Hanumant S. Sawant

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.

Discovery of Variation in Solar Coronal Rotation with Altitude

Here we report the first measure in radio emission of differential rotation as a function of height in the solar corona. This is derived from the disk-integrated simultaneous daily measurements of solar flux at 11 radio frequencies in the range of 275-2800 MHz. Based on the model calculations, these radio emissions originate from the solar corona in the estimated average height range of ~(6-15) × 104 km above the photosphere. The investigations indicate that the sidereal rotation period at the highest frequency (2800 MHz), which originates from the lower corona around 6 × 104 km, is ~24.1 days. The sidereal rotation period decreases with height to ~23.7 days at the lower frequency (405 MHz), which originates at ~13 × 104 km. It is difficult to identify clearly the rotational modulation at 275 MHz, perhaps because these emissions are significantly affected by the turbulence in the intervening medium. Since these investigations are based on disk-integrated solar flux at radio frequencies, it is difficult to say whether these systematic variations in sidereal rotation period are partly due to the latitudinal differential rotation of the solar corona. It will be interesting to investigate this possibility in the future.

Short-Term Periodicities in the Time Series of Solar Radio Emissions at Different Solar Altitudes

A spectral analysis of the time series of daily values of 12 parameters, namely, ten solar radio emissions in the range 275–1755 MHz, 2800 MHz solar radio flux, and sunspot numbers for six continuous intervals of 132 values each during June 1997–July 1999 showed considerable differences from one interval to the next, indicating a nonstationary nature. A ∼ 27-day periodicity was noticed in Interval 2 (∼ 26.8 days), 3 (∼ 27.0 days), 5 (∼ 25.5 days), 6 (∼ 27.0 days). Other periodicities were near 11.4, 12.3, 13.3, 14.5, 15.5, 16.5, 35, 40, 50–70 days. Periodicities were very similar in a large vertical span of the coronal region corresponding to 670–1755 MHz. Above this region, the homogeneity disappeared. Below this region, there were complications and distortions due to localized solar surface phenomena.

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.

Interplanetary scintillation observations for the solar wind disappearance event of May 1999

In this article we present ground-based interplanetary scintillation (IPS) measurements at 103 and 327 MHz for the period of the solar wind disappearance event of May 1999 as seen by various space probes. The solar wind velocity measurements at 327 MHz showed a variable solar wind velocity during this period at a distance of ∼0.5 AU from the Sun. The average solar wind velocity from three radio sources varied in the range of 200–300 km s−1. The scintillation index measurements at 103 MHz indicate that plasma density was very low in the interplanetary medium closer to the Earth and that the density was normal away from it during May 11–13. The scintillation index was enhanced significantly on May 14 after the disappearance event. The comparison with the in situ observations shows that the effect is dramatic in IPS observations. IPS and in situ measurements show that a large, tenuous, and slow plasma cloud engulfed our planet around this time, which could be because of a corotating low-density narrow stream. From the source (Sun) point of view, this was mostly a normal plasma flow in most of the interplanetary medium.

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.

Hard X-rays and associated weak decimetric bursts

AbstractIn 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:(1)In at least 13 cases we can identify the associated hard X-ray and decimetric RS bursts.(2)On average, the X-ray peaks were delayed from the peak of the RS bursts at 1.6 GHz by ∼ 400 ms although a delay as long as 1 s was observed in one case. 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.

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.

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.Low-level blips have typical duration ∼ 350 ms, excitation time ∼ 200 ± 25 ms, decay time ∼ 130 ± 25 ms and a low degree of circular polarization of about 15%. Detailed investigations of decay times of the blips have been carried out in terms of collisional damping and Landau damping. Observed decay times of the blips seem to favour the hypothesis of collisional damping. This investigation suggests that blips probably originate at second harmonic by beam plasma interaction as that of metric type III bursts. Also, low-level ms-spikes with the half power duration in the range of 5 to 20 ms suggest that source sizes be smaller than 50 km if the process of emission is electron-cyclotron maser.

The simplest solar microbursts flux and circular polarization at 22 GHz

The simplest solar microwave microbursts detected with high sensitivity may be the response to the simpler energetic burst injections. Seventeen events from this category were identified in a series of more than 150 bursts recorded in 21–26 November, 1982. This first systematic study suggest that microbursts e-folding rise times concentrate into two classes of time scales, 0.05 s < t ≪ 1 s and 0.5 s ⪝ t ⪝ 2 s. Microbursts’ circular polarization present a dominant steady or slowly varying component that sets in before maximum emission. In some cases a faster component of polarization was found superimposed, which is not always well correlated in time with flux.