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Microflares and the Statistics of X-ray Flares  

Amir Caspi   Submitted: 2011-10-28 12:21

This review surveys the statistics of solar X-ray flares, emphasising the newviews that RHESSI has given us of the weaker events (the microflares). The newdata reveal that these microflares strongly resemble more energetic events inmost respects; they occur solely within active regions and exhibithigh-temperature/nonthermal emissions in approximately the same proportion asmajor events. We discuss the distributions of flare parameters (e.g., peakflux) and how these parameters correlate, for instance via the Neupert effect.We also highlight the systematic biases involved in intercomparing datarepresenting many decades of event magnitude. The intermittency of theflare/microflare occurrence, both in space and in time, argues that thesediscrete events do not explain general coronal heating, either in activeregions or in the quiet Sun.

Authors: I. G. Hannah, H. S. Hudson, M. Battaglia, S. Christe, J. Kasparova, S. Krucker, M. R. Kundu, A. Veronig
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Hannah, I. G., et al. 2011, Space Sci. Rev., 159, 263; DOI 10.1007/s11214-010-9705-4
Last Modified: 2011-11-17 14:20
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The Relationship Between Solar Radio and Hard X-ray Emission  

Amir Caspi   Submitted: 2011-10-28 12:20

This review discusses the complementary relationship between radio and hardX-ray observations of the Sun using primarily results from the era of theReuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focusof joint radio and hard X-ray studies of solar flares uses observations ofnonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlunghard X-rays to study the properties of electrons accelerated in the main flaresite, since it is well established that these two emissions show very similartemporal behavior. A quantitative prescription is given for comparing theelectron energy distributions derived separately from the two wavelengthranges: this is an important application with the potential for measuring themagnetic field strength in the flaring region, and reveals significantdifferences between the electrons in different energy ranges. Examples of theuse of simultaneous data from the two wavelength ranges to derive physicalconditions are then discussed, including the case of microflares, and thecomparison of images at radio and hard X-ray wavelengths is presented. Therehave been puzzling results obtained from observations of solar flares atmillimeter and submillimeter wavelengths, and the comparison of these resultswith corresponding hard X-ray data is presented. Finally, the review discussesthe association of hard X-ray releases with radio emission at decimeter andmeter wavelengths, which is dominated by plasma emission (at lower frequencies)and electron cyclotron maser emission (at higher frequencies), both coherentemission mechanisms that require small numbers of energetic electrons. Thesecomparisons show broad general associations but detailed correspondence remainsmore elusive.

Authors: Stephen M. White, Arnold O. Benz, Steven Christe, Frantisek Farnik, Mukul R. Kundu, Gottfried Mann, Zongjun Ning, Jean-Pierre Raulin, Adriana V. R. Silva-Valio, Pascal Saint-Hilaire, Nicole Vilmer, Alexander Warmuth
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- White, S. M., et al. 2011, Space Sci. Rev., 159, 225; DOI 10.1007/s11214-010-9708-1
Last Modified: 2011-11-17 14:19
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Properties of Energetic Ions in the Solar Atmosphere from γ-Ray and Neutron Observations  

Amir Caspi   Submitted: 2011-10-28 12:19

Gamma-rays and neutrons are the only sources of information on energetic ionspresent during solar flares and on properties of these ions when they interactin the solar atmosphere. The production of {gamma}-rays and neutrons resultsfrom convolution of the nuclear cross-sections with the ion distributionfunctions in the atmosphere. The observed {gamma}-ray and neutron fluxes thusprovide useful diagnostics for the properties of energetic ions, yieldingstrong constraints on acceleration mechanisms as well as properties of theinteraction sites. The problem of ion transport between the accelerating andinteraction sites must also be addressed to infer as much information aspossible on the properties of the primary ion accelerator. In the last coupleof decades, both theoretical and observational developments have led tosubstantial progress in understanding the origin of solar {gamma}-rays andneutrons. This chapter reviews recent developments in the study of solar{gamma}-rays and of solar neutrons at the time of the RHESSI era. Theunprecedented quality of the RHESSI data reveals {gamma}-ray line shapes forthe first time and provides {gamma}-ray images. Our previous understanding ofthe properties of energetic ions based on measurements from the former solarcycles is also summarized. The new results-obtained owing both to the gain inspectral resolution (both with RHESSI and with the non solar-dedicatedINTEGRAL/SPI instrument) and to the pioneering imaging technique in the{gamma}-ray domain-are presented in the context of this previous knowledge.Still open questions are emphasized in the last section of the chapter andfuture perspectives on this field are briefly discussed.

