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Cosmic-ray intensity variations

M. A. POMERANTZ

Bartol Research Foundation of The Franklin InstituteUniversity of Delaware

Newark, Delaware 19716

Although several discoveries have resulted this year from ouranalytical and theoretical studies of data recorded at our polarstations during earlier years, the outstanding solar cosmic-rayevent, which occurred on 16 February 1984, was over-whelmingly the most dramatic in "real time." The largest record-ed magnitude (205 percent) was, of course, at South Pole Station(figure 1, left side) which houses the world's most sensitiveground-based detector of solar cosmic rays. The rise time wasremarkably rapid: the peak was attained in only 8 minutes,indicating very rapid ejection of the gigaelectronvolt protons.

Although the flare was beyond the limb on the Sun's invisibledisk, we know from observations of solar radio emissions,which originate high in the corona, and hence are visible fromEarth, that particle acceleration commenced at 0858 universaltime. The particles observed at South Pole Station were clearlyaccelerated over a very short interval, since they had to propa-gate around one-sixth of the Sun's circumference, from about30° beyond the limb to the foot of the magnetic field line at 60°Wheliolongitude that connects the Sun to the Earth. The com-parison with the corresponding record for McMurdo (figure 1,right side) reveals that the solar cosmic ray flux was exceedinglyanisotropic. Finally, this event is the first for which 10-secondresolution data were available, thanks to the University ofMaryland recorder, to which, happily, the cosmic-ray detectorwas attached.

We estimate that the peak flux of particles with relativisticenergies (above 1 gigaelectronvolt) which reached the Earth at alocation which looked along the sun-earth interplanetary mag-

SOUTH POLE FEB. 16, 1984(N. M. )2 MINUTES

netic field line during this remarkable ground-level enhance-ment (GLE) was about 300-400 percent above the pre-eventlevel. No event approaching this magnitude has occurred since1956, when the fifth GLE exceeding 600 percent was observed.Thus the 28-year hiatus in blockbuster solar flare particle eventsmay be nearing an end.

It is also notable that the last four GLE producing flares oc-curred in the Sun's southern hemisphere, whereas only five outof the 33 earlier events were associated with southern hemi-sphere flares. If this persists, it would be indicative of a changein the Sun's internal structure.

A theoretical breakthrough (Bieber and Pomerantz 1984) wasachieved when analysis of the exceedingly small "steady state"north-south anisotropy, determined from the neutron monitorobservations over the period 1965-1982 at McMurdo Station,Antarctica and Thule, Greenland, showed that the relative in-tensity difference depends upon the polarity of the interplane-tary magnetic field (IMF). The higher intensity is observed atThule when the interplanetary sector is pointing toward thesun and at McMurdo when it is pointing away. This is consistentwith the origin of the effect arising from B x n drift, where B isthe IMF intensity, and An is the cosmic-ray radial gradient.However, there was no indication of a dependence on the phaseof the solar cycle or on the polarity of the solar poloidal magnet-ic field (figure 2). Modulation models in which particle driftsplay a predominant role predict that the radial gradient, andhence the steady state north-south anisotropy, should differradically between epochs of positive and negative solar polarity;however, this result shows that drifts, a highly controversialmechanism in cosmic-ray modulation theories, are not a domi-nant factor in the transport of 10 gigaelectronvolt cosmic rays.

Further study with Australian colleagues (Jacklyn andPomerantz 1984, in press; Jacklyn, Pomerantz, and Duldig 1984)of 27-day waves in the intensity of high energy cosmic raysdetected at Mawson Station, has yielded the significant newresult that there are two components to this strange effect thathad originally been discovered at lower energies from analysisof the neutron monitor data from McMurdo Station (Duggaland Pomerantz 1979-a, 1979-b; Duggal et al. 1981; Jacklyn and

MOMURDO FEB. 16, 1984(N. M.)2 MINUTES

Figure 1. (Left) The ground-level enhancement of 16 Febraury 1984, as observed by the neutron monitor at South Pole Station. Each data pointrepresents counts accumulated in 2 minutes. (Right) Same for McMurdo, showing the extremely anisotropic character of this event. ("N.M."denotes neutron monitor; "UT" denotes universal time.)

