Imaging observations of radio bursts at meter-decameter wavelengths

In this article, we review some of the recent results obtained with imaging observations of the Sun at meter-decameter wavelengths, using the Clark Lake multifrequency radioheliograph. We briefly discuss the use of imaging observations to study the large scale structure of the upper corona. We discuss non-flare associated type II/type IV bursts associated with a coronal streamer disruption event associated with a slow (100 Km/s) CME. We discuss meter-decameter microbursts, which occur at coronal heights, often without any surface activity. Finally, we discuss a correlated type III burst whose emission originates almost simultaneously from two widely separated ( 10⁵ Km) locations.     

Ionospheric refraction in radio source observations at long radio wavelengths

Ionospheric refraction effects encountered in radio source observations in the 30 to 75 MHz range with the Clark Lake TPT telescope are discussed. It is found that simple calibration procedures are sufficient to provide positions of unknown sources with an accuracy of approximately one arcmin. Observations made near sunrise, or during disturbed ionospheric conditions must be discarded. If no corrections are applied, RMS errors of a few arcmin are to be expected. On leave at Radiosterrenwacht Dwingeloo, The Netherlands     

Imaging observations of radio bursts at meter-decameter wavelengths

Solar Physics - In this article, we review some of the recent results obtained with imaging observations of the Sun at meter-decameter wavelengths, using the Clark Lake multifrequency...     

VLA observations of a radio plage at centimeter wavelengths

VLA observations of a solar plage region at 6 and 20 cm wavelengths are presented. The high frequency 6 cm emission correlates well with the associated sunspots, whereas 20 cm emission shows good correlation with the H plage. Large temperature variations over a period of one day are observed in the plage associated component without any significant changes in the sunspots. The dominant emission mechanisms at 6 and 20 cm are found to be gyroresonance radiation and bremstrahlung respectively. It is concluded that the coronal condensation above the chromospheric H plage has an electron density of 5 × 10⁹ cm^–3 and it extends to a height of 5 × 10⁴ km.     

High resolution interferometric observations of Venus at three radio wavelengths

We present new 6.0 and 21.1 cm interferometric observations of Venus. When combined with our previous 3.12 cm work they provide s self-consistent set of high-resolution observations at three wavelengths covering a range in which the opacity of the Venus atmosphere varies by a factor of 50. Model calculations indicate that a model atmosphere of CO₂ in adiabatic equilibrium containing uniformly mixed gaseous absorbers surrounding a dielectric sphere cannot simultaneously and adequately predict the radio interferometric measurements at all wavelengths together with the radar and radio occultation measurements.     

Sakurai's object: the ionized nebula at radio wavelengths

Sakurai's object (V4334 Sgr) is a planetary nebula nucleus which is undergoing its final helium shell flash. This is the first of these rare and important events to be observable with non-optical instruments. We report the first radio detection, using a short (2-h) observation with the Very Large Array (VLA) at 4.86 GHz. The radio emission structure is coincident with the 34-arcsec diameter planetary nebula seen in optical emission lines. We find a statistical distance ∼ 3.8 ± 0.6 kpc, with a range of 1.9 <  D  < 5.3 kpc, depending on the planetary nebula (PN) mass. While we have no direct evidence for a new (post-flash) stellar wind, we estimate an upper limit to the mass-loss rate due to any such wind of 1.7 × 10⁻⁷ M⊙ yr⁻¹. The number of emitting knots in the radio-visible nebula indicates an electron density of ∼ 2 × 10⁸ m⁻³ in those knots, and a total emitting ionized mass of ∼ 0.15 M⊙, at an assumed distance of 3.8 kpc. The radio flux density indicates an Hβ flux of ∼ 6 × 10⁻¹⁶ W m⁻², suggesting an extinction E ( B  −  V ) ∼ 1.15, comparable with reddening estimates in the direction of V4334 Sgr.     

Solar radio bursts with a spectral flattening at millimeter wavelengths

An analysis of solar radio burst spectra in the range 3–80 GHz is carried out using measurements of the observatories at Bern and Nobeyama supplemented by data from worldwide network stations. Special interest was focused on strong events at frequencies above 30 GHz. It is found that there exists an extended group of events with a flattening of the spectra at millimeter wavelengths. In particular, two types of flattening are observed: (i) a high-frequency flattening either following a monotonic spectral flux increase at cm-waves or forming a flat broad-band spectrum at mm-wavelengths ; (ii) a millimetric flattening as a decrease of the slope (i.e., a hardening) of the descending branch of the spectrum having a peak in the microwave range. Besides this, in complicated bursts a strong temporal evolution of millimeter spectra may occur resulting in either type of the flattening. Some factors capable of producing the millimeter flattening are considered: (1) superposition of multiple source regions of gyrosynchrotron radiation, (2) gyromagnetic radiation from a two-component energy spectrum of the accelerated electrons at high energies, or by a temporal hardening of the electron spectrum during extended flares, and (3) optically thin bremsstrahlung of evaporated plasma.Presented at the CESRA Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Fine Structure of the solar atmosphere from near-limb observations in three wavelengths

