Cosmic-ray intensity variations
Cosmic-ray intensity variations M. A. POMERANTZ Bartol Research Foundation of The Franklin Institute University of Delaware Newark, Delaware 19716
Although several discoveries have resulted this year from our analytical and theoretical studies of data recorded at our polar stations during earlier years, the outstanding solar cosmic-ray event, which occurred on 16 February 1984, was overwhelmingly the most dramatic in "real time." The largest recorded magnitude (205 percent) was, of course, at South Pole Station (figure 1, left side) which houses the world's most sensitive ground-based detector of solar cosmic rays. The rise time was remarkably 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 invisible disk, we know from observations of solar radio emissions, which originate high in the corona, and hence are visible from Earth, that particle acceleration commenced at 0858 universal time. The particles observed at South Pole Station were clearly accelerated over a very short interval, since they had to propagate around one-sixth of the Sun's circumference, from about 30° beyond the limb to the foot of the magnetic field line at 60°W heliolongitude that connects the Sun to the Earth. The comparison with the corresponding record for McMurdo (figure 1, right side) reveals that the solar cosmic ray flux was exceedingly anisotropic. Finally, this event is the first for which 10-second resolution data were available, thanks to the University of Maryland recorder, to which, happily, the cosmic-ray detector was attached. We estimate that the peak flux of particles with relativistic energies (above 1 gigaelectronvolt) which reached the Earth at a location which looked along the sun-earth interplanetary mag-
(%) am
netic field line during this remarkable ground-level enhancement (GLE) was about 300-400 percent above the pre-event level. No event approaching this magnitude has occurred since 1956, when the fifth GLE exceeding 600 percent was observed. Thus the 28-year hiatus in blockbuster solar flare particle events may be nearing an end. It is also notable that the last four GLE producing flares occurred in the Sun's southern hemisphere, whereas only five out of the 33 earlier events were associated with southern hemisphere flares. If this persists, it would be indicative of a change in the Sun's internal structure. A theoretical breakthrough (Bieber and Pomerantz 1984) was achieved when analysis of the exceedingly small "steady state" north-south anisotropy, determined from the neutron monitor observations over the period 1965-1982 at McMurdo Station, Antarctica and Thule, Greenland, showed that the relative intensity difference depends upon the polarity of the interplanetary magnetic field (IMF). The higher intensity is observed at Thule when the interplanetary sector is pointing toward the sun and at McMurdo when it is pointing away. This is consistent with the origin of the effect arising from B x n drift, where B is the IMF intensity, and An is the cosmic-ray radial gradient. However, there was no indication of a dependence on the phase of the solar cycle or on the polarity of the solar poloidal magnetic field (figure 2). Modulation models in which particle drifts play a predominant role predict that the radial gradient, and hence the steady state north-south anisotropy, should differ radically between epochs of positive and negative solar polarity; however, this result shows that drifts, a highly controversial mechanism in cosmic-ray modulation theories, are not a dominant factor in the transport of 10 gigaelectronvolt cosmic rays. Further study with Australian colleagues (Jacklyn and Pomerantz 1984, in press; Jacklyn, Pomerantz, and Duldig 1984) of 27-day waves in the intensity of high energy cosmic rays detected at Mawson Station, has yielded the significant new result that there are two components to this strange effect that had originally been discovered at lower energies from analysis of the neutron monitor data from McMurdo Station (Duggal and Pomerantz 1979-a, 1979-b; Duggal et al. 1981; Jacklyn and
MOMURDO FEB. 16, 1984 (N. M.) 2 MINUTES
SOUTH POLE 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 point represents 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
+0)5
$ N 4, 4,
N
H
+0.10
(%)
+
+005 0 -0.05
-------65
-( 2NAGNN - NAGSS-NAGEE)'2 %
III
70
75
80
1.0 LOCAL DIRECTIONAL N
NAGOYA
.4
(a) w (b) BI-HEMISPHERE N MISATOUG
1..
0
b) 0
S
\y'\.,'/(b)
SURFACE -UNDERGROUND
A1=MAWUGN-.14MCMURDONM 6
(a)
MAWSONUG McMURDONM Scaled Down
Pomerantz 1983). One is a north-south asymmetry component which 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 confirms this conclusion and which it is hoped will provide a basis for theoretical understanding of this phenomenon. During the 1983-1984 season, the winter observers were David Clements and Richard Dyson at South Pole Station, and Alexander Anger at McMurdo Station. John Bieber of Bartol and Robert Jacklyn and Marc Duldig, Antarctic Division, Department of Science and Technology, Australia, have been actively involved in carrying out this work. This work was supported in part by the National Science Foundation under grant DPP 83-00544.
