High frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future

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1 SPACE WEATHER, VOL. 5,, doi: /2006sw000295, 2007 High frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future K. G. McCracken 1 Received 12 October 2006; revised 23 January 2007; accepted 25 January 2007; published 26 July [1] It is shown that the >4 GeV fluence of large solar energetic particle events was a factor of 10 greater and the frequency of occurrence a factor of four greater prior to 1958 than during the space era. There were two events in 1946 and 1949 with >4 GeV fluences similar to that of 23 February 1956, suggesting that the >4 GeV fluence of the largest probable event for a similar period would be 3 times greater. The historic cosmic ray and glaciological records indicate that the fluences and probability of occurrence of such large events at both high and low energies are greatest in periods of low long-term solar activity and anticorrelated with the estimated strength of the heliospheric magnetic field. A working model is proposed where the factors controlling the occurrence of large SEP events are (1) the occurrence of a large solar flare or CME and (2) the heliomagnetic field being <6.5 nt near Earth. A theoretical basis for this model is discussed. It is proposed that the next Gleissberg minimum of solar activity will lead to a repetition of the pre-1958 era of high-frequency, high-fluence GLE and SEP events. Two predictive indicators of higher SEP activity are proposed: (1) the Climax neutron monitor rate at solar minimum rises >5% compared to 1954 and (2) the measured heliomagnetic field near Earth remains <6.5 nt for 2 years into a new solar cycle, or alternatively it is substantially less than 5 nt at sunspot minimum. Citation: McCracken, K. G. (2007), High frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future, Space Weather, 5,, doi: /2006sw Introduction [2] The solar energetic particle (SEP) event is a prominent component of space weather. The particle fluences can be very high; substantial fluxes of relativistic ions are sometimes produced; and large fluxes can be attained with very little warning (<1 hour). Over the course of the space era, it has been recognized that many large SEP events cause substantial degradation of satellite assets, and possibly pose a threat to astronauts and to the crews and passengers of high flying aircraft in the polar caps of the Earth. Discussions of the likely exposures are frequently made in terms of two extreme events ; the ground level enhancement (GLE) of 23 February 1956 and the multiple events of August The energy spectra of these two events were sharply different; the former had a hard spectrum and resulted in exceptional fluences at high energies (>500 MeV) but a relatively small fluence at low energies. The latter had a soft spectrum; the 1 Institute for Physical Science and Technology, University of Maryland, College Park, Maryland, USA. fluence was small >500 MeV, but the >30 MeV fluence was exceptionally large [Shea and Smart, 1990]. [3] This paper commences by examining the occurrence of the SEP events that produce relativistic cosmic rays observed at the surface of Earth (GLE). It examines, in particular, the GLE that exhibit significant fluences at energies >4 GeV. It shows that the GLE had high-energy fluences that were a factor of 10 greater and occurred with a four-fold greater frequency in the 20 years prior to the space era than from 1958 to date. It then reviews evidence from the glaciological record that shows that the frequency of occurrence of SEP events with high >30 MeV fluences was up to a factor of six times greater in the past, compared to the space era. These independent observations at widely separated energies lead to the conclusion that the frequency of occurrence of large SEP events is smallest for the most active solar cycles. A theoretical basis is outlined that is consistent with this anticorrelation. [4] Given that SEP events are presently a significant cause of deterioration in space assets and of concern to humans at high altitudes, it is clear that a factor of 10 increase in fluence and a higher frequency of occurrence Copyright 2007 by the American Geophysical Union 1of7

