IPS observations of short-time scale interplanetary activity

We have carried out a program of continuous Interplanetary Scintillation (IPS) monitoring of the interplanetary activity using Ooty Radio Telescope (ORT). From May 1990 to March 1991, during the 22^nd, solar maximum, a few radio sources were monitored to provide long stretches of IPS data with a high-time resolution of few minutes. These observations covered 0.3 to 0.8 AU region (12° to 70° elongations) around the sun at several heliographic latitudes. During the observation, we detected 33 short-time scale IPS events which had significant variation in the scintillation index and solar wind velocity. These were considered to be due to travelling interplanetary disturbances.A multi-component model of plasma density enhancement was developed to estimate the geometry and physical properties of these IPS events. Detailed analysis of 20 of these events suggests, 1. fast IPS events were interplanetary signatures of Coronal Mass Ejections (CMEs), 2. the average mass and energy of these events was 10¹⁶ gm and 10³³ erg respectively,3. 80% of IPS events were associated with X-ray flares on the sun and 50% were associated with geomagnetic activity at earth. Detailed study of the multicomponent model suggests IPS observations at smaller elongations (hence at higher radio frequencies) are more suited to detect fast-moving interplanetary disturbances such as produced by CMEs.     

Synoptic Maps of Solar Wind: Comparison of IPS Data and the NOAA/SEC Version of the Wang and Sheeley Model

Since the 1970s, the Solar-Terrestrial Environment Laboratory, Japan, has been publishing synoptic maps of solar wind velocity prepared using the technique of interplanetary scintillation. These maps, known as V-maps, are useful to study the global distribution of solar wind in the heliosphere. As the Earth-orbiting satellites are unable to probe regions outside the ecliptic, it is important to exploit the scope of interplanetary scintillation to study the solar wind properties at these regions and their relation with coronal features. It has been shown by Wang and Sheeley that there exists an inverse correlation between rate of magnetic flux expansion and the solar wind velocity. The NOAA/Space Environment Center daily updated version of the Wang and Sheeley model has been used to produce synoptic maps of solar wind velocity and magnetic field polarity for individual Carrington rotations. The predictions of the model at 1 AU have been found to be in good agreement with the observed values of the same. The present work is a comparison of the synoptic maps on the source surface using the interplanetary scintillation measurements from Japan and the NOAA/SEC version of the Wang and Sheeley model. The two results agree near the equatorial regions and the slow solar wind locations are consistent most of the times. However, at higher latitudes within ±60°, the wind velocities differ considerably. In the Wang and Sheeley model the highest speed obtained is 600 km s^–1 whereas in the IPS results velocities as high as 800 km s^–1 have been detected. The paper discusses the possible causes for this discrepancy and suggestion to improve the agreement between the two results.     

Measurements of radio emission from the compact source in the Crab nebula with the uran-1 interferometer at 16.7, 20 and 25 MHz

This paper presents the results of measurements of the Crab nebula in the decametre range with an interferometer whose baseline is 2.4–3.5×10³ of the wavelength. Visibility function values, which in these observations determine the contribution by the compact source to the total nebula flux, have been measured at frequencies 16.7, 20 and 25 MHz to be 0.64±0.07, 0.43±0.04 and 0.31±0.03, respectively. The spectral index of the spectrum obtained for the compact source in the range 16.7–122 MHz is 2.09±0.04. Flattening of the nebula spectrum without the compact source has been confirmed for the decametre range.     

Long-Term Radio Scintillation Variations in the Circumsolar Plasma

Long-term scintillation measurements of the solar wind formation zone at solar elongations ranging from 1°–8° (Sun impact parameters: 4–30 R ) were recorded using the water maser source IRC-20431 at the wavelength =1.35 cm during its annual solar occultations in December 1981–1998. Dramatic changes in the spatial dependence of the scintillation index were recorded over the course of the 11-year solar cycle. Markedly diminished scattering, attributed to a pronounced heliolatitude effect, was observed at the closest solar approach distances in the years around solar activity minimum. From parallel investigations of the solar magnetic field structure it was determined that the field strength at the source of the solar wind streamlines is the governing factor for the solar wind acceleration process. Particularly apparent in the scintillation data during solar activity minimum is the increasing role of the polar coronal holes with their associated open magnetic field structure. The dependence of the solar scattering intensity on heliolatitude fades in the years of high solar activity as the level of scintillations increases at polar latitudes.     

Power spectrum of small-scale irregularities in the solar wind

Day to day observations of interplanetary scintillation on ten strongly scintillating radio sources over a period of twelve months show that the power spectrum of the small scale irregularities flattens considerably at temporal frequencies ν < 0·5 Hz. This flattening defines a scale which increases uniformly between 0·1 and 1·0 a.u. from the Sun. The implication of this result will be discussed.     

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.     

