193 research outputs found
Vertical migration maintains phytoplankton position in a tidal channel with residual flow
A tidal channel can retain phytoplankton, despite a residual flow, if the phytoplankton migrate vertically with a daily rhythm. Tidal currents are slowed down by bed friction and so plankton experience faster flow when higher in the water column. The lateral movement of the plankton depends on the nature of the vertical migration, particularly the time spent near the surface and the phase of the tide. A model of this process accorded with observations of chlorophyll derived from in situ fluorescence at a mooring in a tidal channel. Peaks in chlorophyll at the end of the flood tide indicated the presence of a phytoplankton bloom downstream of the mooring. Peaks in chlorophyll at the ends of the morning flood tides were 3 to 4 times larger than at the ends of the evening floods, over several days. In contrast, well-mixed particulates were removed from the channel by the residual flow in just 2 d. Both the day-night asymmetry and the sustained presence of chlorophyll were explained by allowing for vertical migration of the phytoplankton and constraining the period during which they were near the surface. Tidal channels retaining phytoplankton that migrate vertically can be ecologically more diverse and yield higher commercial output of farmed bivalves. The natural timings of some phytoplankton blooms in tidal channels are controlled by the nature of the migration. Although a by-product of vertical migration, longer residence in the tidal channel affords the phytoplankton more nutrients than phytoplankton that advect offshore
A physics-enabled flow restoration algorithm for sparse PIV and PTV measurements
The gaps and noise present in particle image velocimetry (PIV) and particle tracking velocimetry (PTV) measurements affect the accuracy of the data collected. Existing algorithms developed for the restoration of such data are only applicable to experimental measurements collected under well-prepared laboratory conditions (i.e. where the pattern of the velocity flow field is known), and the distribution, size and type of gaps and noise may be controlled by the laboratory set-up. However, in many cases, such as PIV and PTV measurements of arbitrarily turbid coastal waters, the arrangement of such conditions is not possible. When the size of gaps or the level of noise in these experimental measurements become too large, their successful restoration with existing algorithms becomes questionable. Here, we outline a new physics-enabled flow restoration algorithm (PEFRA), specially designed for the restoration of such velocity data. Implemented as a 'black box' algorithm, where no user-background in fluid dynamics is necessary, the physical structure of the flow in gappy or noisy data is able to be restored in accordance with its hydrodynamical basis. The use of this is not dependent on types of flow, types of gaps or noise in measurements. The algorithm will operate on any data time-series containing a sequence of velocity flow fields recorded by PIV or PTV. Tests with numerical flow fields established that this method is able to successfully restore corrupted PIV and PTV measurements with different levels of sparsity and noise. This assessment of the algorithm performance is extended with an example application to in situ submersible 3D-PTV measurements collected in the bottom boundary layer of the coastal ocean, where the naturally-occurring plankton and suspended sediments used as tracers causes an increase in the noise level that, without such denoising, will contaminate the measurements
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
Multiwavelength Constraints on the Origin of a Nearby Repeating Fast Radio Burst Source in a Globular Cluster
Since fast radio bursts (FRBs) were discovered, their precise origins have
remained a mystery. Multiwavelength observations of nearby FRB sources provide
one of the best ways to make rapid progress in our understanding of the
enigmatic FRB phenomenon. We present results from a sensitive, broadband
multiwavelength X-ray and radio observational campaign of FRB 20200120E, the
closest known extragalactic repeating FRB source. At a distance of 3.63 Mpc,
FRB 20200120E resides in an exceptional location, within a ~10 Gyr-old globular
cluster in the M81 galactic system. We place deep limits on both the persistent
X-ray luminosity and prompt X-ray emission at the time of radio bursts from FRB
20200120E, which we use to constrain possible progenitors for the source. We
compare our results to various classes of X-ray sources and transients. In
particular, we find that FRB 20200120E is unlikely to be associated with:
ultraluminous X-ray bursts (ULXBs), similar to those observed from objects of
unknown origin in other extragalactic globular clusters; giant flares, like
