6,740 research outputs found
Reduction of Ion Heating During Magnetic Reconnection by Large-Scale Effective Potentials
The physical processes that control the partition of released magnetic energy
between electrons and ions during reconnection is explored through
particle-in-cell simulations and analytical techniques. We demonstrate that the
development of a large-scale parallel electric field and its associated
potential controls the relative heating of electrons and ions. The potential
develops to restrain heated exhaust electrons and enhances their heating by
confining electrons in the region where magnetic energy is released.
Simultaneously the potential slows ions entering the exhaust below the
Alfv\'enic speed expected from the traditional counterstreaming picture of ion
heating. Unexpectedly, the magnitude of the potential and therefore the
relative partition of energy between electrons and ions is not a constant but
rather depends on the upstream parameters and specifically the upstream
electron normalized temperature (electron beta). These findings suggest that
the fraction of magnetic energy converted into the total thermal energy may be
independent of upstream parameters
Kinetic signatures of the region surrounding the X-line in asymmetric (magnetopause) reconnection
Kinetic particle-in-cell simulations are used to identify signatures of the
electron diffusion region (EDR) and its surroundings during asymmetric magnetic
reconnection. A "shoulder" in the sunward pointing normal electric field (EN >
0) at the reconnection magnetic field reversal is a good indicator of the EDR,
and is caused by magnetosheath electron meandering orbits in the vicinity of
the x-line. Earthward of the X-line, electrons accelerated by EN form strong
currents and crescent-shaped distribution functions in the plane perpendicular
to B. Just downstream of the X-line, parallel electric fields create
field-aligned crescent electron distribution functions. In the immediate
upstream magnetosheath, magnetic field strength, plasma density, and
perpendicular electron temperatures are lower than the asymptotic state. In the
magnetosphere inflow region, magnetosheath ions intrude resulting in an
Earthward pointing electric field and parallel heating of magnetospheric
particles. Many of the above properties persist with a guide field of at least
unity.Comment: Submitted to Geophysical Research Letter
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Smart, secure and seamless access control scheme for mobile devices
Smart devices capture users' activity such as unlock failures, application usage, location and proximity of devices in and around their surrounding environment. This activity information varies between users and can be used as digital fingerprints of the users' behaviour. Traditionally, users are authenticated to access restricted data using long term static attributes such as password and roles. In this paper, in order to allow secure and seamless data access in mobile environment, we combine both the user behaviour captured by the smart device and the static attributes to develop a novel access control technique. Security and performance analyses show that the proposed scheme substantially reduces the computational complexity while enhances the security compared to the conventional schemes
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection
conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta
(ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic
shear (strong guide field). Changing the simulation domain size, we find that
the ion response varies greatly. For reconnecting regions with scales
comparable to the ion Larmor radius, the ions do not respond to the
reconnection dynamics leading to ''electron-only'' reconnection with very large
quasi-steady reconnection rates. The transition to more traditional
''ion-coupled'' reconnection is gradual as the reconnection domain size
increases, with the ions becoming frozen-in in the exhaust when the magnetic
island width in the normal direction reaches many ion inertial lengths. During
this transition, the quasi-steady reconnection rate decreases until the ions
are fully coupled, ultimately reaching an asymptotic value. The scaling of the
ion outflow velocity with exhaust width during this electron-only to
ion-coupled transition is found to be consistent with a theoretical model of a
newly reconnected field line. In order to have a fully frozen-in ion exhaust
with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width
of at least several ion inertial lengths is needed. In turbulent systems with
reconnection occurring between magnetic bubbles associated with fluctuations,
using geometric arguments we estimate that fully ion-coupled reconnection
requires magnetic bubble length scales of at least several tens of ion inertial
lengths
Faxen relations in solids - a generalized approach to particle motion in elasticity and viscoelasticity
A movable inclusion in an elastic material oscillates as a rigid body with
six degrees of freedom. Displacement/rotation and force/moment tensors which
express the motion of the inclusion in terms of the displacement and force at
arbitrary exterior points are introduced. Using reciprocity arguments two
general identities are derived relating these tensors. Applications of the
identities to spherical particles provide several new results, including simple
expressions for the force and moment on the particle due to plane wave
excitation.Comment: 11 pages, 4 figure
Detection of lithium in nearby young late-M dwarfs
Late M-type dwarfs in the solar neighborhood include a mixture of very
low-mass stars and brown dwarfs which is difficult to disentangle due to the
lack of constraints on their age such as trigonometric parallax, lithium
detection and space velocity.
