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Acceleration of energetic particles is a fundamental and ubiquitous mechanism in space and astrophysical plasmas. One of the open questions is the role of the sheath region behind the shock in the acceleration process. We analyze observations by Solar Orbiter, BepiColombo and the L1 spacecraft to explore the structure of a coronal mass ejection (CME)-driven sheath and its relation to enhancements of energetic ions that occurred on April 19-20, 2020. Our detailed analysis of the magnetic field, plasma and particle observations show that the enhancements were related to the Heliospheric Current Sheet crossings related to the reconnecting current sheets in the vicinity of the shock and a mini flux rope that was compressed at the leading edge of the CME ejecta. This study highlights the importance of smaller-scale sheath structures for the energization process. These structures likely formed already closer to the Sun and were swept and compressed from the upstream wind past the shock into the sheath. The upcoming observations by the recent missions (Solar Orbiter, Parker Solar Probe and BepiColombo) provide an excellent opportunity to explore further their role.

Coronal Mass Ejections (CMEs) and their interplanetary counterparts (ICMEs) are key drivers of space weather throughout the heliosphere. Observational studies are used to understand their evolution and for developing existing models and theory in space weather forecasting. Motivated by the future exploration of the solar high-latitudes by Solar Orbiter and complimented by Parker Solar Probe, we aim to contribute to the understanding of high-latitude CMEs as they develop into ICMEs. We examine a high-latitude CME and its subsequent ICME using data from STEREO, Ulysses, and near-Earth spacecraft. We apply a triangulation method to the remote-sensing images from the twin STEREO spacecraft and conduct a multi-spacecraft analysis using the in-situ Ulysses, STEREO, and near-Earth spacecraft data. The Ulysses observations, supported by the other spacecraft, provides a clear picture of the ICME geometry and structure: a shock, followed by a sheath region, and a magnetic flux rope followed by a high-speed stream. This ICME differs from the known ‘over-expanding’ types observed in the high-latitudes by the Ulysses mission, in that it straddles a region between the slow and fast solar winds which in itself drives a shock.

We present observations of the same magnetic cloud made near Earth by the Advance Composition Explorer (ACE), Wind, and the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission comprising the Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and THEMIS C spacecraft, and later by Juno at a distance of 1.2 AU. The spacecraft were close to radial alignment throughout the event, with a longitudinal separation of $3.6^{\circ}$ 3.6 ∘ between Juno and the spacecraft near Earth. The magnetic cloud likely originated from a filament eruption on 22 October 2011 at 00:05 UT, and caused a strong geomagnetic storm at Earth commencing on 24 October. Observations of the magnetic cloud at each spacecraft have been analysed using minimum variance analysis and two flux rope fitting models, Lundquist and Gold–Hoyle, to give the orientation of the flux rope axis. We explore the effect different trailing edge boundaries have on the results of each analysis method, and find a clear difference between the orientations of the flux rope axis at the near-Earth spacecraft and Juno, independent of the analysis method. The axial magnetic field strength and the radial width of the flux rope are calculated using both observations and fitting parameters and their relationship with heliocentric distance is investigated. Differences in results between the near-Earth spacecraft and Juno are attributed not only to the radial separation, but to the small longitudinal separation which resulted in a surprisingly large difference in the in situ observations between the spacecraft. This case study demonstrates the utility of Juno cruise data as a new opportunity to study magnetic clouds beyond 1 AU, and the need for caution in future radial alignment studies.

Understanding the evolution of interplanetary coronal mass ejections (ICMEs) as they propagate through the heliosphere is essential in forecasting space weather severity. Much of our knowledge of ICMEs has been gained using in-situ measurements from single spacecraft, although the increasing number of missions in the inner heliosphere has led to an increase in multi-spacecraft studies improving our understanding of the global structure of ICMEs. Whilst most such recent studies have focused on the inner heliosphere within 1 AU, Juno cruise phase data provides a new opportunity to study ICME evolution over greater distances. We present analysis of ICMEs observed in-situ both by Juno and at least one other spacecraft within 1 AU to investigate their evolution as they propagate through the heliosphere. Investigation of the sheath region and timing considerations between spacecraft allows for the general shape of the shock front to be reconstructed. Combining in-situ observations and results of flux rope fitting techniques determines the global picture of the ICME as it propagates. However, effects on in-situ observations due to radial evolution and due to the longitudinal separation between multi-spacecraft remain hard to separate. We note the importance of the interplanetary environment in which the ICME propagates and the need for caution in radial alignment studies.

International audience; The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described.

