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Solar Flares and Energetic Particles  

Nicole Vilmer   Submitted: 2012-10-18 01:31

Solar flares are now observed at all wavelengths from γ-rays to decametre radio waves. They are commonly associated with efficient production of energetic particles at all energies. These particles play a major role in the active Sun because they contain a large amount of the energy released during flares. Energetic electrons and ions interact with the solar atmosphere and produce high-energy X-rays and γ-rays. Energetic particles can also escape to the corona and interplanetary medium, produce radio emissions (electrons) and may eventually reach the Earth's orbit. I shall review here the available information on energetic particles provided by X-ray/γ-ray observations, with particular emphasis on the results obtained recently by the mission Reuven Ramaty High-Energy Solar Spectroscopic Imager. I shall also illustrate how radio observations contribute to our understanding of the electron acceleration sites and to our knowledge on the origin and propagation of energetic particles in the interplanetary medium. I shall finally briefly review some recent progress in the theories of particle acceleration in solar flares and comment on the still challenging issue of connecting particle acceleration processes to the topology of the complex magnetic structures present in the corona.

Authors: N. Vilmer
Projects: RHESSI

Publication Status: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 370, issue 1970, pp. 3241-3268 Published in Jul 2012
Last Modified: 2012-10-24 12:40
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Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun-Earth connection  

Nicole Vilmer   Submitted: 2008-11-07 09:50

This paper will review the input of 65 years of radio observations to our understanding of solar and solar?terrestrial physics. It is focussed on the radio observations of phenomena linked to solar activity in the period going from the first discovery of the radio emissions to present days. We shall present first an overview of solar radio physics focussed on the active Sun and on the premices of solar?terrestrial relationships from the discovery to the 1980s. We shall then discuss the input of radioastronomy both at metric/decimetric wavelengths and at centimetric/millimetric and submillimetric wavelengths to our understanding of flares. We shall also review some of the radio, X-ray and white-light signatures bringing new evidence for reconnection and current sheets in eruptive events. The input of radio images (obtained with a high temporal cadence) to the understanding of the initiation and fast development in the low corona of coronal mass ejections (CMEs) as well as the radio observations of shocks in the corona and in the interplanetary medium will be reviewed. The input of radio observations to our knowledge of the interplanetary magnetic structures (ICMEs) will be summarized; we shall show how radio observations linked to the propagation of electron beams allow to identify small scale structures in the heliosphere and to trace the connection between the Sun and interplanetary structures as far as 4AU. We shall also describe how the radio observations bring useful information on the relationship and connections between the energetic electrons in the corona and the electrons measured in-situ. The input of radio observations on the forecasting of the arrival time of shocks at the Earth as well as on Space Weather studies will be described. In the last section, we shall summarize the key results that have contributed to transform our knowledge of solar activity and its link with the interplanetary medium. In conclusion, we shall indicate the instrumental radio developments at Earth and in space, which are from our point of view, necessary for the future of solar and interplanetary physics.

Authors: Monique Pick, Nicole Vilmer
Projects: None

Publication Status: Astron. Astrophys. Rev (on line publication) Volume 16
Last Modified: 2008-11-07 11:21
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Time delay between gamma ray lines and hard X-ray emissions during the 23 July 2002 interpreted by a trap plus precipitation model  

Nicole Vilmer   Submitted: 2007-03-14 00:42

The 23 July 2002 event was the first gamma-ray flare observed by the Ramaty Energy Solar Spectroscopic Imager (RHESSI). Analysis of the time profiles of this flare shows a time delay between hard X-ray at 150 keV and gamma ray line emissions. Aim: We aim to interpret this delay in terms of the transport of the particles accelerated during the flare Method: In this paper we focus on the interpretation of this delay in the context of a trap plus precipitation model for energetic particles. Results: The time profiles of the hard X-ray and gamma-ray line profiles can be reproduced given that electrons and ions are injected and partially trapped in different coronal loop systemswith slightly different characteristics such as density and length, and that the energetic electron to ion ratio varies from peak to peak during the flare. Conclusions: the results obtained from this analysis are discussed with respect to the constraints provided by the hard X-ray and gamma-ray images previously obtained, as well as with previuously published analysis of the same event.

