Elsässer variable analysis of fluctuations in the ion foreshock and undisturbed solar wind

Abstract

MHD fluctuations in the solar wind have been investigated previously by use of Elsässer variables. In this paper, we present a comparison of the spectra of Elsässer variables in the undisturbed solar wind at 1 AU and in the ion foreshock in front of the Earth. Both observations take place under relatively strong solar wind flow speed conditions (≈ 600 km s−1). In the undisturbed solar wind we find that outward propagating Alfvén waves dominate, as reported by other observers. In the ion foreshock the situation is more complex, with neither outward nor inward propagation dominating over the entire range investigated (1‐10 mHz). Measurements of the Poynting vectors associated with the fluctuations are consistent with the Elsässer variable analysis. These results generally support interpretations of the Elsässer variables which have been made based strictly on solar wind data and provide additional insight into the nature of the ion foreshock turbulence.