Context.On 2020 April 19 a coronal mass ejection (CME) was detected in situ by Solar Orbiter at a heliocentric distance of about 0.8 AU. The CME was later observed in situ on April 20 by the Wind and BepiColombo spacecraft whilst BepiColombo was located very close to Earth. This CME presents a good opportunity for a triple radial alignment study, as the spacecraft were separated by less than 5° in longitude. The source of the CME, which was launched on April 15, was an almost entirely isolated streamer blowout. The Solar Terrestrial Relations Observatory (STEREO)-A spacecraft observed the event remotely from −75.1° longitude, which is an exceptionally well suited viewpoint for heliospheric imaging of an Earth directed CME.Aims.The configuration of the four spacecraft has provided an exceptionally clean link between remote imaging and in situ observations of the CME. We have used the in situ observations of the CME at Solar Orbiter, Wind, and BepiColombo and the remote observations of the CME at STEREO-A to determine the global shape of the CME and its evolution as it propagated through the inner heliosphere.Methods.We used three magnetic flux rope models that are based on different assumptions about the flux rope morphology to interpret the large-scale structure of the interplanetary CME (ICME). The 3DCORE model assumes an elliptical cross-section with a fixed aspect-ratio calculated by using the STEREO Heliospheric Imager (HI) observations as a constraint. The other two models are variants of the kinematically-distorted flux rope (KFR) technique, where two flux rope cross-sections are considered: one in a uniform solar wind and another in a solar-minimum-like structured solar wind. Analysis of CME evolution has been complemented by the use of (1) the ELEvoHI model to compare predicted CME arrival times and confirm the connection between the imaging and in situ observations, and (2) the PREDSTORM model, which provides an estimate of theDstindex at Earth using Solar Orbiter magnetometer data as if it were a real–time upstream solar wind monitor.Results.A clear flattening of the CME cross-section has been observed by STEREO-A, and further confirmed by comparing profiles of the flux rope models to the in situ data, where the distorted flux rope cross-section qualitatively agrees most with in situ observations of the magnetic field at Solar Orbiter. Comparing in situ observations of the magnetic field between spacecraft, we find that the dependence of the maximum (mean) magnetic field strength decreases with heliocentric distance asr−1.24 ± 0.50(r−1.12 ± 0.14), which is in disagreement with previous studies. Further assessment of the axial and poloidal magnetic field strength dependencies suggests that the expansion of the CME is likely neither self-similar nor cylindrically symmetric.

Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation, and the evolution of ICME magnetic field structure during propagation, are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east-west direction with heliocentric distance., Comment: 35 pages, 10 figures

We report recent observations of energetic particles at energies 1–40 MeV/n made by the COSPIN/LET instrument onboard the Ulysses spacecraft during the period of intense solar activity in August/September 2005 during the declining phase of solar cycle 23. Ulysses, having started its climb to high southern latitudes for the third time, was located at ~5 AU, at a helio-latitude of ~30 degrees south. It detected the arrival of a solar wind compound stream resulting from the merging of a series of fast halo CMEs ejected from the Sun in late August and early September 2005 and their interaction with the pre-existing pattern of solar wind Stream Interaction Regions (SIRs) in the ambient medium through which they propagated. The heavy ion intensities are observed by COSPIN/LET to remain elevated for at least 20 days following the very intense X17.0/3B solar flare on 7 September and its associated very fast CME (plane of sky projected CME speed ~2400 km s−1). We carry out an analysis of the composition of the particle increases observed at the location of the spacecraft. Although the composition signatures were predominantly Solar Energetic Particle (SEP)-like, after the passage of the compound stream over Ulysses, in association with a characteristic forward and reverse shock pair, the observations showed evidence of an enhanced He content.

During November 1992, a series of forward and reverse shocks passed the ULYSSES spacecraft. Spectral and anisotropy measurements are reported for protons and alpha particles between 0.28 and 6 MeV observed by the Energetic Particle Composition Experiment, data recorded by the Magnetometer Experiment and the high-energy (2.7–300 MeV) proton data from the Kiel Electron Telescope. An analysis of energetic particle, plasma and magnetometer data from ULYSSES has allowed a unique study of the corresponding arrival of fare particles, particles within a corotating interaction region and particles transported with a coronal mass ejection. We present an analysis of these data in terms of possible diffusive shock acceleration but conclude that this is likely to be incompatible with the short transit time of the particles. Shock drift acceleration of particles with energies 0.3 MeV/nucleon or solar acceleration followed by particle trapping behind the shock front are alternative possibilities.

The B-field structures of two ICMEs each observed by a pair of spacecraft close to radial alignment have been analysed. The ICMEs were observed in situ by MESSENGER and STEREO-B in Nov. 2010 and Nov. 2011, while the spacecraft were separated by more than 0.6 AU in heliocentric distance, less than 4{\deg} in heliographic longitude, and less than 7{\deg} in heliographic latitude. Both ICMEs took around 2 days to travel between the spacecraft. The ICME B-field profiles observed by MESSENGER have been mapped to the heliocentric distance of STEREO-B and compared directly to the profiles observed by STEREO-B. Figures that result from this mapping allow for easy qualitative assessment of similarity in the profiles. Macroscale features in the profiles that varied on timescales of 1 hour, and which corresponded to the underlying flux rope structure of the ICMEs, were well correlated in the solar east-west and north-south directions, with Pearson's correlation coefficients of around 0.85 and 0.95, respectively; microscale features with timescales of 1 minute were uncorrelated. Overall correlation values in the profiles of one ICME were increased when an apparent change in the flux rope axis direction between the observing spacecraft was taken into account. The high degree of similarity seen in the B-field profiles may be interpreted in in two ways. If the spacecraft sampled the same region of each ICME (i.e. if the spacecraft angular separations can be neglected), the similarity indicates that there was little evolution in the underlying structure of the sampled region during propagation. Alternatively, if the spacecraft observed different, nearby regions within the ICMEs, it indicates that there was spatial homogeneity across those different regions. The field structure similarity observed in these ICMEs points to the value of placing in situ space weather monitors well upstream of the Earth., Comment: 22 pages, 9 figures

