INTERPLANETARY DISCONTINUITIES AND ALFVEN WAVES AT HIGH HELIOGRAPHIC LATITUDES - ULYSSES

This paper presents the results of the first statistical study of inte rplanetary directional discontinuities at both low and high heliograph ic latitudes measured by the Ulysses magnetometer. There is a gradual decrease in the rate of occurrence of interplanetary discontinuities ( ROIDs) with increasing radial distance. From 1 to 5 AU, an e(-(r-1)/5) dependence is derived. Much of this decrease is believed to be an art ifact due to the discontinuity thickening with decreasing \B\, falling outside the detection criteria. As Ulysses goes from the ecliptic pla ne to high (-80 degrees) heliographic latitudes, the ROID value increa ses dramatically. The increase is about a factor of 5 as Ulysses moves from Jupiter at 5 AU to 2.5 AU over the south pole. There is a one-to -one correspondence between high ROID values and high-speed streams. T his is particularly dramatic just after the Jovian encounter when ther e are similar to 25.4-day period corotating streams present. Thus the increase with latitude is primarily due to Ulysses spending an increas ing percentage of time within a high-speed stream emanating from the s olar coronal hole. High-speed streams are characterized by the presenc e of nonlinear Alfven waves with peak-to-peak transverse fluctuations as large as \Delta (B) over bar\/\B\ of 1 to 2. Over the south pole, t he normalized transverse wave power can be characterized by P = 2.5 x 10(-4)f(-1.6) Hz(-1) and the compressional power 1.8 x 10(-4)f(-1.2) H z(-1) for frequencies between 10(-5) and 10(-2) Hz. The normalized wav e power spectra in different regions of the polar coronal hole streams , from midlatitudes to high heliographic latitudes, appear to be quite similar. The wave power in the ecliptic plane is somewhat lower, perh aps due to contamination from low-speed streams. The Alfven waves in t he high-speed stream are found to be propagating outward from the Sun, even at these large heliocentric distances (2.5-5.0 AU). The waves ty pically have arclike polarizations and conserve field magnitude to fir st order. Directional (rotational) discontinuities often form the edge s of the phase-steepened Alfven waves, thus offering a natural explana tion for the high ROID rates within high-speed streams.