Cosmic ray intensity variations in Antarctica

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Cosmic ray intensity variations in Antarctica

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MARTIN A. POMERANTZ and SHAKTI P. DUGGAL

Bartol Research Foundation of The Franklin Institute

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0.32J" 2 can be used to study the entry of low energy solor protons into the earth's atmosphere. The expected values of the 30- and 50-megaHertz riometer absorptions have been calculated for the larger events between June 1969 and September 1970 using data from the McDonnell Douglas charged particle experiment on the polar-orbiting OGO-6 satellite. Several times during each event the satellite passed over the McDonnell Douglas arctic and antarctic geophysical observatories. The calculated total absorption (using 2-minute averages of the data) agrees well with the measured absorption for the passes. The alpha particle and electron contributions during the events usually amount to less than a few percent of the proton absorption. However, during the large November 2, 1969, event, the electrons produced the major part of the absorption before the peak and a significant contribution during virtually its entire duration. The calculated and measured 30-megaHertz absorptions during this event are shown in fig. 2. The OGO-3 and IMP-F study was a joint effort with S. R. Kane of the University of Minnesota. The OGO-3 experiment (at the University of Minnesota) and the OGO-6 experiment were supported by the National Aeronautics and Space Administration. The McDonnell Douglas polar station program is supported by National Science Foundation contract C-393 and the McDonnell Douglas Independent Research and Development Program. We are indebted to the Canadian Government for allowing operation of the Shepherd Bay, N.W.T., Station. Reference Kane, S. R., and A. J . Masley. 1972. The Relationship Be-

tween Polar Riometer and Space Measurements during Solar Cosmic Ray Events. McDonnell Douglas Astronautics Co. Paper WD-1030. 24 p.

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The cosmic ray flux in interstellar space (galactic cosmic radiation) is known to be essentially invariant and isotropic. However, in the interplanetary region the galactic cosmic ray intensity is strongly modulated by the sun, which, in addition, spawns its own brand of energetic cosmic rays on certain occasions. The signature of the solar disturbances and the multifarious manifestations of solar activity can be identified in the resulting complicated temporal variations in the energetic particle intensity in the inner solar system. Consequently, both galactic and solar cosmic rays are eminently well adapted for investigating the solar-induced electromagnetic state of the interplanetary medium, as well as of the immediate environs of both the sun and the earth. Observations from the crucially disposed Bartol network of polar stations (South Pole , McMurdo, and Thule) have recently established that a northsouth anisotropy is a characteristic feature of every cosmic ray storm (Pomerantz and Duggal, 1972; Duggal and Pomerantz, 1971). An unusual type of north-south anisotropy was discovered last year during the cosmic ray storm of January 27, 1971 (fig. 1). In this event no reduction in intensity was observed at antarctic stations for 24 hours (period E in fig. 1)

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January, 197 I Figure 1. Nucleonic intensity at a pair of northern (N) and a pair of southern (S) stations displaying an unusual axial anisotropy. The inset shows the fit of fractional Intensity data to a straight line as predicted by a theoretical model. X represents the effective asymptotic laiude of the stations. From left to right along the abscissa, the stations are McMurdo, Dumont d'Urville, South Pole, Swarthmore, Kerguelen, Sulphur Mountain, Calgary, Deep River, Kid, Uppsala, Goose Bay, Kiruna, Inuvik, Thule. and Alert.

ANTARCTIC JOURNAL



in contrast to the arctic stations. As is shown in the inset, the fractional change in intensity A I/I versus sin X*, where X* is the effective asymptotic direction of viewing of the station, fits a straight line in accordance with the prediction by a theoretical model that quantitatively describes the mechanism responsible for this phenomenon (Nagashima et al., 1968). On January 24, 1971, just before the onset of the aforementioned storm, so-called relativistic solar cosmic rays, with energies up to approximately 5 gigaelectron-volts (GeV), were detected in a ground level event (GLE). The most prominent feature of the January 24, 1971, GLE is that it displayed a heretofore unobserved sectorial pattern of anisotropy that was limited to a narrow and stable region that was confined to the antarctic stations (Pomerantz and Duggal, in press; Duggal and Pomerantz, in press b). Two distinct solar flares, of Importance 313 and 113, with onset times differing by only 1 minute, were eligible as sources for these relativistic particles. An analysis based on the theory of anisotropic diffusion of solar particles in the interplanetary space was conducted in the hope of distinguishing between these two potential sources. The results are shown in fig. 2. The shaded bars, predicted from the analysis, indicate the interval during which the particles were actually being injected from the source. It is clear that the solar cosmic rays were not released in conjunction with the onset of either solar flare. In fact, the precise time of release of the relativistic particles corresponded closely to the maximum phase of the 313 flare. An unusual solar particle event, characterized by U.T. 0l3 , SOLAR FLARES

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References Duggal, S. P., and M. A. Pomerantz. 1971. Cosmic ray anisotropies perpendicular to the equatorial plane. Proceedings of the 12th International Conference on Cosmic Rays, 2: 723. Duggal, S. P., and M. A. Pomerantz. In press a. Relativistic solar cosmic rays from the invisible disk on September 1-2, 1971. World Data Center A, UAG Report, compiled by J. Virginia Lincoln, Boulder, Colorado. Duggal, S. P., and M. A. Pomerantz. In press b. Sectorial anisotropy of solar cosmic rays. Solar Physics. Nagashima, K., S. P. Duggal, and M. A. Pomerantz. 1968. Cosmic ray anisotropy in three-dimensional space. Planetary and Space Sciences, 16(1): 29-46. Pomerantz, M. A., and S. P. Duggal. 1972. North-south anisotropies in the cosmic radiation. Journal of Geophysical Research, 77: 263. Pomerantz, M. A., and S. P. Duggal. In press. Relativistic solar cosmic rays on January 24-25, 1971. World Data Center A, UAG Report, compiled by J. Virginia Lincoln, Boulder, Colorado.

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a source on the back side of the sun (only the third observed thus far), was recorded last year (Duggal and Pomerantz, in press a). This event (September 1-2 5 1971) was abnormal in that it represented the first arrival of energetic particles from the invisible disk in which the particle flux was distinctly and unexpectedly not isotropic. As is shown in fig. 3, the pronounced asymmetry of the intensity-time profiles at north and south pointing stations persisted for more than three hours. Analysis is still in progress to determine the various characteristics of this remarkable event, including a study aimed at ascertaining the precise location and conditions of the source of the solar cosmic rays. This work was partially supported by National Science Foundation grants GA-21175 and GV-28839.

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Figure 2. Time of maximum Tm plotted as a function of assumed injection time Ti, as theoretically predicted by an anistrophic diffusion model, on the basis of data from Thule and South Pole Stations. To is the latest time at which particles following the interplanetary magnetic field lines could have left the sun and reached the earth at the observed onset. The most probable Ti, 2320 + 0001 U.T., is determined by the overlap of the shaded bars, which represent the intervals in which the predicted Tm is within 20of the observed Tm for each station

September-October 1972

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SEPTEMBER I, 1971

Figure 3. Plot on an expanded time scale 16 minute intervals) of the solar cosmic ray intensity at northern (Thule) and southern (McMurdo) polar stations. The indicated anisotropy is completely contrary to expectation in the case of this unusual event arising from a solar cosmic ray source on the invisible disk of the sun.

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