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Ann. Geophys., 22, 689-696, 2004
www.ann-geophys.net/22/689/2004/
© European Geosciences Union 2004
Compressive fluctuations in high-latitude solar wind
B. Bavassano1, E. Pietropaolo2, and R. Bruno1
1Istituto di Fisica dello Spazio Interplanetario (C.N.R.), Roma, Italy
2Dipartimento di Fisica, Università di L’Aquila, L’Aquila, Italy
Abstract. Solar wind compressive fluctuations at MHD scales have been extensively
studied in the past using data from spacecraft on the ecliptic plane. In the
present study, based on plasma and magnetic field measurements by Ulysses, a
statistical analysis of the compressive fluctuations observed in the
high-latitude solar wind is performed. Data are from the first
out-of-ecliptic orbit of Ulysses, when the Sun's activity is low and the
high-latitude heliosphere is characterized by the presence of a fast and
relatively steady solar wind, the polar wind. Our analysis is based on the
computation of hourly-scale correlation coefficients for several pairs of
solar wind parameters such as velocity, density, temperature, magnetic field
magnitude, and plasma pressures (thermal, magnetic, and total). The behaviour
of the fluctuations in terms of their amplitude has been examined, too, and
comparisons with the predictions of existing models have been performed. The
results support the view that the compressive fluctuations in the polar solar
wind are mainly a superposition of MHD compressive modes and of
pressure-balanced structures. Nearly-incompressible effects do not seem to
play a relevant role. In conclusion, our results about compressive
fluctuations in the polar wind do not appear as a break with respect to
previous low-latitude observations. However, our study clearly indicates that
in a homogeneous environment, as the polar wind, the pressure-balanced
fluctuations tend to play a major role.
Key words. Interplanetary physics (MHD waves and turbulence;
solar wind plasma) – Space plasma physics (turbulence)
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