On the intermediate drift burst model

A modification of the presently existing intermediate drift burst model by Kuijpers (1975) and Bernold (1983) is suggested. It is shown that whistler solitons cannot be responsible for intermediate drift bursts. Here, they are interpreted as the radio signature of localized formstable whistler wave packets propagating along the magnetic field in a coronal loop. In the frame of this modified model, the magnetic field strengths derived from fiber burst data agree with previous estimates by Dulk and McLean (1978).     

一个复杂太阳射电爆发及其微波源区特征的初步研究

我国“太阳射电宽带频谱仪(0．7～7．6GHz)”于2001年10月19日观测到的复杂太阳射电大爆发，呈现许多有趣的特征。本文结合NoRH的高空分辨率成像观测资料，分析了该爆发的微波射电源区的演化特征及与射电辐射特征的关系。还发现微波源的缓慢运动，这可能与爆发所伴随的CME的形成有关。     

Fiber bursts concurrent with a weak noise storm

A new kind of radio burst is described and identified as quasi-fiber burst according to some striking similarities with fiber bursts. Its interpretation is discussed in terms of Kuijpers' whistler model and an explanation for a broken variety of. the observed burst is given. The derived magnetic field strength in the source is 4 G at a plasma level of 300 MHz.     

Dynamics of electron beams in a coronal loop and the hard X-ray burst

A numerical simulation has been made for the dynamics of non-thermal electrons (> 10keV) injected with spatial, temporal and velocity distributions into a model coronal loop. The time variations of the spatial intensity distribution and the spectrum for the expected hard X-rays are computed for many models in order to find the important physical parameters for those characteristics.The most important one is the column density of plasma, CD, along the loop. If CD is smaller than 10²⁰ cm^–2, the expected X-rays behave like the solar impulsive hard X-ray bursts, that is the spatial maximum of X-rays shifts to the top of the loop in the later phase of the burst accompanying a spectral softening. On the other hand, if CD is greater than this value, quasi-steady decay appears in the later phase. In this case the intensity distribution of X-rays above about 20 keV along the loop shows a broad maximum away from the loop top giving an extended spatial distribution of hard X-rays, and spectral hardness is kept constant. These characteristics are similar to the solar gradual hard X-ray bursts (the so-called extended burst which is not a hot thermal gradual burst).     

A complex solar radio burst and the characteristics of its related microwave sources and EUV coronal loops

On 2001 Oct. 19, a very complex solar radio burst with a host of interesting features was observed with a broadband (0.7–7.6 GHz) solar radio spectrometer. Combining with the data of NoRH (Nobeyama Radio Heliograph) and TRACE (Transition Region and Corona Explorer), the spectral features of the radio burst, the evolution of the microwave radio sources, and relations with the complex EUV coronal loops are analyzed. The burst is the radio manifestation of a large double-ribbon flare; it consisted of two stages. The earlier stage was dominated bya broadband burst in the centimeter-meter waveband from gyro-synchrotron emission of sources at the footpoints of the loop. The later stage was dominated by a narrow-band decimeter wave burst in the decimetermeter waveband, from a combination of plasma emission and gyro-resonance emission from sources in the top of the loop.     

Hard X-ray imaging of a solar gradual hard X-ray burst on April 1, 1981

An intense solar X-ray burst occurred on April 1, 1981. X-ray images of this gradual hard X-ray burst were observed with the hard X-ray telescope aboard the Hinotori satellite for the initial ten minutes of rise and maximum phases of the burst. The hard X-ray images (13–29 keV) look like a large loop without considerable time variation of an elongated main source during the whole observation period. The main X-ray source seems to lie along a ridge of a long coronal arcade 2 × 10⁴ km above a neutral line, while a tangue-like sub-source may be another large coronal loop although the whole structure of the X-ray source looks like a large semi-circular loop. Both nonthermal and hot thermal (3–4 × 10⁷ K) electrons are contributing to the source image. The ratio of these components changed in a wide range from 2.3 to 0.4 during the observation, while the image was rather steady. It suggests that both heating and accelerations of electrons are occurring simultaneously in a common source. Energetic electrons of 15–30 keV would be collisionally trapped in the coronal magnetic loops with density of the order of 10¹¹ cm^–3.     

Radio and visible-light observations of a coronal arcade transient

We discuss simultaneous visible-light and radio observations of a coronal transient that occurred on 9 April, 1980. Visible-light observations of the transient and the associated erupting prominence were available from the Coronagraph/Polarimeter carried aboard SMM, the P78-1 coronagraph, and from the Haleakala Observatory. Radio observations of the related type III-II-IV bursts were available from the Clark Lake and Culgoora Observatories. The transient was extremely complex; we suggest that an entire coronal arcade rather than just a single loop participated in the event. Type III burst sources observed at the beginning of the event were located along a nearby streamer, which was not disrupted, but was displaced by the outmoving loops. The type II burst showed large tangential motion, but unlike such sources usually do, it had no related herringbone structure. A moving type IV burst source can be associated with the most dense feature of the white-light transient.     

