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COSMIC RAY MEASUREMENTS IN THE ATMOSPHEREY.I. Stozhkov, N.S. Svirzhevsky, and V.S. MakhmutovLebedev Physical Institute, Russian Academy of Sciences, Moscow, RussiaAbstractWe present the main characteristics of cosmic ray population in the atmosphereand its variability (11-and 22-year solar cycle variations, solar protonsoriginating from powerful solar flares, energetic electron precipitation duringgeomagnetic disturbances and Forbush decreases of cosmic rays). Theexperimental data were obtained from the long-term cosmic ray monitoring inthe atmosphere from 1957 to now. The relationship between cosmic ray fluxes,and atmospheric processes are also discussed.1. METHOD OF REGULAR MEASUREMENTS OF COSMIC RAYS IN THE ATMOSPHERERegular cosmic ray measurements of cosmic rays are carried out with different instruments: ionizationchambers, meson telescopes, and neutron monitors on the ground. The idea of cosmic ray monitoring inthe atmosphere with radiosondes was suggested by Prof. S.N. Vernov in the middle of fifties and wasrealized by him and Prof. A.N. Charakhchyan in 1957. Now the cosmic ray monitoring cover a wide rangeof cosmic ray energy spectrum. It is schematically shown in Fig. 1.5cosmic ray fluxSCR4GCR3atm.2nm1789101112lg E, eVFig. 1 Schematic view of galactic and solar cosmic ray spectra (GCR, SCR, thick and thin curves accordingly). Thedotted vertical lines show the minimal energy of primary particles, which are detected by radiosondes in theatmosphere (E 0.1 GeV, upper arrow labeled atm) and by neutron monitors on the ground level (E 1.5 GeV, arrowwith nm). The ground-based ionization chambers and meson telescopes record the primary particles with E 9 GeV.The cosmic ray measurements in the atmosphere are made with standard radiosondes in which thecharged particle detectors are Geiger counter (hereafter counter) and telescope consisting of two countersand with 7 mm Al plate between them. Single counter records charged particles (electrons with energy

E 0.2 MeV, protons with E 5 MeV) and g-rays with E 0.02 MeV (efficiency 1 %). Telescope recordselectrons with E 5 MeV, protons with E 30 MeV and is not sensitive to g-rays. For the isotropic angulardistribution of particles in the upper hemisphere the geometrical factors of these detectors are 15.1 cm2and 17.8 cm2 sr.The long-term cosmic ray measurements in the atmosphere have been started at the several latitudeswith the different geomagnetic cutoff rigidities Rc in the middle of the last century and they are continuedtill now [1]. Every day or several times per week balloon flights have been made. Also several seaexpeditions had been organized where the measurements of cosmic ray fluxes in the atmosphere in a widerange of Rc had been made. Till now more than 70.000 balloon flights have been performed. In Table 1the sites and periods of observations are given. The cosmic ray fluxes are measured from the ground levelup to 30-35 km.Table 1. The sites and periods of cosmic ray measurements in the atmosphere.Site of observationsRc, GVPeriod of observationsMirny, AntarcticaGeographiccoordinates66 33 S; 93 00 0.0403.1963 - present timeTixie71 33 N; 128 54 0.402.1978 – 09.1987Murmansk region68 59 N; 33 05 0.607.1957 - present timeNorilsk69 00 N; 88 00 0.611.1974 – 06.1982Moscow region55 28 N; 37 19 2.407.1957 - present timeAlma-Ata43 12 N; 76 56 6.703.1962 – 02.1992Erevan40 10 N; 44 30 7.601.1976 – 06.1989Sea expeditions60 N - 60 S0.1-171963-65; 1968-72; 1975-76; 1986-87In the atmosphere the main part of charged particles is secondary ones except of altitudes h 20 kmin polar regions where there are low energy primary protons. Below h 20 km cosmic rays mainly consistof secondary electrons and muons.2. GALACTIC COSMIC RAYSTo study galactic cosmic ray flux variations in different energy intervals the magnetic field of the Earthand the atmosphere are used as separators of c ionship is given in Fig. 11. The correlation coefficient R(Nmax, IMF) -0.71 0.04.4572.5IMF,63-2Nmax, cm s-13.589210111.55565758595Year (after 190Fig.11. Time dependence of maximum cosmic ray flux in the polar atmosphere, Nmax (black curve) and IMF (graycurve). The monthly averaged data smoothed with the period of T 3 months were used. The data on IMF were takenfrom INTERNET: http://nssdc.gsfc.nasa.gov/omniweb/.3025h, km2015Start time, UT, dateGCR100822 11.09.01053 11.09.051942 11.09.00326 11.10.000.11.010.0-2100.0-1N, cm sFig. 12. Cosmic ray fluxes recorded in the atmosphere at Mirny in the Antarctic during the solar proton event on 9November 2000. Solid curve is a charged particle flux background produced by galactic cosmic rays in theatmosphere (GCR). Different symbols show the data obtained during the different flights of radiosondes (the dateand start time of launchings are given in the insert).

