Decameter storm radiation,I

A log-periodic array, 3 km long in the E-W direction is in operation at the Clark Lake Radio Observatory. The solar brightness distribution is swept once per second in the 65-20 MHz frequency range. The analysis of the interferometer records allows the determination of one dimensional solar burst positions, to an accuracy of 0.1 R at 60 MHz and 0.3 R at 30 MHz, approximately.Six long duration noise storms have been observed over an eight month period, extending from January to September, 1971. The storms are described and their relation to chromospheric active regions and flares is discussed. Decametric storms are found to be related to complexes of interacting active regions. The interaction is studied in terms of the number of simultaneous flares observed to occur in the various active regions. On the average, twice as many simultaneous flares are observed than would be expected if flares occurred at random. An analysis of coronal magnetic field maps computed from longitudinal photospheric fields shows magnetic arcades and some divergent field lines at the site of storm regions. Decimeter and meter wavelength sources are found to be associated with all decameter storms. At decimeter wavelengths double or multiple sources are often seen above individual active regions forming part of the chromospheric complex.     

Solar Decameter Spikes

We analyze and discuss the properties of decameter spikes observed in July?–?August 2002 by the UTR-2 radio telescope. These bursts have a short duration (about one second) and occur in a narrow frequency bandwidth (50?–?70 kHz). They are chaotically located in the dynamic spectrum. Decameter spikes are weak bursts: their fluxes do not exceed 200?–?300 s.f.u. An interesting feature of these spikes is the observed linear increase of the frequency bandwidth with frequency. This dependence can be explained in the framework of the plasma mechanism that causes the radio emission, taking into account that Langmuir waves are generated by fast electrons within a narrow angle θ≈13^°?–?18^° along the direction of the electron propagation. In the present article we consider the problem of the short lifetime of decameter spikes and discuss why electrons generate plasma waves in limited regions.     

Decameter Type III-Like Bursts

We report the first observations of Type III-like bursts at frequencies 10 – 30 MHz. More than 1000 such bursts during 2002 – 2004 have been analyzed. The frequency drift of these bursts is several times that of decameter Type III bursts. A typical duration of the Type III-like bursts is 1 – 2 s. These bursts are mainly observed when the source active region is located within a few days from the central meridian. The drift rate of the Type III-like bursts can take a large value by considering the velocity of Type III electrons and the group velocity of generated electromagnetic waves.     

Filament eruption and storm radiation at meter-decameter wavelengths

We report the study of a weak noise storm observed by the Clark Lake multifrequency radioheliograph at four frequencies. The noise storm onset was associated with a filament eruption and a gradual rise and fall in soft X-rays. We compare the noise storm emission with related emissions in other wavelengths to develop a composite scenario of the event. Using the properties of the quiet corona inferred from the simultaneously observed quiet-Sun radiation, we estimate the brightness temperature of the storm continuum, which seems to be consistent with the observations reported in Solar Geophysical Data. Superthermal particles with a temperature that is ten times the coronal electron temperature and a density of 10^–3 times the coronal density are adequate to explain the observed radiation. Since the noise storm observations were made at four frequencies, we were able to obtain a brightness temperature spectrum of the storm radiation. If the storm radiation is affected in the same way as the quiet-Sun emission by inhomogeneities, the observed spectrum can be interpreted as due to propagation effects. Since the Clark Lake instrument can observe both quiet Sun and weak bursts simultaneously, we were able to estimate the propagation effects from the quiet-Sun observations and use it to correct the brightness temperature of storm radiation.     

A note on the emission mechanism of storm radiation

A new mechanism has been proposed for the continuum and burst components of solar storm radiation by Genkin, Erukhimov, and Levin (1989a, b). In this paper, we point out that while bursts can be explained by the proposed mechanism of scattering on plasma turbulence generated density fluctuations, the continuum cannot be explained by sattering on thermal ion density fluctuations. The reason is, under the same coronal conditions, second harmonic emissions will dominate over the fundamental emission due to scattering on thermal ion density fluctuations in contradiction to observations. We also note that the range of plasma wave densities needed for this mechanism may not be realistic for the case of plasma wave generation due to loss cone instability. It is further argued that coalescence of plasma waves with low-frequency waves still seems to be the plausible mechanism.     

Decameter type IV bursts associated with coronal transients

Recent observations of neutral line absorbing features in the solar atmosphere may give an important clue to the mechanism whereby both type III solar radiobursts and solar flares are triggered. It is suggested that as new satellite magnetic flux emerges at the edge of an active region in an area of opposite polarity a neutral sheet builds up between the new and old flux. When the sheet has a length of about a megametre its thermal insulation from the surrounding plasma is effective enough for a thermal instability to occur. The resulting compression and inflow of plasma is observed in H on the disc as a neutral line absorbing feature. Furthermore, the electric field of the accompanying collisionless tearing mode instability in a thin slab near the centre of the sheet exceeds the runaway field; it may therefore accelerate electrons to high enough energies to produce the type III burst which usually occurs at the same time as the absorbing feature. Perhaps the flare which sometimes ensues is triggered when the quasi-equilibrium state is destroyed by the development of turbulence in the neutral sheet.     

