The Identification and Characterization of Alfvénic Fluctuations in ULYSSES Data at Midlatitudes

Abstract

Ulysses magnetic field and plasma data recorded during 1992 and 1993 as the spacecraft moved through mid-heliolatitudes, have been used to identify intervals dominated by Alfvénic fluctuations in the presence of corotating interaction regions. A wavelet frame representation of Elsässer variables was used to calculate the Alfvénicity of the fluctuations as a function of time and frequency, and to describe their occurrence primarily within trailing edges of the corotating interaction regions that occurred within the interval. It was confirmed that, in general, if a dominant population of Alfvén waves was present, then these waves were outward propagating, but the power in the waves was found to be highly variable in time. A decrease in the power in the Alfvén waves was found for frequencies lower than ~1 × 10^-5 Hz, corresponding to a scale between 0.3 and 0.5 AU. A dominant population of outward-propagating Alfvén waves was typically found to occur during the trailing edge of high-speed streams, as expected from previously published studies, but they did not persist for the whole of the trailing edge. They generally occurred within the first half and ended in a sharp cutoff across all frequencies. Dominant outward-propagating Alfvénic fluctuations were also identified during some low-speed streams, but they did not persist between rotations. No latitudinal dependence of the purity of the fluctuations was found, but the fluctuations became more persistent at higher latitudes.