Results of IPS Observations in the Period Near Solar Activity Minimum

IPS observations with the Big Scanning Array of Lebedev Physical Institute (BSA LPI) radio telescope at the frequency 111 MHz have been monitored since 2006. All the sources, about several hundred daily, with a scintillating flux greater than 0.2 Jy are recorded for 24 hours in the 16 beams of the radio telescope covering a sky strip of 8^° declination width. We present some results of IPS observations for the recent period of low solar activity considering a statistical ensemble of scintillating radio sources. The dependences of the averaged over ensemble scintillation index on heliocentric distance are considerably weaker than the dependence expected for a spherically symmetric geometry. The difference is especially pronounced in the year 2008 during the very deep solar activity minimum period. These features are explained by the influence of the heliospheric current sheet that is seen as a strong concentration of turbulent solar wind plasma aligned with the solar equatorial plane. A local maximum of the scintillation index is found in the anti-solar direction. Future prospects of IPS observations using BSA LPI are briefly discussed.     

Low Frequency Radio Emission and Star Formation in the Nuclei of Infrared Galaxies

Results of radio observations of luminous and ultraluminous infrared galaxies are discussed. The declining or flattening of the radio spectra of compact and (in some cases) extended sources found using interplanetary scintillation observations at 102 MHz is accounted for by absorption in thermal plasma, in agreement with the observed brightness of optical emission lines. It is argued that a region of violent star formation occupies the area of a molecular disc 1 kpc in size. The discrepancy between the linear sizes of the compact radio sources and the star forming regions is discussed.     

A search for compact decametric radio sources in supernova remnants using the interplanetary scintillation technique

Interplanetary scintillation observations of eleven supernova remnants and the pulsar J1939+2134, around which the existence of a supernova remnant remains obscure, were carried out with the largest in the world decameter radio telescope UTR-2 at 20, 25 and 30 MHz to determine if any of them contain compact radio sources with the angular size θ<5″. The sample included the young Galactic remnants and the other powerful SNRs. The interplanetary scintillations of the compact radio source in the Crab Nebula associated with the well-known pulsar J0534+2200 and the pulsar J1939+2134 were observed. Apart from the Crab Nebula, we have not detected a compact radio source in supernova remnants with the angular size θ<5″ and the flux density more than 10 Jy. The observations do not confirm the existence of the low frequency compact source in Cassiopeia A that has remained controversial.     

High-Redshift Radio Galaxies from Deep Fields

Most of the radio galaxies with z > 3 have been found using the red-shift spectral index correlation. We have started a programme with the Giant Metrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels about 100 times deeper than the known high-redshift radio galaxies, with an aim to detect candidate high-redshift radio galaxies. Here we present results from the deep 150 MHz observations of LBDS-Lynx field, which has been imaged at 327, 610 and 1412 MHz with the Westerbork Synthesis Radio Telescope (WSRT) and at 1400 and 4860 MHz with the Very Large Array (VLA). We find about 150 radio sources with spectra steeper than 1. About two-thirds of these are not detected in Sloan Digital Sky Survey (SDSS), hence are strong candidate high-redshift radio galaxies, which need to be further explored with deep infra-red imaging and spectroscopy to estimate the red-shift.     

Quiet Sun and slowly varying component at meter and decameter wavelengths

Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nançay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations.     

Monitoring of interplanetary and ionospheric scintillation of an ensemble of radio sources

The results of a series of 24-hour observations of radio-source interplanetary and ionospheric scintillation performed on April 4–10, 2006, at the Pushchino Radio Astronomy Observatory are presented. The observations were carried out with the Large Phased Array radio telescope of the Lebedev Institute of Physics, Russian Academy of Sciences, at a frequency of 110 MHz. The scintillating fluxes of all radio sources that fall within a field of sky between declinations +28° and +31° were automatically recorded applying eight beams of the reception pattern operating simultaneously. All of the sources with flux densities of 0.2 Jy or higher were detected. The structure functions of the flux fluctuations were measured for time shifts 1 and 10 s, which characterize the interplanetary (1 s) and ionospheric (10 s) scintillation, respectively. The mean scintillation index m _IPP (on a characteristic time scale of 1 s) of an ensemble of radio sources located within a sky band 4° wide in declination and 1 h wide in right ascension was measured as the parameter that characterizes the interplanetary plasma. Diurnal variations of the interplanetary scintillation index were determined. The maximum m _IPP value at daytime equals 0.3, and the minimum value at nighttime equals 0.10. Weak interday variations of the mean daytime and nighttime scintillation indices were detected. The ionospheric scintillation indices m _Ion are small compared to m _IPP at daytime, but m _Ion ? m _IPP at nighttime. On the whole, both the interplanetary plasma and ionosphere were quiet during the observations.     