2 may have serious consequences. The paper concludes with a discussion of several observations that may provide advance warning that the intensities, and frequencies of occurrence of large SEP events may be returning to the situation evident prior to Instrumental Record of Ground Level Enhancements (GLE) [5] In 1936, the Carnegie Institution of Washington established continuously recording ionization chambers in the USA, New Zealand, Peru, Mexico, and Greenland. The first recognized GLE were observed on 28 February and 7 March Two more were observed on 25 July 1946 and 19 November 1949 [Forbush, 1946; Forbush et al., 1950]. [6] Neutron monitors were first introduced in 1951 and a substantial number were in operation when the next large GLE of the ionization chamber era occurred on 23 February [Meyer et al., 1956]. Comparison demonstrated that a neutron monitor is a factor of 20 times more sensitive to a GLE than an ionization chamber. Two GLE in November 1960 ( % in the neutron data) were not seen (<2%) by the ionization chamber network, and the majority of ionization chambers were shut down over the following decade without any more GLE being seen by them. The neutron monitor network has observed 70 GLE since 1956; however, all but three were one or two orders of magnitude less intense than the GLE of 23 February The comprehensive ionization chamber and neutron monitor data for the GLE of 23 February 1956 provides an intercalibration between the ionization and neutron monitor records of GLE, and threshold levels were further refined using the data obtained during the GLE in November 1960 [Steljes et al., 1961]. [7] The ionization chamber was primarily sensitive to cosmic rays of energy >4 GeV. The >4 GeV fluence is defined as the time integral of the increase in intensity observed by an ionization chamber near sea level and was computed for all the GLE observed up until 1960 by the ionization chamber network. It is now well known that the cosmic radiation is often highly anisotropic during the first hour of a GLE [e.g., Lovell et al., 1998]; however, for the purposes of this paper, the >4 GeV fluences were averaged over all the ionization chambers that saw the GLE. The >4 GeV fluences may therefore be underestimated by 20%. The >4 GeV fluences (in percent increasehours) were then normalized to that observed for the GLE of 23 February 1956 and plotted in Figure 1a. [8] Comparison of the neutron monitor and ionization chamber data from the GLE of 1956 and 1960 has allowed the equivalent ionization chamber >4 GeV fluences to be estimated for the GLE from 1960 to the present, as are also plotted in Figure 1a if the relative >4 GeV fluence is >10%. Several muon telescopes were operating at the time of the large GLE of 29 September 1989 [e.g., Lovell et al., 1998], and these provide an independent estimate of the equivalent ionization chamber response. The intercalibration also provided the equivalent neutron monitor fluence scale (geomagnetic cutoff of 1 GV) given in Figure 1a. [9] There are few in the space community with personal experience of the Carnegie Institution ionization chambers, and some may query their accuracy and long-term stability. The ionization chamber used no electronics; it did not use gas multiplication to increase the ionization current; it used a balanced configuration designed to minimize the influence of environmental conditions and changes in operating voltages. It used an electrostatic galvanometer and a light beam lever to record its data on a photographic strip with a time resolution of 1 min. The station diaries (recently examined by the author who at one time built and operated a similar ionization chamber) indicate that meticulous records were taken of the operating parameters and the timing errors between the recorder clock and Universal Time. All of these factors contribute to a high degree of confidence in the reliability and stability of the instruments. Forbush did not publish any results unless the GLE was observed by all high latitude ionization chambers that were operating at the time. It is arguable that the Carnegie Institution ionization chambers were as stable as any of the modern cosmic ray instruments; with the exception that some of the chambers appear to have been contaminated with a radioactive isotope with a half life of order several decades [Forbush, 1958]. While this affects their use in study of the long-term changes in the galactic cosmic rays, it has no impact upon studies of the short lived GLE. As a further check, the author has recently examined and rescaled all of the original photographic records of the GLE observed by Forbush and confirmed the results published in the 1940s and 1950s. This examination also showed that there was a previously unreported response to the GLE of 4 May 1960 (>4 GeV fluence a hundredth of that for 23 February 1956), and no significant response to any of the GLE in November Intensity and Frequency of Occurrence of GLE, [10] The >4 GeV fluences of the GLE prior to the sunspot maximum of 1958 were up to a factor of 10 greater than those after that date, as shown in Figure 1a. It is to be noted, further, that the greatest >4 GeV fluence was for the GLE of 25 July 1946 and that the >4 GeV fluence for the event of 19 November 1949 approximated that of 23 February That is, the GLE of 23 February 1956 cannot be regarded as an exceptional event, as is sometimes assumed in space weather discussions. [11] There were five large GLE in the 22-year period , yielding a frequency of occurrence of 0.23 per year (Figure 1a). For the period there were three; indicating a frequency of 0.06 yr 1. Using binomial statistics, the observed inequality of occurrence has a probability of 0.03 of occurring by chance. Figure 1a therefore shows that both the >4 GeV fluences and the 2of7

