The high spectral resolution and high signal to noise capabilities of the Goddard High Resolution Spectrograph (GHRS) have permitted very accurate measurements of the gas phase abundances and physical conditions in interstellar clouds found in the Galactic disk and low halo and of the matter in several Galactic high velocity clouds. The interstellar gas phase abundances provide important clues about the composition of dust grain mantles and cores, and about the origins of intermediate and high velocity gas in the Galactic disk and halo. The processes that circulate gas from the disk into the low halo do not destroy dust grain cores. The gas in Complex C in the direction of Mrk 290 has a metallicity of 0.089 ± 0.024 solar, which implies the accretion of low metallicity gas by the Milky Way at a rate per unit area sufficient to solve the long standing Galactic G-dwarf problem. GHRS studies of interstellar Si IV, C IV, and N V absorption toward stars and AGNs have yielded measures of the 3 to 5 kpc extension of hot gas into the halo of the Milky Way. The GHRS results coupled with new measurements from the Far-Ultraviolet Spectroscopic Explorer (FUSE) satellite of O VI absorption by hot halo gas permit a study of the physical conditions in the hot Galactic Corona originally envisioned by Lyman Spitzer in his classic 1956 paper “On a Possible Interstellar Galactic Corona.”

HST observed Mars during all 5 oppositions between 1990 and 1999, providing unique new observations of the planet's atmosphere and surface during seasons which are typically poorly-observed telescopically and in wavelength regions or at spatial scales that are not at all observed by spacecraft. HST observations also filled a crucial gap in synoptic observations of Mars prior to 1998, during a time when no spacecraft were observing the planet. HST data have provided important new insights and understanding of the Martian atmosphere, surface, and satellites, and they continue to fulfill important spacecraft mission support functions, including atmospheric aerosol characterization, dust storm monitoring, and instrument cross-calibration.

Introduction

Mars has been the subject of intense telescopic observations for centuries (see, for example, reviews by Martin et al. 1992 and Sheehan 1988). Interest in the red planet stems partly from its prominent appearance in the night sky as a bright extended object roughly every 26 months, and also from historic telescopic observations and more recent spacecraft encounters that have revealed many similarities between Mars and the Earth in terms of surface and atmospheric characteristics and climatic histories. While cold and arid today and probably inhospitable to most forms of life, evidence exists indicating that Mars once may have had a much more clement climate, during a postulated “warm and wet” epoch early in solar system history (e.g. Pollack et al. 1987; Carr, 1998).

The Galactic Center has been the subject of a variety of HST observing programs, mainly since the installation of NICMOS. The observational strengths of NICMOS lie with its sensitivity and very stable point spread function which enables a variety of studies including sensitive searches for variable sources and accurate colors across the 1 to 2.5 µm region. The emission line filters in NICMOS enable studies of the interstellar medium and a search for [SiVI] emission as a ‘smoking gun’ for gas clouds near a black hole powered accretion disk.

Introduction

The center of the Milky Way is of course the closest galaxy nucleus and is a natural area to choose to study in detail. The discovery of a peculiar radio source, SgrA*, and the subsequent demonstration that it is a black hole has only heightened interest in the center. Figure 1 shows a contour plot at 1.04 µm compared to a NICMOS image at 1.45 µm which clearly shows why the Galactic Center requires use of infrared instrument like NICMOS with Av ∼ 30 while AK ∼ 3.3.

The Galactic Center has been studied with HST from the first observing cycle using WFPC proposed in an era where the nature of many of the stars was not understood, and the existence of a cluster in very close proximity to the black hole, SgrA*, was unknown.

This review aims to give an overview of the contribution of the Hubble Space Telescope to our understanding of the detailed properties of Local Group dwarf galaxies and their older stellar populations. The exquisite stable high spatial resolution combined with photometric accuracy of images from the Hubble Space Telescope have allowed us to probe further back into the history of star formation of a large variety of different galaxy types with widely differing star formation properties. It has allowed us to extend our studies out to the edges of the Local Group and beyond with greater accuracy than ever before. We have learned several important things about dwarf galaxy evolution from these studies. Firstly we have found that no two galaxies have identical star formation histories; some galaxies may superficially look the same today, but they have invariably followed different paths to this point. Now that we have managed to probe deep into the star formation history of dwarf irregular galaxies in the Local Group it is obvious that there are a number of similarities with the global properties of dwarf elliptical/spheroidal type galaxies, which were previously thought to be quite distinct. The elliptical/spheroidals tend to have one or more discrete episodes of star formation through-out their history and dwarf irregulars are characterized by quasi-continuous star-formation.

