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Non-thermal particles in the solar atmosphere during flare developments and in the absence of flares
Adv. Space Re:. Vol. 13, No.9, pp. (9)221—(9)231, 1993 Printed in GreatBritain. All rights reserved.
0273—1 l77i~3$24.00
Copyright © 1993 COSPAR
NON-THERMAL PARTICLES IN THE SOLAR ATMOSPHERE DURING FLARE DEVELOPMENTS AND IN THE ABSENCE OF FLARES N. Vilmer Observatoire de Paris-Meudon DASOP and CNRS-UR.4-324, 92195 Meudon, France
ABSTRACT Non-thermal particles play a major role in the active Sun since they contain a large amount of the energy released during flares and since dekakeV electrons are produced in the absence of flares in association with active regions. These particles are detected either in the interplanetary space or through the radiations they produce in the solar atmosphere during flares (radio, X-ray/v-ray bursts) and outside flares (e.g. radio noise storms). We focus on the particles interacting at the Sun. After a brief discussion on emission processes, we discuss some evidences for the fragmentation of the non- thermal energy release during flares and present some observations relevant to the onset of noise storms. New results obtained on the more energetic flares are then summarized: relative amount of accelerated electrons and ions, particle transport from the acceleration site to the emitting regions, hard X-ray and ~y-ray coronal contributions from partially trapped partides, high energy ( 10 MeV) radiation from relativistic electrons and 100 MeV-1 GeV ions. INTRODUCTION The last 10 years have brought a lot ofinformation on the particles accelerated during solar flares and outside flares (during e.g. radio noise storms).Although a complete knowledge of the way in which the Sun accelerates particles has not been obtained yet, there have been considerable advances in the understanding of the following fundamental questions: What are the first signatures of non-thermal particles at the onset of flares? Where is the particle acceleration region located and where are the interaction sites? Is the large number of particles required to explain the solar flare X-ray and 7-ray observations (typically 10~electrons above 30 keY, 1032 10:33 protons above 30 MeV) accelerated in one single volume of the solar atmosphere or does it result from the combination of many weaker energy releases in a multitude of smaller regions? Is there a link between microflares, flares and other manifestations of non-thermal particles? What is the relationship between ion and electron acceleration? Is there some evidence for long duration particle storage after a flare? -What is the low-energy cutoff and the energy content of the non-thermal particle populations? What is the link between the particles interacting at the Sun and those escaping in the interplanetary medium?
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The subject of flare precursors and triggering as well as the relations between particles at the Sun and in the interplanetary space are reviewed elsewhere in this issue. The acceleration mechanisms will not be discussed here. This review deals with the different radiations produced by the energetic particles when interacting with the solar atmosphere. We focus on the constraints deduced from the observations on the characteristics of the energetic partides and on the conditions in the regions where particles propagate and interact. Energetic electrons (a few tens to a few hundreds of keV) produce radio emission in the low corona through gyrosynchrotron emission in the centimeter and millimeter wavelengths , coherent plasma radiation in the metric-decinietric domain at greater heights (10~-ion kms above the photosphere ) while X-rays and 7-rays are produced by electrons and ions in the denser and lower regions. The impact of non-thermal electron and proton beams (9)221
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on deeper layers of the solar atmosphere may be at the origin of e.g. Ha enhancements or of whitelight flares. The good relationship sometimes observed between Ha time profiles in a kernel and hard X-ray and microwave emissions favor an energy transport by the means of electron beams /1/. The origin of white light flares is still controversial since they may be related either to the direct bombardment of the atmosphere by protons above 100 MeY or by electrons above a few hundred keY impinging on the chromosphere followed by a radiative heating of the upper photosphere and temperature minimum region/2/. The determination of the low-energy cutoff of the non-thermal electron and proton population is a challenging problem since it determines the energy budget of the flare. In most cases, the existence of a thermal component at lower X-rays energies and the usual lack of spectral resolution of the X-ray observations make difficult the estimate of the cutoffof the non-thermal electrons. However, recent observations of an occulted X-ray flare have shown that the electron low-energy cutoff could be as low as 5 keV /3/, thus increasing by several orders of magnitude the non-thermal electron energy content. Some evidence of low-energy proton beams impinging on the deep layers have been obtained by the detection of linear polarization in Ha /4,5/. However, the deduced initial energy of the beam is dependent on the proton transport between the acceleration site and the impinging point (from 100 keY up to 10 MeV) /4,6/. This makes the estimate of the total proton content uncertain. An alternative way for determining the presence of protons below 1 MeY could be through the detection of weak radiative capture 7-ray lines /7/. The remainder of this paper concerns the most direct diagnostics of the accelerated particles, namely X-ray, 7-ray and radio observations, the latter ones providing also constraints on the magnetic topology of the corona. HARD X-RAY AND 7-RAY EMISSION MECHANISM Figure 1 shows an X-ray/7-ray spectrum observed from 300 keV to 10 MeY in the gradual phase of an intense, long duration flare /8,9/. It consists of several nuclear de-excitation lines from Fe, Mg, Ne, Si, C, 0, of the delayed lines from neutron radiative capture at 2.2 MeV (n) and from electron- positron annihilation at 511 keV (e +). These lines are superposed on a continuum made of the same kinetically broadened de-excitation lines and of a bremsstrahlung component extending above 1 MeV. For other events, this last component may extend to energies greater than 10 MeY (10% of the flares observed from 1980 to 1989). The 7-ray lines are unambiguously
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produced by nuclear reactions of energetic ions ( 10 MeV/nudeon) with the ambient medium. These reactions result in excited nuclei with short lifetimes ( 10_il sec) producing the de-excitation “prompt “ 7-ray lines and in secondary products such as positrons, pions( for protons 200 MeV) and neutrons giving rise respectively to the delayed 511 keV annihilation line, a broad emission around 70 MeV and the delayed 2.2 MeY line /10/. For higher energetic primary ions ( 100 MeV/nucleon) the energetic neutrons produced can also be directly detected in the interplanetary space ( 50 MeV neutrons) or produce neutron decay protons( 20 MeY neutrons) /11/. The primary particles emitting the bremsstra..blung component above a few keY are generally thought to be electrons with energies greater than 30 keY. However, although a good agreement is obtained between the numbers of microwave and X-ray emitting electrons, and although this hypothesis naturally explains the correlation found between hard X-rays and different kinds of radio emission (see e.g. /12,13/), problems arise from the large number of electrons needed to produce observed X-ray fluxes (e.g. /14/). This leads to an alternative interpretation where X-rays are produced by more efficient energetic protons ( 60 MeV) (see e.g. /15,16/). The number of energetic particles required to produce the X-rays is thus reduced by a factor ~ 2000. However, there is still an unresolved discrepancy between the protons deduced from the continuum or from the 7-ray lines. It could be reduced if there is a break in the proton spectrum between the low energy 7-ray producing part ( 10 MeV ) and the higher energy part ( 60 MeV ) responsible for the X-ray continuum. Here, we shall consider that the X-ray continuum is produced by energetic electrons. NON-THERMAL ELECTRON ENERGY RELEASE DURING AND OUTSIDE FLARES Localization of the 10-100 keV Electron Acceleration Site Some indications concerning the electron acceleration site come from the study of the frequency drift of decimetric and centimetric type ifi bursts. Most of the type Ill bursts observed above 3 GHz have a frequency drift towards high frequencies, while below 1 GHz, the frequency drift is towards low frequencies. Electron beams