3 Figure 1. The occurrence of large ground level enhancement (GLE) and their correlation with other data, (a) The GLE that produced >4 GeV fluences equal to >1/10th of that produced by the GLE of 23 February The >4 GeV fluence of the later is taken as 100%. The neutron fluence is the estimated integral of the percentage increase over time based on the intercalibration of ionization chambers and neutron monitors as discussed in the text. (b) The observed ( ; connected line) and estimated Climax neutron monitor counting rate ( ; small circles) normalized to 100% in August The times of occurrence of the GLE in Figure 1a are indicated by the large spots, where their Y coordinate is the estimated Climax counting rate when they occurred (two spots superimposed in 1942). (c) The estimated averages of the strength of the heliomagnetic field near Earth, obtained by inversion of the Climax data in Figure 1b. The large spots are the estimated field strengths at the time of the large GLE. (d) The international sunspot number. frequency of occurrence of GLE were significantly greater prior to 1958 than has been observed throughout the space era. [12] Figure 1b presents the observed Climax neutron monitor data and the estimated values for , based on the balloon-borne ionization observations of Neher [1971, and references therein] and the ground level ionization measurements of Forbush [1958]. The estimation process used the well inter-calibrated Neher ionization measurements to compensate for the long term drifts in the Forbush data as described by McCracken and Heikkila [2003] and further refined by McCracken and Beer [2007]. [13] Note that the estimated Climax counting rate decreased substantially between the sunspot minima of 1933 and The independent measurements of the concentration of 10 Be in polar ice confirm this decreasing trend [McCracken et al., 2004a]. Note that all five of the large GLE prior to 1958 occurred when the estimated Climax neutron monitor counting rate was >95% of the highest rate 3of7

4 observed at the solar minimum of That is, all five occurred at times when the modulation of the galactic cosmic radiation (GCR) was weak compared to that during the periods of solar activity after Note also the small amplitudes and infrequent occurrence of >4 GeV GLE after 1958, when the Climax counting rate was consistently <95% except near sunspot minimum. In particular, there were no large >4 GeV GLE near the five sunspot maxima after 1954, despite the frequent occurrence of intense solar flares and CME at those times. In summary, large >4 GeV GLE have avoided the periods of strongest cosmic ray modulation in the space era, while occurring during the earlier solar cycles that exhibited higher GCR intensities (i.e., less modulation). The modulation of the GCR is determined by the strength, turbulence, and configuration of the HMF, and we now investigate whether there is an interrelation between the occurrence pattern of the >4 GeV GLE, and the strength of the HMF. [14] In recent years, three independent methods have been developed to estimate the secular changes in the heliomagnetic field over time. Lockwood et al. [1999] used a method based on changes in the characteristics of the short-term disturbances in the geomagnetic field for the interval Wang et al. [2000], Solanki et al. [2002], and Schrijver et al. [2002] used forward models of the magnetohydrodynamic transport of magnetic fields from the Sun s active regions based on the historic sunspot record, Caballero-Lopez et al. [2004] and McCracken [2007] used an inversion of the observed cosmic ray record based on the cosmic-ray transport equation [Parker, 1965; Jokipii, 1991]. Figure 2b presents the 11-year running averages of the results of three of these studies, and good overall agreement is evident. Thus the low values near 1890 (and 1810 for two of them), higher values in the space age compared to the period of high solar activity between 1840 and 1875, and the rapid increase between 1933 and 1954 are evident in all three. In particular, each indicated an approximately twofold increase in the field between the vicinity of 1900 and 1954 onward. The complete independence of the three procedures