Impulsive Microwave Bursts Research Articles

The Solar Origin of an In Situ Type III Radio Burst Event

Solar type III radio bursts are generated by beams of energetic electrons that travel along open magnetic field lines through the corona and into interplanetary space. However, understanding the source of these electrons and how they escape into interplanetary space remains an outstanding topic. Here we report multi-instrument, multiperspective observations of an interplanetary type III radio burst event shortly after the second perihelion of the Parker Solar Probe (PSP). This event was associated with a solar jet that produced an impulsive microwave burst event recorded by the Expanded Owens Valley Solar Array. The type III burst event also coincided with the detection of enhanced in situ energetic electrons recorded by both PSP at 0.37 au and WIND at 1 au, which were located very closely on the Parker spiral longitudinally. The close timing association and magnetic connectivity suggest that the in situ energetic electrons originated from the jet’s magnetic reconnection region. Intriguingly, microwave imaging spectroscopy results suggest that the escaping energetic electrons were injected into a large opening angle of about 90°, which is at least nine times broader than the apparent width of the jet spire. Our findings provide an interpretation for the previously reported, longitudinally broad spatial distribution of flare locations associated with prompt energetic electron events and have important implications for understanding the origin and distribution of energetic electrons in interplanetary space.

Magnetic Reconnection during the Post-impulsive Phase of a Long-duration Solar Flare: Bidirectional Outflows as a Cause of Microwave and X-Ray Bursts

Abstract Magnetic reconnection plays a crucial role in powering solar flares, production of energetic particles, and plasma heating. However, where the magnetic reconnections occur, how and where the released magnetic energy is transported, and how it is converted to other forms remain unclear. Here we report recurring bidirectional plasma outflows located within a large-scale plasma sheet observed in extreme-ultraviolet emission and scattered white light during the post-impulsive gradual phase of the X8.2 solar flare on 2017 September 10. Each of the bidirectional outflows originates in the plasma sheet from a discrete site, identified as a magnetic reconnection site. These reconnection sites reside at very low altitudes (<180 Mm, or 0.26 R ⊙) above the top of the flare arcade, a distance only <3% of the total length of a plasma sheet that extends to at least 10 R ⊙. Each arrival of sunward outflows at the loop-top region appears to coincide with an impulsive microwave and X-ray burst dominated by a hot source (10–20 MK) at the loop top and a nonthermal microwave burst located in the loop-leg region. We propose that the reconnection outflows transport the magnetic energy released at localized magnetic reconnection sites outward in the form of kinetic energy flux and/or electromagnetic Poynting flux. The sunward-directed energy flux induces particle acceleration and plasma heating in the post-flare arcades, observed as the hot and nonthermal flare emissions.

The calculation of coronal magnetic field and density of nonthermal electrons in the 2003 October 27 microwave burst

Based on Dulk and Marsh's approximate theory about nonthermal gyrosynchrotron radiation, one simple impulsive microwave burst with a loop-like structure is selected for radio diagnostics of the coronal magnetic field and column density of nonthermal electrons, which are calculated from the brightness temperature, polarization degree, and spectral index, as well as the turnover frequency, observed by using the Nobeyama Radioheliograph and the Nobeyama Radio Polarimeters, respectively. Very strong variations (up to one or two orders of magnitude) of the calculated transverse and longitudinal magnetic fields with respect to the line-of-sight, as well as the calculated electron column density, appear in the looptop and footpoint sources during the burst. The absolute magnitude and varied range of the transverse magnetic field are evidently larger than those of the longitudinal magnetic field. The time evolution of the transverse magnetic field is always anti-correlated with that of the longitudinal magnetic field, but positively correlated with that of the electron column density. These results strongly support the idea that quantifying the energy released in a flare depends on a reconstruction of the coronal magnetic field, especially for the transverse magnetic field, and they are basically consistent with the recent theoretical and observational studies on the photospheric magnetic field in solar flares.

Time Evolution of the Turnover Frequency for Diagnosis of the Coronal Magnetic Field

Two impulsive microwave bursts observed by Owens-Valley Solar Arrays (OVSA) are studied. The fast time variation of the turnover frequency in these bursts is quite different from the constant value in the earlier conclusion. The observational turnover frequencies are consistent with the calculations using the non-thermal gyrosynchrotron radiation model. We find the turnover frequency may play an important role for calculating the coronal magnetic field on the basis of Dulk and Marsh's approximations.

