Flare Arcade Research Articles

Discovery of a Temperature-Dependent Upflow in the Plage Region during a Gradual Phase of the X-Class Flare

Abstract We present Hinode/EIS raster scan observations of the plage region taken during the gradual phase of the GOES X3.2 flare that occurred on 2006 December 13. The plage region is located 200${}^{\prime\prime}$ east of the flare arcade. The plage region has a small transient coronal hole. The transient coronal hole is strongly affected by an X-class flare, and upflows are observed at its boundary. Multi-wavelength spectral observations allow us to determine velocities from the Doppler shifts at different temperatures. Strong upflows along with stationary plasma have been observed in the Fe XV line 284.2 Å (log$T/$ K $=6.3$) in the plage region. The strong upflows reach almost 150 km s$^{-1}$, which was estimated by a two-component Gaussian fitting. On the other hand, at a lower corona/transition region temperature (He II, 256.3 Å, log$T/$ K $=$ 4.9), very weak upflows, almost stationary, have been observed. We find that these upflow velocities clearly depend on the temperature with the hottest line, Fe XV, showing the fastest upflow velocity and the second-highest line, Fe XIV, showing the second-highest upflow velocity (130 km s$^{-1}$). All velocities are below the sound speed. The trend of the upflow dependence on temperature dramatically changes at 1 MK. These results suggest that heating may have an important role for strong upflow.

The Width of a Solar Coronal Mass Ejection and the Source of the Driving Magnetic Explosion: A Test of the Standard Scenario for CME Production

We show that the strength (B(sub F1are)) of the magnetic field in the area covered by the flare arcade following a CME-producing ejective solar eruption can be estimated from the final angular width (Final Theta(sub CME)) of the CME in the outer corona and the final angular width (Theta(sub Flare)) of the flare arcade: B(sub Flare) approx. equals 1.4[(Final Theta(sub CME)/Theta(sub Flare)] (exp 2)G. We assume (1) the flux-rope plasmoid ejected from the flare site becomes the interior of the CME plasmoid; (2) in the outer corona (R > 2 (solar radius)) the CME is roughly a spherical plasmoid with legs shaped like a lightbulb; and (3) beyond some height in or below the outer corona the CME plasmoid is in lateral pressure balance with the surrounding magnetic field. The strength of the nearly radial magnetic field in the outer corona is estimated from the radial component of the interplanetary magnetic field measured by Ulysses. We apply this model to three well-observed CMEs that exploded from flare regions of extremely different size and magnetic setting. One of these CMEs was an over-and-out CME, that is, in the outer corona the CME was laterally far offset from the flare-marked source of the driving magnetic explosion. In each event, the estimated source-region field strength is appropriate for the magnetic setting of the flare. This agreement (1) indicates that CMEs are propelled by the magnetic field of the CME plasmoid pushing against the surrounding magnetic field; (2) supports the magnetic-arch-blowout scenario for over-and-out CMEs; and (3) shows that a CME's final angular width in the outer corona can be estimated from the amount of magnetic flux covered by the source-region flare arcade.

Theoretical Predictions of X‐Ray and Extreme‐UV Flare Emissions Using a Loss‐of‐Equilibrium Model of Solar Eruptions

In this paper, we present numerical simulations of solar flares that couple a loss-of-equilibrium solar eruption model with a one-dimensional hydrodynamic model. In these calculations, the eruption is initiated by footpoint motions that disrupt the balance of forces acting on a flux rope. After the eruption begins, a current sheet forms and an arcade of flare loops is created by reconnecting magnetic fields. Thermal energy input into the flare loops is found by assuming the complete thermalization of the Poynting flux swept into the current sheet. This thermal energy is input into a one-dimensional hydrodynamic code for each loop formed in the multithreaded flare arcade. We find that a density enhancement occurs at the loop top when the two evaporating plasma fronts in each leg of the loop collide there. Simulated flare images show that these loop-top density enhancements produce bars of bright emission similar to those observed in the Transition Region and Coronal Explorer (TRACE) 195 A bandpass and loop-top knots of bright emission seen in flare observations by the Soft X-Ray Telescope (SXT) on Yohkoh. We also simulate flare spectra from the Bragg Crystal Spectrometer (BCS) on Yohkoh. We find that during the early stages of flare initiation, there are significant blueshifts in the Ca XIX line, but the intensities are too faint to be observed with BCS. In general, the results of this model simulate observed flare emissions quite well, indicating that the reconnection model of solar flares is energetically consistent with observations.

