Francisco Fernandes

Radio bursts with rapid frequency variations-Lace bursts

The Ondřejov radiospectrograph operating in the 0.8-2.0 GHz frequency range recorded in recent years (1998-2000), three (August 10, 1998; August 17, 1999; June 27, 2000) unique bursts with rapid frequency variations (lace bursts) lasting for several minutes. On August 17, 1999, the same burst was recorded simultaneously by the Brazilian Solar Spectroscope in the 1.0-2.5 GHz frequency range. The frequency variations of these bursts in four time intervals were analyzed by the Fourier method and power-law spectra with power-law indices close to -2 were found. The Fourier spectra show the presence of frequency variations in the 0.01-3.0 Hz interval which indicate fast changes of plasma parameters in the radio source. Due to the similarities in the line features of these bursts with zebra pattern lines, a model similar to that of the zebra pattern was suggested. The model radio spectra, computed using this model with a turbulent state of the solar flare atmosphere, are similar to those observed by the radiospectrographs.

Observation of harmonically related solar radio zebra patterns in the 1-4 GHz frequency range

A unique case of two zebra patterns related harmonically with ratio of1:2 was observed by distant radio telescopes at Sao Jose dos Campos and Ondrejov Observatories. Accompanied zebras show that the ratio of frequencies of the neighboring zebra lines is in the range of 1.009-1.037. There is a tendency of a decrease of this ratio with decreasing frequency within the specific zebra pattern. Both facts speak in favour of plasma emission models for the zebra pattern fine structure in radio burst continua.

Decimetric radio dot emissions

Context. We study a rare type of solar radio bursts called decimetric dot emissions. Aims. In the period 1999−2001, 20 events of decimetric dot emissions observed by the Brazilian Solar Spectroscope (BSS) in the frequency range 950−2640 MHz are investigated statistically and compared with radio fine structures of zebras and fibers. Methods. For the study of the spectral characteristics of the dot emissions we use specially developed Interactive Data Language (IDL) software called BSSView and basic statistical methods. Results. We have found that the dm dot emissions, contrary to the fine structures of the type IV bursts (i.e. zebras, fibers, lace bursts, spikes), are not superimposed on any background burst emission. In the radio spectrum, in most cases the dot emissions form chains that appear to be arranged in zebra patterns or fibers. Because some zebras and fibers, especially those observed with high time and high spectral resolutions, also show emission dots (but superimposed on the background burst emission), we compared the spectral parameters of the dot emissions with the dots being the fine structure of zebras and fibers. For both these dots, similar spectral characteristics were found. Some similarities of the dot emissions can be found also with the lace bursts and spikes. For some events the dot emissions show structural evolution from patterns resembling fibers to patterns resembling zebras and vice versa, or they evolve into fully chaotic patterns. Conclusions. For the first time, we present decimetric dot emissions that appear to be arranged in zebra patterns or fibers. We propose that these emissions are generated by the plasma emission mechanism at the locations in the solar atmosphere where the double resonance condition is fulfilled.

Phenomenological dynamics of coronal loops using a neural network approach

Abstract The objective of this study is to simulate the X-ray 2D-coronal dynamics using an artificial neural network — multilayer backpropagation algorithm with inputs of Energy Fragmentation Patterns obtained from Yohkoh images in soft and hard X-rays. Details of a single loop structure have been investigated for initial analysis. The images are spatio-temporal series showing the loop-top in Soft X-ray (SXR) and foot points in Hard X-ray (HXR). Using a square electron density gradient model, we have characterized the spatio-temporal loop dynamics concerning its twister-relaxation regime. The performance of this trained network model has been tested with classical image statistics applied to the Yohkoh data. In this paper we show preliminary results indicating that this technique can be useful for coronal dynamics analysis.

Solar decimetric type III bursts in semi-closed magnetic field structures

Aims. We investigate statistically seventeen groups of solar type III bursts, observed in the frequency range 950−2500 MHz using the Brazilian Solar Spectroscope (BSS) during the period 1999−2002. Methods. Using specially-developed Interactive Data Language (IDL) software called BSSView, the spectral characteristic properties of the bursts were investigated. For illustration a semi-closed magnetic field structure with trapped electron beams was modelled using the particle-in-cell model. Results. Most of the type III bursts studied in this paper have starting frequencies in the range 1100−1400 MHz and their average bandwidth, half-power duration, and interval between adjacent bursts are 116 ± 59 MHz, 108 ± 41 ms, and 647 ± 643 ms, respectively. The number of bursts with negative frequency drift, which is on average −914 ± 684 MHz s −1 , is greater than the number with positive frequency drift, which is on average +807 ± 675 MHz s −1 . The type III bursts have power-law distribution function for their half-power duration H(fstart) = 4.79 × 10 8 f −2.145±0.015 start , bandwidth B1(fstart) = 1.95 × 10 7 f −1.697±0.022 start and B2(fstart) = 7.07 × 10 5 f −1.154±0.015 start ,a nd frequency drift D(fstart) = 0.43 f 1.040±0.031 start . Most of the groups of type III bursts were also found to exhibit either positive or negative group frequency drifts of on average +53 ± 38 MHz s −1 and −46 ± 42 MHz s −1 , respectively. The detailed statistical analysis suggests that in the frequency range studied there are two categories of type III bursts, the average parameters of which we provide in brackets for each type: a) numerous narrowband bursts (bandwidth ∼100 MHz) with small frequency drift (∼±500 MHz s −1 ) in groups with visible group drift (∼±50 MHz s −1 ) and b) less numerous broader band bursts (bandwidth 171 MHz) with higher frequency drift (∼±1800 MHz s −1 ) in groups without group drift. The statistics of the parameters of these drifting groups are presented for the first time. The drifting groups of type III bursts are interpreted as due to electron beams trapped in moving plasmoids, which are semi-closed magnetic field structures.

