Spectral characteristics of solar S bursts

Observations of the solar radio spectrum have been made with high time and frequency resolution. Spectra were recorded over six 3-MHz bands between 30 and 82 MHz. The receivers used were capable of time and frequency resolutions of 1 ms and 2 kHz, respectively. A large number of radio bursts exhibiting a variety of find spectral structure were recorded.The bursts, referred to here as S bursts, were observed throughout the 30–82 MHz frequency range but were most numerous in the 33–44 MHz band and were very rare at 80 MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines with the centre frequency of each burst decreasing with time. The rate of frequency drift was about 1/3 that of type III bursts. Most bursts were observed over only a limited frequency range (< 5 MHz) but some drifted for more than 10 MHz. The durations measured at a single frequency and the instantaneous bandwidths of S bursts were small; t = 49 ± 34 ms and f = 123 ± 56 kHz for bursts observed near 40 MHz. A significant number had t 20 ms. Flux densities of S burst sources were estimated to fall in the range 10²³-5 × 10²¹ Wm^–1 Hz^–1.A small proportion (1–2%) of bursts showed a fine structure in which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands or fringes. The fringe spacing increased with wave frequency and was f - 90 kHz for fringes near 40 MHz. The bandwidths of fringes was narrow, often less than 30 kHz and in some cases down to 10–15 kHz.New address: Astronomy Program, University of Maryland, College Park, MD, U.S.A.     

Radio emission of the NGC 1499 nebula at decametric wavelengths

Observations of the ionized hydrogen region NGC 1499 have been carried out with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20 and 25 MHz. The half-power resolution of the instrument to zenith is 28×34 at 25 MHz. The average volume density of the non-thermal radio emission between the Sun and the nebula (1.75×10^–40 W m^–3 Hz^–1 ster^–1 at 25 MHz), the electron temperature of the HII nebula (T _e =4400 K), the measure of emission (ME=1500 cm^–6 pc) and other parameters have been obtained. Maps of brightness distribution over the source are presented for each observation frequency. The results are compared with previously obtained data.     

Observations of the Rosette nebula NGC 2237 at decametric wavelengths

The results of observations of the Rosette emission nebula NGC 2237 with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20.0 and 25.0 MHz are given in the shape of contours of constant brightness temperature. The half-power beamwidth of the telescope to zenith at 25.0 MHz was 28×38. Density weighted mean values for the non-thermal radio emissivity between the Sun and the source (7.9×10^–41 W m^–3 Hz^–1 ster^–1 at 25.0 MHz) and the ratio of the intensity of emissivity generated before the area and the intensity of galactic radio emissivity appearing beyond the area equal to 1.3 have been obtained. The electron temperatureT _e=3600 K, the optical depth (about ten at 25 MHz), the measure of emission (ME=3500 cm^–6 pc), the electron densityN _e=8 cm^–3 and the nebular mass 16.6×10⁺³ M have been determined. A comparison with other observation results has been made.     

Upper limits for high energy γ-rays in association with Vela bursts

A sea-level cosmic ray burst detector was in operation at the time of occurrence of 16 Vela bursts, between 1970 and 1973. No events were seen. Upper limits for primary -rays of 10¹¹ eV or higher were set at 10^–29 erg cm^–2 Hz^–1.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Radio and X-ray burst from PSR 0950+08

This article describes in detail a burst from PSR 0950+08 on July 29, 1992. This event was observed by two radio telescopes (separated by ~ 200 km) operating at 103 MHz. There exists a very convincing indirect evidence that at the same time the pulsar also emitted large X-ray flux. The X-ray flux during the event compares with that during a solar X-ray flare. During the event the Sun was extraordinarily quiet as the solar X-ray flux 3 · 10^–7 W/m² only was observed. The cause for the burst is quite unknown and may be complex. However, a possibility of accretion of a comet-like object by pulsar may provide reasonable explanation of the observations. These results open some interesting questions about the pulsar physics.     

