A review of solar proton events during the 22nd solar cycle
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Adv. Space Res. Vol. 30, No. 4, pp. 1033-1044, 2002 Published by Elsevier Science Ltd on behalf of COSPAR
Printed in Great Britain 0273-1177/02 $22.00 + 0.00
A REVIEW OF SOLAR PROTON EVENTS DURING THE 22ND SOLAR CYCLE
D. F. Smart and M. A. Shea
Air Force Research Laboratory (VSBS), 29 Randolph Road, Hanscom AFB, MA, 01731, USA.
Solar cycle 22 had significant, large fluence, energetic particle events on a scale reminiscent of the 19th solar cycle. Examination of the characteristics of these large events suggests that some of the old concepts of spectral form, in- tensity-time envelope and energy extrapolations, used to estimate the dose from large events that occurred during previous solar cycles should be re-evaluated. There has also been a dramatic change in perspective regarding the source of solar protons observed in interplanetary space. Very large fluence events are associated with powerful fast interplanetary shocks. The elemental composition and charge state of these events is suggestive of a dominate source in the solar corona and not from a very hot plasma. Furthermore, there is a strong suggestion that the inten- sity-time profile observed in space is dominated by the connection of the observer to an interplanetary shock source rather than to a unique location near the surface of the sun. These concepts will be examined from the perspective of energetic panicles contributing to the dose experienced by an astronaut on an interplanetary space mission. Published by Elsevier Science Ltd on behalf of COSPAR.
This paper is oriented toward the viewpoint of radiation effects to human blood-forming organs (vital organ dose). We will discuss the energetic particle radiation capable of penetrating the shell of a space vehicle and the human body to reach the blood forming organs (at least 5 cm of tissue). From this viewpoint, the energetic charged pani- cle radiation must have a minimum energy of 30 MeV per nucleon.
From a solar astrophysics point of view, the increased energetic panicle sensor sensitivity to very low flux levels that has occurred over the past decades provides for hitherto unavailable information on the particle composition and possible origin. However, from a radiation dose point of view, small events of limited duration with maximum particle fluxes of a few per cm 2 per hour are not very significant. Therefore we will not put much emphasis on the very small events routinely measured by the improved instrumentation.
OVERVIEW OF SOLAR PROTON EVENTS DURING SOLAR CYCLE 22
Figure 1 illustrates the >30 MeV solar proton events during solar cycle 22. The horizontal lines on this figure indi- cate one sigma in a log-normal distribution of the solar proton event fluence distribution over the last four solar cy- cles. This one sigma characterization of the fluence distribution derived by Nymmik (1993) from the data of Feynman et al., (1993) seems to be one reasonable way of categorizing solar proton events. Stassinopoulos et al. (1996) adopted an order of magnitude classification system categorizing solar proton events as a matter of conven- ience. The lower limit of this figure, an omnidirectional fluence of 105 per cm 2 is comparable to the daily exposure from the integral flux of galactic cosmic ray protons. The solar cycle 22 solar proton events and fluences at ener- gies of>50, >60 and >100 MeV are shown in Figures 2 - 4.
In Figure 5 we display some of the very large events of the last solar cycle from the perspective of the size of a cell in the human body. We have used 100 square microns as the unit of area. The scale on the left of this figure indi- cates the number of proton "hits" per hour during these large events. Also indicated by horizontal lines are the flux levels that correspond to "hits" per day and "hits" per week.
1034 D.F. Smart and M. A. Shea
n t - o
> 80 MeV EVENT INTE~tATEO PROTON O~(IDIREC'rtONAL R .UBKE
. ' .1 . ' . ' . ' .= ' ;2 ! ; : ;~ . : ' ' . , . ' . ' ' .= . ' . ' . ' .= . ' - . ! ; ' ;= . ' . ' . " ] 101o
107 LU O Z lo0 u.I ::) .. J u. lOS
87 88 8g go gl g2
93 94 95 96
Fig. 1. The >30 MeV solar proton events during solar cycle 22, The horizontal lines indicate one sigma of the log-normal distribution.
,'+" E U W e-
o 0 :
Z uJ ::) . J i,
> IB0 MeV EVENT INTEGRATED PROTON ~ FLUENCE
1010 . . , . . . , . . . , . . . , . . . , . . . , . . . , . .+ , . . . , . . .
10s I . . . . . . I t : ' . 86 87 88 89 g0 91 92 g3
I i| . . m .+ 94 95 96
Fig. 2. The > 50 MeV solar proton events during solar cycle 22.
