Radio Emission of Flares and Coronal Mass Ejections

We review recent progress on our understanding of radio emission from solar flares and coronal mass ejections (CMEs) with emphasis on those aspects of the subject that help us address questions about energy release and its properties, the configuration of flare?–?CME source regions, coronal shocks, particle acceleration and transport, and the origin of solar energetic particle (SEP) events. Radio emission from electron beams can provide information about the electron acceleration process, the location of injection of electrons in the corona, and the properties of the ambient coronal structures. Mildly relativistic electrons gyrating in the magnetic fields of flaring loops produce radio emission via the gyrosynchrotron mechanism, which provides constraints on the magnetic field and the properties of energetic electrons. CME detection at radio wavelengths tracks the eruption from its early phase and reveals the participation of a multitude of loops of widely differing scale. Both flares and CMEs can ignite shock waves and radio observations offer the most robust tool to study them. The incorporation of radio data into the study of SEP events reveals that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish.     

Effect of Collisions and Magnetic Convergence on Electron Acceleration and Transport in Reconnecting Twisted Solar Flare Loops

We study a model of particle acceleration coupled with an MHD model of magnetic reconnection in unstable twisted coronal loops. The kink instability leads to the formation of helical currents with strong parallel electric fields resulting in electron acceleration. The motion of electrons in the electric and magnetic fields of the reconnecting loop is investigated using a test-particle approach taking into account collisional scattering. We discuss the effects of Coulomb collisions and magnetic convergence near loop footpoints on the spatial distribution and energy spectra of high-energy electron populations and possible implications on the hard X-ray emission in solar flares.     

Hard x-ray and Metric/Decimetric Radio Observations of the 20 February 2002 Solar Flare

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

Sub-terahertz emission from solar flares: The plasma mechanism of chromospheric emission

We consider the plasma mechanism of sub-terahertz emission from solar flares and determine the conditions for its realization in the solar atmosphere. The source is assumed to be localized at the chromospheric footpoints of coronal magnetic loops, where the electron density should reach n ≈ 10¹⁵ cm^?3. This requires chromospheric heating at heights h ? 500 km to coronal temperatures, which provides a high degree of ionization needed for Langmuir frequencies ν _p ≈ 200–400 GHz and reduces the bremsstrahlung absorption of the sub-THz emission as it escapes from the source. The plasma wave excitation threshold for electron-ion collisions imposes a constraint on the lower density limit for energetic electrons in the source, n ₁ > 4 × 10⁹ cm^?3. The generation of emission at the plasma frequency harmonic ν ≈ 2ν _p rather than the fundamental tone turns out to be preferred. We show that the electron acceleration and plasma heating in the sub-THz emission source can be realized when the ballooning mode of the flute instability develops at the chromospheric footpoints of a flare loop. The flute instability leads to the penetration of external chromospheric plasma into the loop and causes the generation of an inductive electric field that efficiently accelerates the electrons and heats the chromosphere in situ. We show that the ultraviolet radiation from the heated chromosphere emerging in this case does not exceed the level observed during flares.     

Energetic Particle Acceleration and Propagation in Strong CME-Less Flares

Flares and coronal mass ejections (CMEs) contribute to the acceleration and propagation of solar energetic particles (SEP) detected in the interplanetary space, but the exact roles of these phenomena are yet to be understood. We examine two types of energetic particle tracers related with 15 CME-less flares that emit bright soft X-ray bursts (GOES X class): radio emission of flare-accelerated electrons and in situ measurements of energetic electrons and protons near 1 AU. The CME-less flares are found to be vigorous accelerators of microwave-emitting electrons, which remain confined in low coronal structures. This is shown by unusually steep low-frequency microwave spectra and by lack of radio emission from the middle and high corona, including dm?–?m wave type IV continua and metre-to-hectometre type III bursts. The confinement of the particles accelerated in CME-less flares agrees with the magnetic field configuration of these events inferred by others. Two events produced isolated metric type II bursts revealing coronal shock waves. None of the seven flares in the western hemisphere was followed by enhanced particle fluxes in the GOES detectors, but one, which was accompanied by a type II burst, caused a weak SEP event detected at SoHO and ACE. Three of the CME-less flares were followed within some hours by SEP-associated flares from the same active region. These SEP-producing events were clearly distinct from the CME-less ones by their association with fast and broad CMEs, dm?–?m wave radio emission, and intense DH type III bursts. We conclude that radio emission at decimetre and longer waves is a reliable indication that flare-accelerated particles have access to the high corona and interplanetary space. The absence of such emission can be used as a signal that no SEP event is to be expected despite the occurrence of a strong soft X-ray burst.     