The angular resolution of HST has provided stunning images of star forming regions, circumstellar disks, protostellar jets, and outflows from young stars. HST has resolved the cooling layers behind shocks, and enabled the determination of outflow proper motions on time scales less than the post-shock coolingtime. Observations of the best studied region of star formation, the Orion Nebula, has produced many surprises. HST's superior resolution led to the identification of many new outflow systems based on their proper motions, the discovery of dozens of microjets from young stars, and the detection of wide-angle wind-wind collision fronts. HST has also produced spectacular images of circumstellar disks which have led to a rethinking of some aspects of planet formation. It now appears that most stars in the sky are born in environments similar to the Orion Nebula where within a few hundred thousand years after their formation, proto-planetary disks are subjected to the intense radiation fields of nearby massive stars. As a result, Orion's disks are rapidly evaporating. But at the same time their dust grains appear to be growing. Multi-wavelength images indicate that most of the solid mass in these disks may already be in large grains, possibly larger than a millimeter in size. The formation frequency of planets and the architectures of planetary systems will be determined by the competition between grain growth and photo-evaporation.

Globular clusters represent only a small fraction of the total mass in their parent galaxy, but provide a vast array of tests for stellar physics, dynamics, and galaxy formation. This review discusses the prominent accomplishments of HST-based programs:

1. The definition of precise fiducial sequences in the HR diagram, extending down to the hydrogen-burning limit,

2. Discovery of the upper white dwarf cooling sequence in several clusters,

3. Discovery of a highly consistent IMF on the lower main sequence,

4. Definitive age measurements for the oldest clusters in the outermost halo of the Milky Way, the Magellanic Clouds, and the dwarf elliptical satellites of the Milky Way,

5. Elucidation of the innermost structure of M15 and other core-collapsed clusters,

6. Discovery of surprisingly large “anomalous” populations of stars within dense cluster cores: extended blue horizontal-branch stars, blue stragglers, and others,

7. The first reliable color-magnitude studies for globular clusters in M31, M33, and other outlying Local Group members,

8. Discovery of massive young clusters in starburst galaxies with ages as small as 1 Myr,

9. Measurement of metallicity distribution functions among globular cluster systems in many giant E galaxies—bimodality is common, but details differ strongly, and

10. Deep imaging of cluster luminosity distributions in gE galaxies in Virgo, Fornax, and other Abell clusters as distant as Coma.

A decade of observing with HST also coincides with the completion of the last of the initial three Key Projects for HST, the measurement of the Hubble constant, H0. Here we present the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant, summarizing our method, the results and the uncertainties. The Key Project results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc. Based on the Key Project Cepheid calibration and its application to five secondary methods (type Ia supernovae, the Tully-Fisher relation, surface brightness fluctuations, type II supernovae, and the fundamental plane for elliptical galaxies), a combined value of H0 = 72 ± 8 km/sec/Mpc is obtained. An age conflict is avoided for current estimates of globular clusters and H0 if we live in A-dominated (or other form of dark energy) universe.

Introduction

When planning HST, pinning down H0 was one of the scientific programs that drove the design and construction of the telescope. Although the original plans for a Large Space Telescope were scaled down during the mid-1970s, one of the primary arguments for an aperture of at least 2.4m was to enable the detection of Cepheid variables in the Virgo cluster (Smith 1989), a goal that was achieved within months of the corrective optics being installed in HST in December, 1993.

One of the brightest and most variable UV emissions in the solar system comes from Jupiter's UV aurora. The auroras have been imaged with each camera on HST, starting with the pre-COSTAR FOC and continuing with increasing sensitivity to the present with STIS. This paper presents a short overview of the scientific results on Jupiter's aurora obtained from HST UV images and spectra, plus a short discussion of Saturn's aurora.