producing type ifi bursts seem then to originate from a medium where the plasma frequency is around a few GHz, thus leading to an energy release in the low corona ( n ~ 1010 to 1011 cm /12/). Some electron acceleration can also take place higher in the corona, about one solar radius above the photosphere. Such a coronal acceleration is revealed by the observations of radio bursts drifting towards high frequencies at metric wavelengths, and corresponding to two distinct emitting sources in the corona which brighten in close simultaneity /17/. This coronal acceleration may explain the observations of electron spectra in space without any spectral break from a few tens of keY to 2 keV /18/ and can be related with the high coronal flares /19/. Observational Evidences for the Fragmentation of the Flare Energy Release A lot of problems are encountered when it is assumed that the flare energy release takes place in a single loop. In particular, upper limits on the acceleration volume and on the electron acceleration rate /14/ provide strong constraints on the number of electrons which can be accelerated in a single volume. This has led to the question whether a flare results from a collective process of many micro energy releases (see e.g. /20/), or whether it can be described in terms of “avalanche catastrophes” as known in non-linear, chaotic systems (see e.g. /21/). Evidence of this fragmented energy release has been searched in fine temporal, spatial and spectralfeatures in the most direct radiations from 10-100 keY electrons ( radio emissions /12/ and X- rays ) 1. A large number of narrowband (typical bandwidth of an individual spike of 1% to 2% ) millisecond spikes are observed in the 0.5-1 GHz frequency domain at the onset and during the impulsive phase of flares (see Figure 2). These emissions are assumed to arise from regions with densities typical of the acceleration sites and may indicate many localized rapid accelerations giving rise to the flare /12,22/. JASR 13:9-P
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2. Rapid time features are found in the radio and hard X-ray global fluxes. Although subsecond time structures are detected in less than 10% of the hard X-ray bursts above 30 keY observed by HXRBS/SMM /23/, fast time variations (of the order of 500ms ) are seen in many events observed with a higher sensitivity /24/ or systematically detected in short duration flares above 100 keY /25,26/. However, some care must be taken when inferring from rapid time structures evidence for a fragmented energy release (see e.g. /27/). -particle transport effects (for e.g. X-ray radiation) can smear short time scales of the acceleration process while non-linear emission processes for radio coherent radiation may produce rapidly variable emission even for a steady state electron population. These two facts explains why many more time structures are seen at decimetric wavelengths than in the X-ray domain (see e.g. Figure 2). -the total emission can originate from several regions with completely different temporal evolutions (see e.g. /28/). 3.The flare energy release is generally linked with the appearance of multiple interacting magnetic structures (see e.g. /29,30,31,32/). Of course the detection of individual reconnection events wifi require substantially higher sensitivity or spatial resolution in e.g. hard X-rays. 4.A close correlation is observed between the metric type III bursts or narrowband spikes rate and the hard X-ray count-rate in the main phase of a flare /33,34/. Each radio burst or spike could thus be a measure of an individual acceleration process estimated from the X-ray measurements to be respectively of the order of 1028 ergs and 1024 ~1025 ergs.The impulsive phase of a flare comprises then hundreds or thousands of subevents of similar size. The distributions of flares and of their elementary structures as a function of peak rates or energy dissipation at the peak provide other arguments.It was found from a statistical study of 12000 flares observed by SMM/HXRBS that the frequency distribution of the dissipated energy in flares,assuming thick target production, is well represented by a power law with a slope of 1.53
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drawn m the left corner of the first and the last maps. Note at 15:10 UT that both the noise storm at 92 cm and a new source at 21 cm have appeared while the
the position of the noise storm at different wavelengths compared to the motionless 21 cm new source(from /42/).
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and that this slope is not strongly varying with the solar cycle /35/. Furthermore, a similar study of fast time structures in hard X-ray flares has shown that, if the peak count-rate for structures with a given time scale is proportional to the dissipated energy, then the frequency distribution of elementary structures has the same slope as the above flare distribution /36/. The slope of the flare distribution of the peak hard X-ray flux above 20 keY is also consistent /35/ with the slope of the same distribution for the microflares obtained at similar periods of solar activity /37,38/. They are also in agreement with the predicted slope of event energy release from computer simulations of avalanche models governed by the principle of self-organized criticality /21/. As a conclusion, even if there are some arguments in favour of a fragmented energy release from radio and hard X-ray observations, more evidences should be searched for with e.g. spatially resolved observations obtained with a high time resolution. The similitude of the slopes of the frequency distributions of flares, of elementary structures in flares and of microflares can also act in favour of this idea. However, the physical meaning of this slope must be understood and the parameters characterizing the magnitude of the non-thermal energy release must be determined. Non-Thermal Energy Release Outside Flares Based on the slope of frequency distributions and on the observations that hard X-ray microflares appear in Ha as very small flares /39/ and can be associated with weak radio bursts /40/, microflares could be considered as a low energy extrapolation of flares. Other microphenomena associated with active regions are reported in the literature such as centimeter microbursts /41/, but the relation to known radio bursts or hard X-ray microflares and thus the energy content of such microbursts are still unknown. The oldest evidence of electron acceleration (from a few keY to several tens of keV )in the solar corona outside flares are metric and decimetric noise storms for which the radiation mechanism and the energy source are still not well identified. The only consensus is that the emission is radiated at or close to the plasma frequency and is thus produced in a region of a given density and thus coronal height. No systematic association with chromospheric or photospheric phenomena are reported. However, recent simultaneous from the Yery Large Array (VLA) and the Nançay RadioHeliograph (NRH) have shown that the onset of a noise storm in the middle corona occurs in close temporal and spatial coincidence with the appearance of a new source at centimeter wavelength in the active region (Figure 3)/42/, the delay between the two phenomena being of the order of 10 minutes.Such a close relationship between the noise
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storm and the emission in the active region has been reported elsewhere /43,44,45/. The delayed onset of the noise storm at longer wavelengths i.e. at different coronal heights (see Figure 3) as well as its slow movement (a few tens of km s~1)at a given wavelength with a significant component perpendicular to the magnetic field lines are interpreted as due to the emission of non-thermal electrons in an arcade of expanding loops of different densities. The close coincidence between the appearance of the new source in the low atmosphere and the onset of the noise storm argues in favour of the expansion being triggered by this new source .This could be the first indication of some magnetic flux emergence by which the overlying large scale magnetic structure is forced to evolve, thus giving rise to the noise storm. There could then be some similarity between the onset of some noise storms and the onset of flares. For both phenomena a new structure is observed, which may imply the emergence of a new magnetic flux and the interaction of different magnetic structures. However, the difference lies in the time scales (minutes for noise storms instead of seconds for flares) between the appearance of the new structure and the onset of the phenomenon and in the energy budget much smaller for noise storms. Furthermore, the relationship which could exist between the processes giving rise to electron acceleration in a flare or in association