6
(b)
YEAR
Figure 2. Yearly average north-south anisotropy (N-s) expressed in a percentage, determined from the observations at McMurdo Station and Thule. This is an exceedingly small effect requiring sophisticated analytical procedures to extract It from the data. The 17-year average is 0.059 ± 0.006. The times of polarity reversals of the solar poloidal magnetic field are marked at the top.
.6
(a)
(b)
176 166 196 206 216 226 236 246 256 266 1982
Figure 3. In the detection of anisotroplc waves, shown as (b), the local directional and bi-hemisphere difference methods remove any isotropic variations (a) completely. The surface-underground difference method, as used here, removes selectively Forbush type variations having a p rigidity dependence. In application (right) to the period July and August 1982, anisotroplc waves of north-south asymmetry, dependent on the Interplanetary magnetic field polarity, can be seen in the two upper figures. The more pronounced waves seen In the lower figure çilve evidence of superimposed isotropic waves having a flat rigidity dependence. ("UG" denotes underground; "NM" denotes neutron monitor.)
References Bieber, J.W., and M.A. Pomerantz. 1984. Cosmic ray north-south anisotropy 1965-1980: Implications to modulation theories. Transactions of the American Geophysical Union, 65(16), 255. Duggal, S.P., and M.A. Pomerantz. 1979-a. Isotropic waves during cosmics ray storms. (Proceedings of the 16th International Cosmic Ray Conference, Kyoto, Japan, 6-18 August 1979.) Duggal, S.P., and M.A. Pomerantz. 1979-b. Enhanced isotropic cosmic ray intensity waves during cosmic ray storms. Journal of Geophysical Research, 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 17th International Cosmic Ray Conference, Paris, 13-25 July 1981.)
216
Jacklyn, R.M., and M.A. Pomerantz. 1983. Anisotropic intensity waves observed underground at Mawson with a proportional counter telescope. (Proceedings of the 18th International Cosmic Ray Conference, Bangalore, 22 August to 3 September 1983.) Jacklyn, R.M., and M.A. Pomerantz. 1984. Cosmic ray intensity waves and the north-south anisotropy. Transactions of the American Geophysical Union, 65(16) 255. Jacklyn, R.M., and M.A. Pomerantz. In press. Cosmic ray intensity waves and the north-south anisotropy. (Proceedings of the Astronomical Society of Australia, May 1984.) Jacklyn, R.M., M.A. Pomerantz, and M.L. Dulding. 1984. Waves of high energy cosmic ray intensity observed worldwide. (Proceedings of the Annual Meeting of the Astronomical Society of Australia, May 1984.)
ANTARCTIC JOURNAL
Although several discoveries have resulted this year from our analytical and theoretical studies of data recorded at our polar stations during earlier years, the outstanding solar cosmic-ray event, which occurred on 16 February 1984, was overwhelmingly the most dramatic in "real time." The largest recorded magnitude (205 percent) was, of course, at South Pole Station (figure 1, left side) which houses the world's most sensitive ground-based detector of solar cosmic rays. The rise time was remarkably 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 invisible disk, we know from observations of solar radio emissions, which originate high in the corona, and hence are visible from Earth, that particle acceleration commenced at 0858 universal time. The particles observed at South Pole Station were clearly accelerated over a very short interval, since they had to propagate around one-sixth of the Sun's circumference, from about 30° beyond the limb to the foot of the magnetic field line at 60°W heliolongitude that connects the Sun to the Earth. The comparison with the corresponding record for McMurdo (figure 1, right side) reveals that the solar cosmic ray flux was exceedingly anisotropic. Finally, this event is the first for which 10-second resolution data were available, thanks to the University of Maryland recorder, to which, happily, the cosmic-ray detector was attached. We estimate that the peak flux of particles with relativistic energies (above 1 gigaelectronvolt) which reached the Earth at a location which looked along the sun-earth interplanetary mag-
(%) am
netic field line during this remarkable ground-level enhancement (GLE) was about 300-400 percent above the pre-event level. No event approaching this magnitude has occurred since 1956, when the fifth GLE exceeding 600 percent was observed. Thus the 28-year hiatus in blockbuster solar flare particle events may be nearing an end. It is also notable that the last four GLE producing flares occurred in the Sun's southern hemisphere, whereas only five out of the 33 earlier events were associated with southern hemisphere flares. If this persists, it would be indicative of a change in the Sun's internal structure. A theoretical breakthrough (Bieber and Pomerantz 1984) was achieved when analysis of the exceedingly small "steady state" north-south anisotropy, determined from the neutron monitor observations over the period 1965-1982 at McMurdo Station, Antarctica and Thule, Greenland, showed that the relative intensity difference depends upon the polarity of the interplanetary magnetic field (IMF). The higher intensity is observed at Thule when the interplanetary sector is pointing toward the sun and at McMurdo when it is pointing away. This is consistent with the origin of the effect arising from B x n drift, where B is the IMF intensity, and An is the cosmic-ray radial gradient. However, there was no indication of a dependence on the phase of the solar cycle or on the polarity of the solar poloidal magnetic field (figure 2). Modulation models in which particle drifts play a predominant role predict that the radial gradient, and hence the steady state north-south anisotropy, should differ radically between epochs of positive and negative solar polarity; however, this result shows that drifts, a highly controversial mechanism in cosmic-ray modulation theories, are not a dominant factor in the transport of 10 gigaelectronvolt cosmic rays. Further study with Australian colleagues (Jacklyn and Pomerantz 1984, in press; Jacklyn, Pomerantz, and Duldig 1984) of 27-day waves in the intensity of high energy cosmic rays detected at Mawson Station, has yielded the significant new result that there are two components to this strange effect that had originally been discovered at lower energies from analysis of the neutron monitor data from McMurdo Station (Duggal and Pomerantz 1979-a, 1979-b; Duggal et al. 1981; Jacklyn and
MOMURDO FEB. 16, 1984 (N. M.) 2 MINUTES
SOUTH POLE 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 point represents 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
+0)5
$ N 4, 4,
N
H
+0.10
(%)
+
+005 0 -0.05
-------65
-( 2NAGNN - NAGSS-NAGEE)'2 %
III
70
75
80
1.0 LOCAL DIRECTIONAL N
NAGOYA
.4
(a) w (b) BI-HEMISPHERE N MISATOUG
1..