Calculations of the low-cutoff energy of non-thermal electrons in solar microwave and hard X-ray bursts

Two independent methods are proposed to estimate the low-cutoff energy of non-thermal electrons in solar microwave and hard X-ray bursts. At first, the observed ratio of radio flux at two frequencies (or hard X-ray count at two energies) are compared with the models based on the non-thermal gyro-synchrotron (or bremsstrahlung) radiations, excited by non-thermal electrons with a single power-law distribution, in which the low-cutoff energy is considered as the only free parameter. Some other parameters are cancelled by the ratio (such as the coefficient of the power-law distribution of non-thermal electrons), or fixed due to the insensitivity (such as the high-cutoff energy, the ambient density and temperature), or calculated from the other observations (such as the magnetic field). Secondly, the cross-point of the power-law distributions at different time of microwave or hard X-ray bursts is suggested to estimate the low or high-cutoff energies of non-thermal electrons. Two impulsive microwave bursts by Owens-Valley Solar Arrays (OVSA) associated with hard X-ray bursts by YOHKOH/HXT are selected to eliminate the contribution of thermal component around the peak time with plasma temperature of 10 6–10 7 K. The energy at the cross-points in two hard X-ray bursts is comparable with the low-cutoffs to fit the microwave and hard X-ray data at different times of the impulsive phase, i.e., 10–50 keV. These two methods can be applied only when a single power-law distribution is acceptable between the two frequency (or energy) channels in microwave (or hard X-ray) bursts.

First detection of the impulsive and extended phases of a solar radio burst above 200 GHz

We present a detailed analysis of radio observations obtained at 212 and 405 GHz during the 2000 March 22 Hα 2N flare that occured in AR8910 at ~1834 UT. These data are compared with microwave, soft X-ray and hard X-ray measurements of this flare. While the flare emission is not clearly detected at 405 GHz, the time profile of the 212 GHz emission exhibits an impulsive burst, associated in time with the 1–18 GHz impulsive microwave burst and a long-lasting thermal burst which finishes at about the same time as the soft X-ray emission but reaches its maximum later. The 212 GHz impulsive emission and the lack of detection at 405 GHz are consistent with synchrotron radiation from a population of ultrarelativistic electrons in an average magnetic field of 400–600 G. This radiating population of electrons has a hard energy spectrum (power law index ≈2.7). The expected >1 MeV gamma-ray continuum emission from the radio emitting electrons is comparable to that detected for mid-size electron-dominated events and the hard X-ray flux they would produce at 100 keV is consistently lower than the upper limit inferred from the observations. It is shown that the 212 GHz thermal source has to be different from that radiating the soft X-ray and microwave thermal emission. The present observations of a solar burst provide the first evidence of the extension of the gyrosynchrotron spectrum of an impulsive radio burst in the synchrotron domain above 200 GHz.

Imaging spectroscopy of solar microwave radiation. 1: Flaring emission

We present observations of an impulsive microwave burst on the Sun with both high spatial and spectral resolution, made with the Solar Array at the Owens Valley Radio Observatory (OVRO). We used the measured brightness temperature spectrum to infer the emission process responsible for each microwave source, and to derive physical conditions in the source region. We confimed our predictions using soft X-ray measurements from Geostationary Operational Environmental Satellite (GOES), soft X-ray images from Yohkoh, and H-alpha flare images together with sunspots and magnetogram images from the Big Bear Solar Observatory.

Radio observations of the M8.1 solar flare of 23 June, 1988: Evidence for energy transport by thermal processes

The Very Large Array (VLA) and the frequency agile interferometer at the Owens Valley Radio Observatory (OVRO) were used to observe the M8.1 flare of 23 June, 1988. The VLA obtained images prior to and during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency agile interferometer at Owens Valley obtained interferometer amplitude and total power spectra of the flare at 45 frequencies between 1 and 18 GHz. The observations were supplemented by radiometer measurements made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray observations, by USAF SOON Hα filtergrams, and by a KPNO photospheric magnetogram. The radio data reveal a wide variety of phenomena, including: (i) a multiply impulsive microwave burst that is essentially thermal in character; (ii) stationary discrete components at 1.5 GHz, associated temporally and spatially with distant brightenings in Ha; (iii) a dynamical component at 1.5 GHz associated with hot plasma moving subsonically into the corona; (iv) the appearance of intense, short-lived, decimetric burst activity near the lead sunspot in the active region at 1.5 GHz, indicative of a high degree of inhomogeneity in the source. The unusually complete radio coverage allows us to investigate the transport of energy from the initial site to sites of distant Hα brightenings. The transport of energy appears to be most consistent with slow, thermal processes, rather than rapid transport by nonthermal electron beams.