with noise storms is far from being understood. RELATIVISTIC ELECTRONS AND NON-THERMAL IONS IN SOLAR FLARES Electron and Ion Acceleration SMM observations have shown that ion acceleration at energies between 10-100 MeV/nucleon is relatively common during flares and that a rough correlation is observed between the electron content (estimated from the bremsstrahlung total flux above 270 keV), and the nuclear content (estimated from the 4-8 MeV excess above bremsstrahlung) /46/. The flare acceleration mechanism must be efficient and rapid since a simultaneous production of energetic electrons ( few 100 keV) and ions (10-100 MeV/nucleon), as detected by the 4-6 MeV flux assumed to be mainly produced by nuclear radiation, is generally observed on a time scale of 1 second (e.g. /47/). Moreover, the direct neutron detection in space shows that nucleons are accelerated up to a few GeV/nucleons on time scales of a few tens of seconds at the beginning of a flare /48/. On the other hand, a few intense events are observed with very weak nuclear line emission and a strong continuum with a cutoff at about 60 MeV (“Electron Dominated Events” /9/). This is the case for a peak in the initial phase of the flare of 6 Mar 1989, the gradual phase of which exhibits strong 7-ray lines (Figure 1) /49/. Such events are also observed by the 7-ray spectrometer aboard YOHKOH /50/. The SIGMA and PHEBUS observations aboard GRANAT have shown that the flare of 24 May 1990, which produced the largest increase due to solar neutrons yet recorded by neutron monitors /51/, shows no clear evidence of any excess in the nuclear 7-ray line domain during the most intense part of the event in the 10- 15 MeV range/52/. However, the PHEBUS observations above 10 MeV /25/ indicate pion radiation at the end of this phase. This observation, together with the detection of the 2.2 MeY line in the late phase of the flare /53/ and of neutrons indicate that high energy protons are accelerated in the flare, even if the content of the 7-ray line emitting protons is too low compared to the electron content to produce a measurable prompt 7-ray line excess. The above observations thus indicate that the relative content of energetic electrons above 300 keV and protons above 30 MeV can vary from one phase of an event to another, and from flare to flare, even if a rough correlation between both contents is observed. Of course, X-ray and 7-ray images would be a powerful tool to check whether the different phases of a flare exhibiting different behaviours come from the same region. This would help to determine whether the relative amount of accelerated electrons and ions depend on the acceleration mechanism or on the conditions in the acceleration site. Electron and Ion Transport in the Solar Atmosphere. The main difficulty to deduce the characteristics of the accelerated particles from the observation of their radiations, is to evaluate the effects of the particle transport from the acceleration region ( assumed in the low corona) to the usually dense chromospheric X-ray and 7- ray interaction sites. Apart from some recent papers in which particle acceleration and transport are treated simultaneously /e.g. 54,55/, most of the transport models do not look at the details of particle acceleration. The different X-ray/7-ray models differ by the methods used to study the relationship
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between the energetic particles at the interaction site to the injection spectrum produced by the “non-modeized” acceleration mechanism: -Monte Carlo simulation of a Fokker-Planck equation (e.g. /56,57,58,59/) -Analytical or numerical solutions of the Fokker-Planck equation (e.g. /60/) -Analytical or numerical solutions of a continuity equation( e.g. /61,62,63,64,65/) They also differ by the exact assumptions on the geometry on the region where the transport takes place: open or closed magnetic field, magnetic field convergence in the corona or below. They generally consider that the particles are injected at the top of the coronal structure. Some of the particles precipitate in the loss cone created by the converging magnetic field and thus can reach the dense layers of the atmosphere. The loss cone may be fed by scattering due to wave particle interactions in the ionized coronal trap. However, the effects of wave diffusion vary from pitch-angle scattering without energy transfer to energy gain in the case of strong diffusion (see e.g. /55/). In all the models the waves and the particles are not treated in a self-consistent way. Energy losses (Coulomb, gyrosynchrotron) of the energetic partides in the corona and during their propagation towards denser layers are taken into account. Many predictions have been made: -decay phase of 7-ray lines /e.g. 56,66/ -temporal and spectral evolution of neutron production and pion decay radiation /e.g. 58,59/ -relative X-ray and 7-ray timing in the case of a long duration injection and of a simultaneous injection of electrons and ions /63/ -height distribution of the X-rays and 7-rays /e.g. 56,57,67/ -center to limb variations of X-rays and 7-rays /e.g. 61,62,68/ Hard X-Ray and 7-Ray Coronal Contributions from Partially Trapped Particles. The application of the transport model predicting the relative X-ray/7-ray timing /63/ to one main peak of the 27 Apr 1981 event, where a delay of forty seconds is observed between the ray/x-ray maxima shows that the temporal evolutions of the X-ray fluxes at different energies and that of the delayed 4.1-6.4 MeV (considered as a good indication of the timing of the prompt 7-ray line) are reproduced without a second step acceleration of energetic ions.Electrons and ions must be injected simultaneously and continuously in the same magnetic ioop of mean density n=5 1010 cm~3). Particle transport acting differently on electrons and ions is responsible for the different temporal evolutions (see /69/). This does not rule out completely the existence of a delayed acceleration but shows that the particle transport effects must necessarily be taken into account for any detailed analysis of X-ray/7-ray time profiles. For that event, X-ray emission comes predominantly from the trapped electrons, while 7-ray lines are produced either essentially by trapped protons or by trapped and precipitated protons. The available data do not allow to decide between both interpretations. However, even in the second case, at least 30% of the 7-ray line flux is produced in the corona. Such a localization of the 7-ray source could explain why some of the abundances deduced from 7-ray line spectroscopy are more consistent with coronal than with photospheric values /70/. ~-
Another evidence for long term particle trapping comes from the Egret/Compton observations of the 11 Jun 1991 flare. The production of GeV 7-rays 1S observed as late as 8 hours after the impulsive phase /71/. The high energy spectrum in the late phase of the event as well as the
temporal decay of the 50 MeY flux can be explained by ions and electrons accelerated during the impulsive phase of the flare and subsequently trapped in magnetic loops under the condition that the strength of the magnetic field and the level of the plasma turbulence in the corona are sufficiently low (mirror ratio of the order of 10, and coronal density less than 1011 cm3) /72/. The detailed comparison of the time history of the prompt (4 to 7 MeV) and the delayed 2.23 MeV 7-ray line observed by PHEBUS in the impulsive phase allowed to deduce preliminary values of the 3He/H ratio and of the hardness of the ion spectrum /73/. The 3He/H value is consistent with previous observations. The absence of any significant delay between hard X-ray and prompt 7-ray line time profiles implies that coronal trapping of energetic particles is not dominant during the impulsive phase, and that electron and proton interaction during that phase takes place in the chromosphere as is generally observed. The proton spectral indexes found in the impulsive phase and from the interpretation of the late phase are similar. A further study is however necessary to understand the link between the impulsive and the late phases. In particular, if the particle