Brazilian Decimetric Array

Abstract A radio heliograph operating in the frequency range of 1200–1700 MHz is planned by INPE, Brazil, for investigations of time evolution of active regions, which will lead to better understanding of the physics of the flares energy release and particle acceleration, in order to suggest better criteria for the prediction of solar flares, Coronal Mass Ejections (CME), and solar terrestrial relations, such as geomagnetic storms and radio blackouts. In the first phase, the Brazilian Decimetric Array (BDA) will be a T shaped array 256 m by 144 m, consisting of 26 parabolic dish antennas of 4 m diameter. This array will produce full disk images of the sun with a spatial resolution of 3 by 5 arc minutes at 1420 MHz with a time resolution of 100 ms and sensitivity of ∼ 10 Jy. In the second phase, in addition to the compact T array there will be 6 more 7 m diameter antennas on an East-West baseline of 2560 m to obtain higher spatial resolution and better sensitivity. Thus, finally this radioheliograph will have wide field of view and couple of arcsec spatial resolution and high time resolution (100 ms).

Microwave Type III-RS Bursts

A decimetric (1600 ± 100 MHz), high-sensitivity digital spectroscope has been put into regular operation at Sao Jose dos Campos, Brazil, since 1991 March. The dynamic spectra are recorded on 35 mm film while 20 frequencies each separated by 10 MHz are digitized with time resolution of 100 ms. So far, about 350 groups of solar bursts have been observed by this system. These have been classified as microwave type and decimetric reverse slope (RS) type III bursts. Here, we report statistical investigations of the properties of the 44 moderately strong groups of decimetric type III-RS bursts. The average total duration of these bursts is ~0.7 s, the drift rate varied between ~400 and 3000 MHz s −1 , and average decay time is ~120 ms. These observed drifts and timescales suggest that decimetric type III-RS bursts are generated by beam plasma interaction at second harmonic in the inhomogeneous chromosphere, and that the acceleration region is located at a density level of ≤10 9 cm −3 . Subject heading : Sun: radio radiation

Characterization of local self-similarity and criticality in the solar active regions

From solar radio burst data we computed wavelet transforms and frequency distribution for investigation of self-similar temporal variability and power-laws, as the fundamental conditions for characterization of dynamical criticality (self or forced) in the solar active regions. The main result indicates that, as for the global activity, the local coronal magnetic field, in millisecond time scales, can be in a critical state where the dynamics of solar active regions works as avalanches of many small intermittent particle acceleration events.

High sensitivity digital decimetric spectroscope

Abstract Decimetric Solar Radio Spectroscope has been put into operation since March 1991 at Sao Jose dos Campos, Brazil. Presently, it is operating over frequency range of (1.6 ± 0.1) GHz, with time resolution of 100 ms and frequency resolution of (0.3–3) MHz, in conjunction with a 9 m diameter polar mounted antenna. The dynamic spectra are recorded on 35 mm film, while 20 frequencies each separated by 10 MHz are digitized with time resolution of 100 ms. So far, various types of bursts have been observed by this system. Morphological studies of these bursts are presented. Particularly interesting are reverse slope (RS) type III bursts. Soon this system will start operating over a frequency range of (0.2–2.5) GHz, simultaneously in two circular polarizations. Flexibility of this system permits to select desired frequency range with resolution in time of (1–250) ms and in frequency of (0.1–3) MHz.

A LOW COST STEERABLE RADIO-TELESCOPE

We present the technical characteristics of a low-cost radio telescope for solar/non solar observations at decimetric (1200–1700 MHz) and centimetric (2700 and 5000 MHz) wavelengths known as Brazilian Decimetric Array (BDA). The technical specifications of the antenna, tracking system, log-periodic feed, preamplifier and the frequency-synthesised receiver with a Single Side Band (SSB) video output of 2.5 MHz are given.