RADIO EMISSION FROM ACCELERATION SITES OF SOLAR FLARES (original) (raw)
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The Astrophysical Journal, 2013
Based on detailed analysis of radio and X-ray observations of a flare on 2002 April 11 augmented by realistic 3D modeling, we have identified a radio emission component produced directly at the flare acceleration region. This acceleration region radio component has distinctly different (i) spectrum, (ii) light curves, (iii) spatial location, and, thus, (iv) physical parameters from those of the separately identified, trapped or precipitating electron components. To derive evolution of physical parameters of the radio sources we apply forward fitting of the radio spectrum time sequence with the gyrosynchrotron source function with 5 to 6 free parameters. At the stage when the contribution from the acceleration region dominates the radio spectrum, the X-ray-and radio-derived electron energy spectral indices agree well with each other. During this time the maximum energy of the accelerated electron spectrum displays a monotonic increase with time from ∼ 300 keV to ∼ 2 MeV over roughly one minute duration indicative of an acceleration process in the form of growth of the power-law tail; the fast electron residence time in the acceleration region is about 2 − 4 s, which is much longer than the time of flight and so requires a strong diffusion mode there to inhibit free-streaming propagation. The acceleration region has a relatively strong magnetic field, B ∼ 120 G, and a low thermal density, n e 2 · 10 9 cm −3 . These acceleration region properties are consistent with a stochastic acceleration mechanism.
Solar flare electron acceleration: Comparing theories and observations
Advances in Space Research, 2003
A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.
Radio Emission from Solar Flares
Annual Review of Astronomy and Astrophysics, 1998
Radio emission from solar flares offers a number of unique diagnostic tools to address long-standing questions about energy release, plasma heating, particle acceleration, and particle transport in magnetized plasmas. At millimeter and centimeter wavelengths, incoherent gyrosynchrotron emission from electrons with energies of tens of kilo electron volts to several mega electron volts plays a dominant role. These electrons carry a significant fraction of the energy released during the impulsive phase of flares. At decimeter and meter wavelengths, coherent plasma radiation can play a dominant role. Particularly important are type III and type III-like radio bursts, which are due to upward-and downwarddirected beams of nonthermal electrons, presumed to originate in the energy release site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory, the relationship between radio emission and energetic photon emissions has been clarified. In this review, recent progress on our understanding of radio emission from impulsive flares and its relation to X-ray emission is discussed, as well as energy release in flare-like phenomena (microflares, nanoflares) and their bearing on coronal heating.
Gamma-ray and microwave evidence for two phases of acceleration in solar flares
Solar Physics, 1976
Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, we evaluate in detail the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. We also obtain information on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-toelectron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma-ray continuum, we conclude that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates~< 100 keV electrons in flares.
2014
We develop a model for stochastic acceleration of electrons in solar flares. As in several previous models, the electrons are accelerated by turbulent fast magnetosonic waves ("fast waves") via transit-time-damping (TTD) interactions. (In TTD interactions, fast waves act like moving magnetic mirrors that push the electrons parallel or anti-parallel to the magnetic field). We also include the effects of Coulomb collisions and the waves' parallel electric fields. Unlike previous models, our model is two-dimensional in both momentum space and wavenumber space and takes into account the anisotropy of the wave power spectrum FkF_kFk and electron distribution function frmef_{\rm e}frme. We use weak turbulence theory and quasilinear theory to obtain a set of equations that describes the coupled evolution of FkF_kFk and frmef_{\rm e}frme. We solve these equations numerically and find that the electron distribution function develops a power-law-like non-thermal tail within a restricted range of energies Ein(Ermnt,Ermmax)E\in (E_{\rm nt}, E_{\rm max})Ein(Ermnt,Ermmax). We obtain approximate analytic expressions for ErmntE_{\rm nt}Ermnt and ErmmaxE_{\rm max}Ermmax, which describe how these minimum and maximum energies depend upon parameters such as the electron number density and the rate at which fast-wave energy is injected into the acceleration region at large scales. We contrast our results with previous studies that assume that FkF_kFk and frmef_{\rm e}frme are isotropic, and we compare one of our numerical calculations with the time-dependent hard-x-ray spectrum observed during the June 27, 1980 flare. In our numerical calculations, the electron energy spectra are softer (steeper) than in models with isotropic FkF_kFk and frmef_{\rm e}frme and closer to the values inferred from observations of solar flares.