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precipitation in the loss cone is due to waves, the waves and the particles should be treated in a consistent way. If the conditions for particle storage in a loop are related to the total number of trapped particles above a given energy /67,74/, the diffusion rate of particles due to waves may decrease with time after the impulsive phase. This could explain why in the impulsive phase, a lot of particles precipitate giving rise to chromospheric emission while in the late phase the emission may come from partially trapped high energy protons. The main problem of this interpretation is to prevent the particles to drift from the magnetic loop on these long time scales. Either a twisted magnetic field could prevent this effect from being important or the particles may remain trapped in arcades of loops /72/. Such arcades of loops are considered to explain other observations: the comparisons of unocculted and partially occulted X-ray emissions observed with PVO, ICE, GOES together with the application of a transport model shows that the impulsive hard X-ray and soft X-ray emission sources can extend to coronal altitudes of 2 i0~kin above the photosphere leading with an average density of i0~cm3. Such a to a volume the hard X-ray source of 1030 cm3corona large volume of suggests a large diffuse source in the covering several magnetic loops /3/. All these observations give support to the models considering partial electron and proton trapping. However, images at high energies will be the only means to measure the exact height distributions of high energy X-ray and 7-ray emissions. Emission above 10 MeV from Relativistic Electrons and High Energy Ions. Emission 10 MeV is detected in the most energetic flares. However such events are unfrequent since less than 30 events with 10 MeV emission have been observed during 10 years ofcontinuous observations with SMM /75,76,77/. High energy emission up to 1 GeV has now been detected for a few events with GAMMA1 or the Compton Observatory. For one impulsive event, this emission could originate exclusively from electron bremsstrahlung, while pion radiation is the dominant emission above 100 MeV in the decay phase of larger flares lasting from one to several hours /71,78/. There is a tendency of the high energy flares of Cycle 2lto be associated with limb events /75,76/. However, during the rise towards Cycle 22 SMM has detected several events located well on the disk /9/. Even with this detection of disk events, the observed limb fraction (ratio of the number of flares with heliocentric angle such as sinO 0.9 to the total number) still shows a statistically significant excess compared to the prediction for isotropic radiation, in good consistency with the one observed for Cycle 21 /77/. Other high energy events have been observed with SIGMA /79/, PHEBUS/25/,GAMMA1 /78/ and COMPTON/71/ some of them being also associated with disk flares. Before achieving a new center-to-limb distribution, the intercalibration of the different experiments must be realized to compare the relative detection thresholds. Indeed, the center-tolimb variation must be deconvolved from threshold effects in order to deduce constraints from the different models. The different components of the high energy radiation must also be distinguished since they present different anisotropies and thus center-to-limb variations. In particular, the angular distribution of high energy radiation from pion decay is not sufficiently anisotropic to explain the observed limb brightening even if the transport of primary ions and secondary electrons or positrons in a magnetic loop is considered/59/. This confirms that in most events the high energy emission is mainly produced by electrons. At high energies the electron radiation pattern is strongly directed along its velocity. The limb dustering thus indicates that the emitting electron velocity shows an anisotropy in a direction parallel to the photosphere. Electrons partially trapped in a converging magnetic field exhibit such an anisotropy near the footpoint, if the magnetic field is nearly normal to the photosphere. The radiation anisotropy and its center-to-limb variations have been computed in different transport models /61,62,65,66,68/. Apart from MacKinnon and Brown /62/, all the models consider a radial magnetic field at the mirror points. The anisotropy of the radiation can also result from the electron injection itself and it is difficult to estimate from the observations the relative importance of both effects. To be able to make such an estimate and a distinction between the different kinds of magnetic field convergence (e.g. /62/) the accelerated electrons must be followed from the injection site to the denser interaction sites. In the frame of these models, the observations of some events close to the disk center indicate that the electron bremsstrahlung should come from upwardly moving electrons, probably reflected at mirror points below the transition region /76/.
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Another possibility which is not considered in the models is that the magnetic field is not radial at all or that the emission comes from a complicated magnetic structure. Coronal and Chromospheric Signature of High Energy Emission Recent studies have looked at the magnetic environments of high energy disk flares in the case where the high energy emission is dominated by electron bremsstralilung. For the few studied events, the 10 MeV enhancement is systematically associated with the appearance of either a new radio source in the corona /80,81/, or with a new eruptive Ha feature /81,82/: the high energy radiation probably originates from a complex magnetic structure where the magnetic field is far from being simple and radial. The observations of a new source emerging in close coincidence with the high energy radiation can also be related to the observations previously mentioned that the flare energy release occurs when different magnetic structures interact. This could be another evidence of the fragmentation of the energy release. The importance of a flare could be governed by the complexity of the magnetic overall structure revealed by the number of sources of emission. Here again, images obtained at high energies would provide fruitful constraints for the interpretation. CONCLUSION New ideas on the non-thermal particles in the solar atmosphere have emerged from the large amount of observations recorded during the last 10 years. -The idea of the fragmentation of the flare energy release or of a flare being the consequence of “avalanche catastrophes” has gained some interest. Observations have been analyzed to support this idea. Such a work must be pursued with the available past and new data sets to firmly establish such an idea since individual reconnection events are far from being observed. The slope of the frequency distribution of the hard X-ray microflares as a function of peak energy indicates that they could be the low energy counterpart of the flares. Alike in flares, the energization of non-thermal particles at the origin of noise storms may imply the emergence of a new magnetic flux interacting with previous magnetic structures but this result must be checked on a bigger sample. -The importance of the effects of the transport of the energetic particles from the acceleration site to the interaction sites has been systematically studied and many models including transport have been developed. A few models have also included the study of particle acceleration and such an effort should be pursued. On the other hand, some effort should be undertaken in order to treat in a self consistent way the wave particle interactions which are often considered in the transport models. -High energy radiation up to 1 GeV has been observed. This radiation can slowly decay over hours suggesting a long duration storage of high energy particles (in particular ions). Other observational evidences have been obtained for the production of hard X-rays and 7-ray lines by partially trapped electrons and protons. -Recent observations have shown that the relative amount of 300 keY electrons and 30 MeY ions can vary from flare to flare, and even during a flare. The consistency with the previously observed rough correlation between these numbers must be understood. -The production of above 10 MeV impulsive emission from energetic electrons seems to be related to the appearance of a new magnetic structure revealed by a new radio or Ha signature. There seems to be a common observational finding from the onset of noise storms to the triggering of high energy pulses and through the onset of flares: all these phenomena are associated with the appearance of new emission sources. Of course, from one phenomenon to the other, the time scales involved as well as the energy released are completely different. If such a finding is verified on a bigger sample, this would be another argument in favour of the interaction of multiple structures in the acceleration of non- thermal particles in the solar corona and would act in favour of the fragmentation of the energy release. The parameters leading to a more or less energetic or complicated phenomenon should be determined. Some of these parameters could be the complexity and the overall size of the magnetic structures involved in the energy release. To make progress,the path of the accelerated particles as close as possible from their energization site throughout the whole phase of their energy losses and radiations in the solar atmosphere should be traced. One
