FLARE-INDUCED MAGNETIC FIELD CHANGES IN THE CHROMOSPHERE

BARRY J. LaBONTE Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA.

ABSTRACT. It is well known that flares cause changes in the azimuthal direction of chro- mospheric magnetic field fines (e.g. Zirin, 1983). It is less well known that flares also cause changes in the inclination angle of chromospheric magnetic field lines (Bmzek, 1975). Inclina- tion angle changes are notable in that horizontal field lines take the form of fibrils, while verti- cal field lines take the form of plages (Marsh, 1976). This study examines a complete sample of large flares to determine when the field inclination cimnges during the flare. The Hard X- ray Burst Spectrometer Event Listing (Dennis et. al, 1985) was searched for events with total counts > 10 ~ and start times > 14 UT but <24 UT. Big Bear Solar Observatory Ha films were examined to identify which X-ray events show large horizontal ribbon motion over regions of fibrils. Of the 7 events found, 6 contain areas of the chromosphere that have the magnetic field direction turned from horizontal to vertical. The change in field direction at a given loca- tion occurs after the arrival of the ribbon, often 103 to 104 s after the flare start. No change in the chromosphere is seen before the ribbon arrival.

These observations show that field lines involved in the flare do not spend more than a few minutes in "open" configurations before reconnecting to new parmers, independent of the flare duration. The transit time of an Alfvdn wave down a magnetic field line is only ---102 s. Flare models that postulate field line disconnection or opening which lasts longer than the Alfvdn time are ruled out, since the chromospheric footpoints are observed not to change before the ribbon arrival (field line energization). The captive filament eruption model (Moore and LaBonte, 1980) also implies field line alterations lasting the duration of the flare, and is thus ruled out. Successful flare models should have the behavior of a brushfire, with the free energy in one bush (field fine) released only after the burning (flare energy deposition) in its neighbor raises its own temperature over the ignition point (inductively raises the free energy density over the flare threshold). The flare continues until the inductive energy density increase is small enough to be contained by the normal nonflare processes.

A full description of the observations and their implications will appear in a future paper. This work is supported by NASA Grant NSG 7536.

REFERENCES

Bmzek, A.: 1975, Solar Phys. 42, 215. Dennis, B. R., Orwig, L. E., Kiplinger, A. L., Gibson, B. R., Kennard, G. S., and Tolbert, A.

K.: 1985, NASA Tech. Memo. 86236. Marsh, K. A.: 1976, Solar Phys. 50, 37. Moore, R. L., and LaBonte, B. J.: 1980, in M. Dryer and E. Tandberg-Hanssen (eds.), 'Solar

and Interplanetary Dynamics', IAU Symp. 91, 207. Zirin, H.: 1983, Astrophys. J. 274, 900.

Solar Physics 113 (1987) 285-288. �9 1987 by D. Reidel Publishing Company.

286 BARRYJ. L~ONTE

DISCUSSION

HIRAYAMA: As a strong believer in the classical two-ribbon filament- involved flare model, I don't think there is any difficulty in changing the fibrils outside of the two ribbons. You remember that the

prominence eruption starts very slowly and takes a long time to go, as

you have seen many many times since 1910 or so. It has produced many, many papers. At the same time as the erupting prominence, the outer

edges of the two ribbons are probably moving slowly at first - you must

look carefully - and then finally @hs~s~h like that!

LABONTE: I don't think so. I've looked at the movies just for that, and it occurred to me that that's a critical observation. Maybe what I

should say is that it's OK to go flare-chasing. You have to look very

critically at the decay phase - this may still tell us something even though usually we think of that as being a waste of time.

SPICER: I'd like to say that there is a cartoon model which works like that, if you take sheared magnetic fields, twisted or otherwise, and

you get more than one tearing mode they interact with one another; it

processes a large volume and it spreads. That's very hard to draw ....

LABONTE: .... but that's essentially of a form where the gross field

doesn't move very much, it just sits there and it just eats away at

itself. I don't see any large scale loading of field lines.

SPICER: But there's a model like that.

SVESTKA: I must say that I like very much your observations, not so

much the interpretation! I remember seeing so many flares which change completely the pattern of the active region so that even 12 hours later

the plage had the same form as the flare which already had decayed many

hours before. I do not see why this should be an argument against the opening of the field, for example in the Kopp and Pneuman mechanism.