The images taken by the Heliospheric Imager (HI) instruments, part of the SECCHI imaging package on board the pair of STEREO spacecraft, provide information on the radial and latitudinal evolution of the plasma transported by coronal mass ejections (CMEs). In this case study, a CME, appearing near 15 UT on 2007 November 15 in SECCHI coronagraph images, leads to the formation of two out-flowing density structures (DSs) in the heliosphere. The analysis of time-elongation maps constructed from images obtained by the HI instruments shows that these DSs were propagating along the Sun-Earth line. A direct comparison of HI images and in situ measurements taken near Earth could therefore be performed. These two DSs are separated by a cavity associated with little brightness variation or equivalently little electron density variation. In situ measurements made in the solar wind near Earth on 2007 November 20 show that this cavity corresponds to a magnetic cloud (MC). While the leading DS is related to the sheath in front of the MC, the second DS is located on the sunward side of the MC where high-speed solar wind from a coronal hole catches up and interacts with the MC. We conclude that HI observes the sub-structures of a merged interaction region (MIR), a region of the interplanetary medium where the total solar wind pressure is greatly enhanced by the interaction of an MC with the ambient solar wind. This MIR caused the largest geomagnetic storm in 2007.

We present the first observational evidence of the near-Sun distortion of the leading edge of a coronal mass ejection (CME) by the ambient solar wind into a concave structure. On 2007 November 14, a CME was observed by coronagraphs onboard the STEREO-B spacecraft, possessing a circular cross section. Subsequently the CME passed through the field of view of the STEREO-B Heliospheric Imagers where the leading edge was observed to distort into an increasingly concave structure. The CME observations are compared to an analytical flux rope model constrained by a magnetohydrodynamic solar wind solution. The resultant bimodal speed profile is used to kinematically distort a circular structure that replicates the initial shape of the CME. The CME morphology is found to change rapidly over a relatively short distance. This indicates an approximate radial distance in the heliosphere where the solar wind forces begin to dominate over the magnetic forces of the CME influencing the shape of the CME.

This report assesses the current status of research relating the origin at the Sun, the evolution through the inner heliosphere and the effects on the inner heliosphere of the interplanetary counterparts of coronal mass ejections (ICMEs). The signatures of ICMEs measured by in-situ spacecraft are determined both by the physical processes associated with their origin in the low corona, as observed by space-borne coronagraphs, and by the physical processes occurring as the ICMEs propagate out through the inner heliosphere, interacting with the ambient solar wind. The solar and in-situ observations are discussed as are efforts to model the evolution of ICMEs from the Sun out to 1 AU.

While interplanetary coronal mass ejections (ICMEs) are understood to be the heliospheric counterparts of CMEs, with signatures undeniably linked to the CME process, the variability of these signatures and questions about mapping to observed CME features raise issues that remain on the cutting edge of ICME research. These issues are discussed in the context of traditional understanding, and recent results using innovative analysis techniques are reviewed.

In February 2004 the Ulysses spacecraft had its second flyby at Jupiter at 0.8 AU distance from the planet. Twenty-eight dust streams emanating from the jovian system were measured between November 2002 and August 2005 while the spacecraft was within 4 AU of the planet, scanning jovigraphic latitudes from + 75 ∘ to - 25 ∘ . The highest dust fluxes were measured in mid 2004 at the passage of the equatorial plane of the planet when more than 2000 impacts per day were measured. The grain impact direction is correlated with the polarity and strength of the interplanetary magnetic field. At high jovigraphic latitudes, the impact rates show a periodicity of 26 days, closely matching the solar rotation period, while at the jovian equator the streams fluctuate with twice this period. The 14-day subharmonic streams alternate in arrival direction and are correlated with the pointing of the interplanetary magnetic field. Dust fluxes measured above and below the equatorial plane roughly decrease with the inverse square of the distance from the planet while along the equatorial plane dust fluxes are enhanced by up to 2 orders of magnitude.

