PoS(ICRC2019)1148 http://pos.sissa.it/ 1 GV. In the . ∗ [email protected] [email protected] Presenting Author. first four decades of its operation,decline a may secular have decline leveled in off recently. the Environmental neutronhousing effects, rates platform including and have snow the been build emergence observed. up and around relocation The the ofout structures as at the causes South of Pole the have been decline.can ruled be A ignored recent at study the challenged South Pole, theazimuth in assumption and particular that at for geomagnetic large cosmic zenith effects rays angles. approachingcould from This be select work regions important confirms in for that ignoring theinclude geomagnetic particle South cutoff propagation Pole effects in Neutron the Monitorthe Polar South rates. atmosphere, Pole and over the We several evolution extend decades. ofdecline the A in cosmic connection investigation neutron ray is monitor to cutoffs made rates. at between the evolving cutoffs and the Neutron monitors are among the mostsensitive, robust with and high reliable precision, detectors to of modulationsbe GeV deployed in for cosmic Galactic extended rays periods Cosmic of and Rays time, are (GCR). decades,over and Hence, many are they solar able can to cycles. observe the The modulation Southwhich of Pole GCR is Neutron both high Monitor, latitude located and at high the altitude, geographic has South an Pole, atmospheric cutoff of around 0 ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). 36th International Cosmic Ray Conference -ICRC2019- July 24th - August 1st, 2019 Madison, WI, U.S.A. Surujhdeo Seunarine University of Wisconsin-River Falls E-mail: Laura Rose Rosen Washington State University E-mail: Evolution of geomagnetic cutoffs at theand South neutron Pole monitor rates PoS(ICRC2019)1148 3 He or BF 3 1 Normalized pressure and efficiency corrected rates of several lower rigidity cutoff neutron Cosmic rays can be observed indirectly by neutron monitors [1] on the earth’s surface. The monitors: South Pole(SOPO), McMurdo(MCMU), Kerguelen(KERG), Sanae(SNAE),Adelie(TERA), Thule(THUL), and Terre Oulu(OULU). The blueexcept reference SOPO’s. The curve rates is are found normalized by to averaging the the peak rate rates in of 1987. all stations Figure 1: detector component of a neutron monitor is a proportional counter containing either Evolution of geomagnetic cutoffs at the South Pole 1. Apparent Decline in South Pole Neutron Monitor Rates gas. Depending onpolyethylene, the which design, acts to the reflectsecondary central environmental neutrons. counter neutrons A or layer is of moderate lead surrounded theand acts by as neutrons energy a result of layers producer in cosmic in of the which ray the a production paraffin nuclear proportional of wax reaction evaporation counter. between or neutrons, the Modulation which lead onone are different the being time neutrons the scales observed 11-year in are periodicityThe readily in neutron observed, intensity the monitor associated dominant at withfor the the two Amundsen-Scott reversal brief in South periods polarity Polealtitude when of Station of it the has 2820 sun. was operated m. shutOf since down. the The 1964, dozens It effective save of vertical sitsMonitor neutron cutoff on (SOPO) monitor rigidity is the stations the is glacial only operating assumed icefor one around to over at that the five the be has decades world, experienced South constant [2] an thehas Pole at [3] apparent South changed at 0.1 steady [4]. Pole and decline an GV. Neutron it In inchanges its the has [4] count years been as rate since relocated the SOPO at causefor. has the of been [3] Pole. the operating, showed decline. that the the A evolutionparticle monitor Snow of detailed propagation, accumulation type geomagnetic may investigation cutoffs and be for has structures detection important ruled at in haveby the explaining out been South carrying the accounted Pole, these decline. out as We well detailed continue as simulations to study of the geomagnetic decline fields for the past five decades, and include PoS(ICRC2019)1148 and above with ◦ longitude from a range of ◦ shows what is being investigated. 