Physics – Geophysics
Scientific paper
Oct 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006jgra..11110109p&link_type=abstract
Journal of Geophysical Research, Volume 111, Issue A10, CiteID A10109
Physics
Geophysics
23
Interplanetary Physics: Mhd Waves And Turbulence (2752, 6050, 7836), Mathematical Geophysics: Spectral Analysis (3205, 3280), Nonlinear Geophysics: Turbulence (3379, 4568, 7863), Interplanetary Physics: Solar Wind Plasma, Interplanetary Physics: Interplanetary Magnetic Fields
Scientific paper
It is well known that the power spectrum of magnetic field fluctuations in the solar wind exhibits a Kolmogorov spectrum f-α in the inertial range of the turbulence with a power law exponent α near 5/3. The power spectrum of velocity fluctuations has not been as well studied, partly because of the lack of high time resolution measurements needed to resolve a significant fraction of the inertial range. In situ measurements in the ecliptic plane at 1 AU acquired by the 3DP instrument on board the Wind spacecraft near solar minimum in 1995 are used to determine power spectra of the proton bulk velocity fluctuations between 10-5 and 10-1 Hz. The spectrum for the proton kinetic energy (the sum of the spectra for the individual components Vx, Vy, and Vz) obtained using 3-s velocity data is found to possess the spectral exponent α = 1.50 in the inertial range of the turbulence. A similar calculation of the magnetic energy spectrum yields the exponent α = 1.67. The Alfvén ratio, the ratio of the kinetic to magnetic energy spectrum, is a slowly increasing function of frequency throughout the inertial range increasing from approximately 0.5 to 1 in the frequency interval from 10-4 to 10-2 Hz. This indicates that the partition of energy between small-scale velocity and magnetic field fluctuations is frequency-dependent, contrary to some theories. The total energy spectrum (kinetic plus magnetic) has the power law exponent 3/2. A brief investigation of high- and low-speed solar wind streams is also performed, which shows that different spectral exponents for velocity and magnetic field fluctuations are observed in both high- and low-speed wind.
Goldstein Michel L.
Podesta John J.
Roberts Daniel A.
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