1,242 research outputs found
Superflares on Ordinary Solar-Type Stars
Short duration flares are well known to occur on cool main-sequence stars as
well as on many types of `exotic' stars. Ordinary main-sequence stars are
usually pictured as being static on time scales of millions or billions of
years. Our sun has occasional flares involving up to ergs which
produce optical brightenings too small in amplitude to be detected in
disk-integrated brightness. However, we identify nine cases of superflares
involving to ergs on normal solar-type stars. That is,
these stars are on or near the main-sequence, are of spectral class from F8 to
G8, are single (or in very wide binaries), are not rapid rotators, and are not
exceedingly young in age. This class of stars includes many those recently
discovered to have planets as well as our own Sun, and the consequences for any
life on surrounding planets could be profound. For the case of the Sun,
historical records suggest that no superflares have occurred in the last two
millennia.Comment: 16 pages, accepted for publication in Ap
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
Data Mapping Cornwall
This document provides a summary of spatial investigations into Cornwall, conducted by Masters students at the Manchester School of Architecture. The work is the output of a unit with two primary goals: To explore and develop methodological approaches which reveal cross-cutting insights into the operation and performance of space and place. To use the findings of these spatial explorations to explore novel approaches to design which tackle multi-dimensional problems in relation to service and physical infrastructure. The work published within this document comprises exploratory mappings which compare and give spatial context to publically available, existing data. With an overarching goal to understand the context and infrastructure supporting healthcare in the UK, students were tasked with investigating four key themes within the context of Cornwall: Health and Lifestyle Energy and Power Demographics and Economy Networks and Connectivity The summaries provided here are extracts from lengthy atlases which compare datasets that are often viewed in isolation of each other. This is an approach that has been developed over a number years at the School of Architecture, and which has, in many cases, proven to provide a critical understanding of how space is used and places are perceived. This is a significant insight in understanding how architectural and urban design can help support the infrastructural networks which are often culturally invisible
Imaging extrasolar planets by stellar halo suppression in separately-corrected color bands
Extra-solar planets have not been imaged directly with existing ground or
space telescopes because they are too faint to be seen against the halo of the
nearby bright star. Most techniques being explored to suppress the halo are
achromatic, with separate correction of diffraction and wavefront errors.
Residual speckle structure may be subtracted by differencing images taken
through narrowband filters, but photon noise remains and ultimately limits
sensitivity. Here we describe two ways to take advantage of narrow bands to
reduce speckle photon flux and to obtain better control of systematic errors.
Multiple images are formed in separate color bands of 5-10% bandwidth, and
recorded by coronagraphic interferometers equipped with active control of
wavefront phase and/or amplitude. In one method, a single deformable pupil
mirror is used to actively correct both diffraction and wavefront components of
the halo. This yields good diffraction suppression for complex pupil
obscuration, with high throughput over half the focal plane. In a second
method, the coronagraphic interferometer is used as a second stage after
conventional apodization. The halo from uncontrollable residual errors in the
pupil mask or wavefront is removed by destructive interference made directly at
the detector focal plane with an "anti-halo", synthesized by spatial light
modulators in the reference arm of the interferometer. In this way very deep
suppression may be achieved by control elements with greatly relaxed, and thus
achievable, tolerances. In both examples, systematic errors are minimized
because the planet imaging cameras themselves also provide the error sensing
data.Comment: Accepted by ApJ
Spectral line shape of resonant four-wave mixing induced by broad-bandwidth lasers
We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate Î, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of â2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4Î, whereas in the limit of dominant homogeneous broadening, the predicted width is 2Î. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, "modeless", laser resonant with the Q1 (6) line of the A2 ÎŁ - X2 Î (0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory
Predictability of large future changes in a competitive evolving population
The dynamical evolution of many economic, sociological, biological and
physical systems tends to be dominated by a relatively small number of
unexpected, large changes (`extreme events'). We study the large, internal
changes produced in a generic multi-agent population competing for a limited
resource, and find that the level of predictability actually increases prior to
a large change. These large changes hence arise as a predictable consequence of
information encoded in the system's global state.Comment: 10 pages, 3 figure
Report of Committee to Study Existing State Constitution, Including Minority Reports, April 12, 1950
Report of Committee to Study Existing State Constitution, Including Minority Reports, April 12, 1950
Lunar Lander Offloading Operations Using a Heavy-Lift Lunar Surface Manipulator System
This study investigates the feasibility of using a heavy-lift variant of the Lunar Surface Manipulator System (LSMS-H) to lift and handle a 12 metric ton payload. Design challenges and requirements particular to handling heavy cargo were examined. Differences between the previously developed first-generation LSMS and the heavy-lift version are highlighted. An in-depth evaluation of the tip-over risk during LSMS-H operations has been conducted using the Synergistic Engineering Environment and potential methods to mitigate that risk are identified. The study investigated three specific offloading scenarios pertinent to current Lunar Campaign studies. The first involved offloading a large element, such as a habitat or logistics module, onto a mobility chassis with a lander-mounted LSMS-H and offloading that payload from the chassis onto the lunar surface with a surface-mounted LSMS-H. The second scenario involved offloading small pressurized rovers with a lander-mounted LSMS-H. The third scenario involved offloading cargo from a third-party lander, such as the proposed ESA cargo lander, with a chassis-mounted LSMS-H. In all cases, the analyses show that the LSMS-H can perform the required operations safely. However, Chariot-mounted operations require the addition of stabilizing outriggers, and when operating from the Lunar surface, LSMS-H functionality is enhanced by adding a simple ground anchoring system
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