ICMEs at High Latitudes and in the Outer Heliosphere

Interplanetary coronal mass ejections (ICMEs) propagate into the outer heliosphere, where they can have a significant effect on the structure, evolution, and morphology of the solar wind, particularly during times of high solar activity. They are known to play an important role in cosmic ray modulation and the acceleration of energetic particles. ICMEs are also believed to be associated with the large global transient events that swept through the heliosphere during the declining phases of solar cycles 21 and 22. But until recently, little was known about the actual behavior of ICMEs at large heliographic latitudes and large distances from the Sun. Over the past decade, the Ulysses spacecraft has provided in situ observations of ICMEs at moderate heliographic distances over a broad range of heliographic latitudes. More recently, observations of alpha particle enhancements, proton temperature depressions, and magnetic clouds at the Voyager and Pioneer spacecraft have begun to provide comparable information regarding the behavior of ICMEs at extremely large heliocentric distances. At the same time, advances in modeling have provided new insights into the dynamics and evolution of ICMEs and their effects on cosmic rays and energetic particles.     

Solar Imprint on ICMEs, Their Magnetic Connectivity, and Heliospheric Evolution

Interplanetary outflows from coronal mass ejections (ICMEs) are structures shaped by their magnetic fields. Sometimes these fields are highly ordered and reflect properties of the solar magnetic field. Field lines emerging in CMEs are presumably connected to the Sun at both ends, but about half lose their connection at one end by the time they are observed in ICMEs. All must eventually lose one connection in order to prevent a build-up of flux in the heliosphere; but since little change is observed between 1 AU and 5 AU, this process may take months to years to complete. As ICMEs propagate out into the heliosphere, they kinematically elongate in angular extent, expand from higher pressure within, distort owing to inhomogeneous solar wind structure, and can compress the ambient solar wind, depending upon their relative speed. Their magnetic fields may reconnect with solar wind fields or those of other ICMEs with which they interact, creating complicated signatures in spacecraft data.     

Motion of the heliospheric termination shock at high heliographic latitude

We expect the mean distance of the heliospheric termination shock to be greater (smaller) at polar latitudes than at equatorial latitudes, depending on whether the mean dynamic pressure of the solar wind is greater or smaller at high latitudes. The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions, so that at any particular instant the termination shock will resemble a distorted asymmetric balloon with some parts moving inward and others moving outward. If the shock is a gasdynamic or magnetohydrodynamic shock the results of the analysis depend only very weakly on the nature of the upstream disturbance; typical speeds of the disturbed shock are 100 to 200 km/s. In the absence of a significant latitude gradient of the typical magnitude of solar wind disturbances typical motions of the disturbed shock at polar latitudes would be about twice as fast, due to the higher speed of the high-latitude wind. If the dynamics of the termination shock are dominated by acceleration of the aromalous component of the cosmic rays, the motion of the shock in response to a given disturbance is substantially slower than in the gasdynamic case. Conceivably, particle acceleration might be a less important effect at higher latitudes, and we envision the possibility of a termination shock that is dominated by particle acceleration at lower latitudes and is an MHD shock at high latitudes. In this event high latitude solar wind disturbances would produce substantially larger inward and outward motions of the shock in the polar regions.     

Pioneer and Voyager observations of large-scale spatial and temporal variations in the solar wind

The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were launched in 1972, 1974, and 1977, respectively. While these three spacecraft are all at compartively low heliographic latitudes compared with Ulysses, their observation span almost two solar cycles, a range of heliocentric distances from 1 to 57 AU, and provide a unique insight into the long-term variability of the global structure of the solar wind. We examine the spatial and temporal variation of average solar wind parameters and fluxes. Our obsevations suggest that the global structure of the outer heliosphere during the declining phase of the solar cycle at heliographic latitudes up to 17.5°N was charaterized by two competing phenomena: 1) a large-scale increase of solar wind density, temperature, mass flux, dynamic pressure, kinetic energy flux, and thermal enery flux with heliographic latitude, similar to the large-scale latitudinal gradient of velocity seen in IPS observations, 2) a small-scale decrease in velocity and temperature, and increase in density near the heliospheric current sheet, which is associated with a band of low speed, low temperature, and high density solar wind similar to that observed in the inner heliosphere.     

