NuSTAR hard X-ray observation of a sub-A class solar flare

We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only 5 counts per second per detector observed by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4x10^27 ergs. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as "flarelike" properties.Comment: Status: Accepted by the Astrophysical Journal, 2017 July 1

NuSTAR detection of X-ray heating events in the quiet Sun

The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2–4.1 MK and emission measures of (0.6–15) × 1044 cm−3 describe their spectra well, resulting in thermal energies in the range (2–6) × 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares

Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/XRT on 2015 April 29 at ~11:29 UT. This microflare shows heating of material to several million Kelvin, observed in Soft X-rays (SXRs) with Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA. For three of the four NuSTAR observations of this region (pre-, decay, and post phases) the spectrum is well fitted by a single thermal model of 3.2-3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find the pre-flare DEM peaks at ~3 MK and falls off sharply by 5 MK; but during the microflare's impulsive phase the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about $2.5 \times 10^{25}$ erg s$^{-1}$. As the NuSTAR spectrum is purely thermal we determined upper-limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of the heating requires a power-law spectrum of $\delta \ge 7$ with a low energy cut-off $E_{c} \lesssim 7$ keV. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.Comment: Accepted for publication in ApJ. 14 pages with 12 figures and 1 tabl

Reduced Radial Displacement of the Gastrocnemius Medialis Muscle After Electrically Elicited Fatigue

Context: Assessments of skeletal muscle functional capacity often necessitate maximal contractile effort, which exacerbates muscle fatigue or injury. Tensiomyography (TMG) has been investigated as a means to assess muscle contractile function following fatigue; however observations have not been contextualised by concurrent physiological measures. Objective: The aim of the present investigation was to measure peripheral fatigue-induced alterations in mechanical and contractile properties of the plantar flexor muscles through non-invasive TMG concurrently with maximal voluntary contraction (MVC) and passive muscle tension (PMT) in order to validate TMG as a gauge of peripheral fatigue. Design: Pre- and post-test intervention with control. Setting: University laboratory. Participants: Twenty-one healthy male volunteers. Interventions: Subjects plantar flexors were tested for TMG parameters, along with MVC and PMT, before and after either a 5 minute rest period (control) or a 5 minute electrical stimulation intervention (fatigue). Main Outcome Measures: Temporal (contraction velocity) and spatial (radial displacement) contractile parameters of the Gastrocnemius Medialis were recorded through TMG. MVC was measured as an indicator of muscle fatigue and PMT was measured to assess muscle stiffness. Results: Radial displacement demonstrated a fatigue-associated reduction (3.3 ± 1.2 vs. 4.0 ± 1.4 mm vs, p=0.031), while contraction velocity remained unaltered. Additionally, MVC significantly declined by 122.6 ± 104 N (p<0.001) following stimulation (fatigue). PMT was significantly increased following fatigue (139.8 ± 54.3 vs. 111.3 ± 44.6 N, p=0.007).  Conclusion: TMG successfully detected fatigue, evident from reduced MVC, by displaying impaired muscle displacement, accompanied by elevated PMT. TMG could be useful in establishing fatigue status of skeletal muscle without exacerbating the functional decrement of the muscle

Ultraviolet writing of channel waveguides in proton-exchanged LiNbO₃

We report on a direct ultraviolet (UV) writing method for the fabrication of channel waveguides at 1.55 µm in LiNbO3 through UV irradiation of surface and buried planar waveguides made by annealed proton exchange and reverse proton exchange. A systematic study of the guidance properties as a function of the UV writing conditions is presented

Order alpha_s^2 beta_0 Correction to the Charged Lepton Spectrum in b \to c \ell \bar\nu_\ell decays

We compute the \alpha_s^2\beta_0 part of the two-loop QCD corrections to the charged lepton spectrum in b \to c \ell \bar\nu_\ell decays and find them to be about 50\% of the first order corrections at all lepton energies, except those close to the end point. Including these corrections we extract the central values \bar\Lambda=0.33 GeV and \lambda_1=-0.17 GeV^2 for the HQET matrix elements and use them to determine the $\overline{\rm MS}$ b and c quark masses, and |V_{cb}|.Comment: 15 pages, 1 Postscript figur

Size-selective predation by three estuarine zooplanktivorous fish species

Zooplanktivorous fish are a key link between abundant zooplankton and higher trophic levels but the foraging behaviour of zooplanktivorous fish is not fully understood. Selective feeding behaviours have been observed, with many species of planktivorous fish targeting certain species and sizes of zooplankton for prey. However, why certain size classes of zooplankton are preferred remains unclear. Aim. This study investigated prey selection by three zooplanktivorous fish species through the lens of optimal foraging theory. Methods. We assessed the size structure of zooplankton in the environment and compared this with the size distribution of zooplankton in gut contents from three zooplanktivorous fish. Key results. The targeted prey size of Atypichthys strigatus and Scorpis lineolata aligns with the prey size classes in the environment that contain the highest overall biomass. Trachurus novaezelandiae showed little evidence of targeting these size classes. Conclusions. These prey sizes therefore represent the most efficient prey to target because the return on foraging effort is greatest. By contrast, T. novaezelandiae showed only an underselection of large and small prey. Implications. By incorporating this information on this key trophic link between zooplankton and fish, ecosystem models could better resolve the size dependant predation, particularly in size-based models

Behavioral variation across the days and lives of honey bees

In honey bee colonies, workers generally change tasks with age (from brood care, to nest work, to foraging). While these trends are well established, our understanding of how individuals distribute tasks during a day, and how individuals differ in their lifetime behavioral trajectories, is limited. Here, we use automated tracking to obtain long-term data on 4,100+ bees tracked continuously at 3 Hz, across an entire summer, and use behavioral metrics to compare behavior at different timescales. Considering single days, we describe how bees differ in space use, detection, and movement. Analyzing the behavior exhibited across their entire lives, we find consistent inter-individual differences in the movement characteristics of individuals. Bees also differ in how quickly they transition through behavioral space to ultimately become foragers, with fast-transitioning bees living the shortest lives. Our analysis framework provides a quantitative approach to describe individual behavioral variation within a colony from single days to entire lifetimes

Age and growth of Pomatomus saltatrix in the south-western Pacific Ocean (eastern Australia), with a global comparison

Context: Pomatomus saltatrix is one of few globally distributed pelagic mesopredators that is exploited heavily throughout its range. Despite the implementation of management strategies, the south-western Pacific Ocean (eastern Australian) population has few published estimates of the key life-history parameters including growth. Aims: To estimate the age and growth of P. saltatrix in the south-western Pacific and compare these with the age and growth in other populations. Methods: Age estimates were made using whole otolith readings and an age–length key was used with a length frequency distribution to estimate the age structure of the population. Eight different growth models were compared within a Bayesian framework for both juvenile and overall growth. Key results: The Schnute growth equation provided the best fit for overall growth and yielded parameter values of a = −0.15, b = 2.56, Size-at-age 1 = 24.38-cm fork length (FL) and Size-at-age 4 = 47.36 cm FL. Conclusions: P. saltatrix in the south-western Pacific has a growth rate similar to that in other populations of P. saltatrix. Implications: Despite its geographically and genetically distinct populations, P. saltatrix demonstrates a generally consistent life-history strategy of fast growth and high mortality, except for the north-western Atlantic population, which has lower mortality