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Publications and source records attributed to Neugebauer, M..
The alpha particles at high latitudes are traveling significantly faster than the effective wave speed, unlike the in-ecliptic situation and the discrepancy between effective wave speed and the Alfvspeed calculated with all known corrections is also larger than in the ecliptic.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.
ISEE-3 and Ulysses plasma and magnetic field data are used to study features in the trailing regions of interplanetary plasma clouds resulting from coronal mass ejections (CMEs). Approximately one quarter of those events contain periods greater or equal to 6 hours for which the interplanetary magnetic field is quiet and nearly radial. Arguments are presented that this is a causal relation, rather than coincidence.
Observations by the Ulysses SWOOPS plasma experiment are used to investigate spatial and temporal gradients during the mission, with emphasis on more recent high latitude opbservations including the recent South Pole to North Pole passage during solar minimum.
Reported are observations from the Ulysses SWOOPS experiment which provided measurements of the differential streaming between protons and alphas as a function of heliocentric distance and latitude. The data reported are of observations for those periods when Ulysses sampled only the flows from the solar polar coronal holes. All of the high-latitude results differ from the outbound, in-ecliptic data.
Data from the Ulysses spacecraft was used for studying solar tangential discontinuities (TDs) at high latitudes. Significant numbers of TDs seem to exist at these latitudes.
For many years, solar-wind physicists have been using plasma and field data acquired near the ecliptic plane together with data on the scintillation of radio sources and remote sensing of structures in the solar corona to estimate the properties of the high-latitude solar wind, Because of the highly successful Ulysses mission, the moment of truth is now here. This talk summarizes the principal differences between the high and low latitude solar winds at the declining phase of the solar-activity cycle and between the Ulysses observations and expectations.
The Ulysses spacecraft trajectory includes a peak southern latitude of -80.2 deg, reached during September 1994, and perihelion in the ecliptic plane at 134 AU in March 1995. The near-perihelion mission phase features a rapid scan through solar latitude, with rates approaching one degree per day. We will present observations through mid-May 1995, when the spacecraft will be near 1.5 AU and +50 deg solar latitude. At the time of this writing, observations from the solar wind plasma experiment have been examined through -40 deg solar latitude. At that latitude Ulysses was still immersed in fast solar wind from the south polar coronal hole, with wind speeds of 700 to 800 km/s and with a variety of fine structure. Expectations for near-perihelion measurements include times of slow, dense wind characteristic of the near-equatorial heliomagnetic streamer belt. A non-zero tilt of the streamer belt would produce recurrent intervals of fast coronal hole wind and corotating interaction regions (CIRs) caused by fast wind overtaking slow wind. Forward and reverse shock waves bounding the CIRs, routinely observed at low and intermediate latitudes beyond approximately 2 AU, will likely be absent during the northward transit due to proximity to the Sun we will summarize solar wind plasma results concerning meridional gradients in fluid parameters, large-scale and fine structure, and transient events such as coronal mass ejections.
Since passing essentially continuously into regions of solar wind from the southern polar coronal hole at approximately 36 deg S, Ulysses has observed frequent structures lasting from several hours to several days. In addition to Alfven waves and coronal mass ejections, which have been discussed by previous authors, two other sorts of structures are routinely evident. This paper provides the first report of these structures in the high latitude solar wind: (1) small scale compressional structures, and (2) pressure balance structures. The compressional structures are driven by faster solar wind overtaking the slower solar wind ahead of it and exhibit the plasma and field properties expected for compressions. However, unlike large scale stream interaction regions observed in and near the ecliptic plane, these structures are much smaller scale and are transient, not recurring from one rotation to the next. The pressure balance structures are indicated by roughly equal increases in the plasma pressure and decreases in the magnetic field pressure. These structures, which are several degrees across, are more dense and have higher plasma pressures and betas than the surrounding solar wind. These pressure balance structures seem to be likely manifestations of 'polar plumes.'
The radio and plasma wave receivers on the Ulysses spacecraft have detected thousands of short-duration bursts of waves at approximately the electron plasma frequency. These wave events believed to be Langmuir waves are usually less than approximately 5 minutes in duration. They occur in or at the boundaries of depletions in the magnetic field amplitude known as magnetic holes. Using the 16 sec time resolution provided by the plasma frequency receiver, it is possible to examine the density structure inside of magnetic holes. Even higher time resolutions are sometimes available from the radio receiver data. The Ulysses observations show that these wave bursts occur more frequently at high heliographic latitudes; the occurrence rates depend on both latitude and distance from the Sun. We review the statistics for the wave events, compare them to magnetic and plasma parameters, and review the reasons for the more frequent occurrence at high heliographic latitudes.
