Brown, Daniel

Schrijver, Carolus

English

The magnetic field in the outer solar atmosphere is frequently distorted by flares. In some cases, a fraction of the field exhibits a rapidly damped oscillation (Schrijver et al.; Aschwanden et al.). If this is a resonating wave trapped in the field, then the rapid damping requires a viscosity or resistivity that is at least 108 times larger than expected (Nakariakov et al.). We propose instead that some of the field lines are so sensitive to the source positions that rocking motions of the photospheric plasma associated with some solar flares (Kosovichev &amp; Zharkova) cause a few loops to oscillate in (anti)phase in the fundamental mode, with a period and decay rate that are determined largely by the characteristics of the photosphere, saying little about the high atmosphere

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Aberystwyth UniversityOscillations in the magnetic field of the solar corona in response to flares nearthe photosphereBrown, Daniel; Schrijver, CarolusPublished in:Astrophysical JournalPublication date:2000Citation for published version (APA):Brown, D., & Schrijver, C. (2000). Oscillations in the magnetic field of the solar corona in response to flares nearthe photosphere. Astrophysical Journal, 537, L69-L72.General rightsCopyright and moral rights for the publications made accessible in the Aberystwyth Research Portal (the Institutional Repository) areretained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights.            • Users may download and print one copy of any publication from the Aberystwyth Research Portal for the purpose of private study orresearch.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the Aberystwyth Research PortalTake down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.tel: +44 1970 62 2400email: is@aber.ac.ukDownload date: 18. Apr. 2020L69The Astrophysical Journal, 537:L69–L72, 2000 July 1q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.OSCILLATIONS IN THE MAGNETIC FIELD OF THE SOLAR CORONAIN RESPONSE TO FLARES NEAR THE PHOTOSPHERECarolus J. SchrijverStanford-Lockheed Institute for Space Research, Department L9-41, Building 252,3251 Hanover Street, Palo Alto CA 94304; schryver@lmsal.comandDaniel S. BrownDepartment of Mathematical Sciences, St. Andrews University, St. Andrews,KY16 9SS, Scotland, UK; daniel@mcs.st-andrews.ac.ukReceived 2000 March 20; accepted 2000 May 24; published 2000 June 27ABSTRACTThe magnetic field in the outer solar atmosphere is frequently distorted by flares. In some cases, a fraction ofthe field exhibits a rapidly damped oscillation (Schrijver et al.; Aschwanden et al.). If this is a resonating wavetrapped in the field, then the rapid damping requires a viscosity or resistivity that is at least 108 times larger thanexpected (Nakariakov et al.). We propose instead that some of the field lines are so sensitive to the source positionsthat rocking motions of the photospheric plasma associated with some solar flares (Kosovichev & Zharkova)cause a few loops to oscillate in (anti)phase in the fundamental mode, with a period and decay rate that aredetermined largely by the characteristics of the photosphere, saying little about the high atmosphere.Subject headings: Sun: activity — Sun: corona — Sun: magnetic fields1. INTRODUCTIONThe Transition Region and Coronal Explorer (TRACE) space-craft offers an unprecedented resolution of the hot outer atmo-sphere of the Sun (Schrijver et al. 1999): the extreme-ultraviolet(EUV) radiation originating from the K coronal6(1–2) # 10plasma is recorded with an angular resolution equivalent to ≈700km on the Sun and a cadence that is often substantially fasterthan one exposure per minute. But although this major stepforward in solar observations led to the discovery of transverseoscillations of the coronal magnetic field (Schrijver et al. 1999),only two cases of clear oscillations are known thus far: one isassociated with a flare at 12:55 UT on 1998 July 14, analyzedby Aschwanden et al. (1999), the other with a flare at 08:21 UTon 1999 July 4, first reported on here.These oscillations appear to occur in association with a co-ronal blast wave or rapid field reconfiguration, but the infre-quency with which loop oscillations are observed implies thatthe process is extremely sensitive to the conditions of the triggerand to the configuration of the magnetic field. The more com-mon coronal response to a flare is a field distortion that isfollowed by a rapid relaxation to the initial configuration, notinfrequently leading to a Moreton-Ramsey wave that can coverpart or most of the solar disk (Thompson et al. 1997; Wills-Davey & Thompson 1999). Many more examples of this re-sponse are known than of transverse field oscillations; the co-ronal magnetic field is apparently not easily driven to oscillateat