M. Neugebauer

Solar wind observations over Ulysses' first full polar orbit

This study examines solar wind plasma and magnetic field observations from Ulysses first full polar orbit in order to characterize the high-latitude solar wind under conditions of decreasing and low solar activity. By comparing observations taken over nearly all heliolatitudes and two different intervals covering the same radial distances, we are able to separate the radial and latitudinal variations in the solar wind. We find that once the radial gradients are removed, none of the high-latitude solar wind parameters show much latitudinal variation, indicating that the solar wind emanating from the polar coronal holes is extremely uniform. In addition, by examining nearly 6 years of data starting in the declining phase of the last solar cycle and extending through the most recent solar minimum, we are able to address hemispheric asymmetries in the observations. We find that these asymmetries are most likely driven by differences in the solar wind source over the solar cycle and indicate that more energy goes into the polar solar wind during the declining phase of the solar cycle than around minimum. Because the mass flux is larger in the declining phase while the speeds are very similar, we conclude that this energy is introduced at an altitude below the solar wind acceleration critical point. Finally, we provide details of the statistics of over 20 solar wind parameters so that upcoming observations from Ulysses second polar orbit, during much more active times on the Sun, can be readily compared to the quieter first orbit results.

Spatial structure of the solar wind and comparisons with solar data and models

Data obtained by instruments on the Ulysses spacecraft during its rapid sweep through >90° of solar latitude, crossing the solar equator in early 1995, were combined with data obtained near Earth by the Wind spacecraft to study the spatial structure of the solar wind and to compare to different models of the interplanetary magnetic field derived from solar observations. Several different source-surface models matched the double sinusoidal structure of the heliospheric current sheet (HCS) but with differences in latitude as great as 21°. The source-surface model that included an interplanetary current sheet gave poorer agreement with observed current-sheet crossings during this period than did the other source-surface models or an MHD model. The differences between the calculated and observed locations of the HCS were minimized when 22° of solar rotation was added to the constant-velocity travel time from the source surface to the spacecraft. The photospheric footpoints of the open field lines calculated from the models generally agreed with observations in the He 10,830 A line of the locations of coronal holes with the exceptions that (1) in some places, open field lines originated outside the coronal hole boundaries and (2) the models show apparently closed-field regions just inside some coronal hole boundaries. The patterns of mismatches between coronal hole boundaries and the envelopes of open field lines persisted over at least three solar rotations. The highest-speed wind came from the polar coronal holes, with the wind originating deeper within the hole being faster than the wind coming from near the hole boundary. Intermediate and slow streams originated in smaller coronal holes at low latitudes and from open field regions just outside coronal hole boundaries. Although the HCS threaded regions of low speed, low helium abundance, high ionization temperature, and a high ratio of magnesium to oxygen densities (a surplus of an element with low first-ionization potential), there was a great deal of variation in these parameters from one place to another along the HCS. The gradient of speed with latitude varied from 14 to 28 km s−1 deg−1.

Sources of the solar wind at solar activity maximum

[1]xa0The photospheric sources of solar wind observed by the Ulysses and ACE spacecraft from 1998 to early 2001 are determined through a two-step mapping process. Solar wind speed measured at the spacecraft is used in a ballistic model to determine a foot point on a source surface at a solar distance of 2.5 solar radii. A potential-field source-surface model is then used to trace the field and flow from the source surface to the photosphere. Comparison of the polarity of the measured interplanetary field with the polarity of the photospheric source region shows good agreement for spacecraft latitudes equatorward of 60°. At higher southern latitudes, the mapping predicts that Ulysses should have observed only outward directed magnetic fields, whereas both polarities were observed. A detailed analysis is performed on four of the solar rotations for which the mapped and observed polarities were in generally good agreement. For those rotations, the solar wind mapped to both coronal holes and active regions. These findings for a period of high solar activity differ from the findings of a similar study of the solar wind in 1994–1995 when solar activity was very low. At solar minimum the fastest wind mapped to the interior of large polar coronal holes while slower wind mapped to the boundaries of those holes or to smaller low-latitude coronal holes. For the data examined in the present study, neither spacecraft detected wind from the small polar coronal holes when they existed and the speed was never as high as that observed by Ulysses at solar minimum. The principal difference between the solar wind from coronal holes and from active regions is that the O7+/O6+ ion ratio is lower for the coronal hole flow, but not as low as in the polar coronal hole flow at solar minimum. Furthermore, the active-region flows appear to be organized into several substreams unlike the more monolithic structure of flows from coronal holes. The boundaries between plasma flows from neighboring sources are marked by large magnetic holes, plasma sheets, and low entropy, independent of whether the sources have the same or opposite magnetic polarities. The evolution of solar wind properties between 1 AU and the 1.6–5.4 AU solar distance of Ulysses is also briefly discussed.

