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Planetary Long-Term Changes of Cosmic Ray Geomagnetic Cut-Off Rigidities

This study explores the long-term changes in the geomagnetic cut-off rigidities of cosmic rays using trajectory calculations. The results reveal anomalies in the rigidity of the geomagnetic cut-off and provide insights into the future variations.

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Planetary Long-Term Changes of Cosmic Ray Geomagnetic Cut-Off Rigidities

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  1. SH34 Planetary long term changes of the cosmic ray geomagnetic cut off rigidities. B.B.Gvosdevsky1, A.V. Belov2, R.T. Gushchina2, E.A. Eroshenko2, V.G. Yanke2 1Polar Geophysical Institute, Russian Academy of Sciences, PGI, Apatity, Russia, 184209 2Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, IZMIRAN, Moscow, Russia, 108840 In the work by the method of trajectory calculations, the average annual values of the geomagnetic cutoff rigidity of cosmic rays are obtained for the vertical (NMs) and inclined detectors (muon telescopes) of the World Network. The calculations were performed for the period 1950 -2015, as well as for the predicted values of the magnetic field up to 2050 on the basis of the International Geomagnetic Reference Field (IGRF 12). The obtained results reveal two World anomalies: in one - the rigidity of the geomagnetic cutoff decreases, in the other – increases along the time, but globally rigidity of the geomagnetic cutoff decreases by about 7% per century. For inclined directions (according to the data of muon telescopes), an irregular course is observed in the time dependence of the rigidity of geomagnetic cutoff: for northwest directions for detectors of the zone of the South Atlantic anomaly and directions south for the detectors of the zone of the North Atlantic anomaly. Such an irregular course is most likely due to the large variability of the penumbra an irregular course is most likely due to the large variability of the penumbra zone for such inclined directions. gvozdevsky@pgia.ru , A.V. Belov abelov@izmiran.ru , E.A. Eroshenko erosh@izmiran.ru , rgus@izmiran.ru , V.G. Yanke yanke@izmiran.ru. B.B. Gvosdevsky R.T. Gushchina 1

  2. Long term variations of the dipole and non-dipole components of the magnetic field. Over the half-century observational period of cosmic rays, the geomagnetic field has decreased by an average of 4%, but in different regions the change is at different rates. e of 4%, but in different rWhat What is even more impressive, the contribution of high harmonics of the geomagnetic field over this period, on the contrary, increased by one third. During the 20th century, the non-dipole field drifted westward at an average rate of 0.15 ° / year [Wei Zi- Gang et al., 2001]. Transferring period of the inversion is about 2000 years. Dipole component decreases (left side), but dipoles of the higher orders (quadrupole and oktapole …-right side) – increase [Glatzmaier el al., 2005]. In the nearest future it is possible to exspect the continuation of these processes. AF - near Africa, 0.26 °/a SAT - near South Atlantic, 0.04 °/a EA - in Europe and Asia, 0.12 °/a 0.23 °/a NAM - in North America, AUS - near Australia, NAT - near North Atlantic, 0.13 °/a 0.09 °/a атмосферны  е первичные вариации   космически   х лучей        W  R (  ,  h 0  )  m  R (  h ,  dR )      W  R (  h ,  ) 0  J   (  R )  dR магнитосфе рные        c W R   c R  h ,  N  ( ) ( , ) ( ) N m R h  J R dR  c R   0 0 0 N m J R R c c Tasks: 1) To obtain the geomagnetic cut off rigidities for cosmic rays for the whole period of their monitoring, and in the first turn, after 2000 s when such the calculations have not been carried out. 2) To estimate long term changes of the CR cut off rigidities (retrospectively and in prognosis). 3) To estimate the expected CR variations caused by geomagnetic variations for the neutron and muon components. 2

  3. Result 1. Changes of the vertical geomagnetic cut off rigidities (neutron monitors). [Animation_Rc] Changes of the geomagnetic cut off rigidities for vertical component CR relatively to the epoch 1950 for two groups of the cosmic ray stations. The first group - the stations from the region of North Atlantic anomaly and its Eastern tail. The second group - stations in the zone of the South Atlantic anomaly and its western plume. 3

  4. Result 2. Prognosis of the changes of vertical geomagnetic cut off rigidities up to 2050 for some stations. Changes of the vertical geomagnetic cut off rigidities for the CR stations relatively to epoch 1950 and forecasting up to 2050. Prognosis up to 2050 of the changes of the planetary distribution of the vertical geomagnetic cut off rigidities relatively to the epoch 1950. [Animation_planetary_Rc] Forecasting up to 2050 showed that two zones are developing further but nothing extreme during this small period is expected. 4