Authors: Nicole Vilmer, Alec L. MacKinnon, Gordon J. Hurford
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Vilmer, N., et al. 2011, Space Sci. Rev., 159, 167; DOI 10.1007/s11214-010-9728-x
Last Modified: 2011-11-17 14:19
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Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares  

Amir Caspi   Submitted: 2011-10-28 12:16

High-energy X-rays and gamma-rays from solar flares were discovered just overfifty years ago. Since that time, the standard for the interpretation ofspatially integrated flare X-ray spectra at energies above several tens of keVhas been the collisional thick-target model. After the launch of the ReuvenRamaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-rayspectra and images have been of sufficient quality to allow a greater focus onthe energetic electrons responsible for the X-ray emission, including theirorigin and their interactions with the flare plasma and magnetic field. Theresult has been new insights into the flaring process, as well as morequantitative models for both electron acceleration and propagation, and for theflare environment with which the electrons interact. In this article we reviewour current understanding of electron acceleration, energy loss, andpropagation in flares. Implications of these new results for the collisionalthick-target model, for general flare models, and for future flare studies arediscussed.

Authors: Gordon D. Holman, Markus J. Aschwanden, Henry Aurass, Marina Battaglia, Paolo C. Grigis, Eduard P. Kontar, Wei Liu, Pascal Saint-Hilaire, Valentina V. Zharkova
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Holman, G. D., et al. 2011, Space Sci. Rev., 159, 107; DOI 10.1007/s11214-010-9680-9
Last Modified: 2011-11-17 14:18
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An Observational Overview of Solar Flares  

Amir Caspi   Submitted: 2011-10-28 12:15

We present an overview of solar flares and associated phenomena, drawing upona wide range of observational data primarily from the RHESSI era. Following anintroductory discussion and overview of the status of observationalcapabilities, the article is split into topical sections which deal withdifferent areas of flare phenomena (footpoints and ribbons, coronal sources,relationship to coronal mass ejections) and their interconnections. We alsodiscuss flare soft X-ray spectroscopy and the energetics of the process. Theemphasis is to describe the observations from multiple points of view, whilebearing in mind the models that link them to each other and to theory. Thepresent theoretical and observational understanding of solar flares is far fromcomplete, so we conclude with a brief discussion of models, and a list ofmissing but important observations.

Authors: Lyndsay Fletcher, Brian R. Dennis, Hugh S. Hudson, Säm Krucker, Ken Phillips, Astrid Veronig, Marina Battaglia, Laura Bone, Amir Caspi, Qingrong Chen, Peter Gallagher, Paolo C. Grigis, Haisheng Ji, Ryan O. Milligan, Manuela Temmer
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Fletcher, L., et al. 2011, Space Sci. Rev., 159, 19; DOI 10.1007/s11214-010-9701-8
Last Modified: 2011-11-17 14:18
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High-Energy Aspects of Solar Flares: Overview of the Volume  

Amir Caspi   Submitted: 2011-10-28 12:13

In this introductory chapter, we provide a brief summary of the successes andremaining challenges in understanding the solar flare phenomenon and itsattendant implications for particle acceleration mechanisms in astrophysicalplasmas. We also provide a brief overview of the contents of the other chaptersin this volume, with particular reference to the well-observed flare of 2002July 23

Authors: Brian R. Dennis, A. Gordon Emslie, Hugh S. Hudson
Projects: GONG,Hinode/EIS,Hinode/SOT,Hinode/XRT,Nobeyama Radioheliograph,Owens Valley Solar Array,RHESSI,SDO-AIA,SDO-HMI,SDO-EVE,SoHO-EIT,SoHO-MDI,SoHO-CDS,SoHO-LASCO,SoHO-SUMER,STEREO,TRACE

Publication Status: Published -- Dennis, B. R., et al. 2011, Space Sci. Rev., 159, 3; DOI 10.1007/s11214-011-9802-z
Last Modified: 2011-11-17 14:17
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Origin of the submillimeter radio emission during the time-extended phase of a solar flare  

Amir Caspi   Submitted: 2011-09-27 18:45

Solar flares observed in the 200-400 GHz radio domain may exhibit a slowly varying and time-extended component which follows a short (few minutes) impulsive phase and which lasts for a few tens of minutes to more than one hour. The few examples discussed in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present a detailed analysis of the time-extended phase of the 2003 October 27 (M6.7) flare, combining 1-345 GHz total-flux radio measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, radiated by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7-16 MK and 1-3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 104 K. These results, which may also apply to other millimeter-submillimeter radio events, are not consistent with the expectations from standard semi-empirical models of the chromosphere and transition region during flares, which predict observable radio emission from the chromosphere at all frequencies where the corona is transparent.