Investigation of long-duration arcade flares

Aims. We present the analysis of the three well-observed long-duration arcade flares (LDAFs). We aim to find general properties of LDAFs. In this paper results concerning morphology and physical parameters in these flares are shown. Methods. Yohkoh observations, Kitt Peak Vacuum Telescope (KPVT) magnetograms and GOES /SEM measurements are used to determine morphology and evolution of the analysed flares and physical parameters of their loop-top kernels (temperature, density, altitude). Results. We found that: (1) a so-called arcade channel may exist in LDAFs; (2) the energy release in LDAFs occurs in several places simultaneously and the reconnection process is not uniform along the flaring arcade; (3) a presence of $20{-}30$ MK hot plasma during the decay phase suggests that the reconnection occurs after the LDAF maximum.

Investigation of long-duration arcade flares

Aims. We analyse three well-observed long-duration arcade flares (LDAFs) to find their general properties. Results concerning energy release in LDAFs (energy balance and diagnostic diagrams) are shown. Methods. Yohkoh observations and Kitt Peak Vacuum Telescope magnetograms are used to determine physical parameters of flare loop-top kernels (temperature, density, altitude). Using these parameters the energy release is studied in detail. Results. We found that: (1) the energy release in LDAFs occurs during their whole decay phase; (2) the kernel heating ( E H ) is of the order of 0.01–1 erg cm -3 s -1 ; (3) the kernels usually cannot be distinguished in SXT images when E H is drops below a particular value. This value depends on the altitude of the kernels.

Energy Release Rates along Hα Flare Ribbons and the Location of Hard X‐Ray Sources

Local reconnection and energy release rates for an X3.8 flare that occurred on 2005 January 17 are derived. In particular, we distinguish between Hα flare ribbon segments that were accompanied by RHESSI hard X-ray (HXR) footpoints and those without HXRs. We find that the reconnection and energy release rates are not uniform along the flare ribbons but much larger at the locations where the HXR footpoints are observed. The difference is about 2 orders of magnitude in the case of the energy release rates and 1 order of magnitude for the reconnection rates (with peak values up to 8 kV m-1). These differences are enough to explain the different flare morphologies typically observed in HXRs (compact footpoints) and Hα/UV (extended ribbons) by the limited dynamic range of present HXR instruments. Our results are consistent with a scenario in which the electrons are accelerated primarily along a certain subsystem of magnetic loops as outlined by the HXR footpoints, and only a minor fraction (for the 2005 January 17 flare estimated to be about 1/15) go into the large flare arcade outlined by the Hα ribbons and EUV postflare loops.

Measurement of the Energy Release Rate and the Reconnection Rate in Solar Flares

By using the method presented by Isobe et al. (2002), the non-dimensional reconnection rate Vin/Va has been determined for the impulsive phase of three two-ribbon flares, where Vin is the velocity of the reconnection inflow and Va is the Alfven velocity. The non-dimensional reconnection rate is important to make a constraint on the theoretical models of magnetic reconnection. In order to reduce the uncertainty of the reconnection rate, it is important to determine the energy release rate of the flares from observational data as accurately as possible. To this end, we have carried out one dimensional hydrodynamic simulations of a flare loop and synthesized the count rate detected by the soft X-ray telescope (SXT) aboard Yohkoh satellite. We found that the time derivative of the thermal energy contents in a flare arcade derived from SXT data is smaller than the real energy release rate by a factor of 0.3 - 0.8, depending on the loop length and the energy release rate. The result of simulation is presented in the paper and used to calculate the reconnection rate. We found that reconnection rate is 0.047 for the X2.3 flare on 2000 November 24, 0.015 for the M3.7 flare on 2000 July 14, and 0.071 for the C8.9 flare on 2000 November 16. These values are similar to that derived from the direct observation of the reconnection inflow by Yokoyama et al. (2001), and consistent with the fast reconnection models such as that of Petschek (1964).

