The cosmic ray storm of July, 1961

BARTOL RESEARCH FOUNDATION MARTIN A. POMERANTZ, DIRECTOR

THE COSMIC RAY STORM OF JULY, 1961" BY S. P. DUGGAL AND M. A. P O M E R A N T Z

Since the discovery by Forbush (1) 1 25 years ago of sudden reductions in the rate at which cosmic radiation reaches the surface of the earth, many theories have been proposed for explaining events of this type which are now designated Forbush decreases or cosmic ray storms. In a typical cosmic ray storm, the sea level intensity of the nucleonic component at high latitudes decreases by about 4-10 per cent in a few hours. This is followed by a recovery phase extending over a period of several days or even longer. These decreases are generally associated with solar flares occurring about one day earlier, and with class IV radio outbursts. Thus, the reduction in intensity of the galactic cosmic radiation stems from solar-initiated phenomena. Theories describing the modulation mechanism fall into two categories: (a) geocentric theories, in which the reduction in intensity is attributed to the solid earth or its magnetic field (in this case, the extent of the region in which the intensity diminution occurs is limited to a distance of about 10 earth radii) ; (b) heliocentric theories, in which the intensity reduction is envisaged as occurring in a large volume of the inner solar system as a consequence of the action of magnetic fields ultimately of solar origin. The recent observations by means of space probe and satellite of sudden cosmic ray intensity decreases at distances as far as 8 million kilometers from the earth have revealed that the basic modulation mechanism responsible for reductions in the intensity of galactic cosmic rays is independent of the presence of the earth or of the terrestrial magnetic field (2). Some cosmic ray storms display a pattern of fine structure which may manifest itself as: (1) (2) (3) (4)

a decrease and recovery before onset; a small increase just prior to onset; temporary increases during the cosmic ray storm ; post-commencement oscillations.

It has been shown that the pre-commencement decrease represents a short-lived and highly directional anisotropy of the primary radiation (3, 4). The pre-commencement increase may be the result of a * This research was supported in part by the U. S. Air Force Geophysics Research Directorate and the National Science Foundation. The boldface numbers in parentheses refer to the references appended to this note. 32~

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gain of energy by the cosmic ray particles upon reflection from the front portion of the solar plasma, or it may be caused by the shock wave front associated with the plasma (5). Temporary increases have been ascribed to directional anisotropies, or to the reduction of the normal geomagnetic cutoff produced by magnetic storms (6, 7). Models proposed for explaining post-commencement oscillations include the possibility that trapped protons drifting in a distended solar dipole-like field periodically encounter the earth, or, alternatively, that quasi-periodic fluctuations of the magnetic field (resulting from a "ringing" behind the hydromagnetic shock front) moving with the plasma varies the maximum rigidities of protons trapped within these travelling-field regions (8). The necessity for increasing the counting rates of cosmic ray detectors in order to reduce statistical uncertainties in the investigation of small intensity fluctuations has become increasingly clear. Furthermore, the need for comparisons of data recorded at a number of stations in analyzing small intensity variations has been demonstrated (9). To conduct studies of short-time intensity variations in the polar regions, large area plastic scintillator meson telescopes have been designed in this laboratory for operation at field stations in the Arctic and the Antarctic. With counting rates of approximately 1.5 million counts per hour, the statistical standard deviation in data recorded during a one hour interval is less than 0.1 per cent. Two such instruments were in operation during July, 1961--one at McMurdo, Antarctica, and the other at Swarthmore, Pennsylvania. The latter was subsequently installed at Thule, Greenland, upon the completion of the period of test operation at Swarthmore. The month of July, 1961, was marked by a sequence of events, some of which have already been described. The injection of solar cosmic rays occurred on July 18 and July 20 (I0). A remarkable progressive rotation of the diurnal variation vector which advanced around the clock to earlier hours over a period of 8 consecutive days between July 17 and July 25 was detected by the meson telescope at Swarthmore (11). The present note reports details of the complicated fine structure in the cosmic ray intensity as recorded at the stations listed in Table I inlmediately preceding this period. Tam.E I.--Location of Stations. Station

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Coordinates Long.

Geomagnetic Lat.

Coordinates Long.