Authors: G. Trottet, J.-P. Raulin, G. Gim?nez de Castro, T. L?thi, A. Caspi, C.H. Mandrini, M. L. Luoni, & P. Kaufmann
Projects: RHESSI,TRACE

Publication Status: Published -- Trottet, G., et al. 2011, Sol. Phys., 273, 339; DOI 10.1007/s11207-011-9875-6
Last Modified: 2011-11-17 14:11
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RHESSI Line and Continuum Observations of Super-hot Flare Plasma  

Amir Caspi   Submitted: 2010-12-01 11:27

We use RHESSI high-resolution imaging and spectroscopy observations from ~5 to 100 keV to characterize the hot thermal plasma during the 2002 July 23 X4.8 flare. These measurements of the steeply falling thermal X-ray continuum are well fit throughout the flare by two distinct isothermal components: a super-hot (Te > 30 MK) component that peaks at ~44 MK and a lower-altitude hot (Te <~ 25 MK) component whose temperature and emission measure closely track those derived from GOES measurements. The two components appear to be spatially distinct, and their evolution suggests that the super-hot plasma originates in the corona, while the GOES plasma results from chromospheric evaporation. Throughout the flare, the measured fluxes and ratio of the Fe and Fe-Ni excitation line complexes at ~6.7 and ~8 keV show a close dependence on the super-hot continuum temperature. During the pre-impulsive phase, when the coronal thermal and non-thermal continua overlap both spectrally and spatially, we use this relationship to obtain limits on the thermal and non-thermal emission.

Authors: A. Caspi & R. P. Lin
Projects: RHESSI

Publication Status: Published -- Caspi, A. & Lin, R. P. 2010, ApJ, 725, L161; DOI 10.1088/2041-8205/725/2/L161
Last Modified: 2014-05-30 14:00
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Super-hot (T > 30 MK) Thermal Plasma in Solar Flares  

Amir Caspi   Submitted: 2010-08-19 16:41

The Sun offers a convenient nearby laboratory to study the physical processes of particle acceleration and impulsive energy release in magnetized plasmas that occur throughout the universe, from planetary magnetospheres to black hole accretion disks. Solar flares are the most powerful explosions in the solar system, releasing up to 1032-1033 ergs over only 100-1,000 seconds. These events can accelerate electrons up to hundreds of MeV and can heat plasma to tens of MK, exceeding ~40 MK in the most intense flares. The accelerated electrons and the hot plasma each contain tens of percent of the total flare energy, indicating an intimate link between particle acceleration, plasma heating, and flare energy release. X-ray emission is the most direct signature of these processes; accelerated electrons emit hard X-ray bremsstrahlung as they collide with the ambient atmosphere, while hot plasma emits soft X-rays from both bremsstrahlung and excitation lines of highly-ionized atoms. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes this emission from ~3 keV to ~17 MeV with unprecedented spectral, spatial, and temporal resolution, providing the most precise measurements of the X-ray flare spectrum and enabling the most accurate characterization of the X-ray-emitting hot and accelerated electron populations. RHESSI observations show that ''super-hot'' temperatures exceeding ~30 MK are common in large flares but are achieved almost exclusively by X-class events and appear to be strictly associated with coronal magnetic field strengths exceeding ~170 Gauss; these results suggest a direct link between the magnetic field and heating of super-hot plasma, and that super-hot flares may require a minimum threshold of field strength and overall flare intensity. Imaging and spectroscopic observations of the 2002 July 23 X4.8 event show that the super-hot plasma is both spectrally and spatially distinct from the usual ~10-20 MK plasma observed in nearly all flares, and is located above rather than at the top of the loop containing the cooler plasma. It exists with high density even during the pre-impulsive phase, which is dominated by coronal non-thermal emission with negligible footpoints, suggesting that particle acceleration and plasma heating are intrinsically related but that, rather than the traditional picture of chromospheric evaporation, the origins of super-hot plasma may be the compression and subsequent thermalization of ambient material accelerated in the reconnection region above the flare loop, a physically-plausible process not detectable with current instruments but potentially observable with future telescopes. Explaining the origins of super-hot plasma would thus ultimately help to understand the mechanisms of particle acceleration and impulsive energy release in solar flares.

Authors: A. Caspi
Projects: RHESSI

Publication Status: Published -- Ph.D. Dissertation (Univ. of California, Berkeley); arXiv: 1105.1889
Last Modified: 2014-05-30 14:01
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