upper limits on the total radiant energy of solar flares

UPPER LIMITS ON THE TOTAL RADIANT ENERGY OF SOLAR FLARES

H. S. Hudson Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, U.S.A.

and

R. C. Willson Jet Propulsion Laboratory, Pasadena, CA 91103, U.S.A.

ABSTRACT. We establish limits on the total radiant energy of solar flares during the period 1980 February - November, using the solar-constant monitor (ACRIM) on board the Solar Maximum Mission. Typical limits amount to 6 x 1029 erg/s for a 32-second integration time, with 5o statistical significance, for an impulsive emission; for a gradual component, about 4 x I0 Bi ergs total radiant energy. The limits lie about an order of magnitude higher than the total radiant energy estimated from the various known emission components, suggesting that no heretofore unknown dominant component of flare radiation exists.

I. INTRODUCTION

A solar flare consists of an explosive release of radiation and mechanical energy near the surface of the sun, with copious acceler- ation of non-thermal particles. The Solar Maximum Mission made the first systematic effort to observe simultaneously all of the important radiations, and hence no accurate knowledge of the total radiant energy release previously existed. Indeed, we have no observations of some regions of the spectrum, and of others so little that we cannot estimate their contributions. Bruzek (1967) and others have described the magnitudes of the known components of the observed energy distri- bution. The mechanical energy release represented by the inter- planetary shock wave (Hundhausen, 1972), probably contains more energy than the electromagnetic radiation; in any case the particle radiation ("solar cosmic rays") makes a negligible contribution energetically (see, for example, the review by Svestka, 1976). A direct measurement of the total excess irradiance produced by a solar flare - by bolometric techniques - will reveal any radiation that had heretofore remained hidden in an unobserved gap in the spectrum, and at the same time define the radiant part of the flare energy exactly as an aid in understanding of the flare mechanisms.

The Solar Maximum Mission includes in its instrument complement a sensitive radiometer, the Active Cavity Radiometer Irradiance Monitor or ACRIM (Willson, 1979). Although intended primarily to define the

SolarPhysics 86 (1983) 123-130. 0038-0938/83/0861-0123 $ 01.20. �9 1983 by D. Reidel Publishing Co., Dordreeht and Boston

124 H.S. HUDSON AND R. C. WILLSON

solar irradiance precisely as an input to climatological studies, this instrument has properties that make it useful for the study of solar variations of many causes. Sunspots in particular produce marked effects (Willson et al., 1981), resulting in a "missing flux" that represents a substantial fraction of the 6.4 x i0 I0 erg/cmis total radiant flux at the photosphere. The sunspot flux deficits cause variations in the "solar constant" at the level of tenths of a percent.

This paper examines the ACRIM data for selected important flares that occurred during its interval of good pointing, essentially 1980 February - November. We compare the upper limits thus obtained with estimated energy fluxes in Ha and soft X-rays. At present we have an incomplete survey, omitting many flares and not using the highest time resolution of the ACRIM detector.

2. THE ACTIVE CAVITY RADIOMETER

The ACRIM sensor consists of a black conical cavity in a carefully-designed thermal environment (Willson, 1979). Measurement of the absolute solar irradiance relies on a determination of the electrical power needed to maintain the cavity temperature when a shutterblocks off the solar input. The resulting absolute measurement has relatively good accuracy, but for the present purposes we only care about the relative precision. The ACRIM instrument has excellent DC stability (Willson et al., 1981) and observes solar variations on all time scales up to the Nyquist frequency of 3.815 mHz (Woodard et al., 1982).

The ACRIM, a thermal detector, has an extremely broad-band spectral response. The absorbing cone has a specular black finish with an absorptance near unity in the visible wavelengths. Although no measurements exist at other wavelengths of interest for solar flares - infrared, ultraviolet, or XUV down to a few angstroms - we may assume that the absorptance remains essentially independent of wavelength from millimeter waves to soft X-rays. The ACRIM measurement thus yields a true bolometric magnitude.

ACRIM has a thermal time constant of about i s. It operates normally in a shutter cycle of 131.072 s duration, with one quarter of this interval used for prime measurements. In this paper we analyze

only this kind of data, consisting of 32.768 s integrations spaced at 131.072 s intervals. Information also exists at the level of indi- vicual 1.024 s samples, but we have not yet studied these data and will describe them later. A typical orbit (~ 96 min) contains about 27 shutter cycles. The instrument thus obtains some 864 individual measurements. An analysis of these data yields an "orbital mean" together with a statistical standard deviation; typical standard deviations lie in the range i0 - 15 parts per million of the mean irradiance.