Fractal dimensions of the time variation of solar radio emission

The fractal dimensions of solar radio fluxes at 245, 410, 610, 1415, 2695, 2800, 4995, 8800, and 15400 MHz are calculated for the data period 1976–1990. The fractal dimension used here is an index to quantify the time variability of radio emission. The fractal dimensions were found to have values in the range of 1.2–2.0 for time scales of 10 days, 1–10 months, and 10 months. The lowest values were found around 3 GHz. The annual variations of fractal dimensions are small and are not in concert with the solar cycle for most of the fractal dimension at the analyzed frequencies except those for 4995 and 8800 MHz. The annual variations of the fractal dimensions are similar for the sunspot number and radio emission around 3 GHz; this implies a close relation between them. According to a simulation, larger fractal dimensions correspond to shorter e-folding time constants in the distribution of radio-source lifetimes.     

A high resolution study in time,position, intensity,and frequency of the radio event of January 14, 1971

A high resolution study in time, frequency, position, and intensity was made at 169 MHz and neighbouring frequencies of the solar radio event of 1971, January 14, 11^h 20^m–30^m UT. The event consisted of two closely resembling groups of type III bursts and type II like details.Before, during, and after the outburst a stationary type IV continuum was seen, with small amplitude pulsating structure. The size of the pulsating structure (which was located inside the continuum) was considerably smaller than the continuum size, and in agreement with an explanation by fluctuating magnetic inhomogeneities inside the continuum source.The continuum moved outward after each outburst at a high speed (2–4000 km s^–1). After the second event the continuum source returned inward slowly ( 450 km s^–1). The outward motion is discussed. It can be explained by a combination of the impact of a fast magnetohydrodynamic shock and the injection of highly energetic particles during the event, the required number being also necessary to account for the observed radio flux. The slow returning motion can be related to mhd restoring of the magnetic field configuration.     

Evolution of the spectral fine structure of Jupiter's decametric S-storms

High-resolution spectra of Jupiter's decametric S-storms are studied with an acousto-optical radio spectrograph operating over the frequency range 20–30 MHz. In 1985–1989 20 S-storms were recorded in the Io-B region. There is only a slight average zoning effect of certain types of fine structure in the Io-B region, with sporadic S-bursts occurring most often in the early CML values, and S-trains in the late values. Emissions of type N and its variants occur at lower values of the Io phase than S-emissions and their variants. There is no exact storm-to-storm correspondence, nor any Io-B-centered zones in which the various types of fine structure could be accurately placed. Every storm is different and has a signature of its own. An important exception is formed by the wide-range quasi-periodic FDS-S storms that occur at the edge of the Io-B region with Io phase values greater than 80 . These are outstanding storms in which the individual bursts may extend across the full spectral width of 20–30 MHz and be repeated in rapid succession at quasi-periodic rates of 20–40 s^–1. It is suggested that these be referred to as type Q storms. It is estimated that only 10% of the S-burst types are recorded so far.     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

Discovery of two cosmic X-ray bursts in 1970

Two bursts of high-energy photons have been discovered during analysis of 2 1/2 years of data from NRL's solar X-ray detector on OSO-6. Both bursts were simultaneously observed by the OGO-5 hard X-ray spectrometer (Kane, 1975). The bursts occurred at about 18 087 s UT on 25 January, 1970, and about 56 532 s UT on 1 October, 1970. The October event was also observed by Vela 5A; however, none of the Vela detectors observed the January event which had an intensity of about 2×10^–5 ergs cm^–2. Based on these new data, the number of bursts with intensities above about 10^–5 ergs cm^–2 appears to be about 50% higher than the Vela data alone would indicate.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

On the spectra of type-III solar radio bursts observed at low frequencies

The spectra of strong bursts observed at low frequencies by OGO-5 during 1968–1970 are presented. They usually exhibit an intense main peak between 100 kHz and 1 MHz, and sometimes a less intense secondary peak between 1 and 3.5 MHz. Main peaks of 10^–12 Wm^–2 Hz^–1 or more were obtained in very strong events, but because of antenna calibration problems those could be one or two orders of magnitude too high. Recently published work supports the finding that type III bursts at low frequencies can be at least four orders of magnitude more intense than at ground-based frequencies of observation. It is found that the energy received at the Earth increases with decreasing frequency approximately as f ^–n, where 3 n 4.     

