UV studies and the solar wind

The solar wind carves a cavity in the flow of interstellar H atoms through the solar system by charge-exchange ionization. The resulting Ly- sky pattern depends on the latitude distribution of the solar wind flux and velocity. We review how the solar wind characteristics (mass flux latitude distribution) can be retrieved from Ly- observations, yielding a new remote sensing method of solar wind studies, through UV optical measurements.     

The Solar Wind - Inner Heliosphere

The solar wind in the inner heliosphere, inside ~ 5 AU, has been almost fully characterized by the addition of the high heliographic latitude Ulysses mission to the many low latitude inner heliosphere missions that preceded it. The two major omissions are the high latitude solar wind at solar maximum, which will be measured during the second Ulysses polar passages, and the solar wind near the Sun, which could be analyzed by a Solar Probe mission. Here, existing knowledge of the global solar wind in the inner heliosphere is summarized in the context of the new results from Ulysses.     

The Rotation of the Fe XIV Solar Corona During the Recent Solar Activity Minimum

We analyze data observed by the LASCO C1 coronagraph on board the SOHO spacecraft during the solar minimum activity from April 1996 to March 1997. Using the phase dispersion technique, we investigate the periodicity and recurrence of Fe XIV emission structures with heliospheric latitude and distance above the Sun's surface with high spatial resolution. We find no significant deviation from a rigidly rotating Fe XIV corona with latitude or with distance from the Sun even on these small scales. In agreement with earlier work, the coronal rotation period at solar minimum is about 27.5 ± 1 days. This revised version was published online in June 2006 with corrections to the Cover Date.     

Remote Sensing of H from Ulysses and Galileo

We model interplanetary H Lyman-α (Lα) observations from Galileo UVS (Ultraviolet Spectrometer) and EUVS (Extreme Ultraviolet Spectrometer) (Hord et al., 1992) and the Ulysses interstellar neutral gas (GAS) instrument (Witte et al., 1992). EUVS measurements near solar maximum (max) in 1990–1992 have a peaked brightness maximum upwind due to a rather isotropic solar wind charge-exchange ionization pattern (A=0–0.25). GAS measurements from solar minimum (min) in 1997 have a plateau in the upwind direction that we model using Ulysses SWOOPS (solar wind plasma experiment) solar min data on solar wind density and velocity at different heliographic latitudes. The isotropic ionization pattern deduced from EUVS at solar max may be consistent with recent SWOOPS results (McComas et al., 2000b, c) that high speed solar wind is absent at high latitudes during solar max. Galileo and Ulysses Lα data favor higher H temperatures (15 000–18 000 K) than previous models. This revised version was published online in August 2006 with corrections to the Cover Date.     

What Can Be Learned from X-ray Spectroscopy Concerning Hot Gas in the Local Bubble and Charge Exchange Processes?

Both solar wind charge exchange emission and diffuse thermal emission from the Local Bubble are strongly dominated in the soft X-ray band by lines from highly ionized elements. While both processes share many of the same lines, the spectra should differ significantly due to the different production mechanisms, abundances, and ionization states. Despite their distinct spectral signatures, current and past observatories have lacked the spectral resolution to adequately distinguish between the two sources. High-resolution X-ray spectroscopy instrumentation proposed for future missions has the potential to answer fundamental questions such as whether there is any hot plasma in the Local Hot Bubble, and if so what are the abundances of the emitting plasma and whether the plasma is in equilibrium. Such instrumentation will provide dynamic information about the solar wind including data on ion species which are currently difficult to track. It will also make possible remote sensing of the solar wind.     

Fine Structure in the Corona at High Latitudes at Solar Maximum

Microstreams and pressure balance structures in fast solar wind were more easily detected at Ulysses at 2.2 AU over the poles than at Helios at 0.3 AU. This is because solar rotation leads to dynamic interactions between different speed regimes at a rate that depends on latitude for the same size features. Dynamic interactions make structures more difficult to detect with increasing distance from the Sun. At solar maximum, Ulysses will sample high latitude solar wind coming from streamers, providing information on fine structure at the tops of streamers and on the source of slow solar wind. Examples are given here of the detectability of various sized structures at Ulysses when it is over the polar regions of the Sun. This revised version was published online in August 2006 with corrections to the Cover Date.     

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ∼5 AU to ∼67° S latitude at ∼3 AU. Using observations of >∼30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ∼30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ulysses observations of solar wind plasma parameters in the ecliptic from 1.4 to 5.4 AU and out of the ecliptic

We report observations of radial and latitudinal gradients of Ulysses plasma parameters. The solar wind velocity increased rapidly with latitude from 0° to 35°, then remained approximately constant at higher latitudes. Solar wind density decreased rapidly from 0° to 35° of latitude, and also was approximately constant beyond that latitude. The mass flux similarly decreased away from the equator (but less than the density), whereas the momentum flux was relatively constant. The radial gradient of the entropy at high latitude indicated a value for the polytrope index of about 1.72 (close to adiabatic); the in-ecliptic estimates of radial gradients for temperature and entropy may be biased by temporal variation. A striking increase in the alpha particle-proton velocity difference with latitude is found.     

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.     

