4,679 research outputs found
Characteristics of Magnetohydrodynamic Oscillations Observed with Michelson Doppler Imager
We report on the spatial distribution of magnetogram oscillatory power and
phase angles between velocity and magnetogram signals as observed with the
Michelson Doppler Imager. The dataset is 151.25 arcsec times 151.25 arcsec
containing sunspot from Dec 2, 1997 with a temporal sampling interval of 60
seconds and spatial sampling of 0.605 arcsec. Simultaneously observed continuum
intensity and surface velocity accompany the magnetic information. We focus on
three frequency regimes: 0.5-1.0, 3.0-3.5 and 5.5-6.0 mHz corresponding roughly
to timescales of magnetic evolution, p-modes and the 3 minute resonant sunspot
oscillation. Significant low frequency magnetogram power is found in lower flux
pixels, 100-300 Gauss, in a striking ring with filamentary structure
surrounding sunspot. Five minute magnetogram power peaks in extended regions of
flux 600-800 Gauss. The 3 minute oscillation is observed in sunspot umbra in
pixels whose flux measures 1300-1500 Gauss. Phase angles of approximately -90
degrees between velocity and magnetic flux in the 3.0-3.5 and 5.5-6.0 mHz
regimes are found in regions of significant cross amplitude.Comment: 4 Pages, 4 Figures -- For better Figure files see:
http://www.astro.ucla.edu/~norton/pub_list.htm
Computing the entropy of user navigation in the web
Navigation through the web, colloquially known as "surfing", is one of the main activities of users during web interaction. When users follow a navigation trail they often tend to get disoriented in terms of the goals of their original query and thus the discovery of typical user trails could be useful in providing navigation assistance. Herein, we give a theoretical underpinning of user navigation in terms of the entropy of an underlying Markov chain modelling the web topology. We present a novel method for online incremental computation of the entropy and a large deviation result regarding the length of a trail to realize the said entropy. We provide an error analysis for our estimation of the entropy in terms of the divergence between the empirical and actual probabilities. We then indicate applications of our algorithm in the area of web data mining. Finally, we present an extension of our technique to higher-order Markov chains by a suitable reduction of a higher-order Markov chain model to a first-order one
Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits
The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric
Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8.
Contact times between Mercury and the solar limb have been used since the 17th
century to derive the Sun's size but this is the first time that high-quality
imagery from space, above the Earth's atmosphere, has been available. Unlike
other measurements this technique is largely independent of optical distortion.
The true solar radius is still a matter of debate in the literature as measured
differences of several tenths of an arcsecond (i.e., about 500 km) are
apparent. This is due mainly to systematic errors from different instruments
and observers since the claimed uncertainties for a single instrument are
typically an order of magnitude smaller. From the MDI transit data we find the
solar radius to be 960".12 +/- 0".09 (696,342 +/- 65 km). This value is
consistent between the transits and consistent between different MDI focus
settings after accounting for systematic effects.Comment: Accepted for publication in The Astrophysical Journal (2012 March 5
Magnetic Characterization of Nanocrystalline Nickel Ferrite Films Processed by a Spin-Spraying Method
Highly crystalline nickel ferrite films with different chemical compositions were processed via the spin-spraying method and their morphological, structural and magnetic properties were subsequently investigated. Regardless of the chemical composition, films with variable thicknesses are constructed by 200–400 nm spherical grains grown in the direction normal to the substrate surface. Magnetization measurements show that the spinel ferrite films present a hysteretic behaviour at room temperature with a randomly oriented in-plane easy axis and an anisotropy constant K1 ≈ −2.5 × 104 erg cm−3 Furthermore, the absence of an angular dependence of the coercivity for the in-plane measurement of magnetization coupled with the ‘M’-shaped angular dependence of the out-of-plane measurement, indicates that the anisotropy of this film is predominantly crystalline shape anisotropy
B-T phase diagram of CoCr2O4 in magnetic fields up to 14 T
We have measured the magnetization and specific heat of multiferroic CoCr2O4
in magnetic fields up to 14 T. The high-field magnetization measurements
indicate a new phase transition at T* = 5 - 6 K. The phase between T* and the
lock-in transition at 15 K is characterized by magnetic irreversibility. At
higher magnetic fields, the irreversibility increases. Specific-heat
measurements confirm the transition at T*, and also show irreversible behavior.
We construct a field-temperature phase diagram of CoCr2O4.Comment: 4 page
On-Orbit Performance of the Helioseismic and Magnetic Imager Instrument onboard the Solar Dynamics Observatory
The Helioseismic and Magnetic Imager (HMI) instrument is a major component of
NASA's Solar Dynamics Observatory (SDO) spacecraft. Since beginning normal
science operations on 1 May 2010, HMI has operated with remarkable continuity,
e.g. during the more than five years of the SDO prime mission that ended 30
September 2015, HMI collected 98.4% of all possible 45-second velocity maps;
minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology.
HMI velocity, intensity, and magnetic-field measurements are used in numerous
investigations, so understanding the quality of the data is important. We
describe the calibration measurements used to track HMI performance and detail
trends in important instrument parameters during the mission. Regular
calibration sequences provide information used to improve and update the HMI
data calibration. The set-point temperature of the instrument front window and
optical bench is adjusted regularly to maintain instrument focus, and changes
in the temperature-control scheme have been made to improve stability in the
observable quantities. The exposure time has been changed to compensate for a
15% decrease in instrument throughput. Measurements of the performance of the
shutter and tuning mechanisms show that they are aging as expected and continue
to perform according to specification. Parameters of the tunable-optical-filter
elements are regularly adjusted to account for drifts in the central
wavelength. Frequent measurements of changing CCD-camera characteristics, such
as gain and flat field, are used to calibrate the observations. Infrequent
expected events, such as eclipses, transits, and spacecraft off-points,
interrupt regular instrument operations and provide the opportunity to perform
additional calibration. Onboard instrument anomalies are rare and seem to occur
quite uniformly in time. The instrument continues to perform very well.Comment: 50 pages, 18 figures, 20 table
Combining the Hybrid Functional Method with Dynamical Mean-Field Theory
We present a new method to compute the electronic structure of correlated
materials combining the hybrid functional method with the dynamical mean-field
theory. As a test example of the method we study cerium sesquioxide, a strongly
correlated Mott-band insulator. The hybrid functional part improves the
magnitude of the pd-band gap which is underestimated in the standard
approximations to density functional theory while the dynamical mean-field
theory part splits the 4f-electron spectra into a lower and an upper Hubbard
band.Comment: 5 pages, 2 figures, replaced with revised version, published in
Europhys. Let
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