The Suess-Urey mission (return of solar matter to Earth)

The Suess-Urey (S-U) mission has been proposed as a NASA Discovery mission to return samples of matter from the Sun to the Earth for isotopic and chemical analyses in terrestrial laboratories to provide a major improvement in our knowledge of the average chemical and isotopic composition of the solar system. The S-U spacecraft and sample return capsule will be placed in a halo orbit around the L1 Sun-Earth libration point for two years to collect solar wind ions which implant into large passive collectors made of ultra-pure materials. Constant Spacecraft-Sun-Earth geometries enable simple spin stabilized attitude control, simple passive thermal control, and a fixed medium gain antenna. Low data requirements and the safety of a Sun-pointed spinner, result in extremely low mission operations costs.     

Ulysses observations of latitude gradients in the heliospheric magnetic field: Radial component and variances

The radial component of the magnetic field at Ulysses, over latitudes from –10° to –45° and distances from 5.3 to 3.8 AU, compares very well with corresponding measurements being made by IMP-8 in the ecliptic at 1AU. There is little, if any, evidence of a latitude gradient. Variances in the field, normalized to the square of the field magnitude, show little change with latitude in variations in the magnitude but a large increase in the transverse field variations. The latter are shown to be caused by the presence of large amplitude, long period Alfvénic fluctuations. This identification is based on the close relation between the magnetic field and velocity perturbations including the effect of anisotropy in the solar wind pressure. The waves are propagating outward from the Sun, as in the ecliptic, but variance analysis indicates that the direction of propagation is radial rather than field-aligned. A significant long-period component of 10 hours is present.     

Genesis on-board determination of the solar wind flow regime

Some of the objectives of the Genesis mission require the separate collection of solar wind originating in different types of solar sources. Measurements of the solar wind protons, alpha particles, and electrons are used on-board the spacecraft to determine whether the solar-wind source is most likely a coronal hole, interstream flow, or a coronal mass ejection. A simple fuzzy logic scheme operating on measurements of the proton temperature, the alpha-particle abundance, and the presence of bidirectional streaming of suprathermal electrons was developed for this purpose. Additional requirements on the algorithm include the ability to identify the passage of forward shocks, reasonable levels of hysteresis and persistence, and the ability to modify the algorithm by changes in stored constants rather than changes in the software. After a few minor adjustments, the algorithm performed well during the initial portion of the mission. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solar wind corotating stream interaction regions out of the ecliptic plane: Ulysses

Ulysses plasma observations reveal that the forward shocks that commonly bound the leading edges of corotating interaction regions (CIRs) beyond 2 AU from the Sun at low heliographic latitudes nearly disappeared at a latitude of S26°. On the other hand, the reverse shocks that commonly bound the trailing edges of the CIRs were observed regularly up to S41.5°, but became weaker with increasing latitude. Only three CIR shocks have been observed poleward of S41.5°; all of these were weak reverse shocks. The above effects are a result of the forward waves propagating to lower heliographic latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a global model of solar wind flows that originate in a simple tilted-dipole geometry back at the Sun.     

Ulysses solar wind observations to 56° south

In the 25 months since Jupiter flyby, the Ulysses spacecraft has climbed southward to a heliolatitude of 56°. This transit has been marked by an evolution from slow, dense coronal streamer belt solar wind through two regions where the rotation of the Sun carried Ulysses back and forth between streamer belt and polar coronal hole flows, and finally into a region of essentially continuous fast, low density solar wind from the southern polar coronal hole. Throughout these large changes, the momentum flux normalized to 1 AU displays very little systematic variation. In addition, the bulk properties of the polar coronal hole solar wind are quite similar to those observed in high speed streams in the ecliptic plane at 1 AU. Coronal mass ejections and forward and reverse shocks associated with corotating interaction regions have also been observed at higher heliolatitudes, however they are seen less frequently with increasing southern heliolatitude. Ulysses has thus far collected data from 20° of nearly contiguous solar wind flows from the polar coronal hole. We examine these data for characteristic variations with heliolatitude and find that the bulk properties in general show very little systematic variation across the southern polar coronal hole so far.     