1 South latitude and 0 2 ◦ MAGNETOCOSMICS [5] is a GEANT4 [6] based particle tracking code for the Earth’s mag- In our simulations, we used the Tsyganenko 89 [7] model for the external magnetic field and PLANETOCOSMICS [10] is a GEANT4 based simulation that transports particles through Keeping the same definition for the altitude of the atmosphere, external field, internal field, netosphere. It allows one tomodels: compute IGRF, the Tsyganenko. trajectory The of magneticof charged field fields particles models for in are selected two configurable, years. geomagnetic allowing Thus,directions, field the one in can simulation particular simulate for the evolution the ofTOCOSMICS location rigidity accepts of cutoffs user SOPO. and defined asymptotic When latitude calculatingand and the direction longitude of cutoff coordinates, the rigidity, altitude primary MAGNE- particle in thatdinates. the is atmosphere, entering The the simulations atmosphere calculate relative to cutoffthe the rigidity, minimum geographic the energy coor- location cosmic dependent ray that quantity can that be determines observed at the earth’sthe surface. IGRF 10 [8]above model sea for level. the Simulated internal protons magnetic arrive field. at 90 The atmosphere is defined to be 20 km the earth’s atmosphere. It includesof effects particles due that to reach the a detector geomagnetic at field. user It defined also altitude and computesdate, atmospheric the depth. flux and time; wefour simulated zenith-azimuth combinations the that corresponded flux to offocused the on greatest protons increases large in in values cutoff 1965, of rigidity.monitors 1975, We rigidity observe 1985, because cosmic that’s rays 1995, where fromTen and the above thousand 2005 largest the galactic for increases cutoffs, cosmic the occurred. so rays,and it limited produced Neutron is in a worthwhile our cascade to of simulation[11]. examine secondary to particles. that only To space. focus The protons, only atmospheric hit on model theenergies rigidities used of atmosphere the was that protons NRLMSISE-00 could ranged contribute from the todetermined cutoff differences of from 1965, in the 1975, neutron first 1985, monitor part or 1995Pole. rates, of to this the the study. 2010 cutoff in The was 2005, protons as leftincident were directions out simulated showed to of increases arrive this in above some partwould the bins South of not and the decreases impact in study our others, because and analysis. excluding the one change The year in energy rigidity distribution across of different protons produced in the simulations Data, corrected for pressureFor and convenience efficiency, of for presentation, several datasolar are low cycle normalized is cutoff evident, to neutron but the there monitorsdecades. peak is are an intensity Past approximate shown. in 2000, 10% 1987. it decrease appearsrates in The are that SOPO’s 11 still rates the systematically over year decline the lower in when first rates compared few to is other not monitors. as strong as2. in earlier Simulations years, but the zenith and azimuth directions. Thisour results was to done [9]. for After the afocused preliminary years only scan 1969 on across larger and a zenith wide 2005 angles range to that ofin showed be zenith 1965, significant and in able increases azimuth, five in to year attention rigidity compare intervals, was cutoffs. wethe Beginning calculated full the cutoff range rigidity of for azimuth. zenithin angles Then, 75 rigidity keeping cutoffs for 2005 each as year our simulated. reference year, we calculated the difference Evolution of geomagnetic cutoffs at the South Pole a study of particle propagation in the atmosphere. Figure PoS(ICRC2019)1148 , the 2 04 MeV and − 0e . 3 show the main results on the evolution of geomagnetic cutoffs. In Figure 3 and 2 Difference in rigidity cutoffs, with respect to 2005, for years 1965, 1975, 1985, and 1995. There Difference in rigidity cutoffs, with respect to 2005, for the year 1969. It can be seen that there are 03 MeV. Figures + 0e . 