The Heliospheric Magnetic Field at Solar Maximum: Ulysses Observations

At solar maximum, the large-scale structure of the heliospheric magnetic field (HMF) reflects the complexity of the Sun's coronal magnetic fields. The corona is characterised by mostly closed magnetic structures and short-lived, small coronal holes. The axis of the Sun's dipole field is close to the solar equator; there are also important contributions from the higher order terms. This complex and variable coronal magnetic configuration leads to a much increased variability in the HMF on all time scales, at all latitudes. The transition from solar minimum to solar maximum conditions, as reflected in the HMF, is described, as observed by Ulysses during its passage to high southern heliolatitudes. The magnetic signatures associated with the interaction regions generated by short-lived fast solar wind streams are presented, together with the highly disordered period in mid-1999 when there was a considerable reorganisation in coronal structures. The magnetic sector structure at high heliolatitudes shows, from mid-1999, a recognisable two-sector structure, corresponding to a highly inclined Heliospheric Current Sheet. A preliminary investigation of the radial component of the magnetic field indicates that it remains, on average, constant as a function of heliolatitude. Intervals of highly Alfvénic fluctuations in the rarefaction regions trailing the interaction regions have been, even if intermittently, identified even close to solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Near-Earth Plasma Environment

An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth’s plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances.     

Magnetic Field Observations of Transient Events at Ulysses, 1996–2000

During the years 1996–2000 solar activity has been gradually rising and is now close to maximum. At the same time the Ulysses spacecraft has performed a north to south traverse of the low latitude regions of the heliosphere and is now once again travelling through high southerly latitudes. We show some examples and report on the occurence rates of transient solar wind disturbances which have been identified by their magnetic field signatures. ‘Magnetic clouds’ remain more common at low (compared to high) latitudes despite the rise in solar activity. However, more events were observed at high latitudes than at solar minimum. This revised version was published online in August 2006 with corrections to the Cover Date.     

ICMEs in the Inner Heliosphere: Origin, Evolution and Propagation Effects

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.     

The heliospheric magnetic field

The heliospheric magnetic field (HMF) is an important component of the heliospheric medium. It has been the subject of extensive studies for the past thirty five years. There is a very large observational data base, mostly from the vantage point of the ecliptic plane, but now also from the solar polar regions, from the Ulysses mission. This review aims to present its most important large scale characteristics. A key to understand the HMF is to understand the source functions of the solar wind and magnetic fields close to the sun. The development of new modelling techniques for determining the extent and geometry of the open magnetic field regions in the corona, the sources of the solar wind and the HMF has provided a new insight into the variability of the source functions. These are now reasonably well understood for the state of the corona near solar minimum. The HMF at low-to-medium heliolatitudes is dominated, near solar minimum, by the Corotating Interaction Regions (CIRs) which arise from the interaction of alternating slow and fast solar wind streams, and which, in turn, interact in the outer heliosphere to form the large scale Merged Interaction Regions. The radial component of the HMF is independent of heliolatitude; the average direction is well organised by the Parker geometry, but with wide distributions around the mean, due, at high latitudes, to the presence of large amplitude, Alfvénic fluctuations. The HMF at solar maximum is less well understood, due in part to the complexity of the solar source functions, and partly to the lack of three dimensional observations which Ulysses is planned to remedy at the next solar maximum. It is suggested that the in-ecliptic conditions in the HMF, largely determined by the dynamics of transients (Coronal Mass Ejections) may also be found at high latitudes, due to the wide latitude distribution of the CMEs.     

Formation and Evolution of Corotating Interaction Regions and their Three Dimensional Structure

Corotating interaction regions are a consequence of spatial variability in the coronal expansion and solar rotation, which cause solar wind flows of different speeds to become radially aligned. Compressive interaction regions are produced where high-speed wind runs into slower plasma ahead. When the flow pattern emanating from the Sun is roughly time-stationary these compression regions form spirals in the solar equatorial plane that corotate with the Sun, hence the name corotating interaction regions, or CIRs. The leading edge of a CIR is a forward pressure wave that propagates into the slower plasma ahead, while the trailing edge is a reverse pressure wave that propagates back into the trailing high-speed flow. At large heliocentric distances the pressure waves bounding a CIR commonly steepen into forward and reverse shocks. Spatial variation in the solar wind outflow from the Sun is a consequence of the solar magnetic field, which modulates the coronal expansion. Because the magnetic equator of the Sun is commonly both warped and tilted with respect to the heliographic equator, CIRs commonly have substantial north-south tilts that are opposed in the northern and southern hemispheres. Thus, with increasing heliocentric distance the forward waves in both hemispheres propagate toward and eventually across the solar equatorial plane, while the reverse shocks propagate poleward to higher latitudes. This paper provides an overview of observations and numerical models that describe the physical origin and radial evolution of these complex three-dimensional (3-D) heliospheric structures. This revised version was published online in August 2006 with corrections to the Cover Date.     