1984 REVIEW 215

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Figure 2. Yearly average north-south anisotropy (N-s) expressed ina percentage, determined from the observations at McMurdo Stationand Thule. This is an exceedingly small effect requiring sophisti-cated analytical procedures to extract It from the data. The 17-yearaverage is 0.059 ± 0.006. The times of polarity reversals of the solarpoloidal magnetic field are marked at the top.

Pomerantz 1983). One is a north-south asymmetry componentwhich exhibits reversal of phase between the two hemispheres,while the other is an isotropic component of constant phase.Figure 3 illustrates the nature of the analysis which confirmsthis conclusion and which it is hoped will provide a basis fortheoretical understanding of this phenomenon.

During the 1983-1984 season, the winter observers wereDavid Clements and Richard Dyson at South Pole Station, andAlexander Anger at McMurdo Station. John Bieber of Bartol andRobert Jacklyn and Marc Duldig, Antarctic Division, Depart-ment of Science and Technology, Australia, have been activelyinvolved in carrying out this work. This work was supported inpart by the National Science Foundation under grant DPP83-00544.

References

Bieber, J.W., and M.A. Pomerantz. 1984. Cosmic ray north-southanisotropy 1965-1980: Implications to modulation theories. Transac-tions of the American Geophysical Union, 65(16), 255.

Duggal, S.P., and M.A. Pomerantz. 1979-a. Isotropic waves duringcosmics ray storms. (Proceedings of the 16th International CosmicRay Conference, Kyoto, Japan, 6-18 August 1979.)

Duggal, S.P., and M.A. Pomerantz. 1979-b. Enhanced isotropic cosmicray intensity waves during cosmic ray storms. Journal of GeophysicalResearch, 84(12), 7382-7385.

Duggal, S.P., R.M. Jacklyn, M.A. Pomerantz, and C.H. Tsao. 1981.Cosmic ray intensity waves at high rigidities. (P'oceedings of the 17thInternational Cosmic Ray Conference, Paris, 13-25 July 1981.)

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176 166 196 206 216 226 236 246 256 2661982

Figure 3. In the detection of anisotroplc waves, shown as (b), thelocal directional and bi-hemisphere difference methods remove anyisotropic variations (a) completely. The surface-underground dif-ference method, as used here, removes selectively Forbush typevariations having a p rigidity dependence. In application (right) tothe period July and August 1982, anisotroplc waves of north-southasymmetry, dependent on the Interplanetary magnetic field polarity,can be seen in the two upper figures. The more pronounced wavesseen In the lower figure çilve evidence of superimposed isotropicwaves having a flat rigidity dependence. ("UG" denotes under-ground; "NM" denotes neutron monitor.)

Jacklyn, R.M., and M.A. Pomerantz. 1983. Anisotropic intensity wavesobserved underground at Mawson with a proportional counter tele-scope. (Proceedings of the 18th International Cosmic Ray Con-ference, Bangalore, 22 August to 3 September 1983.)

Jacklyn, R.M., and M.A. Pomerantz. 1984. Cosmic ray intensity wavesand the north-south anisotropy. Transactions of the AmericanGeophysical Union, 65(16) 255.

Jacklyn, R.M., and M.A. Pomerantz. In press. Cosmic ray intensitywaves and the north-south anisotropy. (Proceedings of the Astro-nomical Society of Australia, May 1984.)

Jacklyn, R.M., M.A. Pomerantz, and M.L. Dulding. 1984. Waves of highenergy cosmic ray intensity observed worldwide. (Proceedings of theAnnual Meeting of the Astronomical Society of Australia, May 1984.)

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