April 4, 20111

Neutron Monitor:The Once and Future CosRay

Paul EvensonUniversity of Delaware

A-118-S

April 4, 20112

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, 20113

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, 20114

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, 20115

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 (106K) region known as the solar corona

April 4, 20116

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, 20117

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

April 4, 20118

OBSERVATION OF COSMIC RAYSWITH 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, 20119

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, 201110

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, 201111

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 CODESIN: Inuvik, Canada FS: Fort Smith, Canada PE: Peawanuck, Canada NA: Nain, Canada BA: Barentsburg, NorwayMA: Mawson, Antarctica AP: Apatity, Russia NO: Norilsk, Russia TB: Tixie Bay, Russia CS: Cape Schmidt, Russia TH: Thule, Greenland MC: McMurdo, Antarctica

April 4, 201112

The Role of Pole

• Pole “looks” near the equator, like most Spaceship Earth Stations

• High altitude permits measurement of particle spectra

STATION CODESFS: Fort Smith, CanadaTH: Thule, GreenlandMC: McMurdo, AntarcticaNA: Nain, CanadaSP: South Pole,

AntarcticaBA: Barentsburg, NorwayMA: Mawson, AntarcticaAP: Apatity, RussiaNO: Norilsk, RussiaTB: Tixie Bay, RussiaCS: Cape Schmidt, RussiaIN: Inuvik, Canada

April 4, 201113

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, 201114

Neutron Monitors

• 100 MeV hadrons interact with 208Pb to produce multiple low energy “evaporation” neutrons

• Neutrons “thermalize” in polyethylene

• Detected by fission proportional counters– BF3 “BP-28”

n + 10B → α + 7Li– 3He:

n + 3He → p + 3H• Both types are called

“NM64”Neutron Monitor in Nain, Labrador

Construction completed November 2000

April 4, 201115

• 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, 201116

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, 201117

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, 201118

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, 201119

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, 201120

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, 201121

Extra Slides

Expanded Discussion of Various Items

April 4, 201122

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, 201123

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, 201124

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, 201125

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, 201126

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, 201127

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

April 4, 201128

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, 201129

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, 201130

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, 201131

IceTop and PAMELA

• IceTop and PAMELA compared at equivalent times

• !!Preliminary!!– Takao Kuwabara– Galina Bazilevskaya  

April 4, 201132

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