Direct characterisation of tuneable few-femtosecond dispersive-wave pulses in the deep UV

Dispersive wave emission (DWE) in gas-filled hollow-core dielectric waveguides is a promising source of tuneable coherent and broadband radiation, but so far the generation of few-femtosecond pulses using this technique has not been demonstrated. Using in-vacuum frequency-resolved optical gating, we directly characterise tuneable 3fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses.Comment: 5 pages, 4 figure

What does 'supporting parents' mean? - parents' views

This paper reports on the views of a community sample of 428 parents with primary school-aged children. In a previous study parents had identified that they needed 'support'. This study was designed to try to understand what types of support parents already have and what support they think needs to be available to them. Most parents use informal support of family and friends and have limited awareness of what is available to them in the way of locally based services. They propose services which are already available, like Parentline, but of which they are unaware. There seems to be a need for universal, non-stigmatising services which design their programmes with parents and can refer to more specialised services, e.g. Social Services or Family Centres. These services need to be located in agencies which parents frequent and are comfortable with, such as schools and health settings

To burn or not to burn: Comparing reintroducing fire with cutting an encroaching conifer for conservation of an imperiled shrub‐steppe

Woody vegetation has increased on rangelands worldwide for the past 100– 200 years, often because of reduced fire frequency. However, there is a general aversion to reintroducing fire, and therefore, fire surrogates are often used in its place to reverse woody plant encroachment. Determining the conservation effectiveness of reintroducing fire compared with fire surrogates over different time scales is needed to improve conservation efforts. We evaluated the conservation effectiveness of reintroducing fire with a fire surrogate (cutting) applied over the last ~30 years to control juniper (Juniperus occidentalis Hook.) encroachment on 77 sagebrush‐steppe sites. Critical to conservation of this imperiled ecosystem is to limit juniper, not encourage exotic annual grasses, and promote sagebrush dominance of the overstory. Reintroducing fire was more effective than cutting at reducing juniper abundance and extending the period of time that juniper was not dominating the plant community. Sagebrush was reduced more with burning than cutting. Sagebrush, however, was predicted to be a substantial component of the overstory longer in burned than cut areas because of more effective juniper control. Variation in exotic annual grass cover was explained by environmental variables and perennial grass abundance, but not treatment, with annual grasses being problematic on hotter and drier sites with less perennial grass. This suggests that ecological memory varies along an environmental gradient. Reintroducing fire was more effective than cutting at conserving sagebrush‐steppe encroached by juniper over extended time frames; however, cutting was more effective for short‐term conservation. This suggests fire and fire surrogates both have critical roles in conservation of imperiled ecosystems

The Sage-Grouse Habitat Mortgage: Effective Conifer Management in Space and Time

AbstractManagement of conservation-reliant species can be complicated by the need to manage ecosystem processes that operate at extended temporal horizons. One such process is the role of fire in regulating abundance of expanding conifers that disrupt sage-grouse habitat in the northern Great Basin of the United States. Removing conifers by cutting has a beneficial effect on sage-grouse habitat. However, effects may last only a few decades because conifer seedlings are not controlled and the seed bank is fully stocked. Fire treatment may be preferred because conifer control lasts longer than for mechanical treatments. The amount of conservation needed to control conifers at large temporal and spatial scales can be quantified by multiplying land area by the time needed for conifer abundance to progress to critical thresholds (i.e., “conservation volume”). The contribution of different treatments in arresting conifer succession can be calculated by dividing conservation volume by the duration of treatment effect. We estimate that fire has approximately twice the treatment life of cutting at time horizons approaching 100 yr, but, has high up-front conservation costs due to temporary loss of sagebrush. Cutting has less up-front conservation costs because sagebrush is unaffected, but it is more expensive over longer management time horizons because of decreased durability. Managing conifers within sage-grouse habitat is difficult because of the necessity to maintain the majority of the landscape in sagebrush habitat and because the threshold for negative conifer effects occurs fairly early in the successional process. The time needed for recovery of sagebrush creates limits to fire use in managing sage-grouse habitat. Utilizing a combination of fire and cutting treatments is most financially and ecologically sustainable over long time horizons involved in managing conifer-prone sage-grouse habitat

High-energy Behavior of the Double Photoionization of Helium from 2 to 12 keV

We report the ratio of double-to-single photoionization of He at several photon energies from 2 to 12 keV. By time-of-Aight methods, we find a ratio consistent with an asymptote at 1.5%±0.2%, essentially reached by h v≈4 keV. Fair agreement is obtained with older shake calculations of Byron and Joachain [Phys. Rev. 164, 1 (1967)], of Aberg [Phys. Rev. A 2, 1726 (1970)], and with recent many-body perturbation theory (MBPT) of Ishihara, Hino, and McGuire [Phys. Rev. A 44, 6980 (1991)]. The result lies below earlier MPBT calculations by Amusia et al. [J. Phys. B 8, 1248 (1975)] (2.3%), and well above semiempirical predictions of Samson [Phys. Rev. Lett. 65, 2861 (1990)], who expects no asymptote and predicts ơ(He2+)/ơ (He+) =0.3% at 12 keV

