T H E E X P L O S I V E P H A S E OF S O L A R F L A R E S

KAREN L. HARVEY Lockheed Solar Observatory, Saugus, Calif, U.S.A.

(Received 19 June, 1970)

Abstract. The explosive phase of a flare can be defined by a simple photometric measurement of Ha film records of the flare development. Using the quantitative definition, improved correlations are found between the start of the explosive phase and the start of 10.7 cm radio bursts and Sudden Frequency Deviations compared to earlier correlations of the same data using visual estimates of the start of the explosive phase. Explosive development may be confined to only part of a flare.

1. Introduction

The term 'explosive phase' has been applied to a period of rapid development exhib- ited by some flares. Athay and Moreton (1961) and Moreton (1961, 1965) describe the explosive phase as a period of an abrupt increase in brightness of the flare with a rapid expansion of the flare borders. Moreton (1964) finds that this period lasts

from 10 to 30 sec. Several studies of the temporal relation of the start of the explosive phase and flare

related phenomena have demonstrated the importance of the explosive phase in flare development; e.g., X-rays (Moreton, 1964), centimeter-wavelength radio bursts (Covington and Harvey, 1961 ; Angle, 1968), flare waves (Athay and Moreton, 1961 ; Smith and Angle, 1968) and some ionospheric disturbances (Hansen and Kleczek, 1962; Donnelly, 1967, 1968; Davies and Donnelly, 1966). For these studies, the existence and onset of the explosive phase was determined by visually scanning a sequence of Ha filtergrams and subjectively evaluating whether the flare exhibits

explosive development. The term 'flash phase' occasionally is used interchangeably with 'explosive phase'.

In this paper a distinction will be made between the two terms. As first used by Ellison (1949), a flash phase applies to the rapid broadening of the HT. line profile during the flare rise. The flash phase is, thus, a spectroscopic characteristic of some flares. The explosive phase refers to the rapid development phase of flares observed on fixed-wavelength filtergrams, specifically those obtained with the Ha line.

This paper is intended to define the explosive phase and its onset in terms of a quantitative photometric measurement of flare development.

2. A Quantitative Measure of Flare Development

Quantitative studies of flare development by means of photometry have been made previously by several investigators (Ellison, 1949, 1952; Ellison et al., 1960; Abra- menko et al., 1960; Dizer, 1969). Dodson (1953) and Dodson et al. (1956) studied

Solar Physics 16 (1971) 423-430. All Rights Reserved Copyright �9 1971 by D. ReideI Publishing Company, Dordrecht-Holland

424 KAREN L.HARVEY

the light curves of chosen points in flares measured with a densitometer on Ha spectroheliograms. The flare intensity was measured relative to the quiet sun and was then converted into units of continuous spectrum at 6590 A. More recently Tallent (1967, 1970) has applied a video-meter to measuring in real time the inte- grated intensity, peak intensity and area of flares as observed with a 0.5 A Ha filter.

Most of the methods that have been employed in flare photometry require either a large amount of work for the information obtained or complicated equipment. In order to study a large number of flares, a simple photometric measurement of flare development that required a minimum of equipment was sought. After some experi- mentation, the following procedure was finally adopted for the study of Ha filter- grams of flares.

All measurements were made on an enlarged projected image of the sun. For each flare to be analyzed, a mask was prepared with an aperture the size and configuration of the flare at maximum area. The mask was placed over the phototube of a 'Densi- chron' densitometer. The phototube was positioned at the location of the flare on the projected image of the solar disk and the logarithm of the light transmitted through the film was measured in the area defined by the mask. The resultant photo- metric quantity will henceforth be referred to as 'F ' . Each measurement of F for the flare was normalized by subtracting it from a similar measure of an adjacent quiet region of the sun. By measuring F from frame to frame of the cinematographic record of the flare, a quantitative time history of the flare (F-curve) was determined. While F is measured in units of film density, the physical meaning of F (unimportant in the present study) is a complicated function of flare intensity and area. But because of the high contrast of the film used, we are primarily measuring a change in area.