Soft X-ray analysis of a loop flare on the Sun

We present the results of an analysis of soft X-ray images for a solar flare which occurred on 1992 July 11. This flare, as seen in Yohkoh Soft X-ray Telescope (SXT) images was of comparatively simple geometry, consisting of two bright footpoints early in the flare with a bright loop seen later in the flare. We examine how closely this flare compares with the supposed paradigm of a confined simple-loop flare. Closer examination of the SXT images reveals that the flare structure consisted of at least two adjacent loops, one much fainter than the other. We examine the brighter of the two soft X-ray loops. The SXT images reveal an apparent slow, northward motion of this loop (roughly transverse to its major axis). Examination of derived emission measure and temperature images also indicate an apparent northward motion. In addition, we find an increase in the cross-sectional width at the top of the loop with time. Emission measure maps derived from the SXT images also indicates an apparent broadening of the loop-top region. We infer that the apparent northward motion and the apparent broadening of the soft X-ray emission can be explained in a reconnection scenario where successive magnetic field structures do not lie in a plane but are tilted to the south of the line of sight but with successively brightening loops oriented at less tilted angles. Halpha images for this flare reveal an evolution from a few brilliant points to a short two- ribbon-like appearance. Comparison of the SXT images with the Halpha images shows that the Halpha patches are aligned with the footpoints of the soft X-ray loops, suggesting the presence of a small arcade structure. There is no clear evidence for an eruptive signature in our observations nor in reports from other observations. The lack of an eruptive signature could suggest that the flare may have been a confined simple-loop flare, but this is not compelling due to a gap in the coronal observations prior to and early in the event. Analysis of our observations indicate that the flare exhibited characteristics suggesting that it may be better understood as a mini-arcade flare. These results casts doubt on the validity of the supposed paradigm of a confined simple-loop flare, at least for this flare. They indicate that even an apparently simple-loop flare may be considered to be a variety of arcade flare. We also find an effect which, to our knowledge, has not been reported before: the hot flaring regions later become cooler than the surrounding quiescent corona. That is, the flare loops do not evolve into bright active region loops, but into cooler loops. This may indicate an increase in the efficiency of the cooling mechanism or a transformed equilibrium state within the flaring loops.

TRACE Observation of an Arcade Flare Showing Evidence Supporting Quadruple Magnetic Source Model for Arcade Flares

Abstract The result of analyses of an arcade flare on 1999 July 19, observed by the Satellite TRACE, is reported, and the significance of the following findings is stressed. It is clearly seen that four magnetic regions of alternating polarities [named A($+$), B($-$), C($+$), and D($-$) from the East in the flaring region] are involved in this arcade flare event in an essential way. The high structure covering the triangular region between A and the northern part of D disappears when the dark filament lying between B and C erupts. The X-ray arcade is formed only between B and C. The overlying structure connecting regions A and the northern part of D disappears and the entire region is involved in the flare. Low-lying loops connect the inner sources to the outer sources. The field lines from regions B to A and C to D are not much affected by the eruption of the dark filament, itself, but the loops near the upper surfaces of the closed loop regions are clearly seen to move toward each other (from both sides to the central line) as the arcade flare progresses between the inner pair, B and C; some of them appear to be converted into a flare arcade. The observed behavior can best be understood in terms of the quadruple magnetic source model advocated by one of the authors (YU). The long-conceived “reclosing of once opened bipolar magnetic arcade” models by the rising dark filament (called CSHKP model) find it difficult to explain this.

Shock-excited radio burst from reconnection outflow jet?