Altitude (meters)

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76.6°N 39.9°N 77.9°S

68.8°W 75.4°W 166.6°E

+88.0 ° +51.4 ° - 79.0 °

1.1 o 352.2 ° 294.3 °

260 89 48

The region of the sun designated H71 produced flares of hnportance 3 and 3q- on July 11 and July 12, respectively. Major radio outbursts

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and short-wave radio fadeouts were associated with these sudden chromospheric eruptions. Both a sudden commencement geomagnetic storm and a Forbush decrease started about 1100 U T on July 13. i

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The neutron intensity at Thule and McMurdo, and the meson intensity at M c M u r d o and Swarthmore during the period July 11-15, 1961, are plotted in Fig. 1. The cosmic ray intensity is expressed as a percentage of the pre-storm level, here defined as the mean intensity between 0000 and 0300 U T on July 13. The meson data have been corrected for pressure only (barometric coefficient = 0.22% per ram. Hg) as have the nucleonic intensities (barometric coefficient = - 0.98~o per mm. Hg). A plot of the hourly readings of the horizontal c o m p o n e n t of the geomagnetic field, It, scaled from the Fredericksburg magnetogram, is also shown in this figure. A decrease of about 2 per cent in the intensity of the meson component at Swarthmore occurred on July 12. This cannot be accounted for by changes in the geomagnetic field. Actually, the small decrease in H observed at this time is qualitatively in the wrong sense for producing a reduction in the cosmic ray intensity. This decrease in the flux of mesons observed at Swarthmore was not detected at McMurdo. On the other hand, the neutron monitor at Mina Aguilar also displayed a decrease on this day. It is therefore possible t h a t the intensity reduction on July 12 m a y be a manifestation of enhanced daily variation (11). Because of the nature of the intensity fluctuations near onset, it is difficult to specify the precise start of the cosmic ray storm. Actually, this is rather a general problem in the analysis of Forbush decreases, and renders the comparison of onset times at different locations rather ambiguous in m a n y cases. For the present purposes, the onset of the storm as observed at Swarthmore is defined as the hour after which the intensity decreased for three consecutive hours in a manner such t h a t the intensity in any one of these hours was lower by at least 2¢ than t h a t in the preceding hour. The dotted line labelled A in Fig. 1 corresponds to onset at Swarthmore, in accordance with this definition. Following onset, the record at Swarthmore is characterized by three distinct peaks. The first occurred 8 hours after onset, the second (indicated by the dotted vertical line labelled B) followed the first peak by about 14 hours, and the third (dotted line C) was reached about 20 hours after the second peak. The first peak was also observed at McMurdo. On the other hand, although there appears to be a t e m p o r a r y halt in the decrease in intensity at Thule at the time of the first peak, a significant m a x i m u m is not present. Data from a n u m b e r of other stations (cf. Fig. 3) reveal a similar pattern. The second peak (B) is evident at both polar stations, and hence, despite the fact t h a t a magnetic storm was in progress at this time, the amplitude of the increases (about 7 per cent) cannot be accounted for in terms of reduction in cutoff. The third peak (C) appears in the Thule neutron monitor record,

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b u t was not detected by either the neutron monitor or nleson telescope at M c M u r d o . No large changes in the horizontal c o m p o n e n t of the geomagnetic field were observed at Fredericksburg on July 15 at the time of this t e m p o r a r y increase (cf. Fig. 2). Hence, the difference m u s t be a t t r i b u t e d to a directional anisotropy in the modulation mechanism. Since the horizontal intensity, as plotted in Fig. 1, refers to the instantaneous values of H every hour, the intervals marked T1 and T2 have been replotted on an expanded scale in Fig. 2. Here, the instani

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taneous values of H every 15 minutes during the periods of the rising portions of peaks B and C indicate t h a t the changes were small, and hence not responsible for the t e m p o r a r y increases in cosmic ray intensity. In an earlier s t u d y of short-term increases during cosmic ray storms, it was proposed t h a t a homogeneous field of opposite direction added to the dipole magnetic field during the main phase of the magnetic storm could account for the observed increases (7). The present observations are not compatible with this model. Furthermore, if the effect were attributable to a reduction of the geomagnetic field, the increase at widely distributed stations should occur at the same universal time.

