A magnetohydrodynamical (MHD) shock is a discontinuity with a plasma flow across it. Conservation of mass, momentum, and energy requires the corresponding fluxes to be equal on both sides of the discontinuity. The tangential components of the electric field do not change either. A collisionless shock is not a discontinuity. Collisionless shocks have nontrivial transition regions, determined by the nonlinear interaction of charged particles with the electromagnetic fields. Whatever the interaction and structure may be, the conservation laws and the electrodynamic constraints must be maintained throughout. Depending on the incident plasma parameters, the shock structure evolves to adjust itself to the conservation laws. Self-regulation means that the required structure develops even if we do not understand the specific microscopic mechanisms of the structure development. We review here the basic field profiles and ion behavior of self-regulating shocks.
Ultraviolet (UV) observations offer unique insights into transient and variable astrophysical phenomena, from the Sun and nearby stars to supernovae and star-forming galaxies. Here, we summarise and review the diagnostic power of the UV spectrum for probing the physical conditions, dynamics and energetics of these events. We trace the historical development of UV astronomy from early satellite missions to the current landscape shaped by the Hubble Space Telescope, the GALaxy Evolution eXplorer, the Neil Gehrels Swift Observatory and AstroSat/UVIT. Particular emphasis is placed on resonance lines such as Mg II h and k, C II, Si IV, C IV and N V, which probe the (solar and) stellar chromosphere and transition region, revealing magnetic heating, shocks and mass loss across stellar types. We examine UV diagnostics of stellar flares, pre-main-sequence activity and rotation–activity correlations, linking them to exoplanetary irradiation and habitability. We then explore the UV view of supernova shock breakout and early-time emission, as well as the far-UV mapping of young stellar populations and hierarchical star formation in the Magellanic Clouds. Together, these aspects illustrate how the UV connects small-scale plasma processes to galactic-scale star formation. Despite remarkable progress, UV transient astronomy remains limited by instrument availability and cadence. Forthcoming missions promise to transform this field by capturing the rapid, luminous phenomena that define the dynamic Universe.
Neutron star low-mass X-ray binaries (NS LMXBs) are systems in which a neutron star accretes matter from a Roche-lobe-filling companion via an accretion disc, releasing gravitational energy with an efficiency of order 10–20
Remote-sensing spectroscopy of interstellar absorption lines towards nearby stars allows us to infer the elemental composition of dust in the Local Interstellar Cloud (LIC), whereas in-situ mass spectrometry of LIC dust entering the heliosphere directly determines the composition. These two approaches are, therefore, highly complementary to each other in constraining elemental abundances in both gaseous and solid phases of the LIC. In this review, we present a comprehensive synthesis of current knowledge on the composition of the LIC derived from remote-sensing gas measurements and in-situ and laboratory dust measurements. We demonstrate that the chemical composition of LIC dust measured in situ acts as a promising tool to determine the gas-phase abundances and ionization states of the elements inaccessible through spectroscopic observations. We also show that the isotopic dust composition can be estimated by in-situ mass spectrometry and provides key information on the age of dust and gas in the LIC. The identification of the smallest dust size in in-situ data gives a clue about the magnetic field vector of the LIC and the structure of the heliospheric boundaries. In-situ data on the mass and spatial distributions of LIC dust in combination with dust dynamics calculations are accounted for by dust coagulation growth in the interstellar medium. In light of the ongoing IMAP mission and the forthcoming DESTINY+ mission, in-situ mass spectrometry of LIC dust is emerging as a powerful tool for probing the physical and chemical properties of the LIC.
The number of rocky extrasolar planets being discovered continues to increase, but so too does the apparent diversity of such worlds. As we work to understand the likely bulk properties and thermal, geological, and climatological attributes we might expect of rocky exoplanets, we can look to the Solar System for guidance. Here, we review the interior, surface, and atmospheric characteristics of modern Earth, and discuss how our homeworld has changed through Solar System history. We then visit in turn Venus, Mars, Mercury and Earth’s Moon, and Io, noting how these terrestrial planets are alike, how they are different, and how they have evolved through time. Finally, we consider some examples of the types of rocky worlds known or suspected to exist without direct Solar System counterparts—but we argue that even then, for all the variety we might expect, they are still variations on a common theme. Perhaps the most important lesson the Solar System can teach us is that terrestrial bodies change through time, sometimes dramatically, and that rocky worlds of similar size and mass can have vastly different planetary outcomes.
