What is the role of magnetic null points in large flares?

We have performed the analysis of the magnetic topology of active region NOAA 10486 before two large flares occurring on October 26 and 28, 2003. The 3D extrapolation of the photospheric magnetic field shows the existence of magnetic null points when using two different methods. We use TRACE 1600 Å and 195 Å brightenings as tracers of the energy release due to magnetic reconnections. We conclude on the three following points:

1. The small events observed before the flares are related to low lying null points. They are long lasting and associated with low energy release. They are not triggering the large flares.

2. On October 26, a high altitude null point is found. We look for bright patches that could correspond to the signatures of coronal reconnection at the null point in TRACE 1600 Å images. However, such bright patches are not observed before the main flare, they are only observed after it.

3. On October 28, four ribbons are observed in TRACE images before the X17 flare. We interpret them as due to a magnetic breakout reconnection in a quadrupolar configuration. There is no magnetic null point related to these four ribbons, and this reconnection rather occurs at quasi-separatrix layers (QSLs).

We conclude that the existence of a null point in the corona is neither a sufficient nor a necessary condition to give rise to large flares.     

Converging motion of conjugate flaring kernels during two large solar flares

In this paper, we analyze the footpoint motion of two large solar flares using observations made by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The two flares are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They are both classical two-ribbon flares as observed in TRACE 1600 or 171 Å images and have long-duration conjugate hard X-ray (HXR) footpoint emission. We use the ‘center-of-mass’ method to locate the centroids of the UV/EUV flare ribbons. The results are: (1) The conjugate UV/EUV ribbons and HXR footpoints of the two flares show a converging (inward) motion during the impulsive phase. For the two flares, the converging motion lasts about 3 and 10 min, respectively. The usual separation (outward) motion for the flare ribbons and footpoints take place only after the converging motion. (2) During the inward and the outward motion, the conjugate ribbons and footpoints of the two events exhibit a strong unshear motion. In obtaining above results, TRACE UV/EUV and RHESSI HXR data show an overall agreement. The two events demonstrate that the magnetic reconnection for the flares occurs in highly sheared magnetic field. Furthermore, the results support the magnetic model constructed by Ji et al. [Ji, H., Huang, G., Wang, H. Astrophys. J. 660, 893–900, 2007], who proposed that the contracting motion of flaring loops is the signature of the relaxation of sheared magnetic fields.     

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.     

The radio signature of a kink unstable loop

Based upon multi-wavelength observations outlined by Huang et al. [Solar Phys. 213 (2003) 341], especially the dynamic spectrum at 4.5–7.5 GHz, we study the physical nature of the radio fine structures (FS) during the flare on August 25, 1999 in AR 8674 (S28E21). The main results are: (1) the helical loop of the event related to the FS is unstable for m = 1 kink mode; (2) the time interval between the beginning of reconnection and relaxation of the unstable loop, inferred from the observation, is quantitatively consistent with the results of the numerical simulations on kink unstable loop; (3) the magnetic field strength estimated from the fast kink standing wave is basically of the same order as that estimated from the photospheric magnetic field, which provides strong support to our analysis.     

Hard X-ray footpoint motions in solar flares: Comparing magnetic reconnection models with observations

Hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of the October 29, 2003 GOES X10 two-ribbon flare are used together with magnetic field observations from the Michelson Doppler Imager (MDI) onboard SoHO to compare footpoint motions with predictions from magnetic reconnection models. The temporal variations of the velocity v of the hard X-ray footpoint motions and the photospheric magnetic field strength B in footpoints are investigated. The underlying photospheric magnetic field strength is generally higher (B ∼ 700–1200 G) in the slower moving (v ∼ 20–50 km s⁻¹) western footpoint than in the faster (v ∼ 20–100 km s⁻¹) moving eastern source (∼100–600 G). Furthermore, a rough temporal correlation between the HXR flux and the product vB² is observed.     