Corotating Interaction Regions at High Latitudes

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°S shows 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°S shows 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 interaction region are presented. They include direct magnetic field connection across latitude due to stochastic field line weaving or to systematic weaving caused by solar differential rotation combined with non-radial expansion of the fast wind. Another possible mechanism is particle diffusion across the average magnetic field, which includes stochastic field line weaving. A constraint on connection to a distant portion of the CIR is energy loss in the solar wind, which is substantial for the relatively slow-moving accelerated ions. Finally, the weaker northern CIR is compared with the southern CIR. It is weak because the inclination of the streamer belt and HCS decreased as Ulysses traveled to lower latitudes so that the spacecraft remained at about the maximum latitudinal extent of the HCS. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stream Interaction Regions at High Heliographic Latitudes During Ulysses12/22/2004 6:25PM Second Polar Orbit

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. This revised version was published online in August 2006 with corrections to the Cover Date.     

X-Rays From Mars

X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity, confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states. With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying processes in the Martian exosphere on a global scale.     

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5R and 200R have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.     

Energy Deposition in Planetary Atmospheres by Charged Particles and Solar Photons

We discuss here the energy deposition of solar FUV, EUV and X-ray photons, energetic auroral particles, and pickup ions. Photons and the photoelectrons that they produce may interact with thermospheric neutral species producing dissociation, ionization, excitation, and heating. The interaction of X-rays or keV electrons with atmospheric neutrals may produce core-ionized species, which may decay by the production of characteristic X-rays or Auger electrons. Energetic particles may precipitate into the atmosphere, and their collisions with atmospheric particles also produce ionization, excitation, and heating, and auroral emissions. Auroral energetic particles, like photoelectrons, interact with the atmospheric species through discrete collisions that produce ionization, excitation, and heating of the ambient electron population. Auroral particles are, however, not restricted to the sunlit regions. They originate outside the atmosphere and are more energetic than photoelectrons, especially at magnetized planets. The spectroscopic analysis of auroral emissions is discussed here, along with its relevance to precipitating particle diagnostics. Atmospheres can also be modified by the energy deposited by the incident pickup ions with energies of eV’s to MeV’s; these particles may be of solar wind origin, or from a magnetospheric plasma. When the modeling of the energy deposition of the plasma is calculated, the subsequent modeling of the atmospheric processes, such as chemistry, emission, and the fate of hot recoil particles produced is roughly independent of the exciting radiation. However, calculating the spatial distribution of the energy deposition versus depth into the atmosphere produced by an incident plasma is much more complex than is the calculation of the solar excitation profile. Here, the nature of the energy deposition processes by the incident plasma are described as is the fate of the hot recoil particles produced by exothermic chemistry and by knock-on collisions by the incident ions.     

Radial and meridional trends in solar wind thermal electron temperature and anisotropy: Ulysses

Ulysses plasma measurement from 1.15 to 5.31 AU and from S6.4° to S48.3° solar latitude are used to assess the trends in the solar wind thermal electron temperature and anisotropy. Improved spacecraft potential corrections and data products have been incorporated. The radial temperature gradient is steeper than in previous determinations, but flatter than adiabatic. When normalized to 1 AU, temperature decrease with increasing latitude. Little change in the average thermal anisotropy has been seen during the mission.     

Ulysses' Second Orbit: Remarkably Different Solar Wind

By the time of the 34th ESLAB symposium, dedicated to the memory of John Simpson, Ulysses had nearly reached its peak southerly latitude in its second polar orbit. The global solar wind structure observed thus far in Ulysses' second orbit is remarkably different from that observed over its first orbit. In particular, Ulysses observed highly irregular solar wind with less periodic stream interaction regions, much more frequent coronal mass ejections, and only a single, short interval of fast solar wind. Ulysses also observed the slowest solar wind seen thus far in its ten-year journey (∼270 km s⁻¹). The complicated solar wind structure undoubtedly arises from the more complex coronal structure found around solar activity maximum, when the large polar coronal holes have disappeared and coronal streamers, small-scale coronal holes, and frequent CMEs are found at all heliolatitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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.     

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.     

Three-dimensional Solar Modulation of Cosmic Ray and Anomalous Components in the Inner Heliosphere

Our picture of modulation in the inner heliosphere has been greatly affected by observations from the Ulysses mission, which since 1992 has provided the first comprehensive exploration of modulation as a function of latitude from 80° S to 80° N heliographic latitude. Among the principal findings for the inner heliosphere are: a) the cosmic ray intensity depends only weakly on heliographic latitude; b) for the nuclear components, and especially for the anomalous components, the intensity increases towards the poles, qualitatively consistent with predictions of drift models for the current sign of the solar magnetic dipole; c) no change in the level of modulation was observed across the shear layer separating fast polar from slow equatorial solar wind near 1 AU; d) 26-day recurrent variations in the intensity persist to the highest latitudes, even in the absence of clearly correlated signatures in the solar wind and magnetic field; e) the surface of symmetry of the modulation in 1994-95 was offset about 10° south of the heliographic equator; f) the intensity of electrons and of low energy (< 100 MeV) protons showed essentially no dependence on heliographic latitude.