S. Ananthakrishnan

Publications of S. Ananthakrishnan

• Occultation of the radio source 2019+098 (3C411) by comet 1983e (Sugano-Saigusa-Fujikawa)

  Authors: S. Ananthakrishnan, Dilip G. Banhatti

  03/2009;

  [This cometary occultation observation from June 1983 remained to be formally reported due to other preoccupations of the authors. It was presented in seminars to colleagues at Ooty, Bangalore and[This cometary occultation observation from June 1983 remained to be formally reported due to other preoccupations of the authors. It was presented in seminars to colleagues at Ooty, Bangalore and elsewhere. We now write it up as we have been asked about it by various colleagues at various times, and feel we owe it to them to put it firmly on record.] We planned and observed with Ooty Radio Telescope the occultation with Comet 1983e Sugano-Saigusa-Fujikawa of the extragalactic radio source 2019+098 = 3C411. The results are presented formally for the first time, along with a brief account of other cometary occultations and general background of planning, execution and interpretation of such observations which will be useful for other future observers. The occultation occurred at 07:52 IST on 12th June 1983. It amounted to 25% peak to peak fluctuation in the flux density of the radio source. The rough predicted occultation time was 07:24 IST. We interpret the results after refining the occultation time to allow for various effects.

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• Very long baseline IPS observations of the solar wind speed in the fast polar streams

  Authors: A. Pramesh Rao, S. Ananthakrishnan, V. Balasubramanian, William A. Coles

  07/1995;

  Observations of intensity scintillation (IPS) with two or more spaced antennas have been widely used to measure the solar wind velocity. Such methods are particularly valuable in regions whichObservations of intensity scintillation (IPS) with two or more spaced antennas have been widely used to measure the solar wind velocity. Such methods are particularly valuable in regions which spacecraft have not yet penetrated, but they are also very useful in improving the spatial temporal sampling of the solar wind, even in regions where spacecraft data are available. The principle of the measurement is to measure the time delay tau(sub d) between the scintillations observed with an antenna baseline b. The velocity estimate is just V = b/tau(sub d). The error in estimation of the time delay delta tau(sub d) is independent of the baseline length, thus the error in the velocity estimate delta V given by delta(V)/V approximately equals to (delta tau(sub d))/tau(sub d) is inversely proportional to tau(sub d) and hence to b. However the use of a long baseline b has a less obvious advantage; it provides a means for separating fast and slow contributions when both are present in the scattering region. Here we will present recent observations made using the large cylinder antenna at Ooty in the Nilgiri Hills of South India, and one of the 45 m dishes of GMRT near Pune in West-Central India. The baseline of 900 km is, by a considerable margin, the longest ever used for IPS and provides excellent velocity resolution. These results compared with the ULYSSES observations and other IPS measurements made closer to the sun with higher frequency instruments such as EISCAT and the VLBA will provide a precise measure of the velocity profile of the fast north-polar stream.

• Interplanetary scintillation observations of an unbiased sample of 90 Ooty occultation radio sources at 326.5~MHz

  Authors: Dilip G. Banhatti, S. Ananthakrishnan

  Monthly Notice of the Royal Astronomical Society. 01/1989; 240:117-127.

  We present 327-MHz interplanetary scintillation (IPS) observations of an unbiased sample of 90 extragalactic radio sources selected from the ninth Ooty lunar occultation list. The sources areWe present 327-MHz interplanetary scintillation (IPS) observations of an unbiased sample of 90 extragalactic radio sources selected from the ninth Ooty lunar occultation list. The sources are brighter than 0.75 Jy at 327 MHz and lie outside the Galactic Plane (|b| >= 10 degrees). We derive values of mu, the fraction of scintillating flux density, and psi, the equivalent Gausian diamete for the scintillating structure. Various correlations are found between the observed parameters. In particular, the scintillating component weakens and broadens with increasing largest angular size (LAS), and stronger scintillators have more compact scintillating components. The correlation between mu and the total source flux density found by previous workers is extended to smaller flux densities, but the three new points do not permit a distinction between a continuing upward trend or a levelling off of mu in the relation between the median values.

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• Metrewavelength fine structure in 30 extragalactic radio sources with sizes of a few arcsec

  Authors: Dilip G. Banhatti, S. Ananthakrishnan, A Pramesh Rao

  Monthly Notices of the Royal Astronomical Society. 01/1983; 205:585-591.

  We report interplanetary scintillation (IPS) observations at 327 MHz of an unbiased sample of 30 extragalactic radio sources having overall size between 1 and 4 arcsec, and flux densities greaterWe report interplanetary scintillation (IPS) observations at 327 MHz of an unbiased sample of 30 extragalactic radio sources having overall size between 1 and 4 arcsec, and flux densities greater than 1 Jy at 327 MHz. From VLBI observations, these sources have been reported to contain compact components of sizes <~ 0.02 arcsec contributing on an average about 25% of the total emission at 5 GHz. The IPS observations show that about 45% of the total emission at 327 MHz arises from structures of sizes between 0.05 & 0.5 arcsec (corresponding typically to 0.5 to 5 kpc). A comparison of the VLBI and IPS results indicates that the VLBI & IPS components probably refer to the same physical features in these sources; and that these features are probably either hotspots or steep-spectrum kiloparsec cores for about 2/3rds of the sources and may be flat-spectrum compact components for the rest.

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