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0273—1 l77i~3$24.00
Copyright © 1993 COSPAR
NON-THERMAL PARTICLES IN THE SOLAR ATMOSPHERE DURING FLARE DEVELOPMENTS AND IN THE ABSENCE OF FLARES N. Vilmer Observatoire de Paris-Meudon DASOP and CNRS-UR.4-324, 92195 Meudon, France
ABSTRACT Non-thermal particles play a major role in the active Sun since they contain a large amount of the energy released during flares and since dekakeV electrons are produced in the absence of flares in association with active regions. These particles are detected either in the interplanetary space or through the radiations they produce in the solar atmosphere during flares (radio, X-ray/v-ray bursts) and outside flares (e.g. radio noise storms). We focus on the particles interacting at the Sun. After a brief discussion on emission processes, we discuss some evidences for the fragmentation of the non- thermal energy release during flares and present some observations relevant to the onset of noise storms. New results obtained on the more energetic flares are then summarized: relative amount of accelerated electrons and ions, particle transport from the acceleration site to the emitting regions, hard X-ray and ~y-ray coronal contributions from partially trapped partides, high energy ( 10 MeV) radiation from relativistic electrons and 100 MeV-1 GeV ions. INTRODUCTION The last 10 years have brought a lot ofinformation on the particles accelerated during solar flares and outside flares (during e.g. radio noise storms).Although a complete knowledge of the way in which the Sun accelerates particles has not been obtained yet, there have been considerable advances in the understanding of the following fundamental questions: What are the first signatures of non-thermal particles at the onset of flares? Where is the particle acceleration region located and where are the interaction sites? Is the large number of particles required to explain the solar flare X-ray and 7-ray observations (typically 10~electrons above 30 keY, 1032 10:33 protons above 30 MeV) accelerated in one single volume of the solar atmosphere or does it result from the combination of many weaker energy releases in a multitude of smaller regions? Is there a link between microflares, flares and other manifestations of non-thermal particles? What is the relationship between ion and electron acceleration? Is there some evidence for long duration particle storage after a flare? -What is the low-energy cutoff and the energy content of the non-thermal particle populations? What is the link between the particles interacting at the Sun and those escaping in the interplanetary medium?
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The subject of flare precursors and triggering as well as the relations between particles at the Sun and in the interplanetary space are reviewed elsewhere in this issue. The acceleration mechanisms will not be discussed here. This review deals with the different radiations produced by the energetic particles when interacting with the solar atmosphere. We focus on the constraints deduced from the observations on the characteristics of the energetic partides and on the conditions in the regions where particles propagate and interact. Energetic electrons (a few tens to a few hundreds of keV) produce radio emission in the low corona through gyrosynchrotron emission in the centimeter and millimeter wavelengths , coherent plasma radiation in the metric-decinietric domain at greater heights (10~-ion kms above the photosphere ) while X-rays and 7-rays are produced by electrons and ions in the denser and lower regions. The impact of non-thermal electron and proton beams (9)221
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on deeper layers of the solar atmosphere may be at the origin of e.g. Ha enhancements or of whitelight flares. The good relationship sometimes observed between Ha time profiles in a kernel and hard X-ray and microwave emissions favor an energy transport by the means of electron beams /1/. The origin of white light flares is still controversial since they may be related either to the direct bombardment of the atmosphere by protons above 100 MeY or by electrons above a few hundred keY impinging on the chromosphere followed by a radiative heating of the upper photosphere and temperature minimum region/2/. The determination of the low-energy cutoff of the non-thermal electron and proton population is a challenging problem since it determines the energy budget of the flare. In most cases, the existence of a thermal component at lower X-rays energies and the usual lack of spectral resolution of the X-ray observations make difficult the estimate of the cutoffof the non-thermal electrons. However, recent observations of an occulted X-ray flare have shown that the electron low-energy cutoff could be as low as 5 keV /3/, thus increasing by several orders of magnitude the non-thermal electron energy content. Some evidence of low-energy proton beams impinging on the deep layers have been obtained by the detection of linear polarization in Ha /4,5/. However, the deduced initial energy of the beam is dependent on the proton transport between the acceleration site and the impinging point (from 100 keY up to 10 MeV) /4,6/. This makes the estimate of the total proton content uncertain. An alternative way for determining the presence of protons below 1 MeY could be through the detection of weak radiative capture 7-ray lines /7/. The remainder of this paper concerns the most direct diagnostics of the accelerated particles, namely X-ray, 7-ray and radio observations, the latter ones providing also constraints on the magnetic topology of the corona. HARD X-RAY AND 7-RAY EMISSION MECHANISM Figure 1 shows an X-ray/7-ray spectrum observed from 300 keV to 10 MeY in the gradual phase of an intense, long duration flare /8,9/. It consists of several nuclear de-excitation lines from Fe, Mg, Ne, Si, C, 0, of the delayed lines from neutron radiative capture at 2.2 MeV (n) and from electron- positron annihilation at 511 keV (e +). These lines are superposed on a continuum made of the same kinetically broadened de-excitation lines and of a bremsstrahlung component extending above 1 MeV. For other events, this last component may extend to energies greater than 10 MeY (10% of the flares observed from 1980 to 1989). The 7-ray lines are unambiguously
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produced by nuclear reactions of energetic ions ( 10 MeV/nudeon) with the ambient medium. These reactions result in excited nuclei with short lifetimes ( 10_il sec) producing the de-excitation “prompt “ 7-ray lines and in secondary products such as positrons, pions( for protons 200 MeV) and neutrons giving rise respectively to the delayed 511 keV annihilation line, a broad emission around 70 MeV and the delayed 2.2 MeY line /10/. For higher energetic primary ions ( 100 MeV/nucleon) the energetic neutrons produced can also be directly detected in the interplanetary space ( 50 MeV neutrons) or produce neutron decay protons( 20 MeY neutrons) /11/. The primary particles emitting the bremsstra..blung component above a few keY are generally thought to be electrons with energies greater than 30 keY. However, although a good agreement is obtained between the numbers of microwave and X-ray emitting electrons, and although this hypothesis naturally explains the correlation found between hard X-rays and different kinds of radio emission (see e.g. /12,13/), problems arise from the large number of electrons needed to produce observed X-ray fluxes (e.g. /14/). This leads to an alternative interpretation where X-rays are produced by more efficient energetic protons ( 60 MeV) (see e.g. /15,16/). The number of energetic particles required to produce the X-rays is thus reduced by a factor ~ 2000. However, there is still an unresolved discrepancy between the protons deduced from the continuum or from the 7-ray lines. It could be reduced if there is a break in the proton spectrum between the low energy 7-ray producing part ( 10 MeV ) and the higher energy part ( 60 MeV ) responsible for the X-ray continuum. Here, we shall consider that the X-ray continuum is produced by energetic electrons. NON-THERMAL ELECTRON ENERGY RELEASE DURING AND OUTSIDE FLARES Localization of the 10-100 keV Electron Acceleration Site Some indications concerning the electron acceleration site come from the study of the frequency drift of decimetric and centimetric type ifi bursts. Most of the type Ill bursts observed above 3 GHz have a frequency drift towards high frequencies, while below 1 GHz, the frequency drift is towards low frequencies. Electron beams producing type ifi bursts seem then to originate from a medium where the plasma frequency is around a few GHz, thus leading to an energy release