and their overall agreement provides the basis for the following comparison of the occurrence of large GLE with the estimated HMF. [15] Figure 2b shows that the 11-year average of the HMF near Earth was 3.0 nt near 1900 AD (a period of low peak sunspot numbers; see Figure 2c), rising to >7.0 nt from 1964 onward. Examination of Figure 1c shows that all the large GLE prior to 1958 occurred when the estimated strength of the HMF near Earth was less than 6.5 nt. Thus the estimated HMF was less than 6.5 nt for the majority of the two solar cycles , and four large GLE occurred at times ranging throughout those solar cycles; i.e., throughout the period of highest solar activity After 1954, the HMF was in the range 7--9 nt during the most active periods of the five solar cycles up to 2005 and despite the frequent occurrence of many large solar flares and CME, no large >4 GeV GLE were observed. The only large >4 GeV GLE in the 50 year period occurring in 1956 while the estimated HMF was still near its sunspot minimum value. In summary, the evidence suggests that large >4 GeV GLE occur preferentially when the HMF strength is less than 6.5 nt near Earth. 4. Evidence From the Glaciological Record [16] McCracken et al. [2004b, and references therein] have shown that thin layers of nitrate in polar ice record the occurrence of large SEP events. They further showed that the probability of occurrence of an SEP event with 30 MeV fluence > cm 2 is anticorrelated with the long term level of solar activity. For example, while there were only three SEP events with 30 MeV fluences > cm 2 between 1960 and 2000, there were five in the five year interval The >30 MeV fluence of each was comparable to or greater than the multiple SEP events of August 1972, often regarded as the benchmark for large SEP events. In summary, they showed that the fluences and frequency of occurrence of large SEP events was substantially greater in the vicinity of 1900, decreasing thereafter to the lower values observed during the space era. They further demonstrated an inverse correlation between the occurrence of SEP events and the estimated strength of the HMF. That is, a similar inverse correlation is evident for GLE (Figure 1a) and the SEP event data derived from the glaciological record. [17] Figure 2 further displays this anti-correlation. Figure 2a presents the observed and estimated Climax neutron monitor rate, Figure 2b displays the secular change in the HMF during this period estimated using the three independent methods discussed above. The occurrence of SEP events with 30 MeV fluences > cm 2 is also given in the second panel (data from McCracken et al. [2001] and Shea and Smart [1990]). Comparison of Figures 2a and 2b shows that a higher occurrence of SEP is associated with higher GCR intensities (Figure 2a) and less intense HMF. In particular, there was a substantial decrease in the frequency of occurrence of large SEP events following the 50% increase in the HMF between 1933 and Discussion [18] Satellite and other measurements have shown that the SEP energy spectrum varies considerably from event to event. In general, SEP events that originate on the western portion of the solar disk have hard spectra; while the fluences are large at relativistic energies, they may be quite small at low energies. By way of contrast, SEP events that originate near the center of the solar disk have soft spectra and may be seen only at low energies (<100 MeV). The GLE and glaciological evidence therefore refers to two different aspects of the SEP events observed at Earth. For example, the GLE of 20 January 2005 was one of the largest 4of7

5 Figure 2. The occurrence of solar energetic particles (SEP) events with >30 MeV fluences > cm 2. (a) The observed ( ; connected line) and estimated Climax neutron monitor counting rate based on the ionization chamber ( ; small open circles), and cosmogenic 10 Be ( ; small filled circles) record, normalized as in Figure 1b (see text). (b) The 11-year running averages of the strength of the heliomagnetic field near Earth estimated using three independent methods. The occurrence of SEP events with >30 MeV fluences > cm 2 is given at the bottom of Figure 1b. (c) The international sunspot number. in the past 50 years at relativistic energies, while it was small in the vicinity of 30 MeV. [19] Together, the forgoing sections show that the occurrence of SEP events at both relativistic and low energies is anticorrelated with the strength of the HMF near Earth. We now consider the implications of these observed correlations in terms of the dependence of particle acceleration upon magnetic field strength, previously discussed by McCracken et al. [2004b]. [20] Wang and Sheeley [2002] have shown that the magnetohydrodynamic