John W. Bieber University of Delaware jwbieber@bartol.udel

1. Neutron Monitor Community WorkshopCurrent and Future State of the Neutron Monitor NetworkOctober 24-25, 2015 HonoluluDomestic Perspective on Neutron Monitors John W. Bieber University of Delaware jwbieber@bartol.udel.edu Visit our Website: http://neutronm.bartol.udel.edu/

2. Invention of the Neutron Monitor • The neutron monitor was invented in 1948 by Professor John Simpson of the University of Chicago • The instrument’s key features are its inherent sensitivity, stability, and capabilities • Impelled in part by the International Geophysical Year, neutron monitor usage grew quickly. By the end of 1957, there were 51* operating worldwide (*) Source: Shea, M. A., and D. F. Smart, Fifty Years of Cosmic Radiation Data, Space Sci. Rev., 93, 229-262, 2000. John Simpson (1916 – 2000) with an early (circa 1950) neutron monitor. Photo from Simpson, J. A., The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor, Space Sci. Rev.,93, 11-32, 2000.

3. THE MODERN (“NM64”) NEUTRON MONITOR • A large instrument, weighing ~32 tons (standard 18-tube NM64) • Detects secondary neutrons generated by collision of primary cosmic rays with air molecules • Enclosures of lead and polyethylene amplify the signal from cosmic secondaries, and suppress environmental neutrons • Detection Method: • Proportional counter filled with BF3: n + 10B → α + 7Li • Proportional counter filled with 3He: n + 3He → p + 3H Neutron Monitor in Nain, Labrador Construction completed November 2000 with NSF/MRI support

4. Long-Term Stability of Neutron Monitors • Typical stability: ~0.05%/year • Ideal for cosmic ray studies over sunspot cycle (~11 yr) or magnetic cycle (~22 yr) time scales • Important tool for inter-normalizing spacecraft instruments over different epochs • Table based upon Oh et al., JGR, Vol 118, pp 5431–5436, doi:10.1002/jgra.50544, 2013

5. SOLAR MODULATION OF GALACTIC COSMIC RAYS:THE LONG VIEW • The ~11 yr variation of cosmic rays in anticorrelation with solar activity is clearly evident • There is a also a ~22 yr magnetic cycle variation, with alternating “pointy” and “flat-topped” cosmic ray peaks: a key early support for the role of drifts in solar modulation • Modulated cosmic rays reached a new space age high during the recent cosmic ray maximum (2009), pointing to possible longer term variations

6. Solar Diurnal Anisotropy:Here, the magnetic cycle (~22 yr) dominates • Phase (local time of maximum) of solar diurnal variation • ~22 yr variation with minima in positive solar polarity (e.g., the 1950’s and 1970’s) • Larger effect for higher cutoff stations • Effect is owing to a systematic variation of the product of the parallel mean free path and radial gradient • Consistent with magnetic cycle variation of radial gradient predicted by drift models • Also consistent with magnetic cycle dependence of mean free path from, e.g., magnetic helicity Vertical lines denote solar minima Source: Bieber & Chen, ApJ, 372, 301-313, 1991.

7. The instrument is the arrayThe scientific value of neutron monitors is greatly enhanced when multiple monitors are linked together in coordinated arrays • Viewing-direction arrays for measuring 3D cosmic ray angular distribution. • Example: Spaceship Earth,1 optimized for solar cosmic rays • Cutoff arrays for measuring the Galactic cosmic ray spectrum • Moraal et al.2 discuss what could be achieved by a calibrated array with a suitable distribution in cutoff rigidity • Direct neutron arrays (equatorial, high-altitude monitors) for measuring relativistic solar neutrons • Major gaps in existing station distribution • Realtime arrays for space weather applications • Partially realized by NMDB (www.nmdb.eu) • Bieber et al., Spaceship Earth Observations of the Easter 2001 Solar Particle Event, Astrophys. J. (Lett.), 601, L103-L106, 2004. • Moraal et al., Design and Co-Ordination of Multi-Station International Neutron Monitor Networks, Space Sci. Rev., 93, 285-303, 2000.

8. Ground Level Enhancements (GLE) The January 20, 2005 GLE • Rare events: typically ~15 GLE per solar cycle • But only 1, so far, in the current cycle • Caused by >500 MeV protons accelerated near the Sun • February 1956 GLE1 remains the largest (out of 71) detected to date • Established scattering and diffusion theory as pre-eminent tools for understanding cosmic ray transport • January 2005 GLE (illustrated) was second largest • Terre Adelie increase (not shown) was 46X over 6 min • Event was enormously anisotropic: Neutron rate increase at other high-latitude stations was an order of magnitude smaller – “only” 3X or so (1) Meyer, P., E. N. Parker, and J. A. Simpson, Solar Cosmic Rays of February, 1956 and Their Propagation through Interplanetary Space, Phys. Rev., 104, 768, 1956

