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

2. February 7, 2010 2 Why Crawl When You Can Fly? Cosmic rays 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 see 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 the geomagnetic field.

3. 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)

4. February 7, 2010 4 Neutron Monitors Older type “BP28” – proportional counter filled with BF 3 : n + 10 B → α + 7 Li Modern type – counter filled with 3 He: n + 3 He → p + 3 H Both types are called “ NM64 ” Neutron Monitor in Nain, Labrador Construction completed November 2000

5. Differential Response f n. Counting Rate geomagnetic T ransmission Y ield function Assuming L is a limiting rigidity, T is a step function Varying Geomagnetic Transmission Determines the Energy Spectrum Page 9 heliospheric M odulation 1 0 P T STEP FUNCTION GCR spectrum PcPc The complicated geomagnetic transmission can be well represented by a step function, defining the effective “Geomagnetic Cutoff Rigidity”: P c

6. Latitude Survey February 7, 2010 6 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

7. Spectra from Multiple Neutron Monitors: Keep the Network Alive!

8. University of Delaware Operated Stations Swarthmore / Newark (1964) South Pole (1964) [NSF ANT-0838839] Thule (1957) McMurdo / Jang Bogo (1960) [NSF ANT-1245939] Inuvik (1964) Fort Smith (2000) Peawanuck (2000) Nain (2000) February 7, 2010 8

9. Partner Stations Doi Inthanon, Thailand –David Ruffolo, Alejandro Saiz -- Mahidol U –Established in 2008 –Former Morioka NM with BRI electronics Daejeon, South Korea –Yu Yi, Suyeon Oh – Chungnam National U –Established in 2012 –Former Goose Bay NM with BRI electronics River Falls, Wisconsin –James Madsen, Surujhdeo Seunarine -- U Wisconsin –Established 2014? –Base for transportable neutron monitors –Home station for undergraduate operation of neutron monitors February 7, 2010 9

10. 10 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)

11. February 7, 2010 11 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

12. February 7, 2010 12 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

13. February 7, 2010 13 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

14. February 7, 2010 14 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

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

16. 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) = c 0 + c 1 exp(b μ) with μ being the cosine of pitch angle, and c 0, c 1, and b free parameters. The symmetry axis from which pitch angles are measured is also a free parameter.

18. The Move 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 group at Chungnam National University directly into the Spaceship Earth collaboration February 7, 2010 18

19. South Pole Counting Rate Long Term Decline February 7, 2010 19

20. South Pole Cutoff Change February 7, 2010 20