Measurements of the properties of solar wind plasma relevant to studies of its coronal sources

Interplanetary measurements of the speeds, densities, abundances, and charge states of solar wind ions are diagnostic of conditions in the source region of the solar wind. The absolute values of the mass, momentum, and energy fluxes in the solar wind are not known to an accuracy of 20%. The principal limitations on the absolute accuracies of observations of solar wind protons and alpha particles arise from uncertain instrument calibrations, from the methods used to reduce the data, and from sampling biases. Sampling biases are very important in studies of alpha particles. Instrumental resolution and measurement ambiguities are additional major problems for the observation of ions heavier than helium. Progress in overcoming some of these measurement inadequacies is reviewed.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Scientific results from the pioneer Saturn infrared radiometer

The Pioneer 11 Infrared Radiometer instrument made observations of Saturn and its rings in broadband channels centered at 20 and 45 μm and obtained whole-disk information on Titan. A planetary average effective temperature of 96.5±2.5 K implies a total emission 2.8 times the absorbed sunlight. Correlation with radio science results implies that the molar fraction of H₂ is 90±3% (assuming the rest is He). Temperatures at the 1 bar level are 137 to 140 K; regions appearing cooler may be overlain by a cloud acting as a 124 K blackbody surface. A minimum temperature averaging 87 K is reached near 0.06 bars. Ring boundaries and optical depths are consistent with those at optical wavelengths. Ring temperatures are 64–86 K on the south (illuminated) side, ～54 K on the north (unilluminated) side, and at least 67 K in Saturn's shadow. There is evidence for a south to north drop in ring temperatures. Titan's 45 μm brightness temperature is 75±5 K.     

Magnetic holes in the solar wind and their relation to mirror-mode structures

Data obtained by the Ulysses magnetometer and solar wind analyzer have been combined to study the properties of magnetic holes in the solar wind between 1 and 5.4 AU and to 23° south latitude. Although the plasma surrounding the holes was generally stable against the mirror instability, there are indications that the holes may have been remnants of mirror mode structures created upstream of the points of observation. Those indications include: (1) For the few holes for which proton or alpha-particle pressure could be measured inside the hole, the ion thermal pressure was always greater than in the plasma adjacent to the holes. (2) The plasma surrounding many of the holes was marginally stable for the mirror mode, while the plasma environment of all the holes was significantly closer to mirror instability than was the average solar wind. (3) The plasma containing trains of closely spaced holes was closer to mirror instability than was the plasma containing isolated holes. (4) The near-hole plasma had much higher ion (ratio of thermal to magnetic pressure) than did the average solar wind.     

Propagating Shocks

A survey of propagating interplanetary shocks reveals the following properties: (1) More shocks are observed around solar-activity maximum than minimum. (2) The maximum frequency of shock occurrence is between 2 and 5 AU. (3) Shocks slow down in the inner solar system, but in the outer solar system some may get a boost in speed when overtaken by a following shock. (4) The average shock strength (as measured by either the Mach number or the density ratio) also reaches peak values at distances of 2–5 AU. (5) Shocks are stronger at their noses than at their flanks. (6) At 1 AU, there are many more quasiperpendicular than quasiparallel shocks with the frequency of occurrence roughly constant with the cosine of the angle between the upstream field and the shock normal.     

Advanced techniques for plasma composition and dynamics measurements

Magnetic and RF mass spectrometers have been used routinely in ionospheric research, while traditional ionospheric, magnetospheric, and interplanetary plasma measurements have been made with several types of electrostatic analyzers. Proper interpretation of these data is possible if the spectral peaks are well defined, although ambiguities between fast, light ions and slow, heavy ions cannot always be satisfactorily resolved. Recent and planned experiments involve the study of plasmas which are sufficiently energetic that the spectral peaks overlap. Furthermore, these studies of ionosphere/magnetosphere coupling and of the interaction of the solar wind with the atmospheres of Venus and comets require unambiguous identification of the ion masses with simultaneous mapping of the three-dimensional velocity distribution function of each ion species. This challenge has been partially met by several new types of instruments; the two most common types involve either (1) sequential electrostatic and magnetic analyses or (2) sequential electrostatic and time-of-flight analyses. Some new instruments have also incorporated measurements of total kinetic energy, electric charge, or secondary emission coefficients as diagnostic tools. This paper reviews these recent advances and points out areas where further development is expected and needed.