Pergamon Adv. Space Res. Vol. 14, No. 10, pp. (10)631-(10)638, 1994

1994 COSPAR Printed in Great Bntain. All rights reserved.

0273-1177/94 $7.00 + 0.00

SIGNIFICANT PROTON EVENTS OF SOLAR CYCLE 22 AND A COMPARISON WITH EVENTS OF PREVIOUS SOLAR CYCLES

M. A. Shea and D. F. Smart

Space Physics Division, Geophysics Directorate/PL, 29 Randolph Road, Hanscom AFB, Bedford, MA 01731-3010, U.S.A.

ABSTRACT

The sun has produced several high energy and large fluence solar proton events during solar cycle 22. This recent activity is similar to activity that occurred in the 19th solar cycle before the advent of rou- tine space measurements. In a review of the recent events and a comparison with significant solar pro- ton events of previous solar cycles, it appears that the 20th and 21st solar cycles were deficient in the total fluence of solar particles as detected at the earth. Therefore, when models of maximum solar proton fluxes to be encountered for deep space missions are developed, solar proton data acquired dur- ing the present solar cycle should be incorporated.

HISTORICAL BACKGROUND

The knowledge that solar-initiated processes can accelerate particles to energies greater than a few MeV has been known for almost 50 years. Over the years these solar particle events have been referred to by a number of descriptive names such as solar cosmic ray events, solar proton events, groundqevel events, polar cap absorption events, and solar electron events. Each of these names was associated with a particular detection technique.

The first instances where the sun was unambiguously identified as the source of particles detected at the earth were on 28 February and 7 March 1942. However, it was not until the 25 July 1946 ground-level event that Forbush /I/ published both these and his earlier measurements of the 1942 events suggesting that these sudden increases were associated with the emission of energetic particles by a solar flare/2/. This suggestion was viewed with skepticism; however, the event of 19 November 1949, measured by both an experimental neutron monitor in Manchester, UK as well as by ionization chambers, gave re- spectable scientific credence to the explanation of solar flare accelerated particles detected on the earth /3-5/.

The initial observations of "solar cosmic rays" relied on ground-based measurements of secondary par- tides generated at the "top" of the earth's atmosphere. Ground-based ionization chambers or muon de- tectors respond to the muons generated by incident protons above - 4 GeV. The development of the neutron monitor lowered the detection threshold to ~ 500 MeV protons at high latitude locations. During the International Geophysical Year (IGY), the cosmic radiation flux through and above the at- mosphere was recorded by balloon payloads; later these detectors were adapted for the initial earth-or- biting satellites. Concurrently, it was discovered that the newly developed riometer was very sensitive to particle energy deposition in the ionosphere directly above the instruments. Although neutron mon- itors recorded the most energetic solar particle events during the 19th solar cycle, most of the solar particle fux and fluence data available from the 19th solar cycle were derived from "proxy" ionospheric measurements in the earth's polar regions. Figure 1 gives a conceptual history of the availability and energy threshold of the various techniques used to detect solar cosmic-ray events since 1933.

(10)631

(10)632 M.A. Shea and D. F. Smart

I 0 0 0 0

~m I 0 0 0

)- o n,'

z I 0 0 btJ

IO

M U O N

NEUTRON MONITORS

B A L L O O N S . ,

SATELLITES

(.9

W Z bd

I0

IONOSPHERIC

DETECTION T E C H N I Q U E S

1930 1940 1950 1960 1970 1980

YEAR

Fig. 1. Conceptual history of the detection thresholds of solar cos- mic ray events. The thickness of the lines indicates the relative number of each type of detector in use. The differences in shading in the ionospheric section indicates changes in detection technique.

The major ground-level event of 23 February 1956, recorded by the world wide cosmic ray network being established for the IGY, resulted in considerable interest in these relatively unusual events. However, at the end of the IGY and the IGC (1957-1959) with only seven relativistic solar proton events reported since 1942, our knowledge of these events was extremely limited as illustrated in Figure 2. At that time the folklore assumed that major solar proton events were associated with major flares on the western limb of the sun; major flares on the eastern hemisphere of the sun, as viewed from the earth, were not considered to be producers of major proton events at the earth.

Fig. 2. Artist's concept of the limited knowledge of solar proton events during the 1950's. The directional sign indicates a flare on the eastern or western hemisphere of the sun (as viewed from the earth). Major relativistic solar proton events were assumed to originate from large flares on the western hemisphere of the sun.

