Decimetric Bursts Research Articles

Fluctuation analysis of solar radio bursts associated with geoeffective X-class flares

High temporal resolution solar observations in the decimetric range (1–3 GHz) can provide additional information on solar active regions dynamics and thus contribute to better understanding of solar geoeffective events as flares and coronal mass ejections. The June 6, 2000 flares are a set of remarkable geoeffective eruptive phenomena observed as solar radio bursts (SRB) by means of the 3 GHz Ondrejov Observatory radiometer. We have selected and analyzed, applying detrended fluctuation analysis (DFA), three decimetric bursts associated to X1.1, X1.2 and X2.3 flare-classes, respectively. The association with geomagnetic activity is also reported. DFA method is performed in the framework of a radio burst automatic monitoring system. Our results may characterize the SRB evolution, computing the DFA scaling exponent, scanning the SRB time series by a short windowing before the extreme event. For the first time, the importance of DFA in the context of SRB monitoring analysis is presented.

EVIDENCE OF CHROMOSPHERIC EVAPORATION IN THE 2004 DECEMBER 1 SOLAR FLARE

In this paper, we present the radio and hard X-ray evidence of chromospheric evaporation during an M1.0 flare which occurred on 2004 December 1. The radio emission was observed by the Solar Broadband Radio Dynamic Spectrometer in China, which yielded dynamic spectra of decimetric emission. The hard X-ray emission was observed by RHESSI. In the radio spectra, the burst is characterized by two groups of parallel-drifting structures, some of which change their drifting rates from positive to negative. Based on the standard flare model, we may explain these decimetric bursts in terms of chromospheric evaporation. On the other hand, RHESSI observations show that the hard X-ray emission in the energy range of 10-15 keV tends to rise from two footpoints to the looptop and eventually merges into a single looptop source, which is accepted as evidence of hard X-ray chromospheric evaporation. Such processes happened twice in this event. The drifting radio structures occurred between them, at the same time as the third hard X-ray peak was observed at 25-50 keV.

Unusual Zebra Patterns in the Decimeter Wave Band

An analysis of new observations showing fine structures consisting of narrow- band fiber bursts as substructures of large-scale zebra-pattern stripes is carried out. We study four events using spectral observations taken with a newly built spectrometer located at the Huairou station, China, in the frequency range of 1.1 - 2.0 GHz with extremely high fre- quency and time resolutions (5 MHz and 1.25 ms). All the radio events were analyzed by using the available satellite data (SOHO LASCO, EIT, and MDI, TRACE, and RHESSI). Small-scale fibers always drift to lower frequencies. They may belong to a family of ropelike fibers and can also be regarded as fine structures of type III bursts and broadband pulsations. The radio emission was moderately or strongly polarized in the ordinary wave mode. In three main events fiber structure appeared as a forerunner of the entire event. All four events were small decimeter bursts. We assume that for small-scale fiber bursts the usual mechanism of coalescence of whistler waves with plasma waves can be applied, and the large-scale zebra pattern can be explained in the conventional double plasma resonance (DPR) model. The appearance of an uncommon fine structure is connected with the following special features of the plasma wave excitation in the radio source: Both whistler and plasma wave insta- bilities are too weak at the very beginning of the events (i.e., the continuum was absent), and the fine structure is almost invisible. Then, whistlers generated directly at DPR levels "highlight" the radio emission only from these levels owing to their interaction with plasma waves.

A complicated solar eruption event on 2003 October 26

We present multi-wavelength observations of a complicated solar eruption event to associate with an X1.2 flare and a Coronal Mass Ejection (CME) on 2003 October 26. The soft X-ray profile shows a possibility for occurrence of two flares with peaks around 06:20 and 07:00 UT. According to our observations, there are many evidences to show that they are corresponding to two energy releases. The first one produces type II, type III, moving type IV continua, a decimetric burst (DCIM) and strong emissions at Hα, 195 and 1600 A; While the second energy release only produces a group of RS-III bursts, DCIM and Hα emissions. It appears that the first energy release is associated with a CME, while the second CME is quiet. Such observational difference between two energy releases is found indicating two magnetic reconnection processes occurrence with different plasma situation.