SO MeV EVENT INTEGRATED PROTON OMNIDBECT1ONAL FLUBCCE
3 u .
l O P O . . . . , . . . , . . . , . . . , , , , , . , , , . , , , . . . i . . . , .
1+ L lOa 107
86 87 88 89 90 91 lil I i .... g2 93 g4 95 96
Fig. 3. The >60 MeV solar proton events during solar cycle 22.
M t- O
w 0 z w . J U.
) 100 MIV LrVENT ~ 1 1 D PIIOlrON ~ ~
100 . . . , . . . , . . . , . . . , . . . , . . . , . . . , . . . , . . , . . .
1. ,lll 1~ "" . . . . . . .
86 87 88 89
I .L . I |= . . J . | . . .
90 91 92 9 -'1 94 95 96
YEAR Fig. 4. The > 100 MeV solar proton events during solar cycle 22.
1 0 0 ~
o . . . . . . . 1.0 Proton per Day
1 0-4 1 9 20 21 22 23 24 25 26 27 28 29 30 31
OCTOBER 1989 (UT DAY) Fig. 5. The very large events of October 1989 displayed from the perspective of the size of a cell ( -100 square microns).
> 30 MeV
> 50 MeV
> 60 MeV
> 100 MeV
Review of Solar Proton Events 1035
THE PARTICLE SOURCE
Both the classical solar flare source scenario and the shock acceleration scenario have enthusiastic advocates. Many of the "large" solar particle events are associated with "large" solar flares. However, many of the proton events observed at the Earth cannot be unambiguously and confidently time associated with specific solar flares. Furthermore, measurement of the elemental and isotopic composition of solar particles during large flux and flu- ence events was found to be consistent with the particles having passed through less than 30 mg cm -2 of matter from the acceleration site to their detection location (Mason, 1987). Thus, the solar photosphere is eliminated as the source region of the particles observed in space since there is no evidence of fragmentation due to interaction with the significant mass of the solar atmosphere. The intensity-time profiles of solar energetic particles leave no doubt that interplanetary shocks accelerate ions. The fast coronal mass ejection (CME) generated interplanetary shock is now considered to be the principal source of the ions observed in space (see Reames, 1995, for a review). There is also the concept of a composite solar particle event to which both sources contribute flux (Cliver, 1996).
A relatively recent grouping of solar particle events is to classify them according to the associated solar flare X-ray activity (see Table 1). This results in a classification of either "impulsive" (those associated with impulsive soft X- ray events) or "gradual" (those associated with long duration soft X-ray events). The "impulsive" solar particle events are the group that appears to be associated with the solar flare process. The long duration (i.e. "gradual") X- ray event is believed to be a proxy indicative of a coronal mass ejection, and hence this class of events is assumed to be the result of the shock acceleration process.
An extensive study of the "impulsive-flare" associated energetic particle events by Cane et al. (1986) and Reames et al. (1994) shows that these events are generally small flux and fluence events measurable in space over a restricted heliolongitudinal range, about one radian of heliolongitudinal distance centered on the most favorable propagation path from the solar flare location. The charge state of solar energetic ions that are associated with the impulsive soft X-ray flare is consistent with a multi-million degree hot (-2 x 107 OK) plasma source.
Table 1. Properties of Solar Proton Events Associated with Impulsive and Gradual soft X-ray events. (Adapted from Reames, 1995)
Impulsive X-ray event and associated particle event characteristics
Gradual X-ray event and associated particle event characteristics
Particles Electron rich 3He/4He Fe/O H/He
Charge state of Iron DurationHours Heliolongitude range
Associated Solar Radio type Associated Solar X-ray emission CME association Associated Interplanetary shock Events per year (At solar maximum)
Proton rich ~1 ~1 ~ 10 ~20 Days
1036 D.F. Smart and M, A. Shea
Large solar particle events seem to be associated with the occurrence of a fast coronal mass ejection or, in the absence of actual CM]~ observations, CME proxies. This association is complicated by the fact that large fast coronal mass ejoctions also seem to have an association with "big" solar flares. The long duration solar X-ray event is considered to be an excellent CME proxy. Figure 6 illustrates the fast CME associated with the 24 October 1989 solar cosmic ray event observed at the Earth. Large solar particle events tend to have a "normal" composition that is relatively consistent with abundances found in the solar corona or in the solar wind