Observations of preburst heating and magnetic field changes in a coronal loop at 20 cm wavelength

The Very Large Array (VLA) has been used at 20 cm wavelength to study the evolution of a burst loop with 4 resolution on timescales as short as 10 s. The VLA observations show that the coronal loop began to heat up and change its structure about 15 min before the eruption of two impulsive bursts. The first of these bursts occurred near the top of the loop that underwent preburst heating, while the second burst probably occurred along the legs of an adjacent loop. These observations evoke flare models in which coronal loops twist, develop magnetic instabilities and then erupt. We also combine the VLA observations with GOES X-ray data to derive a peak electron temperature of T _e = 2.5 × 10⁷ K and an average electron density of N _e 1 × 10¹⁰ cm^–3 in the coronal loop during the preburst heating phase.     

Interpretation of a special fine structure in type-IV solar radio bursts

A special fine structure (slowly drifting chains of narrowband fiber bursts), firstly observed during the solar type-IV radio burst on April 24, 1985, is interpreted as the radio signature of whistler waves periodically excited by a switch-on/switch-off process of a loss-cone instability in a localized wave packet of the fast magnetoacoustic mode.     

Simultaneous microwave observations of solar flares at 6 and 20 cm wavelengths using the VLA

Using the Very Large Array, solar burst observations have been carried out simultaneously at 6 and 20 cm. Structural changes and preheating have been observed in the flaring regions on time scales of minutes to tens of minutes before the onset of the burst impulsive phase. The 6 cm burst sources are located close to the neutral line, or near the legs of a flaring loop. The 20 cm burst sources show complex and extended structures spatially separated from both the preburst emission and the gradual decay phase of the burst. We interpret the observations in terms of a two-component flare model (bulk heating as well as acceleration of particles) and derive the physical parameters of the burst sources.On leave of absence from Indian Institute of Astrophysics, Bangalore, India.     

Fiber Fine Structures Superposed on the Solar Continuum Emission Near 3 GHz

On April 21, 2002, a broadband solar radio burst was observed at about 01:00 – 03:00 UT with the digital spectrometers of National Astronomical Observatories of China (NAOC). Also many fiber bursts superposed on the continuum bursts were detected in the frequency range of 2.6 – 3.8 GHz during the time interval. After data processing, some parameters of the fibers such as frequency drift rate, duration, bandwidth, and relative bandwidth were determined. The mean value of the frequency drift was in the range of 42.3 – 87.4 MHz s⁻¹ (negative). A theoretical interpretation for the fibers was presented based upon a model of the velocity of Alfvén solitons. In this model, the source of the fiber emission was considered as the ducting of the solitons within the magnetic-mirror loop. Then the magnetic field strength of the fiber source was estimated to be about 130 ≤ B₀ ≤ 270 G. Also a comparison of the magnetic field estimation was made with another model of whistler group velocity.     

MICROWAVE EMISSION FROM CORONAL HEIGHTS: STUDY OF A NON-THERMAL RADIO FLARE

Two small radio flares following the great gamma-ray burst on 11 June 1991 are studied. We analyse the different association of emission features at microwaves, decimeter waves, and soft and hard X-rays for the events. The first flare has well-defined emission features in microwaves and soft and hard X-rays, and a faint decimetric signature well after the hard X-ray burst. It is not certain if the decimetric event is connected to the burst features. The second event is characterized by an almost simultaneous appearance of hard X-ray burst maxima and decimetric narrowband drift bursts, but soft X-ray emission is missing from the event. With the exception of the possibility that the soft X-ray emission is absorbed along the way, the following models can explain the reported differences in the second event: (1) Microwave emission in the second event is produced by 150 keV electrons spiraling in the magnetic field relatively low in the corona, while the hard X-ray emission is produced at the beginning of the burst near the loop top as thick-target emission. If the bulk of electrons entered the loop, the low-energy electrons would not be effectively mirrored and would eventually hit the footpoints and cause soft X-ray emission by evaporation, which was not observed. The collisions at the loop top would not produce observable plasma heating. The observed decimetric type III bursts could be created by plasma oscillations caused by electron beams traveling along the magnetic field lines at low coronal heights. (2) Microwave emission is caused by electrons with MeV energies trapped in the large magnetic loops, and the electrons are effectively mirrored from the loop footpoints. The hard X-ray emission can come both from the loop top and the loop footpoints as the accelerated lower energy electrons are not mirrored. The low-energy electrons are not, however, sufficient to create observable soft X-ray emission. The type III emission in this case could be formed either at low coronal heights or in local thick regions in the large loops, high in the corona.     