325-MHz observations of the ELAIS-N1 field using the Giant Metrewave Radio Telescope

We present observations of the European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz using the Giant Metrewave Radio Telescope (GMRT), with the ultimate objective of identifying active galactic nuclei and starburst galaxies and examining their evolution with cosmic epoch. After combining the data from two different days we have achieved a median rms noise of  ≈40 μJy  beam⁻¹, which is the lowest that has been achieved at this frequency. We detect 1286 sources with a total flux density above  ≈270 μJy  . In this paper, we use our deep radio image to examine the spectral indices of these sources by comparing our flux density estimates with those of Garn et al. at 610 MHz with the GMRT, and surveys with the Very Large Array at 1400 MHz. We attempt to identify very steep spectrum sources which are likely to be either relic sources or high-redshift objects as well as inverted-spectra objects which could be Giga-Hertz Peaked Spectrum objects. We present the source counts, and report the possibility of a flattening in the normalized differential counts at low flux densities which has so far been reported at higher radio frequencies.     

A sharper view of the outer Galaxy at 1420 and 408 MHz from the Canadian Galactic Plane Survey – I. Revisiting the KR catalogue and new Gigahertz Peaked Spectrum sources

Arcminute-resolution radio continuum images at 408 and 1420 MHz from the Canadian Galactic Plane Survey (CGPS) have been used to re-examine radio sources listed in the Kallas & Reich catalogue. This catalogue is of particular interest to Galactic studies as it lists both extended and compact radio sources found in the second Galactic quadrant. We have determined the nature (extended versus compact, Galactic versus extragalactic) of all of these bright radio sources. A number of large H  ii regions with no optical counterparts are highlighted along with a sample of large radio galaxies. Many sources previously thought to be extended Galactic objects are shown to be point sources. A sample of point sources with flat or rising spectra between 408 and 1420 MHz has been compiled, and within this sample likely Gigahertz Peaked Spectrum sources have been identified.     

IPS observations at stelab for the fruitful coming decade

We have been carrying out solar wind measurements using the interplanetary scintillation (IPS) method. Our IPS observation system is operated at a frequency of 327MHz and consists of four stations located at Toyokawa, Fuji, Sugadaira and Kiso. The present system, however, has insufficient sensitivity to measure enough IPS sources for observing the solar wind with adequate spatial and temporal resolution. Therefore we have been excuting the upgrade project since 1994 in order to observe a larger number of compact radio sources. The Fuji system has been improved successfully and has achieved sensitivity by a factor over five compared with the previous system. The upgrade project is now in progress for the Toyokawa and Sugadaira station.     

Scintillation Measurements of the Solar Wind Velocity in Strong Scattering near the Sun

The multi-antenna scintillation method of measuring the solar-wind velocity has been very effective, particularly near the Sun and at high heliographic latitudes where direct measurements are rare or non-existent. However, scintillation observations inherently involve an LOS integration. Several methods have been used to deal with this problem, but they all require the basic assumption that contributions from different parts of the LOS add linearly. This assumption is valid for weak scintillations where the Born approximation holds, but it is not correct for strong scintillations. In this article we compare simultaneous observations of the same radio source, and therefore the same solar wind, at radio wavelengths of 32 cm and 92 cm. The 32-cm observations at the European Incoherent Scatter Radar (EISCAT) were made in weak-scattering and those at 92 cm at the Solar-Terrestrial Environment Laboratory (STEL) were made in strong-scattering mode. The results showed no significant bias in velocity caused by strong scattering, confirming that the LOS inversion techniques can be extended into the strong-scattering regime.     

Space Weather and Solar Wind Studies with OWFA

In this paper, we review the results of interplanetary scintillation (IPS) observations made with the legacy system of the Ooty Radio Telescope (ORT) and compare them with the possibilities opened by the upgraded ORT, the Ooty Wide Field Array (OWFA). The stability and the sensitivity of the legacy system of ORT allowed the regular monitoring of IPS on a grid of large number of radio sources and the results of these studies have been useful to understand the physical processes in the heliosphere and space weather events, such as coronal mass ejections, interaction regions and their propagation effects. In the case of OWFA, its wide bandwidth of 38 MHz, the large field-of-view of ~27^° and increased sensitivity provide a unique capability for the heliospheric science at 326.5 MHz. IPS observations with the OWFA would allow one to monitor more than 5000 sources per day. This, in turn, will lead to much improved studies of space weather events and solar wind plasma, overcoming the limitations faced with the legacy system. We also highlight some of the specific aspects of the OWFA, potentially relevant for the studies of coronal plasma and its turbulence characteristics.     

A search for local sources of the S-component at decameter wavelengths

In an effort to find local sources of the Slowly Varying Component (SVC), an analysis is made of the episodical observations carried out since 1972 during periods of low solar activity at 20 and 25 MHz. In contrast to other writers who reported on successful observations of such sources (Kundu et al., 1977; Sastry et al., 1981, 1983), we have not found local sources, though we used the UTR-2 radio telescope to observe from several to several tens brightness distributions of the quiet Sun per day.The multiple daily measurements allowed tracing the dynamics of the burst development. As has been found, bursts of high intensity can give rise to nonthermal radiation from the region of generation, thus producing a considerable increase in the maximum brightness temperature of scans across the Sun. The relaxation time of this process is 20 to 30 min. It is not improbable that this is the effect responsible for large variations in the maximum brightness temperatures of the scans that Sastry et al. (1981) connected with the SVC.25 MHz radioheliograms of April 29 and 30, 1976, are presented which supplement the data of observations at Clark Lake (Kundu et al., 1977). It is shown that the sources observed there on April 27 and April 29, 1976, were most likely of nonthermal nature.We conclude that at present, in spite of reports of some workers, there is no convincing evidence for the existence of local SVC sources at decameter wavelengths. Their reality could be confirmed or denied by more observations with better radio telescopes and a better account of the specific features of the decameter band.Formely Division of Radio Astronomy, Institute of Radiophysics and Electronics.     

