Aims. To study preflare X-ray brightenings as diagnostics of the destabilisation of flare-associated erupting filaments/prominences. Methods. We combine new observations from the Transition Region and Coronal Explorer (TRACE) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), as well as revisit events reported in the literature to date, in order to scrutinise the preflare activity during eight flare-associated filament eruptions. Results. The preflare activity occurs in the form of discrete, localised X-ray brightenings observed between 2 and 50 min before the impulsive phase of the flare and filament acceleration. These transient preflare brightenings are situated on or near (within 10 of) the polarity inversion line (PIL), coincident with emerging and/or canceling magnetic flux. The filaments begin to rise from the location of the preflare brightenings. In five out of eight events, the preflare brightenings were observed beneath the filament channel, close to the filament footpoint first observed to rise. Both thermal and nonthermal hard X-ray emissions during the preflare enhancement were detected with RHESSI, suggesting that both plasma heating and electron acceleration occurred at this time. The main energy release during the impulsive phase of the flare is observed close to (within 50 of) the preflare brightenings. The fast-rise phase of the filament eruption starts at the same time as the onset of the main flare or up to 5 min later. Conclusions. The preflare brightenings are precursors to the flare and filament eruption. These precursors represent distinct, localised instances of energy release, rather than a gradual energy release prior to the main flare. The X-ray precursors represent clearly observable signatures in the early stages of the eruption. Together with the timing of the filament fast-rise at or after the main flare onset, the X-ray precursors provide evidence for a tether-cutting mechanism initially manifested as localised magnetic reconnection being a common trigger for both flare emission and filament eruption.

Aims. We study the physical properties of a recurring solar active region jet observed in X-rays and extreme-ultraviolet (EUV). Methods. Multi-wavelength data from all three instruments on board Hinode were analysed. X-ray imaging and spectroscopy of the microflaring emission associated with the jets was performed with the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). Associated EUV jets were observed with the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)/Extreme Ultraviolet Imager (EUVI) on board STEREO. Results. We found a correlation between recurring magnetic flux cancellation close to a pore, the X-ray jet emission, and associated Ca II H ribbon brightenings. We estimated the lower limit for the decrease in magnetic energy associated with the X-ray jet emission at 3 × 10 29 erg. The recurring plasma ejection was observed simultaneously at EUV and X-ray temperatures, associated with type III radio bursts and microflaring activity at the jet footpoint. Conclusions. The recurring jet (EUV and X-ray) emissions can be attributed to chromospheric evaporation flows due to recurring coronal magnetic reconnection. In this process, the estimated minimum loss in the magnetic energy is sufficient to account for the total energy required to launch the jet.

Aims. We aim to examine the precursor phases and early evolution of a prominence eruption associated with a M4-class flare and a partial halo coronal mass ejection (CME) observed on 2005 July 27. Our main goal is to investigate the precursor eruption signatures observed in EUV, X-ray and microwave emission and their relation to the prominence destabilisation. Methods. We perform a multi-wavelength study of the prominence morphology and motion using high-cadence and spatial resolution EUV 171 Å images from the TRACE satellite. The high-temperature flare radiative emission in soft and hard X-rays are analysed through imaging and spectral modeling with RHESSI. Complementary microwave images (17 GHz and 34 GHz) from NoRH are also investigated.Results. The activation of the filament proceeds from one anchored footpoint. We observe "pre-eruption" brightenings in X-ray and EUV images, close to the erupting footpoint of the prominence, being temporally correlated to the point when the prominence first enters a slow-rise phase, and then an accelerated fast-rise phase. The brightness temperature (T b ) of the prominence at 34 GHz is increasing during the eruption. We also find very good correlation between the prominence height-time profile and the spatially integrated soft X-ray (SXR) emission. Conclusions. We discuss the observed precursor brightenings with respect to possible mechanisms that might be responsible for the prominence destabilisation and acceleration. Our observations suggest that reconnection events localised beneath the erupting footpoint may eventually destabilise the entire prominence, causing the eruption.

Aims. We study the physical properties of an active region (AR) jet in order to probe the mechanisms responsible for it. Methods. We report 2007 January 15/16 observations of a recurring jet situated on the west side of NOAA AR 10938. Multiwavelength data from all three instruments onboard Hinode were analysed. This paper focuses on one instance of a jet observed with the Hinode/EUV Imaging Spectrometer (EIS). Using EIS raster data we measured the temperatures, Doppler shifts, density, and filling factor. Results. A strong blue-shifted component and an indication of a weak red-shifted component at the base of the jet was observed around Log T e = 6.2. The up-flow velocities exceeded 150 km s −1 . The jet component was seen over a range of temperatures between 5.4 and 6.4 in Log T e . Using Fe xii λ186 and λ195 line ratios, we measured densities above Log N e = 11 for the high-velocity up-flow component. We found that the density of the high-velocity up-flow increases with velocity. We estimate the filling factor in the jet upflow to be <0.03. With the Hinode/Solar Optical Telescope (SOT), we observed recurrent (quasi periodic) magnetic flux cancelations just before the recurrent jet emission was seen in images taken with the X-ray Telescope (XRT).Conclusions. The high-velocity up-flows, together with the density dependence on velocity, support an evaporation scenario for the acceleration of this jet. The high density and small filling factor, coupled with the high Doppler velocities are strongly suggestive of multiple small-scale magnetic reconnection events being responsible for the production of both EUV and X-ray jets.

