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[1] Using observations from the FAST small explorer spacecraft, we present fields and plasma observations above the dayside auroral oval showing the erosion of ionospheric plasmas from the topside ionosphere by the action of Alfvén waves. Using interferometric techniques, the waves are shown to approximately obey the expected dispersion for Alfvén waves with transverse scales extending from greater than electron inertial lengths down to ion gyroradii. Measurements of the plasma density where these waves are observed show that over latitudinal widths exceeding 100 km total depletion of the cold ionospheric plasma can occur. These depleted regions or cavities are populated by magnetosheath plasmas, upgoing transversely accelerated ionospheric ions, and downgoing field-aligned electrons. The ionospheric ions and field-aligned electrons are distributed as conics and beams, respectively. Poynting flux observations on the density gradients comprising the cavity walls show that these waves are directed downward and focused inward toward regions of lower density. Wave phase velocity measurements, while subject to significant uncertainty, show that the wave vector is directed transversely outward from the cavity. These observations suggest a feedback model for Alfvén wave focusing and ion heating on density gradients that can lead to intense ion outflow from the ionosphere and subsequent depletion of ionospheric plasmas.

Observations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large‐amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large‐amplitude (~100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.

Evapotranspiration (ET), or the sum of water released to the atmosphere from ground surfaces, intercepts canopy precipitation through evaporation and plant transpiration and is one of the most significant components in the water cycle. In this study, Moderate Resolution Imaging Spectroradiometer (MODIS) 16 global terrestrial ET products were validated at 17 flux tower locations in Asia. Overall, overestimations due to energy balance misclosure distorted the trend of the data at nine locations [r: 0.27-0.82; bias: -21.41-2.38 mm 8-d -1 ; Root Mean Square Error (RMSE): 6.12-21.81 mm 8-d -1 ]. Regardless of variation in the scattering patterns, good agreements between MODIS-based ET and ET measured at the flux towers were observed at five locations (r: 0.50-0.76; bias: -1.42-1.99 mm 8-d -1 ; RMSE: 1.99-8.96 mm 8-d -1 ). Underestimation at one site (r = 0.28, bias = -17.00 mm 8-d -1 , RMSE = 17.41 mm 8-d -1 ) was accompanied by mismatches at two sites (r = 0.12-0.18; bias = -4.19 − -0.04 mm 8-d -1 , RMSE = 5.76-7.66 mm 8-d -1 ). The best performances of the MOD16 ET algorithm were observed at sites with forested land cover, but no substantial differences were found under a variety of climate conditions. This study is the first comprehensive trial to validate global terrestrial MODIS ET in Asia, showing that a MODIS global terrestrial ET product can estimate actual ET with reasonable accuracy. We believe that our results can be used as baseline ET values for satellite image-based ET mapping research in South Korea.

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Key Points• Soil hydrologic parameters including field saturation, field capacity, initiation of plant water stress and plant extraction limits can be reliably determined from electronic soil moisture sensor records.• Soil profile wetting and drying occurs along a regular continuum of soil moisture following the advance of the wetting from to the effective base of the soil profile.• Frozen soil conditions and interactions between energy and water limited water balances complicate interpretations of fluxes in the soil-plant-atmosphere continuum.Soil moisture is an important control on hydrologic function, as it governs vertical fluxes from and to the atmosphere, groundwater recharge, and lateral fluxes through the soil. Historically, the traditional model parameters of saturation, field capacity, and permanent wilting point have been determined by laboratory methods. This approach is challenged by issues of scale, boundary conditions, and soil disturbance. We develop and compare four methods to determine values of field saturation, field capacity, plant extraction limit (PEL), and initiation of plant water stress from long term in-situ monitoring records of TDR-measured volumetric water content ( ). The monitoring sites represent a range of soil textures, soil depths, effective precipitation and plant cover types in a semi-arid climate. The records exhibit attractors (high frequency values) that correspond to field capacity and the PEL at both annual and longer time scales, but the field saturation values vary by year depending on seasonal wetness in the semi-arid setting. The analysis for five sites in two watersheds is supported by comparison to values determined by a common pedotransfer function and measured soil characteristic curves. Frozen soil is identified as a complicating factor for the analysis and users are cautioned to filter data by temperature, especially for near surface soils.

[1] FAST electric field data and ion drift moments are combined to allow full DC E ? (electric field perpendicular to the geomagnetic field) studies of auroral return current regions. Statistical comparison of 71 return current potential structures showed several differences between sheetlike structured perpendicular E ? field events, where the ratio of the two E ? components remains constant during the spacecraft crossing, and curved structures where the ratio varies. Sheetlike structures can be interpreted as straight arcs, but curved structures require gradients in another dimension. We define a parameter h, which is a proxy for the ratio of the potential at the spacecraft and the upgoing electron characteristic energy. Thus h is a measure of the extent to which the potential contours are closed below the spacecraft. Statistical comparison shows that U-shaped closedpotential models are mostly consistent with curved events and ionospheric effects are dominant in sheetlike structures. This result implies that the spatial structure of the events, as indicated by the ratio of the E ? components, allows us to distinguish ionospheric fields and U-shaped potentials. Statistical studies of scale sizes, magnitudes of electric fields and magnetic perturbations, and downward current density, sorted by the parameter h, reveal various interesting features. We attempt to explain these properties on the basis of different potential closure models for sheetlike and curved structures, which have important implications for models of the formation and evolution of potential structures for downward current regions.

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