those observed from Galactic and extragalactic magnetars; or most intermediate
flares and very bright short X-ray bursts, similar to those seen from magnetars
in the Milky Way. We show that FRB 20200120E is also unlikely to be powered by
a persistent or transient ultraluminous X-ray (ULX) source or a young,
extragalactic pulsar embedded in a Crab-like nebula. We also provide new
constraints on the compatibility of FRB 20200120E with accretion-based FRB
models involving X-ray binaries and models that require a synchrotron maser
process from relativistic shocks to generate FRB emission. These results
highlight the power that multiwavelength observations of nearby FRBs can
provide for discriminating between potential FRB progenitor models.Comment: 58 pages, 10 figures, 7 tables, submitte
LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity from FRB 20180916B
FRB 20180916B is a well-studied repeating fast radio burst source. Its
proximity (~150 Mpc), along with detailed studies of the bursts, have revealed
many clues about its nature -- including a 16.3-day periodicity in its
activity. Here we report on the detection of 18 bursts using LOFAR at 110-188
MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are
seen down to the lowest-observed frequency of 110 MHz, suggesting that their
spectra extend even lower. These observations provide an order-of-magnitude
stronger constraint on the optical depth due to free-free absorption in the
source's local environment. The absence of circular polarization and nearly
flat polarization angle curves are consistent with burst properties seen at
300-1700 MHz. Compared with higher frequencies, the larger burst widths
(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due
to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure
that may be correlated with the activity cycle of the source. We compare the
LOFAR burst arrival times to those of 38 previously published and 22 newly
detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).
Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected
by LOFAR. We find that the burst activity is systematically delayed towards
lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results
in the context of a model in which FRB 20180916B is an interacting binary
system featuring a neutron star and high-mass stellar companion.Comment: Accepted for publication by ApJ
Gravity Recovery and Interior Laboratory (GRAIL): Extended Mission and End-Game Status
The Gravity Recovery and Interior Laboratory (GRAIL) [1], NASA s eleventh Discovery mission, successfully executed its Primary Mission (PM) in lunar orbit between March 1, 2012 and May 29, 2012. GRAIL s Extended Mission (XM) initiated on August 30, 2012 and was successfully completed on December 14, 2012. The XM provided an additional three months of gravity mapping at half the altitude (23 km) of the PM (55 km), and is providing higherresolution gravity models that are being used to map the upper crust of the Moon in unprecedented detail
Periodic activity from a fast radio burst source
Fast radio bursts (FRBs) are bright, millisecond-duration radio transients
originating from extragalactic distances. Their origin is unknown. Some FRB
sources emit repeat bursts, ruling out cataclysmic origins for those events.
Despite searches for periodicity in repeat burst arrival times on time scales
from milliseconds to many days, these bursts have hitherto been observed to
appear sporadically, and though clustered, without a regular pattern. Here we
report the detection of a day periodicity (or possibly a
higher-frequency alias of that periodicity) from a repeating FRB
180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment
Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September
16th, 2018 through February 4th, 2020, we find that all bursts arrive in a
5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our
results suggest a mechanism for periodic modulation either of the burst
emission itself, or through external amplification or absorption, and disfavour
models invoking purely sporadic processes
Preliminary Results on Lunar Interior Properties from the GRAIL Mission
The Gravity Recovery and Interior Laboratory (GRAIL) mission has provided lunar gravity with unprecedented accuracy and resolution. GRAIL has produced a high-resolution map of the lunar gravity field while also determining tidal response. We present the latest gravity field solution and its preliminary implications for the Moon's interior structure, exploring properties such as the mean density, moment of inertia of the solid Moon, and tidal potential Love number k2. Lunar structure includes a thin crust, a deep mantle, a fluid core, and a suspected solid inner core. An accurate Love number mainly improves knowledge of the fluid core and deep mantle. In the future GRAIL will search for evidence of tidal dissipation and a solid inner core
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