13,874 research outputs found
The mass of unimodular lattices
The purpose of this paper is to show how to obtain the mass of a unimodular
lattice from the point of view of the Bruhat-Tits theory. This is achieved by
relating the local stabilizer of the lattice to a maximal parahoric subgroup of
the special orthogonal group, and appealing to an explicit mass formula for
parahoric subgroups developed by Gan, Hanke and Yu.
Of course, the exact mass formula for positive defined unimodular lattices is
well-known. Moreover, the exact formula for lattices of signature (1,n) (which
give rise to hyperbolic orbifolds) was obtained by Ratcliffe and Tschantz,
starting from the fundamental work of Siegel. Our approach works uniformly for
the lattices of arbitrary signature (r,s) and hopefully gives a more conceptual
way of deriving the above known results.Comment: 15 pages, to appear in J. Number Theor
Electron acceleration by cascading reconnection in the solar corona I Magnetic gradient and curvature effects
Aims: We investigate the electron acceleration in convective electric fields
of cascading magnetic reconnection in a flaring solar corona and show the
resulting hard X-ray (HXR) radiation spectra caused by Bremsstrahlung for the
coronal source. Methods: We perform test particle calculation of electron
motions in the framework of a guiding center approximation. The electromagnetic
fields and their derivatives along electron trajectories are obtained by
linearly interpolating the results of high-resolution adaptive mesh refinement
(AMR) MHD simulations of cascading magnetic reconnection. Hard X-ray (HXR)
spectra are calculated using an optically thin Bremsstrahlung model. Results:
Magnetic gradients and curvatures in cascading reconnection current sheet
accelerate electrons: trapped in magnetic islands, precipitating to the
chromosphere and ejected into the interplanetary space. The final location of
an electron is determined by its initial position, pitch angle and velocity.
These initial conditions also influence electron acceleration efficiency. Most
of electrons have enhanced perpendicular energy. Trapped electrons are
considered to cause the observed bright spots along coronal mass ejection
CME-trailing current sheets as well as the flare loop-top HXR emissions.Comment: submitted to A&
Protocluster Discovery in Tomographic Ly Forest Flux Maps
We present a new method of finding protoclusters using tomographic maps of
Ly Forest flux. We review our method of creating tomographic flux maps
and discuss our new high performance implementation, which makes large
reconstructions computationally feasible. Using a large N-body simulation, we
illustrate how protoclusters create large-scale flux decrements, roughly 10
Mpc across, and how we can use this signal to find them in flux maps.
We test the performance of our protocluster finding method by running it on the
ideal, noiseless map and tomographic reconstructions from mock surveys, and
comparing to the halo catalog. Using the noiseless map, we find protocluster
candidates with about 90% purity, and recover about 75% of the protoclusters
that form massive clusters (). We
construct mock surveys similar to the ongoing COSMOS Lyman-Alpha Mapping And
Tomography Observations (CLAMATO) survey. While the existing data has an
average sightline separation of 2.3 Mpc, we test separations of 2 - 6
Mpc to see what can be tolerated for our application. Using
reconstructed maps from small separation mock surveys, the protocluster
candidate purity and completeness are very close what was found in the
noiseless case. As the sightline separation increases, the purity and
completeness decrease, although they remain much higher than we initially
expected. We extended our test cases to mock surveys with an average separation
of 15 Mpc, meant to reproduce high source density areas of the BOSS
survey. We find that even with such a large sightline separation, the method
can still be used to find some of the largest protoclusters.Comment: 18 pages, 12 figure
Frequency-sweep examination for wave mode identification in multimodal ultrasonic guided wave signal
This article has been made available through the Brunel Open Access Publishing Fund.Ultrasonic guided waves can be used to assess and monitor long elements of a structure from a single position. The greatest challenges for any guided wave system are the plethora of wave modes arising from the geometry of the structural element which propagate with a range of frequency-dependent velocities and the interpretation of these combined signals reflected by discontinuities in the structural element. In this paper, a novel signal processing technique is presented using a combination of frequency-sweep measurement, sampling rate conversion, and Fourier transform. The technique is applied to synthesized and experimental data to identify different modes in complex ultrasonic guided wave signals. It is demonstrated throughout the paper that the technique also has the capability to derive the time of flight and group velocity dispersion curve of different wave modes in field inspections. © 2014 IEEE
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