0
b) 0
S
\y'\.,'/(b)
SURFACE -UNDERGROUND
A1=MAWUGN-.14MCMURDONM 6
(a)
MAWSONUG McMURDONM Scaled Down
Pomerantz 1983). One is a north-south asymmetry component which 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 confirms this conclusion and which it is hoped will provide a basis for theoretical understanding of this phenomenon. During the 1983-1984 season, the winter observers were David Clements and Richard Dyson at South Pole Station, and Alexander Anger at McMurdo Station. John Bieber of Bartol and Robert Jacklyn and Marc Duldig, Antarctic Division, Department of Science and Technology, Australia, have been actively involved in carrying out this work. This work was supported in part by the National Science Foundation under grant DPP 83-00544.
6
(b)
YEAR
Figure 2. Yearly average north-south anisotropy (N-s) expressed in a percentage, determined from the observations at McMurdo Station and Thule. This is an exceedingly small effect requiring sophisticated analytical procedures to extract It from the data. The 17-year average is 0.059 ± 0.006. The times of polarity reversals of the solar poloidal magnetic field are marked at the top.
.6
(a)
(b)
176 166 196 206 216 226 236 246 256 266 1982
Figure 3. In the detection of anisotroplc waves, shown as (b), the local directional and bi-hemisphere difference methods remove any isotropic variations (a) completely. The surface-underground difference method, as used here, removes selectively Forbush type variations having a p rigidity dependence. In application (right) to the period July and August 1982, anisotroplc waves of north-south asymmetry, dependent on the Interplanetary magnetic field polarity, can be seen in the two upper figures. The more pronounced waves seen In the lower figure çilve evidence of superimposed isotropic waves having a flat rigidity dependence. ("UG" denotes underground; "NM" denotes neutron monitor.)
References Bieber, J.W., and M.A. Pomerantz. 1984. Cosmic ray north-south anisotropy 1965-1980: Implications to modulation theories. Transactions of the American Geophysical Union, 65(16), 255. Duggal, S.P., and M.A. Pomerantz. 1979-a. Isotropic waves during cosmics ray storms. (Proceedings of the 16th International Cosmic Ray Conference, Kyoto, Japan, 6-18 August 1979.) Duggal, S.P., and M.A. Pomerantz. 1979-b. Enhanced isotropic cosmic ray intensity waves during cosmic ray storms. Journal of Geophysical Research, 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 17th International Cosmic Ray Conference, Paris, 13-25 July 1981.)
216
Jacklyn, R.M., and M.A. Pomerantz. 1983. Anisotropic intensity waves observed underground at Mawson with a proportional counter telescope. (Proceedings of the 18th International Cosmic Ray Conference, Bangalore, 22 August to 3 September 1983.) Jacklyn, R.M., and M.A. Pomerantz. 1984. Cosmic ray intensity waves and the north-south anisotropy. Transactions of the American Geophysical Union, 65(16) 255. Jacklyn, R.M., and M.A. Pomerantz. In press. Cosmic ray intensity waves and the north-south anisotropy. (Proceedings of the Astronomical Society of Australia, May 1984.) Jacklyn, R.M., M.A. Pomerantz, and M.L. Dulding. 1984. Waves of high energy cosmic ray intensity observed worldwide. (Proceedings of the Annual Meeting of the Astronomical Society of Australia, May 1984.)
ANTARCTIC JOURNAL
