Hari Om Vats

Comparison between the extended-medium and the phase-screen scintillation theories

Abstract By numerically solving the fourth moment equation, comparisons are made between (i) situations in which fluctuations of refractive index extend uniformly from the transmitter to the receiver and (ii) situations in which the fluctuations are confined to a phase screen. The comparisons are made for inverse power-law phase spectra having a total mean square fluctuation of phase (ΔΦ) 2 , an outer scale L0 and an inner scale 0.01 L0. Four values of the spectral index are investigated, including the Kolmogoroff value. The comparisons are made for a wavelength such that the Fresnel scale is intermediate between the outer scale and the inner scale, so that both refractive and diffractive scattering are involved. The study extends from situations dominated by weak diffractive scattering up to ones dominated by multiple refractive scattering. It is found that, provided that the equivalent screen (i) is centrally located, (ii) has the same type of fluctuation spectrum and (iii) generates the same value of (ΔΦ) 2 as the extended medium, then the scintillation index and the intensity autocorrelation function in the reception plane are approximately the same in the two cases. Results are very dependent on how many refractive scatterings occur between the transmitter and the receiver, but they are not very dependent on how these refractive scatterings are distributed along the path.

Amplitude scintillations of ATS-6 radio beacon signals within the equatorial electrojet region (Ootacamund, dip 4° N)

The recordings of the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25% of cases) around 0900 hr and 1400 hr during the daytime.During the daytime, the scintillation decreases approximately as the inverse of the frequency for higher frequencies while for lower frequencies the law is valid till scintillation index at 40 MHz does not exceed 0.9. The temporal variation of daytime scintillation shows impulsive character, the duration of activity lasts for 1–2 hours at a time.During the nighttime, the scintillation decreases inversely with frequency for weak and moderate scintillation activity. The scintillation index at 360 MHz becomes independent of that at 140 MHz when the index at 140 MHz exceeds 0.85. For the set of frequencies 40–140 MHz, on some occasions scintillation index at 40 MHz is seen to be less than that at 140 MHz. The nighttime scintillations are in general stronger and remain so for extended length of time.The daytime scintillations are suggested to be due to blanketing or some other non-q type of sporadicE layer. The nighttime scintillations are most probably due to spreadF condition and the abnormal frequency variation of the scintillations may be due to multiple scattering layer during periods of intense spreadF.

Quantitative explanation of strong multi-frequency intensity scintillation spectra using refractive scattering

Abstract Under evening equatorial conditions, strong intensity spectra observed simultaneously on transmissions through the ionospheric F-region at 40, 140 and 360 MHz from a stationary satellite are explained quantitatively in terms of refractive scattering using the approach of Booker and Majidiahi (1981). Use is made of an outer scale (wavelength/2gp) of 50 km, an inner scale of 5 m and an integrated mean square fluctuation of ionization density [ ∝ (ΔN) 2 dz ] of 1028 m−5. The spectral index required to fit the observations is 3, and no major departure from this value is permissible either way. This produces the correct spectral behavior at high fluctuation frequencies and the correct ratios of the upper roll-off frequencies at the three wave frequencies. The RMS fluctuation of phase is about 130 rad at 360 MHz, 340 rad at 140 MHz and 1200 rad at 40 MHz. At 40MHz the scale of the intensity fluctuation at ground level is about 10m, and this is caused by refractive scattering in the ionosphere at scales of the order of the outer scale; removal of practically all fluctuations in the ionosphere at scales below the Fresnel scale leaves the fine structure of the intensity spectrum at ground level virtually unaffected.

Differential rotation measurement of soft X-ray corona

The aim of this paper is to study the latitudinal variation in the solar rotation in soft X-ray corona. The time series bins are formed on different latitude regions of the solar full disc (SFD) images that extend from 80°S to 80°N. These SFD images are obtained with the soft X-ray telescope (SXT) on board the Yohkoh solar observatory. The autocorrelation analyses are performed with the time series that track the SXR flux modulations in the solar corona. Then for each year, extending from 1992 to 2001, we obtain the coronal sidereal rotation rate as a function of the latitude. The present analysis from SXR radiation reveals that: (i) the equatorial rotation rate of the corona is comparable to the rotation rate of the photosphere and the chromosphere, (ii) the differential profile with respect to the latitude varies throughout the period of the study; it was more in the year 1999 and least in 1994, and (iii) the equatorial rotation period varies systematically with sunspot numbers and indicates its dependence on the phases of the solar activity cycle.

EMPIRICAL DETERMINATION OF CONVECTION PARAMETERS IN WHITE DWARFS. I. WHOLE EARTH TELESCOPE OBSERVATIONS OF EC14012-1446 ∗

We report on an analysis of 308.3?hr of high-speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 ?Hz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l = 1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004-2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41?s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1?m, McDonald 2.1?m, and KPNO 2?m telescopes. The fits indicate a time-averaged convective response timescale of ?0 = 99.4 ? 17?s, a temperature exponent N = 85 ? 6.2, and an inclination angle of ? i = 329 ? 32. We present our current empirical map of the convective response timescale across the DA instability strip.