9. TRANSPORT MODELING WITH BOLTZMANN EQUATION • Data points in top three panels are the first three coefficients of a Legendre polynomial expansion of the neutron monitor data • We could not fit this event with a standard Parker field; instead we had to invoke a downstream magnetic bottleneck • Quantities derived from modeling: • Scattering mean free path • Injection profile • Distance to downstream bottleneck • Reflection coefficient of bottleneck

10. INTERPRETATION OF NEUTRON MONITOR DATAIS ENHANCED BY AVAILABILITY OF SPACECRAFT DATAExample: January 20, 2005 Type III Burst Observed aboard WIND

11. INJECTION PROFILE AT SUN DERIVED FROM COSMIC RAYSCOMPARED WITH SOLAR RADIO BURST Top: Injection function derived from neutron monitor data (magnetic bottleneck scenario) Bottom: Solar radio waves at 500 kHz (red) and at 5 MHz (black) observed by WIND Solar Time

12. SPACE WEATHER APPLICATIONS NEUTRON MONITORS ARE WELL SITUATED TO ALERT / MONITORRADIATION HAZARD ON POLAR AIRLINE ROUTES Line shows Chicago-Beijing great circle route. Squares are neutron monitorstations.

13. GLE ALARM SYSTEMS: OPERATIONAL NOW ! BARTOL / UNIV DELAWARE SYSTEM • A GLE alert is issued when 3 stations of Spaceship Earth (plus South Pole) record a 4% increase in 3-min averaged data • With 3 stations, false alarm rate is near zero • GLE Alert precedes SEC (now SWPC, Space Weather Prediction Center) Proton Alert by ~ 10-30 min • For details, see Kuwabara et al., Space Weather, 4, S10001, 2006. OPERATIONAL GLE ALARM SYSTEMS • Bartol / Univ Delaware: http://www.bartol.udel.edu/~takao/neutronm/glealarm/ • IZMIRAN:http://cr0.izmiran.ru/GLE-AlertAndProfilesPrognosing/ • NMDB: http://www.nmdb.eu/?q=node/19

14. NEUTRON MONITORS FOR SPACE WEATHER FORECASTING AND SPECIFICATION NM Prediction (open symbols) • Neutron Monitor Prediction of Solar Energetic Particle Spectra1 • Realtime Mapping of Radiation Intensity in Polar Regions • Realtime Galactic Cosmic Ray Spectrum > 1 GeV(illustration lower right2 ) • ICME Warning from “Loss Cone” Precursor Anisotropy3,4 • BZ Prediction5 • Oh et al.,Space Weather, 10, S05004, 2012. • http://neutronm.bartol.udel.edu/~pyle/SpectralPlot.png • Belovet al., Proc. 27th Int. Cosmic-Ray Conf. (Hamburg), 9, 3507, 2001. • Leerungnavarat et al.,Astrophys. J.,593, 587-596, 2003. • Bieber et al., American Geophysical Union, Fall Meeting, abstract #SH53A-2146, 2013. GOES Observed (closed symbols) http://neutronm.bartol.udel.edu/~pyle/SpectralPlot.png

15. NEUTRON MONITORS IN THE 21ST CENTURY:Whither ?… or Wither ? “… if NSF invests in long term observations for specific research purposes then there must be the possibility of NSF stopping support at some point.” – NSF, 2007 NSF funding of neutron monitors has declined from 14 supported instruments in 2000 to 1+ supported instruments today. (The “1” instrument is at South Pole. The “+” refers to McMurdo, which is in the process of being transferred to the Korean station Jang Bogo.)

16. Neutron Monitors in the 21st Century?Ten Reasons Why the Answer Is “Yes!” (# 1-5) • Neutron monitor arrays are the state-of-the-art method for observing the intensity and angular distribution of GeV cosmic rays. • Nothing flown in space is competitive in this energy range. • Modeling interplanetary transport of solar cosmic rays provides key information on how charged particles are scattered by magnetic turbulence (parallel mean free path). • GLE particles provide the clearest picture of the particle injection at the Sun (injection onset and time profile). • Modeling GLE provides insight into the role of magnetic mirroring in cosmic ray transport. • Neutron monitors can provide the first space weather alert of some major proton events.

17. Neutron Monitors in the 21st Century?Ten Reasons Why the Answer Is “Yes!” (# 6-10) • High-altitude equatorial monitors sometimes observe direct relativistic solar neutrons, an important window into the acceleration site, because neutrons travel unimpeded by the magnetic field. • High-altitude polar monitor combined with “bare counter” provides clean measurement of relativistic solar particle spectrum. • Space weather application: The relativistic spectrum (which can be measured earliest) has been shown to be predictive of lower energies. • Other space weather applications are emerging • Realtime mapping of ground radiation intensity (especially polar regions) • Realtime GCR spectrum • ICME warning from loss cone anisotropy • BZ prediction • Neutron monitors provide unique insight into 22-yr (magnetic cycle) variations of the solar modulation of cosmic rays. • Neutron monitors provide insight into even longer-term change, such as the recent unusual solar minimum, which resulted in a record level of Galactic cosmic rays (3% above the previous high count rate from Galactic cosmic rays).