We search for young brown dwarf candidates among a sample of 28 nearby late-M
dwarfs with spectral types between M5.0 and M9.0, and we also search for debris
disks around three of them.
Based on theoretical models, we used the color , the -band absolute
magnitude and the detection of the Li I 6708 doublet line as a strong
constraint to estimate masses and ages of our targets. For the search of debris
disks, we observed three targets at submillimeter wavelength of 850 m.
We report here the first clear detections of lithium absorption in four
targets and a marginal detection in one target. Our mass estimates indicate
that two of them are young brown dwarfs, two are young brown dwarf candidates
and one is a young very low-mass star. The closest young field brown dwarf in
our sample at only 15 pc is an excellent benchmark for further studying
physical properties of brown dwarfs in the range 100150 Myr. We did not
detect any debris disks around three late-M dwarfs, and we estimated upper
limits to the dust mass of debris disks around them.Comment: 10 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Ion Larmor Radius Effects Near A Reconnection X Line At The Magnetopause: Themis Observations And Simulation Comparison
We report a Time History of Events and Macroscale Interactions during Substorms (THEMIS-D) spacecraft crossing of a magnetopause reconnection exhaust ~9 ion skin depths (di) downstream of an X line. The crossing was characterized by ion jetting at speeds substantially below the predicted reconnection outflow speed. In the magnetospheric inflow region THEMIS detected (a) penetration of magnetosheath ions and the resulting flows perpendicular to the reconnection plane, (b) ion outflow extending into the magnetosphere, and (c) enhanced electron parallel temperature. Comparison with a simulation suggests that these signatures are associated with the gyration of magnetosheath ions onto magnetospheric field lines due to the shift of the flow stagnation point toward the low-density magnetosphere. Our observations indicate that these effects, ~2–3 di in width, extend at least 9 di downstream of the X line. The detection of these signatures could indicate large-scale proximity of the X line but do not imply that the spacecraft was upstream of the electron diffusion region
Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Sco OB association
Aims: Low-resolution optical spectroscopic observations for 78 very low-mass
star and brown dwarf candidates that have been photometrically selected using
the DENIS survey point source catalogue. Methods: Spectral types are derived
for them using measurements of the PC3 index. They range from M6 to L4. H_alpha
emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths
are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH
molecular features are also reported. Results: A rule-of-thumb criterion to
select young very low-mass objects using the NaI doublet equivalent width is
given. It is used to confirm seven new members of the Upper Sco OB association
and two new members of the R Cr-A star-forming region. Four of our field
objects are also classified as very young, but are not members of any known
nearby young association. The frequency of lower-gravity young objects in our
field ultracool sample is 8.5%. Our results provide the first spectroscopic
classification for 38 ultracool dwarfs in the solar vicinity with
spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L
dwarfs within 20 pc).Comment: 25 pages, 10 figures, 7 tables, Accepted by A&
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB10 and LSR1835+32 and the Possible Role of Rotation
[Abridged] As part of our on-going investigation of magnetic activity in
ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical
observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations
of VB10 (M8), both with a duration of about 9 hours. LSR1835+32 exhibits
persistent radio emission and H-alpha variability on timescales of ~0.5-2 hr.
The detected UV flux is consistent with photospheric emission, and no X-ray
emission is detected to a deep limit of L_X/L_bol<10^-5.7. The H-alpha and
radio emission are temporally uncorrelated, and the ratio of radio to X-ray
luminosity exceeds the correlation seen in F-M6 stars by >2x10^4. Similarly,
L_Halpha/L_X>10 is at least 30 times larger than in early M dwarfs, and
eliminates coronal emission as the source of chromospheric heating. The lack of
radio variability during four rotations of LSR1835+32 requires a uniform
stellar-scale field of ~10 G, and indicates that the H-alpha variability is
dominated by much smaller scales, <10% of the chromospheric volume. VB10, on
the other hand, shows correlated flaring and quiescent X-ray and UV emission,
similar to the behavior of early M dwarfs. Delayed and densely-sampled optical
spectra exhibit a similar range of variability amplitudes and timescales to
those seen in the X-rays and UV, with L_Halpha/L_X~1. Along with our previous
observations of the M8.5 dwarf TVLM513-46546 we conclude that late M dwarfs
exhibit a mix of activity patterns, which points to a transition in the
structure and heating of the outer atmosphere by large-scale magnetic fields.
We find that rotation may play a role in generating the fields as evidenced by
a tentative correlation between radio activity and rotation velocity. The X-ray
emission, however, shows evidence for super-saturation at vsini>25 km/s.Comment: Submitted to Ap
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