The Earth's aurora: Present understanding

With a long history of ground-based and spacecraft measurements, we now have some understanding of the physics of the Earth's auroral processes. A general picture of the nature of auroral activity on the Earth has evolved, without a complete understanding of the many details. In general, auroral emissions are produced by high energy charged particles precipitating into the Earth's upper atmosphere from the magnetosphere (the region of space where the motions of particles are governed by the Earth's magnetic field). It is well established that the Earth's auroral activity is related to solar activity, and more specifically to conditions in the solar wind reaching the Earth. The precipitating charged particles are accelerated to high energies in the Earth's magnetosphere, with some acceleration occurring in the magnetotail region and some occurring by fieldaligned potentials in the topside ionosphere.

Magnetohydrodynamics, or MHD in short, describes the macroscopic behavior of an electrically conducting fluid – usually an ionized gas called a plasma –, which forms the basis of this book. By macroscopic we mean spatial scales larger than the intrinsic scale lengths of the plasma, such as the Debye length λD and the Larmor radii ρj of the charged particles. In this chapter we first derive, in a heuristic way, the dynamic equations of MHD and discuss the local thermodynamics (Section 2.1). Since most astrophysical systems rotate more or less rapidly, it is useful to write the momentum equation also in a rotating reference frame, where inertial forces appear (Section 2.2). Then some convenient approximations are introduced, in particular incompressiblity and, for a stratified system, the Boussinesq approximation (Section 2.3). In MHD theory the ideal invariants, i.e., integral quantities that are conserved in an ideal (i.e., nondissipative) system, play a crucial role in turbulence theory; these are the energy, the magnetic helicity, and the cross-helicity (Section 2.4). Though this book deals with turbulence, it is useful to obtain an quick overview of magnetostatic equilibrium configurations, which are more important in plasmas than stationary flows are in hydrodynamics (Section 2.5). Also the zoology of linear modes, the small-amplitude oscillations about an equilibrium, is richer than that in hydrodynamics (Section 2.6). Finally, in Section 2.7 we introduce the Elsässer fields, which constitute the basic dynamic quantities in MHD turbulence. In this chapter we write the equations in dimensional form, using Gaussian units, to emphasize the physical meaning of the various terms.

In this chapter we talk about accretion disks, a widespread phenomenon in astrophysics, wherein magnetic turbulence is present not just as a byproduct but rather is essential for its very existence, as is now generally believed. Accretion, the accumulation of mass onto a central object due to gravitational attraction, naturally leads to the formation of disk-like structures, since the infalling matter, due to conservation of angular momentum, tends to rotate about the center of gravity. The system is in approximate equilibrium, in that the radial component of the gravitational force is balanced by the centrifugal force and the axial component by the pressure gradient. Since the disk material moves on Keplerian orbits with angular velocity Ω(r) α r−3/2, the angular momentum α r2Ω decreases with decreasing radius. Hence, when the matter moves inward, conservation of angular momentum requires that the excess is transferred outward. Thus the rate of accretion of mass is determined by the transport of angular momentum, which therefore becomes the crucial issue for understanding the dynamics of these systems. If a transport mechanism is provided, material is spiraling in toward the central object (just as conservation of vorticity leads to a spiraling flow of the water from a bathtub).

One important aspect of turbulence theory is the need to understand how obviously random motions are generated from a smooth flow. There are essentially three approaches to this problem: the dynamic systems approach; the development of singular solutions of the ideal fluid equations, in particular the question of finite-time singularities; and the excitation of instabilities and their effects. The dynamic systems approach, i.e., the transition to a chaotic temporal behavior in some low-order nonlinear dynamic model such as the Lorentz model of thermal convection, had once been considered a very promising way to describe also the transition to turbulence in a fluid. However, these expectations have largely been frustrated, mainly because the low-order approximations of the fluid equations ignore the most important aspect of turbulence, namely the excitation and interactions of a broad range of different spatial scales. We will therefore not discuss dynamic systems theory in this treatise.