The high latitude heliospheric magnetic field

As the Ulysses spacecraft approaches its first pass under the south pole of the sun, it is an appropriate time to review our current knowledge and predictions regarding the three dimensional behaviour of the heliospheric magnetic field, in particular at high heliographic latitudes. Optical techniques for measuring the photospheric magnetic field and observations of coronal brightness structures provide indications of the behaviour of the source of the heliospheric field in the corona. As the coronal fields are carried out into the heliosphere by the solar wind, from Parker's model we would expect that the spiral field observed in the equatorial plane should gradually unwind with latitude leading to open, approximately radial, field lines over the polar regions. Predictions of departures from, and models extending this simple picture are discussed. Both the Pioneer and Voyager spacecraft have spent brief periods in the regions above the maximum latitude of the heliospheric current sheet-relevant results from these missions are reviewed as well as results from the early stages of the out-of-ecliptic phase of the Ulysses mission. The configuration of the coronal magnetic field exhibits a strong dependence on the phase of the solar activity cycle. While the forthcoming Ulysses polar passes take place near to solar minimum, the different conditions which might be encountered on a second orbit of the sun at solar maximum are described.     

A summary of solar wind observations at high latitudes: Ulysses

The Ulysses mission has provided the first in-situ observations of the solar wind covering all solar latitudes from the equator to the poles in both hemispheres. The measurements from the first polar passes, made at near-minimum solar activity conditions, have confirmed the basic picture established on the basis of remote sensing techniques: the high-latitude wind is fast, and originates in the polar coronal holes. The detailed in-situ observations have, however, revealed a number of features related to the global solar wind structure that were not expected: the transition between slow and fast wind was relatively abrupt, followed by a slight increase in speed toward the poles; the mass flux is almost independent of latitude, with only a modest increase at the equator; the momentum flux is significantly higher over the poles than near the equator, suggesting a non-circular cross-section for the flanks of the heliosphere.     

The Solar Wind in the Outer Heliosphere

The solar wind evolves as it moves outward due to interactions with both itself and with the circum-heliospheric interstellar medium. The speed is, on average, constant out to 30 AU, then starts a slow decrease due to the pickup of interstellar neutrals. These neutrals reduce the solar wind speed by about 20% before the termination shock (TS). The pickup ions heat the thermal plasma so that the solar wind temperature increases outside 20–30 AU. Solar cycle effects are important; the solar wind pressure changes by a factor of 2 over a solar cycle and the structure of the solar wind is modified by interplanetary coronal mass ejections (ICMEs) near solar maximum. The first direct evidences of the TS were the observations of streaming energetic particles by both Voyagers 1 and 2 beginning about 2 years before their respective TS crossings. The second evidence was a slowdown in solar wind speed commencing 80 days before Voyager 2 crossed the TS. The TS was a weak, quasi-perpendicular shock which transferred the solar wind flow energy mainly to the pickup ions. The heliosheath has large fluctuations in the plasma and magnetic field on time scales of minutes to days.     

Properties of Interplanetary Coronal Mass Ejections

Interplanetary coronal mass ejections (ICMEs) originating from closed field regions on the Sun are the most energetic phenomenon in the heliosphere. They cause intense geomagnetic storms and drive fast mode shocks that accelerate charged particles. ICMEs are the interplanetary manifestations of CMEs typically remote-sensed by coronagraphs. This paper summarizes the observational properties of ICMEs with reference to the ordinary solar wind and the progenitor CMEs.     

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.     