transport of magnetic field from the active sunspot regions of the Sun determines the strength of the HMF throughout the heliosphere. Consequently, the strength of the HMF at Earth is determined by the strength of the coronal magnetic field. This in turn determines the magnitude of the Alfven speed in the corona, according to V A = B/(4pn 0 M) 0.5, where n 0 is the plasma number density, and M the mean mass number of the plasma particles. The lower estimated HMF prior to 1954 and near 1900 evident in Figures 1 and 2 therefore implies lower Alfven speeds in the corona prior to the space age. For the purposes of this paper, the acceleration of SEP is assumed to occur in a supersonic shock wave, initiated by either a solar flare, or a coronal mass ejection (CME). The efficiency of acceleration of charged particles in supersonic shocks is strongly dependent on the compression ratio in 5of7

6 the shock [e.g., Jones and Ellison, 1991], which is in turn determined by the Alfven Mach number of the shock. Thus the Alfven Mach number of a supersonic shock of velocity V will be greater when the coronal magnetic fields are of low strength, decreasing as the coronal field increases. Thus the efficiency of acceleration of SEP is expected to have decreased as the strength of the coronal field, and hence the HMF, increased between 1900 and 1954 [McCracken et al., 2004b]. [21] The following working model is therefore proposed to account for the observation of the large >4 GeV fluence SEP events shown in Figure 1. It postulates that the frequency of occurrence and the fluences produced by SEP events are strongly influenced by two factors: (1) the frequency of occurrence of large solar flares or large CME and (2) the acceleration efficiency of the supersonic shocks they produce. Webb and Howard [1994] have shown that the rates of occurrence of CME and solar flares are proportional to the sunspot number. However as solar activity increases (i.e., the sunspot number increases), the direct satellite observations, and the several independent estimates discussed previously show that the strength of the HMF also increases, and in the present era, quickly exceeds a value of 6.5 nt near Earth, above which the model postulates that the higher Alfven velocities have reduced the compression ratios, and significant >4 GeV fluxes are no longer produced. Reference to the HMF in Figure 1c shows that during the space age the HMF was <6.5 nt for 1 year at the start of the solar cycle and 2 year at the end. The low sunspot numbers and low frequency of occurrence of solar flares and CME during these periods therefore implies a low probability for the occurrence of very large SEP events. By way of contrast, Figure 1c shows that the estimated HMF was <6.5 nt for the majority of the sunspot cycle and for all the previous cycle. That is, on the basis of the working model, the combination of frequent solar flares and CME near sunspot maximum, and low Alfven velocities, would imply that substantial >4 GeV fluences would be produced by the largest solar events during those two cycles. Clearly, the working model is a simplification of the situation; other factors, such as the plasma density and the spatial dimensions of the shock, will also influence the acceleration process as well. [22] Figure 2c shows that solar activity (as indicated by peak sunspot number) was relatively low in the periods and ; these being the minima of the 80-year Gleissberg quasi- periodicity in solar activity. The Gleissberg cycle is persistent in the historical sunspot record and in the cosmogenic 10 Be and 14 C records, suggesting that the next Gleissberg minimum after that of could occur in the relatively near future. Svalgaard and Cliver [2005] have made an equivalent prediction based on the variation in the strength of the Sun s polar fields over the past four solar cycles. Figure 2a, and the several estimates of the HMF discussed previously and given in Figure 2b suggests that the Climax neutron monitor counting rate will then rise to the vicinity of 115% (as in 1900 in Figure 2a), and the 11-year average strength of the HMF decrease to 3 nt. The experimental evidence in Figures 1 and 2, and the working model, imply that there would then be a repetition of the era of high >4 GeV fluence SEP events observed between 1936 and 1958, at a similar high frequency of occurrence. It is noted that the historical record shows that the frequency of occurrence of large SEP events began to increase in the 1 or 2 cycles prior to the Gleisberg minimum, for example (Figure 2b) and [McCracken et al., 2001]. 