A large number of magnetic holes have been found in the Ulysses data during its cruise in the ecliptic. They are interpreted as convecting structures, probably caused by the mirror instability which exists in high beta plasmas with anisotropic temperatures. The characteristics of the holes reflect the solar wind condition of the region in which the holes are formed, and the point of observation may be far removed from where the instability occurs. A preliminary survey appears to indicate that the number of holes has no significant radial dependence. However, the number of holes does appear to increase with increasing heliographic latitude. Yet the large scale solar wind structures with their compression regions disappeared at approximately 57 deg south latitude. Thus any causal relationship between the holes and large scale solar wind structures is questionable. The temperature anisotropy and high beta required by the mirror instability must be generated by other mechanisms. In order to tie the magnetic holes and the mirror instability to their cause, the evolution of their characteristics with heliocentric distance and latitude needs to be investigated. With the progression of Ulysses around the sun a survey will be conducted to ascertain the characteristics of the magnetic holes as a function of heliographic latitude and heliocentric distance. A comparison of the results with the solar wind conditions may lead to the identification of the magnetic hole generating mechanism(s).
Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles do not participate in the wave motions because they 'surf' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the 'observed' wave speed, B(sub o)(delta v squared / delta B squared)exp 1/2, as determined from the ratio of the amplitudes of the velocity and magnetic fluctuations. It is proposed that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from asymmetric distributions of interstellar pickup ions. If the proposed explanation is correct, it indicates that scattering of pickup ions onto a (bi)spherical shell may not be as complete as generally supposed.
The Ulysses spacecraft has been observing the properties of the high-latitude solar wind during a period of declining and minimum solar activity when the Sun displayed well-developed polar coronal holes. Poleward of approximately 40 deg, Ulysses remained continuously within the flow from the polar coronal hole. All was not quiet within that flow, however. In addition to coronal mass ejections seen at latitudes as high as 60 deg, there were mini-high velocity streams with amplitudes as large as 100 km/s and well developed compression regions on their leading edges and there were large-amplitude Alfven waves. The alpha-particles participate in the mini-stream variations, but not in the Alfven waves, which provides a method of separating the two phenomena. The duration of the mini-streams was typically a few days whereas the periods of the Alfven waves were usually less than 12 hours. Other properties of the mini-streams will be summarized.
Density fluctuations with periods 10 minutes to 1 hour have been investigated in ISEE 3 plasma measurements of solar wind flows at l AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results support the interpretation of the large-scale variations in density fluctuations observed by Doppler scintillation measurement techniques inside 0.2 AU. The highest absolute and relative density fluctuations occur ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi-stationary solar wind, density and relative density fluctuations are highest around the heliospheric current sheet and lowest in the high-speed coronal flow. Superposed epoch analysis shows that the region of enhanced density fluctuations and its abrupt boundaries observed in the vicinity of the heliospheric current sheet near the Sun persists to l AU, providing further support for the filamentary nature of the extensions of coronal streamers. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows with differing origins at the Sun and as a detector of propagating interplanetary disturbances.
Several parameters measured by Ulysses as it traveled southward to heliographic latitudes of -50 deg are presented and analyzed. The radial component of the magnetic field, averaged over 5 deg latitude increments and extrapolated back to 1 AU, is found to agree with baseline measurements provided by IMP-8. There is little, if any, evidence of a latitude gradient, a result consistent with the dominance of the magnetic field associated with the heliospheric current sheet and with recent models which include the effect of the current sheet as well as of source surface fields. Thus far, the spiral angle agrees with the Parker spiral assuming a rate of rotation of the field lines at the Sun equal to the equatorial value. No evidence is seen of either a change in rotation rate with latitude or an unwinding of the spiral as suggested by a recent analysis. Hourly variances in the field magnitude and in the sum of the variances in the components, normalized to the square of the observed field strenght, show the former to be independent of latitude while the latter shows a strong increase with latitude. These two observations are shown to be associated with Alfven waves that are continuously present at high latitudes. The waves have large amplitudes, extend to long periods, and have important implications for galactic cosmic rays and the solar wind.
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