3. SOLAR COSMIC RAYS, PRECIPITATION, AND RADIOACTIVITYSince the beginning of cosmic ray measurements in the atmosphere in 1957 several tens of solar protonevents were recorded (e.g. [5]). As a rule such events are observed in the polar atmosphere where Rc arerather low (see Table 1). During solar proton events total cosmic ray flux at high altitudes in theatmosphere increases in several (sometimes in tens) times. As example, the solar proton fluxes generatedby solar flare on November 9, 2000 and recorded in the Earth’s atmosphere are given in Fig. 12. Solarproton fluxes were observed in the atmosphere at h 17 km and their values increase with altitude.100J( E), cm-2 s-1 sr-111032140.11001000E, MeVFig. 13. The energy spectra of solar protons on 9 November 2000. The start times of radiosonde launchings andexponents g of solar proton energy spectra I( E) E-g are given below: 1 –Date - 11.09.00, Start time (UT) - 8:22,g 7.2; 2 – Date - 11.09.00, Start time (UT) - 10:53, g 5.8; 3 – Date - 11.09.00, Start time (UT) - 13:36, g 7.8; 4 –Date – 11.10.00, Start time (UT) - 19:42, g 4.9.sunspot number2001081506100450019552solar proton event numberFrom these data the fluxes and energy spectra of solar flare particles in the energy range of E 100500 MeV were obtained. They are depicted in Fig. 13. In this solar proton event the particles with E 500MeV were not observed and this event was not recorded by neutron monitors. We note that the observedsolar proton “soft” energy spectra could be due to the additional acceleration of particles in theinterplanetary space by shock waves as it was happened in the past during the solar proton events inAugust 1972 [6].01965197519851995YearFig. 14. The time dependences of yearly number of the solar proton events recorded in the atmosphere (black pointsand right axes) and yearly sunspot number (open points and left axes).

353530302525h, kmh, kmFor the 45-year period of cosmic ray monitoring in the atmosphere 105 solar proton events wererecorded. In Fig. 14 the time dependence of the yearly number of these events and solar activity level(sunspot number) are shown. It is seen that the solar proton events mainly occur during the ascending anddescending phases of solar activity.20151009301510Start time UT Date125005.03.0520Start time UT Date125005.03.0505.05.00930005.05.000123-2 -1N, cm s4012-23-1N, cm sFig. 15. Precipitation of high energy electrons into the northern polar atmosphere recorded by single counter on 5March 2000 (left panel, black points). The background from galactic cosmic rays is shown by open points. Thetelescope recorded galactic cosmic ray background only (right panel). The inserts show the dates of radiosondeflights and launching times.During the geomagnetic disturbed periods in the polar atmosphere at high altitudes high-energyelectron precipitation events are detected [7, 8, 9]. Our northern polar station in Murmansk region is nearthe polar oval (McIlwain’s parameter L 5.6) where the precipitations are observed rather often. In Fig. 15the example of precipitation detected on November 2000 is shown. The counter records secondary g-raysproduced by precipitating electrons in the atmosphere.But at the same time the telescope records only the background from galactic cosmic rays and itallows to separate precipitation from solar proton events. It is significant that g-rays recorded in theatmosphere at h 25-35 km are produced by the precipitating electrons with E several MeV. The timedependence of the yearly precipitation number and the sunspot number are given in Fig. 16. Into theprecipitation data the corrections for patrol efficiency were introduced. The data obtained show that theprecipitation take place most often during the descending phase of solar activity (remind that solar protonevents are observed most often during the ascending and descending solar activity phases, see Fig. 14).This fact was established earlier in other papers [10]. The total number of precipitation recorded at thestation in Murmansk region during 1957-2000 equals 549 events. For almost the same period (1963-2000)at the Antarctic station Mirny (66 33 S; 93 00 ; Rc 0.04 GV; Lª11) 10 precipitation events wererecorded only.In Table 2 the yearly precipitation numbers for 20 - 23rd solar cycles are given. The last line in thisTable includes the ascending phase of 23rd solar activity cycle only.The regular monitoring of charged particle fluxes in the atmosphere provides the prompt control ofradiation conditions and allows to detect radiation clouds from nuclear explosions or nuclear plantfailures. In Fig. 17 and 18 the observations of radioactive clouds in the polar northern atmosphere and

over Moscow are shown as an examples. The excesses of particles over the cosmic ray backgroundrecorded by single counter were due to the radioactivity particles. The telescope data showed normalcount rate from cosmic rays only.A powerful surface nuclear explosion was produced near lake Lobnor in China on the 14th ofOctober 1970. In the atmosphere radioactive clouds were observed near Murmansk on October 25-26 (seeFig. 17) and on November 11-12, near Moscow and Alma-Ata on November 5 and 6. In the polar region itwas observed in the altitude range hª15-25 km and charged particle flux was increased in 7-8 times incomparison with the cosmic ray background. At the first registration the cloud had the shape of a disk withvertical size of nearly 4 km and horizontal size along the wind direction of 1100 km. The maximalactivity was equal 10-3 Bq/cm3 as measured at hª18-22 km. After 26 October the radioactive cloudpassed away from the observation site.A radioactive cloud in the atmosphere over Moscow was recorded on 12-14 April 1993 (see Fig.18). The cloud was seen at hª10-30 km and had the horizontal extension of nearly 1000 km and maximumactivity 10-4 Bq/cm3. We do not know this cloud origin but on 6 April the failure at the chemical plant inSiberia (Seversk town near Tomsk) occurred [11].A more dense cloud was observed over Moscow on the 27th of October 1999, the maximumactivity being 7 10-3 Bq/cm3 at h 15 km. The cloud horizontal extension was about 200 km. The source ofthis last cloud is unknown also.200801506010040502001955sunspot numberprecipitation number10001965197519851995YearFig. 16. The time variations of the yearly number of precipitation (black points) and sunspot number (open points). Theobservations were made at the northern polar station in Murmansk region (68 59 N; 33 05 ; Rc 0.6 GV; L 5.6) during the timeinterval of (6-12) UT.Table 2. The precipitation number in different solar activity cycles.solar cycle number,period20 (1964-1975)21 (1976-1985)22 (1986-1995)23 (1996-2000)sunspot numberper year58.882.978.546.9precipitationnumber (total)14414011885Precipitationnumber per year12141217