Spectral Exponents of Kinetic and Magnetic Energy Spectra in Solar Wind Turbulence

Abstract

Kinetic and magnetic energy spectra in the ecliptic plane near 1 AU are found to exhibit different power-law behaviors in the inertial range, with the magnetic spectrum often having a power-law exponent near 5/3 and the kinetic energy spectrum often having a power-law exponent near 3/2 (the inertial range extends from approximately 5 × 10⁻⁴ to 10⁻¹ Hz). The total energy, kinetic plus magnetic, has a power-law exponent that lies between 3/2 and 5/3, with a value near 1.6. The Alfvén ratio, the ratio of kinetic to magnetic energy, is found to be a slowly increasing function of frequency in the inertial range, increasing from roughly 0.5 to 0.9 in the frequency range from 10⁻³ to 10⁻¹ Hz. These conclusions are based on the analysis of four distinct time intervals of solar wind magnetic field and plasma data obtained by the Wind spacecraft near the end of solar cycle 22 and at different times throughout solar cycle 23. Three 54 day intervals and one 81 day interval are used to compute power spectra in the range from 10⁻⁵ to 1.7 × 10⁻¹ Hz. Power-law exponents are estimated from linear least-squares fits to the logarithm of the power spectral density versus the logarithm of the frequency over the frequency interval from 10⁻³ to 10⁻² Hz. To prevent errors due to spectral aliasing, the last decade of the spectrum is omitted from the calculation of the power-law exponents. The results show that a measurable difference exists between the power-law exponents of velocity and magnetic field fluctuations and that this difference persists throughout the solar cycle.