3,013 research outputs found
Small-scale magnetic flux emergence in the quiet Sun
Small bipolar magnetic features are observed to appear in the interior of
individual granules in the quiet Sun, signaling the emergence of tiny magnetic
loops from the solar interior. We study the origin of those features as part of
the magnetoconvection process in the top layers of the convection zone. Two
quiet-Sun magnetoconvection models, calculated with the
radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching
from the top layers of the convection zone to the corona, are analyzed. Using
3D visualization as well as a posteriori spectral synthesis of Stokes
parameters, we detect the repeated emergence of small magnetic elements in the
interior of granules, as in the observations. Additionally, we identify the
formation of organized horizontal magnetic sheets covering whole granules. Our
approach is twofold, calculating statistical properties of the system, like
joint probability density functions (JPDFs), and pursuing individual events via
visualization tools. We conclude that the small magnetic loops surfacing within
individual granules in the observations may originate from sites at or near the
downflows in the granular and mesogranular levels, probably in the first 1 or
1.5 Mm below the surface. We also document the creation of granule-covering
magnetic sheet-like structures through the sideways expansion of a small
subphotospheric magnetic concentration picked up, and pulled out of the
interior, by a nascent granule. The sheet-like structures we found in the
models may match the recent observations of Centeno et al. (2017).Comment: 9 pages, 5 figures, Published in The Astrophysical Journal Letter
Optimised solutions to the last-mile delivery problem in London using a combination of walking and driving
Inspired by actual parcel delivery operations in London, this paper describes a two-echelon distribution system that combines the use of driving and walking as part of last-mile deliveries in urban areas for a single driver. The paper presents an optimisation model that explicitly treats and integrates the driving and walking elements, and describes a branch-and-cut algorithm that uses new valid inequalities specifically tailored for the problem at hand. Computational
results based on real instances obtained from a courier operating in London are presented to show the performance of the algorithm
Multi-Fluid Simulations of Upper Chromospheric Magnetic Reconnection with Helium-Hydrogen mixture
Our understanding of magnetic reconnection (MR) under chromospheric
conditions remains limited. Recent observations have demonstrated the important
role of ion-neutral interactions in the dynamics of the chromosphere.
Furthermore, the comparison between spectral profiles and synthetic
observations of reconnection events suggest that current MHD approaches appear
to be inconsistent with observations. First, collisions and multi-thermal
aspects of the plasma play a role in these regions. Second, hydrogen and helium
ionization effects are relevant to the energy balance of the chromosphere. This
work investigates multi-fluid multi-species (MFMS) effects on MR in conditions
representative of the upper chromosphere using the multi-fluid Ebysus code. We
compare an MFMS approach based on a helium-hydrogen mixture with a two-fluid
MHD model based on hydrogen only. The simulations of MRs are performed in a
Lundquist number regime high enough to develop plasmoids and instabilities. We
study the evolution of the MR and compare the two approaches including the
structure of the current sheet and plasmoids, the decoupling of the particles,
the evolution of the heating mechanisms, and the composition. The presence of
helium species leads to more efficient heating mechanisms than the two-fluid
case. This scenario, which is out of reach of the two-fluid or single-fluid
models, can reach transition region temperatures starting from upper
chromospheric thermodynamic conditions, representative of a quiet Sun scenario.
The different dynamics between helium and hydrogen species could lead to
chemical fractionation and, under certain conditions, enrichment of helium in
the strongest outflows. This could be of significance for recent observations
of helium enrichment in the solar wind in switchbacks and CMEs
Dynamic Pricing for Vehicle Ferries: using Packing and Simulation to Optimize Revenues
We propose an heuristic approach to the vehicle ferry revenue management problem, where the aim is to maximize the revenue obtained from the sale of vehicle tickets by varying the prices charged to different vehicle types, each occupying a different amount of deck space. Customers arrive and purchase tickets according to their vehicle type and their willingness-to-pay, which typically increases over time because customers purchasing tickets closer to departure tend to accept higher prices. The optimization problem can be solved using dynamic programming but the possible states in the selling season are the set of all feasible vehicle mixes that fit onto the ferry. This makes the problem intractable as the number of vehicle types and ferry size increases. We propose a state space reduction, which uses a vehicle ferry loading simulator to map each vehicle mix to a remaining-space state. This reduces the state space of the dynamic program. Our approach allows the value function to be approximated rapidly and accurately with a relatively coarse discretization of states. We present simulations of the selling season using this reduced state space to validate the method. The vehicle ferry loading simulator was developed in collaboration with a vehicle ferry company and addresses real-world constraints such as manoeuvrability, elevator access, strategic parking gaps, vehicle height constraints and ease of implementation of the packing solutions
Intermittent reconnection and plasmoids in UV bursts in the low solar atmosphere
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena
in the solar atmosphere. Yet the detailed physical mechanisms driving
reconnection are difficult to discern in the remote sensing observations that
are used to study the solar atmosphere. In this paper we exploit the
high-resolution instruments Interface Region Imaging Spectrograph (IRIS) and
the new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST)
to identify the intermittency of magnetic reconnection and its association with
the formation of plasmoids in so-called UV bursts in the low solar atmosphere.