Significant Proton Events of Solar Cycle 22 (l 0)633

Throughout the next two solar cycles, from the start of the 20th cycle in November 1964 to the end of the 21st solar cycle in September 1986, our knowledge of solar proton events greatly increased, partic- ularly with almost continuous measurements of lower energy solar protons on earth-orbiting satellites. These measurements, together with the "proxy" measurements of the 19th solar cycle, were combined to assemble a data base of solar proton events at the earth/6/. The criteria used for identification of these "significant" events were a peak flux > 10 protons/cm2-sec-ster for protons above 10 MeV. The results of this study indicated that although significant solar proton events occurred between the second and eighth year of the solar cycle, there was no repeatable pattern of the occurrence of solar proton events from one cycle to the next.

SOLAR PROTON EVENTS IN SOLAR CYCLE 22

Number of Events

The 22nd solar cycle started in October 1986 with a rapidly rising sunspot number t o a maximum in July 1989. By this time there had been one major solar proton event, in March 1989, and several other lesser events which still qualified for the "significant event" criteria/6/. Starting in July 1989 and con- tinuing for the next two years, the sun produced several episodes of solar activity, the aggregate of which exceeded anything recorded since the space era began. Eleven relativistic solar particle events were recorded within an eleven month period including the largest event since 23 February 1956.

Using the same criteria as used for s o l a r cycles 19-21, the preliminary total number of events for the first five years of solar cycle 22 already exceeds the total number of events for the 20th solar cycle and is only a few events less than the total number recorded during the 21st cycle. Figure 3 illustrates the number of these events as a function of each 12-month period within each solar cycle from solar cycle 19 to the present time. The start of each solar cycle was selected as the month after the minimum in the smoothed sunspot number/7/. The large number of events during the third year of the 22nd solar cycle is clearly evident from this figure.

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I 5 I0 YEARS FROM SUNSPOT MINIMUM

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YEARS FROM SUNSPOT MINIMUM

3o~ 2s

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Fig. 3. The number of significant discrete solar proton events for each 12-month period after solar minimum (circles) and the 12-month mean sunspot number for the corresponding period (histograms) for solar cycles 19-21 and the first five years of solar cycle 22. The data for the present solar cycle are preliminary.

(10)634 M.A. Shea and D. F. Smart

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40

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D Z

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YEARS FROM SUNSPOT MINIMUM

Fig. 4. Summation of significant discrete solar proton events for cycles 19-21 (solid cir- cles) and the first five years of cycle 22 (open circles). The corresponding 12-month average sunspot number is shown by the histogram. The data are organized in 12-month periods beginning with the month after sunspot minimum as defined by the statistically smoothed sunspot number. The data for the present solar cycle are preliminary.

Solar Proton Event Fluence

Quite frequently, when a solar active region passes across the solar disk, it may produce many major solar flares several of which release particles associated with the aggregate particle event observed at the earth. One of the most noteworthy events of this type is the August 1972 sequence of solar activity in cycle 20. This same phenomenon of episodes of solar proton events is also present in the 22nd solar cycle.

Table 1 summarizes the number of events and the proton fluence above 10 and 30 MeV for solar cycles 19-21. Also listed are the > 10 and > 30 MeV fluence for the eight major sequences of activity that have occurred within the first five years of solar cycle 22. The fluenee for solar cycle 22 has already exceeded that for either cycle 20 or 21; it is approximately half the value estimated for cycle 19. Of special interest is the sequence of activity in October 1989 which generated more fluence above 10 and 30 MeV than was measured at the earth during the entire 21st solar cycle. This event was comparable to the August 1972 solar particle event which is usually quoted as a "worst case" example. However, unlike the August 1972 event, the October 1989 event contained a significant flux of protons above 500 MeV with three major relativistic solar proton events included in the episode of activity.

Significant Proton Events of Solar Cycle 22 (10)635

Table 1. Summary of Solar Proton Events for Solar Cycles

Cycle Start* End

19 May 54 Oct 64

20 Nov 64 Jun 76

21 Jul 76 Sep 86

NO. of

Months in

Cycle

126

22

140

123

No. of No. of Discrete

Discrete Proton proton Producing Events Regions

65 47

72 56

81 57

Solar Cycle Integrated Solar Proton Fluence

particles (cm -2)

> 10 MeV > 30 MeV

7.2 x 101° 1.8 x 101°

2.2 x 101° 6.9 x 109

1.8 x 1010 2.8 x 109

* The start of each solar cycle was selected as the month after the minimum in the smoothed sunspot number/7/.

