OBSERVATIONAL EVIDENCE FOR THE RELATIONSHIP BETWEEN WALÉN SLOPE AND AMPLITUDE RATIO OF INWARD TO OUTWARD ALFVÉN WAVES IN THE SOLAR WIND

Abstract

ABSTRACT Assuming that the observed Alfvén waves in the solar wind are the superposition of inward- and outward-propagating Alfvén waves, we obtain an analytical relation from which the observed Walén slope R W can give a theoretical estimate of the amplitude ratio of inward to outward waves. From the Wind data at 1 AU, we select 37 Alfvén wave events classified observationally as three kinds: dominant outward Alfvén waves with (Class A), dominant outward Alfvén waves with (Class B), and dominant inward Alfvén waves with (Class C). For Class A events the theoretical predictions of based on R W deviate from the wave amplitude observations, but for Class B and C events the theoretical predictions agree well with related observations, being a direct observational evidence that the superposition of inward and outward Alfvén waves can cause the subunity of R W . A simple simulation is made with a white Gaussian noise to demonstrate that a small noise could reproduce the observed properties of all three kinds of events cause the measured parameters of waves with in Class A to deviate significantly from the true values more than waves with . The simulation implies that the observational results based on wave amplitudes seem reliable only for waves with . The ratios calculated from the analytical relation based on R W are closer to true values than those obtained from wave amplitude observations.