60,389 research outputs found
Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines
Oscillations were observed across the whole solar disk using the Doppler
shift and line depth of spectral lines from the CO molecule near 4666~nm with
the National Solar Observatory's McMath/Pierce solar telescope. Power,
coherence, and phase spectra were examined, and diagnostic diagrams reveal
power ridges at the solar global mode frequencies to show that these
oscillations are solar p-modes. The phase was used to determine the height of
formation of the CO lines by comparison with the IR continuum intensity phase
shifts as measured in Kopp et al., 1992; we find the CO line formation height
varies from 425 < z < 560 km as we move from disk center towards the solar limb
1.0 > mu > 0.5. The velocity power spectra show that while the sum of the
background and p-mode power increases with height in the solar atmosphere as
seen in previous work, the power in the p-modes only (background subtracted)
decreases with height, consistent with evanescent waves. The CO line depth
weakens in regions of stronger magnetic fields, as does the p-mode oscillation
power. Across most of the solar surface the phase shift is larger than the
expected value of 90 degrees for an adiabatic atmosphere. We fit the phase
spectra at different disk positions with a simple atmospheric model to
determine that the acoustic cutoff frequency is about 4.5 mHz with only small
variations, but that the thermal relaxation frequency drops significantly from
2.7 to 0 mHz at these heights in the solar atmosphere
Four Soviets Walk the Dog-Improved Bounds for Computing the Fr\'echet Distance
Given two polygonal curves in the plane, there are many ways to define a
notion of similarity between them. One popular measure is the Fr\'echet
distance. Since it was proposed by Alt and Godau in 1992, many variants and
extensions have been studied. Nonetheless, even more than 20 years later, the
original algorithm by Alt and Godau for computing the Fr\'echet
distance remains the state of the art (here, denotes the number of edges on
each curve). This has led Helmut Alt to conjecture that the associated decision
problem is 3SUM-hard.
In recent work, Agarwal et al. show how to break the quadratic barrier for
the discrete version of the Fr\'echet distance, where one considers sequences
of points instead of polygonal curves. Building on their work, we give a
randomized algorithm to compute the Fr\'echet distance between two polygonal
curves in time on a pointer machine
and in time on a word RAM. Furthermore, we show that
there exists an algebraic decision tree for the decision problem of depth
, for some . We believe that this
reveals an intriguing new aspect of this well-studied problem. Finally, we show
how to obtain the first subquadratic algorithm for computing the weak Fr\'echet
distance on a word RAM.Comment: 34 pages, 15 figures. A preliminary version appeared in SODA 201
Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets
We present a new method to detect planetary transits from time-series
photometry, the Transit Least Squares (TLS) algorithm. TLS searches for
transit-like features while taking the stellar limb darkening and planetary
ingress and egress into account. We have optimized TLS for both signal
detection efficiency (SDE) of small planets and computational speed. TLS
analyses the entire, unbinned phase-folded light curve. We compensate for the
higher computational load by (i.) using algorithms like "Mergesort" (for the
trial orbital phases) and by (ii.) restricting the trial transit durations to a
smaller range that encompasses all known planets, and using stellar density
priors where available. A typical K2 light curve, including 80d of observations
at a cadence of 30min, can be searched with TLS in ~10s real time on a standard
laptop computer, as fast as the widely used Box Least Squares (BLS) algorithm.
We perform a transit injection-retrieval experiment of Earth-sized planets
around sun-like stars using synthetic light curves with 110ppm white noise per
30min cadence, corresponding to a photometrically quiet KP=12 star observed
with Kepler. We determine the SDE thresholds for both BLS and TLS to reach a
false positive rate of 1% to be SDE~7 in both cases. The resulting true
positive (or recovery) rates are ~93% for TLS and ~76% for BLS, implying more
reliable detections with TLS. We also test TLS with the K2 light curve of the
TRAPPIST-1 system and find six of seven Earth-sized planets using an iterative
search for increasingly lower signal detection efficiency, the phase-folded
transit of the seventh planet being affected by a stellar flare. TLS is more
reliable than BLS in finding any kind of transiting planet but it is
particularly suited for the detection of small planets in long time series from
Kepler, TESS, and PLATO. We make our Python implementation of TLS publicly
available.Comment: A&A accepted. Code, documentation and tutorials at
https://github.com/hippke/tl
On-board B-ISDN fast packet switching architectures. Phase 2: Development. Proof-of-concept architecture definition report
For the next-generation packet switched communications satellite system with onboard processing and spot-beam operation, a reliable onboard fast packet switch is essential to route packets from different uplink beams to different downlink beams. The rapid emergence of point-to-point services such as video distribution, and the large demand for video conference, distributed data processing, and network management makes the multicast function essential to a fast packet switch (FPS). The satellite's inherent broadcast features gives the satellite network an advantage over the terrestrial network in providing multicast services. This report evaluates alternate multicast FPS architectures for onboard baseband switching applications and selects a candidate for subsequent breadboard development. Architecture evaluation and selection will be based on the study performed in phase 1, 'Onboard B-ISDN Fast Packet Switching Architectures', and other switch architectures which have become commercially available as large scale integration (LSI) devices
- …