Gerard Trottet

Microwave diagnostics of energetic electrons in flares

Electrons accelerated during solar flares are revealed by their electromagnetic radiation in different spectral ranges, emitted at different heights in the solar atmosphere. The observational analysis points to a common and continuous injection of particles. Based on this result, a quantitative investigation of the hard X-ray and microwave emissions observed during the 29 June, 1980 flare at 11: 40 UT has been performed. This is the first modelisation that takes into account both the inhomogeneity of the microwave source region and the dynamical evolution of the electron population. First results of our model computations demonstrate that during the most energetic phase of the event both hard X-rays and microwaves are described by electron populations resulting from the same injection function, and that the total numbers of electrons required for both emissions are compatible. Account for the inhomogeneity of the microwave source is shown to be a necessary condition for the interpretation of observed spectra.

The Giant 1991 June 1 Flare: Evidence for Gamma-Ray Production in the Corona and Accelerated Heavy Ion Abundance Enhancements from Gamma-Ray Spectroscopy

We investigated the implications of the gamma-ray line emission observed with Granat/PHEBUS from the behind-the-limb flare on 1991 June 1. We showed that thin target interactions are required to account for the very high observed ratio of the nuclear line emission in the 1.1-1.8 MeV and 4.1-7.6 MeV energy bands, which are populated predominantly by gamma rays from de-excitations of Ne-Fe and C-O, respectively. We found that the composition of the accelerated particles that produce gamma rays in this extremely powerful gradual flare clearly shows the heavy-element abundance enhancements characteristic of acceleration via resonant wave-particle interactions. Furthermore, the heavy-element abundance enhancements increase with time, reaching, toward the end of the flare, the highest values observed in space from impulsive solar energetic particle events. We showed that the energy deposited in the coronal interaction region is comparable to the largest total accelerated particle energy content in previously observed gamma-ray flares. However, this deposited energy is small in comparison with the energy content in greater than 20 keV electrons inferred from hard X-ray observations with Ulysses for which the 1991 June 1 flare was in full view. Approximate energy equipartition between accelerated electrons and ions then implies that the total ion energy content is much larger than the portion deposited in the corona, requiring a short ion residence time τ and a low ambient density nH in the coronal interaction region (nHτ 2.5 × 109 cm-3 s).

Association between gradual hard X-Ray emission and metric continua during large flares

X-ray radiation is used to study coronal phenomena in conjunction with meter wave observations during some large solar flares. It is found that metric flare continua and moving type IV bursts are associated with gradual and long lasting (a few tens of minutes) microwave and hard X-ray emissions. The detailed temporal analysis reveals that although metric and hard X-ray sources are located at very different heights, both kinds of emission result from a common and continuous/repetitive injection of electrons in the corona. The late part of the metric event (stationary type IV burst) is only associated with soft X-ray radiation. This indicates that the mean energy of the radiating electrons is lower during stationary type IV bursts than during the earlier parts of the event.

The orientation of pre-transient coronal magnetic fields

Loop-like white light coronal transients are generally believed to be nearly planar sheets which are thin compared to the loop extent; however, this picture may be questioned since virtually no observations (of the more than 100 transient events observed during 1973–74 Skylab period) show such loops edge-on. From the group of transient events studied by Munro etal. (1979) for which definite surface associations exist, we find loop transients are strongly correlated with filament regions where the filament axis was oriented north-south. From direct soft X-ray observations of an expanding arch, the possible identification of the soft X-ray signature of footpoints of transient loops, and monochramatic observations of low coronal loops, we infer that loop-like coronal transients have their origin in low-lying coronal loops nearly co-planar with the north-south aligned filament axis. The situation with respect to non-loop events is less clear; such events apparently often arise from more complex filament geometries. Possible reasons for the preference of transients to arise from north-south filament-oriented regions are discussed.

Energetic electron injection into the high corona during the gradual phase of flares: Evidence against acceleration by a large scale shock

Large scale coronal shock waves are observed by their radio signature associated with many solar flares. In this contribution it is demonstrated that such coronal shock waves are inefficient accelerators of electrons observed both in the downstream region (<1 R0 above the photosphere) and the upstream region (≳10 R0), as compared to other, though unspecified, mechanisms which act in the lower solar atmosphere.

Multiple-wavelength analysis of energy release during a solar flare - Thermal and nonthermal electron populations

Collaborative solar investigations by Tufts University and the Observatoire de Paris have resulted in simultaneous radio observations with the Very Large Array (VLA) and the Nancay Radioheliograph (NR), comparisons of this radio data with X-ray observations, and theoretical interpretations of the dominant radiation mechanisms during a weak impulsive solar flare observed on May 28, 1988. The VLA has mapped the flaring structures at time intervals of 3.3 s, showing that the preflash and flash-phase components of the impulsive emission originate in spatially separated sources. The 20.7 cm preflash source is ascribed to thermal gyroresonance emission from coronal loops with typical magnetic field strengths of up to 270 G; this emission is associated with heating and exhibits no detectable hard X-ray radiation above 30 keV. The flash-phase 20.7 cm source and the hard X-ray emission are attributed to nonthermal electrons in the coronal and chromospheric portions of a magnetic loop. The combination of imaging observations at 20.7 and 91.6 cm excludes emission from a confined hot plasma during the flash phase. 42 refs.