3. Determination of the Explosive Phase of a Sample of Flares

For this study, F-curves of 109 flares selected from the Lockheed Solar Observatory records from the period March 1959 to August 1967 were measured. Selection for the flare sample was made on the following basis: (1) the film record of each flare was complete, (2) the flares were located at less than 60 ~ east or west of central meridian, and (3) the area of the flare was greater than 1.5 square degrees. Because we are investigating the explosive phase, the flare sample was purposely biased to include a large number of rapidly growing flares. The flare sample is, thus, not representative of a randomly selected sample of flares. The time resolution of the F-curves during the rise phase of the flares ranges from 10 sec to 2 rain.

Figure 1 shows the F-curves of two Importance 1-flares of approximately the same area and total change in F during flare development. The solid curve is a flare having a fairly normal rise period of 9 min. The dashed curve is of a flare that was subjectively determined to exhibit explosive development. The comparison suggests that the period of explosive development (the explosive phase) may be defined in terms of the slope of the F-curve.

For each flare in the sample, the maximum value of dF/dt during the rise phase

T H E EXPLOSIVE PHASE OF SOLAR FLARES 425

was determined. Since the curves only represent a change in the F-level of the flare, dF/dt was normalized with respect to the total increase in F from flare start to flare maximum. Figure 2 is a histogram of the resulting slope distribution of the flare sample. Those flares that have been subjectively classified as exhibiting an explosive phase are indicated by shading. It is interesting to note that although our sample is biased towards faster rising flares, we find no obvious separation of the slope distri- bution according to whether or not a flare exhibits explosive development.

Flares were defined as having an explosive phase if (dF/dt) .... was greater than 25% of the change in F from flare start to flare maximum where t is in minutes. The

Fig. 1.

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Histogram showing the normalized slope of the 109 flares included in this study.

426 KAREN L. HARVEY

explosive phase lasts for the duration that the normalized slope exceeds 0.25 rain-1. Based on this criterion, 52 of the flares in the sample were determined to exhibit an explosive phase. The average duration of the explosive phase is 113 sec with a range of 30 to 190 sec.

In F i g u r e 3, for the 52 flares having an explosive phase, we have compared the

Fig. 3.

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Comparison of the times of flare maximum, flare start and the start of the explosive phase determined visually with those determined by the F-curve.

times of flare start, flare maximum and the start of the explosive phase determined from the visual inspection of flare development on H-alpha filtergrams and from the measured F-curves. The visual determinations of flare start and maximum were made according to the IAU standards (Trans. 1/tU, 1964). Flare start was defined from the F-curve at the time the normalized slope exceeded 0.02 rain-1 and flare maximum at the time of maximum F measured for the flare. In each grouping of the flare sample in Figure 3, zero represents the F-curve determination of flare maximum, flare start and the start of the explosive phase respectively. The histograms show these same critical times determined from the visual inspection of the flares as a distribution about the F-curve determination. On the average, the time of maximum defined by the F-curve slightly follows the visual determination; the start of the flare defined by the F-curve tends to occur before the visual estimation. For 7 0 ~ of the flares, flare start and flare maximum determined by both methods coincide to within _ 1 min.

THE EXPLOSIVE PHASE OF SOLAR FLARES 427

The start of the explosive phase determined from the F-curves occurs on average of

22 sec before the visual estimate.

4. The Explosive Phase and Associated Flare Effects

From Figure 3, it is not obvious whether the subjective or F-curve ,determination of

the start of the explosive phase is more significant. A valid test would be to compare

the start of the explosive phase, determined by each method, with flare-related phenomena presumed to be associated with the explosive phase. In this section, we consider three flare effects (10.7 cm radio bursts, hard X-ray bursts and Sudden

Frequency Deviations) which statistically have been shown to be related to the explosive phase.

