Microwave Spikes Research Articles

Evolvement of microwave spike bursts in a solar flare on 2006 December 13

Solar radio spikes are one of the most intriguing spectral types of radio bursts. Their very short lifetimes, small source size and super-high brightness temperature indicate that they should be involved in some strong energy release, particle acceleration and coherent emission processes closely related to solar flares. In particular, for the microwave spike bursts, their source regions are much close to the related flaring source region which may provide the fundamental information of the flaring process. In this work, we identify more than 600 millisecond microwave spikes which recorded by the Solar Broadband Radio Spectrometer in Huairou (SBRS/Huairou) during an X3.4 solar flare on 2006 December 13 and present a statistical analysis about their parametric evolution characteristic. We find that the spikes have nearly the same probability of positive and negative frequency drifting rates not only in the flare rising phase, but also in the peak and decay phases. So we suppose that the microwave spike bursts should be generated by shock-accelerated energetic electrons, just like the terminational shock (TS) wave produced by the reconnection outflows near the loop top. The spike bursts occurred around the peak phase have the highest central frequency and obviously weak emission intensity, which imply that their source region should have the lowest position with higher plasma density due to the weakened magnetic reconnection and the relaxation of TS during the peak phase. The right-handed polarization of the most spike bursts may be due to the TS lying on the top region of some very asymmetrical flare loops.

Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

Parametric decay instabilities (PDIs) lead to the generation of strong frequency-shifted radiation when powerful X-mode polarized microwaves, injected for electron cyclotron resonance heating (ECRH) of fusion plasmas, cross a region of non-monotonic electron density near the second-harmonic upper hybrid resonance (UHR). For the standard second-harmonic X-mode ECRH scenarios used at the ASDEX Upgrade tokamak, the second-harmonic UHR occurs near the plasma edge, meaning that PDIs occur in connection with phenomena leading to non-monotonic edge electron density profiles, e.g. blobs, edge-localized modes (ELMs), and inter-ELM modes. We present the first study of strong signals near half the ECRH frequency in fusion-relevant plasmas and demonstrate their PDI-like features, such as an ECRH power threshold required for their occurrence, through analog modulations of the ECRH power. The signals near half the ECRH frequency are found to be far more prevalent than expected based on previous theories, and we hence present a qualitative explanation of their origin, in terms of cross-polarized microwaves generated through non-linear interactions of the waves involved in the PDIs, which captures the basic features of the experimental observations. We additionally present examples of the PDI-related microwave signals just below the ECRH frequency which indicate the location of the microwave source. Furthermore, based on a non-linear magnetohydrodynamics simulation of an ELM crash in ASDEX Upgrade, performed using the JOREK code, the duration of the PDI-like microwave spikes observed in connection with ELMs is shown to match the transit time of an ELM filament though an ECRH beam. Finally, the PDI power threshold expected theoretically, calculated using the electron density and temperature profiles from the JOREK simulation, is shown to match the experimentally observed values.

The effect of electron holes on cyclotron maser emission driven by horseshoe distributions

This Brief Communication presents a quantitative investigation for the effect of electron holes on electron-cyclotron maser (ECM) driven by horseshoe distributions. The investigation is based on an integrated distribution function for the horseshoe distributions with electron holes. Results show that the presence of electron holes can significantly enhance the ECM growth rate by 2–3 times in a very narrow waveband. The present study suggests that these electron holes probably are responsible for some fine structures of radiations, such as narrowband events in auroral kilometric radiation and solar microwave spikes.