On the influence of solar wind turbulent flows on decameter wavelength scintillations

Expressions for cross-correlation functions and spectra of weak interplanetary scintillations are deduced taking into account the solar wind flow structure. The paper discusses the influence of large-scale currents and small-scale velocity fluctuations distributed under the normal and lognormal laws.     

Turbulence in planetary occultations.: IV. Power spectra of phase and intensity fluctuations

Power spectra of phase and intensity scintillations during occultation by turbulent planetary atmospheres are significantly affected by the inhomogeneous background upon which the turbulence is superimposed. Such coupling is particularly pronounced in the intensity, where there is also a marked difference in spectral shape between a central and a grazing occultation. While the former has its structural features smoothed by coupling to the inhomogeneous background, such features are enhanced in the latter. Indeed, the latter power spectrum peaks around the characteristic frequency that is determined by the size of the free-space Fresnel zone and the ray velocity in the atmosphere; at higher frequencies strong fringes develop in the power spectrum. A confrontation between the theoretical scintillation spectra computed here and those calculated from the Mariner 5 Venus mission by R. Woo, A. Ishimaru, and W. B. Kendall (1974, J. Atmos. Sci.31, 1698–1706) is inconclusive, mainly because of insufficient statistical resolution. Phase and/or intensity power spectra computed from occultation data may be used to deduce characteristics of the turbulence and to distinguish turbulence from other perturbations in the refractive index. Such determinations are facilitated if observations are made at two or more frequencies (radio occultation) or in two or more colors (stellar occultation).     

The Coefficient of Asymmetry of Radio-Source Interplanetary Scintillation

It is proposed to use the coefficient of asymmetry of the distribution function of fluctuations of a scintillating source flux density as a parameter that characterizes interplanetary turbulent plasma. It is demonstrated that this parameter can be measured with a differential method and that its informative capacity is equivalent to that of the source scintillation index. A series of test observations of scintillations was performed with the Large Phased Array antenna of the Lebedev Institute of Physics, Russian Academy of Sciences, simultaneously with measurements of the source scintillation indices and coefficients of asymmetry. Comparative analysis of the measured quantities showed that the coefficient of asymmetry within a numerical coefficient equals the source scintillation index, normalized to the flux density of the scintillating component. The coefficient of asymmetry makes it possible to restore scintillation indices when the radio sources are weak and it is difficult to measure their mean flux densities, and, hence, it enlarges the number of observable scintillating sources and makes the exploration of interplanetary plasma by means of the mapping of scintillation indices more efficient.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 375–380.Original Russian Text Copyright © 2005 by Shishov, Tyul’bashev, Artyukh, Subaev, Chashei, Chernikov.     

Life-time of ionospheric scintillations from multi-satellite radio transmission recordings

Amplitude recordings from five U.S. Navy Navigational Satellites (NNSS), at a frequency of 150 MHz, were used to estimate the life-time of scintillation producing ionization patches in the F-region. The median life-time of scintillations, which was found to be 5 hr, differs to a considerable degree from the life-time of ionospheric reflections reported elsewhere. It is suggested that various spatial resolutions of ionospheric sounding techniques may account for the difference in the computed life-times.     

Equatorial scintillation

Equatorial scintillations have been observed at Legon, Ghana for nearly 20 yr. The occurrence characteristics of the scintillations are reviewed, and the physical characteristics of the electron density irregularities summarized. A much more comprehensive summary of the seasonal variation of scintillation is given, and it is found to be remarkably similar to the variations in thermospheric temperature. Evidence for the suppression of scintillation during magnetic disturbances is given. Curves for the daily variation of the Faraday rotation angle φ are presented and their unusual post-sunset behaviour noted. It is suggested that this can be explained in terms of a theory presented by Rishbeth, in which the F region ionization moves with nearly the full velocity of the neutral atmospheric wind at night, after the E region conductivity has fallen to a relatively low value. This can account for the observed drift velocity of the irregularities. The rapid increase in the post-sunset horizontal velocity of the ionization together with the observed vertical rise, can account for the variations of φ. It is further suggested that the large gradients in the density and drift velocity of the ionization resulting from the mechanism suggested by Rishbeth give rise to the production of the observed F region irregularities in electron density which cause equatorial scintillation.     

The velocity of pulsars and interstellar irregularities in the scintillation pattern

The scintillation theory is developed for application to the interstellar medium taking into account both the movement of the pulsars and the movement of the interstellar irregularities.It is shown that the velocity of the drifting pattern differs essentially from that for the pulsars. This difference is due to the medium extent effect and to the motion of the irregularities. The pulsar velocityv ₀ and the parameters of the motion of the irregularities ( , ) can be derived from the obtained formulae, using the known parameters of the cross-correlation function of scintillations (V _ef, ₁,S).In contrast with the interplanetary scintillation, the asymmetry of the form of the cross-correlation function of the interstellar scintillations is caused not only by the motion of the interstellar irregularities, but also by the movement of the source itself.     