Electron acceleration by electric fields near the footpoints of current-carrying coronal magnetic loops

We analyze the electric fields that arise at the footpoints of a coronal magnetic loop from the interaction between a convective flow of partially ionized plasma and the magnetic field of the loop. Such a situation can take place when the loop footpoints are at the nodes of several supergranulation cells. In this case, the neutral component of the converging convective flows entrain electrons and ions in different ways, because these are magnetized differently. As a result, a charge-separating electric field emerges at the loop footpoints, which can efficiently accelerate particles inside the magnetic loop under appropriate conditions. We consider two acceleration regimes: impulsive (as applied to simple loop flares) and pulsating (as applied to solar and stellar radio pulsations). We have calculated the fluxes of accelerated electrons and their characteristic energies. We discuss the role of the return current when dense beams of accelerated particles are injected into the corona. The results obtained are considered in light of the currently available data on the corpuscular radiation from solar flares.     

Energies of Electrons Accelerated in Turbulent Reconnecting Current Sheets in Solar Flares

Yohkoh observations strongly suggest that electron acceleration in solar flares occurs in magnetic reconnection regions in the corona above the soft X-ray flare loops. Unfortunately, models for particle acceleration in reconnecting current sheets predict electron energy gains in terms of the reconnection electric field and the thickness of the sheet, both of which are extremely difficult to measure. It can be shown, however, that application of Ohm's law in a turbulent current sheet, combined with energy and Maxwell's equations, leads to a formula for the electron energy gain in terms of the flare power output, the magnetic field strength, the plasma density and temperature in the sheet, and its area. Typical flare parameters correspond to electron energies between a few tens of keV and a few MeV. The calculation supports the viewpoint that electrons that generate the continuum gamma-ray and hard X-ray emissions in impulsive solar flares are accelerated in a large-scale turbulent current sheet above the soft X-ray flare loops.     

Radial oscillations of current-carrying coronal loops

The radial oscillations of coaxial magnetic flux tubes with an azimuthal field in the shell modeling current-carrying coronal loops are studied in the cool plasma approximation. Since the concept of current-carrying coronal loops provides a theoretical basis for studying simple loop flares, finding their parameters by means of coronal seismology is a topical problem of modern solar physics. The dispersion equation for radial oscillations is derived and the dispersion curves are constructed. Oscillations with arbitrarily long periods are shown to exist at the fundamental radial mode.     

Nonthermal Electron Energy Deposition in the Chromosphere and the Accompanying Soft x-ray Flare Emission

We analyse four solar flares which have energetic hard X-ray emissions, but unusually low soft X-ray flux and GOES class (C1.0–C5.5). These are compared with two other flares that have soft and hard X-ray emission consistent with a generally observed correlation that shows increasing hard X-ray accompanied by increasing soft X-ray flux. We find that in the four small flares only a small percentage of the nonthermal electron beam energy is deposited in a location where the heating rate of the electron beam exceeds the radiative cooling rate of the ambient plasma. Most of the beam energy is subsequently radiated away into the cool chromosphere and so cannot power chromospheric evaporation thus reducing the soft X-ray emission. We also demonstrate that in the four small flares the nonthermal electron beam energy is insufficient to power the soft X-ray emitting plasma. We deduce that an additional energy source is required, and this could be provided by a DC-electric field (where quasi-static electric field channels in the coronal loops accelerate electrons, and those electrons with velocity below a critical velocity will heat the ambient plasma via Joule heating) in preference to a loop-top thermal source (where heat flux deposited in the corona is conducted along magnetic field lines to the chromosphere, heating the coronal plasma and giving rise to further chromospheric evaporation).     

