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Remote sensing of inland waters can provide timely and global water quality information to a wide variety of stakeholders. One of the parameters that determines the feasibility of using optical space-based instruments for monitoring inland waters is the ground sampling distance (GSD), defined as the width of a pixel projected on the Earth’s surface. We assume that to analyze a body of water with optical imagery, its characteristic width must be larger than 3 times the GSD to obtain an ‘unmixed’ pixel that doesn’t contain signal from the adjacent land. Here we obtain the size distribution of river lengths, river areas, and lake areas—as a function of width—for rivers and lakes in the Western United States (US) and in Australia. We base this analysis on the Surface Water and Ocean Topography River Database (SWORD) and HydroLAKES databases, extrapolated to 5 m-wide features. We show that the fraction of river length and river area larger than a certain width increases sharply as the width decreases, indicating that even small decreases in the GSD result in significant increases in the number of bodies that can be surveyed. On the other hand, the distribution of lake areas shows a ‘knee’ at around 400 m, indicating that gains from GSDs smaller than 130 m will be modest. We found that a satellite instrument with a GSD capability of 18 m can provide coverage of 4.4% of total river lengths, 38% of total river area, and 94% of total lake area within the study areas. We argue that decreasing the GSD incurs penalties associated with loss of signal-to-noise, larger instrument, smaller swath, and longer revisit times.

The theory of binary star formation predicts that close binaries (a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory. [ABSTRACT FROM AUTHOR]

The transfer of matter between a circumbinary disk and a young binary system remains poorly understood, obscuring the interpretation of accretion indicators. To explore the behavior of these indicators in multiple systems, we have performed the first systematic time-domain study of young binaries in the ultraviolet. We obtained far- and near-ultraviolet HST/COS spectra of the young spectroscopic binaries DQ Tau and UZ Tau E. Here we focus on the continuum from 2800 to 3200 Å and on the C iv doublet (λλ1548.19, 1550.77 Å) as accretion diagnostics. Each system was observed over three or four consecutive binary orbits, at phases ∼0, 0.2, 0.5, and 0.7. Those observations are complemented by ground-based U-band measurements. Contrary to model predictions, we do not detect any clear correlation between accretion luminosity and phase. Further, we do not detect any correlation between C iv flux and phase. For both stars the appearance of the C iv line is similar to that of single Classical T Tauri Stars (CTTSs), despite the lack of stable long-lived circumstellar disks. However, unlike the case in single CTTSs, the narrow and broad components of the C iv lines are uncorrelated, and we argue that the narrow component is powered by processes other than accretion, such as flares in the stellar magnetospheres and/or enhanced activity in the upper atmosphere. We find that both stars contribute equally to the narrow component C iv flux in DQ Tau, but the primary dominates the narrow component C iv emission in UZ Tau E. The C iv broad component flux is correlated with other accretion indicators, suggesting an accretion origin. However, the line is blueshifted, which is inconsistent with its origin in an infall flow close to the star. It is possible that the complicated geometry of the region, as well as turbulence in the shock region, are responsible for the blueshifted line profiles. [ABSTRACT FROM AUTHOR]