Event Date and Year Mar 7-25, 89 0.12 x Aug 12-18, 89 0.76 x Sept 29 - Oct 2, 89 0.38 x Oct 19-30, 89 1.9 x Dec 30, 89-Jan2 , 90 0.21 x May 21-31, 90 0.035x Mar 22-26, 91 0.96 x June 4-21, 91 0.32 x

10 lo 0.03 x 109 101° 1.4 x 10 9 10 lo 1.4 x 10 9 101o 4.2 x 10 9 10 l° 0.13 x 109 1010 0.14 x 109 101° 1.8 x 109 1010 0.79 x 109

Totals: 4.68 x 10 lO 9.9 x 10 9

RELATIVISTIC SOLAR PROTON EVENTS

The longest and most homogeneous data base of solar proton events is of relativistic solar protons as detected by the world-wide network of neutron monitors. Protons having energies greater than ap- proximately 450 MeV can be detected by neutron monitors in the earth's polar regions (above an L- shell of 4). For detectors at mid and equatorial latitudes, the threshold energy is determined by the earth's magnetic field. Protons must have energies of approximately 15 GeV or greater to be detected in the vertical direction in the equatorial regions. Figure 5 shows the temporal distribution of these rel- ativistic solar proton events since 1954 and the location of the flare associated with each event. Eleven of the 48 events have been associated with solar activity (i.e. an assumed flare) behind the west limb of the sun.

COMPARISON WITH PREVIOUS SOLAR CYCLES

Dcration of Events

The relativistic solar proton events are the easiest to compare from solar cycle to solar cycle since this is the most homogeneous data base. One of the most striking differences is the duration of events dur- ing the 22rid solar cycle compared with the duration of events that occurred during the 20th and 21st solar cycles. Figure 6 illustrates the duration of two comparable events as recorded by the neutron monitor located at sea level on Kerguelen Island in the Indian Ocean. This monitor is located at a verti- cal geomagnetic cutoff rigidity of 1.1 GV (i.e. equivalent proton energy of 508 MeV). The event on 7 May 1978 had the highest increase for all relativistic solar proton events in either the 20th or 21st solar cycles with a recorded increase of 214%. The event had a duration of just over an hour. The event on 29 September 1989 with its comparable increase lasted more than 10 hours. For unknown reasons, the events of this 22rid solar cycle have been of considerably longer duration than similar events of the pre- vious two solar cycles. However, three of the major relativistic solar proton events of the 19th solar cycle (23 February 1956, 12 November 1960 and 15 November 1960) were long duration events. From these rather limited statistics it would appear that the relativistic solar proton events of the 22rid solar cycle were similar in duration to those of the 19th solar cycle - but not the 20th or the 21st cycles.

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(10)636 M.A. Shea and D. F. Smart

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RELATIVISITIC SOLAR PROTON EVENTS

220 CYCLE 19 CYCLE 20 CYCLE 21 CYCLE 22 200 / " t 18o l 14O 120 i X / '~ ! I

I '° 4O 2 . . i . . . .

54 59 64 69 74 79 84 89 94

YEAR

g GLE FREQUENCY

-, 0' H I F E ~ r [ - [ i F] , , , , i

Z , , , = . . . .

54 59 64 69 74 79 84 89 94 Y E A R

Fig. 5. The relativistic solar proton events observed since 1955. The smoothed curve in the center of the figure shows the smoothed sunspot number; the bottom part of the figure shows the number of high energy solar proton events each year. At the top of the figure is a circle depicting the sun. The indi- vidual dots are placed at the solar longitude (as viewed from the earth) of each flare considered to be the source region of the high energy solar proton accelero~on for the events hetween 1956 and 1992.

l ( e r g l l e l m i l , ~ l a l l d

290 -

~ 90-

'°i - I 0 - ! ! ! - " , ' . , ' . , ' . , ' . , ' . , : , : , : , : , : ,

RELATIVE TIME (HOURS)

Fig. 6. The durution of the largest relativistic solar proton event of the 22rid solar cycle (29 SepterNJer 1989) compared with the largest event of the 20th and 21st solar cycles (7 May 1978). The individual tic marks on the x-axis represent 30 minutes. The Kerguelen Island neutron monitor is located at 49.35 ° S, 70.27 ° E and measures the cosmic radiation above approximately 508 MeV.