  5.                   Changes of the cut off rigidities for inclined directions of cosmic rays. Detector Lat,˚ Lon,˚ Unstable directions Operating ground level muon telescopes for cosmic rays. Nagoya SaoMartinho Kuwait Hobart Mustang/Greifswald Mustang/Kiel Yakutsk YangBaJing Guangzhou Adelaide/ /BucklandPark Leonsito Putre NewMoscowOpto Moscow/URAGAN Novosibirsk GRAPES SierraNegra Norikura Yerevan2000 Ottawa Bure Moussala Blagoevgrad ZhongShan 35.12 -29.44 29.24 -42.9 54.08 54.35 62.02 30.11 23.60 -34.93 -35.00 -31.80 -18.2 55.47 55.65 54.48 11.38 18.99 36.10 40.37 45.44 44.63 42.18 42.01 -69.4 136.97 n1, n3, w1, w3, w4, nw2 -53.81 e3, e4, se2, s1, s3, sw2 47.97 n1, w4, nw2 147.33 s1, s3, s4, w1, e3, e4, se2 13.38 - 10.11 129.72 - 90.53 n1, n3 113.18 - 138.58 - 140.00 s1, w1 -69.30 s1 -69.55 - 37.32 - 37.67 e3, e4, ne2, nw2 83 - 76.67 - -97.31 n1, n4, ne2,e4 136.60 n1, w1 44.25 n3, nw2, nw4 -75.68 5.91 23.59 23.1 -76.4 s1, w1 For rigidities of geomagnetic cutoff of some north-western and south-eastern directions of the detectors (the left column of the table), an irregular time course is observed. Independent trajectory confirm this result. This possibility was pointed out by Valarta and Lemaitre [Lemaitre et al., 1936], who discovered the penumbra effect. The shadow cone, which is filled with particles that cross the surface of the Earth; this region lies near the horizon, faces the north or south pole and is most important for large zenith angles. 5

  6. Result3. Features of the behavior of Rc at the detectors of North- Atlantic and South-Atlantic zones. Below graphical results are presented for 25 muon detectors. From their analysis it may be concluded: 1) Irregular behavior of geomagnetic cut off rigidities is observed only for inclined directions north-west at the detectors of South- Atlantic anomaly and its western шлейфа (Nagoya, Kuwait, YangBaJing, Yerevan2000, Mustang/Greifswald, Moscow, Moussala, Blagoevgrad, Guangzhou, Novosibirsk, Yakutsk, ZhongShan, Putre, Leonsito). 2) Irregular behavior of geomagnetic cut off rigidities is observed only for inclined directions south –east (not always for all) for detectors of North-Atlantic anomaly and its eastern шлейфа (Hobart, GRAPES, Ottawa, Mustang/Kiel, BucklandPark). These directions are related to the low rigidities for detectors listed in 1 and 2 and the range of these rigidities is 8-10 GV. Norikura, SierraNegra, Bure, Adelaida & 6

  7. Telescope Adelaida & BucklandPark Illustration of the Penumbra role for explaining the irregular nature of the time dependence for the direction of BucklandPark.s1 1975 1985 1995 7

  8. Trajectory and penumbra. Hyro radius for 10 GV protons is equal to ρ= 10·109/(300·0.33)=1000 km Animation of 1975  /( 300 ) [ ] [ ] /[ ] cm  V G R B   ftp://crsb.izmiran.ru/MagEffect/Vertical/CutOff/GRID_ Start1950/Plot_2020/Animation_2020.gif] Animation: Penumbra_BucklandPark.s1 ftp://crsb.izmiran.ru/MagEffect/Inclined/Penumbra/Animation_Penumbra_BucklandPark.s1.gif 8

  9. Calculator “CutOff Rigidity” This program calculate the geomagnetic cut off rigidity for fixed date and fixed geographic point. It can calculate trajectories by one of model of magnetosphere: dipole, IGRF, Tsyganenko models IGRF+T89, IGRF+T96, IGRF+T02. In result the program gives lowest RS, upper RH,effective Rсgeomagnetic cut off rigidities and penumbra – set of allowed and forbidden rigidities between Rs and RHwith the step of 0.001 GV. Screenshot of the interface of calculator “CutOff Rigidity” http://crsv.izmiran.ru/cutoff 9

  10. Conclusions 1) The reason of essential change of geomagnetic cut off rigidity is total decrease of the Earth magnetic field, on which a background specific “contrast’ arises as two anomalous zones: North-Atlantik anomaly and its eastern plume, and South-Atlantik anomaly and its western plume. 2) Time changes of the cut off rigidities for vertical incident particles at the World wide network stations completely agree with the behavior due to these two anomalous zones. 3) Over the observable period of the CR the mean planetary geomagnetic cut off rigidity for vertical incident particles decreased on 0.2 GV or 3.4 %, that leads to increase of measured flux on ~1 % per 50 years. In the epicenter of the anomalies the flux changes on about 12 % during the same period. 4) The decrease in the dipole component of the magnetic field, but with an increase in the contribution of high harmonics of the magnetic field, as well as changes in the displacement vector of the poles and anomalies, leads to unexpected effects. This is the significant variability of the penumbra, the instability of the inclined trajectories of the particles 5 ) The instability of inclined trajectories of particles leads to an irregular course in the time dependence of the rigidity of the geomagnetic cutoff of inclined north-west directions for the detectors of the zone of the South Atlantic anomaly and its western plume (Nagoya, Kuwait, YangBaJing, Yerevan2000, Norikura, SierraNegra, Mustang/Greifswald, Moscow, Moussala, Blagoevgrad, Guangzhou, Novosibirsk, Yakutsk, ZhongShan, Putre, Leonsito). 6) The instability leads to an irregular course for inclined south-east directions for the detectors of the zone of the North Atlantic anomaly and its eastern plume (Hobart, GRAPES, Ottawa, Mustang/Kiel, Bure, Adelaida & BucklandPark). 7) To study the anomalous behavior of inclined trajectories, it is necessary to continue a detailed study of the penumbra and the causes of its variability (jerk, harmonics). ftp://crsb.izmiran.ru/MagEffect 10

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