New observations of solar noise storm radiation at decameter wavelengths

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping ( _c ) is more than the growth (- _A ), there is a steady emission responsible for the continuum, and whenever _c = - _A (which may be satisfied randomly) there is a sudden jump in T _b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n _b /n _e 10^–10–10^–9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Unusual Solar Radio Burst Observed at Decameter Wavelengths

An unusual solar burst was observed simultaneously by two decameter radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) on 3 June 2011 in the frequency range of 16?–?28 MHz. The observed radio burst had some unusual properties, which are not typical for the other types of solar radio bursts. Its frequency drift rate was positive (about 500 kHz?s^?1) at frequencies higher than 22 MHz and negative (100 kHz?s^?1) at lower frequencies. The full duration of this event varied from 50 s up to 80 s, depending on the frequency. The maximum radio flux of the unusual burst reached ≈10³ s.f.u. and its polarization did not exceed 10 %. This burst had a fine frequency-time structure of unusual appearance. It consisted of stripes with the frequency bandwidth 300?–?400 kHz. We consider that several accompanied radio and optical events observed by SOHO and STEREO spacecraft were possibly associated with the reported radio burst. A model that may interpret the observed unusual solar radio burst is proposed.     

Solar Drift Pair Bursts in the Decameter Range

The results of observations of solar decametric drift pair bursts are presented. These observations were carried out during a Type III burst storm on July 11–21, 2002, with the decameter radio telescope UTR-2, equipped with new back-end facilities. High time and frequency resolution of the back-end allowed us to obtain new information about the structure and properties of these bursts. The statistical analysis of more than 700 bursts observed on 13–15 July was performed separately for “forward” and “reverse” drift pair bursts. Such an extensive amount of these kind of bursts has never been processed before. It should be pointed out that “forward” and “reverse” drift pair bursts have a set of similar parameters, such as time delay between the burst elements, duration of an element, and instant bandwidth of an element. Nevertheless some of their parameters are different. So, the absolute average value of frequency drift rate for “forward” bursts is 0.8 MHz s⁻¹, while for “reverse” ones it is 2 MHz s⁻¹. The obtained functional dependencies “drift rate vs. frequency” and “flux density vs. frequency” were found to be different from the current knowledge. We also report about the observation of unusual variants of drift pairs, in particular, of “hook” bursts and bursts with fine time and frequency structure. A possible mechanism of drift pairs generation is proposed, according to which this emission may originate from the interaction of Langmuir waves with the magnetosonic waves having equal phase and group velocities.     

Decameter studies of the 5 September 1973 flare

We discuss the spectra and positions of the meter-decameter wavelength radio sources associated with the 5 September 1973 flare. We discuss the evolution of the size of the type II burst source and show that it fluctuates by a factor of 10, or larger. Consequently, the potential and kinetic energies associated with the shock are uncertain by the same factor. By comparing the positions of the type II and type III sources we conclude that while the shock wave associated with the type II was guided along high loops, the type III electrons were injected along open field lines which diverged within a short height in the corona. The characteristics of a particularly interesting type III burst with a low-frequency cut-off are discussed. We argue that nearby loops were not disrupted by the shock and that the energetic electrons produced during the event must have been injected at several sites and guided along open field lines at large distances from the flare to produce type III bursts.     

Ionizing radiation detected by pioneer II

The total ionizing component of cosmic radiation was measured up to an altitude of 1550 km. An upper bound to the ratio of average-to-minimum specific ionization was determined by comparing the ionization with the count-rate observations from the Explorer IV satellite. This result implied that protons are the predominant species which gave rise to the observed ionization.     

Observations of the Rosette Nebula using the Decameter Wave Radio Telescope at Gauribidanur

A map of Rosette Nebula in continuum absorption is made at 34.5 MHz using the Decameter Wave Radio Telescoe at Gauribidanur, India, with a resolution of 26×40, is presented. These observations are combined with the 2700 MHz measurements of Grahamet al. (1982) to derive the electron temperature distribution across the nebula. It is found that the temperatures in the southeastern parts of the nebula are around 5000 K and increase up to 8000 K towards the northwestern regions. It is suggested that the lower electron temperatures in the southeastern regions are due to the presence of more dust there compared to other regions in the nebula.     