Models of dynamic (two-ribbon-, arcade) flares involve the formation of a system of standing slow and possibly also fast mode shock waves associated with the fast reconnection process below the erupting filament. These shocks are anticipated theoretically, but are not unambiguously confirmed by observations. In this paper we identify for the first time the radio signature of a fast mode outflow termination shock in a dynamic radio burst spectrogram. The standing fast mode shock is revealed by a zero-drift type II burst recorded between 300 and 400 MHz. It started almost 1 hour after the impulsive phase of the 7 April 1997 flare and lasted for more than 30 min. The burst shows a characteristic herringbone fine structure and a band split of (10 ′ 6)% of the emission frequency. No fundamental-harmonic pattern was observed, and we argue that the feature is fundamental mode emission. Simultaneous imaging observations (Ha, Yohkoh SXT, SOHO EIT) show a relaxed postflare loop arcade with a bright soft X-ray cusp commonly interpreted as a typical reconnection pattern. Conditions for termination shock formation and excitation of radio emision are investigated. Favourable circumstances for the radio detection of a termination shock in the reconnection outflow are a comparatively large height of the diffusion region, a low plasma to magnetic pressure ratio β upstream of the slow shocks, and a small angle between the reconnecting field lines. Finally, we discuss why similar radio signatures are not observed more frequently, and why it appeared so late in the event. We stress the implications and point to some inconsistencies which might be a consequence of commonly practiced ad hoc application of idealized model results to realistic conditions.

Investigation of the energy release in long-duration flares

We investigate an example of long-duration arcade flare of 14/15 May 1993. Its spatial structure can be reliably determined from the Yohkoh X-ray images and Kitt Peak magnetograms. Temperatures, densities and heights of the loop-top flare kernels have been estimated from these data and also the kernel heating has been investigated in detail. We have found that (1) The energy release in the loop-top flare kernels occurs also during the whole flare decay phase; (2) The kernel evolution during the decay phase is quasi-steady-state, i.e. it is determined by the slow decrease of the heating rate, E H(t).

High Spatial Resolution Observations of a Hot Region in a Solar Flare with the [ITAL]Transition Region and Coronal Explorer[/ITAL

The Transition Region and Coronal Explorer (TRACE) provides some of the highest spatial resolution images ever taken of hot solar flare plasma. The TRACE 195 A channel is particularly sensitive to high-temperature flare plasma because of the presence of the Fe XXIV λ192 resonance line in this bandpass. The TRACE 171 A channel observes emission from thermal bremsstrahlung during a flare. Since this emission is generally weak, it is usually not possible to derive electron temperatures for flare plasma from TRACE observations. In this Letter, we present analysis of the 2000 March 24 X1.8 limb flare that produced high count rates in both the TRACE 195 and 171 A channels. We find evidence for a small, high-temperature region near the top of the flare arcade. This hot region appears to lie at the base of the cusp-shaped structure that extends above the arcade. The TRACE observations are consistent with a strong enrichment of Fe over its photospheric value in the hot region that suggests in situ heating of this plasma. We also find that multithermal simulations of flare evolution reproduce the observations much better than an isothermal model does.

Downflows and structure above LDE arcades: Possible signatures of reconnection?

We will introduce a set of observations made during the decay phase of long duration event (LDE) flares on the Sun. In a number of events, the soft X-ray images from Yohkoh SXT indicate a downward directed flow field in the region immediately above the flare arcade. These are tentatively identified as evidence of reconnection outflows. The prototypical event is the M5 flare of 20 January 1999, presented by McKenzie and Hudson (1999); since the time that paper was completed, sixteen more events have been found in the interval April 1998 to February 2000. As in the 20 January event, the speeds of downflow are 35–500 km s−1, lower than both the freefall speed and the typically assumed Alfven speed. The motion is evidenced by both dark and bright (i.e., X-ray emitting) features, some of which may have a looplike morphology; no cool counterparts have been detected in Hα or EUV observations. Movies depicting some of the LDE downflows were presented at this meeting; these also appear in the journal Solar Physics (McKenzie, 2000), since they cannot be presented in these Proceedings.