Ulysses observations of microstreams in the solar wind from coronal holes

During its south polar passage in 1994, the Ulysses spacecraft continuously sampled the properties of the solar wind emanating from the south polar coronal hole. At latitudes poleward of ∼−60°, the solar wind speed had an average value of 764 km/s and a range of 700–833 km/s. The principal variations in the vector velocity were associated with either outward propagating Alfven waves with periods up to about half a day or with longer-period high- or low-speed “microstreams.” The microstreams had an amplitude of ∼40 km/s and a mean half width of 0.4 days, and they recurred on timescales of 2–3 days (power spectral peaks at 1.9 and 3.3 days). The density and temperature profiles showed the expected evidence of pileup and compression on the leading edges of high-speed microstreams, although no forward or reverse shocks were observed. The particle fluxes were nearly the same for both the fast and slow microstreams. The higher-speed microstreams had higher proton temperatures and higher alpha-particle abundances than did the slower microstreams. The absence of latitude variations in the thickness or the recurrence rate suggests that the microstreams are caused by temporal rather than long lived (> a few days) spatial variations in the source region at the Sun. Some speculations are made about the possible cause of the microstreams.

Ion distributions in large magnetic holes in the fast solar wind

Magnetic holes are isolated localized depressions in the magnitude of the interplanetary magnetic field. Six examples of long-duration (9 to 30 min) magnetic holes observed by the Ulysses spacecraft at high heliographic latitudes near 3 AU are studied in detail. Each of the holes was in pressure balance with its surroundings. The proton-proton and alpha-proton differential streaming typically observed in the fast solar wind dropped to very low levels within the holes. The ion temperatures perpendicular to the magnetic field also increased, leading to marginal mirror-mode stability. Possible causes of these high-latitude magnetic holes are discussed. A new method of deconvolving the three-dimensional angular distributions of the protons and alpha particles from the ion instruments angular response is described in an appendix.

Ulysses observations of differential alpha-proton streaming in the solar wind

Data from the solar wind spectrometer on the Ulysses spacecraft are used to study the differential streaming between the alpha particles and protons in the solar wind over the heliographic distance range of 1.3 to 5.4 AU and latitudes from 0° to ±80° during the period December 1990 through September 1995. The study is based on 6-hour averages of the parameter Vαp = |Vα - Vp| where Vα and Vp are the vector velocities of the alpha particles and protons, respectively. It is found that Vαp decreases with increasing distance from the Sun and with decreasing solar wind speed. The distance and velocity dependencies can be combined into a single dependence on travel time Tfrom the Sun to the point of observation, with Vαp declining, on the average, as T−0.70±0.07. After normalization by this travel time factor, there is no residual dependence of Vαp, on heliographic latitude thus ruling out any rotational effects on either the acceleration or deceleration of the alphas relative to the protons. There is also no significant difference in the normalized values of Vαp between quasi-stationary and transient (coronal mass ejection) flows. The ratios Vαp/VA, where VA is the Alfven speed, and Vαp/Vwave, where Vwave is the observed propagation speed of Alfvenic fluctuations, both decline with increasing distance from the Sun, but Vαp/Vwave remains in the range of 1.0 to 1.5 out to a travel time of 5 or 10 days. There are weak correlations between the normalized value of Vαp and the amplitudes of fluctuations in both the magnitude and the direction of the interplanetary magnetic field. Although Vαp anticorrelates strongly with the ratio of the Coulomb collision time to the solar wind expansion time, it is believed that the correlation is not evidence of a cause and effect relation between those two parameters over much of the solar wind regime observed by Ulysses. Where comparisons are possible, the Ulysses data closely agree with extrapolations of the Helios data to greater solar distances.