We have identified 91 Petschek-type exhausts associated with local, quasi-stationary magnetic reconnection in the solar wind in plasma and magnetic field data from the Ulysses spacecraft obtained over a wide range of heliocentric distances (1.4-5.4 AU) and latitudes (S79°-N65°). The characteristic signature of an exhaust was a brief (minutes) interval of accelerated or decelerated plasma flow within a bifurcated current sheet in which changes in magnetic field and flow velocity were correlated at one edge and anticorrelated at the other. Transitions from outside to inside the exhausts were always slow-mode-like, the exhausts appearing to an observer as encounters with closely spaced, forward-reverse, slow-mode wave (shock) pairs. The exhausts almost universally occurred in low-speed or interplanetary coronal mass ejection plasma having low proton β (

Recent observations from the $\it Ulysses$ spacecraft (Simnett 2003, Sol. Phys., 213, 387) have measured the energy spectrum of the ambient ions associated with the passage of a fast magnetosonic quasi-perpendicular shock driven by a coronal mass ejection (CME). Near the shock front the energy spectrum of the accelerated ions, which are predominantly protons, exhibits features (maxima) at about 250 keV-1.5 MeV. We show that these are the right order of magnitude for protons which have been accelerated by the surfatron mechanism at a fast magnetosonic shock wave propagating perpendicular to the ambient magnetic field. The in situ measurements therefore illustrate directly that this mechanism could possibly be responsible for interplanetary particle acceleration. The shock is also accompanied by an increase in the near-relativistic electron intensity of almost two orders of magnitude. We present a similar event observed by the ACE spacecraft.

From all the transient events identified in interplanetary space by in-situ measurements, Magnetic Clouds (MCs) are among the most intriguing ones. They are a special kind of Interplanetary Coronal Mass Ejections (ICMEs), characterized by a well-defined magnetic field configuration. We use a list of 40 MCs detected by Ulysses to study bidirectional flows of protons in the 0.5 MeV energy range. Solar wind ions are also analysed in order to compare cloud to non-cloud ICMEs.The enhancement in freezing-in temperatures inside the clouds, obtained with data from the SWICS instrument, provides insights into processes occurring early during the ejection of the material and represents a complementary tool to differentiate cloud from non-cloud ICMEs. At higher energies, directional information for protons obtained with the EPAC instrument allows a comparison with previous results concerning bidirectional suprathermal electrons. The findings are qualitatively comparable. Apparently, the portion of bidirectional flows inside magnetic clouds is neither heavily dependent on distance from the Sun nor on parameters obtained from a flux rope model.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The first passage of the Ulysses spacecraft to high latitudes was during the declining phase of solar cycle 22. The recent second passage to high latitudes was close to the maximum of solar cycle 23. The axis of the dipolar component of the Sun's magnetic field was close to 90° from the spin axis, and the coronal neutral line extended up to high latitudes. A variable but generally slow solar wind was observed all the way up to the highest latitudes reached by Ulysses, as was the sector structure of the magnetic field. The high-latitude heliosphere was populated with intensities of energetic particles with energies around 1 MeV several orders of magnitude above background. We show how the changes in the Sun's magnetic field, the coronal holes, and the configuration of the heliosphere could be responsible for the differences between particle observations in the two orbits.

Observations during the recent ascent of Ulysses to the south polar region are reported. Large variations in field magnitude are associated principally with solar wind interaction regions. The observed spiral angle agrees reasonably well with the Parker model. The magnetic flux, the product of the radial field component and the square of the radial distance (r² BR), is independent of latitude implying non-radial expansion of the solar wind. Two magnetic sectors are present up to a latitude of ≈ −78° above which a single negative (inward) polarity is observed consistent with the polar field not yet having reversed. Crossings between sectors, extrapolated back to the sun, appear consistent with a single warped current sheet tilted slightly to the rotation axis. Field lines in the polar region passing near the current sheet originate at low latitudes. They provide access of high energy solar particles to the polar region.

Charge state distributions of heavy solar wind ions measured in interplanetary space can be used to probe the physical conditions in the solar corona. This paper presents a study of the charge state distributions and the magnetic topology of 56 coronal mass ejections (CMEs) observed in interplanetary space by the Ulysses spacecraft. The analysis of the data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument and the Vector Helium Magnetometer (VHM) experiment onboard Ulysses shows a clear correlation between the charge state distributions and the magnetic topology of CMEs. Almost all CMEs whose charge state distributions are shifted to higher charge states with respect to the ambient solar wind have the structure of magnetic clouds, whereas CMEs with the same charge state distributions as the surrounding solar wind do not show magnetic cloud structure. This correlation is found for CMEs observed at low, mid, and high solar latitudes. On the basis of the numerical solution of the ionization/recombination equations for oxygen and silicon, it is investigated which changes of the electron temperature, electron density, and the speed of the ions in the source region of the CMEs can reproduce the observations. It is shown that the main reason for the observed enhancement of higher charge states in the cloud CMEs is an increased electron temperature. However, the evolution of the density and velocity of the CMEs before the charge states freeze in cannot be neglected.

Pioneer and Ulysses observations of energetic particles and solar wind plasma and magnetic fields are analyzed to study particle transport in corotating interaction regions (CIRs) and near stream interfaces (SIs). Energetic particle diffusion with a lowered coefficient near the SIs as compared with the free solar wind may account for the energetic particle profiles in CIRs and for the modulation of Jovian electrons. Our analyses include examination of the magnetic power and variances perpendicular and parallel to the magnetic field for a range of wave numbers and for a range of conditions within two CIRs. Since the magnetic field immediately on either side of a SI, but near it, is not connected to either the forward shock or reverse shock, it is the cross-field motion of the energetic particles that is relevant. Since the cross-field transport is thought to be related to field line mixing, or random walk, we examined magnetic field fluctuations normal to the average field to determine whether any signature of reduced perpendicular particle transport could be found. Evidence consistent with reduced particle transport near the SI was indeed found when we examined time series of the 1-min averages of the magnetic field components. A planar magnetic structure was associated with the trailing unshocked layer following the SIs (i.e., SI to SI + 12 hours) and also with the entire CIRs, but it was not present in the free solar wind. We have quantitatively examined the effects of shear and compression on particle transport and conclude that the effects of shear and compression reduce particle transport in CIRs and in the vicinity of SIs.

