MAGNETIC-FLUX TUBES AT 3 AU

We present an analysis of plasma and magnetic field data acquired by t he Ulysses spacecraft on May 1994. Our study is motivated by the resul t of Poletto ei al. (1996) who found some evidence for a peak in the p ower spectrum of magnetic pressure at a frequency nu approximate to 2 x 10(-5) Hz, during that period. A re-evaluation of the plasma pressur e power spectrum, on the basis of better data than used in the previou s work, gives only marginal evidence for a peak at that frequency. If both spectra had excess power in the same spectral range, one might hy pothesize that the Pressure Balanced Structures (PBS) detected in the data trace periodically distributed coronal structures which maintain their identity up to large distances. A careful data analysis, however , shows that this interpretation is hardly tenable. Hence, we consider the alternative hypotheses that the observed PBS are either a bundle of magnetic Bur tubes, with no characteristic periodicity, in pressure equilibrium with the ambient, or the manifestation, at large distance s, of waves generated close to the Sun. To prove the latter case, we m ade a test simulation of the evolution with heliocentric distance of a n ensemble of Alfven and slow mode waves, generated close to the Sun, and show that structures similar to those we analyzed may form in the interplanetary medium. Our simulations also seem to show that together with PBS, magnetic holes, frequently observed in the Ulysses data, co uld also originate from the nonlinear evolution of large amplitude slo w waves in quasi-perpendicular propagation We conclude-that the observ ed PBS most likely arise via an irt situ generation mechanism, rather than being remnants of solar structures.