GEOPHYSICALRF•EARCH LETTERS, VOL. 20, NO. 24, PAGES 2789-2792,DECEMBER 23, 1993 LATITUDINAL VARIATION OF SOLAR WIND COROTATING STREAM INTERACTION REGIONS: ULYSSES J. T. Gosling, 1S.J. Bame, 1 D.J. McComas, 1 J. L. Phillips, 1 V. J. Pizzo 2B.E.Goldstein, 3 and M. Neugebauer 3 ' Abstract. Ulysses'initialtransit to highheliographic latitudes at a heliocentric distance of -5 AU has revealed systematic effects in the latitudinalevolutionof corotating interaction regions (CIRs). At a latitude corresponding roughlyto, but slightly lessthan, the inferred tilt of the coronal streamer belt and embedded heliospheric current sheet, the strong forward shockscommonly associated with CIRs at lower latitudes disappeared almost entirely; however, the reverse shocks associated with these CIRs persisted to latitudes ~10ø above the streamer belt. Systematic meridional flow deflections observed in association with the forward and reverse waves bounding the CIRs demonstrate that the above effectis the resultof the fact thatthe forward wavespropagate to lower latitudes and the reverse waves to higher latitudeswith increasing heliocentric distance. Theseobservational results are in excellent agreement with the predictions of a three- dimensional model of corotating solar wind flows that originate in a tilteddipole geometry back at theSun. Introduction Whenthe coronal expansion is relatively time-stationary, the solar wind at low latitudestends to be organized into alternating streams of high andlow speed flows thatcorotate withthe Sun. With increasing distance from theSunthe high speedstreams steepen and overtake the slower solar wind ahead, producing compressive, corotating interaction regions, CIRs, on the rising speed portions of these streams. The leading edge of a CIR is a forward wave thatpropagates into theslow solar wind ahead, while the trailing edge is a reverse wave that propagates backward into the high speed flow. Beyond ~ 2 AU from the Sun the forward andreverse waves commonly steepen into a pair of shocks thatboundthe CIR [e.g., Hundhausen and Gosling, 1976; Smith and Wolfe, 1976]. CIRs have been extensivelystudied at low heliographic latitudes; however, as spacecraft observations well removed fromtheecliptic plane have not previously been available,it has not heretofore been possible to examine directly latitudinal variations in theevolution of CIRs. The Ulyssesspacecraft was launched on October 6, 1990 into an orbit that included a Jupiter swingbyand that will eventually take thespacecraft nearly over the poles of theSun. The Ulysses payload includes a Los Alamos ion experiment that measures 3-dimensional distributions over an 1 1 Los Alamos Nat'onal Laboratory, Los Alamos, New Mexico 2San Juan Institute, San Juan Capistrano, California (also Guest Researcher at NOAA/SEL, Boulder, Colorado) 3Jet Propulsion Laboratory, Pasadena, California Copyright 1993by the American Geophysical Union. Papernumber 93GL03116 0094-8534/93/93 GL-03116503.00 energy/charge range extending from 0.257 to 35.0 keV/charge and an electron experimentthat measures3-dimensional distributions over a nominal energy/charge range from 0.81 to 862 eV [Bame et al., 1992]. Our purpose hereis to present the results of a preliminaryanalysis of the Ulysses plasma observations of CIRs over a latitudinal range of S13.2 ø- S37.9øand covering a heliocentric distance range of 5.33-4.37 AU. The time period of the study, from early July 1992 through late August1993, encompasses an interval when the solarwind at moderate southern heliographic latitudes at - 5 AU was dominated by a single,largehigh speed stream that persisted for at least15 solar rotations. Observations An overviewof the stream structure observed by Ulysses as it traveled out of the ecliptic planeis provided in Bameet al. [1993]. Following the Jupiter encounter in February 1992 until July 1992 whenUlysses wasstill equatorward of ~S13 ø the solar wind speed varied within a narrowrange of-350 to -550 km s -1. However, a single, broad high speed stream with maximum speed (after July) in excess of 700 km s -1has been observed everysolar rotation since July 1992. Minimum speeds in the troughs between the reappearances of the stream were -400 km s- until Ulysses passed poleward of -S28 ø in April 1993, at which time the minima roseto values of -550 km s-1. These observations have been used to infer that the coronalstreamer belt and its embedded heliospheric current sheet were tiltedat ~29 øto the solarequatorial planeat this time. A similar conclusion has been drawn from magnetic field observations from Ulysses [Smith et al., 1993]. The CIR associated with this large amplitude stream was usually bounded by a forward-reverse shock pair. Figure 1 provides a summary of the Ulysses shock observations in the July 1992 - August 1993 interval.The figure shows a plotof the ratio of downstream to upstream densityminus 1 as a function of solar latitude. (The density ratio is a measure of shock "strength" andis inversely proportional to the ratioof upstream to downstream normal flow speeds in the shock frame.) There are several interesting aspects to the data shown in Figure 1, the most obvious being the change in shock occurrence at a latitude of -S26 ø, corresponding roughly to, but slightly less than, the inferred tilt of the streamer belt and current sheet at that time. Equatorward of S26 ø there were 15 forward shocks but only 9 reverse shocks, whereas poleward of S26 øtherewere 15 reverse shocks but only 2 definite forward shocks (the 3 forward events encountered near S29 ø were extremely weak and may not actuallybe shocks). Shocksequatorward of S26 ø, both forward and reverse, also tended to be stronger than those polewardof that latitude. Sevenout of ten CIRs observed equatorward of S26 ø were bounded by a forward-reverse shockpair;in two cases the forward wave was not a shock and in the other case the reverse wave was not. On the other 2789