Ulysses observed well-defined stream interaction regions, SIRs, associated with solar wind stream structure up to a latitude of S65° and shocks to at least a latitude of S71° during the second polar orbit. These SIRs and shocks produced a substantial heliospheric processing of the solar wind. Only a subset of the SIRs recurred on successive solar rotations and only about half of the well-defined SIRs observed poleward of S9.8° were bounded by forward-reverse shock pairs. The majority of the SIRs had local magnetic topologies and azimuthal orientations similar to, but meridional tilts different from, those observed in the first polar orbit when most SIRs corotated with the Sun. The irregular meridional tilts presumably were a consequence of a complex coronal geometry and the temporally evolving nature of the solar wind flow at this time. A lack of reverse shocks poleward of S54° (with one exception) and a lack of well defined SIRs poleward of S65° is evidence that SIRs develop more slowly with distance at high latitudes.

We present observations of energetic ions from the Cosmic Ray and Solar Particle Investigation (COSPIN) and Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energy (HISCALE) instruments on board the Ulysses spacecraft. We examine in detail the onset of energetic ions observed during the March 1991 series of solar events. Active region 6555, at E28 at 2247 UT on March 22, 1991 (day 81), was most likely responsible for the high-energy protons subsequently seen at Ulysses. A slow increase in the high-energy (∼50 MeV) proton intensity, made up of many small but rapid intensity increases, began around 9 hours after the onset of the event, taking 1.5 days to reach maximum intensity. On the flank, and in front of a coronal mass ejection (CME), coming from a previous event, we observed a region with a drop in the magnetic field strength. This region was populated with hot shocked plasma and, owing to the low magnetic field pressure, had an ion plasma beta >1. Mirror mode waves, with discontinuities appearing at the edges of the waves, produced a larger-than-average number of magnetic field discontinuities, a large fraction of the discontinuities being aligned with their normals oriented along the radial direction, suggesting that they were being convected outward by the CME. These discontinuities acted as a series of barriers to the propagation of the energetic solar particles coming from the solar event, delaying substantially the onset of the particle event at Ulysses.

In this paper, magnetic and plasma measurements are used to analyze 17 interplanetary coronal mass ejections (CMEs) identified by Ulysses during its in-ecliptic passage to Jupiter. We focus on the expansion characteristics of these CMEs (as inferred from the time rate of change of the velocity profiles through the CMEs) and the properties of 14 forward shocks unambiguously associated with these CMEs. We highlight radial trends from 1 to 5.4 AU. Our results indicate that the CMEs are generally expanding at all heliocentric distances. With regard to the shocks preceding these ejecta, we note the following: (1) There is a clear tendency for the shock speed (in the upstream frame of reference) to decrease with increasing heliocentric distance as the CMEs transfer momentum to the ambient solar wind and slow down; (2) 86% of the shock fronts are oriented in the ecliptic plane such that their normals point westward (i.e., in the direction of planetary motion about the Sun), (3) 86% of the shocks are propagating toward the heliographic equator; and (4) no clear trend was found in the strength of the shocks versus heliocentric distance. These results are interpreted using simple dynamical arguments and are supported by fluid and magnetohydrodynamic (MHD) simulations.

Ulysses encountered two separate sequences of corotating interaction regions (CIRs) in 1992–3 and 1996–7 when the spacecraft was in the southern and northern solar hemispheres, respectively. We have examined magnetometer data from one event observed in each hemisphere. Using two separate analytical methods we provide evidence for the magnetic field vectors within these CIRs being strongly confined to a series of near-parallel planes. The distribution of the field within the interaction region is characteristic of that observed in planar magnetic structures. This planar order is evident over both small (3-hour) and large (whole event) scales. The CIRs display opposed north-south tilts that are consistent with previous authors' analysis of solar wind plasma data. These results are in direct agreement with the tilted dipole model of three-dimensional corotating stream interactions.

SUMMARY We develop statistical testing methodology for closely spaced spectral lines based on multitaper spectrum estimates and apply it to a time series of solar magnetic field magnitude data recorded by the Ulysses spacecraft. The test is formulated through complex-valued weighted least squares. Using this test in combination with the test for well-separated lines, we can accurately detect the solar equatorial rotation frequency of the sun, and its harmonics, from data recorded by Ulysses’s magnetometer at solar mid-latitudes; this is a potentially important result for solar physicists, since it has implications for the structure and dynamics of magnetic fields in the solar corona.