Measurement of duration and signal‐to‐noise ratio of astronomical transients using a Spectral Kurtosis spectrometer

AbstractFollowing our prior theoretical and instrumental work addressing the problem of automatic real‐time radio frequency interference (RFI) detection and excision from astronomical signals, the wideband Spectral Kurtosis (SK) spectrometer design we proposed is currently being considered as an alternative to the traditional spectrometers when building the new generation of radio instruments. The unique characteristic of an SK spectrometer is that it accumulates both power and power‐squared, which are then used to compute an SK statistical estimator proven to be very effective in detecting and excising certain types of RFI signals. In this paper we introduce a novel measurement technique that exploits the power and power square statistics of an SK spectrometer to determine durations and signal‐to‐noise ratios of transient signals, whether they are RFI or natural signals, even when they are below the time resolution of the instrument. We demonstrate this novel experimental technique by analyzing a segment of data recorded by the Expanded Owens Valley Solar Array Subsystem Testbed (EST) during a solar radio burst in which microwave spike bursts occurred with durations shorter than the 20 ms time resolution of the instrument. The duration of one well‐observed spike is quantitatively shown to be within a few percent of 8 ms despite the 20 ms resolution of the data.

SMALL-SCALE MICROWAVE BURSTS IN LONG-DURATION SOLAR FLARES

Solar small scale microwave bursts (SMBs), including microwave dot, spike, and narrow band type III bursts, are characterized with very short timescales, narrow frequency bandwidth, and very high brightness temperatures. Based on observations of the Chinese Solar Broadband Radio Spectrometer at Huairou with superhigh cadence and frequency resolution, this work presents an intensive investigation of SMBs in several flares occurred in active region NOAA 10720 during 2005 Jan 14-21. Especially long-duration flares, SMBs occurred not only in early rising and impulsive phase, but also in the flare decay phase, and even in time of after the flare ending. These SMBs are strong bursts with inferred brightness temperature at least 8.18*10^11 - 1.92*10^13 K, very short lifetime of 5-18 ms, relative frequency bandwidths of 0.7-3.5%, and superhigh frequency drifting rates. Together with their obviously different polarizations from the background emission (the quiet Sun, and the underlying flaring broadband continuum), such SMBs should be individual independent strong coherent bursts which is related to some non-thermal energy releasing and production of energetic particles in small scale source region. These facts show the existence of small scale strong non-thermal energy releasing activities after the flare maxima, which is meaningful to the prediction of space weather. Physical analysis indicates that plasma mechanism may be the most favorable candidate for the formation of SMBs. From plasma mechanism, the velocities and kinetic energy of fast electrons can be deduced, and the region of electron acceleration can also be tracke.

高压下LaBr3电子结构与光学性质的第一性原理

In some events, weak fast solar bursts (near the level of the quiet Sun) were observed in the background of numerous spikes in emission and absorption. In such a case, the background contains the noise signals of the receiver. In events on 2005 September 16 and 2002 April 14, the solar origin of fast bursts was confirmed by simultaneous recording of the bursts at several remote observatories. The noisy background pixels in emission and absorption can be excluded by subtracting a higher level of continuum when constructing the spectra. The wavelet spectrum, noisy profiles in different polarization channels and a spectrum with continuum level greater than zero demonstrates the noisy character of pixels with the lowest levels of emission and absorption. Thus, in each case, in order to judge the solar origin of all spikes, it is necessary to determine the level of continuum against the background of which the solar bursts are observed. Several models of microwave spikes are discussed. The electron cyclotron maser emission mechanism runs into serious problems with the interpretation of microwave millisecond spikes: the main obstacles are too high values of the magnetic field strength in the source (omega_(Pe) ≤ omega_(Be)). The probable mechanism is the interaction of plasma Langmuir waves with ion-sound waves (l + s t) in a source related to shock fronts in the reconnection region.

Concerning spikes in emission and absorption in the microwave range

In some events, weak fast solar bursts (near the level of the quiet Sun) were observed in the background of numerous spikes in emission and absorption. In such a case, the background contains the noise signals of the receiver. In events on 2005 September 16 and 2002 April 14, the solar origin of fast bursts was confirmed by simultaneous recording of the bursts at several remote observatories. The noisy background pixels in emission and absorption can be excluded by subtracting a higher level of continuum when constructing the spectra. The wavelet spectrum, noisy profiles in different polarization channels and a spectrum with continuum level greater than zero demonstrates the noisy character of pixels with the lowest levels of emission and absorption. Thus, in each case, in order to judge the solar origin of all spikes, it is necessary to determine the level of continuum against the background of which the solar bursts are observed. Several models of microwave spikes are discussed. The electron cyclotron maser emission mechanism runs into serious problems with the interpretation of microwave millisecond spikes: the main obstacles are too high values of the magnetic field strength in the source (ωPe ≤ ωBe). The probable mechanism is the interaction of plasma Langmuir waves with ion-sound waves (l + s → t) in a source related to shock fronts in the reconnection region.

