Neutron Monitor Community Workshop Current and Future State of the Neutron Monitor Network October 24-25, 2015 Honolulu Domestic Perspective on Neutron Monitors John W. Bieber University of Delaware Visit our Website:

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, , 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.

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 BF 3 : n + 10 B → α + 7 Li – Proportional counter filled with 3 He: n + 3 He → p + 3 H Neutron Monitor in Nain, Labrador Construction completed November 2000 with NSF/MRI support

Long-Term Stability of Neutron Monitors NM StationSensitivity Variation [% / year] Sensitivity % Change Apatity Hermanus Inuvik Jungfraujoch Kerguelen Lomnicky McMurdo Moscow Newark Oulu Rome Sanae Terre Adelie Thule Yakutsk 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: /jgra.50544, 2013

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

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 Source: Bieber & Chen, ApJ, 372, , Vertical lines denote solar minima

The instrument is the array The 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 ( (1)Bieber et al., Spaceship Earth Observations of the Easter 2001 Solar Particle Event, Astrophys. J. (Lett.), 601, L103-L106, (2)Moraal et al., Design and Co-Ordination of Multi-Station International Neutron Monitor Networks, Space Sci. Rev., 93, , 2000.

Ground Level Enhancements (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 GLE 1 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 The January 20, 2005 GLE (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

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

INTERPRETATION OF NEUTRON MONITOR DATA IS ENHANCED BY AVAILABILITY OF SPACECRAFT DATA Example: January 20, 2005 Type III Burst Observed aboard WIND

INJECTION PROFILE AT SUN DERIVED FROM COSMIC RAYS COMPARED 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

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

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 ~ min For details, see Kuwabara et al., Space Weather, 4, S10001, OPERATIONAL GLE ALARM SYSTEMS Bartol / Univ Delaware: IZMIRAN: NMDB:

NEUTRON MONITORS FOR SPACE WEATHER FORECASTING AND SPECIFICATION Neutron Monitor Prediction of Solar Energetic Particle Spectra 1 Realtime Mapping of Radiation Intensity in Polar Regions Realtime Galactic Cosmic Ray Spectrum > 1 GeV (illustration lower right 2 ) ICME Warning from “Loss Cone” Precursor Anisotropy 3,4 B Z Prediction 5 (1)Oh et al., Space Weather, 10, S05004, (2) (3)Belov et al., Proc. 27th Int. Cosmic-Ray Conf. (Hamburg), 9, 3507, (4)Leerungnavarat et al., Astrophys. J.,593, , (5)Bieber et al., American Geophysical Union, Fall Meeting, abstract #SH53A-2146, NM Prediction (open symbols) GOES Observed (closed symbols)

NEUTRON MONITORS IN THE 21 ST 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.)

Neutron Monitors in the 21 st Century? Ten Reasons Why the Answer Is “Yes!” (# 1-5) 1.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. 2.Modeling interplanetary transport of solar cosmic rays provides key information on how charged particles are scattered by magnetic turbulence (parallel mean free path). 3.GLE particles provide the clearest picture of the particle injection at the Sun (injection onset and time profile). 4.Modeling GLE provides insight into the role of magnetic mirroring in cosmic ray transport. 5.Neutron monitors can provide the first space weather alert of some major proton events.

Neutron Monitors in the 21 st Century? Ten Reasons Why the Answer Is “Yes!” (# 6-10) 6.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. 7.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. 8.Other space weather applications are emerging – Realtime mapping of ground radiation intensity (especially polar regions) – Realtime GCR spectrum – ICME warning from loss cone anisotropy – B Z prediction 9.Neutron monitors provide unique insight into 22-yr (magnetic cycle) variations of the solar modulation of cosmic rays. 10.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).