity) –Better resolution 1-10 GV (IceTop, PAMELA) –Search for upper cutoff (HAWC, Auger) (Lopate) GLE are “iron rich” –Composition at high energy (IceTop/NM/Bare) Is there a qualitative difference between flares that do and do not produce GeV particles –Small event search (IceTop) Reported two phases: acceleration or propagation? –Better angular and temporal separation (NM network augmented with PAMELA, Auger, HAWC and IceTop) Development of large Cherenkov detectors (water or propylene glycol) to characterize terrestrial background radiation –McMurdo as a reference base station –Continued “latitude surveys” on the icebreakers

April 4, Summary CosRay has been part of Pole for 45 years Continuous evolution of its role in the global neutron monitor network has kept it in the forefront of solar particle research With IceTop, CosRay will make more exciting contributions to our knowledge of heliospheric processes I have focused on solar particle events because they are easier to talk about, but there is also a new window opening on small scale disturbances in the solar wind.

April 4, Extra Slides Expanded Discussion of Various Items

April 4, 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

April 4, 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

April 4, 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

April 4, The First Extraterrestrial Event Detected by IceCube Dec 14, 2006 photograph of auroras near Madison, WI Dec 13, 2006 X3-Class Solar Flare (SOHO) IceTop and Spaceship Earth Observations of the Solar Flare

April 4, 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 at the end of day 347 are clear

April 4, Why IceTop Works as a GeV Particle Spectrometer The IceTop detectors are thick (90 g/cm 2 ) 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

April 4, 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 assuming that the composition is the same as galactic cosmic rays The lines show this fit and the one sigma (systematic) errors

April 4, Solar Particle Spectrum Determination (II) IceTop proton spectrum (heavy blue line with one sigma error band). Black line is the assumed background cosmic-ray proton spectrum Points are maximum proton fluxes from GOES spacecraft data.

April 4, 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 Cutoffs and exact spectral shape are diagnostic of particle acceleration mechanisms.

April 4, IceTop and PAMELA IceTop and PAMELA compared at equivalent times !!Preliminary!! –Takao Kuwabara –Galina Bazilevskaya

April 4, Vladimir: “If IceTop Is So Great, Why Do We Still Need Cosray??”