Hα Ribbons Research Articles

Parallel Motions of Coronal Hard X-Ray Source and Hα Ribbons

During solar flares Hα ribbons form and often move away from the local magnetic polarity inversion line (PIL). While the motion perpendicular to the PIL has been taken as evidence for coronal magnetic reconnection in the so-called CSHKP standard model, the other velocity component parallel to the PIL is much less adopted as a property of the magnetic reconnection process. In this Letter we report an event in which the motion parallel to the PIL is found in both Hα ribbons and a thermal hard X-ray source. Such commonality would indicate a link between the coronal magnetic reconnection and footpoint emissions as in the standard solar flare model. However, its direction implies a reconnection region that is increasing in length, a feature missing from the standard two-dimensional model. We present a modified framework in which the variation along the third dimension is allowed, in order to assess the effect of such a proper motion on estimation of the magnetic reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hα filtergrams of Big Bear Solar Observatory (BBSO), and the SOHO Michelson Doppler Imager (MDI) magnetogram obtained for the 2004 March 30 flare.

Hard X-Ray Intensity Distribution along Hα Ribbons

Unusual ribbon-like hard X-ray sources were found with the RHESSI observation of a 2B/M8.0 flare on 2005 May 13. We use this unique observation to investigate the spatial distribution of the hard X-ray intensity along the ribbons and compare it with the local magnetic reconnection rate and energy release rate predicted by the standard magnetic reconnection model for two ribbon flares. In the early phase of the flare, the hard X-ray sources appear to be concentrated in strong field regions within the Hα ribbons, which is explicable by the model. At and after the flare maximum phase, the hard X-ray sources become spatially extended to resemble Hα ribbons in morphology, during which the spatial distribution of hard X-ray intensity lacks a correlation with that of the local magnetic reconnection rate and energy release rate predicted by the model. We argue that the magnetic reconnection during this event may involve the rearrangement of magnetic field along the magnetic arcade axis that is inevitably overlooked by the two-dimensional model and suggest that this type of three-dimensional reconnection will be best seen in so-called sigmoid-to-arcade transformations.

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.

Non-LTE diagnostics of velocity fields during the gradual phase of a solar flare

We perform an analysis of the velocity field within the Hα ribbons during the gradual phase of an M 1.0 solar flare observed on October 22, 2002. We use spectroscopic observations performed with the German VTT (Vacuum Tower Telescope) working in the MSDP (Multichannel Subtractive Double Pass spectrograph) observing mode. From these obser- vations the Hα line profiles in chosen areas of the flare ribbons were reconstructed and these observational profiles were compared with a grid of synthetic Hα line profiles calculated by the non-LTE radiative-transfer code. This code allows us to calculate different models of the chromosphere with a prescribed velocity field. By optimising the best fit between the observed and synthetic profiles we find the most appropriate models of the chromosphere and vertical structure of the velocity field in the analysed areas of the flare ribbons. By means of the non-LTE radiative-transfer calculations we show that in most analysed areas of the Hα flare ribbons the chromospheric plasma exhibited upward motion with a mean velocity of a few km s −1 .T hese results are consistent with previous estimates and support the scenario of a gentle evaporation during the gradual phase.

Flare-related changes in the profiles of six photospheric spectral lines

The profiles of six photospheric absorption spectral lines (Fe i 5250 A, Fe i 5324 A, Fe i 5576 A, Ca i 5590 A, Ca i 6103 A and Fe i 6165 A), measured in the kernel of a 2N solar flare and in a quiet-Sun area, were compared. The observations were carried out with an echelle spectrograph of the Crimean Astrophysical Observatory. It was shown that, compared to the quiet-Sun profiles, the flare profiles are shallower in the line core and are less steep in the wings. Therefore, measurements of the longitudinal magnetic field made with a magnetograph system which uses the Ca i 6103 A spectral line, can be underestimated by 18–25% in areas of bright Hα ribbons of a moderate solar flare. Modeling of the solar photosphere performed by using a synthesis method showed that, in a solar flare, the enhanced core emission seems to be related to heating of the photosphere by the flare, whereas the decrease of the slope of the wings was presumably caused by the inhomogeneity of the photospheric magnetic field.