Nighttime sensitivity of ionospheric VLF measurements to X‐ray bursts from a remote cosmic source

On 22 January 2009, a series of X-ray bursts were emitted by the soft gamma ray repeater SGR J1550-5418. Some of these bursts produced enhanced ionization in the nighttime lower ionosphere. These ionospheric disturbances were studied using X-ray measurements from the Anti-Coincidence Shield of the Spectrometer for Integral onboard the International Gamma-Ray Astrophysics Laboratory and simultaneous phase and amplitude records from two VLF propagation paths between the transmitter Naval Radio Station, Pearl Harbor (Hawaii) and the receivers Radio Observatorio do Itapetinga (Brazil) and Estacao Antarctica Commandante Ferraz (Antarctic Peninsula). The VLF measurements have been obtained with an unprecedented high time resolution of 20 ms. We find that the illumination factor I (illuminated path length times the cosine of the zenith angle), which characterizes the propagation paths underlying the flaring object, is a key parameter which determines the sensitivity threshold of the VLF detection of X-ray bursts from nonsolar transients. For the present VLF measurements of bursts from SGR J1550-5418, it is found that for I ≥ 1.8 Mm, all X-ray bursts with fluence in the 25 keV to 2 MeV range larger than F25_min ~ 1.0 × 10−6 erg/cm2 produce a measurable ionospheric disturbance. Such a lower limit of the X-ray fluence value indicates that moderate X-ray bursts, as opposed to giant X-ray bursts, do produce ionospheric disturbances larger than the sensitivity limit of the VLF technique. Therefore, the frequency of detection of such events could be improved, for example by increasing the coverage of existing VLF receiving networks. The VLF detection of high-energy astrophysical bursts then appears as an important observational diagnostic to complement their detection in space. This would be especially important when space observations suffer from adverse conditions, like saturation, occultation from the Earth, or the passage of the spacecraft through the South Atlantic anomaly.

Recent Progress in Understanding Energy Conversion and Particle Acceleration in the Solar Corona

We report on results of the working group sessions at the CESRA 2001 workshop on “Energy Conversion and Particle Acceleration in the Solar Corona” which was focused on radio observations and related modeling. Progress reached in the following areas is summarized: (1) diagnostics of coronal magnetic fields and the morphology of the field in flares and filament eruptions; (2) evidence of magnetic reconnection and MHD turbulence in radio emissions; (3) acceleration site, propagation, and trapping of radio-emitting energetic particles in flares; (4) the sites of particle acceleration in long duration events, as evidenced by the 2000 July 14 (“Bastille Day”) flare; (5) radio imaging of CMEs and filament eruptions; (6) the relationship of coronal and interplanetary shock waves to flares, CMEs, and other coronal waves; and (7) the origin of solar energetic particles.

Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT

The hard X-ray/gamma-ray (HXR/GR) impulsive burst on 1991 June 30 (0255 UT) was associated with a flare which occured between 2 and 12 behind the east limb of the Sun. The partially occulted HXR/GR emission from this flare was detected at up to 100 MeV by three instruments on Earth-orbiting spacecraft: the Burst and Transient Source Experiment (BATSE) and the Energetic Gamma-Ray Experiment (EGRET) on CGRO and by the Payload for High Energy Burst Spectroscopy (PHEBUS) on GRANAT. As seen from the two spacecraft in Earth orbit, the size of the burst corresponds to that of a moderate electron-dominated GR event (Dingus et al. 1994; Vilmer et al. 1999). However, this event is one of the giant flares reported by Kane et al. (1995). It was observed by the Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses, located 135 east of the Earth-Sun line. GRB measured the total>28 keV HXR emission from the flare. In this paper we combine HXR observations by GRB and BATSE in order to determine the time evolution of the power-law index of the photon spectrum of the partially occulted HXR emission seen by BATSE and of the fraction R of the partially occulted to the total>28 keV emission. decreased from 5.4 to 2.6 and R varied from 20% at the beginning of the event down to<1% at its maximum. These results indicate that the spatial distribution of the HXR sources was complex, and evolved in the course of the event. While the HXR emission detected by GRB was almost entirely produced at the footpoints of loops by thick-target interactions, a fraction of the HXR emission seen by BATSE likely originated in the unocculted, low density, portion of the HXR emitting loops. The data also show that a small fraction (10%) of the HXR emission detected by BATSE in Earth orbit was radiated by a thick-target source on the visible disk.

Pairs of non fundamental-harmonic type III bursts

Different forms of pairs of type III bursts have been discussed in the literature. We report here a new aspect revealed by high time resolution radioheliography. In some groups of these bursts, each element appears to be split into two components. These pairs recur with a characteristic time, and in a given group the time splitting of the two components of each pair is the same (one second or less). The nature of these pairs is discussed: the fundamental-harmonic hypothesis is excluded. Alternative interpretations are reviewed.