Compressive fluctuations in the solar wind and their polytropic index

Magnetohydrodynamic compressive fluctuations\udof the interplanetary plasma in the region from 0.3 to 1 AU have been\udcharacterized in terms of their polytropic index. Following Chandrasekhar's\udapproach to polytropic fluids, this index has been determined through a fit of\udthe observed variations of density and temperature. At least three different\udclasses of fluctuations have been identified: (1) variations at constant thermal\udpressure, in low-speed solar wind and without a significant dependence on\uddistance, (2) adiabatic variations, mainly close to 1 AU and without a relevant\uddependence on wind speed, and (3) variations at nearly constant density, in fast\udwind close to 0.3 AU. Variations at constant thermal pressure are probably a\udsubset of the ensemble of total-pressure balanced structures, corresponding to\udcases in which the magnetic field magnitude does not vary appreciably throughout\udthe structure. In this case the pressure equilibrium has to be assured by its\udthermal component only. The variations may be related to small flow-tubes with\udapproximately the same magnetic-field intensity, convected by the wind in\udconditions of pressure equilibrium. This feature is mainly observed in\udlow-velocity solar wind, in agreement with the magnetic topology (small open\udflow-tubes emerging through an ensemble of closed structures) expected for the\udsource region of slow wind. Variations of adiabatic type may be related to\udmagnetosonic waves excited by pressure imbalances between contiguous flow-tubes.\udSuch imbalances are probably built up by interactions between wind flows with\uddifferent speeds in the spiral geometry induced by the solar rotation. This may\udaccount for the fact that they are mainly found at a large distance from the\udsun. Temperature variations at almost constant density are mostly found in fast\udflows close to the sun. These are the solar wind regions with the best examples\udof incompressible behaviour. They are characterized by very stable values for\udparticle density and magnetic intensity, and by fluctuations of Alfvénic type.\udIt is likely that temperature fluctuations in these regions are a remnant of\udthermal features in the low solar atmosphere. In conclusion, the polytropic\udindex appears to be a useful tool to understand the nature of the compressive\udturbulence in the interplanetary plasma, as far as the frozen-in magnetic field\uddoes not play a crucial role