Numerical simulation of a solar flare produced by the emergence of new magnetic flux

A scenario for the production of a current sheet above an active region during the emergence of new magnetic flux is considered. The formation of a current sheet is demonstrated via a numerical solution of a system of MHD equations with dissipative terms. The flare energy is stored in the magnetic field of the current sheet. The decay of the current sheet can account for a number of solar-flare phenomena, including the observed divergence of Hα ribbons.

Magnetic Field Evolution Leading to Solar Flares I. Cases with Low Magnetic Shear and Flux Emergence

Abstract We picked up five solar flares larger than GOES X-ray class M/Hα importance 1, and studied the magnetic evolution of the corresponding active regions. In particular, we investigated the spatial relationships among Hα flare ribbons, soft X-ray (SXR) flare loops, and magnetic configurations. Our results show that all of these flares are not associated with high magnetic shear, but with emerging flux, indicating that flux emergence is a common phenomenon in this kind of solar flare with low magnetic shear. The maximum values of the transverse magnetic fields on the neutral lines responsible for these flares are 300-800 G. All of these flares but one have multiple (more than two) Hα ribbons, and almost all of these ribbons are located at the footpoints of SXR flare loops. The flare-related shear changes, which can be both positive and negative, may be determined by the balance of the energy carried by the emerging flux and that required to power the flare.

3-D magnetic configurations for filaments and flares: The role of “magnetic dips” and “bald patches”

The 3-D magnetic configuration of a filament and of a low energy flare is reconstructed, using linear magnetohydrostatic ( lmhs) extrapolations. In both cases, we find observational signatures of energy release at the locations of computed “bald patches” separatrices, characterised by field lines which are tangent to the photosphere. The filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si iv with SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed dipped field lines are in good agreement with the shape of the filament and its feet observed in Hα. Some “bald patches” (BPs) are present where the distribution of dips reaches the photosphere. We show that some of the large scale field lines rooted in BPs can be related to bright fine structures in Si iv. We propose that the plasma there is heated by ohmic dissipation from the currents expected to be present along the BP separatrices. The flare was observed on May 18, 1994, in soft X-rays with Yohkoh/SXT, and in Hα at Mitaka (Japan). The magnetic field is directly extrapolated from a photospheric magnetogram from Kitt Peak Observatory. The intersections with the photosphere of the computed separatrices match well the bright Hα ribbons. The later are associated to three BPs, with overlaying dipped field lines. We show that enhanced densities are present in these dips, which can be correlated with dark Hα fibrils. Both cases show the importance of dipped field lines and BPs in the solar atmosphere. Energy release via ohmic dissipation as well as reconnection along BP separatrices is proposed to provide heating observed as UV brightenings in filament channels and even as small flares.

Evidence for Chromospheric Evaporation in the Late Gradual Flare Phase from [ITAL]SOHO[/ITAL]/CDS Observations

Using extreme-ultraviolet (EUV) spectroheliograms from the first intentional postflare observations with the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we determine relative line-of-sight velocities and their temporal evolution during the gradual flare phase of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams in lines of O V, Fe XVI, and Fe XIX, with formation temperatures Tmax of, respectively, 0.25, 2.0, and 8.0 MK show strong velocity gradients coincident with the Hα ribbons, visible in Big Bear Solar Observatory (BBSO) images. These gradients are perpendicular to and moving with the Hα ribbons. Bright downflowing plasma seems to be prevalent in the regions, between the ribbons and the magnetic neutral line, that coincide with the ends of postflare loops seen with the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma on the outer side of the ribbons is less bright in the EUV but shows strong relative blueshifts. This pattern of upflows and downflows demonstrates, for the first time in transition region and coronal lines, the existence of chromospheric evaporation during the late gradual phase of a flare and provides evidence for ongoing reconnection.

