45,190 research outputs found
Global Distribution of Water Vapor and Cloud Cover--Sites for High Performance THz Applications
Absorption of terahertz radiation by atmospheric water vapor is a serious
impediment for radio astronomy and for long-distance communications.
Transmission in the THz regime is dependent almost exclusively on atmospheric
precipitable water vapor (PWV). Though much of the Earth has PWV that is too
high for good transmission above 200 GHz, there are a number of dry sites with
very low attenuation. We performed a global analysis of PWV with
high-resolution measurements from the Moderate Resolution Imaging Spectrometer
(MODIS) on two NASA Earth Observing System (EOS) satellites over the year of
2011. We determined PWV and cloud cover distributions and then developed a
model to find transmission and atmospheric radiance as well as necessary
integration times in the various windows. We produced global maps over the
common THz windows for astronomical and satellite communications scenarios.
Notably, we show that up through 1 THz, systems could be built in excellent
sites of Chile, Greenland and the Tibetan Plateau, while Antarctic performance
is good to 1.6 THz. For a ground-to-space communication link up through 847
GHz, we found several sites in the Continental United States where mean
atmospheric attenuation is less than 40 dB; not an insurmountable challenge for
a link.Comment: 15 pages, 23 figure
Understanding the white-light flare on 2012 March 9 : Evidence of a two-step magnetic reconnection
We attempt to understand the white-light flare (WLF) that was observed on
2012 March 9 with a newly constructed multi-wavelength solar telescope called
the Optical and Near-infrared Solar Eruption Tracer (ONSET). We analyzed WLF
observations in radio, H-alpha, white-light, ultraviolet, and X-ray bands. We
also studied the magnetic configuration of the flare via the nonlinear
force-free field (NLFFF) extrapolation and the vector magnetic field observed
by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO). Continuum emission enhancement clearly appeared at the 3600
angstrom and 4250 angstrom bands, with peak contrasts of 25% and 12%,
respectively. The continuum emission enhancement closely coincided with the
impulsive increase in the hard X-ray emission and a microwave type III burst at
03:40 UT. We find that the WLF appeared at one end of either the sheared or
twisted field lines or both. There was also a long-lasting phase in the H-alpha
and soft X-ray bands after the white-light emission peak. In particular, a
second, yet stronger, peak appeared at 03:56 UT in the microwave band. This
event shows clear evidence that the white-light emission was caused by
energetic particles bombarding the lower solar atmosphere. A two-step magnetic
reconnection scenario is proposed to explain the entire process of flare
evolution, i.e., the first-step magnetic reconnection between the field lines
that are highly sheared or twisted or both, and the second-step one in the
current sheet, which is stretched by the erupting flux rope. The WLF is
supposed to be triggered in the first-step magnetic reconnection at a
relatively low altitude.Comment: 4 pages, 4 figures, published in A&A Lette
Physical mechanism of superluminal traversal time: interference between multiple finite wave packets
The mechanism of superluminal traversal time through a potential well or
potential barrier is investigated from the viewpoint of interference between
multiple finite wave packets, due to the multiple reflections inside the well
or barrier. In the case of potential-well traveling that is classically
allowed, each of the successively transmitted constituents is delayed by a
subluminal time. When the thickness of the well is much smaller in comparision
with a characteristic length of the incident wave packet, the reshaped wave
packet in transmission maintains the profile of the incident wave packet. In
the case of potential-barrier tunneling that is classically forbidden, though
each of the successively transmitted constituents is delayed by a time that is
independent of the barrier thickness, the interference between multiple
transmitted constituents explains the barrier-thickness dependence of the
traversal time for thin barriers and its barrier-thickness independence for
thick barriers. This manifests the nature of Hartman effect.Comment: 9 pages, 3 figures, Some comments and suggestions are appreciate
First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3
Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be
3-dimentional (3D) strong topological insulators. In this paper, based on
ab-initio calculations, we study in detail the topological nature and the
surface states of this family compounds. The penetration depth and the
spin-resolved Fermi surfaces of the surface states will be analyzed. We will
also present an procedure, from which highly accurate effective Hamiltonian can
be constructed, based on projected atomic Wannier functions (which keep the
symmetries of the systems). Such Hamiltonian can be used to study the
semi-infinite systems or slab type supercells efficiently. Finally, we discuss
the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.Comment: 8 pages,17 figure
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