Aims. This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, as well as hosting the potential of a rich return in further science. Methods. SO/PHI measures the Zeeman effect and the Doppler shift in the Fe i 617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO 3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders (LCVRs). The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope (FDT), covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope (HRT), can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line. Results. SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission's highly elliptical orbit.

No abstract

Rotor-layer wind resource and turbine available power uncertainties prior to wind farm construction may contribute to significant increases in project risk and costs. Such uncertainties exist in part due to limited offshore wind measurements between 40 and 250 m and the lack of empirical methods to describe wind profiles that deviate from a priori, expected power law conditions. In this article, we introduce a novel wind profile classification algorithm that accounts for nonstandard, unexpected profiles that deviate from near power law conditions. Using this algorithm, offshore Doppler wind lidar measurements in the Mid-Atlantic Bight are classified based on goodness-of-fit to several mathematical expressions and relative speed criteria. Results elucidate the limitations of using power law extrapolation methods to approximate average wind profile shape/shear conditions, as only approximately 18% of profiles fit well with this expression, while most consist of unexpected wind shear. Further, results demonstrate a relationship between classified profile variability and coastal meteorological features, including stability and offshore fetch. Power law profiles persist during unstable conditions and relatively weaker northeasterly flow from water (large fetch), whereas unexpected classified profiles are prevalent during stable conditions and stronger southwesterly flow from land (small fetch). Finally, the magnitude of the discrepancy between hub-height wind speed and rotor equivalent wind speed available power estimates varies by classified wind-profile type. During unexpected classified profiles, both a significant overprediction and underprediction of hub-height wind available power is possible, illustrating the importance of accounting for site-specific rotor-layer wind shear when predicting available power. KEYWORDS atmospheric stability, coastal meteorology, Doppler wind lidar, offshore wind resource, rotor equivalent wind speed, turbine available power INTRODUCTIONOffshore wind (OSW) power provides an enormous clean electricity resource to help mitigate anthropogenic climate change and stimulate the economy. There is a significant opportunity to generate OSW power since most of the global resource is untapped with 90% of the 12 GW installed capacity worldwide concentrated in the relatively small geographic region of Northern Europe. 1 Global investment in OSW development reached a record high in 2016, signaling an important transition in the industry toward accelerating expertise 2 ; however, preconstruction energy yield uncertainty is a reoccurring challenge that contributes to significant project risk and could delay the technology's cost competitiveness. 3Inaccuracies in estimating turbine performance prior to wind farm construction contributes to preconstruction energy yield uncertainty. 3 It is well known that vertical wind speed shear across a turbine's rotor layer contributes to power performance uncertainty (eg, previous studies 4-8 ). To help reduce this uncertainty, a rotor equivalent wind speed (REW...