observable amplitudes at periods of a few minutes.The field-line (or magnetic loop) oscillations have been in-terpreted as a resonance of trapped waves, possibly of fast kinkmode (Aschwanden et al. 1999), in essentially the fundamentalresonance of the field in which the loop footpoints form thefixed nodes. If the phenomenon is indeed a coronal resonance,then the damping timescale is the most telling quantity (Na-kariakov et al. 1999). Nakariakov et al. (1999) take the waveto be trapped in the coronal loop and assume that dissipationoccurs only through viscous or resistive processes. The damp-ing timescale then requires either the viscosity or the resistivityto be a factor of 108–109 higher than expected on theoreticalarguments (Nakariakov et al. 1999). If correct, this would easethe traditional problems with coronal heating mechanisms sig-nificantly, while requiring a substantial rethinking of our un-derstanding of magnetohydrodynamic processes.In this Letter, we propose an alternative mechanism for theoscillation and its rapid decay. The driving and decay in thiscase are determined by photospheric properties rather than thecoronal resonant response.2. OBSERVATIONS OF CORONAL LOOP OSCILLATIONSFollowing the flare on 1998 July 14, approximately 10 dis-tinct bundles of magnetic field lines seen in the 171 A˚ TRACEpassband (characteristic of ≈106 K) were observed to oscillate(Aschwanden et al. 1999). The average peak amplitude of theoscillations was 4100 km. The periods for all these loops wereclose to 5 minutes, and they were in phase to within approx-imately 1 minute (roughly the cadence of the observing se-quence) throughout the ≈15 minute interval during which theoscillations persisted.The flare on 1999 July 4, shown in Figure 1, occurred duringan orbital phase in which TRACE experienced background ra-diation that was too high for the relatively long EUV exposures,so that the initial phase was observed only at UV wavelengths.This C6 flare, in a region some 407 southeast of disk center,was first seen in the 1600 A˚ channel at 08:20 UT and occurredat the leading edge of the central bipolar structure (in whichtwo groups of opposite-polarity pores temporarily coagulateinto a sunspot-like configuration, but encompassing two po-larities rather than one). The TRACE 171 A˚ observations re-sumed at 08:33 UT, i.e., after approximately one damping time-scale as observed for the 1998 July 4 flare (Nakariakov et al.1999). After that, only one oscillation is observed in a fan ofloops enclosed by the two examples shown in the bottom panelof Figure 1. The period is consistent with 5 minutes but isobviously poorly determined; the observed displacements havea maximum amplitude of 3000 km (for the upper loop) to 6700km (for the lower loop), suggesting substantially larger am-plitudes in the initial phase. Interestingly, they move in anti-L70 OSCILLATIONS IN MAGNETIC FIELD Vol. 537Fig. 1.—Oscillations of the magnetic field in the solar outer atmosphere.Top: Magnetogram (Michelson Doppler Imager [MDI] on SOHO). The circlemarks the location of the initial flare brightening at 08:20 UT in the TRACE1600 A˚ channel. Bottom: TRACE image in the 171 A˚ channel [characteristicof K] after oscillations have damped. The set of oscillating6(1–1.5) # 10coronal loops is bounded by the solid white lines (at 08:36 UT). The dashedlines show the initial positions of the loops (at 08:33 UT). These two loopsare in antiphase. A faint loop in between these (dotted gray line) is slightlyout of phase with respect to the upper marked loop. The bright flare ridge,near which the oscillating loops end, is marked by a curve that is shown inboth panels. The field of view has sides of 290 Mm.Fig. 2.—Configuration of the magnetic field susceptible to oscillations. Thefigure shows a simple configuration of magnetic charges in a plane; the positivesources are shown as plus signs; the negative sources are shown as opendiamonds. The null points where the magnetic field vanishes are shown asfilled circles. The field lines shown demarcate the surface that forms the as-ymptote (called the separatrix fan of null point N) for field lines from eitherof the two nearest charges. The two line segments in the plane connecting thenull point to the charges are called spine field lines.phase. A faint loop between these two appears to lag slightlybehind the nearby upper loop, but by no more than approxi-mately 1 minute.The common properties of these two instances in whichoscillating loops have been seen are that (1) their periods lieclose to 5 minutes, (2) the damping timescale is of order 15minutes, and (3) the oscillations of all loops are very nearlyin phase or in antiphase.3. DISCUSSION AND CONCLUSIONSInstead of the resonance mechanism for coronal oscillationthat was proposed by Nakariakov et al. (1999), we propose analternative