in the low corona ( n ~ 1010 to 1011 cm /12/). Some electron acceleration can also take place higher in the corona, about one solar radius above the photosphere. Such a coronal acceleration is revealed by the observations of radio bursts drifting towards high frequencies at metric wavelengths, and corresponding to two distinct emitting sources in the corona which brighten in close simultaneity /17/. This coronal acceleration may explain the observations of electron spectra in space without any spectral break from a few tens of keY to 2 keV /18/ and can be related with the high coronal flares /19/. Observational Evidences for the Fragmentation of the Flare Energy Release A lot of problems are encountered when it is assumed that the flare energy release takes place in a single loop. In particular, upper limits on the acceleration volume and on the electron acceleration rate /14/ provide strong constraints on the number of electrons which can be accelerated in a single volume. This has led to the question whether a flare results from a collective process of many micro energy releases (see e.g. /20/), or whether it can be described in terms of “avalanche catastrophes” as known in non-linear, chaotic systems (see e.g. /21/). Evidence of this fragmented energy release has been searched in fine temporal, spatial and spectralfeatures in the most direct radiations from 10-100 keY electrons ( radio emissions /12/ and X- rays ) 1. A large number of narrowband (typical bandwidth of an individual spike of 1% to 2% ) millisecond spikes are observed in the 0.5-1 GHz frequency domain at the onset and during the impulsive phase of flares (see Figure 2). These emissions are assumed to arise from regions with densities typical of the acceleration sites and may indicate many localized rapid accelerations giving rise to the flare /12,22/. JASR 13:9-P
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2. Rapid time features are found in the radio and hard X-ray global fluxes. Although subsecond time structures are detected in less than 10% of the hard X-ray bursts above 30 keY observed by HXRBS/SMM /23/, fast time variations (of the order of 500ms ) are seen in many events observed with a higher sensitivity /24/ or systematically detected in short duration flares above 100 keY /25,26/. However, some care must be taken when inferring from rapid time structures evidence for a fragmented energy release (see e.g. /27/). -particle transport effects (for e.g. X-ray radiation) can smear short time scales of the acceleration process while non-linear emission processes for radio coherent radiation may produce rapidly variable emission even for a steady state electron population. These two facts explains why many more time structures are seen at decimetric wavelengths than in the X-ray domain (see e.g. Figure 2). -the total emission can originate from several regions with completely different temporal evolutions (see e.g. /28/). 3.The flare energy release is generally linked with the appearance of multiple interacting magnetic structures (see e.g. /29,30,31,32/). Of course the detection of individual reconnection events wifi require substantially higher sensitivity or spatial resolution in e.g. hard X-rays. 4.A close correlation is observed between the metric type III bursts or narrowband spikes rate and the hard X-ray count-rate in the main phase of a flare /33,34/. Each radio burst or spike could thus be a measure of an individual acceleration process estimated from the X-ray measurements to be respectively of the order of 1028 ergs and 1024 ~1025 ergs.The impulsive phase of a flare comprises then hundreds or thousands of subevents of similar size. The distributions of flares and of their elementary structures as a function of peak rates or energy dissipation at the peak provide other arguments.It was found from a statistical study of 12000 flares observed by SMM/HXRBS that the frequency distribution of the dissipated energy in flares,assuming thick target production, is well represented by a power law with a slope of 1.53
Non-Thermal Particles
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drawn m the left corner of the first and the last maps. Note at 15:10 UT that both the noise storm at 92 cm and a new source at 21 cm have appeared while the
the position of the noise storm at different wavelengths compared to the motionless 21 cm new source(from /42/).
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and that this slope is not strongly varying with the solar cycle /35/. Furthermore, a similar study of fast time structures in hard X-ray flares has shown that, if the peak count-rate for structures with a given time scale is proportional to the dissipated energy, then the frequency distribution of elementary structures has the same slope as the above flare distribution /36/. The slope of the flare distribution of the peak hard X-ray flux above 20 keY is also consistent /35/ with the slope of the same distribution for the microflares obtained at similar periods of solar activity /37,38/. They are also in agreement with the predicted slope of event energy release from computer simulations of avalanche models governed by the principle of self-organized criticality /21/. As a conclusion, even if there are some arguments in favour of a fragmented energy release from radio and hard X-ray observations, more evidences should be searched for with e.g. spatially resolved observations obtained with a high time resolution. The similitude of the slopes of the frequency distributions of flares, of elementary structures in flares and of microflares can also act in favour of this idea. However, the physical meaning of this slope must be understood and the parameters characterizing the magnitude of the non-thermal energy release must be determined. Non-Thermal Energy Release Outside Flares Based on the slope of frequency distributions and on the observations that hard X-ray microflares appear in Ha as very small flares /39/ and can be associated with weak radio bursts /40/, microflares could be considered as a low energy extrapolation of flares. Other microphenomena associated with active regions are reported in the literature such as centimeter microbursts /41/, but the relation to known radio bursts or hard X-ray microflares and thus the energy content of such microbursts are still unknown. The oldest evidence of electron acceleration (from a few keY to several tens of keV )in the solar corona outside flares are metric and decimetric noise storms for which the radiation mechanism and the energy source are still not well identified. The only consensus is that the emission is radiated at or close to the plasma frequency and is thus produced in a region of a given density and thus coronal height. No systematic association with chromospheric or photospheric phenomena are reported. However, recent simultaneous from the Yery Large Array (VLA) and the Nançay RadioHeliograph (NRH) have shown that the onset of a noise storm in the middle corona occurs in close temporal and spatial coincidence with the appearance of a new source at centimeter wavelength in the active region (Figure 3)/42/, the delay between the two phenomena being of the order of 10 minutes.Such a close relationship between the noise
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storm and the emission in the active region has been reported elsewhere /43,44,45/. The delayed onset of the noise storm at longer wavelengths i.e. at different coronal heights (see Figure 3) as well as its slow movement (a few tens of km s~1)at a given wavelength with a significant component perpendicular to the magnetic field lines are interpreted as due to the emission of non-thermal electrons in an arcade of expanding loops of different densities. The close coincidence between the appearance of the new source in the low atmosphere and the onset of the noise storm argues in favour of the expansion being triggered by this new source .This could be the first indication of some magnetic flux emergence by which the overlying large scale magnetic structure is forced to evolve, thus giving rise to the noise storm. There could then be some similarity between the onset of some noise storms and the onset of flares. For both phenomena a new structure is observed, which may imply the emergence of a new magnetic flux and the interaction of different magnetic structures. However, the difference lies in the time scales (minutes for noise storms instead of seconds for flares) between the appearance of the new structure and the onset of the phenomenon and in the energy budget much smaller for noise storms. Furthermore, the relationship which could exist between the processes giving rise to electron acceleration in a flare or in association with noise storms is far from being understood. RELATIVISTIC ELECTRONS AND NON-THERMAL IONS IN SOLAR FLARES Electron