Authors: C. Dauphin, N. Vilmer
Projects: RHESSI

Publication Status: A&A in press
Last Modified: 2007-03-15 04:44
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Observations of a soft X-ray rising loop associated with a type II burst and a coronal mass ejection in the 03 November 2003 X-ray flare  

Nicole Vilmer   Submitted: 2006-05-10 08:02

We report observations of a type II burst-the signature of a shock wave- starting at the unusual high frequency of 650 MHz during the 03 November 2003 flare. This flare is associated with the propagation of a soft X-ray coronal loop and with a coronal mass ejection. We study the origin of the shock wave in the low corona and present a kinematics analysis of the soft X-ray coronal loop and of the CME observed a few tens of minutes later. We study the spatial and temporal relation between the soft X-ray rising loop observed by GOES/SXI, the type II sources observed by the Nançay Radioheliograph (NRH) and the CME observed by SOHO/LASCO. The type II burst observed during this flare is driven by the X-ray rising loop and the kinematics analysis shows that the X-ray coronal loop and the CME are related.

Authors: C. Dauphin, N. Vilmer, S. Krucker
Projects: RHESSI

Publication Status: A&A in press
Last Modified: 2006-05-10 08:18
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Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003  

Nicole Vilmer   Submitted: 2005-05-09 09:29

The GOES X3.9 flare on 03 November 2003 at 9:45 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 three minutes and long duration emission (partially observed by RHESSI) separated in time by four minutes. 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

Authors: C. Dauphin, N. Vilmer, T. Lüthi, G. Trottet , S. Krucker, A. Magun
Projects: RHESSI

Publication Status: Advances in Space Research (accepted)
Last Modified: 2005-05-09 09:29
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Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT  

Nicole Vilmer   Submitted: 2003-09-12 09:10

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 gamma 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. gamma 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 this complex 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's orbit was radiated by a thick-target source on the visible disk.

Authors: G. Trottet, R.A. Schwartz, K. Hurley, J.M. McTiernan, S.R. Kane, N. Vilmer
Projects: None

Publication Status: Astronomy and Astrophysics, 403,1157, 2003
Last Modified: 2003-09-12 09:10
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High energy particles accelerated during the large solar flare of 1990 May 24 : X/gamma observations  

Nicole Vilmer   Submitted: 2003-09-11 09:30

The PHEBUS experiment aboard GRANAT observed gamma-ray line emission and gamma-ray continuum above 10 MeV from the 24 May, 1990 solar flare. Observations and interpretation of the high-energy continuum have been discussed previously. Here we re-examine these, combining the PHEBUS observations above 10 MeV with calculations of the pion decay continuum to quantitatively constrain the accelerated ion energy distribution at energies above 300 MeV. The uncertainty in the determination of the level of the primary electron bremsstrahlung as well as the lack of measurements on the gamma-ray emission above ~ 100 MeV combine to allow rather a wide range of energy distribution parameters (in terms of the number of protons above 30 MeV, the spectral index of the proton distribution and the high energy cut-off of the energetic protons). Nevertheless we are able to rule out some combinations of these parameters. Using the additional information provided by the gamma-ray line observations we discuss whether it is possible to construct a consistent picture of the ions which are accelerated in a wide energy range during this flare. Our findings are discussed with respect to previous works on the spectrum of energetic protons in the 10 MeV to GeV energy range.

Authors: N. Vilmer, A.L. MacKinnon, G. Trottet, C. Barat
Projects: None

Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2003-09-11 09:30
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Hard X-ray and metric/decimetric spatially resolved observations of the 10 April 2002 solar flare  

Nicole Vilmer   Submitted: 2003-09-09 02:47

The GOES M8.2 flare on 10 April 2002 at ~ 1230 UT was observed at X-ray wavelengths by RHESSI and at metric/decimetric wavelengths by the Nançay Radioheliograph (NRH). We discuss the temporal evolution of X-ray sources together with the evolution of the radio emission sites observed at different coronal heights by the NRH. While the first strong HXR peak at energies above 50 keV arises from energy release in compact magnetic structures (with spatial scales of a few 104 km) and is not associated with strong radio emission, the second one leads to energy release in magnetic structures with scales larger than 105 km and is associated with intense decimetric/metric and dekametric emissions. We discuss these observations in the context of the acceleration sites of energetic electrons interacting at the Sun and of escaping ones.