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 1782-1797, doi:10.1121/1.3666003.Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.This work was supported by Office of Naval Research (ONR) Grants Nos. N00014-05-1-0482 and N00014-11-1- 0194 to T.F.D., ONR Grant No. N00014-04-1-0146 to J.F.L., and an ONR Ocean Acoustics Postdoctoral Fellowship awarded to J.M.C. under Professor William Carey at Boston University

Remote Sensing Experiments Using the Rogue-alpha,beta CubeSats as a Constellation: High Frame Rate Environmental Observations from Agile, Taskable, Infrared and Visible Sensors in Low Earth Orbit

The Aerospace Corporation’s Rogue-alpha,beta program built and launched two 3-Unit CubeSats in 18-months, each equipped with modified commercial infrared camera payloads, visible context cameras, laser communications and precision pointing capabilities. Launched on November 2, 2019, the two spacecraft (Rogue-alpha and beta) were boosted and released from the International Space Station Cygnus NG-12 robotic resupply spacecraft on January 31, 2020 into a circular 460-km, 52° inclined orbit. The primary Rogue IR sensor is a 1.4-micron band, 640x512 pixel, 28° field of view, InGaAs short wavelength infrared (SWIR) camera. It is accompanied by a panchromatic, 10-megapixel, 37° field of view visible context camera. In addition, the narrow- and wide-field-of-view star sensors may also be utilized as nighttime sensors. During the first two years of spaceflight, the Rogue satellites conducted a series of experiments using both spacecraft to conduct cooperative remote sensing observations and to test the capabilities of the 1.4-micron water overtone band. These included: 1) fore-aft pointing using two spacecraft for stereo observations of cloud structure and altitude, 2) horizon-pointed imaging in all directions relative to the spacecraft orbit (fore, aft, port, and starboard) to maximize the imaged field of view, 3) pre-programmed point-and-stare imaging, 4) nadir-pointed operations for vicarious calibration with other satellites. All of these modes of operation are usually conducted in multi-frame collections at 1-20 frames-per-second for dozens to thousands of frames. During the mission we investigated different modes of collecting data, taking advantage of the evolving orbital spacing of the pair of CubeSats. Initial close satellite spacing allowed along-track fore-aft stereo observations of weather formations, as well as pre-programmed tip-and-queue observations, and sequential point-and-stare experiments aimed at collecting minutes of data on targets of interest. Cloud altitude was measured on weather events by simultaneous stereo observations, and by mono observations using the changing view angles during a constant point along track or slewing during a pass. Observations were collected on hurricanes, typhoons, thunderstorms, monsoon storms, and forecasted tornadic weather. Unique observations of severe wildfires were collected, exploring the capability for our 1.4micron band to detect fires during daytime, and to characterize pyrocumulonimbus clouds. Nighttime observations were also made of human lighting, infrared sources, and moonlight-illuminated clouds, including observations utilizing the Rogue satellites’ star sensors for remote sensing tests. These experiments collectively explored the possibilities for dynamically tasked, high-frame-rate, low-earth-orbit sensors to carry out weather and environmental monitoring missions in ways that differ from traditional scanned or push-broom satellite sensor systems. We will present a summary of our tasking ConOps, observations of weather events and fires, and highlight results and techniques for cloud height characterization by our two CubeSat constellation during its first two years on orbit. Our results with two satellites demonstrate possibilities for future missions using cooperative tasking in larger constellations of dynamically tasked sensors in low Earth orbit

Landsat Imagery from a CubeSat: Results and Operational Lessons from the R3 Satellite\u27s First 18 Months in Space

R3 is a 3-U CubeSat launched on a RocketLab Electron into a 500 km circular orbit at 85° inclination on December 16th, 2018. The spacecraft flies a multispectral sensor that takes data in the six Landsat visible and near infrared bands. The R3 sensor mates a custom refractive telescope with a Materion Precision Optics Landsat filter, and an ON Semiconductor fast-framing high-sensitivity Si CMOS array, to produce 50-km wide, 44-m resolution Landsat-like image strips. Data are taken in push-broom mode and are downlinked via a 100Mbps compact lasercom system. Frames are then co-added on the ground in time-delay-integration (TDI) fashion to increase signal-to-noise ratio and create multi-spectral Earth images from the compact sensor. The system is an engineering concept demonstration of a compact multispectral sensor in CubeSat form. We describe our ConOps, flight operations, sensor focus and alignment, initial imaging check out, and initial comparisons of R3 data to Landsat-8 imagery of the same Earth locations. RGB, color infrared, and normalized differential vegetation index (NDVI) products are compared between CUMULOS and Landsat-8. Results show good multispectral image quality from the CubeSat sensor, and illustrate the ability of R3 to detect vegetation and other features in a manner similar to Landsat, as well as the challenge in perfectly exposing all 6 VIS/NIR Landsat bands using our commercial 10-bit CMOS array. We also highlight the performance of the compact laser communications system which enabled the successful performance of this mission