The field lines are already pretty steep there when they open. The only

effect of the opening is that some material escapes upwards. It's more an extension upwards, rather than an opening, and the fibrils there

will not brighten, I suppose. They will brighten only when the field

reconnects and some energy is added. That is when the ribbons arrive there, and after that it doesn't return to the state it was in before.

That's strange.

LABONTE: The trick is, that if the field line are lifted and moved in

some substantial way, why doesn't this footpoint respond to that at

all? It only responds to the thermal wave that comes down when they get reconnected. That seems unreasonable to me. If you look around active regions, you see that the fields in general evolve over timescales of days, and pieces of plage stand up and fibrils lay down and what not. If this large event has transpired and a Kopp and Pnewman kind of cartoon occurs, lifting everything and changing it really ... formerly

FLARE-INDUCED MAGNETIC FIELD CHANGES IN THE CHROMOSPHERE 287

the two footpoints could exchange information, and now all of a sudden they can't. You would think that that would make a difference to the

footpoints, that in an Alfven time they would know that something was

different, and I'm saying that i00 Alfen times later they haven't learned it yet, in some cases. That seems odd to me.

ZIRIN: What is at the base of those fibrils that you're talking about. Are they coming from regions of enhanced magnetic fields?

LABONTE: No, these are anchored right in a piece of regular magnetic

field. If you looked at a magnetogram, you'd find that part of that magnetic path was covered with bright plage, the other part was overlaid by the tail ends of these fibrils. After the flare they've stood up straight and the whole thing is now plage.

ZIRIN: In other words, the magnetogram would not change, only the angle of the ...

LABONTE: It's the angle in the chromosphere that has changed.

MOORE: The usual response to a new observation that attacks or upsets a cherished idea is to attack the observation, or its interpretation. I would like to take the usual tack here. You say that the plage stood up. That's the basic assumption here. The alternative would be that it really didn't change that much, but the heating changed. And maybe the guy lays almost like this, but if he's down here, he's lower and you're being fooled. But maybe we can test this with sensitive vector magnetograms.

LABONTE: Yes, but the simple test I have, is to look at the picture the next day or the day after that. July 29 is easy, because it's not evolving fast. These other flares occurred in evolving regions, so even the next day things may be different. It may be hard to decipher but in virtually every case that plage is still there. There may be another flare going on but the plage is there; it outlasts the soft x-ray signature of the flare which I take to be roughly the time in which there is enhanced heat conduction from the corona.

MOORE: I'd like to echo Hal, though: it's a beautiful idea but it needs to be tested.

FISHER: I just wanted to ask a question about your derivation of the angle with respect to the vertical by the length of the fibril. How sensitive do you think that is, I mean how accurate do you think that is, basically.

LABONTE: Well, I think that should be treated with some suspicion, however what Ken Marsh did in his paper was to notice that fibrils, long fibrils, measured by thier maximum length as they grow and decay, were also long-lived fibrils, and by plotting lifetime versus length, he could get some set of data points. Then he assumed it to be ballistic, that the stuff was just going up and down as though it had

288 BARRY J. LaBONTE

been fired out of a cannon at a single velocity. Then one measures a

velocity - in fact one could got a velocity in there any way, even with direct Doppler measurements if you wanted. The curve fits right through

with no free parameters and so the observations of the properties of

fibrils suggest that they are well matched by a simple ballistic model

and in that case the length you see is just the measure of how far over

they are tilted. If they are standing up vertical you just don't seem

them for very long if they're lying down horizontal they go out along way before they slide back down. So that's questionable but that is the

observed property of fibrils.

I~TJDSON: I just wanted to comment that these may look like plage, but

they may be different in the sense that they have faculae beneath them.

Because if they had faculae, I can think of two problems: first you

would probably see a bigger flare effect in ACRIM because we see

faculae quite well and we see no flare effects in ACRIM. Second the standard cartoon for why faculae are bright has to do with looking down in the hot walls of these little holes in the chromosphere or the

photosphere and I don't see how that would change that would depend on

the angle of the field at all.

LABONTE= We're looking in the chromosphere and it's much higher. The

facular signal that would show up in ACRIM, for example, is much lower

than this.

HUDSON: What I'm saying is that the usual relationship between plage

and faculae is different for your cases here, and it may not ought to

be called plage at all.

LABONTE: Well, Hal calls them sun spots in cases when they are overlain

with Hu stuff that goes horizontal; when that goes away you see a spot. What I've got is the weaker-field case of fibrils sitting here and when they go away it turns into a plage. So it's the analogous thing a

couple orders of magnitude down.