MULTISPECTRAL OBSERVATIONS OF AN ERUPTIVE FLARE

We analyze the pre-flare and impulsive phase of an eruptive (two-ribbon) flare at several wavelengths. The total energy (mechanical plus radiative) released by the flare is 8 x 1030 erg, about a factor 6 higher than the free magnetic energy (1.3 1030 erg) estimated from the non-potentiality of the magnetic field configuration in the flare area. During the impulsive phase, we find a very good time coincidence between the hard X-ray light curve and the light curves for 2 small areas (≃ 4″ in size) in the red wing of the Hα line and in the He-D3 line center. This temporal coincidence is compatible with the interpretation that hard X-ray emission is produced by bremsstrahlung of accelerated electron beams striking these dense areas. For the other regions of the Hα ribbons we find more gradual light curves, suggesting a different energy transport mechanism such as conduction.

Reconnection driven by an erupting filament in the May 14, 1981 flare

We present observations of the flare of May 14, 1981, which can be classified as a three-ribbon flare. After a detailed analysis in metric, decimetric, microwave, optical, and X-ray ranges we propose that the event was caused by a reconnection process driven by erupting filament. The energy was liberated in the current sheet above the filament in the region between the erupting flux and the overlying field. It is shown that plasma microinstabilities develop as the plasma enters the current sheet. The observations indicate that during the precursor phase a certain low-frequency turbulence, such as ion-accoustic turbulence had to be present. The reconnection rate was growing due to the increasing tension of the stretched overlying field. It is shown that the reconnection proceeded in the Sonnerup-Petschek regime during the precursor, and changed to the pile-up regime in the fast reconnection phase, when the maximal lateral expansion (50 km s−1) of the Hα ribbons was observed. The proposed process of reconnection driven by an erupting filament can be applied to three- and four-ribbon flares.

Evidence for interacting loop process in a phase of the May 16, 1981 flare

The behaviour of the flare in the period of enhancement and maximum of hard X-ray, microwave and decimetric type IV continuum is analysed. The elongation of the Hα ribbons and microwave source disclose that the energy release site was shifting through a system of loops with a velocity less than 200 km s-1, and that the energy was carried down the field lines with a velocity of about 1000 km s-1, implying the thermal conduction front mechanism of energy transport. Several processes of energy release are considered and it is concluded that an explanation in terms of succeeding interactions of neighbouring loops, involving fast reconnection of their poloidal components is in best agreement with the observations.

Behaviour of the polarization at dm-m wavelengths during the evolution of five two-ribbon flares

The general behaviour of the circular polarization at dm-m wavelengths during the evolution of five two-ribbon flares is investigated. The changes of polarization, if present, occurred 10 to 20 min after the impulsive phases. Increases of the radio and X-ray fluxes occurred at the moments when the Hα ribbons started to extend over spot umbrae.

Role of magnetic reconnection in solar flares

The behaviour of the magnetic field in the solar atmosphere is governed by the equations of MHD rather than the electromagnetism of wires. In particular, the roles of magnetic reconnection are: to create small flares; to trigger large flares; and to release magnetic energy in large flares. Observational evidence for reconnection is reviewed together with recent theory and observation of (post-) flare loops, which enable one to deduce the reconnection electric field from the motion of either the flare loops or Hα ribbons. Also, an account is given of a new unified theory for fast steady-state reconnection which includes the Sweet-Parker, Petschek and Sonnerup-like models as special cases.

Spectrograph, filtergraph and magnetograph observations of the two-ribbon flare of 29 July, 1973

We present high resolution detailed observations of the class 3N two-ribbon flare of 1973, July 29 (McMath 12461), which was associated with the disappearance of a large filament (‘disparition brusque’). This flare occurred in a diffuse bipolar magnetic region completely devoid of sunspots, and was further associated with a type IV radio burst and a soft X-ray event. Extensive Hα filtergraph, spectrograph and magnetograph records during the main phase of the flare suggest that downfalling and streaming material is present on both ribbons for several hours during the Hα emission enhancement, but only at a small number of points located both on and off the ribbons. We find a poor spatial correspondence between bright emission knots in the Hα ribbons and the positions of the observed downward motion. We conclude that the model of infall-impact of Hyder (1967a, b) is not consistent with our filtergraph and spectrograph observations.