Modulation Near Solar Maximum at High Solar Latitudes Observations From the Ulysses Cospin High Energy Telescope

The three-dimensional structure of the solar maximum modulation of cosmic rays in the heliosphere can be studied for the first time by comparing observations from Ulysses at high solar latitudes to those from in-ecliptic spacecraft, such as IMP-8. Observations through mid-2000 show that changes in modulation remain well correlated at Earth and Ulysses up to latitudes of ∼60° south. The observed changes seem to be best correlated with changes in the inclination of the heliospheric current sheet. The spectral index of the proton spectra at energies <100 MeV in the ecliptic and at high latitudes remain roughly consistent with the T ⁺¹ spectrum expected from modulation models, while the spectral index of the helium spectrum at both locations has changed smoothly from the flat or even negative index spectra characteristic of anomalous component fluxes toward the T ⁺¹ galactic spectrum with increasing modulation. Intensities near the equator and at high latitude remain nearly equal, and latitudinal gradients for nucleonic cosmic rays thus remain small (<1% deg⁻¹) at solar maximum. In the most recent data fluxes of protons and helium with energies less than ∼100 MeV nucl⁻¹ measured by Ulysses are smaller than those measured at IMP-8, suggesting that the gradients may have switched to become negative toward the poles even before a clear reversal of polarity for the solar magnetic dipole has been completed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic Fields in the Inner Heliosphere

The structure of Heliospheric Magnetic Field (HMF) is a function of both the coronal conditions from which it originates and dynamic processes which take place in the solar wind. The division between the inner and outer regions of the heliosphere is the result of dynamic processes which form large scale structures with increasing heliocentric distance. The structure of the HMF is normally described in the reference frame based on Parker's geometric model, but is better understood as an extension of potential field models of the corona. The Heliospheric Current Sheet (HCS) separates the two dominant polarities in the heliosphere; its large scale geometry near solar minimum is well understood but its topology near solar maximum remains to be investigated by Ulysses. At solar minimum, Corotating Interaction Regions (CIRs) dominate the near-equatorial heliosphere and extend their influence to mid-latitudes; the polar regions of the heliosphere are dominated by uniform fast solar wind streams and large amplitude, long wavelength, mostly transverse magnetic fluctuations. Coronal Mass Ejections (CMEs) introduce transient variability into the large scale heliospheric structure and may dominate the inner heliosphere near solar maximum at all latitudes.     

Latitudinal and radial variation of shock associated ≥30 keV ion spectra and anisotropies at Voyagers 1 and 2

The spectra and anisotropies of ions 30 keV have been measured by the Low Energy Charged Particle experiment on Voyagers 1 and 2 in the vicinity of interplanetary shocks between radial distances of 1–55 AU and heliographic latitudes 11° S-32° N. The spectra and anisotropies associated with a recent corotating (CIR) event at low latitude observed at Voyager 2 (36.6 AU, –9°) are similar to those of another event at high latitude observed at Voyager 1 (49.8 AU, 33.5°). An earlier CIR event observed at Voyager 2 (14 AU) associated with the previous solar cycle produced spectra and anisotropies remarkably similar to the more recent events. The anisotropies are used to calculate the solar wind velocity downstream of shocks where possible using the Compton-Getting effect, allowing the determination of previously unknown velocities at the locations of Voyager 1. For the large shock event observed at Voyagers 1 (38 AU, 30°) and 2 (29 AU, 3°) in mid-1989, the postshock spectra and anisotropies are well described by convected power law distributions. The Voyager 1 and 2 postshock spectra 4 days after the shock passage are nearly identical. The preshock anisotropies at low energy are similar, despite differences in the magnetic field orientation and the low energy spectrum. We find that the 30 keV ion anisotropies are generally well described by convective distributions downstream but not in the upstream region for shocks and many other shock events at Voyagers 1 and 2.     

Latitudinal Variation of the Underlying Heliospheric Magnetic Field Direction: Comparison of the Ulysses First and Second Orbits

We discuss the underlying direction of the heliospheric magnetic field measured by Ulysses in the latitude range 6° S-65° S by examining distributions of the magnetic field azimuthal angle with respect to the simple Parker spiral model. During the first Ulysses traversal of this latitude range in 1992–1994, while solar activity was declining, the shape of the distributions obtained at high latitudes in the fast solar wind differed from that at lower latitudes. In the present data set, obtained during rising solar activity, both field polarities are present at all latitudes and the peaks of the distributions agree with the predicted spiral direction to first approximation. However, compared to the first orbit, a significantly greater percentage of the observed field vectors have large deviations from the spiral direction. This revised version was published online in August 2006 with corrections to the Cover Date.