3.1 Geomagnetic Cutoff Evolution are regions of small azimuth bands atincreases high are zenith gradual. where the cutoffs increase significantly, and these temporal full range of difference in rigidity cutoffs between 1969 and 2005 is shown. Analyzing more years Figure 3: Figure 2: increases in cutoff rigidity for higher zenith angles. followed a spectral index500 of MV, 2.65 roughly the average [10]altitude over of a 3 km, solar [12] the cycle. altitude of We [13] SOPO recorded on with the the glacial the flux ice, of solar with neutrons energies modulation between arriving 1 parameter at an set to Evolution of geomagnetic cutoffs at the South Pole 1 3. Results PoS(ICRC2019)1148 3 1985 2010 24.31 24.53 99.89 99.00 99.27 243.98 146.32 203.19 shows the differences in 3 represent neutrons that would 1980 2005 24.11 24.36 99.88 98.83 99.23 3 244.69 147.98 203.38 1975 2000 23.95 24.21 99.88 98.69 99.07 99.96 99.20 244.85 1970 1995 4 23.51 23.78 99.88 98.56 24.84 24.88 99.94 99.16 0.05 1965 1990 23.51 23.78 99.87 24.55 24.70 99.92 99.12 84 52 87 84 52 87 112 112 Azimuth [degrees] , the non-zero entries indicate that neutrons produced under the simulated . No cut is made on the location of the neutrons at the Polar surface. Table 2 3 shows the theoretical flux that we would have seen in 2005 if it were not due to 90 88 87 89 90 88 87 89 and 3 Zenith 2 [degrees] at 10 a.m. for each year. . While other studies have corrected any cutoff rigidity values above 40 GV to equal th 1 Calculated cutoff rigidities, in GV, throughout the years for the zenith and azimuth combinations The non-zero entries in the Normalized Flux column in Table The numerical values of the calculated rigidities, along with their differences are displayed The flux of neutrons reaching the surface for the four greatest increases in cutoff rigidities are shows the theoretical flux of neutrons that would have reached the surface of the earth for each the increase in geomagnetic cutoffs. conditions were reaching the glacial ice of the Polar surface before the increases in cutoffs. of the chosen zenith-azimuth combinations during the years 1965, 1975, 1985, and 1995. Table 2 uses the same energy rangesyear and 2005. conditions Table for simulating primary particles, except is run in the in Table 3.2 Particles at the Surface given in Tables corresponding to the largest cutoffcember rigidity 12 increases between 1965 and 2005. Simulations are run on De- in the regions thatcutoff experienced rigidity large at different increases, zenith and each azimuth,directions panel in the GV, in with change respect Figure in to rigidity 2005.bands is While at negligible for over high most the incident zenith time where perioddue the to shown, rigidity the there increases viewing are significantly. direction small at The azimuth the shapes South of Pole. the bands are likely 40 GV [9], wecombinations have corresponding kept to the the greatest valuesdegrees increases as azimuth; in 90 they cutoff degrees are. rigidity zenith, are:87 112 degrees In degrees 89 zenith, azimuth; descending degrees 52 88 degrees zenith, order, degrees azimuth. 87 the zenith, 84 four degrees zenith-azimuth azimuth; reach 3 km above thesame surface column at in the Table South Pole if the geomagnetic cutoff had not changed. In the Table 1: Evolution of geomagnetic cutoffs at the South Pole PoS(ICRC2019)1148 primary flux] / s / 2 7.097e-05 9.099e-05 5.200e-05 6.586e-05 8.665e-06 1.050e-06 6.155e-06 1.002e-06 8.197e-06 9.837e-06 9.306e-06 7.203e-06 1.116e-03 1.492e-07 5.232e-08 1.282e-08 cm / Normalized Flux [neutrons Primary 98.30 98.30 98.30 98.30 [GeV] 243.75 243.75 243.75 243.75 147.04 147.04 147.04 147.04 202.44 202.44 202.44 202.44 max E 5 South latitude, and in the energy ranges suggested by the Primary ◦ 97.76 98.07 98.23 23.03 23.39 23.92 22.86 23.29 23.61 23.95 98.94 98.95 98.96 99.01 22.59 [GeV] 1.33e-03 min E Year 1965 1975 1985 1995 1975 1985 1995 1965 1975 1985 1995 1965 1975 1985 1995 1965 52 84 87 112 Azimuth [degrees] , at 10:00 a.m. th Values showing the normalized flux of neutrons arriving at the surface of the earth at 3 km alti- 87 90 88 89 While cutoff rigidity depends on direction, typically