A. 10.7 cm RADIO BURSTS

Figure 4 shows the comparison of the start of the 10.7 cm radio bursts with the start

of the explosive phase, determined visually (top) and from the F-curves (bottom), for

48 of the explosive-phase flares for which radio observations were available. The comparison with the visual start shows similar results to those of Covington and

Harvey (1961) in that the 10.7 cm bursts slightly precede the explosive phase start.

However, the bottom graph suggests a better correlation with the start of the explo-

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Fig. 4. Comparison of the start of the 10.7 cm radio burst with the start of the explosive phase determined visually (top) and by the F-curve (bottom).

4 2 8 KAREN L. HARVEY

sive phase defined by the F-curve. This becomes especially evident when we consider that the 11 bursts for which the areas are not shaded in Figure 4, have an active dark filament in the flare area prior to or during the rise of the flare. This suggests that if there is an observed active filament, either the F-measurements are biased due to the dark features or that the 10.7 cm radio bursts do not necessarily occur at the explosive phase. I f we disregard these I 1 cases, we find a good correlation between the start of the explosive phase determined by the F-curve and the start of the 10.7 cm radio burst. Furthermore, the start of the explosive phase is found on an average to slightly precede the onset of the 10.7 cm radio burst contrary to previous results.

B. X-RAY BURSTS < 1/~

The timing of X-ray bursts ( < 1/~) with the start of the explosive phase was first investigated by Moreton (1964). He found that for 9 flares observed to have hard X-ray emission, all had explosive phases. The start of the X-ray bursts followed the start of the explosive phase by an average of 1.5 rain for the 6 flares where the onset of the X-ray burst was observed. For the 0920 UT flare of 21 March 1966, Falciani et al. (1968) found the X-ray event followed the rapid development of the high intensity regions of the flare by 2 rain.

For a small sample of 10 flares, we generally found that the onset of hard X-ray bursts also occurred from 0 to 2 min after the start of the explosive phase determined f rom the F-curve.

C. SUDDEN FREQUENCY DEVIATIONS (SFD)

Davies and Donnelly (1966) and Donnelly (1967) have established a close time rela- tion between the start of the SFD and the start of the explosive phase. They found that the SFD occurred on an average of 1.5 rain prior to the start of the explosive phase as determined from a visual inspection of the flare event on film.

Donnelly (1968) has re-examined this relation using for the explosive phase start the times derived f rom the F-curves. A better correlation was found. The SFD starts at the onset of the explosive phase.

5. Spatial Structure of the Explosive Phase

In the previous sections, we have discussed the explosive phase characteristics exhib- ited by an entire flare. In many flares, especially large complicated ones, explosive development may occur in part of the flare. One good example of this behavior is illustrated in Figure 5. The F-curves of four flare sections, three of which are shown in Figure 5, making up the major flare of 23 May 1967, 1834 UT, were measured. The F-curves demonstrate the independent behavior of various sections of the flare. This behavior has been pointed out by Dodson et aL (1956), Ellison et al. (1960) and Dodson and Hedeman (1968). Based on the criterion defined in Section 3, only Section E of the flare showed explosive development. Section E also appears to be the origin of a flare wave at the onset of the explosive phase (Smith and Angle, 1968).

T H E E X P L O S I V E P H A S E O F S O L A R F L A R E S 429

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6. Conclusions

The explosive phase of a flare has been defined in terms of a pho tomet r i c measure

of flare development . In testing the val idi ty of our definit ion as c o m p a r e d with tha t

de te rmined by visual examina t ion of the film records of the flare, we found a more

precise t ime cor re la t ion of 10.7 cm rad io bursts and Sudden Frequency Devia t ions

with the quant i ta t ive definit ion of the explosive phase start. Stat is t ical ly there was

only a small difference (22 sec) between the s tar t of the explosive phase de te rmined

by the two methods . However , the quant i ta t ive defini t ion of the explosive phase

provides more consistent and rel iable results bo th in defining the onset of the ex-

plosive phase and in corre la t ion of flare re la ted phenomena with the explosive phase.

References

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430 KAREN L.HARVEY

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