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Intensity measurements obtained with neutron monitors and meson telescopes at a number of stations between July 11 and July 15, 1961, have been plotted in Fig. 3. The dotted vertical lines A, B and C °•o 0

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FIG. 3. Cosmic ray intensities at a number of stations arranged according to geographic longitude, July 11-15, 1961. The dotted lines A, B and C correspond with those in Fig. 1.

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correspond with those in Fig. 1. Examination of the figure reveals that peaks B and C occurred earlier at European stations than at American stations. The differences are equal to the local time differentials. All of these considerations lead to the conclusion that the temporary increases B and C arise from a directional anisotropy. This may be caused by turbulent magnetic fields in the solar plasma which produced the Forbush decrease. Although a geocentric mechanism, involving an interaction between the geomagnetic field and the magnetized cloud which produces the basic modulation, is not precluded as an explanation of the temporary increases, a change-in-cutoff model appears to be untenable for describing the fine structure in the present cosmic ray storm. The observed effect could be ascribed either to holes in the modulating region (for example, location of the earth near the boundary of the plasma in which it was immersed), or to local acceleration effects.

Acknowledgments For data which are included in this paper, we wish to thank the following; D. C. Rose, H. Elliot, S. A. Korff, H. S. Ghielmetti, E. Bagge and J. P. Legrande. William W. Fairchild and James Knoten were in charge of the stations at McMurdo and Thule, respectively. Appreciation is expressed to our colleagues at neighboring Scott Base for the Antarctic magnetograms, and to R. E. Gebhardt for providing the Fredericksburg magnetograms. G. Grant Bell, currently at McMurdo, assisted with the meson telescope observations at Swarthmore. We are grateful to the Geophysics Research Directorate of the U. S. Air Force for maintaining the facility at the Geopole Station, Thule, and to the Office of Antarctic Programs, National Science Foundation, for the support of the station at McMurdo as part of the I;. S. Antarctic Research Program. REFERENCES

(1) S. E. FORBUSH, "On the Effects in Cosmic-ray Intensity Observed During the Recent Magnetic Storm," Phys. Rev., Vol. 51, p. 1108 (1937). (2) C. Y. FAn, P. MEYER AND J. A. SIMeSON, "Rapid Reduction of Cosmic Radiation Intensity Measured in Interplanetary Space," Phys. Rev. Letters, Vol. 5, p. 269 (1960). (3) A. G. FENTON, K. G. McCRACKEN, D. C. ROSE AND B. G. WILSON, " T h e O n s e t Times of Forbush-type Cosmic Ray Intensity Decreases," Can. J. Phys., Vol. 37, p. 970 (1959). (4) D. CATTAm, M. GALLI AND P. RANDI, "Diurnal Variation and Forbush Decrease," Nuovo Cimento, Vol. 6, p. 923 (1961). (5) L. I. DORMAN, "On the Energy Spectrum and Duration on Earth of a Cosmic Ray Intensity Increase Caused by Shock Wave and Albedo from the Front Magnetized Face of a Corpuscular Stream," Proc. Int. Conf. Moscow, Vol. IV, 1960, p. 134. (6) Y. L. BLOKn, L. I. DORMANAND N. S. KAMINER, "Investigations of Geophysical Phenomena Associated with the Magnetic Storm of Mid-May 1959 by Means of Cosmic Rays," I U G G Monographs (Helsinki Symposium) No. 12, 1961, p. 158.

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(7) I. KONDO, K. NAGASHIMA,S. YOSHIDA AND M. WADA, "On Worldwide Cosmic Ray Intensity Increases Associated with Cosmic Ray Storms," Proc. Int. Conf. Moscow, Vol. IV, 1960, p. 208. (18) R. L. CHASSON, "Fine Structure of Forbush Decreases," Proc. Kyoto Conference on Cosmic Rays and the Earth Storm, 1961. (9) M.A. POMERANTZ,S. P. DUGGALAND I(. NAGASHIMA,"Solar Diurnal Variation of Cosmic Ray Intensity," Proc. Kyoto Conference on Cosmic Rays and the Earth Storm, 1961. (10) M. A. POMERANTZAND S. P. DUGGnL, "Anisotropy in Solar Particles Incident on Polar Regions," JotSR. FRANKLININST., Vol. 273, p. 242 (1962). (11) S. P. DUGGALAND M. A. POMERANTZ, "Progressive Rotation of Cosmic Ray Diurnal Variation Vector," Phys. Rev. Letters, in press.