19 GHz (1.58 cm) solar radio bursts in the period July 1967 to December 1969

The results of 21/2 yr (July 1967–December 1969) monitoring of solar radio bursts at 19 GHz ( = 1.58 cm) at the Radio and Space Research Station, Slough, are presented. Observations at this frequency are important in helping to define the form of the microwave spectrum of solar bursts since many of the more intense bursts have their spectral peak in the frequency region above 10 GHz. Fifteen bursts with peak flux increases exceeding 1000 × 10^–22 Wm^–2 Hz^–1 were observed during this period.     

Flare fragmentation and type III productivity in the 1980 June 27 flare

We present observations of the solar flare on 1980 June 27, 16:14–16:33 UT, which was observed by a balloon-borne 300 cm² phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200–400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of 7.6 × 10²⁷ photons keV^–1 per type III burst at 20 keV. The occurrence of 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile.The generation of a superhot isothermal HXR component in the decay phase of the flare coincides with the fade-out of type III production.Universities Space Research Associates.ST Systems Corporation.     

Periodicities in the Time Series of Solar Coronal Radio Emissions and Chromospheric uv Emission Lines

A spectral analysis of the time series of daily values of ten solar coronal radio emissions in the range 275–1755 MHz, the 2800 MHz radio flux, several UV emission lines in the chromosphere and in the transition region, and sunspot number, for six successive intervals of 132 days each, during June 1997–July 1999 (26 months) showed that the spectral characteristics were not the same for all intervals. Details are presented for Interval 1, where there was no 27-day oscillation, and Interval 2, where there was a strong 27-day oscillation. In every interval, periodicities were remarkably similar in most of these indices, indicating that the solar atmosphere (chromosphere and corona) rotated as one block, up to a height of 150000 km. Above this height, the periodicities became obscure. Near the solar surface, sunspots showed extra or different periodicities, some of which vanished at low altitudes. For the 27-day feature as also for the long-term rise during 1996–1998, the maximum percentage changes were for radio emissions near 1350–1620 MHz.     

The interpretation of non-linear radio spectra of discrete radio sources by a general mechanism

The analysis of discrete radio sources spectra in the range 10–5000 MHz reveals that deviations from a power law in the low-frequency region may be due to distortion of differential energy spectra of relativistic electrons at low energies. An empirical expression for an energy-spectrum law was found to be in a good agreement with most of the radio spectra measured. The main physical parameters of 92 sources are evaluated. It is concluded that a low-energy electron excess exists with respect to the lawE ^– in most of the discrete sources which radiate non-linear low-frequency spectra.The forms of radio and energy spectra are further considered in a logarithmic scale.     

A model for solar radio pulsations at short centimetric band

In this paper, the observed solar radio pulsations during the bursts at 9.375 GHz are considered to be excited by some plasma instability. Under the condition of the conservation of energy in the wave-particle interaction, the saturation time of plasma instabilities is inversely proportional to the initial radiation intensity, which may explain why the repetition rate of the pulsations is directly proportional to the radio burst flux at 9.375 GHz as well as 15 GHz and 22 GHz. It is also predicted that the energy released in an individual pulse increases with increasing the flux of radio bursts, the modularity of the pulsations decreases with increasing the flux of radio bursts, these predictions are consistent with the statistical results at 9.375 GHz in different events. The energy density of the non-thermal particles in these events is estimated from the properties of pulsation. For the typical values of the ambient plasma density (10⁹ cm^–3) and the ratio between the nonthermal and ambient electrons (10^–4), the order of magnitude of the energy density and the average energy of the nonthermal electrons is 10^–4 erg/cm³ and 10 kev, respectively. It is interesting that there are two branches in a statistical relation between the repetition rate and the radio burst flux in a special event on March 11–17, 1989, which just corresponds to two different orders of magnitude for the quasi-quantized energy released in these five bursts. This result may be explained by the different ratios between the thermal and the nonthermal radiations.     