We examine the relation between the current sheet as derived from observations of the Sun's neutral line, and recurrent particle increases associated with co-rotating interactions regions (CIR) at 4 to 5 AU radial distance as observed by the Ulysses spacecraft during its rst out-of- ecliptic orbit around the Sun. We compare observations during the ascent to high southern latitudes, close to so- lar maximum, with observations during the return to the ecliptic from high northern latitudes, close to solar mini- mum, when the spacecraft was passing from the streamer belt to high-speed solar wind flow or vice-versa every solar rotation, and when regular recurrent particle increases were observed. Although conditions are very dierent, we show that the position of the Sun's neutral line, propagated out to the position of the spacecraft, can be used to explain the recurrence of the particle increases observed at 4-5 AU at Ulysses.

The Whole Sun Month campaign (August 10 to September 8, 1996) brought together a wide range of space-based and ground-based observations of the Sun and the interplanetary medium during solar minimum. The wealth of data collected provides a unique opportunity for testing coronal models. We develop a three-dimensional magnetohydrodynamic (MHD) model of the solar corona (from 1 to 30 solar radii) applicable to the WSM time period, using measurements of the photospheric magnetic field as boundary conditions for the calculation. We compare results from the computation with daily and synoptic white-light and emission images obtained from ground-based observations and the SOHO spacecraft and with solar wind measurements from the Ulysses and WIND spacecraft. The results from the MHD computation show good overall agreement with coronal and interplanetary structures, including the position and shape of the streamer belt, coronal hole boundaries, and the heliospheric current sheet. From the model, we can infer the source locations of solar wind properties measured in interplanetary space. We find that the slow solar wind typically maps back to near the coronal hole boundary, while the fast solar wind maps to regions deeper within the coronal holes. Quantitative disagreements between the MHD model and observations for individual features observed during Whole Sun Month give insights into possible improvements to the model.

In this report, we summarize measurements made by the plasma experiment on the Ulysses spacecraft during the period designated as “Whole Sun Month” (WSM, August 10 to September 8, 1996). This interval coincided with the return of solar wind variability at Ulysses. Ulysses was located at ∼ 28°N heliographic latitude, at a heliocentric distance of 4.25 AU, and on the opposite side of the Sun from Earth. In particular, we explore the evolution of the solar wind between the Sun and Ulysses for several rotations surrounding WSM. Specifically, we map Ulysses measurements back toward the Sun by applying a two-dimensional inverse MHD algorithm. This approach is compared with the commonly used constant speed (or ballistic) approximation. We find that the MHD mapping technique produces substantially better results when compared with solar observations. Both the Ulysses MHD-mapped results and the solar observations are consistent with a picture of a modestly tilted streamer belt (< 10°) that was deformed northward by an active region at 240° –; 270° longitude.

From October 1996 to January 1997, Ulysses was situated roughly above the west limb of the Sun as observed from Earth at a heliocentric distance of about 4.6 AU and a latitude of about 25 deg. This presents the first opportunity to compare Solar and Heliospheric Observatory (SOHO) limb observations of coronal mass ejections (CMEs) directly with their solar wind counterparts far from the Sun using the Ulysses data. During this interval, large eruptive events were observed above the west limb of the Sun by the Large Angle Spectrometric Coronagraph (LASCO) on SOHO on October 5, November 28, and December 21-25, 1996. Using the combined plasma and magnetic field data from Ulysses, the October 5 event was clearly identified by several distinguishing signatures as a CME. The November 28 event was also identified as a CME that trailed fast ambient solar wind, although it was identified only by an extended interval of counterstreaming suprathermal electrons. The December 21 event was apparently characterized by a six-day interval of nearly radial field and a plasma rarefaction. For the numerous eruptive events observed by the LASCO coronagraph during December 23-25, Ulysses showed no distinct, CMEs, perhaps because of intermingling of two or more of the eruptive events. By mapping the Ulysses observations back in time to the Sun assuming a constant flow speed, we have identified intervals of plasma that were accelerated or decelerated between the LASCO and Ulysses observations.

We analyze here the energy spectra of 50-keV to 20-MeV protons accelerated at corotating shocks observed at Ulysses in the southern heliosphere during 1992 and 1993. In general, our results are inconsistent with the predictions of two models based on diffusive shock acceleration theory (Fisk and Lee (1980) and Jones and Ellison (1991)). The main results are the following: (1) The relationships between the spectral indices and the shock compression ratio show significant departures from those predicted by the two models; the observed spectra are substantially harder than predicted; (2) the spectral indices at the reverse shocks depend strongly on the heliographic latitude of Ulysses; the hardest spectra are associated with strong quasi-perpendicular shocks observed between 208S and 308S; (3) the spectral indices at the reverse shocks are anticorrelated with the upstream plasma velocity V9 in the de Hoffman-Teller frame of the shock; and (4) the spectral indices at the forward shocks show no dependence on either the shock parameters or the heliographic latitude. The latitudinal dependence of the reverse shock spectra is probably related to the tilt (;258) of the heliospheric current sheet with respect to the solar equatorial plane during 1992 and 1993. The negative correlation between the spectral indices at the reverse shocks and V9 u indicates that the gradient drift mechanism plays a crucial role in accelerating particles at corotating interaction regions (CIRs). The differences in our observations at the forward and reverse shocks may be due to the presence of a more energetic seed population and an enhancement in the level of magnetic field fluctuations upstream of the trailing edges of the CIRs.