We analyze a time series of high-resolution observations near the limb, in the continuum, in the Mg b ₁ line (-0.4 Å off line center), and in the wings of H (+0.75 Å). The observations were obtained with a CCD camera at the Vacuum Tower telescope of the Sacramento Peak Observatory. We study the association of facular points, as they appear at different heights of the solar atmosphere (continuum, b ₁ - 0.4 Å, H + 0.75 Å) with the feet of fine dark mottles.     

Saturn's rings: Particle composition and size distribution as constrained by observations at microwave wavelengths: II. Radio interferometric observations

The sizes, composition, and number of particles comprising the rings of Saturn may be meaningfully constrained by a combination of radar- and radio-astronomical observations. In a previous paper, we have discussed constraints obtained from radar observations. In this paper, we discuss the constraints imposed by complementary “passive” radio observations at similar wavelengths. First, we present theoretical models of the brightness of Saturn's rings at microwave wavelengths (0.34–21.0 cm), including both intrinsic ring emission and diffuse scattering by the rings of the planetary emission. The models are accurate simulations of the behavior of realistic ring particles and are parameterized only by particle composition and size distribution, and ring optical depth. Second, we have reanalyzed several previously existing sets of interferometric observations of the Saturn system at 0.83-, 3.71-, 6.0-, 11.1-, and 21.0-cm wavelengths. These observations all have spatial resolution sufficient to resolve the rings and planetary disk, and most have resolution sufficient to resolve the ring-occulted region of the disk as well. Using our ring models and a realistic model of the planetary brightness distribution, we are able to establish improved constraints on the properties of the rings. In particular, we find that: (a) the maximum optical depth in the rings is ～ 1.5 ± 0.3 referred to visible wavelengths; (b) a significant decrease in ring optical depth from λ3.7 to λ21.0 cm allows us to rule out the possibility that more than ～30% of the cross section of the rings is composed of particles larger than a meter or so; this assertion is essentially independent of uncertainties in particle adsorption coefficient; and (c) the ring particles cannot be primarily of silicate composition, independently of particle size, and the particles cannot be primarily smaller than ～0.1 cm, independently of composition.     

Extragalactic sources with asymmetric radio structure II. further observations of the quasar B2 1320 + 299

We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299, which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to help clarify the situation.     

Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.     

Radio observations of filaments at metric and decimetric wavelengths

A filament eruption at decimetric wavelength is illustrated here, involving a quiescent filament seen in absorption. A CME occurs in the vicinity of the event. This revised version was published online in July 2006 with corrections to the Cover Date.     

The radio radius of the Sun at millimeter and centimeter wavelengths

The radio radius of the Sun is determined from an analysis of the radio contact times of the 7 March, 1970 and 10 July, 1972 solar eclipses from = 3 mm to = 31 cm. Agreement with other eclipse measurements is good. A best fit curve through the several points gives the radio radius to within approximately ±0.01 of the photosheric radius below -5 cm.     

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.     

Simultaneous multifrequency observations of an eruptive prominence at millimeter wavelengths

Radio images and spectra of an eruptive prominence were obtained from simultaneous multifrequency observations at 36 GHz, 89 GHz, and 110 GHz on May 28, 1991 with the 45-m radio telescope at Nobeyama Radio Observatory (NRO), the National Astronomical Observatory, Japan (NAOJ). The radio spectra indicated that the optical depth is rather thick at 36 GHz whereas it is thin at 89 and 110 GHz. The H data, taken at Norikura Solar Observatory, NAOJ, suggest that the eruption of an active region filament was triggered by an H flare. The shape and position of the radio prominence generally coincided with those of H images. The radio emission is explained with an isothermal cool thread model. A lower limit for the electron temperature of the cool threads is estimated to be 6100 K. The range of the surface filling factors of the cool threads is 0.3–1.0 after the H flare, and 0.2–0.5 in the descending phase of the eruptive prominence. The column emission measure and the electron number density are estimated to be of the order of 10²⁸ cm^–5 and 10¹⁰ cm^–3, respectively. The physical parameters of a quiescent prominence are also estimated from the observations. The filling factors of the eruptive prominence are smaller than those of the quiescent prominence, whereas the emission measures and the electron densities are similar. These facts imply that each cool thread of the prominence did not expand after the eruption, while the total volume of the prominence increased.     

