1. Antarctica and the Global Neutron Monitor Network
Paul Evenson
University of Delaware
Department of Physics and Astronomy
February 7, 2010

2. Energetic Particle Time Variability
This discussion is all about time variability of high energy particles from space that are responsible for nearly all radiation in the atmosphere of the Earth
February 7, 2010

3. 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)
February 7, 2010

4. Why Crawl When You Can Fly?
Cosmic ray particles (electrons, protons, and heavier nuclei) are characterized by their spectrum (or distribution in energy) and their anisotropy (or distribution in arrival direction).
Spacecraft instruments are elegant examples of design that return fantastically detailed information on cosmic ray particles.
These instruments are almost invariably small and therefore cannot measure enough high energy particles to be useful.
Although ground based detectors are crude by comparison, they can be made big.
Spectrum and anisotropy can be determined by using networks of neutron monitors and understanding the geomagnetic field.
February 7, 2010

5. The remainder of my talk ..
… has two main themes
Importance of Antarctica to research using neutron monitors
Filling gaps in “sky” coverage in the study of high energy particles (cosmic rays) in space
Importance of neutron monitors to Antarctica
Understanding radiation environment and radioisotope production
But first I make a few general statements about this observation technique …
February 7, 2010

6. Neutron Monitors Neutron Monitor in Nain, Labrador
Low energy neutrons are produced in blocks of lead by ~ 100 MeV atmospheric neutrons.
These are contained and “thermalized” by polyethylene.
Thermal neutrons are then detected and counted – the dataset is the count rate as a function of time
Detectors are proportional counters
Older type “BP28” – are filled with BF3:
n + 10B → α + 7Li
Newer type (but now very expensive) are filled with 3He:
n + 3He → p + 3H
Both types are called “NM64”
Neutron Monitor in Nain, Labrador
Construction completed November 2000
February 7, 2010

7. Differential Response fn.
Page 9
Components of the Counting Rate
geomagnetic Transmission
The complicated geomagnetic transmission can be well represented by a step function, defining the effective “Geomagnetic Cutoff Rigidity”: Pc 1 P T
STEP FUNCTION Pc
heliospheric Modulation
GCR spectrum
Yield function
Counting Rate
Assuming L is a limiting rigidity, T is a step function
Differential Response fn.

8. Variation with cutoff is illustrated by a “latitude survey”
U.S. Coast Guard icebreakers, the Polar Sea or the Polar Star many times carried a standard 3-NM64 neutron monitor for us from Seattle to McMurdo and back
February 7, 2010

9. Neutron Monitors at Different Cutoffs Follow the Energy Spectrum

10. Sample of Data at Different Geomagnetic Cutoffs
Doi Inthanon – 16.8GV
Athens – 8.5 GV
Rome – 6.3 GV
Newark (USA) 2.1 GV
Oulu (Finland) 0.8 GV
No cutoff but different viewing directions
North: Thule (Greenland)
Equator: South Pole
South: McMurdo/Jang Bogo
May 15-24, 2014
February 7, 2010

11. Multi-national participation:
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:
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)
Chungnam National University (South Korea)
February 7, 2010

12. Why are all the stations at high latitude
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
February 7, 2010

13. Why are all the stations at high latitude
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
February 7, 2010

14. Why are all the stations at high latitude
Why are all the stations at high latitude? Reason 3: Focusing of incident primaries
Particles are focused by the converging polar magnetic field
Primaries with widely divergent local angles of incidence have similar interplanetary asymptotic directions
Calculations are made by following time-reversed trajectories
February 7, 2010

15. Spaceship Earth Viewing Directions
Optimized for solar cosmic rays
Nine stations view equatorial plane at 40-degree intervals
Thule, McMurdo, Barentsburg provide three dimensional perspective
Solid symbols denote station geographic locations. Asymptotic (interplanetary) viewing directions (open squares) and range (lines) are different from station geographic 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
February 7, 2010

16. Neutron Monitor Observations of the December 13, 2006 Ground Level Enhancement (GLE)
This “maverick” GLE occurred near solar minimum, but it was a large event, exceeding a 100% increase at Oulu

17. Antarctic Radiation Environment: Secular Decline of South Pole Counting Rate
February 7, 2010

18. Current Research: Does This Reflect a Cutoff Change?
February 7, 2010

19. Move from McMurdo to Jang Bogo Station
A point is plotted for each hour of a day indicating the asymptotic viewing direction for a 2.0 GV particle vertically incident at four Antarctic neutron monitors. Squares show the geographical locations: McMurdo (red), Terre Adelie (violet), Mawson (blue), and Sanae (green). Jang Bogo (black) will be equivalent to McMurdo as a location for a neutron monitor. This will enhance collaboration between NSF and KOPRI, as well as bring the groups at Chungnam and Chonnam National Universities directly into the Spaceship Earth collaboration
February 7, 2010

20. Event Modeling
Pitch angle is measured from the assumed interplantary magnetic field direction. Individual station data are fitted to an angular distribution of the form
f(μ) = c0 + c1 exp(b μ)
with μ being the cosine of pitch angle, and c0, c1, and b free parameters. The symmetry axis from which pitch angles are measured is also a free parameter.