scenario that offers a natural explanation for theobserved timescales without invoking extraordinary coronalconditions. This mechanism relies solely on the properties ofmagnetic field topologies in which field lines move throughspace with very different velocities as the field sources aremoved about (an example is shown in a movie by Priest &Schrijver 1999). We explore this process using simple vacuumpotential fields; nonvacuum fields are expected to have similarproperties.For clarity, we start from a configuration of magnetic chargesin a plane that crudely mimics the configurations observed forthe two loop oscillations: arcs of a given polarity nested aroundeach other in a central inversion of the polarity pattern (Fig. 2).Between the charges of like polarities, there are null points wherethe field goes to zero. These null points lie on what are knownas the spine field lines that connect them to the nearby charges.The other mathematical extension of these spine field lines de-fines the separatrix (Brown & Priest 1999) that separates thedomains of connectivity between the pairs of charges (Fig. 2).Typically, the two spine field lines going into a null point spreadout into a surface, or fan, around the null point; this separatrixfan acts as an asymptote for field lines that lie close to the spines.Now imagine a field line emanating from a charge (C inFig. 2) in a direction that goes almost but not quite to thenearby null point (N), lying just slightly above the plane andto one side of the null point when seen from above (for ex-ample, one that lies close to the separatrix field line pointedout by the arrow in Fig. 2). If some of the charges in the planemove slightly, so will the null points, but the null points, theseparatrices, and their intersections at separators will generallychange little in position. Any field line passing close to a nullpoint, however, will display a substantial swing in position. Ifthe null point is moved just to the other side of the field-linedirection from the source, the field line will swing very nearlyacross the entire separatrix dome (more or less swinging alongNo. 1, 2000 SCHRIJVER & BROWN L71Fig. 3.—Oscillations in the magnetic field over a magnetic region on thesolar surface. The horizontal plane, cut in the middle, shows the projection oflines of force in a potential magnetic field with sources as in Fig. 2 (opendiamonds and plus signs for the opposite polarities). The vertical plane showsthe temporal evolution of the field at a cut through the projection plane. Smallperiodic displacements of two of the central sources cause all field lines tooscillate. Those that pass close to a null point exhibit a much higher amplitudethan those that give the null points a wider berth, even though they may lieclose to the more strongly oscillating field lines throughout their path to anopposite charge. The oscillation of the field line marked by the arrow is shownin Fig. 4.Fig. 4.—Loop oscillations in the central cut through the field, as in Fig. 3.The displacement (dashed line) of the magnetic field line marked by the arrowin Fig. 3, which has the largest amplitude, is 7 times larger than that of thecentral magnetic charges (solid line). The displacement of the null point (dottedline; right-hand axis) is almost 300 times smaller than the loop-top amplitude.the arc indicated by the dotted line in Fig. 2). This phenomenondoes not necessarily require reconnection to occur on the mov-ing field line (Priest & Schrijver 1999), although some fieldlines moving ahead and/or behind them need to be involvedin that process. The perturbation of the null point may be drivenby the displacement of quite distant charges.We propose that the phenomenon of oscillating loops is relatedto sunquakes that may be triggered by near-surface solar flares(Kosovichev & Zharkova 1998). If the flare is violent enoughor the magnetic configuration favorable, a shock wave can coupleto the solar surface (photosphere). Alternatively, one could imag-ine that a rapid topological reconfiguration in the corona causesfootpoints to adjust rapidly as Lorentz forces propagate the per-turbation down to the photosphere. Either way, the flare caninduce a displacement of the field sources at the surface, whichinduces in turn an oscillatory relaxation of the field as the fieldrocks about in the photospheric gas. Unfortunately, at the timeof the flare on 1998 July 14, the Solar and Heliospheric Ob-servatory (SOHO) was not observing the Sun. For the flare on1999 July 4, no significant photospheric perturbation was ob-served (A. Kosovichev 2000, private communication). Hence,we cannot quantify the magnitude of the oscillations that wereexcited. The mechanism that we propose works, however, evenfor relatively small displacements.The flare that triggers the loop oscillation observed on 1999July 4 