VLF goniometer observations at halley bay,Antarctica—II. Magnetospheric structure deduced from whistler observations

On 26 July 1967, a magnetically quiet day (ΣK_p = 12?) with high whistler activity at Halley Bay, it was found possible, by measurement of whistler nose-frequency and dispersion and the bearings of the whistler exit points, to make a detailed study of the magnetospheric structure associated with the whistler ducts.During the period 0509–2305 UT most of the exit points of whistlers inside the plasmasphere were situated along a strip about 100km wide passing through Halley Bay in an azimuthal direction 30°E of N between 57° and 62° invariant latitude. A mechanism which can give rise to such a well-defined locus which co-rotates with the Earth is not clear. Nevertheless, it does appear that the locus coincides with the contour of solar zenith angle 102° at 1800 UT 25 July. This was also the time of occurrence of a sub-storm and it is suggested that the magnetospheric structure was initiated by proton precipitation along the solar zenith angle 102° contour.At mid-day knee-whistlers observed outside the plasmapause had exit points which were closely aligned along an L-shell at an invariant latitude of 62.5°. They exhibited a marked variation (~ 3:1) in electron tube content over about 12° of invariant longitude and a drift of about 8 msec^?1 to lower L-shells.Throughout the period of observation the plasmapause lay about 2° polewards of the mean position found by Carpenter (1968) for moderately disturbed days.     

Structure of radio sources at wavelengths 10.5 and 12.0 cm from observations of the Solar eclipse on March 29, 2006

The eclipse observations were performed at the Laboratory of Radio Astronomy of the CrAO in Katsiveli with stationary instrumentation of the Solar Patrol at wavelengths of 10.5 and 12.0 cm. The data obtained were used to determine the brightness temperature of the undisturbed Sun at solar activity minimum between 11-year cycles 23 and 24: T _d10.5 = (43.7 ± 0.5) × 10³ K at 10.5 cm and T _d12.0 = (51.8 ± 0.5) × 10³ K at 12.0 cm. The radio brightness distribution above the limb group of sunspots NOAA 0866 was calculated. It shows that at both wavelengths the source consisted of a compact bright nucleus about 50 × 10³ km in size with temperatures T _b10.5 = 0.94 × 10⁶ K and T _b12.0 = 2.15 × 10⁶ K located, respectively, at heights h _10.5 = 33.5 × 10³ km and h _12.0 = 43.3 × 10³ km above the sunspot and an extended halo with a temperature T _b = (230–300) × 10³ K stretching to a height of 157 × 10³ km above the photosphere. The revealed spatial structure of the local source is consistent with the universally accepted assumption that the radiation from the bright part of the source is generated by electrons in the sunspot magnetic fields at the second-third cyclotron frequency harmonics and that the halo is the bremsstrahlung of thermal electrons in the coronal condensation forming an active region. According to the eclipse results, the electron density near the upper boundary of the condensation was N _e ≈ 2.3 × 10⁸ cm^?3, while the optical depth was τ ≈ 0.1 at an electron temperature T _e ≈ 10⁶ K. Thus, the observations of the March 29, 2006 eclipse have allowed the height of the coronal condensation at solar activity minimum to be experimentally determined and the physical parameters of the plasma near its upper boundary to be estimated.     

Further observations of radio sources from the BG survey. II. Mainly extragalactic

New observations of six fields from the low galactic latitude BG survey are presented. The radio sources within each of these fields appear to be of a predominantly extragalactic nature. However, one object is in close positional proximity to the pulsar 1922 + 20 and the possibility of their association is considered. The extended source BGE 0323 + 65 is shown to be unlikely to exist.     

Diffuse radio emission from the Coma cluster of galaxies at decametre wavelengths

We have observed the region of the Coma cluster at 34.5 MHz with a resolution of 26 arcmin × 40 arcmin. A map of the diffuse halo (Coma C) is presented. The size of the halo is found to be 54 arcmin × 30 arcmin. The position angle is 50° ± 10° and the integrated flux is 60 ± 11 Jy. We have also found an extended source to the south of Coma A. The measured half-power widths of this source are 30 arcmin × 40 arcmin. The position angle is 135° and the integrated flux is ~ 15 Jy at 34.5 MHz. The spectral index in the frequency range 408 to 34.5 MHz is -1.0. It is suggested that this source also belongs to the Coma cluster.     

Comparison of the FLUKA calculations with CAPRICE94 data on muons in atmosphere

In order to benchmark the three-dimensional calculation of the atmospheric neutrino flux based on the FLUKA Monte Carlo code, muon fluxes in the atmosphere have been computed and compared with data taken by the CAPRICE94 experiment at ground level and at different altitudes in the atmosphere. For this purpose only two additions have been introduced with respect to the neutrino flux calculation: the specific solar modulation corresponding to the period of data taking and the bending of charged particles in the atmosphere. Results are in good agreement with experimental data, although improvements in the model are possible. At this level, however, it is not possible to disentangle the interplay between the primary flux and the interaction model.