Spectral and spatial observations of microwave spikes and zebra structure in the short radio burst of May 29, 2003

The unusual radio burst of May 29, 2003 connected with the M1.5 flare in AR 10368 has been analyzed. It was observed by the Solar Broadband Radio Spectrometer (SBRS/Huairou station, Beijing) in the 5.2-7.6 GHz range. It proved to be only the third case of a neat zebra structure appearing among all observations at such high frequencies. Despite the short duration of the burst (25 s), it provided a wealth of data for studying the superfine structure with millisecond resolution (5 ms). We localize the site of emission sources in the flare region, estimate plasma parameters in the generation sites, and suggest applicable mechanisms for interpretating spikes and zebra-structure generation. Positions of radio bursts were obtained by the Siberian Solar Radio Telescope (SSRT) (5.7 GHz) and Nobeyama radioheliograph (NoRH) (17 GHz). The sources in intensity gravitated to tops of short loops at 17 GHz, and to long loops at 5.7 GHz. Short pulses at 17 GHz (with a temporal resolution of 100 ms) are registered in the R-polarized source over the N-magnetic polarity (extraordinary mode). Dynamic spectra show that all the emission comprised millisecond pulses (spikes) of 5-10 ms duration in the instantaneous band of 70 to 100 MHz, forming the superfine structure of different bursts, essentially in the form of fast or slow-drift fibers and various zebra-structure stripes. Five scales of zebra structures have been singled out. As the main mechanism for generating spikes (as the initial emission) we suggest the coalescence of plasma waves with whistlers in the pulse regime of interaction between whistlers and ion-sound waves. In this case one can explain the appearance of fibers and sporadic zebra-structure stripes exhibiting the frequency splitting.

О природе всплесков микроволнового излучения в петельных структурах активной области

О природе всплесков микроволнового излучения в петельных структурах активной области

Microwave Millisecond Spike Emission in the Solar Flare of 1991 May 16

Properties, including the time duration, polarization, quasi-periodical oscillations and so on, of the microwave spike emissions observed at 2.5 GHz and 2.6 GHz during the solar flare of 1991 May 16 are analyzed statistically. The left-handed and right-handed circular polarizations of the spike emissions at 2.5, 2.6 and 3.1 GHz are reported in detail. At these 3 frequencies, most of the spikes are superposed on both the rising (and maximum) and the descending phase of the burst. It is noteworthy that spikes also appeared superposed on the small bursts that appeared after the main burst. The spike emission lasted 17 minutes. Polarization reversals on different timescales appearing in the spike emissions at 2.5 and 2.6 GHZ are described. Our statistical analysis indicates that the polarization reversals at 2.5 and 2.6 GHz differ in characters on average, the polarization reversal at 2.5 GHz is earlier than that of 2.6 GHz by about 1.5 minutes, and polarization reversal of the spike emission is more frequent at 2.5 GHZ.

Spike Decomposition Technique: Modeling and Performance Tests

We develop an automated technique for fitting the spectral components of solar microwave spike bursts, which are characterized by narrowband spectral features. The algorithm is especially useful for periods when the spikes occur in densely packed clusters, where the algorithm is capable of decomposing overlapping spike structures into individual spectral components. To test the performance and applicability limits of this data reduction tool, we perform comprehensive modeling of spike clusters characterized by various typical bandwidths, spike densities, and amplitude distributions. We find that, for a wide range of favorable combinations of input parameters, the algorithm is able to recover the characteristic features of the modeled distributions within reasonable confidence intervals. Having model-tested the algorithm against spike overlap, broadband spectral background, noise contamination, and possible malfunction of some spectral channels, we apply the technique to a spike cluster recorded by the Chinese Purple Mountain Observatory (PMO) spectrometer, operating above 4.5 GHz. We study the variation of the spike distribution parameters, such as amplitude, bandwidth, and related derived physical parameters, as a function of time. The method can be further applied to observations from other instruments and to other types of fine structures.