Authors: N. Vilmer, S. Krucker, G. Trottet and R.P. Lin
Projects: RHESSI

Publication Status: Advances in Space Research (in press)
Last Modified: 2003-09-09 02:47
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Hard X-ray and metric/decimetric radio observations of the 20 February 2002 solar flare  

Nicole Vilmer   Submitted: 2002-09-19 10:18

The GOES C7.5 flare on 20 February 2002 at 11:07 UT is one of the first solar flares observed by RHESSI at X-ray wavelengths. It was simultaneously observed at metric/decimetric wavelengths by the Nançay Radioheliograph (NRH) which provided images of the flare between 450 and 150 MHz. We present a first comparison of the hard X-ray images observed with RHESSI and of the radio emission sites observed by the NRH. This first analysis shows that: (1) there is a close occurrence between the production of the HXR radiating most energetic electrons and the injection of radio emitting non-thermal electrons at all heights in the corona, (2) modifications with time in the pattern of the HXR sources above 25 keV and of the decimetric radio sources at 410 MHz are observed occurring on similar time periods, (3) in the late phase of the most energetic HXR peak, a weak radio source is observed at high frequencies, overlaying the EUV magnetic loops seen in the vicinity of the X-ray flaring sites above 12 keV. These preliminary results illustrate the potentiality of combining RHESSI and NRH images for the study of electron acceleration and transport in flares.

Authors: N. Vilmer, S. Krucker, R.P. Lin, RHESSI team
Projects:

Publication Status: Solar Phys in press
Last Modified: 2002-09-19 10:18
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What can be learned about competing acceleration models from multiwavelength observations?  

Nicole Vilmer   Submitted: 2002-07-19 07:45

We review the available evidence from various wavelength ranges, alone and in combination, bearing on solar particle acceleration. Radio, X-ray and gamma-ray observations yield direct information on solar ion and electron acceleration at the Sun. We describe the main spectral features in the X/gamma domain, outline the means by which they yield information on accelerated particles, and summarise results obtained using them on numbers and energies of flare fast ions and electrons. Relative numbers and energy content of electrons and ions may vary from flare to flare, and in the course of a single event. In general, both electronic and ionic species appear to embody significant fractions of the total flare energy and either can be dominant, although there is great uncertainty over accelerated particle minimum energies.Rapid fluctuations in X/gamma-rays point to a fragmented accelerator, acting on timescales of 100 ms or less, even after particle transport effects have been considered. Millimeter wave observations also reveal spatial fragmentation. Together with distributions of overall event size, such fragmentation suggests a scale-invariant energy release process, such as would occur in a state of Self-Organised Criticality. There is good evidence from X/gamma and cm/mm observations for hardening of the electron distribution towards the MeV energy range. Intercomparisons of X/gamma rays and cm/mm wave observations emphasise the importance of MeV energy range electrons in the latter. 'Electron-rich' events, characterised by a hard electron population extending to relativistic energies, may occur during individual flares. Existing instrumental capabilities mean that the absence of gamma-ray lines does not rule out significant, simultaneous ion acceleration. Radio observations indicate these spectral changes are associated with changes in spatial structure. Spatially resolved radio observations indicate that primary particle acceleration takes place moderately high in the corona (107 to 108 m), and have recently been made to yield information on accelerated electron pitch angle distribution. Throughout, we emphasise questions on which the unprecedented capabilities of the RHESSI mission will shed new light.

Authors: Nicole Vilmer and Alexander L. MacKinnon
Projects:

Publication Status: to be published in Lecture Notes in Physics
Last Modified: 2002-07-19 07:45
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Abstracts by Author
Solar Flares and Energetic Particles
Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun-Earth connection
Time delay between gamma ray lines and hard X-ray emissions during the 23 July 2002 interpreted by a trap plus precipitation model
Observations of a soft X-ray rising loop associated with a type II burst and a coronal mass ejection in the 03 November 2003 X-ray flare
Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003
Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT
High energy particles accelerated during the large solar flare of 1990 May 24 : X/gamma observations
Hard X-ray and metric/decimetric spatially resolved observations of the 10 April 2002 solar flare
Hard X-ray and metric/decimetric radio observations of the 20 February 2002 solar flare
What can be learned about competing acceleration models from multiwavelength observations?

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