6. Predictive Indications of a Rise in SEP or GLE Event Fluence and Frequency of Occurrence [23] On the basis of the observations in Figures 1 and 2, and the working model, the onset of a period of higher GLE fluences and frequency of occurrence such as prior to 1958 is likely to be heralded by the following observations [24] 1. That the GCR intensity at sunspot minimum measured by the Climax neutron monitor increases to >105% with respect to the value in This would then approximate the situation during the sunspot cycle after the sunspot minimum of 1944, when two large SEP were observed at times well removed from the sunspot minima. Should the Climax neutron count reach 115% as estimated for the vicinity of 1900 AD, the >4 GeV fluences, and GLE frequency, may become considerably greater than in the period [25] 2. The strength of the HMF near Earth remains below 6.5 nt for 2 years into a new solar cycle or falls below 6.5 nt a number of years prior to the end of the cycle. Alternatively, that the HMF at sunspot minimum is substantially below 5 nt. [26] In both of the above cases, the historical evidence and the working model indicate that there would be a significant probability that a large solar flare or CME would be produced while the coronal magnetic fields were low, approximating the situation that yielded the high >4 GeV fluence GLE observed during the sunspot cycles Depending on geometrical factors (i.e., where the SEP event occurred on the Sun with respect to Earth), the result could be a high >4 GeV fluence event or one with a high fluence at low energies (>30 MeV) but little at relativistic energies. 7. Conclusions [27] It has been shown that the fluences and frequency of occurrence of large SEP have been substantially lower during the space age than during the two sunspot cycles , and during the Gleissberg minimum in the vicinity of During the interval the >4 GeV fluences were a factor of ten greater than during the space era, and the frequency of occurrence 4 times greater. The evidence indicates that these properties are anticorrelated with long-term solar activity, the fluences and frequency 6of7

7 of occurrence of SEP being higher in periods of relatively low cycles of solar activity (e.g., near the Gleissberg minima). [28] Further, it has been shown that the GLE of 23 February 1956 was not the largest in the instrumental record at relativistic energies and that there were three GLE in the interval with >4 GeV fluences equal or greater than observed in that event. Using those observations, and the cumulative probability curve in the work of McCracken et al. [2001], indicates that the probability of occurrence of a three-fold larger >4 GeV GLE in a similar 22 year period would be 0.3 and therefore of practical significance. [29] The historical record indicates that another period of low solar activity such as the Gleissberg minimum of could occur in the near future. When this occurs, the experimental evidence indicates that there could be a ten-fold increase in the >4 GeV fluence, and a four to six fold increase in the frequency of occurrence of large SEP at both high and low energies, compared to the space era. This would cause substantial degradation of satellite assets and possibly pose a threat to astronauts and to the crews and passengers of high flying aircraft in the polar caps of the Earth. [30] On the basis of the cosmic ray record, and our understanding of the heliomagnetic fields, and their modulation of the galactic cosmic radiation, two predictive indicators of enhanced SEP activity are proposed. They are (1) when the Climax neutron monitor counting rate during sunspot minimum exceeds 105% (relative to that of August 1954); and (2) when the observed strength of the HMF near Earth is 6.5 nt at a time of modest to high solar activity, or alternatively, that the HMF at sunspot minimum is substantially less than 5 nt. [31] Acknowledgments. This work was funded by NSF grant ATM The Climax neutron monitor data was supplied by the University of New Hampshire, supported by NSF grant ATM The Carnegie Institution of Washington and Shaun Hardy are thanked for providing access to the original photographic records from their ionization chamber network. F.B. McDonald, M.A. Shea, and D.F. Smart provided important advice and critical comment. References Caballero-Lopez, R. A., H. Moraal, K. G. McCracken, and F. B. McDonald (2004), The heliospheric magnetic field from 850 to 2000AD inferred from 10 Be records, J. Geophys. Res., 109, A12102, doi: /2004ja Forbush, S. E. (1946), Three unusual cosmic-ray increases possibly due to charged particles from the Sun, Phys. Rev., 70, Forbush, S. E. (1958), Cosmic ray intensity variations during two solar cycles, Geophys. Res., 63, Forbush, S. E., T. B. Stinchcomb, and M. Schein (1950), The extraordinary increase of cosmic ray intensity on November 19, 1949, Phys. 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