Millisecond Microwave Spikes: Statistical Study and Application for Plasma Diagnostics

We analyze a dense cluster of solar radio spikes registered at 4.5-6 GHz by the Purple Mountain Observatory spectrometer (Nanjing, China), operating in the 4.5-7.5 GHz range with 5 ms temporal resolution. To handle the data from the spectrometer, we developed a new technique that uses a nonlinear multi-Gaussian spectral fit based on X-2 criteria to extract individual spikes from the originally recorded spectra. Applying this method to the experimental raw data, we eventually identified about 3000 spikes for this event, which allows us to make a detailed statistical analysis. Various statistical characteristics of the spikes have been evaluated, including the intensity distributions, the spectral bandwidth distributions, and the distribution of the spike mean frequencies. The most striking finding of this analysis is the distributions of the spike bandwidth, which are remarkably asymmetric. To reveal the underlaying microphysics, we explore the local-trap model with the renormalized theory of spectral profiles of the electron cyclotron maser (ECM) emission peak in a source with random magnetic irregularities. The distribution of the solar spike relative bandwidths calculated within the local-trap model represents an excellent fit to the experimental data. Accordingly, the developed technique may offer a new tool with which to study very low levels of magnetic turbulence in the spike sources, when the ECM mechanism of the spike cluster is confirmed.

A New Catalogue of Fine Structures Superimposed on Solar Microwave Bursts

The 2.6–3.8 GHz, 4.5–7.5 GHz, 5.2–7.6 GHz and 0.7–1.5 GHz component spectrometers of Solar Broadband Radio Spectrometer (SBRS) started routine observations, respectively, in late August 1996, August 1999, August 1999, and June 2000. They just managed to catch the coming 23rd solar active maximum. Consequently, a large amount of microwave burst data with high temporal and high spectral resolution and high sensitivity were obtained. A variety of fine structures (FS) superimposed on microwave bursts have been found. Some of them are known, such as microwave type III bursts, microwave spike emission, but these were observed with more detail; some are new. Reported for the first time here are microwave type U bursts with similar spectral morphology to those in decimetric and metric wavelengths, and with outstanding characteristics such as very short durations (tens to hundreds ms), narrow bandwidths, higher frequency drift rates and higher degrees of polarization. Type N and type M bursts were also observed. Detailed zebra pattern and fiber bursts at the high frequency were found. Drifting pulsation structure (DPS) phenomena closely associated with CME are considered to manifest the initial phase of the CME, and quasi-periodic pulsation with periods of tens ms have been recorded. Microwave ``patches", unlike those reported previously, were observed with very short durations (about 300 ms), very high flux densities (up to 1000 sfu), very high polarization (about 100% RCP), extremely narrow bandwidths (about 5%), and very high spectral indexes. These cannot be interpreted with the gyrosynchrotron process. A superfine structure in the form of microwave FS (ZPS, type U), consisting of microwave millisecond spike emission (MMS), was also found.

A superfine structure in solar microwave bursts

We have observed in the microwave range (with the radio spectrometer of the Huairu station (Beijing, NAOC) around 3 GHz) the fine structure of solar radio bursts called zebra patterns and fiber bursts (seen drifting on the frequency stripes in emission and in absorption on the background burst continuum emission). In all seven observed bursts we discovered a new effect: zebra stripes have a superfine structure, consisting of numerous fast spikes with duration at a limit of the time resolution of the spectrometer, 8 ms. Since for zebra patterns and microwave spikes different emission mechanisms were proposed, these new observations require us to revise known theories. An alternative model of microwave millisecond spikes is based on the coupling of plasma waves with ion-sound waves : . Since the main features of zebra stripes and fiber bursts are similar, we consider the the zebra pattern of these bursts as whistler manifestations. Whistlers yield a principal contribution in the fine structure radio emission by coupling with Langmuir waves at sum as well as difference frequencies: . Allowance for the conversion of ion-sound waves into whistlers (and inversely in a pulsating regime) enables us to identify the zebra pattern consisting of spikes as a simultaneous manifestation of both those processes ( and ) in radio sources, related to magnetic reconnection above flare regions.