A mechanism for producing plasma radiation in the gigahertz range by precipitating high-energy protons

Gamma-ray observations are discussed to determine the density of protons of about 1 MeV precipitating to the photosphere. It is shown that Coulomb collisions will produce a positive slope in the proton distribution for energies less than 0.1 MeV for traversed column depths greater than 10¹⁸ cm ^–2. This could lead to plasma wave emission and radiation near the plasma frequency for densities 3.1 × 10¹⁰ cm^–3 and temperatures 4.0 × 10⁴ K where collisional and collisionless damping of the plasma waves is sufficiently weak. It is expected that these conditions will only be satisfied sporadically which leads to stationary radio emission limited in frequency and time. Recent radio observations of impulsive phase non-drifting patches in the 1–3 GHz range with duration 2–4 s are presented which could be produced by this mechanism.     

Infrared emission of dust grains in the clusters of galaxies

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 10⁷–10⁸ yr at a gas concentrationn _g 10^–3 cm^–3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300 are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near =100 and the same for the Perseus cluster near 50–70. The total fluxes of IR emission from these clusters are about 10⁵–10⁶ Jy and can be detected by modern observational methods.     

Identification of a solar X-ray source using D layer ionization behavior during an eclipse

Semi-quantitative reports of shortwave radio reception during the 12 November 1966 total solar eclipse have been used to determine the characteristics of a major source of D layer ionization. An effective electron depletion coefficient of 1.2 × 10^–2 sec^–1 was found empirically and used to reduce the data. Analysis of the radio absorption shows the source was located near heliographic coordinates, B = +7°, L = 340° and was probably less than 0.5 of arc in diameter. At the time of the eclipse, the source accounted for 40% of the radio absorption on a single, vertical pass through the D layer. Preoccultation behavior of the signal strength is interpreted by assuming a portion of the source X-ray flux was reflected at grazing incidence from the limb of moon. For point sources, such reflections have specific chromatic characteristics which were used to derive a crude source spectrum in the 3–140 Å range. X-ray absorption edges of N, O, Na, Be, and possible Mg, Ar, Ne, Ca, and K arising from the terrestrial atmosphere have been identified. A source temperature approximately twice that of the rest of the corona is indicated.     

Solar microwave bursts as indicators of the occurrence of solar proton emission

A study has been made of the relation of 19 GHz( = 1.58 cm) solar radio bursts to solar proton emission, with particular reference to the usefulness of relatively long duration bursts with intensities exceeding 50% of the quiet Sun flux (or exceeding 350 × 10^–22 W m^–2 Hz^–1) as indicators of the occurrence of proton events during the four years from 1966–69. 76 to 88% of such bursts are directly associated with solar protons and 60 to 85% of the moderate to large proton events in the four year period could have been predicted from these bursts. The complete microwave spectra of the proton events have also been studied, and have been used to extend the results obtained at 19 GHz to other frequencies, particularly in the 5–20 GHz band. The widely used frequency of 2.8 GHz is not the optimum frequency for this purpose since proton events have a minimum of emission in this region. Most of the radio energy of proton events is at frequencies above 10 GHz. The radio spectra of proton events tend to peak at higher frequencies than most non-proton events, the overall range being 5 to 70 GHz, with a median of 10–12 GHz and a mean of 17 GHz.On leave from the Radio and Space Research Station, Slough, England, as 1969–1970 National Research Council-National Academy of Sciences Senior Post-Doctoral Research Associate at AFCRL.