P. Zlobec

Kinematics and evolution of twist in the eruptive prominence of August 18, 1980

The prominence which erupted at the SE limb on August 18, 1980 is one of the best observed disparition brusque events: high-resolution monochromatic ground-based observations in the Hα line were supplemented by the SMM and Solwind satellite coronographic observations; the radio wavelength range was well covered by single-frequency and spectral observations, and the prominence magnetic fields were measured two days before the eruption.The prominence showed a helical-like internal structure from the pre-eruptive phase, up to the late phases of eruption. The pitch angles of the helical-like threads were measured at several positions 31 along the prominence axis, and the evolution of twist was followed during the eruption. These measurements provide an estimate of the parameters which are directly comparable with theoretical models. The pitch angles of the helical threads decreased during the eruption. A redistribution of twist along the prominence axis could not be detected within the accuracy of measurements, although there are indications that the twist was partly transformed into an external kink-type screw of the prominence axis. The value of the total twist did not change during the eruption within the accuracy of the measurements.The kinematics of the process was followed, and accompanying events in the radio-range and soft X-rays are listed. Measurements of the magnetic field vector in the prominence are reviewed briefly. The observations were compared with predictions of cylindrical models, considering the forces acting at the prominence summit. Observational implications and constraints are discussed, and the decrease of the axial electric current and the mass loss are inferred.

Time scales of the slowly drifting pulsating structure observed during the April 12, 2001 flare

First time scales of high-frequency (500−1500 MHz) slowly drifting pulsating structures observed during the April 12, 2001 flare by the Ondy (800−4500 MHz) and Potsdam (40−800 MHz) radiospectrographs and by the 1420 and 610 MHz Trieste radiopolarimeters (with high time resolution (1 ms)) are studied statistically. The Fourier method reveals pe- riods in the range of 0.9−7.5 s. For shorter periods the power spectra show a power-law form, especially in the interval of about 0.06−0.2 s, where the power-law index is in the 1.3−1.6 range. The results are interpreted using the flare model with plasmoid ejections. For the first time, the multi-scale cascading reconnection process is included in the interpretation. Corresponding time scales are estimated analytically. Further, magnetic reconnection in the bursting regime is simulated in a 2-D MHD model and variations of the dissipation power and radio radiation measure are computed. Fourier spectra of these numerical variations are determined and compared with those obtained from observations.

Ignition of MHD shocks associated with solar flares

We have selected single frequency recordings of 28 ‘high-frequency’ type II bursts characterized by a starting frequency greater than 237 MHz to estimate as accurately as possible the ‘launch-time’ of the flare-associated MHD shocks. We established the time associations between metric type II burst onsets and the time characteristics of the microwave and X-ray fluxes of the associated flares. The associated flares were impulsive events with rise times most often about 1 min in the hard X-ray range and 1–2 min in the microwave wavelength range. The majority of the type II bursts from our sample started about 1 min after the maximum of the microwave burst. Launch times of MHD shocks producing type II bursts were obtained using the 10 × Saito coronal model and shock velocities estimated from burst characteristics at different frequencies. Back-extrapolations of type II recordings indicate that MHD shocks are launched in the time interval prior to the maximum of the first peak in the associated microwave burst, most probably at the beginning of the rapid increase of the microwave burst.

Fine structures in time profiles of type II bursts at frequencies above 200 MHz

We studied the properties of fine structures in 23 type II bursts recorded at the Trieste Astronomical Observatory at frequencies above 200 MHz.The lifetime of a single fine structure is a fraction of a second. The ratio of fine structures intensity vs bulk flux density is different in different type II bursts and it changes during the evolution of a single event; the reported maximum ratio is 3. The polarization of fine structures is nearly the same during the lifetime of an event. There is also no essential difference in polarization between fine structures and bulk emission; this holds also for an example of high-polarization (about 80%) event.At frequencies lower than 200 MHz the analogy between herringbone structure and type III bursts is frequently mentioned in the literature. From the observations we studied, it results, however, that the time profile of single fine-structure elements and their polarization are substantially different from the morphology of type III bursts.The observed fine structures and their characteristics are discussed in the framework of the model by Holman and Pesses (1983).

Classification and Properties of Supershort Solar Radio Bursts

Characteristics of supershort structures (SSSs) occurring in the metric solar type IV radio bursts are described. The most important property of SSSs is their duration, which, at half-power, ranges from 4 to 60 ms and is thus much shorter than generally expected for the bursts in the metric range. The comparison of the distributions of SSS durations with those of the spikes confirms that these are completely different classes of bursts. Our analysis is focused on the frequency range 200-450 MHz, providing us with the one-to-one identification of individual SSSs in single-frequency records of the INAF-Trieste Astronomical Observatory (Italy) and in the high-resolution spectral data of Artemis IV (Greece). The analysis reveals a number of different bursts that are classified as simple broadband, simple narrowband, and complex SSSs. The diversity of SSSs has a resemblance to the variety of the well-known metric radio bursts characterized by a 1 s timescale.