Venus Doppler winds at cloud tops observed with ESPaDOnS at CFHT

We present new wind measurements in Venus’ lower mesosphere from visible spectroscopy during the 2007 worldwide coordinated ground campaign in support of ESA's Venus Express mission. These observations consisted of high-resolution spectra of Fraunhofer lines in the entire visible range (0.37-1.05 μm) to measure the winds near 68 km using the Doppler shift of solar radiation scattered by clouds toward the observer's direction. The observations included various points of the dayside hemisphere at a phase angle of ∼109°. We took advantage of two symmetrical elongations in July and September 2007 at Canada-France-Hawaii's 3.6-m telescope. Kinematical fits to the Doppler winds provide a mean equatorial velocity of (104±10) m s⁻¹ for the zonal retrograde flow. This velocity agrees quite well with the mean value obtained by tracking the UV markings from several spacecraft.     

Ionospheric Disturbances and Their Impact on IPS Using MEXART Observations

We study the impact of ionospheric disturbances on the Earth’s environment caused by the solar events that occurred from 20 April to 31 May 2010, using observations from the Mexican Array Radio Telescope (MEXART). During this period of time, several astronomical sources presented fluctuations in their radio signals. Wavelet analysis, together with complementary information such as the vertical total electron content (vTEC) and the Dst index, were used to identify and understand when the interplanetary scintillation (IPS) could be contaminated by ionospheric disturbances (IOND). We find that radio signal perturbations were sometimes associated with IOND and/or IPS fluctuations; however, in some cases, it was not possible to clearly identify their origin. Our Fourier and wavelet analyses showed that these fluctuations had frequencies in the range ≈?0.01 Hz?–?≈?1.0 Hz (periodicities of 100 s to 1 s).     

Radio scintillations due to plasma irregularities with power law spectra: The interplanetary medium

The scintillation effects which are produced when radiation is incident on a medium containing phase changing irregularities which have power law spectra are discussed. The results are then applied to the interplanetary medium and it is concluded that the small scale sizes (≈100 km) for irregularities measured by interplanetary scintillation represent a genuine ‘outer scale’ distinct from the 10⁶ km scales detected by space probes. It is also shown that the small scale irregularities contain discontinuities which are finite or almost finite, with a sharpness as yet undetermined.     

Three-dimensional properties of interplanetary disturbances in 1978–1981

This paper presents the average three-dimensional configuration of solar flare- or disappearing filament-associated interplanetary disturbances on the basis of IPS (interplanetary scintillation) and spacecraft observations in 1978–1981. The angular distribution of the propagation speed at 1 AU is largely isotropic over the range of 110° in solar longitude centered at the normal of the solar source. In the latitudinal direction, the characteristic angular extent is about 60°. Thus the three-dimensional shape of an interplanetary disturbance can be approximated by a half of an ellipsoid having an axial ratio of about 1.8.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Power Spectrum of Cosmic-ray Fluctuations During Consecutive Solar Minimum and Maximum Periods

Power spectral analysis of cosmic-ray intensity recorded by eight stations was carried out over a wide range of frequencies from 2.3 × 10^–8 Hz to 5.8 × 10^–6 Hz (2–500 days) during the period 1964–1995. Spectrum results of large-scale fluctuations have revealed the existence of a broad peak near 250–285 days and a narrower peak at 45–50 days during the studied epochs as a stable feature in all neutron monitors covering a wide rigidity range. The cosmic-ray power spectrum displayed significant peaks of varying amplitude with the solar rotation period (changed inversely with the particle rigidities) and its harmonics. The amplitudes of 27-day and 13.5-day fluctuations are greater during the positive-polarity epochs of the interplanetary magnetic field (qA>0) than during the qA<0 epochs. The comparison of cosmic-ray power spectra during the four successive solar activity minima have indicated that at the low-rigidity particles the spectrum differences between the qA>0 and qA<0 epochs are significantly large. Furthermore, the spectrum for even solar maximum years are higher and much harder than the odd years. There are significant differences in the individual spectra of solar maxima for different cycles.     

Study of equatorial plasma bubbles using all sky imager and scintillation technique from Kolhapur station: a case study

The nightglow observations of OI 630.0 nm emission carried out from low latitude station Kolhapur using All Sky Imager (ASI) with \(140^{\circ}\) field of view (FOV) for the month of April 2011 are used. The images were processed to study the field aligned irregularities often called as equatorial plasma bubbles (EPBs). The present study focuses on the occurrence of scintillation during the traversal of EPBs over ionospheric pierce point (IPP). Here we dealt with the depletion level (depth) of the EPB structures and its effect on VHF signals. We compared VHF scintillation data with airglow intensities at Ionospheric pierce point (IPP) from the same location and found that the largely depleted EPBs make stronger scintillation. From previous literature, it is believed that the small scale structures are present near the steeper walls of EPBs which often degrades the communication, the analysis presented in this paper confirms this belief.