Chaotic temporal variations in cosmic masers

Electron cyclotron maser emission is an accepted physical mechanism for generating coherent planetary and stellar radio emissions. Observational data has indicated evidence of nonlinear and chaotic temporal variability in some cosmic masers such as solar microwave spikes. The nonlinear and chaotic characteristics of cosmic masers can be attributed to plasma turbulence, such as Alfvén chaos, embedded in the emission region. We report a chaos theory of Alfvén waves which can account for chaotic acceleration of electrons in the source region of cosmic masers. Two types of Alfvén intermittency are identified: Pomeau-Manneville intermittency and crisis-induced intermittency. Since Alfvén waves may be responsible for accelerating electrons that emit maser radiations, the chaotic dynamics of Alfvén waves may be the origin of chaotic time variations of cosmic masers. Hence, we suggest that Alfvén intermittency may cause intermittent temporal fluctuations which can be observed in cosmic masers.

Generation of powerful ultrashort electromagnetic pulses based on superradiance

Experimental results of the observation of superradiation from intense, subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating TM 01 wave are presented. The ultra-short microwave pulses in Ka, W, and G band were generated with repetition frequencies of up to 25 Hz. Observation of RF breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the peak power as high as 60–120 MW for the 300–400 ps pulses at 38 GHz. The initial observation of 75 GHz 10–15 MW radiation pulses with duration less than 150 ps, and of 150 GHz microwave spikes with a risetime of 75ps are also reported. Comparison with simulations is discussed as well.

Ion-sound model of microwave spikes with fast shocks in the reconnection region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+s→t. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.

Polarization observations of solar microwave spike emission at high temporal and spectral resolutions

11 microwave spike events were observed with the 2.6–3.8 GHz solar radio spectrometer of Beijing Astronomical Observatory. The spikes have short durations (tens of ms) and weak flux densities (<500 sfu), similar to the previous reports. Their degrees of polarization span over a large range, some spikes have frequency drifts as high as several GHz/s. Time delays of 8 ms or more are found between the two polarizations of some spikes, and drastic variations in thedegree of polarization with the frequency, even reversal of the polarizations, are found.

A study of the fine structures superimposed on solar bursts in the 2.6–3.8 GHz frequency band

Microwave type III bursts and microwave spikes superimposed on solar bursts observed by Beijing Astronomical Observatory (BAO) are studied in this paper. The bursts and spikes are closely related and together they make up fine structure clusters; but the time resolution of the instrument plays an important role in their relation.

The Generating Region of Bidirectional Electron Beams in the Corona

Metric and decimetric type III bursts and microwave spike emissions with negative and positive frequency drift rates which were observed with radio spectrometers at Yunnan and Beijing Observatories are presented. The frequencies and heights at which the bidirectional electron beams originated are estimated. Three events reveal a separatrix frequency (at 250, 1300, and 2900 MHz) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both upward and downward directions. These cases may indicate that the changeover frequencies of bidirectional electron beams are within a large band from 250 to 2900 MHz and the frequency bands of separatrices are in very small (4 to 100 MHz) and different bands. These type III bursts appear to be a plasma emission phenomenon from a beam of electrons which seem to have widely separated acceleration regions from the high to the low corona. These cases suggest that current sheets that separate open and closed magnetic fluxes in the low corona, and oppositely directed open field lines in the high corona are possible sites for bidirectional electron acceleration. The regions of magnetic topology from closed to open magnetic field structures should be very large (from about 20 000 to 107 000 km above the photosphere).