the rigidity reported for a location refers Zenith [degrees] change in rigidity. Noneutrons cut that was would arrive made at on theThis Polar a surface further specific had supports the location the cutoffs idea on not thatthe the increased evolving South in geomagnetic surface. Pole. the effects given A contribute The directions. full to simulation fluxes thework. of declining represent SOPO’s It rates responses will at to also these be neutrons will worthwhiledid be to not examined explore change in cosmic as future rays much, arriving butthe from where neutron directions it monitors. where might the This better rigidity may overlap reveal with a the larger region deficit of of greatest neutrons. sensitivity to only to the vertical cutoff rigidity. Ourin results demonstrate cutoff that rigidities between 1969 could and have 2005,increases occurred an are increase at quite high significant, zenith meaningmuch angles greater that for energy particles certain to arriving azimuth. pass from2 through Some those shows the the of directions earth’s flux these geomagnetic now of field require neutronsat compared on 20 to the km in surface, in the 3 the km past. upper height, atmosphere Table produced at by 90 cosmic ray protons arriving Table 2: tude. These secondary neutrons aresimulations products to of have primary energies protons between approaching the theyear cutoff South 2005. in Pole, 1965, defined Keeping 1975, in consistent 1985, our orDecember with 1995 12 the and cutoff the rigidity cutoff in simulations, the the reference date and time of the simulations is 4. Summary Evolution of geomagnetic cutoffs at the South Pole PoS(ICRC2019)1148 primary flux] / s / 2 9.517e-05 6.111e-05 8.690e-05 9.211e-05 7.581e-06 1.365e-05 2.054e-06 1.002e-06 7.452e-06 7.891e-06 7.757e-06 1.121e-05 2.073e-03 1.414e-07 4.366e-08 1.605e-08 cm / Normalized Flux , 11, 01 2011. [neutrons , 93:11–32, 04 2012. Primary 98.30 98.30 98.30 98.30 [GeV] 243.75 243.75 243.75 243.75 147.04 147.04 147.04 147.04 202.44 202.44 202.44 202.44 max E 6 Primary Space Science Reviews 97.76 98.07 98.23 23.03 23.39 23.92 22.86 23.29 23.61 23.95 98.94 98.95 98.96 99.01 22.59 [GeV] 1.33e-03 min E Year 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 52 84 87 112 Azimuth [degrees] Values showing the normalized flux of neutrons arriving at the surface of the earth at 3 km alti- 87 90 88 89 Proceedings of the 32nd International Cosmic Ray Conference, ICRC 2011 neutron monitor – (keynote lecture). This work was supported in parts by NSF Integrative Activities in Physics, REU Site: Re- Zenith [degrees] [2] Paul Evenson, John Bieber, John Clem, and Roger Pyle. South pole neutron monitor lives again. [1] John A. Simpson. The cosmic ray nucleonic component: The invention and scientific uses of the search In Neutrino Astrophysics at the1757517, University of and Wisconsin-River NSF Falls, Awards OPP,Nuntiyakul, Antarctic 1460752 and Chiang Education, Mai ATE Award University,Doetinchem, 1341312. for University her of We Hawaii, helpful thank for advicecode; access Dr. and during to Dr. his Rolf this Waraporn modified Buetikofer, project; University and ofTOCSOMICS updated Bern, and Dr. for PLANETOCOSMICS. PLANETOCOSMICS access We thank and Philip Dr. permission von Paul tothis Evenson use project for both in including MAGNE- the discussions weekly of neutronproviding monitor the facilities working needed group to conference work call, on and this UWRF project. physics for References Table 3: tude. These secondary neutrons aresimulations products to of have primary energies protons between approaching the theyear cutoff South 2005. in Pole, 1965, defined Keeping 1975, in consistent 1985, our orDecember with 1995 12, the and 2005 cutoff the at rigidity cutoff 10 in simulations, a.m. the the reference date and time of the5. simulations is Acknowledgements Evolution of geomagnetic cutoffs at the South Pole PoS(ICRC2019)1148 , , , 506(3):250 – Proc. 34th , 118, 11 2013. , 112(A12). Journal of Geophysical He component in the 4 Journal of Geophysical Research: Space , page 143, 08 2015. 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