Analyzing Explosive Volcanic Deposits From Satellite‐Based Radar Backscatter, Volcán de Fuego, 2018

Satellite radar backscatter has the potential to provide useful information about the progression of volcanic eruptions when optical, ground-based, or radar phase-based measurements are limited. However, backscatter changes are complex and challenging to interpret: explosive deposits produce different signals depending on pre-existing ground cover, radar parameters and eruption characteristics. We use high temporal- and spatial-resolution backscatter imagery to examine the emplacement and alteration of pyroclastic density currents (PDCs), lahar and ash deposits from the June 2018 eruption of Volcán de Fuego, Guatemala, using observatory reports and rainfall gauge data to ground truth our observations. We use a temporally dense time series of backscatter data to reduce noise and extract deposit areas. We observe backscatter changes in six drainages, the largest deposit was 11.9-km-long that altered an area of 6.3 urn:x-wiley:21699313:media:jgrb55183:jgrb55183-math-0001 and had a thickness of 10.5 urn:x-wiley:21699313:media:jgrb55183:jgrb55183-math-00022 m in the lower sections as estimated from radar shadows. The 3 June eruption also produced backscatter signal over an area of 40 urn:x-wiley:21699313:media:jgrb55183:jgrb55183-math-0003, consistent with reported ashfall. We use transient patterns in backscatter time series to identify nine periods of high lahar activity in a single drainage system between June and October 2018. We find that the characterisation of subtle backscatter signals associated with explosive eruptions are best observed with (1) radiometric terrain calibration, (2) speckle correction, and (3) consideration of pre-existing scattering properties. Our observations demonstrate that SAR backscatter can capture the emplacement and subsequent alteration of a range of explosive deposits, allowing the progression of an explosive eruption to be monitored

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J-V Hysteresis

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/ mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (&sim;0.2 &mu;C/ cm2) and another, larger charge (40 &mu;C/cm2), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of &sim;5, independent of bias light intensity. The fast decay (&sim;1 &mu;s at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J&minus;V hysteresis. Capacitance vs open circuit voltage (Voc) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the Voc hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at Voc suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J&minus;V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.</div