Preliminary observations of solar radio sources with the Culgoora radioheliograph operating at four frequencies

The Culgoora radioheliograph has been modified for observing at 327.4 MHz, which is in addition to the three frequencies (43.25, 80, and 160 MHz) previously available. At the new frequency the array beamwidth is 56, which represents the highest resolution yet available for metre-wavelength solar mapping.At 327.4 MHz the sources of radio emission are mainly in the lowest layers of the corona. Some preliminary four-frequency observations have been made of type I storms. It is found that the source size generally decreases with increasing observing frequency. This result confirms earlier suggestions that the sources of both type I and type III emission are contained in structures whose boundaries diverge outwards in the corona.     

Decametric survey of discrete sources in the northern sky

The results of the very-low frequency sky survey of discrete sources made with the UTR-2 radio telescope within the declination zone 41° to 52° are presented. The UTR-2 radio source catalogue contains the estimates of the coordinates and flux densities of 432 sources measured at a number of the lowest frequencies used in contemporary radio astronomy within the range from 10 at 25 MHz. The questions of establishing the UTR-2 sky survey sensitivity, completeness and reliability of the resulting catalogue are considered. The coordinates of the sources measured at very low frequencies have been compared with respective data obtained in the 4C survey at 178 MHz.     

Broadband lane structures in the decametric spectra of Jupiter and the scintillation boundary

Dynamic spectra of Jovian noise storms at 20–30 MHz are compared with the scintillation indices of the overhead radio source Cas A at 45 MHz. Of six intense storms observed, four that displayed broadband lane patterns occurred on nights when the scintillation index of Cas A was unusually low, while two that did not display broadband lane structures occurred on nights when the scintillation index was high. It is evident that the scintillation boundary was located north of Oulu on the nights when the broadband lane patterns occurred and was therefore not involved in the lane-producing mechanism.     

Scintillations of cosmic radio sources in the decametre waveband

The effect of fluctuations of the interplanetary plasma and the ionosphere upon the scintillation spectra of radio sources at decametre waves is considered with due regard for the finite antenna aperture, fluctuation anisotropy, and the direction of their drift in space. It has been shown that scintillation due to interplanetary plasma (IPP), can be reliably separated from the ionospheric scintillation background at decametre wavelengths.For elongations between 90° to 150°, the IPP scintillation power spectrum observed in the 12.6–25 MHz waveband is of a power law form with the index 3.1±0.6, which is in close agreement with the values known for smaller elongations. The solar wind velocity projection orthogonal to the line of sight is estimated for elongations about 110° and has been found to be 300±80 km s^–1. As in the case of smaller elongations, the velocity dispersion is significant.At night, wideband spectra of ionospheric scintillations are observed in the decametre band, with the breaking point at approximately 0.01 Hz in the 12 m band, and narrow-band spectra whose cut-off frequency is below 0.01 Hz. The power spectrum of ionospheric scintillations is of a power-law form with the index 3.4±0.5. In some cases steeper spectra are observed.     

Mid-latitude radio-satellite scintillation: The height of the irregularities

The heights of the irregularities in ionospheric refractive index responsible for radio-satellite scintillation have been found from long-term spaced-receiver experiments at mid-latitude and sub-auroral locations using the 40 MHz transmissions of satellite BE-B. The important results relate to the diurnal, seasonal, and latitude variations of the irregularity heights. Night-time scintillation is usually produced in the F-region at all times of the year, and in particular in the top-side of the F-region in Summer. Day-time scintillation is rare at midlatitudes; at sub-auroral latitudes it may be produced at any height. The night-time height distributions show a constancy of height with latitude in Summer and Winter, although there is a slight increase poleward in average Autumn-Spring night-time heights. Only records displaying weak to moderate scintillation at 40 MHz were used in the analysis, and therefore the heights reported are those of irregularities producing such scintillation.     

Upper Limits on the Continuum Emission from Geminga at 74 and 326 MHz

We report a search for radio continuum emission from the gamma-ray pulsar Geminga. We have used the VLA to image the location of the optical counterpart of Geminga at 74 and 326 MHz. We detect no radio counterpart. We derive upper limits to the pulse-averaged flux density of Geminga, taking diffractive scintillation into account. We find that diffractive scintillation is probably quenched at 74 MHz and does not influence our upper limit, S<56 mJy (2 sigma), but that a 95% confidence level at 326 MHz is S<5 mJy. Owing to uncertainties on the other low-frequency detections and the possibility of intrinsic variability or extrinsic variability (refractive interstellar scintillation) or both, our nondetections are nominally consistent with these previous detections.