Third plasma harmonic radiation in type II bursts

We present observational evidence for simultaneous fundamental, second and third harmonic radio emission during an excessively strong type II burst on February 16, 1984. This burst was emitted from an active region behind the limb allowing for fair resolution of the wave bands. If interpreted as a triple harmonic system, three different, nearly equally probable mechanisms for higher harmonic emission are qualitatively discussed. These are a four-wave process which involves very strong Langmuir waves, a decay process first proposed by Cairns (1987) for higher harmonic emission near the Earth's bow shock, and time evolution of the emitted frequency during Langmuir wave collapse. In sufficiently strong coronal shock waves, both of the former mechanisms may be more efficient than under solar wind conditions. In the third mechanism, Langmuir wave collapse may be driven by strong electron beams as are expected to exist in quasiparallel shocks where electron reflection may be strongest. We discuss the differences between the signatures of these mechanisms.     

Decameter solar type III bursts: Data classification with use of cluster analysis

The many-parametric data on decametric type III bursts containing more than 1000 events were classified with use of cluster analysis, i.e., the pattern recognition procedure. For the classification such parameters have been used as intensity, duration and degree of circular polarization derived from burst time profiles. The automatic classification has resulted in division of daily samples of type III bursts into classes of bursts, more homogeneous statistically, which can be associated with different components of type III radiation distinguished by their physical origin. For the classes obtained, statistically reliable dependences of the mean intensity on source position can be found which allow one to conclude about the source structure and evolution, as well as about the characteristics defined by a burst generation mechanism and propagation effects.     

Mathematical Modeling of the Formation of Type IIId Solar Decameter Radio Bursts with Echo Components

Using numerical simulation, we investigate the possibility that echo components of type IIId solar decameter bursts can be produced through refraction of radio emission in a coronal plasma with large-scale regular electron density inhomogeneities. It is shown that the observed time profiles of radio burst intensity with three or four maxima can be related to the presence in the middle corona of streamers and a localized regular nonuniformity whose electron density exceeds background electron density by a factor of more than 3 – 4. The nonuniformity scale in this case is comparable with the optical diameter of the solar disk. The observed radio burst echo components with delays longer than 3 s are explained by the production of additional radio emission propagation modes within a “transverse" refraction waveguide arising between the localized electron density nonuniformity and deeper layers in the corona. As these additional modes are reflected from the streamers, they can reach the Earth. Calculations of the time profile of radio burst intensity take into consideration the influence of scattering by turbulent coronal inhomogeneities and of collisional absorption. Comparison of the modeling results with the observational data shows that the calculated values of some profile parameters differ from the observed values. One of the possible reasons is that the method used does not take into account the diffraction leakage of radio emission through the large-scale nonuniformity.     

The interaction of matter and radiation in the hot model of the Universe,II

Heating of the primaeval plasma prior to the epoch of recombination results in distortions in the Rayleigh-Jeans region of the microwave relic radiation spectrum (1–60 cm, or more exactly =2.5_–7/8 cm). The present observational data allow limits to be set to such energy injection from which follow upper limits to (a) the amount of antimatter in the universe; (b) the parameters of primaeval turbulence; and (c) the adiabatic fluctuation spectrum for small masses (M<10¹¹ M ).If the heating takes place prior to the epocht=10_1012/5 sec (and in particular at the annihilation of electron-positron pairs atT10⁸–10¹⁰ K,t<300 sec), no observable distortions are expected in the relic radiation spectrum. Here =/_crit is the dimensionless average density of matter in the universe.Translated from the Russian by D. F. Smith.     

An approximate analytical solution for self-isothermal flow behind a radiation driven shock wave,II

An analytical solution for a self-similar isothermal flow behind a radiation driven shock wave in non-uniform medium at rest is investigated using the method of Laumbach and Probstein (1969).     

Decameter Type IV Burst Associated with a Behind-the-limb CME Observed on 7 November 2013

We report on the results of observations of a type IV burst made by the Ukrainian Radio interferometer of the Academy of Sciences (URAN-2) in the frequency range 22?–?33 MHz. The burst is associated with a coronal mass ejection (CME) initiated by a behind-the-limb active region (N05E151) and was also observed by the Nançay Decameter Array (NDA) radio telescope in the frequency band 30?–?60 MHz. The purpose of the article is the determination of the source of this type IV burst. After analysis of the observational data obtained with the URAN-2, the NDA, the Solar-Terrestrial Relations Observatory (STEREO) A and B spacecraft, and the Solar and Heliospheric Observatory (SOHO) spacecraft, we come to the conclusion that the source of the burst is the core of a behind-the-limb CME. We conclude that the radio emission can escape the center of the CME core at a frequency of 60 MHz and originates from the periphery of the core at a frequency of 30 MHz that is due to occultation by the solar corona at the corresponding frequencies. We find plasma densities in these regions assuming the plasma mechanism of radio emission. We show that the frequency drift of the start of the type IV burst is governed by an expansion of the CME core. The type III bursts that were observed against this type IV burst are shown to be generated by fast electrons propagating through the CME core plasma. A type II burst was registered at frequencies of 44?–?64 MHz and 3?–?16 MHz and was radiated by a shock with velocities of about \(1000~\mbox{km}\,\mbox{s}^{-1}\) and \(800~\mbox{km}\,\mbox{s}^{-1}\), respectively.