Radio evidence for breakout reconnection in solar eruptive events

Context. Magnetic reconnection is understood to be fundamental to energy release in solar eruptive events (SEEs). In these events reconnection produces a magnetic flux rope above an arcade of hot flare loops. Breakout reconnection, a secondary reconnection high in the corona between this flux rope and the overlying magnetic field, has been hypothesized. Direct observational evidence for breakout reconnection has been elusive, however. Aims. The aim of this study is to establish a plausible interpretation of the combined radio and hard X-ray (HXR) emissions observed during the impulsive phase of the near-limb X3.9-class SEE on 2003 November 03. Methods. We study radio spectra (AIP), simultaneous radio images (Nancay Multi-frequency Radio Heliograph, NRH), and singlefrequency polarimeter data (OAT). The radio emission is nonthermal plasma radiation with a complex structure in frequency and time. Emphasis is on the time interval when the HXR flare loop height was observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to be at its minimum and an X-ray source was observed above the top of the arcade loops. Results. Two stationary, meter-wavelength sources are observed radially aligned at 0.18 and 0.41 R above the active region and HXR sources. The lower source is apparently associated with the upper reconnection jet of the flare current sheet (CS), and the upper source is apparently associated with breakout reconnection. Sources observed at lower radio frequencies surround the upper source at the expected locations of the breakout reconnection jets. Conclusions. We believe the upper radio source is the most compelling evidence to date for the onset of breakout reconnection during a SEE. The height stationarity of the breakout sources and their dynamic radio spectrum discriminate them from propagating disturbances. Timing and location arguments reveal for the first time that both the earlier described above the flare loop top HXR source and the lower radio source are emission from the upper reconnection jet above the vertical flare CS.

Analysis of the polarization of pulsating structures at m-dm wavelengths

We performed an analysis of the polarization of 60 pulsating radio events identified in type IV bursts recorded in the m-dm band during the Solar Cycle XXI at the Trieste Astronomical Observatory.Two major points summarize the results of such an analysis: (i) the emission is totally polarized at the source and the source itself is unique; (ii) the emission occurs in the ordinary magneto-ionic mode for most of the samples, if one accepts the leading-spot hypothesis.The first point confirms what was derived by other authors who anyhow considered a more limited set of samples: highly-polarized events are the most frequent, intense and long-lasting. The intermediate and low polarization observed in other cases are to be attributed to propagation effects, which are effective along the path after the emission, and this interpretation is supported by different observed features such as shorter duration and lower intensity of the events.Our second point differs from a previous work which claims the extraordinary mode, but this discrepancy can be justified by: (a) the small number of events analyzed in that work, which gives poorer statistics; (b) a quite different observing frequency range; (c) the different selection criterion. However, in spite of the relative richness of our data set we cannot give a final answer to the emission mode problem as the leading-spot hypothesis is questionable and we report critical arguments against it based on experimental results.The polarization degree of pulsations looks generally constant during the whole lifetime. As a general trend the selected events show the same polarization features both for the pulsations and the background. A different polarization degree of the continuum is probably the signature of the contemporaneous presence of more than one source.

Delay between the circularly polarized components in fine structures during solar type IV events

We analyzed intermediately polarized (20–80%) fine structures (pulsations, sudden reductions, fiber bursts and zebras) that were recorded in type IV events. The mean polarization degree was practically the same for all the fine structures recorded in an interval lasting a few minutes and it was similar to the polarization of the continuum. A detailed analysis during the evolution of single structures reveals changes in polarization (in particular an ‘undulation’ at flux density minima) even stronger than 20%. They were caused by a delay, up to 0.1 s, between the two circularly polarized components. The weaker polarimetric component was delayed in 2 sets and the stronger one in 1 set. In the event of April 24, 1985 different types of fine structures were sporadically detected in more than one hour long time interval. Short delays of the stronger or of the weaker component were sometimes observed.The events characterized by fine structures are generally totally polarized in the ordinary mode. We assume that this holds also for the phenomena studied here. The observed intermediate polarization therefore requires a depolarization due to propagation effects. We discuss the mode coupling and the reflection of the original radio signal that could also generate the delay of the weaker and the stronger component respectively. The possibility of polarization variation due to the change of the angle between the direction of the propagation and the magnetic field in a quasi-transversal region and in a low intensity magnetic field in a current sheet is also given.

Fine structure in a metric type IV burst: Multi-site spectrographic, polarimetric, and heliographic observations

The spectral fine structure of solar radio continua is thought to reveal wave-particle and wave-wave interactions in magnetic traps in the solar corona. We present observations of spectra, polarization, and spatial characteristics of combined emission/extinction features (‘zebra patterns’) during a decimetric/metric type IV event on 5 June, 1990. Very high modulation depths are observed. The size and location of the sources during emission and extinction are determined for the first time. Two remarkable features are found: (1) The sources of emission stripes have finite size, up to nearly 2′; during extinction stripes the brightness is reduced across the whole extent of the unperturbed continuum, which is slightly larger than 2′. (2) During emission stripes the sources drift over distances up to several × 104 km, with apparent velocities up to 105 km s−1. The observed features are briefly discussed with respect to interpretations based on wave-particle interactions and on the scattering of electromagnetic waves.

Intermediate polarization of type I bursts

A constant polarization degree was found in groups of type I bursts having intermediate polarization for all the duration of the group. The same circumstance was verified also for analogous bursts present during a type IV event. The consequences of this behaviour, considering also the results of some recent papers, is discussed. The polarization amount is probably intrinsic to the source itself.