Discovery of Variation in Solar Coronal Rotation with Altitude

Here we report the first measure in radio emission of differential rotation as a function of height in the solar corona. This is derived from the disk-integrated simultaneous daily measurements of solar flux at 11 radio frequencies in the range of 275-2800 MHz. Based on the model calculations, these radio emissions originate from the solar corona in the estimated average height range of ~(6-15) × 104 km above the photosphere. The investigations indicate that the sidereal rotation period at the highest frequency (2800 MHz), which originates from the lower corona around 6 × 104 km, is ~24.1 days. The sidereal rotation period decreases with height to ~23.7 days at the lower frequency (405 MHz), which originates at ~13 × 104 km. It is difficult to identify clearly the rotational modulation at 275 MHz, perhaps because these emissions are significantly affected by the turbulence in the intervening medium. Since these investigations are based on disk-integrated solar flux at radio frequencies, it is difficult to say whether these systematic variations in sidereal rotation period are partly due to the latitudinal differential rotation of the solar corona. It will be interesting to investigate this possibility in the future.

Periodicities in the coronal rotation and sunspot numbers

This study is an attempt to investigate the long-term variations in coronal rotation by analysing the time-series of the solar radio emission data at 2.8 GHz frequency for the period 1947-2009. Here, daily adjusted radio flux (known as Penticton flux) data are used. The autocorrelation analysis shows that the rotation period varies between 19.0-29.5 sidereal days (mean sidereal rotation period is 24.3 d). This variation in the coronal rotation period shows evidence of two components in the variation: (1) 22-yr component which may be related to the solar magnetic field reversal cycle or Hales cycle; and (2) a component which is irregular in nature, but dominates over the other components. The cross-correlation analysis between the annual average sunspot number and the coronal rotation period also shows evidence of its correlation with 22-yr Hales cycle. The 22-yr component is found to be almost in phase with the corresponding periodicities in the variation of the sunspot number.

Rotational Modulation of Microwave Solar Flux

Time series data of 10.7 cm solar flux for one solar cycle (1985–1995 years) was processed through autocorrelation. Rotation modulation with varying persistence and period was quite evident. The persistence of modulation seems to have no relation with sunspot numbers. The persistence of modulation is more noticeable during 1985–1986, 1989–1990, and 1990–1991. In other years the modulation is seen, but its persistence is less. The sidereal rotation period varies from 24.07 days to 26.44 days with no systematic relation with sunspot numbers. The results indicate that the solar corona rotates slightly faster than photospheric features. The solar flux was split into two parts, i.e., background emission which remains unaffected by solar rotation and the localized emission which produces the observed rotational modulation. Both these parts show a direct relation with the sunspot numbers. The magnitude of localized emission almost diminishes during the period of low sunspot number, whereas background emission remains at a 33% level even when almost no sunspots may be present. The localized regions appear to shift on the solar surface in heliolongitudes.

North-South Asymmetry in the Solar Coronal Rotation

The solar images at 17 GHz by the Nobeyama Radio Heliograph and in X-rays by the soft X-ray telescope (SXT) on board the Yohkoh satellite have been of particular interest for the estimation of solar coronal rotation using the flux modulation approach. These studies established that the solar corona rotates differentially. The radio images estimate an equatorial rotation period lower than those estimated by the X-ray images. The latitude profiles of the coronal rotation have temporal variability. It is very interesting that the space–time plots of sidereal rotation period clearly reveal north–south (NS) asymmetry. The asymmetry appears to change its sign in odd and even activity cycles of the Sun.

Interplanetary scintillation observations for the solar wind disappearance event of May 1999

In this article we present ground-based interplanetary scintillation (IPS) measurements at 103 and 327 MHz for the period of the solar wind disappearance event of May 1999 as seen by various space probes. The solar wind velocity measurements at 327 MHz showed a variable solar wind velocity during this period at a distance of ∼0.5 AU from the Sun. The average solar wind velocity from three radio sources varied in the range of 200–300 km s−1. The scintillation index measurements at 103 MHz indicate that plasma density was very low in the interplanetary medium closer to the Earth and that the density was normal away from it during May 11–13. The scintillation index was enhanced significantly on May 14 after the disappearance event. The comparison with the in situ observations shows that the effect is dramatic in IPS observations. IPS and in situ measurements show that a large, tenuous, and slow plasma cloud engulfed our planet around this time, which could be because of a corotating low-density narrow stream. From the source (Sun) point of view, this was mostly a normal plasma flow in most of the interplanetary medium.