Impulsive microwave burst and solar noise storm emission resolved with the VLA

view Abstract Citations (17) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Impulsive Microwave Burst and Solar Noise Storm Emission Resolved with the VLA Willson, Robert F. ; Lang, Kenneth R. ; Liggett, Margaret Abstract The evolution of a microwave burst at 20.7 cm wavelength and a type I noise storm at 91.6 cm wavelength are examined using VLA. The magnetic loops in the two spectral regions are studied. The sizes and brightness temperatures of the 20.7 cm burst sources are compared with those predicted by multithermal and nonthermal models of microwave burst emission of Dulk and Dennis (1982). The data reveal that: (1) the precursor, impulsive, and post-impulsive phases of the 20.7 cm burst are located in spatially separated sources; (2) the gradual enhancement of a 91.6 cm noise storm continuum source suggests a feed-back mechanism exists between activity in higher and lower lying loops; and (3) the derived magnetic field strengths at 20.7 cm for the nonthermal models are 35-160 G with core field strengths of 125-220 G. Publication: The Astrophysical Journal Pub Date: February 1990 DOI: 10.1086/168437 Bibcode: 1990ApJ...350..856W Keywords: Microwaves; Noise Storms; Solar Radiation; Solar Radio Bursts; Brightness Temperature; Photosphere; Solar Corona; Solar Magnetic Field; Sunspots; Very Large Array (Vla); Solar Physics; INTERFEROMETRY; POLARIZATION; SUN: CORONA; SUN: RADIO RADIATION full text sources ADS |

Kilometric shock-associated events and microwave bursts

The peak times of impulsive microwave bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. We discuss the possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona.

Spotless flares and the associated radio continuum emission

We studied 24 spotless flares of Ha importance ≥ 1 which occurred during the 21st cycle of solar activity. The spotless flares could be grouped in three categories according to their location and time history of the associated active region. Our association of the flares with radio events was based on relative timing and on the flare importances. Weak microwave gradual rise and fall events were frequently recorded during the occurrence of the spotless flares. A few flares from our sample could be associated with impulsive and complex microwave bursts. Only in one case an association of a spotless flare with a significant metric type II/IV event seems to be justified.

Relation between solar narrow-band decimetre-wave bursts and associated X-ray bursts

Theoretical models have been proposed, stimulated by a successful account of the jovian decametric radiation1, for a maser mechanism, or a normal relativistic cyclotron resonance mechanism, operating in solar flares, and for transport of energy across magnetic fields to the electron gas by maser-amplified radio waves2. Much effort has been directed towards observations of solar-flare radio spike bursts3–8. Here we compare solar narrow-band decimetre-wave spike bursts with corresponding X-ray events for 25 solar flares. We find that decimetre spike bursts are 102–103 times stronger than normal impulsive microwave bursts for the same observed amount of hard X-ray emission. This feature, along with the observation of 100% circular polarization, strongly supports a maser origin for the decimetre spike bursts.

Solar filament eruptions and energetic particle events

The 1981 December 5 solar filament eruption that is associated with an energetic (E greater than 50 MeV) particle event observed at 1 AU. The eruption was photographed in H-alpha and was observed by the Solwind whitelight coronagraph on P78-1. It occurred well away from any solar active region and was not associated with an impulsive microwave burst, indicating that magnetic complexity and a detectable impulsive phase are not required for the production of a solar energetic particle (SEP) event. No metric type II or IV emission was observed, but an associated interplanetary type II burst was detected by the low-frequency radio experiment on ISEE 3. The December 5 and two other SEP events lacking evidence for low coronal shocks had unusually steep energy spectra (gamma greater than 3.5). In terms of shock acceleration, this suggests that shocks formed relatively high in the corona may produce steeper energy spectra than those formed at lower altitudes. It is noted that the filament itself maybe one source of the ions accelerated to high energies, since it is the only plausible coronal source of the He(+) ions observed in SEP events.

