Scale dependence of the variance anisotropy near the proton gyroradius scale: Additional evidence for kinetic Alfvén waves in the solar wind at 1 AU

Abstract

The kinetic Alfvén wave (KAW) has the property that for p ∼ 1 the ratio (δB)2/(δB)2 approaches zero in the small wave number limit kp 1 and increases monotonically to values of order unity when kp ∼ 1, where p is the thermal proton gyroradius. In this study, the wave number dependence of the ratio (δB)2/(δB)2 is used to investigate the existence of KAWs in the solar wind near the proton gyroradius scale k p = 1. To facilitate comparisons between theory and observations, solar wind measurements of (δB)2/(δB)2 are restricted to times when the local mean magnetic field B0 is nearly perpendicular to the local flow velocity of the solar wind V. The quantity (δB)2 is the average magnetic power in the B0 direction and (δB)2 is the average magnetic power in the V × B0 direction. The analysis of 20 high speed streams in the ecliptic plane near 1AU from 2007 through 2011 consistently yield quantitatively similar results which all show a steady increase in the ratio (δB) 2/(δB)2 in the neighborhood of kp = 1 that is in reasonable agreement with the theory of KAWs derived from the Vlasov-Maxwell dispersion relation. These results are interpreted as evidence for the existence of an axisymmetric spectrum of KAWs in the fast solar wind at wave numbers near kp = 1 where the energy spectrum of solar wind fluctuations steepens and kinetic physics becomes important. © 2012. American Geophysical Union. All Rights Reserved.