The heliospheric magnetic field over the poles of the sun has been examined to identify intervals when the polarity of the magnetic field deviates significantly from the calculated Parker spiral, using Ulysses observations in 1994 and 1995, near solar minimum. Intervals with deviation > 90° have been identified, corresponding to magnetic field vectors pointing in the hemisphere opposite to that of the dominant polarity in the high speed solar wind originating in the polar coronal holes. The number of inversion periods decreases as the averaging period is increased from 1 to 12 hours. Their distribution was found to vary in time over the northern solar pole, where the average direction also deviated from the Parker direction. The propagation direction of waves during polarity inversions show that these are caused by large-scale folds in the magnetic field, rather than by opposite polarity magnetic flux tubes originating near the sun.

Ulysses observed a stable strong CIR from early 1992 through 1994 during its first journey into the southern hemisphere. After the rapid latitude scan in early 1995, Ulysses observed a weaker CIR from early 1996 to mid-1997 in the northern hemisphere as it traveled back to the ecliptic at the orbit of Jupiter. These two CIRs are the observational basis of the investigation into the latitudinal structure of CIRs. The first CIR was caused by an extension of the northern coronal hole into the southern hemisphere during declining solar activity, whereas the second CIR near solar minimum activity was caused by small warps in the streamer belt. The latitudinal structure is described through the presentation of three 26-day periods during the southern CIR. The first at 24 Ss hows the full plasma interaction region including fast and slow wind streams, the compressed shocked flows with embedded stream interface and heliospheric current sheet (HCS), and the forward and reverse shocks with associated accelerated ions and electrons. The second at 40 S exhibits only the reverse shock, accelerated particles, and the 26-day modulation of cosmic rays. The third at 60 Ss hows only the accelerated particles and modulated cosmic rays. The possible mechanisms for the access of the accelerated particles and the CIR-modulated cosmic rays to high latitudes above the plasma

Corotating interaction regions (CIRs) in the middle heliosphere have distinct morpholog- ical features and associated patterns of turbulence and energetic particles. This report summarizes current understanding of those features and patterns, discusses how they can vary from case to case and with distance from the Sun and possible causes of those variations, presents an analytical model of the morphological features found in earlier qualitative models and numerical simulations, and identifies aspects of the features and patterns that have yet to be resolved.

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind.

The episode of intense solar activity on 4 and 6 November 1997 resulted in large and extended particle events observed by the Ulysses and WIND spacecraft. Ulysses was at a heliocentric distance of 5.34 AU, very close to the solar equator and 100° west in heliolongitude from earth. WIND was near the earth at 0.99 AU from the Sun and in the ecliptic plane. Both spacecraft detected particle events and magnetic field structures related to this intense activity. A likely scenario for these particle events is proposed. The differences in the particle intensity profiles observed by both spacecraft are explained in terms of their location with respect to the solar activity site, the existence of interplanetary structures traveling towards the spacecraft, and the nature of particle transport from the source up to the two spacecraft.

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.

An overall profile of plasma wave activity in a frequency range of 0.2 to 448 Hz in the solar wind is presented using 6 years of Ulysses data which cover a large range of heliographic latitudes (0° to ±80° ) at distances from 1 to 5 AU from the Sun. The spacecraft has continuously observed wave activity with peak power below the local electron cyclotron frequency f ce . Four distinct types of fluctuation phenomena have been observed: (1) enhanced electromagnetic fluctuations associated with the interplanetary shocks and heliospheric current sheet crossings and other solar wind turbulence; (2) enhanced electromagnetic fluctuations associated with compression regions of high-speed stream interfaces, which were observed in periods of increasing solar wind velocity; (3) electric fluctuations associated with the expanding solar wind, which were observed in periods of decreasing solar wind velocity; and (4) enhanced electric fluctuations in the high-latitude fast solar wind plasma. The fourth type of wave activity was observed nearly continuously with the relative power observed peaked near the local f ce . The first three types of waves were observed in the heliomagnetic streamer belt flows, where the spacecraft frequently encountered enhanced solar wind turbulence, interplanetary shocks, and current sheet crossings. The electromagnetic wave bursts (types 1 and 2) are likely to be whistler mode. The occurrences of apparently electrostatic waves during periods of expanding solar wind are coincident with significant reductions in the electron heat flux intensity. The generation mechanism of these electrostatic waves is still under investigation, but the observations may imply that these waves reduce the intensity of the heat flux through enhanced wave particle scattering associated with a heat flux instability.