Analysis of RHESSI 3-10 keV spectra for 27 solar flares is reported. This energy range includes thermal free-free and free-bound continua and two line features, at $6.7 and $8 keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at $6.7 keV to derive the electron temperature T e , the emission measure, and the Fe-line equivalent width as functions of time in each flare. The Fe/H abundance ratio in each flare is derived from the Fe-line equivalent width as a function of T e . To minimize instrumental problems with high count rates and effects associated with multitemperature and nonthermal spectral components, spectra are presented mostly during the flare decay phase, when the emission measure and temperature were smoothly varying. We found flare Fe/H abundance ratios that are consistent with the coronal abundance of Fe (i.e., 4 times the photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7 flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We find evidence that the Fe xxv ion fractions are less than the theoretically predicted values by up to 60% at T e ¼ 12 MK; the observed N(Fe xxv)/N(Fe) values appear to be displaced from the most recent theoretical values by between 1 and 3 MK.

Aims. To perform a benchmark analysis for the recent version of the CHIANTI atomic database (v. 5.2) based on high-resolution solar flare X-ray spectra in the range 3.4-6.1 Å from the RESIK crystal spectrometer on the CORONAS-F spacecraft. Methods. A C5.8 flare occurring on 2003 February 22 was chosen for analysis. RESIK spectra of this flare include emission lines of He-like and H-like K, Ar, S, and Si, with some dielectronic lines. Initially, two independent plasma diagnostic techniques are employed: an emission measure (EM) loci analysis using the line flux and the line contribution function G(T e , N e ), and a new method based on continuum fluxes and contribution functions. We further apply a differential emission measure (DEM) analysis, from which CHIANTI synthetic spectra are derived. The continuum from RESIK spectra is checked against simultaneous RHESSI and GOES observations. Comparisons of CHIANTI synthetic spectra with those from the MEKAL code in the 3.4-6.1 Å range are also presented. Results. The emitting plasma appears multi-thermal, having one dominant temperature component determined independently from the line and continuum EM loci and DEM analyses. Consistency between line and continuum emissions requires photospheric elemental abundances (Asplund et al. 2005), with a depleted sulphur abundance. With the exception of RESIK channel 4 (5.0-6.1 Å), we find overall very good agreement between the calculated and observed intensities. From comparisons with other instruments, RESIK's precision in the continuum level is confirmed to be within the estimated 20% uncertainties in the intensity calibration. We find general agreement between CHIANTI and MEKAL isothermal spectra, but we note that the atomic data for the Si xii and Si xiii ions contained in CHIANTI are more complete. Conclusions. RESIK observations of both lines and continua are suitable for characterising the properties of the flaring plasma such as temperature, emission measure and elemental abundance. These spectra can be used to evaluate any atomic database.

Aims. We study the onset of a solar eruption involving a filament ejection on 2007 May 20. Methods. We observe the filament in Hα images from Hinode/SOT and in EUV with TRACE and STEREO/SECCHI/EUVI. Hinode/XRT images are used to study the eruption in soft X-rays. From spectroscopic data taken with Hinode/EIS we obtain bulkflow velocities, line profiles, and plasma densities in the onset region. The magnetic field evolution was observed in SoHO/MDI magnetograms. Results. We observed a converging motion between two opposite polarity sunspots that form the primary magnetic polarity inversion line (PIL), along which resides filament material before eruption. Positive-flux magnetic elements, perhaps moving magnetic features (MMFs) flowing from the spot region, appear north of the spots, and the eruption onset occurs where these features cancel repeatedly in a negative-polarity region north of the sunspots. An ejection of material observed in Hα and EUV marks the start of the filament eruption (its "fast-rise"). The start of the ejection is accompanied by a sudden brightening across the PIL at the jet's base, observed in both broad-band images and in EIS. Small-scale transient brightenings covering a wide temperature range (Log T e = 4.8−6.3) are also observed in the onset region prior to eruption. The preflare transient brightenings are characterized by sudden, localized density enhancements (to above Log n e [cm −3 ] = 9.75, in Fe xiii) that appear along the PIL during a time when pre-flare brightenings were occurring. The measured densities in the eruption onset region outside the times of those enhancements decrease with temperature.Persistent downflows (red-shifts) and line-broadening (Fe xii) are present along the PIL.Conclusions. The array of observations is consistent with the pre-eruption sheared-core magnetic field being gradually destabilized by evolutionary tether-cutting flux cancelation that was driven by converging photospheric flows, and the main filament ejection being triggered by flux cancelation between the positive flux elements and the surrounding negative field. A definitive statement however on the eruption's ultimate cause would require comparison with simulations, or additional detailed observations of other eruptions occurring in similar magnetic circumstances.

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