Significaat Proton Events of Solar Cycle 22 (10)637

peak Flux vs. Fluence Events

In our study of solar proton events, we have compared those events having a large "peak" solar proton flux with those events associated with large solar proton fiuence. Particle physicists usually refer to the peak flux observed in a specific channel of a solar particle detector. This can be either an integral flux above a specified energy level, in units of partieles/cm2-sec-ster or a differential measurement which specifies the flux at a specific energy in units of particles/cm2-sec-ster-MeV. Individual events are usu- ally compared using identical channels. The peak flux specifies the maximum particle flux. Peak flux is usually used to quantify individual solar particle events.

Fluence is the total number of particles above a selected energy that is experienced throughout an entire event. Fluence may be given in either directional units of particles/cm 2-ster or omni-directional units of particles/cm 2. The fiuence is generally of concern for the total radiation exposure. During an episode of activity there may be a number of individual solar particle events which contribute to the to- tal radiation exposure.

In comparing those events with the largest flux with those having the largest fluence, and utilizing the ionization data acquired in the 17th and 18th solar cycles /8/we have found that the events with the largest peak flux are associated with flares that occur on the western limb of the sun; those with the largest fiuence are associated with solar activity near the center of the sun (as seen from the earth). Most of the events that are associated with solar activity near the center of the sun are of long duration and often have an enhanced particle flux in conjunction with the arrival of the interplanetary shock at the earth. Table 2 lists our estimates of the five largest high-energy peak flux events and the five largest fiuence events since 1942.

RANK

1

2

3

4

Table 2. Evaluation of Major Solar Proton Events

Peak Flux Total Fluence (> 1 GeV) (> 10MeV)

Feb 56 July 46

Nov 49 Nov 60

Sept 89 Oct 89

May 60 July 59 Mar 42(?)

SUMMARY

In our study of the solar proton events that have occurred since 1955, the following conclusions can be made:

(a) There have been 294 significant solar proton events measured at the earth during solar cycles 19-22 (i.e. from May 1954 until October 1991).

(b) Although there is a general relationship between solar proton event occurrence and sunspot number, there is no predictable pattern within the solar cycle.

(c) More than 15% of the events contain relativistic solar protons (E > 450 MeV). (d) Solar proton events occur in episodes of activity from the same solar active region. (e) Most major fiuence events appear to be associated with the arrival of an interplanetary mag-

netic shock at the earth and subsequent geomagnetic storm. (f) Solar cycle 19 has the largest flux and fiuence.

From our observations, and from the knowledge of the powerful events of the 17th and 18th solar cy- cles, it would appear that the 20th and 21st solar cycles were deficient in solar proton production as measured at the earth. Therefore, when models of maximum solar proton fluxes to be encountered for deep space missions are developed, solar proton data acquired during the present solar cycle should be incorporated.

(10)638 M.A. Shea and D. F. Smart

REFERENCES

1. S.E. Forbush, Three unusual cosmic-ray intensity increases due to charged particles from the sun, Phys. Rev. 70, 771 (1946).

. H. Elliot, Cosmic ray intensity variations and the Manchester school of cosmic ray physics, in Early History of Cosmic Ray Studies, ed. Y. Sekido and H. Elliot, Astrophysics and Space Science Li- brary,-ll8, D. Reidel Pub. Co., Dordrecht, Holland, p. 375, 1985.

3. N. Adams, A temporary increase in the neutron component of cosmic rays, Phil. Mag. 41, 503 (1950).

4. N. Adams, and H.J.J. Braddick, A temporary increase in the neutron component of cosmic rays, Phil. Mag. 41, 505, 1950.

5. S.E. Forbush, T.D. Stinchcomb, and M. Schein, The extraordinary increase of cosmic-ray intensity on November 19, 1949, Phys. Rev. 79, 501 (1950).

6. M.A. Shea and D.F. Smart, A summary of major solar proton events, Solar Phys. 127, 297 (1990).

. J.A. McKinnon, Sunspot Numbers: 1610-1986 based on the sunspot activity in the years 1610-1960, UAG-95, National Oceanic and Atmospheric Administration, National Geophysical Data Center, Boulder, Colorado, USA (1987).

. D.F. Smart and M.A. Shea, A comparison of the magnitude of the 29 September 1989 high energy event with solar cycle 17, 18 and 19 events, Proceedings of the 22nd International Cosmic Ray Conference, Dublin, 3, p. 101 (1977).