3. FLARE SELECTION

We have examined a selection of flares culled mainly from the hard X-ray flare catalog (HXRBS team, 1982). Table I summarizes the observed parameters of these flares, of all area classifications but strongly tending towards the "B" brightness classification in Ha, presumably because of the original selection from the hard X-ray list. The list includes most of the very significant flare events during the first year of SMM observation, including the June 21 "white-light prominence" observed visually by Harvey and Duvall (Harvey, 1982). The list includes one importance 3B flare, that of 1980 October 14. None of the events produced a striking variation in the ACRIM reading.

125

We also surveyed for completeness a second list, also taken from the hard X-ray catalog, but for all of the 28 events with hard X-ray fluence exceeding 105 counts during the interval 1980 October 23 - 1980 November 15. Of these there was ACRIM coverage for 19, and in no case did a significant variation occur at the time of the flare.

4. OBSERVATIONS OF SEVERAL MAJOR FLARES

During the period under study, the Sun cooperated with only a single importance 3 flare, that of 1980 October 14. We show orbital means of the ACRIM data corresponding to this period in Figure i.

TABLE I

Flares Examined

Date Ha Soft X-ray

Imp. Location Start Max. End Class

5/21/80 2B S14W15 2051 2107 2144 X1

6/04/80 SB S14E59 0654 0655 0705 M6

6/07/80 SB NI4W70 0309 0314 0320 M7

6/21/80 IB NI7W91 0115 0120 0146 X2

7/01/80 IB S12W38 1618 1628 1642 X2

'7/05/80 IB N28W29 2233 2246 2329 M8

7/23/80 2B S17E13 0051 0103 0209 M8

9/04/80 IB S09W28 2156 2220 2231 M2

10/14/80 3B S09W07 0558 0611 0632 X3

11/06/80 2B S12E72 0329 0352 0533 X9

11/07/80 2B N07WII 0159 0208 0307 X2

11/12/80 IB NIOW72 0445 0452 0517 X2

11/15/80 IB S12W53 1540 1553 1626 XI

126 H.S . HUDSON AND R. C. WILLSON

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UT 1980 OCT. 14


TABLE II

Typical Flare Upper Limits (50)

127

Interval Power Total Energy (s) (ergs/s) (ergs)

32 6 x 1029 2 x 1031

3700* 1.15 x 1029 4.3 x 1032

*Thomas (1981)

Another exceptional flare, an X9 in the GOES soft X-ray classification, occurred on 1980 November 6; we show this flare also in Figure 1 along with the limb flare of June 21 responsible for the white-light prominence and intense y-ray emission. In none of these cases did an obvious excess irradiance occur on a time scale comparable to an orbital period. Accordingly we will treat the data as upper limits.

For simple estimate of the upper limit on the total flare luminosity we take ~ = 12 parts per million as a typical measured standard error (one orbit). Thus for a 5 ~ limit we find Lflar e < 1.15 x 1029 erg/s. We can obtain a limit on the total flare energy by assuming a duration of 3700 s, taken as representative of the He or soft X-ray gradual phase. Then Wflar e < 4.3 x 1032 ergs.

Finally, we can construct a limit on shorter time scales from a single two-minute shutter cycle, corresponding roughly to the impulsive phase of a flare. The resulting 5 ~ limits are Limpulsiv e < 6 x 1029 erg/s or Wimpulsiv e < 2 x 1031 ergs. Figure 2 shows examples of the ACRIM data at higher time resolution; we note that ACRIM provides only 32.768 s of data out of each 131.072 s shutter cycle in its normal mode of operation, so that in any given flare the shutter-open periods may not have intercepted the impulsive emission.

5. CONCLUSIONS The ACRIM sensitivity falls very close to that needed to detect

solar flares in integrated sunlight. For example, Thomas (1981) esti- mates the total He energy as 0.31 - i.I x 1027 erg/s for 2B - 3B flares. Our ACRIM limit lies about two orders of magnitude above this

t

Fig. ]. Time histories of ACRIM orbital means during three major flares: (a) 1980 June 21, responsible for a white-light prominence viewed by Harvey and Duvall, as well as the most important y-ray event of 1980; (b) 1980 October 14, the only importance 3B flare in the prime data set from SMM; and (c) the 1980 Nov. 6 event that produced the largest soft X-ray flux.