Peculiarities of long-wave radio bursts from solar flares preceding strong geomagnetic storms
Cosmic Research, 2009
Radio bursts in the frequency range of 100-1500 kHz, recorded in 1997-2000 on the INTERBALL-1 satellite during the solar flares preceding the strong geomagnetic storms with D st < -100 nT, are analyzed in this paper. The observed long-wave III-type radio bursts of solar origin at frequencies of 1460 and 780 kHz were characterized by large values of the flux S f = 10 -15 -10 -17 W/m 2 Hz and duration longer than 10 min. The rapid frequency drift of a modulated radio burst continued up to a frequency of 250 kHz, which testified that the exciting agent (a beam of energetic electrons) propagated from the Sun to the Earth. All such flares were characterized by the appearance of halo coronal mass ejections, observed by the LASCO/ SOHO , and by the presence of a southward B z -component of the IMF, measured on the ACE and WIND spacecraft. In addition, shortly after radio bursts, the INTERBALL-1 satellite has recorded the fluxes of energetic electrons with E > 40 keV. PACS: 96.50.Uv; 96.50.Qx
Periodic Acceleration of Electrons in the 1998 November 10 Solar Flare
The Astrophysical Journal, 2001
We present an examination of the multi-wavelength observation of a C7.9 flare which occurred on 1998 November 10. This is the first time of imaging observation of the quasi-periodic pulsations (QPPs). Four bursts were observed with the hard X-ray telescope aboard Yohkoh and the Nobeyama Radioheliograph during the impulsive phase of the flare. In the second burst, the hard X-ray and microwave time profiles clearly showed a QPP. We estimated the Alfvén transit time along the flare loop using the images of the soft X-ray telescope aboard Yohkoh and the photospheric magnetgrams, and found that the transit time was almost equal to the period of the QPP. We therefore suggest, based on a shock acceleration model, that variations of macroscopic magnetic structures, such as oscillations of coronal loops, affect the efficiency of particle injection/acceleration.
Electron Acceleration and Transport During the November 5, 1998 Solar Flare At ∼13:34 UT
Solar Physics, 2006
This paper deals with a detailed analysis of spectral and imaging observations of the November 5, 1998 (Hα 1B, GOES M1.5) flare obtained over a large spectral range, i.e., from hard X-rays to radiometric wavelengths. These observations allowed us to probe electron acceleration and transport over a large range of altitudes that is to say within small-scale (a few 10 3 km) and large-scale (a few 10 5 km) magnetic structures. The observations combined with potential and linear force-free magnetic field extrapolations allow us to show that: (i) Flare energy release and electron acceleration are basically driven by loop-loop interactions at two independent, low lying, null points of the active region magnetic field; (ii) <300 keV hard X-ray-producing electrons are accelerated by a different process (probably DC field acceleration) than relativistic electrons that radiate the microwave emission; and (iii) although there is evidence that hard X-ray and decimetric/metric radio-emitting electrons are produced by the same accelerator, the present observations and analysis did not allow us to find a clear and direct magnetic connection between the hard X-ray emitting region and the radio-emitting sources in the middle corona. 76 G. TROTTET ET AL.
Solar microwave bursts ? A review
Space Science Reviews, 1982
We review the observational and theoretical results on the physics of microwave bursts that occur in the solar atmosphere. We particularly emphasize the advances made in burst physics over the last few years with the great improvement in spatial and time resolution especially with instruments like the NRAO three element interferometer, Westerbork Synthesis Radio Telescope and more recently the Very Large Array (VLA).