The Ulysses spacecraft encountered 160 interplanetary shock waves between the time of its launch in October 1990 and the end of 1993 when it reached 48° S heliographic latitude. The onboard radio and plasma wave instrument observed many plasma waves associated with these shocks. Electrostatic waves at frequencies between the ion and electron plasma frequencies are observed intermittently throughout the solar wind but are often strongly enhanced near the shocks. These waves are usually interpreted as ion acoustic waves that have been Doppler shifted to the observed frequencies. They show very diverse characteristics in their amplitude and in their temporal behavior relative to the time of the shock passage at Ulysses. They are most likely to occur within a few minutes of shock passage, but on average they are enhanced above background levels for several hours before and after passage of the shock. The temporal behavior of the probability of occurrence of these waves is roughly symmetric relative to the time of shock passage and furthermore is quite similiar for quasi-parallel and quasi-perpendicular shocks and for forward and reverse shocks. These waves are correlated with several of the local plasma parameters measured by the Ulysses plasma instrument, most strongly with Te/Tp, the ratio of electron temperature to proton temperature, and with the electron heat flux. There is no significant correlation of the waves with the upstream plasma β or the shock strength. The similiarity of the quasi-parallel and quasi-perpendicular cases implies that the mechanism for wave generation is not sensitive to θBn, the angle between the upstream magnetic field and the shock normal. Examination of the particle distribution function at the shocks indicate that beams in the proton distribution function and enhanced electron heat flux may contribute to driving the waves unstable, particularly when Te/Tp is small.

The Ulysses spacecraft crossed from the slow dense solar wind characteristic of the solar streamer belt into the fast, less dense flow from the northern polar coronal hole over a very short interval (several days) in late March 1995. The spacecraft, which was at 1.35 AU and {approximately}19{degree} north heliographic latitude, moving northward in its orbit, remained in the fast solar wind from then through summer 1996. This boundary crossing is unique in that the combination of the spacecraft motion and rotation of the structure past the spacecraft caused Ulysses to move smoothly and completely from one regime into the other. In this study we examine this crossing in detail. The crossing is marked by a region of enhanced pressure, typical of stream interaction regions, which extends {approximately}2{times}10{sup 7}km across. We find that the transition between the slow and fast regimes occurs on several temporal, and hence spatial, scales. On the shortest scale ({lt}8{times}10{sup 4}km) the stream interface is a tangential discontinuity where the proton and core electron densities and ion and electron pressures all drop while the magnetic pressure jumps to maintain a rough pressure balance. The alpha to proton ratio also jumps across the stream interface to reachmore » the comparatively constant polar hole value of {approximately}4.3{percent}. On larger scales (a few {times}10{sup 6}km) the proton and alpha temperatures rise to their high-speed wind values. Finally, on the largest scale ({approximately}10{sup 8}km) the solar wind speed ramps up from {approximately}400kms{sup {minus}1} to {approximately}750kms{sup {minus}1}, typical of polar hole flows. While it seems likely that the stream interface maps back to a sharp boundary near the Sun, the large region of increasing flow speed suggests that there is also an extended gradient in solar wind source speed close to the Sun. {copyright} 1998 American Geophysical Union« less

Ulysses observations at mid and high heliographic latitudes have revealed a solar wind devoid of the large variations in density, temperature, and speed that are commonly observed at low latitudes. One event, however, observed on May 1, 1996, while Ulysses was located at {approximately}3.7AU and 38.5{degree}, stands out in the plasma data set. The structure, which is unique in the Ulysses high-latitude data set, is seen as a drop in proton density of almost an order of magnitude and a comparable rise in proton temperature. The event lasts {approximately}3(1)/(2) hours giving the structure a size of {approximately}9.6{times}10{sup 6}km (0.06 AU) along the spacecraft trajectory. Minimum variance analysis of this interval indicates that the angle between the average magnetic field direction and the minimum variance direction is {approximately}92{degree}, suggesting that the {open_quotes}density hole{close_quotes} may be approximated by a series of planar slabs separated by several tangential discontinuities. We discuss several possible explanations for the origin of this structure, but ultimately the origin of the density hole remains unknown. {copyright} 1998 American Geophysical Union

We have investigated the relationship between the energetic (∼1 MeV) proton intensity (J) and the magnetic compression ratio (C) measured at the trailing edges of corotating interaction regions observed at Ulysses. In general, our results show that the proton intensity was well correlated with the compression ratio, provided that the seed intensity remained constant, consistent with predictions of the Fermi model. Specifically, our results indicate that particles were accelerated to above ∼1 MeV in energy at or near the trailing edges of the compression regions observed in the midlatitude southern heliosphere, irrespective of whether the bounding reverse shocks were present or not. On the basis of this, we conclude that shock acceleration is probably not the only mechanism by which particles are accelerated to above ∼1 MeV in energy at compression or interaction regions (CIRs). On the basis of magnetic field measurements obtained near the trailing edges of several midlatitude CIRs, we propose that particles could have been accelerated via the Fermi mechanism by being scattered back and forth across the trailing edges of the compression regions by large-amplitude Alfven waves. Our results also show that the proton intensity was well correlated with the compression ratio during low solar activity periods but was essentially independent of C during periods of high solar activity. We suggest that the correlation between J and C was not observed during solar active periods because of significant variations in the seed intensity that result from sporadic contributions from transient solar events. In contrast, the correlation was observable during quiescent periods probably because contributions from transients had decreased dramatically, which allowed the CIRs to accelerate particles out of a seed population whose intensity remained relatively unperturbed.