Collaborative VLA, SOHO and RHESSI observations of evolving sources of energy release in the corona above active regions

Very Large Array (VLA) observations at 20 and 91 cm wavelength are compared with data from the SOHO (EIT and MDI) and RHESSI solar missions to investigate the evolution of decimetric Type I noise storms and Type III bursts and related magnetic activity in the photosphere and corona. The combined data sets provide clues about the mechanisms that initiate and sustain the decimetric bursts and about interactions between thermal and nonthermal plasmas at different locations in the solar atmosphere. On one day, frequent, low-level hard X-ray flaring observed by RHESSI appears to have had no clear affect on the evolution of two closely-spaced Type I noise storm sources lying above the target active region. EIT images however, indicate nearly continuous restructuring of the underlying EUV loops which, through accompanying low-level magnetic reconnection, might give rise to nonthermal particles and plasma turbulence that sustain the long-lasting Type I burst emission. On another day, the onset of an impulsive hard X-ray burst and subsequent decimetric burst emission followed the gradual displacement and coalescence of a small patch of magnetic magnetic polarity with a pre-existing area of mixed magnetic polarity. The time delay of the impulsive 20 and 91 cm bursts by up to 20 min suggests that these events were unlikely to represent the main sites of flare electron acceleration, but instead are related to the rearrangement of the coronal magnetic field after the main flare at lower altitude. Although the X-ray flare is associated with the decimetric burst, the brightness and structure of a long-lasting Type I noise storm from the same region was not affected by the flare. This suggests that the reconfiguration of the coronal magnetic fields and the subsequent energy release that gave rise to the impulsive burst emission did not significantly perturb that part of the corona where the noise storm emission was located.

High resolution studies of intermediate drift bursts

SUMMARY We have analyzed IMD bursts recorded in the decimetric frequency band (- 1000 - 2500 MHz) for the first time with high time resolution of 20 - 50 ms and frequency less than 10 MHz. The main characteristics determined for the individual IMD bursts are: (i) the total duration is N 200 - 2000 ms; (ii) the frequency bandwidth, Av 1~ 50 - 150 MHz; (iii) the instantaneous bandwidth, Au/v N 2 - 12 %; (iw) all individual IMD bursts show negative frequency drift rate as ti 21 -30 - -270 MHz s-l; and (u) the ratio b/u ” -0.20 - -0.02 s-l. We discussed the application of Alfven wave models based on plasma emission and electron cyclotron maser emission mechanisms to IMD bursts observations. We find that the model with plasma emission can satisfy the requirements on the density and the magnetic field of the emission region for generation of the decimetric IMD bursts. The magnetic field strength in the source was estimated as N 30 ~ 150 G. Finally, we concluded that the maser model is not operative in the frequency range of our observations and also the whistler wave model predicts a dependence between ti and Y that does not agree with the present observations. ACKNOWLEDGEMENTS The BSS project is supported by INPE, CNPq and FAPESP. F.C.R. Fernandes and V. Krishan ac- knowledge receiving scholarship from MCT-CNPq (PCI-382499/2002-06) and FAPESP (01/06031-8), respec- tively. The participation of H.S. Sawant in 34th COSPAR Scientific Assembly was supported by FAPESP (01/00056-9). Thanks are due to the referees for suggestions and comments. REFERENCES Aurass, H., G. P. Chernov, M. Karlicky, et al., On a sequence of remarkable fine structures in the type IV burst of 24 April, 1985, Solar Phys., 112, 547-357, 1987. Benz, A. O., and G. Mann, Intermediate drift bursts and the coronal magnetic field, As&on.

Very-Large-Array Observations of Evolving Type I Noise Storms and Nonthermal Bursts In Association With Flares And Coronal Mass Ejections

Very-Large-Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the Waves experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths. We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or to the disruption of ambient magnetic fields by the passage of a CME.

Detection of prolonged, extremely faint decimeter bursts on the sun

We report the detection of long-lived sources of radio bursts accompanied by polarized background emission in solar active regions. Both types of radio sources were detected at several decimeter wavelengths in observations on the RATAN-600 radio telescope in one-dimensional scans in intensity and circular polarization with a sensitivity of about 5–10 Jy. The degree of polarization is from 70 to 100%. The microburst and background sources exist for several days and appear at sites of prolonged energy release. The typical duration of an individual microburst is about 1–2 s, and the time interval between microbursts is about 3–5 s. A negative microburst frequency drift of about 100 MHz/s or more is also observed. Some interpretations of the microburst and background sources are discussed. The most probable microburst model involves the generation of radio emission via the plasma mechanism, with the upward propagation of fast electrons above an active region. In this case, the required energy of the Langmuir waves is 2×10−8 of the heat energy of the background plasma. Microbursts appear in different places in an active region. New methods for determining the magnetic-field intensity in the regions of generation of the decimeter-wavelength emission are proposed.

