The spectrum of electron density fluctuations in the solar wind and its variations with solar wind speed

We describe simultaneous interplanetary scintillation (IPS) measurements for the solar distance range 70-185 R⊙ made using the Ooty Radio Telescope and the Solar Terrestrial Environment Laboratory three-antenna system during the first half of the current solar cycle 22. These measurements have been used to establish the spectral characteristics of the electron density fluctuations in the solar wind, and we study the relation of these to the plasma flow speed. At times of low activity, for high-speed streams (Vsw ≈ 600 km s−1) the spectra (sensitive to the IPS temporal frequency range 1-10 Hz) are steep, with a power law exponent α ≈ 3.8. During high levels of activity for high-speed streams, the spectra are less steep with α ≈ 3.4. In the low-speed wind (Vsw ≈ 400 km s−1) the spectra are clearly flatter, α ≈ 2.9, and nearly invariant with solar activity. The average shape of the spectrum obtained from this study is consistent with results obtained via other techniques. By combining the present estimates with earlier spacecraft data, it is possible to consider the shape of the density fluctuation spectrum in the frequency range 10−5-10¹ Hz. It is shown that the spectrum has different slopes in different spectral domains and is well described by a three-component model as proposed by Coles and Harmon [1989]. Moreover, the high-frequency components of the spectra in the low- and high-speed streams involve different spatial frequency ranges.

[1]  P. K. Manoharan.  Three-dimensional structure of the solar wind: Variation of density with the solar cycle , 1993 .

[2]  G. Bourgois,et al.  SOLAR CYCLE CHANGES IN THE TURBULENCE LEVEL OF THE POLAR STREAM NEAR THE SUN , 1992 .

[3]  J. Armstrong,et al.  OBSERVATIONS OF THE ELECTRON DENSITY SPECTRUM NEAR THE SUN DURING SPECTRAL BROADENING TRANSIENTS , 1992 .

[4]  J. Harmon,et al.  The solar wind density spectrum near the Sun: Results from Voyager radio measurements , 1991 .

[5]  B. Rickett,et al.  Evolution of the solar wind structure over a solar cycle: Interplanetary scintillation velocity measurements compared with coronal observations , 1991 .

[6]  N. Sheeley,et al.  Latitudinal distribution of solar-wind speed from magnetic observations of the Sun , 1990, Nature.

[7]  E. Marsch,et al.  Spectral and spatial evolution of compressible turbulence in the inner solar wind , 1990 .

[8]  M. Kojima,et al.  Solar cycle dependence of global distribution of solar wind speed , 1990 .

[9]  B. Rickett,et al.  Observations of field-aligned density fluctuations in the inner solar wind , 1990 .

[10]  P. K. Manoharan,et al.  Determination of solar-wind velocities using single-station measurements of interplanetary scintillation , 1990 .

[11]  J. Harmon,et al.  Propagation observations of the solar wind near the sun , 1989 .

[12]  G. Withbroe.  The temperature structure, mass, and energy flow in the corona and inner solar wind , 1988 .

[13]  B. Bavassano,et al.  Latitudinal dependence of the interplanetary magnetic field spectrum , 1987 .

[14]  N. Armand,et al.  A model of the solar wind turbulence from radio occultation , 1987 .

[15]  M. Goldman,et al.  Aspects of interplanetary plasma turbulence , 1987 .

[16]  A. Richter,et al.  The Role of Alfvenic Fluctuations in MHD Turbulence Evolution between 0.3 and 1.0 AU , 1987 .

[17]  M. McCall,et al.  Voyager Microwave Scintillation Measurements of Solar Wind Plasma Parameters. , 1986 .

[18]  W. Coles,et al.  Changes in the microturbulence spectrum of the solar wind during high-speed streams , 1985 .

[19]  M. Kojima,et al.  Solar wind speed from IPS measurements , 1985 .

[20]  M. Kojima,et al.  THREE-STATION OBSERVATIONS OF INTERPLANETARY SCINTILLATION AT 327 MHZ - I , 1984 .

[21]  L. Celnikier,et al.  A determination of the electron density fluctuation spectrum in the solar wind, using the ISEE propagation experiment , 1983 .

[22]  S. L. Scott,et al.  Solar wind observations near the sun using interplanetary scintillation. , 1983 .

[23]  H. T. Howard,et al.  Radio wave scattering observations of the solar corona First-order measurements of expansion velocity and turbulence spectrum using Viking and Mariner 10 spacecraft , 1981 .

[24]  A. Efimov,et al.  Inhomogeneous structure and velocity of the circumsolar plasma based on data of the Venera-10 station , 1980 .

[25]  John W. Armstrong,et al.  Spacecraft radio scattering observations of the power spectrum of electron density fluctuations in the solar wind , 1979 .

[26]  M. Kojima.  Anisotropic Structure of Solar-Wind Plasma Irregularities Observed with Interplanetary Scintillations , 1979 .

[27]  W. A. Coles,et al.  Solar wind velocity estimation from multi-station IPS , 1978 .

[28]  J. Harmon,et al.  Interplanetary scintillation measurements of the electron density power spectrum in the solar wind , 1978 .

[29]  J. Armstrong,et al.  Interplanetary scintillations of PSR 0531+21 at 74 MHz , 1978 .

[30]  W. Coles.  Interplanetary scintillation , 1978 .

[31]  B. Jackson,et al.  Physical properties of a polar coronal hole from 2 to 5 solar radii , 1977 .

[32]  R. Kopp,et al.  Models of coronal holes above the transition region. , 1977 .

[33]  C. Russell,et al.  On the causes of spectral enhancements in solar wind power spectra , 1976 .

[34]  M. Neugebauer.  The enhancement of solar wind fluctuations at the proton thermal gyroradius , 1975 .

[35]  S. Ananthakrishnan,et al.  Observations of Interplanetary Scintillations at 327 MHz , 1974 .

[36]  M. Neugebauer,et al.  Direct measurements of solar-wind fluctuations between 0.0048 and 13.3 Hz , 1973 .

[37]  P. A. Dennison,et al.  Coronal Broadening of the Crab Nebula 1969-71: Observations , 1972, Publications of the Astronomical Society of Australia.

[38]  A. Readhead.  INTERPLANETARY SCINTILLATION OF RADIO SOURCES AT METRE WAVELENGTHS. II. THEORY. , 1971 .

[39]  V. K. Kapahi,et al.  Large Steerable Radio Telescope at Ootacamund, India , 1971 .

[40]  J. Wolfe,et al.  PRELIMINARY POWER SPECTRA OF THE INTERPLANETARY PLASMA. , 1970 .