1,443 research outputs found
On the Cause of Supra-Arcade Downflows in Solar Flares
A model of supra-arcade downflows (SADs), dark low density regions also known
as tadpoles that propagate sunward during solar flares, is presented. It is
argued that the regions of low density are flow channels carved by
sunward-directed outflow jets from reconnection. The solar corona is
stratified, so the flare site is populated by a lower density plasma than that
in the underlying arcade. As the jets penetrate the arcade, they carve out
regions of depleted plasma density which appear as SADs. The present
interpretation differs from previous models in that reconnection is localized
in space but not in time. Reconnection is continuous in time to explain why
SADs are not filled in from behind as they would if they were caused by
isolated descending flux tubes or the wakes behind them due to temporally
bursty reconnection. Reconnection is localized in space because outflow jets in
standard two-dimensional reconnection models expand in the normal (inflow)
direction with distance from the reconnection site, which would not produce
thin SADs as seen in observations. On the contrary, outflow jets in spatially
localized three-dimensional reconnection with an out-of-plane (guide) magnetic
field expand primarily in the out-of-plane direction and remain collimated in
the normal direction, which is consistent with observed SADs being thin.
Two-dimensional proof-of-principle simulations of reconnection with an
out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion
regions and the necessity of density stratification. Three-dimensional
simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in
August, 2013. This version is the accepted versio
Super-Alfv\'enic propagation of reconnection signatures and Poynting flux during substorms
The propagation of reconnection signatures and their associated energy are
examined using kinetic particle-in-cell simulations and Cluster satellite
observations. It is found that the quadrupolar out-of-plane magnetic field near
the separatrices is associated with a kinetic Alfv\'en wave. For magnetotail
parameters, the parallel propagation of this wave is super-Alfv\'enic
(V_parallel ~ 1500 - 5500 km/s) and generates substantial Poynting flux (S ~
10^-5 - 10^-4 W/m^2) consistent with Cluster observations of magnetic
reconnection. This Poynting flux substantially exceeds that due to frozen-in
ion bulk outflows and is sufficient to generate white light aurora in the
Earth's ionosphere.Comment: Submitted to PRL on 11/1/2010. Resubmitted on 4/5/201
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
The Rb 780-nanometer Faraday anomalous dispersion optical filter: Theory and experiment
The Faraday anomalous dispersion optical filter may provide ultra-high background noise rejection for free-space laser communications systems. The theoretical model for the filter is reported. The experimental measurements and their comparison with theoretical results are discussed. The results show that the filter can provide a 56-dB solar background noise rejection with about a 2-GHz transmission bandwidth and no image degradation. To further increase the background noise rejection, a composite Zeeman and Faraday anomalous dispersion optical filter is designed and experimentally demonstrated
Stability of surfaces in the chalcopyrite system
It has been observed previously that the stable surfaces in chalcopyrites are the polar 112 surfaces. We present an electron microscopy study of epitaxial films of different compositions. It is shown that for both CuGaSe2 and CuInSe2 the 001 surfaces form 112 facets. With increasing Cu excess the faceting is suppressed, indicating a lower surface energy of the 001 surface than the energy of the 112 surface in the Cu rich regime, but the 001 surface is higher in energy than the 112 surface in the Cu poor regime. As both surfaces are polar the stabilization is attributed to defect formatio
Derivation of the Mass Distribution of Extrasolar Planets with MAXLIMA - a Maximum Likelihood Algorithm
We construct a maximum-likelihood algorithm - MAXLIMA, to derive the mass
distribution of the extrasolar planets when only the minimum masses are
observed. The algorithm derives the distribution by solving a numerically
stable set of equations, and does not need any iteration or smoothing. Based on
50 minimum masses, MAXLIMA yields a distribution which is approximately flat in
log M, and might rise slightly towards lower masses. The frequency drops off
very sharply when going to masses higher than 10 Jupiter masses, although we
suspect there is still a higher mass tail that extends up to probably 20
Jupiter masses. We estimate that 5% of the G stars in the solar neighborhood
have planets in the range of 1-10 Jupiter masses with periods shorter than 1500
days. For comparison we present the mass distribution of stellar companions in
the range of 100--1000 Jupiter masses, which is also approximately flat in log
M. The two populations are separated by the "brown-dwarf desert", a fact that
strongly supports the idea that these are two distinct populations. Accepting
this definite separation, we point out the conundrum concerning the
similarities between the period, eccentricity and even mass distribution of the
two populations.Comment: 19 pages, 3 figures, submitted to The Astrophysical Journa
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
IR Detection of Low-Mass Secondaries in Spectroscopic Binaries
This paper outlines an infrared spectroscopic technique to measure the radial
velocities of faint secondaries in known single-lined binaries. The paper
presents our H-band observations with the CSHELL and Phoenix spectrographs and
describes detections of three low-mass secondaries in main-sequence binaries:
G147-36, G164-67, and HD144284 with mass ratios of 0.562+-0.011, 0.423+-0.042,
and 0.380+-0.013, respectively. The latter is one of the smallest mass ratios
derived to date
A Concept for Airborne Precision Spacing for Dependent Parallel Approaches
The Airborne Precision Spacing concept of operations has been previously developed to support the precise delivery of aircraft landing successively on the same runway. The high-precision and consistent delivery of inter-aircraft spacing allows for increased runway throughput and the use of energy-efficient arrivals routes such as Continuous Descent Arrivals and Optimized Profile Descents. This paper describes an extension to the Airborne Precision Spacing concept to enable dependent parallel approach operations where the spacing aircraft must manage their in-trail spacing from a leading aircraft on approach to the same runway and spacing from an aircraft on approach to a parallel runway. Functionality for supporting automation is discussed as well as procedures for pilots and controllers. An analysis is performed to identify the required information and a new ADS-B report is proposed to support these information needs. Finally, several scenarios are described in detail
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