The efficiency of electron acceleration in solar type-IV radio pulsations with a zebra pattern

Based on a comprehensive analysis of the October 25, 1994 event, we consider the balance of energetic particles in a type-IV solar radio emission source with a zebra-type fine structure (in a coronal magnetic loop). The zebra pattern is formed through the injection of fast electrons into a trap and the formation of a ring-type nonequilibrium electron distribution function. We estimated the characteristic zebra-pattern lifetime, which is determined by the escape of fast particles from the trap into the loss cone. In addition, we determined the number of fast particles that must be injected into the trap to provide the observed radio brightness temperature in zebra-pattern stripes by analyzing the plasma emission mechanism responsible for the zebra-pattern generation. As a result, we estimated the efficiency of the electron acceleration mechanism in coronal magnetic loops at the post-flare evolutionary phase of an active region.     

Electron Acceleration in a Dynamically Evolved Current Sheet Under Solar Coronal Conditions

Electron acceleration in a drastically evolved current sheet under solar coronal conditions is investigated via the combined 2.5-dimensional (2.5D) resistive magnetohydrodynamics (MHD) and test-particle approaches. Having a high magnetic Reynolds number (10⁵), the long, thin current sheet is torn into a chain of magnetic islands, which grow in size and coalesce with each other. The acceleration of electrons is explored in three typical evolution phases: when several large magnetic islands are formed (phase 1), two of these islands are approaching each other (phase 2), and almost merging into a “monster” magnetic island (phase 3). The results show that for all three phases electrons with an initial Maxwell distribution evolve into a heavy-tailed distribution and more than 20 % of the electrons can be accelerated higher than 200 keV within 0.1 second and some of them can even be energized up to MeV ranges. The lower-energy electrons are located away from the magnetic separatrices and the higher-energy electrons are inside the magnetic islands. The most energetic electrons have a tendency to be around the outer regions of the magnetic islands or to appear in the small secondary magnetic islands. It is the trapping effect of the magnetic islands and the distributions of E _p that determine the acceleration and spatial distributions of the energetic electrons.     

Hard X-ray emission from the solar corona

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

Radio and soft X-ray investigation of the solar flares of February 4, 1986

In this paper, the 3B flare of February 4, 1986 is studied comprehensively. The escape electrons accelerated to 10–100 keV at the top of coronal loop are confirmed by III type bursts. The energetic electron beams moved downward trigger the eruptions in the low layer of solar atmosphere. The radio and soft X-ray bursts are interpreted, respectively, by the maser mechanism and evaporation effect. Finally, the important role of energetic electron beams in solar flares is pointed out.     

Location of Decimetric Pulsations in Solar Flares

This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nançay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30?–?240 Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal source.     

Very Large Array,SOHO, and RHESSI Observations of Magnetic Interactions and Particle Propagation across Large-Scale Coronal Loops

Very Large Array (VLA) observations at wavelengths of 20 and 91 cm have been combined with data from the SOHO and RHESSI solar missions to study the evolution of transequatorial loops connecting active regions on the solar surface. The radio observations provide information about the acceleration and propagation of energetic electrons in these large-scale coronal magnetic structures where energy release and transport take place. On one day, a long-lasting Type I noise storm at 91 cm was seen to intensify and shift position above the northern hemisphere region following an impulsive hard X-ray burst in the southern hemisphere footpoint region. VLA 20-cm observations as well as SOHO EIT EUV images showed evolving coronal plasma that appeared to move across the solar equator during this time period. This suggests that the transequatorial loop acted as a conduit for energetic particles or fields that may have triggered magnetic changes in the corona where the northern noise storm region was seen. On another day, a hard X-ray burst detected at the limb was accompanied by impulsive 20- and 91-cm burst emission along a loop connecting to an active region in the same hemisphere but about 5′ away, again suggesting particle propagation and remote flare triggering across interconnecting loops.     