The problem of finite-time singularities has evoked considerable discussion. This is primarily a mathematical problem concerning the nature of the solution of the ideal fluid equations, whose relevance for the generation of turbulence in dissipative systems might be debatable. However, similarly to the theory of absolute equilibrium states of the ideal system considered in Section 5.2, which provides valuable information about the cascade dynamics in dissipative turbulence, the way in which the ideal solution becomes singular gives some indication of the spatial structure of eddies encountered in the dissipative system.

In the derivation of spectral laws presented in Chapter 5 we used only certain general properties of the turbulence, in particular the integral invariants, which lead to the spectral cascades. (Only the Alfvén effect resulting in the IK spectrum is based on a specific dynamic process of the MHD system.) Though phenomenological arguments, especially dimensional analysis, are often very powerful and robust, since they represent basic physical principles, they only predict a few scaling laws but cannot, for instance, specify proportionality factors, such as the Kolmogorov constant and the sign of the residual energy spectrum. Morover, these arguments provide little insight into the turbulence dynamics. Such properties must be treated by a statistical theory derived from the basic fluid equations. Here the most practical approach is two-point closure theory. An alternative method, renormalization-group (RNG) theory, which was originally developed in the context of the theory of critical phenomena, has also been applied to hydrodynamic turbulence (e.g., Yakhot and Orszag, 1986) and MHD turbulence (Fournier et al., 1982; Camargo and Tasso, 1992), but there is still a considerable degree of arbitrariness and even inconsistency. We shall therefore not discuss RNG theory any further but restrict the treatment in this chapter to closure theory.

In Chapter 4 we introduced the one-point-closure approximation consisting of the equations for the average fields and some phenomenological expressions for the correlation functions appearing in these equations, which is appropriate for describing large-scale inhomogeneous-turbulence processes. To study intrinsic small-scale properties, for which correlation functions are of primary interest, one has to go one step further in the hierarchy of moment equations.

Turbulence in electrically conducting fluids is necessarily accompanied by magnetic-field fluctuations, which will, in general, strongly influence the dynamics. It is true that, in our terrestrial world, conducting fluids in turbulent motion are rare. In astrophysics, however, material is mostly ionized and strong turbulence is a widespread phenomenon, for instance in stellar convection zones and stellar winds and in the interstellar medium. Turbulent magnetic fields are therefore expected to play an important role. Despite the fact that, on a microscopic level, astrophysical plasmas exhibit rather diverse properties, a unified macroscopic treatment in the framework of magnetohydrodynamics (MHD) to describe the most important magnetic effects is appropriate. Hence there is much interest in MHD turbulence in the astrophysical community. Considerable interest comes also from the side of pure theory, where MHD turbulence introduces new concepts into turbulence theory, as the large number of articles on this topic in the literature shows. However, to date no monograph on MHD turbulence seems to have been written. I therefore believe that a treatise both introducing the field and reviewing the current state of the art could be welcome.

The solar wind provides an almost ideal laboratory for studying high-Reynolds-number MHD turbulence. Turbulence is free to evolve unconstrained and unperturbed by in situ diagnostics, satellite-mounted magnetometers, probes and particle detectors. We will see that many features of homogeneous MHD turbulence discussed in the previous chapters are discovered in solar-wind turbulence, but there are also unexpected and still unexplained features. Since the turbulence varies significantly in the different regions of interplanetary space depending on the local solar-wind conditions, it is useful to first give at least a rough picture of the mean solar-wind properties, before discussing the properties of the turbulent fluctuations about the mean state.

Mean properties of the solar wind

Stars lose not only energy by radiation but also mass (and angular momentum) by a, more or less, continuous radial flow called the stellar wind, the solar wind in the case of the Sun. The origin of this flow is the high temperature in the corona, which means that the coronal plasma is not gravitationally bound and, if it is not confined by magnetic loops, expands into interplanetary space, giving rise to the supersonic solar wind. The flow extends radially out to a distance beyond the planetary system, before it is slowed down by the termination shock expected at roughly 100 AU (1 AU ≃ 1.5 × 108 km is the Earth's orbital radius). It thus forms a bubble in interstellar space, the heliosphere. The solar-wind plasma consisting of fully ionized hydrogen with a small admixture of helium soon reaches supersonic and super-Alfvénic speeds.