High-resolution observations of solar radio bursts at 2, 6, and 20 cm wavelength

The Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe eight solar bursts at 2, 6, or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10⁷ K and 2 x 10⁸ K, and degrees of circular polarization between 10 and 90%. A series of 10 s snapshot maps are presented for the more intense bursts, and superimposed on photospheric magnetograms or Hα photographs. The impulsive phase of the radio bursts is located near the magnetic neutral line of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. The impulsive phase of one 6 cm burst was smaller and spatially separated from both the preburst radio emission and the gradual decay phase of the burst. Another 6 cm burst exhibited preburst heating of the coronal loop in which the burst occurred. The plasma was probably heated at a lower level in the loop, while the burst energy was released several minutes later at a higher level. A multiple-spike 20 cm burst exhibited polarity inversions with degrees of circular polarization of 90%. The rapid changes in circular polarization are attributed to either a magnetically complex region or the emersion of new magnetic flux at coronal heights where magnetic field strengths H ≈ 300 to 400 G.     

Time variations of hard X-ray bursts observed with the Solar X-ray Telescope aboard Hinotori (with a movie)

We have developed a new method for synthesizing hard X-ray maps from the raw data of the Solar X-ray Telescope (SXT) aboard Hinotori. Using this method we analyzed five typical SXT events and summarized their images in a movie with a time resolution of about 8 s (half spin period of the satellite). The movie clearly shows that (1) three different classes of bursts, i.e., the gradual thermal burst, the multiple impulsive burst, and the extended outburst, have different structures and show quite different variations from each other, and that (2) the source of the extended outburst is located in the corona above 10⁴ km and its shape appears to be a large loop.     

Observations of a complex solar radio burst with fine structure on 3 May 1973

The simultaneous high resolution recordings of dynamic spectra in the range 93–220 MHz and polarization at 204 MHz of a complex type II–IV event which started at 08:33 UT on 3 May 1973 shown a sporadic zebra pattern. In contrast with the unpolarized type II burst, the stripes in the emission and absorption of the zebra pattern were fully polarized and most likely corresponded to the ordinary wave. As to spectral and polarization characteristics, the fiber bursts with intermediate frequency drift did not differ from the stripes of the zebra pattern. The microstructure of the type II burst was characterised by a lot of spikes with variable frequency drift, duration 0.1 s and instantaneous bandwidth ≈1 MHz.     

On a sequence of remarkable fine structures in the type IV burst of 24 April, 1985

During the type IV burst on 24 April, 1985 we observed at 234 MHz an untypical, strong, nearly six hours lasting continuum emission incorporating several groups of broadband pulsations, zebra patterns, fiber bursts, and a new fine structure phenomenon. The power spectra of the groups of broadband pulsations reveal no simple structure. There is only one common periodic component between 0.3 s and 0.4 s. Slowly drifting chains of narrowband fiber bursts are described as a new fine structure by spectrograms and simultaneously recorded single frequency intensity profiles. A qualitative model of this new fine structure is suggested.     

The spatial structure of a solar flare in soft X-rays and centimetric wavelengths

High-resolution images of the decay phase of a soft X-ray flare observed by the S-054 experiment on Skylab are compared with interferometric scans of the radio burst obtained simultaneously at 2.8 cm (Felli et al., 1975). The spatial resulution of the radio instrument in one direction, although lower than the X-ray telescope resolution, is high enough for a detailed comparison. The comparison clarifies the relationship between the sources of soft X-ray and thermal radio emission in solar flares. The X-ray emission is localized in a loop-like structure which appears spatially coincident with the rapidly varying component of the radio burst. The more stable components of the radio source, which do not appear to contribute substantially to X-ray emission, are found to be spatially associated with the extremes of the X-ray loop. A model of plasma-filled loops is suggested which accounts for the emissions in both spectral ranges and for their spatial location and temporal development.On leave from Osservatorio Astrofisico di Arcetri, Florence, Italy.     

Closed loop model of turbulent acceleration for the gradual phase of the radio burst of 1981 April 27

We propose a closed loop model of turbulent stochastic acceleration for the gradual phase of the radio burst of 1981 April 27. It can explain the various observed features in the spectrum, the rather long time of rise, the absence of X-ray bursts below 56 keV and of any detectable proton events.     

An explanation of the abnormal features of the 1981 april 27 event

A large limb burst occured on 1981 April 27, 0720 UT. A large eruptive loop prominence was observed by us at 0816–0951. At 0829.5 and 0833 the loop showed an abnormal flat top, and simultaneously, radio emission at 3.2 cm showed an abnormal absorption. To explain these features, we supposed that a dense cloud moved in front of the top of the H_α loop. We showed that if the cloud has a thickness of 10⁴km and if it has an electron density 10?9 times that in the active region, then the abnormal absorption at 3.2 cm can be explained by collisional damping.     

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.