Measurements of Electron Anisotropy in Solar Flares Using Albedo with RHESSI X-Ray Data

The angular distribution of electrons accelerated in solar flares is a key parameter in the understanding of the acceleration and propagation mechanisms that occur there. However, the anisotropy of energetic electrons is still a poorly known quantity, with observational studies producing evidence for an isotropic distribution and theoretical models mainly considering the strongly beamed case. We use the effect of photospheric albedo to infer the pitch-angle distribution of X-ray emitting electrons using Hard X-ray data from RHESSI. A bi-directional approximation is applied and a regularised inversion is performed for eight large flare events to deduce the electron spectra in both downward (towards the photosphere) and upward (away from the photosphere) directions. The electron spectra and the electron anisotropy ratios are calculated for a broad energy range, from about ten up to ～?300 keV, near the peak of the flares. The variation of electron anisotropy over short periods of time lasting 4, 8 and 16 seconds near the impulsive peak has been examined. The results show little evidence for strong anisotropy and the mean electron flux spectra are consistent with the isotropic electron distribution. The 3σ level uncertainties, although energy and event dependent, are found to suggest that anisotropic distribution with anisotropy larger than ～?three are not consistent with the hard X-ray data. At energies above 150?–?200 keV, the uncertainties are larger and thus the possible electron anisotropies could be larger.     

Ion-cyclotron instability and electron acceleration in coronal loops

Quasi-electrostatic electron and ion-cyclotron instabilities are studied. The result indicates that the higher harmonic ion cyclotron instabilities (ICI) can be excited while the fast ions produced from reconnection are injected into a coronal loop. Part of the energetic ions can be dragged out of the magnetic mirror turning points and a negative plasma potential is generated. The plasma potential may directly accelerate the electrons up to the relativistic velocity within a short time. This acceleration is similar to the processes occurring in the magnetic mirror devices of controlled thermonuclear fusion. The spectrum and flux of accelerated electrons have also been obtained. Some observational results during the solar flare might be explained by this acceleration mechanism.     

Fast Magnetoacoustic Waves in a Fan Structure Above a Coronal Magnetic Null Point

We analyze the 26 November 2005 solar radio event observed interferometrically at frequencies of 244 and 611 MHz by the Giant Metrewave Radio Telescope (GMRT) in Pune, India. These observations are used to make interferometric maps of the event at both frequencies with the time cadence of 1 s from 06:50 to 07:12 UT. These maps reveal several radio sources. The light curves of these sources show that only two sources at 244 MHz and 611 MHz are well correlated in time. The EUV flare is more localized with flare loops located rather away from the radio sources. Using SoHO/MDI observations and potential magnetic field extrapolation we demonstrate that both the correlated sources are located in the fan structure of magnetic field lines starting from a coronal magnetic null point. Wavelet analysis of the light curves of the radio sources detects tadpoles with periods in the range P=10?–?83 s. These wavelet tadpoles indicate the presence of fast magnetoacoustic waves that propagate in the fan structure of the coronal magnetic null point. We estimate the plasma parameters in the studied radio sources and find them consistent with the presented scenario involving the coronal magnetic null point.     

Simulation studies of electron acceleration by ion ring distributions in solar flares

Using a 2 1/2-D fully relativistic electromagnetic particle-in-cell code (PIC) we have investigated a potential electron acceleration mechanism in solar flares. The free energy is provided by ions which have a ring velocity distribution about the magnetic field direction. Ion rings may be produced by perpendicular shocks, which could in turn be generated by the super-Alfvénic motion of magnetic flux tubes emerging from the photosphere or by coronal mass ejections (CMEs). Such ion distributions are known to be unstable to the generation of lower hybrid waves, which have phase velocities in excess of the electron thermal speed parallel to the field and can, therefore, resonantly accelerate electrons in that direction. The simulations show the transfer of perpendicular ion energy to energetic electrons via lower hybrid wave turbulence. With plausible ion ring velocities, the process can account for the observationally inferred fluxes and energies of non-thermal electrons during the impulsive phase of flares. Our results also show electrostatic wave generation close to the plasma frequency: we suggest that this is due to a bump-in-tail instability of the electron distribution.