128 H.S. HUDSON AND R, C. WILLSON

700

m E

600

q

500 > 0

~ 400

z 300

o

II I p, WHITE-LIGHT FRO~INENCE i ~

(Horvey & Duvall)

FI~ LIMB FLARE (IB)

065o

Fig~ 2a

GOES X2 ~ - - - - - . - . . ~ _ ~

~bo o,',o o,~o o,~o o,~o UT JUNE 21, 1980

Fig. 2.

4 0 0 0 , , ~ . . . . . , . . . . , . . . . . , . . . . .

3000

2000

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~ 1000 :a::

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0344 0346 0348 0350 Ul NOV. 6, 19BO

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-700

-8oo LJ~..

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0352

Time histories for ACRIM data during two impulsive events, showing individual 32-second integrations spaced at 131-second intervals: (a) 1980 June 21, and (b) 1980 November 6. In the latter case we compare with the hard X-ray light curve for the lower-energy channel (HXRBS team, 1982), which shows the gradual phase. The impulsive energy release occurs during the increase of the X-ray counting rate.

UPPER LIMITSONTHETOTALRADIANTENERGY OFSOLARFLARES

TABLE III

Estimated Component Luminosities

129

Radiation Time Scale Power Total Energy Source (s) (ergs/s) (ergs)

H~ (3B) 3700 i.i x 1027 4.] x 1030 (i)

White light 600 2.8 x 1027 6 x 1030 (2)

EUV flash i000 3 x 1028 3 x 1031 (3)

Soft X-ray 2000 2.8 x 1027 6 x 1030 (4)

Burst

(i) Thomas (1981) (2) Rust and Hegwer (1975) (3) Donnelly (1971) (4) NOAA Solar-Geophysical Data

level. The He line, however, should contain only some small fraction of the total flare luminosity. Unfortunately, we know little quantitatively of the detailed spectral breakdown of flare radiation. Canfield et al. (1980) find that He contributed about 12% of the visible line energy in a small flare observed by Skylab; however the existence of other emission components must reduce the He fraction of the total considerably. Canfield et al. point out, for example, that the visible continuum, usually not detected in flare patrol observations, could make a relatively large contribution. The ACRIM data set limits on the magnitude of such contributions; apparently no major predominant component of flare radiation has escaped detection.

Improved total-irradiance measurements would allow the detection of solar flares in integrated sunlight. Similarly, a panoramic detector with good photometric properties could also make such observations of stellar flares, and the SUCCeSsful observation of solar flares will help us to understand the common mechanisms of the two sets of phenomena. One should note that Livingston and Ye (1982) have already detected flares in integrated solar K-line data; the "K-index" increased by some 3% during an importance 3B flare. Unfortunately we cannot interpret these measurements quantitatively in terms of actual energy flux.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under

grant ATM-81-17355 and by the National Aeronautics and Space Admini- stration under NSG-7161. We would like to thank M. Woodard for helpful discussions.

130 H.S. HUDSON AND R. C. WILLSON

REFERENCES Bruzek, A: 1967, in J. Xanthakis (ed.), Solar Physics (Interscience),

p. 414. Canfield, R.C., Cheng, C.-C., Dere, K.P., Dulk, G.A., McLean, D.J.,

Robinson, R.D., Schmahl, E.J., and Schoolman, S.A.: 1980, in P.A. Sturrock (ed.), Solar Flares (Colorado), p. 451.

Donnelly, R.F.: 1971, Solar Physics 20, 188. Hundhausen, A.J.: 1972, Coronal Expansion and the Solar Wind (Springer,

New York). HXRBS Team: 1982, private communication. Livingston, W.C., and Ye, B.: 1982 Publ. Ast. Soc. Pac. (to be

published). Rust, D. and Hegwer, F.: 1975, Solar Physics 40, 141. Svestka, Z.: 1976, Solar Flares (Reidel, Dordrecht). Thomas, R.J.: 1981, private communication Willson, R.C.: 1979, Applied Optics 18, 179. Willson, R.C., Gulkis, S., Janssen, M., Hudson, H.S., and Chapman, G.A.:

1981, Science 211, 700. Woodard, M., Hudson, H., and Willson, R.: 1982, UCSD-SP-82-32.

DISCUSSION

HIRAYAMA: When you add all the flares together, can you enhancement in the solar irradiance?

find any

HUDSON: We have not yet tried to do a summed epoch analysis, but we plan to do this in the future. It would be much less ambiguous if we were able to make a direct observation of a particular flare.

upper limits on the total radiant energy of solar flares

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