Comment on the abundances of rotational and tangential discontinuities in the solar wind

[1]xa0Several recent studies indicate that there are very few discontinuities in the solar wind that have both the small changes in field magnitude and the large components of the magnetic field normal to the discontinuity surface that are commonly assumed to be signatures of rotational discontinuities (RDs). Those results call into question the validity of earlier studies based on the minimum variance technique for determining the direction of the normal to the discontinuity; many of those studies concluded that RDs occur more frequently than tangential discontinuities (TDs). The new results raise the question of whether the discontinuities with both small magnitude changes and small normal components are TDs with small magnitude changes or RDs with small normal components. Reexamination of an earlier study of discontinuities observed with the ISEE 3 spacecraft and consideration of the plasma jump conditions across some of the discontinuities observed by Cluster fail to give a clear answer. The pros and cons of the two interpretations are summarized.

Generation mechanism for magnetic holes in the solar wind

A new mechanism for generation of magnetic holes in the solar wind is presented. In the high speed solar wind, large-amplitude right-hand polarized Alfvenic wave packets propagating at large angles to the ambient magnetic field are shown to generate magnetic holes (MHs). Characteristics of these holes crucially depend on plasma β (β being the ratio of kinetic pressure to magnetic pressure) and the ratio of electron temperature T c to proton temperature T i . Proton temperature anisotropy is found to be favorable but not essential for the development of MHs. From our simulations we observe MHs with microstructures bounded by sharp gradients (magnetic decreases) in some cases. Tile holes generated by this process have thicknesses of hundreds of ion Larmor radii, typical of many of the solar wind hole observations, the depths of the holes are also comparable. The theory can explain the presence of MHs seen in the solar wind for those cases when anisotropies are not favorable for the development of the mirror mode instability.

Phase-steepened Alfvén waves, proton perpendicular energization and the creation of magnetic holes and magnetic decreases: The ponderomotive force

[1]xa0Solar wind protons detected within Magnetic Holes (MHs) and Magnetic Decreases (MDs) are found to be preferentially heated perpendicular to 0. The MHs/MDs are associated with the phase-steepened edges of nonlinear Alfven waves. The proton anisotropies can lead to the proton cyclotron and mirror mode plasma instabilities. We examine the Ponderomotive Force (PF), a phenomenon due to wave pressure gradients, and show that for this plasma regime and for phase-steepened Alfven waves, the PF proton acceleration/energization will primarily be orthogonal to B0. It is suggested that accelerated ions create the MHs/MDs by a diamagnetic effect.

Solar wind measurements with SOHO: The CELIAS/MTOF proton monitor

The proton monitor, a small subsensor in the Charge, Element, and Isotope Analysis System/Mass Time-of-Flight (CELIAS/MTOF) experiment on the SOHO spacecraft, was designed to assist in the interpretation of measurements from the high mass resolution main MTOF sensor. In this paper we demonstrate that the proton monitor data may be used to generate reasonably accurate values of the solar wind proton bulk speed, density, thermal speed, and north/south flow direction. Correlation coefficients based on comparison with the solar wind measurements from the SWE instrument on the Wind spacecraft range from 0.87 to 0.99. On the basis of the initial 12 months of observations, we find that the proton momentum flux is almost invariant with respect to the bulk speed, confirming a previously published result. We present observations of two interplanetary shock events, and of an unusual solar wind density depletion. This large density depletion, and the correspondingly large drop in the solar wind ram pressure, may have been the cause of a nearly simultaneous large increase in the flux of relativistic magnetospheric electrons observed at geosynchronous altitudes by the GOES 9 spacecraft. Extending our data set with a 10-year time span from the OMNIWeb data set, we find an average frequency of about one large density depletion per year. The origin of these events is unclear; of the 10 events identified, 3 appear to be corotating and at least 2 are probably CME related. The rapidly available, comprehensive data coverage from SOHO allows the production of near-real time solar wind parameters that are now accessible on the World Wide Web.