Low-frequency electric field and density fluctuation measurements on Solar Orbiter

Solar Orbiter will orbit the Sun down to a distance of 0.22 AU allowing detailed in situ studies of important but unexplored regions of the solar wind in combination with coordinated remote sensing of the Sun. In-situ measurements require high quality measurements of particle distributions and electric and magnetic fields. We show that such important scientific topics as the identification of coronal heating remnants, solar wind turbulence, magnetic reconnection and shock formation within coronal mass ejections all require electric field and plasma density measurements in the frequency range from DC up to about 100 Hz. We discuss how such measurements can be achieved using the double-probe technique. We sketch a few possible antenna design solutions.     

An analysis of solar radio burst events on December 1, 2004

We present the analysis of the radio observations of December 1, 2004 from 07:00 UT to 07:40 UT in the 1.100–1.340 GHz band by Solar Broadband Radio Dynamic Spectrometer (SBRS) in Huairou Station. There are three groups of radio fine structures during the impulsive phase of this flare denoted by N1, Z2, and Z3. N1 has several emission lines with mixed fast and slow frequency drift rate which may reflect the conditions of flare loop and fast flows out from reconnection site; Z2 and Z3 are zebra patterns. The radio observations combined with hard X-ray and other observations show that the fine structures are connected with energetic particles. The information about magnetic field and energetic particle during the burst are also estimated based on our model.     

Collaborative VLA,SOHO and RHESSI observations of evolving sources of energy release in the corona above active regions

Very Large Array (VLA) observations at 20 and 91 cm wavelength are compared with data from the SOHO (EIT and MDI) and RHESSI solar missions to investigate the evolution of decimetric Type I noise storms and Type III bursts and related magnetic activity in the photosphere and corona. The combined data sets provide clues about the mechanisms that initiate and sustain the decimetric bursts and about interactions between thermal and nonthermal plasmas at different locations in the solar atmosphere. On one day, frequent, low-level hard X-ray flaring observed by RHESSI appears to have had no clear affect on the evolution of two closely-spaced Type I noise storm sources lying above the target active region. EIT images however, indicate nearly continuous restructuring of the underlying EUV loops which, through accompanying low-level magnetic reconnection, might give rise to nonthermal particles and plasma turbulence that sustain the long-lasting Type I burst emission. On another day, the onset of an impulsive hard X-ray burst and subsequent decimetric burst emission followed the gradual displacement and coalescence of a small patch of magnetic magnetic polarity with a pre-existing area of mixed magnetic polarity. The time delay of the impulsive 20 and 91 cm bursts by up to 20 min suggests that these events were unlikely to represent the main sites of flare electron acceleration, but instead are related to the rearrangement of the coronal magnetic field after the main flare at lower altitude. Although the X-ray flare is associated with the decimetric burst, the brightness and structure of a long-lasting Type I noise storm from the same region was not affected by the flare. This suggests that the reconfiguration of the coronal magnetic fields and the subsequent energy release that gave rise to the impulsive burst emission did not significantly perturb that part of the corona where the noise storm emission was located.     

On some large solar energetic particle events: Direct impulsive component and broad associated coronal mass ejections

Using the proton intensity and X-ray flux data from the GOES, combined with the observations of the associated solar eruptions by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) on board the Solar and Heliospheric Observatory (SOHO), 14 large SEP events occurring in the period 2000 January–2002 April have been studied. It is found that: (1) events with the SEPs increasing shortly after the maximum of their parent flares (<1 h; hereafter prompt events) have rapid and great (up to four orders of magnitude) SEP increments in high-energy channels (> ∼100 MeV); however, for events whose onset of the SEP injection lags the flare maximum for a long time (>3 h; hereafter delayed events), the high-energy SEPs show no obvious enhancements (within one order of magnitude); (2) peak intensity of the prompt events is distinctly larger than that of the delayed events; (3) CMEs associated with the poorly magnetically connected events (source region <W30°) in our survey are all halo CMEs. From these observational differences, we propose a special scenario of the production of the largest SEP events: both CMEs and flares are induced in the same coronal process; high-energy particles accelerated in the reconnection region can escape easily from the open field lines and/or be transported by fast CMEs into interplanetary space, indicating a direct impulsive component in large gradual SEP events. Meanwhile, the broad width of the associated CMEs implies that the CME width is more important in SEP events production than previously considered.     

Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003

The GOES X3.9 flare on 03 November 2003 at ∼09:45 UT was observed from metric to millimetric wavelengths by the Nançay Radioheliograph (NRH), the Radio Solar Telescope Network (RSTN) and by radio instruments operated by the Institute of Applied Physics (University of Bern). This flare was simultaneously observed and imaged up to several 100 keV by the RHESSI experiment. The time profile of the X-ray emission above 100 keV and of the radio emissions shows two main parts, impulsive emission lasting about 3 min and long duration emission (partially observed by RHESSI) separated in time by 4 min. We shall focus here on the modulations of the broad-band radio continua and of the X-ray emissions observed in the second part of the flare. The observations suggest that gyrosynchrotron emission is the prevailing emission mechanism even at decimetric wavelengths for the broad-band radio emission. Following this interpretation, we deduce the density and the magnetic field of the decimetric sources and briefly comment on possible interpretations of the modulations.     

Relationship of solar flare accelerated particles to solar energetic particles (SEPs) observed in the interplanetary medium

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies as in large SEP events, but it appears that a different acceleration process, one associated with fast coronal mass ejections is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty high energy solar spectroscopic imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. Here, preliminary comparisons of the RHESSI observations with observations of both energetic electron and ion near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.     

Multi-wavelength study of a coronal mass ejection: a flare event from AR#9393

The active region, AR#9393, produced a number of intense flares during March–April 2001. In this paper, we report the analysis of an X1.1 flare event of April 2, 2001 and its associated coronal mass ejection. The timing and location of the H_α eruption, radio burst activities, and the onset of mass ejection suggest an energy release that occurred close to the surface of the sun. At this region, as shown by the magnetogram, X-ray and EUV images, the field configuration was complex and the 3-D extrapolation revealed the presence of a magnetic null point. Results also suggest that the energy release is followed by the magnetic reconnection between the low-lying loops near the separator point and outlying loops. This study provides the support for the magnetic break-out process to trigger the energy release in eruptive flare event.     

The temperature map of the loop and coronal sources of an occulted solar flare

An occulted solar flare occurred at about 06:07 UT on 2002, November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 min with a speed of ∼24 km s⁻¹ during the early impulsive phase and then expanded at ∼7 km s⁻¹, while the coronal source presented an upward motion at about 6 km s⁻¹. We obtained the temperature map of the loop source from RHESSI image spectrum. The temperature of the loop increases with altitude, indicating that the reconnection X-point of this flare is located above the loop source. However, the apparent coronal source is the top of another independent large-scale loop.     

The thermal X-ray spectrum of the 2003 April 26 solar flare

Observations and their analysis of the thermal X-ray spectrum of the M2 flare on 2003 April 26 are described. The spectrum observed by the RHESSI mission cover the energy range from ∼5 to ∼50 keV. With its ∼1-keV spectral resolution, intensities and equivalent widths of two line complexes, the Fe line group at 6.7 keV (mostly due to Fe xxv lines and Fe xxiv satellites) and the Fe/Ni line group at 8 keV (mostly due to higher-excitation Fe xxv lines and Ni xxvii lines) were obtained as a function of time through a number of flares. The abundance of Fe can also be determined from RHESSI spectra; it appears to be consistent with a coronal value for at least some times during the flare. Comparisons of RHESSI spectra with those from the RESIK crystal spectrometer on CORONAS-F show very satisfactory agreement, giving much confidence in the intensity calibration of both instruments.     

Evidence for a solar coronal thick-target hard X-ray source observed by RHESSI

We study a solar flare hard X-ray (HXR) source observed by the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) in which the HXR emission is almost entirely in a coronal loop so dense as to be collisionally thick at electron energies up to ∼45−60 keV. This contrasts with most events previously reported in which the HXR emission is primarily from the loop footpoints in the collisionally dense chromosphere. In particular, we show that the high loop column densities inferred from the GOES and RHESSI soft X-ray emission measure and the volume of the flare loop are consistent with the coronal thick-target interpretation of the HXR images and spectra. The high column densities observed already at the very beginning of the impulsive phase are explained by chromospheric evaporation during a preflare which, as Nobeyama 17 GHz radio images reveal, took place in the same set of nested loops as the main flare.     