and Ion Acceleration SMM observations have shown that ion acceleration at energies between 10-100 MeV/nucleon is relatively common during flares and that a rough correlation is observed between the electron content (estimated from the bremsstrahlung total flux above 270 keV), and the nuclear content (estimated from the 4-8 MeV excess above bremsstrahlung) /46/. The flare acceleration mechanism must be efficient and rapid since a simultaneous production of energetic electrons ( few 100 keV) and ions (10-100 MeV/nucleon), as detected by the 4-6 MeV flux assumed to be mainly produced by nuclear radiation, is generally observed on a time scale of 1 second (e.g. /47/). Moreover, the direct neutron detection in space shows that nucleons are accelerated up to a few GeV/nucleons on time scales of a few tens of seconds at the beginning of a flare /48/. On the other hand, a few intense events are observed with very weak nuclear line emission and a strong continuum with a cutoff at about 60 MeV (“Electron Dominated Events” /9/). This is the case for a peak in the initial phase of the flare of 6 Mar 1989, the gradual phase of which exhibits strong 7-ray lines (Figure 1) /49/. Such events are also observed by the 7-ray spectrometer aboard YOHKOH /50/. The SIGMA and PHEBUS observations aboard GRANAT have shown that the flare of 24 May 1990, which produced the largest increase due to solar neutrons yet recorded by neutron monitors /51/, shows no clear evidence of any excess in the nuclear 7-ray line domain during the most intense part of the event in the 10- 15 MeV range/52/. However, the PHEBUS observations above 10 MeV /25/ indicate pion radiation at the end of this phase. This observation, together with the detection of the 2.2 MeY line in the late phase of the flare /53/ and of neutrons indicate that high energy protons are accelerated in the flare, even if the content of the 7-ray line emitting protons is too low compared to the electron content to produce a measurable prompt 7-ray line excess. The above observations thus indicate that the relative content of energetic electrons above 300 keV and protons above 30 MeV can vary from one phase of an event to another, and from flare to flare, even if a rough correlation between both contents is observed. Of course, X-ray and 7-ray images would be a powerful tool to check whether the different phases of a flare exhibiting different behaviours come from the same region. This would help to determine whether the relative amount of accelerated electrons and ions depend on the acceleration mechanism or on the conditions in the acceleration site. Electron and Ion Transport in the Solar Atmosphere. The main difficulty to deduce the characteristics of the accelerated particles from the observation of their radiations, is to evaluate the effects of the particle transport from the acceleration region ( assumed in the low corona) to the usually dense chromospheric X-ray and 7- ray interaction sites. Apart from some recent papers in which particle acceleration and transport are treated simultaneously /e.g. 54,55/, most of the transport models do not look at the details of particle acceleration. The different X-ray/7-ray models differ by the methods used to study the relationship
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between the energetic particles at the interaction site to the injection spectrum produced by the “non-modeized” acceleration mechanism: -Monte Carlo simulation of a Fokker-Planck equation (e.g. /56,57,58,59/) -Analytical or numerical solutions of the Fokker-Planck equation (e.g. /60/) -Analytical or numerical solutions of a continuity equation( e.g. /61,62,63,64,65/) They also differ by the exact assumptions on the geometry on the region where the transport takes place: open or closed magnetic field, magnetic field convergence in the corona or below. They generally consider that the particles are injected at the top of the coronal structure. Some of the particles precipitate in the loss cone created by the converging magnetic field and thus can reach the dense layers of the atmosphere. The loss cone may be fed by scattering due to wave particle interactions in the ionized coronal trap. However, the effects of wave diffusion vary from pitch-angle scattering without energy transfer to energy gain in the case of strong diffusion (see e.g. /55/). In all the models the waves and the particles are not treated in a self-consistent way. Energy losses (Coulomb, gyrosynchrotron) of the energetic partides in the corona and during their propagation towards denser layers are taken into account. Many predictions have been made: -decay phase of 7-ray lines /e.g. 56,66/ -temporal and spectral evolution of neutron production and pion decay radiation /e.g. 58,59/ -relative X-ray and 7-ray timing in the case of a long duration injection and of a simultaneous injection of electrons and ions /63/ -height distribution of the X-rays and 7-rays /e.g. 56,57,67/ -center to limb variations of X-rays and 7-rays /e.g. 61,62,68/ Hard X-Ray and 7-Ray Coronal Contributions from Partially Trapped Particles. The application of the transport model predicting the relative X-ray/7-ray timing /63/ to one main peak of the 27 Apr 1981 event, where a delay of forty seconds is observed between the ray/x-ray maxima shows that the temporal evolutions of the X-ray fluxes at different energies and that of the delayed 4.1-6.4 MeV (considered as a good indication of the timing of the prompt 7-ray line) are reproduced without a second step acceleration of energetic ions.Electrons and ions must be injected simultaneously and continuously in the same magnetic ioop of mean density n=5 1010 cm~3). Particle transport acting differently on electrons and ions is responsible for the different temporal evolutions (see /69/). This does not rule out completely the existence of a delayed acceleration but shows that the particle transport effects must necessarily be taken into account for any detailed analysis of X-ray/7-ray time profiles. For that event, X-ray emission comes predominantly from the trapped electrons, while 7-ray lines are produced either essentially by trapped protons or by trapped and precipitated protons. The available data do not allow to decide between both interpretations. However, even in the second case, at least 30% of the 7-ray line flux is produced in the corona. Such a localization of the 7-ray source could explain why some of the abundances deduced from 7-ray line spectroscopy are more consistent with coronal than with photospheric values /70/. ~-
Another evidence for long term particle trapping comes from the Egret/Compton observations of the 11 Jun 1991 flare. The production of GeV 7-rays 1S observed as late as 8 hours after the impulsive phase /71/. The high energy spectrum in the late phase of the event as well as the
temporal decay of the 50 MeY flux can be explained by ions and electrons accelerated during the impulsive phase of the flare and subsequently trapped in magnetic loops under the condition that the strength of the magnetic field and the level of the plasma turbulence in the corona are sufficiently low (mirror ratio of the order of 10, and coronal density less than 1011 cm3) /72/. The detailed comparison of the time history of the prompt (4 to 7 MeV) and the delayed 2.23 MeV 7-ray line observed by PHEBUS in the impulsive phase allowed to deduce preliminary values of the 3He/H ratio and of the hardness of the ion spectrum /73/. The 3He/H value is consistent with previous observations. The absence of any significant delay between hard X-ray and prompt 7-ray line time profiles implies that coronal trapping of energetic particles is not dominant during the impulsive phase, and that electron and proton interaction during that phase takes place in the chromosphere as is generally observed. The proton spectral indexes found in the impulsive phase and from the interpretation of the late phase are similar. A further study is however necessary to understand the link between the impulsive and the late phases. In particular, if the particle