goes off near the center of the magnetic configuration. Ifwe oscillate the central sources of opposite polarity in our chargeplane, we see that field lines passing very near to null pointsshow much larger oscillations, whereas the others are hardlydisplaced at all (Fig. 3). Comparing our simulations with thescale of the observed magnetic region on the Sun, a displacementof several hundred kilometers of the magnetic charges couldresult in a loop oscillation with an amplitude of several thousandkilometers (Fig. 4), as is observed. In this scenario, the periodof the oscillation is dominated by the photospheric oscillation(typically around 5 minutes), and the damping is simply a con-sequence of the propagation of the photospheric oscillation asin the case of a pebble thrown into a pond (two to three periods),as observed by Kosovichev & Zharkova (1998). The initial am-plitude of 1.5 km s21 observed for the single sunquake observedso far is compatible with the required amplitude of a few hundredkilometers for the source displacements. The delay between theflare and the excitation of the loop oscillations is small—determined by the Alfve´n speed in the corona—as indeedobserved.The real magnetic configuration is much more complex thanthe simplified version used to illustrate the scenario: the flux ineach of the two polarities is concentrated in a multitude of fluxtubes scattered over the opposite-polarity areas within the mag-netic plage. Welsch & Longscope (1999) argue that within anenvironment of N concentrations of the same polarity, there arenull points, provided there are no degenerate nulls causedN 2 1by particular symmetries. Each of these nulls will be affectedsomewhat by the displacement of a charge. All displacementswill be in phase with the rocking motion of a footpoint thatresults from a flare, but the direction of motion of a respondingloop depends on the direction in which the null shifts. As aresult, loops are expected to oscillate either in phase or in an-tiphase with others depending on the displacement of the nullthey approach, as is indeed observed. In the example shown inFigure 1, the loops oscillating in antiphase end at different pointson a ridge. This ridge passes through the center of the trailingpolarity and therefore is necessarily close to a number of differentnull points located between the substantial number of flux tubesin the photosphere. The larger number of nulls leads to smallerseparatrix surface areas, which limits the amplitude of the looposcillations to roughly the spacing of the null points times theexpansion factor of the field between the canopy and the highercorona; the observed amplitudes of some 4000–7000 km areallowed by the expected size of the connectivity volumes for atypical spacing of some 5000 km and a field expansion of afactor of 2–3.The interpretation of loop oscillations as a response to pho-tospheric perturbations, amplified through null-point displace-ments, is supported by our initial explorations using potentialfields. Other magnetic configurations may be much more sen-sitive to small-amplitude charge displacements, while closerL72 OSCILLATIONS IN MAGNETIC FIELD Vol. 537packing of like-polarity charges (as is the case on the real Sun)may increase the fraction of the field involved in the process(although not many field lines should be involved, in view ofthe fact that loop oscillations are a rarely observed phenomenon).Further research is needed to tell what causes the rare photo-spheric oscillations, how large the associated displacement os-cillations are, and how that perturbation propagates into the co-rona. Whichever explanation of the loop oscillations will proveto be correct, their study has already deepened our understandingof the solar corona. If our scenario is correct, however, the coronamay not have the remarkably high viscosity or resistivity arguedfor by Nakariakov et al. (1999).REFERENCESAschwanden, M. J., Fletcher, L., Schrijver, C. J., & Alexander, D. 1999, ApJ,520, 880Brown, D. S., & Priest, E. R. 1999, Proc. R. Soc. London A, 455, 3931Kosovichev, A. G., & Zharkova, V. V. 1998, Nature, 393, 317Nakariakov, V. M., Ofman, L., DeLuca, E. E., & Davila, J. M. 1999, Science,285, 862Priest, E. R., & Schrijver, C. J. 1999, Sol. Phys., 190, 1Schrijver, C. J., et al. 1999, Sol. Phys., 187, 261Thompson, B. J., Newmark, J. S., Gurman, J. B., St. Cyr, O. C., & Stetzelberger,S. 1997, BAAS, 29, 01.30Welsch, B. T., & Longcope, D. W. 1999, ApJ, 522, 1117Wills-Davey, M., & Thompson, B. J. 1999, Sol. Phys., 190, 467

Oscillations in the magnetic field of the solar corona in response to flares near the photosphere

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Oscillations in the magnetic field of the solar corona in response to flares near the photosphere

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