Spectral analysis of a two-ribbon microflare

Using high-resolution Hα, CaII 8542 Å and FeI 6302.5 Å Stokes spectral data obtained simultaneously with THEMIS in 2002 September, we have analyzed the spectra and the characteristics of a two-ribbon microflare (MF). The hard X-ray emission provides evidence of non-thermal particle acceleration in the microflare. The two-ribbons are located on either sides of the magnetic polarity inversion line. The non-thermal characteristics mainly appeared at the outer edges of the flare ribbons. It indicates that the instantaneous magnetic reconnection and the particle acceleration mainly took place at the outer edges of the flare ribbons. Using the Hα and CaII 8542 Å line profiles and the non-LTE calculation, we obtain the semi-empirical atmospheric model for the bright kernel of the MF. The result indicates that the temperature enhancement in the chromosphere is about 2000–2500 K.     

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.     

Where will efficient energy release occur in 3-D magnetic configurations?

The energy needed to power flares is thought to be stored in the coronal magnetic field. However, the energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current sheets can form, and then, reconnection can efficiently occur. This has been confirmed for several flares computing the coronal field and comparing the locations of the flare loops and ribbons to the deduced 3-D magnetic topology. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed, “Quasi-Separatrix Layers” (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. They let us understand where reconnection occurs in a broader variety of flares than separatrices do. The strongest electric field and current are generated at, or close to where the QSLs are thinnest. This defines the region where particle acceleration can efficiently occur. A new feature of 3-D reconnection is the natural presence of fast field-line slippage along the QSLs, a process called “slip-running reconnection”. This is a plausible origin for the motions of the X-ray sources along flare ribbons.     

Two energy release processes for CMEs: MHD catastrophe and magnetic reconnection

It remains an open question how magnetic energy is rapidly released in the solar corona so as to create solar explosions such as solar flares and coronal mass ejections (CMEs). Recent studies have confirmed that a system consisting of a flux rope embedded in a background field exhibits a catastrophic behavior, and the energy threshold at the catastrophic point may exceed the associated open field energy. The accumulated free energy in the corona is abruptly released when the catastrophe takes place, and it probably serves as the main means of energy release for CMEs at least in the initial phase. Such a release proceeds via an ideal MHD process in contrast with nonideal ones such as magnetic reconnection. The catastrophe results in a sudden formation of electric current sheets, which naturally provide proper sites for fast magnetic reconnection. The reconnection may be identified with a solar flare associated with the CME on one hand, and produces a further acceleration of the CME on the other. On this basis, several preliminary suggestions are made for future observational investigations, especially with the proposed Kuafa satellites, on the roles of the MHD catastrophe and magnetic reconnection in the magnetic energy release associated with CMEs and flares.     

Plasmoid formation in eruptive flares

One phenomena Yohkoh has observed is plasmoid eruption in flares. Thus this is a key factor that must be explained in any flare mechanism. In order to understand the dynamics of a plasmoid, we performed a numerical MHD simulation and investigated the evolution of the coronal magnetic field, which is initially a force-free configuration. The main results are as follows. At first, small amount of dissipation, induced by the initial perturbation, occurs in the current sheet where the plasmoid forms. This plasmoid is slowly going upward by magnetic tension force of the reconnected magnetic fields produced by initial dissipation. The crucial point comes when the perpendicular magnetic fields are washed away from the reconnection point, after that the reconnection proceeds effectively so that the magnetic tension force of the reconnected fields becomes strong, which make the plasmoid be rapidly erupted upward. These are consistent with the observational results, which say that before the main energy release the plasmoid slowly rises and when the flare sets in it is rapidly accelerated upward. In this paper, we emphasize on the role that the perpendicular magnetic fields play in the evolution of flare.