Velocity oscillations in the outer heliosphere: A signature of pickup ion temperature variability?

Abstract

[1] Unusual long wavelength (0.5-1 AU), low-frequency ($3 Â 10 À5 s À1 ) velocity oscillations have been observed by Voyager 2 at 48 AU in the outer heliosphere (Paularena et al., 1996). The velocity oscillations appear to be superimposed on weak linear velocity gradients that are typically increasing. To date, no explanation has been advanced for the origin of either the flow gradients or the oscillations. Here we show that pickup ions can excite low-frequency fluctuations and drive the flow field linearly. The model is compared directly to velocity oscillations observed by the Voyager 2 PLS instrument between 40 and 55 AU over the 6-year period 1993-1998.Citation: Zank, G. P., X. Ao, and J. D. Richardson (2005), Velocity oscillations in the outer heliosphere: A signature of pickup ion temperature variability?,

MHD analysis of the velocity oscillations in the outer heliosphere

“…The oscillation is generated at ~50 AU where the corotating solar wind stream is apparently becoming weaker and is ubiquitous in the outer heliosphere out to the termination shock. In applying the linearized relation given by Zank et al [], the phase of the velocity oscillation is anticorrelated with the density perturbation (Figure ) which results in inward propagating fluctuations in the frame moving with the solar wind. The fluctuations are carried outward by the supersonic solar wind in the rest frame of the Sun.…”

“…The wavelength of the velocity oscillation is monochromatic and ~2 AU in this simulation on 1 May 2005 while the wavelength observed by V2/PLS during 1996–1998 was 0.5–1 AU [ Paularena et al , ; Zank et al , ].…”

“…The theoretical analysis by Zank et al [] suggests that the velocity oscillation detected by V2/PLS can be explained by the interaction between solar wind protons and neutral interstellar hydrogen. The linear relation between the perturbed velocity, u 1 , and the perturbed density, ρ 1 , is given by and, where u and ρ are the velocity and density of the solar wind, respectively, C 0 is the sound speed, and ± corresponds to radially outward and inward propagation, respectively; ω ′ and k are the frequency and the wave number in the frame moving with the solar wind.…”

“…The theoretical analysis by Zank et al [2005] suggests that the velocity oscillation detected by V2/PLS can be explained by the interaction between solar wind protons and neutral interstellar hydrogen. The linear relation between the perturbed velocity, u 1 , and the perturbed density, 1 , is given by…”

“…The wavelength of the oscillation is ~2 AU on 1 May 2005 in our simulations which is consistent with the newly observed event in late 2003 (Figure a). However, for the previous events found by Paularena et al [] and Zank et al [] in 1996–1998, the wavelength of the oscillations was ~ 0.5 AU. This difference may suggest that the wavelength possibly depends on the solar activity, that is, global structure and properties of solar wind greatly changed in this decade [e.g., McComas et al , ; Tokumaru et al , ].…”