A Quadruple Magnetic Source Model for Arcade Flares and X‐Ray Arcade Formations outside Active Regions. II. Dark Filament Eruption and the Associated Arcade Flare

A 2.5-dimensional MHD simulation based on the quadruple magnetic source model (Uchida et al.) was performed to deal with a dark filament eruption and the associated arcade flare. The numerical results are summarized as follows:

3D Structure of Arcade-Type Flares Derived from the Homologous Flare Series of 21, 24, and 27 February 1992

In February 1992, three flares, which we consider constitute a homologous flare series (flares having basically the same configuration repeating in the same situation), occurred in the active region NOAA 7070 and were observed by Yohkoh SXT. In the present paper, we first discuss the homology of these three flares, and derive the 3D structure by making use of the information obtained from the three different lines of sight at common phases. The result of this analysis made clear for the first time that the so-called `cusped arcade' at the maximum phase in the well-known 21 February 1992 flare is, contrary to the general belief, an `elongated arch' created at the beginning of the flare, seen with a shallow oblique angle. It is not the `flare arcade' seen on axis as widely conceived. This elongated arch roughly coincides with a diagonal of the main body of the soft X-ray arcade that came up later. The magnetic structure responsible for the flare as a whole turned out to be a structure with quadruple magnetic sources – with the third and fourth sources also playing essential roles. The observationally derived information in our paper provides strong restrictions to the theoretical models of the process occurring in arcade flares.

Investigation of Coronal Mass Ejections. I. Loop-type with Arcade Flare between the Fixed Legs, and Bubble-type Due to Flare Blast Waves

AbstractIn this paper, we give arguments that there are two types of coronal mass ejection (CME).The first type of CME discussed here is the ‘loop-type’, whose occurrence is related to an arcade flare somewhere between the footpoints. It was found that there were pre-event magnetic connections between the flare location and the locations of the footpoints of a CME of this type, and that these connections disappeared after the event. This suggests that the footpoints of loop-type CMEs are special prescribed points, and this was verified by the observation that the footpoints do not move in this type of CME.The other type of CME is the ‘bubble-type’, which is associated with the flare blast from explosive flares. We confirmed the association of this type of CME with the so-called EIT (Extreme Ultra-violet Imaging Telescope) waves, but the velocity of expansion of the bubble is twice or more greater than that of the EIT waves depending on events. Although EIT waves were widely considered to be Moreton waves viewed by SoHO/EIT in the solar activity minimum period, recent simultaneous observations of both have revealed that the EIT wave is something different from the Moreton wave, and propagates separately with a velocity less than half that of a Moreton wave.We therefore propose a new overall picture: the bubble-type CMEs are the flare-produced MHD blast waves themselves, whose skirt is identified as a Moreton wave. EIT waves may be interpreted as follows: the slow-mode gas motions from the source cause secondary longwavelength fast-mode waves which are trapped in the “waveguide” in the low corona. The secondary long-wavelength wave in the fast-mode, which is trapped in the low corona, has a slower propagation velocity due to the nature of the waves trapped in a “waveguide”. This trapped wave induces slow-mode motions of the gas through a mode-coupling process in the high chromosphere, where the propagation velocities of the fastand slow-mode waves match.Three-dimensional MHD simulations for these two types of CME are in progress, and are previewed in this paper.

Microwave Observation of Eruptive Solar Events with and without Flare Activity

We present microwave (17 GHz) observations of eruptive activity in four solar coronal events with the Nobeyama Radioheliograph. These are weak events occurring at or near the solar limb associated with several types of activity: polar crown activity, prominence eruptions, and arcade flares. The morphological evolutions of the microwave sources in these events show the following characteristic features in common. The activity starts as a mound-shaped source (1.0-4.5 × 104 km in height), above which a compact blob (≤1.0 × 104 km in size) appears later and expands horizontally toward the top of another low-lying mound. Finally a faint arch with a filamentary structure is formed, bridging the two mounds (0.2-2.0 × 105 km in horizontal scale size). Thus, the activity seems to propagate through the arch corridor from the initially activated mound to the secondary. During this process, the activity level increases, as indicated by an increase of the brightness temperature of the mound and the blob as well as by the formation of the arch and the eruption of the blob. These common features suggest that basically the same energy buildup and release process takes place in all four events, in spite of the large difference in the total released energy. Here we propose magnetic reconnection progressing in between the blob and the mound as the basic process, as indicated by high-temperature plasma there, seen in soft X-rays. Our findings support a theoretically predicted analogy between filament (or prominence) activity and flare onset.