Investigations of the acceleration region of energetic electrons associated with decimetric type III and X-ray bursts

Preliminary results of the association between type III decimetric bursts, mainly having center frequency above 1000 MHz and recorded by Phoenix radio spectrometer, and seven hard X-ray flares observed by the Yohkoh/HXT from September, 1992 to October, 1993 are reported here. (a) For an assumed improved density model of the solar chromosphere, an average electron beam velocity (∼ 0.16 c) and hence an average electron energy (∼ 7 keV) was inferred from the average frequency drift rate (∼ 1350 MHz/s) of 160 isolated type III bursts. Assuming the electrons lose energy primarily by collisions, the height of injection of the energetic electrons was estimated (1.2 × 10 9 – 4.5 × 10 9 cm). (b) In two flares at the onset of the bursts, the correlation between X-rays and radio was better at lower frequencies (≤ 500 MHz). However, during the time evolution of those flares, the correlation improved for higher and higher frequencies (≥ 800 MHz), suggesting that the acceleration region was displaced towards the photosphere. The estimated velocity of the acceleration region is ∼ 3 × 10 3 – 8 × 10 3 km/s. (c) In two flares, the enhancement of the radio decimetric emission above 500 MHz started earlier (∼ 10 s) than the X-ray emissions, suggesting in case of these two flares that the acceleration region is located near to where the decimetric emission is generated.

First VLA Observations of Nonthermal Metric Bursts Associated with Coronal Mass Ejections Detected by the [ITAL]Solar and Heliospheric Observatory[/ITAL

We present the first observations of nonthermal decimetric burst emission of the Sun using the new 400 cm (74 MHz) system at the VLA. Our VLA observations were carried out in collaboration with the Large Angle Spectroscopic Coronagraph (LASCO) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory spacecraft. Full-disk observations at 400 and 91 cm were used to study the spatial and temporal variations of nonthermal radio bursts during coronal mass ejections (CMEs) detected by LASCO as well as transient extreme-ultraviolet brightenings detected by the EIT. VLA snapshot maps at 400 cm revealed impulsive burst emission in the low corona that began near the estimated start time of the CME activity; that beginning also coincided with a C1.1 GOES X-ray burst and an EIT flarelike brightening. The nonthermal metric bursts then continued sporadically during the next several hours, which included the ejection of spatially separated CME components. The 400 cm bursts are contained within curved, or archlike, sources at a fixed radial distance but with a varying position between two active regions detected by the EIT near the limb, suggesting that they were emitted within large-scale magnetic loops. Our 91 cm observations also show the onset of a long-lasting type I noise storm following the initiation of CME activity, again suggesting an intimate relationship between the production of nonthermal particles and large-scale evolving plasma-magnetic field structures in the corona.

Solar radio bursts and acceleration processes of energetic protons

We have analyzed ground-based and space data of solar radio bursts and proton events and found that solar proton events are closely associated with type IV μ bursts or short decimetric wave bursts while some 24%–30% of the proton events occur with associated type II bursts. These results are contrary to the long-held view that acceleration by shock in type II bursts is necessary for producing the proton events. We argue that both the energetic electrons which generate strong type IV μ bursts and the energetic protons in proton flares are accelerated by stochastic MHD turbulence in the same magnetic loop during the impulsive phase of solar flares.

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 1015 K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.