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precipitation in the loss cone is due to waves, the waves and the particles should be treated in a consistent way. If the conditions for particle storage in a loop are related to the total number of trapped particles above a given energy /67,74/, the diffusion rate of particles due to waves may decrease with time after the impulsive phase. This could explain why in the impulsive phase, a lot of particles precipitate giving rise to chromospheric emission while in the late phase the emission may come from partially trapped high energy protons. The main problem of this interpretation is to prevent the particles to drift from the magnetic loop on these long time scales. Either a twisted magnetic field could prevent this effect from being important or the particles may remain trapped in arcades of loops /72/. Such arcades of loops are considered to explain other observations: the comparisons of unocculted and partially occulted X-ray emissions observed with PVO, ICE, GOES together with the application of a transport model shows that the impulsive hard X-ray and soft X-ray emission sources can extend to coronal altitudes of 2 i0~kin above the photosphere leading with an average density of i0~cm3. Such a to a volume the hard X-ray source of 1030 cm3corona large volume of suggests a large diffuse source in the covering several magnetic loops /3/. All these observations give support to the models considering partial electron and proton trapping. However, images at high energies will be the only means to measure the exact height distributions of high energy X-ray and 7-ray emissions. Emission above 10 MeV from Relativistic Electrons and High Energy Ions. Emission 10 MeV is detected in the most energetic flares. However such events are unfrequent since less than 30 events with 10 MeV emission have been observed during 10 years ofcontinuous observations with SMM /75,76,77/. High energy emission up to 1 GeV has now been detected for a few events with GAMMA1 or the Compton Observatory. For one impulsive event, this emission could originate exclusively from electron bremsstrahlung, while pion radiation is the dominant emission above 100 MeV in the decay phase of larger flares lasting from one to several hours /71,78/. There is a tendency of the high energy flares of Cycle 2lto be associated with limb events /75,76/. However, during the rise towards Cycle 22 SMM has detected several events located well on the disk /9/. Even with this detection of disk events, the observed limb fraction (ratio of the number of flares with heliocentric angle such as sinO 0.9 to the total number) still shows a statistically significant excess compared to the prediction for isotropic radiation, in good consistency with the one observed for Cycle 21 /77/. Other high energy events have been observed with SIGMA /79/, PHEBUS/25/,GAMMA1 /78/ and COMPTON/71/ some of them being also associated with disk flares. Before achieving a new center-to-limb distribution, the intercalibration of the different experiments must be realized to compare the relative detection thresholds. Indeed, the center-tolimb variation must be deconvolved from threshold effects in order to deduce constraints from the different models. The different components of the high energy radiation must also be distinguished since they present different anisotropies and thus center-to-limb variations. In particular, the angular distribution of high energy radiation from pion decay is not sufficiently anisotropic to explain the observed limb brightening even if the transport of primary ions and secondary electrons or positrons in a magnetic loop is considered/59/. This confirms that in most events the high energy emission is mainly produced by electrons. At high energies the electron radiation pattern is strongly directed along its velocity. The limb dustering thus indicates that the emitting electron velocity shows an anisotropy in a direction parallel to the photosphere. Electrons partially trapped in a converging magnetic field exhibit such an anisotropy near the footpoint, if the magnetic field is nearly normal to the photosphere. The radiation anisotropy and its center-to-limb variations have been computed in different transport models /61,62,65,66,68/. Apart from MacKinnon and Brown /62/, all the models consider a radial magnetic field at the mirror points. The anisotropy of the radiation can also result from the electron injection itself and it is difficult to estimate from the observations the relative importance of both effects. To be able to make such an estimate and a distinction between the different kinds of magnetic field convergence (e.g. /62/) the accelerated electrons must be followed from the injection site to the denser interaction sites. In the frame of these models, the observations of some events close to the disk center indicate that the electron bremsstrahlung should come from upwardly moving electrons, probably reflected at mirror points below the transition region /76/.
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Another possibility which is not considered in the models is that the magnetic field is not radial at all or that the emission comes from a complicated magnetic structure. Coronal and Chromospheric Signature of High Energy Emission Recent studies have looked at the magnetic environments of high energy disk flares in the case where the high energy emission is dominated by electron bremsstralilung. For the few studied events, the 10 MeV enhancement is systematically associated with the appearance of either a new radio source in the corona /80,81/, or with a new eruptive Ha feature /81,82/: the high energy radiation probably originates from a complex magnetic structure where the magnetic field is far from being simple and radial. The observations of a new source emerging in close coincidence with the high energy radiation can also be related to the observations previously mentioned that the flare energy release occurs when different magnetic structures interact. This could be another evidence of the fragmentation of the energy release. The importance of a flare could be governed by the complexity of the magnetic overall structure revealed by the number of sources of emission. Here again, images obtained at high energies would provide fruitful constraints for the interpretation. CONCLUSION New ideas on the non-thermal particles in the solar atmosphere have emerged from the large amount of observations recorded during the last 10 years. -The idea of the fragmentation of the flare energy release or of a flare being the consequence of “avalanche catastrophes” has gained some interest. Observations have been analyzed to support this idea. Such a work must be pursued with the available past and new data sets to firmly establish such an idea since individual reconnection events are far from being observed. The slope of the frequency distribution of the hard X-ray microflares as a function of peak energy indicates that they could be the low energy counterpart of the flares. Alike in flares, the energization of non-thermal particles at the origin of noise storms may imply the emergence of a new magnetic flux interacting with previous magnetic structures but this result must be checked on a bigger sample. -The importance of the effects of the transport of the energetic particles from the acceleration site to the interaction sites has been systematically studied and many models including transport have been developed. A few models have also included the study of particle acceleration and such an effort should be pursued. On the other hand, some effort should be undertaken in order to treat in a self consistent way the wave particle interactions which are often considered in the transport models. -High energy radiation up to 1 GeV has been observed. This radiation can slowly decay over hours suggesting a long duration storage of high energy particles (in particular ions). Other observational evidences have been obtained for the production of hard X-rays and 7-ray lines by partially trapped electrons and protons. -Recent observations have shown that the relative amount of 300 keY electrons and 30 MeY ions can vary from flare to flare, and even during a flare. The consistency with the previously observed rough correlation between these numbers must be understood. -The production of above 10 MeV impulsive emission from energetic electrons seems to be related to the appearance of a new magnetic structure revealed by a new radio or Ha signature. There seems to be a common observational finding from the onset of noise storms to the triggering of high energy pulses and through the onset of flares: all these phenomena are associated with the appearance of new emission sources. Of course, from one phenomenon to the other, the time scales involved as well as the energy released are completely different. If such a finding is verified on a bigger sample, this would be another argument in favour of the interaction of multiple structures in the acceleration of non- thermal particles in the solar corona and would act in favour of the fragmentation of the energy release. The parameters leading to a more or less energetic or complicated phenomenon should be determined. Some of these parameters could be the complexity and the overall size of the magnetic structures involved in the energy release. To make progress,the path of the accelerated particles as close as possible from their energization site throughout the whole phase of their energy losses and radiations in the solar atmosphere should be traced. One
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