Supra-arcade Downflows in Long-Duration Solar Flare Events

This report seeks to introduce the reader to a set of observations made during the decay phase of long-duration event (LDE) flares on the Sun. In a number of events, the soft X-ray images from Yohkoh SXT indicate a downward-directed flow field in the region immediately above the flare arcade. The prototypical event is the M5 flare of 20 January 1999, presented by McKenzie and Hudson (1999); since the time that paper was completed, eleven more events, all related to coronal mass ejections, have been found in the interval 1 January 1998 to 8 May 1999. As in the 20 January event, the speeds of downflow are 45–500 km s−1, lower than both the free fall speed and the typically assumed Alfven speed. A comparison is drawn between the SXT observations and the EUV and Hα data, where such are available. Important additions to our knowledge since the first paper are: (i) the motion is evidenced by both dark and bright (i.e., X-ray emitting) features, some of which may have a loop-like morphology; (ii) no cool counterparts have been detected in Hα or EUV observations. The data are compared to the expected appearance according to two likely (and opposing) interpretations: an `above-the-arcade coronal rain' interpretation and a `shrinking magnetic flux tube' interpretation. In the current standard explanation of the formation of post-CME arcades, motion of the latter type may be related to outflow from a reconnection site. Movies depicting some of the flare events are on the accompanying CD-ROM.

Current-sheet-associated radio sources and development of the magnetosphere of an active region revealed from 17 GHz and Yohkoh data

We study the evolution of the active region (AR) NOAA 7321 in which appeared a so-called `neutral-line-associated source' (NLS) on the basis of data of the Nobeyama Radioheliograph and Yohkoh/SXT. We provide a physical interpretation of the NLS in terms of a topological magnetic reconnection model in a quadrupole magnetic configuration and discuss its relation to the evolution of the active region. Two kinds of the NLS were observed at 17 GHz. One of them, `rising NLS', was found in the growth stage. The other was `stationary NLS' detected in the maximal stage of the AR. Their presence was associated with substantial expansion of the active region's magnetosphere and accompanied by gradual development of spine-like structures visible in soft X-rays before homologous long-duration arcade flares. We suggest that the rising 17 GHz NLS corresponded to a fragment of a `horizontal' current sheet moving upward. Bright X-ray spines were boundaries of that current sheet. Almost all bursts observed from 26 to 28 October 1992 which accompanied class C and M flares occurred in the rising NLS. Formation of magnetic X-point singularities is believed to be responsible for the low-lying NLS. Reversal of circular polarization due to the effect of radio wave propagation was detected in that NLS on the limb. The initial stage of the microwave burst of the long-duration X9 class flare on 2 November 1992 occurred in this NLS. We also revealed observational manifestations of the presence of `vertical' non-neutral current sheet in the spatial structure of this NLS before the flare.

A Quadruple Magnetic Source Model for Arcade Flares and X-Ray Arcade Formations outside Active Regions I. Dark Filament Suspension and the Magnetic Structure in the Pre-Event Regions

Abstract The high-sensitivity, wide dynamic-range observations by the Soft X-ray Telescope (SXT) aboard Yohkoh has enabled us to look into the faint pre-event structures of arcade flares, and of even fainter X-ray arcade formation events outside active regions. What we have found in the pre-event structure of the latter, however, was not a sagged simple (bipolar) arcade, as expected in the classical model, but a “dual-arcades„ type structure in which the inside legs of each “arcade„ cross with the other's, landing at the closer part of the domain of the other. Similar features, together with some other features inexplicable in the classical arcade flare model, were also found in strong arcade flares in active regions seen axis-on at the limb. These features raised a severe problem with the classical “reclosing of the once opened simple arcade„ model(s). In the present paper, we propose interpretations of what we discovered by Yohkoh-SXT by reviving a quadruple source model proposed by one of the authors (YU) years ago, pointing out a serious difficulty in the classical model(s). This model, based on the quadruple magnetic sources in the photosphere, has a “neutral sheet„ already in the pre-event phase in the corona above the field polarity-reversal line in the photosphere, and turns out to explain quite nicely the structures of both faint pre-event corona before arcade formation events, and that of arcade flares discovered by Yohkoh-SXT. A dynamic model of arcade flares and arcade formation events based on this dark filament model will be discussed in the forthcoming Paper II of this series.