On the correlation of microwave bursts with the H α flares

High correlation between microwave bursts and H α flares has already been reported. However, Solar Maximum Year cooperation made it possible to do simultaneous observations of selected active regions with good spatial resolution. In this paper the results of the direct comparison of solar microwave recordings at the Metsähovi Radio Research Station and H α films at the Hvar Observatory are reported. This comparison reveals that the correlation of microwave gradual rise and fall (GRF) events (time scale tens of minutes) with H α flares is practically 100 percent. On the other hand the correlation of the impulsive microwave bursts (time scale tens of seconds) with H α flares is low. From this it can be concluded that the main contribution to the microwave GRF bursts comes from thermal radiation.

Spatial characteristics of microwave bursts

The spatial characteristics of microwave bursts are discussed in relation to impulsive and post-burst phases. The existence of two components - a gradual and another spiky - in the impulsive phase is discussed from the WSRT high time and spatial resolution observations. Using the WSRT data, evidence is presented for homologous flares at 6 cm, from the similarity of their spatial structure and their temporal evolution. Preflare changes in microwave active regions are presented and their interpretations in terms of newly emerging flux at the flare site are discussed. High spatial resolution observations of the structure of microwave flaring loops and their interpretation in terms of arcades of loops as the sites of primary energy release are presented. Theoretical interpretation of the confinement of microwave producing energetic electrons in the coronal part of loops is discussed. The relative timing of the peaks of impulsive hard X-ray and microwave burst is discussed. Possible diagnostics of impulsive phase onsets from cm-λ polarization data are presented, and the role of the emergence of new flux and of the current sheet formed between closed loops in producing impulsive energy release at centimeter wavelengrths are analyzed.

Electron Cyclotron Masers during Solar Flares

It has been suggested (Holman et al 1980; Melrose and Dulk 1982a) that solar microwave spike bursts are due to electron cyclotron maser action. These bursts have been observed in the range 1-3 Ghz, and occur in conjunction with flare-associated impulsive microwave and hard X-ray bursts. The bursts have rise times of a millisecond or less (e.g. Slottje 1978).

Magnetic structure of a flaring region producing impulsive microwave and hard X-ray bursts

Using VLA observations of the 1B/M1 flare of June 25, 1980, 6-cm 'snapshot' maps are synthesized. The spatial and temporal resolutions during the 9 minutes of the impulsive phase were, respectively, 1 arcsec x 2 arcsec and 10 s. Some displacement is noted between the locations of the burst source and the preflare loop structures seen in the preflare map. The burst peak occurred on the neutral line of the preflare polarization map, between the two oppositely polarized microwave 'loop' structures approximately 40 arcsec long. Concurrent hard X-ray observations were made of the burst, although these had no spatial resolution. The 6-cm maps show the locations of a number of the X-ray burst spikes. The 6-cm burst was fully resolved into at least eight components, many of which were bipolar.

Second-stage acceleration in a limb-occulted flare

We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred ∼20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an Hα flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.

Radio and X-ray observations of a multiple impulsive solar burst with high time resolution

A well-developed multiple impulsive microwave burst occurred on February 17, 1979 simultaneously with a hard X-ray burst and a large group of type III bursts at metric wavelengths. The whole event is composed of several subgroups of elementary spike bursts. Detailed comparisons between these three classes of emissions with high time resolution of ∼ 0.5 s reveal that individual type III bursts coincide in time with corresponding elementary X-ray and microwave spike bursts. It suggests that a non-thermal electron pulse generating a type III spike burst is produced simultaneously with those responsible for the corresponding hard X-ray and microwave spike bursts. The rise and decay characteristic time scales of the elementary spike burst are ≪ 1 s, ∼ 1 s and ∼ 3 s for type III, hard X-ray and microwave emissions respectively. Radio interferometric observations made at 17 GHz reveal that the spatial structure varies from one subgroup to others while it remains unchanged in a subgroup. Spectral evolution of the microwave burst seems to be closely related to the spatial evolution. The spatial evolution together with the spectral evolution suggests that the electron-accelerating region shifts to a different location after it stays at one location for several tens of seconds, duration of a subgroup of elementary spike bursts. We discuss several requirements for a model of the impulsive burst which come out from these observational results, and propose a migrating double-source model.

The Spatial Structure of Solar Microwave Bursts

Observations of five impulsive microwave bursts were made during July 15–18, 1978 using the VLA at 4.9 GHz with temporal and spatial resolutions of 10 s and up to 1″ respectively. A series of 2-dimensional snapshot synthesis maps was made as a function of time, and compared with Hα photographs and magnetograms. Each flare was characterized by a single source of constant size, located on the magnetic neutral line between the Hα kernels.