The Si IV 1403A UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular shapes,
in addition to signatures of bidirectional flows. Such profiles had previously
been linked, in idealized numerical simulations, to magnetic reconnection
driven by the plasmoid instability. Simultaneous CHROMIS images in the
chromospheric Ca II K 3934A line now provide compelling evidence for the
presence of plasmoids, by revealing highly dynamic and rapidly moving
brightenings that are smaller than 0.2 arcsec and that evolve on timescales of
order seconds. Our interpretation of the observations is supported by detailed
comparisons with synthetic observables from advanced numerical simulations of
magnetic reconnection and associated plasmoids in the chromosphere. Our results
highlight how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are realistic
enough to compare with observations can directly reveal the small-scale
physical processes that drive the wide range of phenomena in the solar
atmosphere.Comment: Accepted for publication in Astrophysical Journal Letters. Movies are
available at http://folk.uio.no/rouppe/plasmoids_chromis
Quantifying environmental and financial benefits of using porters and cycle couriers for last-mile parcel delivery
Parcel carriers face increasingly difficult operating conditions in busy metropolitan areas due to growing consumer demand for ever faster delivery services and having to cope with traffic congestion and city authority measures that may restrict or penalise access for certain types of vehicle. This paper evaluates the potential environmental and financial benefits of switching from traditional van-based deliveries to an alternative operating model, where porters or cycle couriers undertake deliveries supported by a substantially reduced van fleet.
Results using a specially-developed algorithm to model operations of a real carrier in an area of central London, UK, suggested that the carrier could reduce CO2 emissions by 45%, NOx emissions by 33%, driving distance by 78% and curbside parking time by 45%. Overall cost savings to the carrier were estimated to be in the range 34–39%. Scaling up the modelled emissions savings to London’s Central Activities Zone, an area of approximately 30 km2 and with current total annual parcel delivery distance of around 15 million km, could see annual emissions savings in the region of 2 million kg CO2 and 1633 kg NOx if all carriers utilised porters or cycle couriers. The key operating challenges identified were related to sorting and consolidating items by weight and volume, parcel handover arrangements and how to deal with express items and failed deliveries
Homologous Helical Jets: Observations by IRIS, SDO and Hinode and Magnetic Modeling with Data-Driven Simulations
We report on observations of recurrent jets by instruments onboard the
Interface Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory (SDO)
and Hinode spacecrafts. Over a 4-hour period on July 21st 2013, recurrent
coronal jets were observed to emanate from NOAA Active Region 11793. FUV
spectra probing plasma at transition region temperatures show evidence of
oppositely directed flows with components reaching Doppler velocities of +/-
100 km/s. Raster Doppler maps using a Si IV transition region line show all
four jets to have helical motion of the same sense. Simultaneous observations
of the region by SDO and Hinode show that the jets emanate from a source region
comprising a pore embedded in the interior of a supergranule. The parasitic
pore has opposite polarity flux compared to the surrounding network field. This
leads to a spine-fan magnetic topology in the coronal field that is amenable to
jet formation. Time-dependent data-driven simulations are used to investigate
the underlying drivers for the jets. These numerical experiments show that the
emergence of current-carrying magnetic field in the vicinity of the pore
supplies the magnetic twist needed for recurrent helical jet formation.Comment: 15 pages, 10 figures, accepted by Ap
Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares
The physical processes causing energy exchange between the Sun's hot corona
and its cool lower atmosphere remain poorly understood. The chromosphere and
transition region (TR) form an interface region between the surface and the
corona that is highly sensitive to the coronal heating mechanism. High
resolution observations with the Interface Region Imaging Spectrograph (IRIS)
reveal rapid variability (about 20 to 60 seconds) of intensity and velocity on
small spatial scales at the footpoints of hot dynamic coronal loops. The
observations are consistent with numerical simulations of heating by beams of
non-thermal electrons, which are generated in small impulsive heating events
called "coronal nanoflares". The accelerated electrons deposit a sizable
fraction of their energy in the chromosphere and TR. Our analysis provides
tight constraints on the properties of such electron beams and new diagnostics
for their presence in the nonflaring corona.Comment: Published in Science on October 17:
http://www.sciencemag.org/content/346/6207/1255724 . 26 pages, 10 figures.
Movies are available at: http://www.lmsal.com/~ptesta/iris_science_mov
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