Chromospheric evaporation and decimetric radio emission in solar flares

We have discovered decimetric signatures of the chromospheric evaporation process. Evidence for the radio detection of chromospheric evaporation is based on the radio-inferred values of (1) the electron density, (2) the propagation speed, and (3) the timing, which are found to be in good agreement with statistical values inferred from the blueshifted Ca xix soft X-ray line. The physical basis of our model is that free-free absorption of plasma emission is strongly modified by the steep density gradient and the large temperature increase in the upflowing flare plasma. The steplike density increase at the chromospheric evaporation front causes a local discontinuity in the plasma frequency, manifested as almost infinite drift rate in decimetric type III bursts. The large temperature increase of the upflowing plasma considerably reduces the local free-free opacity (due to the T-(exp -3/2) dependence) and thus enhances the brightness of radio bursts emitted at the local plasma frequency near the chromospheric evaporation front, while a high-frequency cutoff is expected in the high-density regions behind the front, which can be used to infer the velocity of the upflowing plasma. From model calculations we find strong evidence that decimetric bursts with a slowly drifting high-frequency cutoff are produced by fundamental plasma emission, contrary to the widespread belief that decimetric bursts are preferentially emitted at the harmonic plasma level. We analyzed 21 flare episodes from 1991-1993 for which broadband (100-3000 MHz) radio dynamic spectra from Phoenix, hard X-ray data from BATSE/CGRO, and soft X-ray data from GOES were available. We detected slowly drifting high-frequency cutoffs between 1.1 and 3.0 GHz, with drift rates of -41 +/- 32 MHz/s, extending over time intervals of 24 +/- 23 s. Developing a density model for type III-emitting flare loops based on the statistically observed drift rate of type III bursts by Alvarez & Haddock, we infer velocities of up to 360 km/s for the upflowing plasma, with an average of v(sub CE) = 236 +/- 130 km /s for episodes with 5-15 s duration. The mean electron density of the upflowing plasma is n(sub e) = 5.2(+/-3.1) x 10(exp 10) /cu cm when it is first detected in radio, at coronal altitudes of h(sub 0) = 9.2 +/- 2.3 Mm.

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.

Electron beams and associated rapid fluctuations in solar flares

Recent observations show that the rapid fluctuations in radio, hard X-ray, and Hα emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.

Hard X-rays and associated weak decimetric bursts

In previous attempts to show one-to-one correlation between type III bursts and X-ray spikes, there have been ambiguities as to which of several X-ray spikes are correlated with any given type III burst. Here, we present observations that show clear associations of X-ray bursts with RS type III bursts between 16:46 UT and 16:52 UT on July 9, 1985. The hard X-ray observations were made at energies above 25 keV with HXRBS on SMM and the radio observations were made at 1.63 GHz using the 13.7m Itapetinga antenna in R and L polarization with a time resolution of 3 ms. Detailed comparison between the hard X-ray and radio observations shows: One possible explanation of the long delays between the RS bursts and the associated X-ray bursts is that the RS burst is produced at the leading edge of the electron beam, whereas the X-ray burst peaks at the time of arrival of the bulk of the electrons at the high density region at the lower corona and upper chromosphere. Thus, the time comparison must be made between the peak of the radio pulse and the start of the X-ray burst. In that case the delays are consistent with an electron travel time with velocity ∼ 0.3 c from the 800 MHz plasma level to the lower corona assuming that the radio emission is at the second harmonic.

Low-level decimetric (1.6 GHz) solar burst activity

Observations of solar bursts at 1.6 GHz were carried out in the month of July 1985 for about two weeks. Five intervals of solar burst activity, each one lasting for a couple of minutes, were observed. Predominantly, two classes of fast bursts were observed: viz: ‘spike’ and ‘blips’. However, some of these bursts were two orders of magnitude less intense than those reported earlier.

Occurrence of solar decimetric spike bursts over a period of 4 years during the 21st cycle

A total number of 460 DCIM events, single or groups, were observed during the period of October 1980 to December 1984 with the Weissenau spectrograph as dynamic film spectra. Several parameters describing these data were evaluated statistically and the main features were discussed in context with associated burst types.

Correlation of solar decimetric radio bursts with X-ray flares

Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 1010 cm−3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process.

VLA observations of narrow-band decimetric burst emission

The Very Large Array was used to observe a multiply-impulsive solar radio burst at several wavelengths near 20 cm. The observations indicate that the impulsive emission was nearly 100% circularly polarized and originated in small regions of ∼ 10″–20″ in size. For one of the impulsive spikes, we find evidence of narrow-band emission that could be attributed to an electron-cyclotron maser. The radio data are also compared with soft X-ray data and interpreted in light of a model in which the coronal plasma is heated by maser burst emission.