442 research outputs found
Hypernatural Numbers as Ultrafilters
In this paper we present a use of nonstandard methods in the theory of
ultrafilters and in related applications to combinatorics of numbers
Subsurface Flows in and Around Active Regions with Rotating and Non-rotating Sunspots
The temporal variation of the horizontal velocity in subsurface layers
beneath three different types of active regions is studied using the technique
of ring diagrams. In this study, we select active regions (ARs) 10923, 10930,
10935 from three consecutive Carrington rotations: AR 10930 contains a
fast-rotating sunspot in a strong emerging active region while other two have
non-rotating sunspots with emerging flux in AR 10923 and decaying flux in AR
10935. The depth range covered is from the surface to about 12 Mm. In order to
minimize the influence of systematic effects, the selection of active and quiet
regions is made so that these were observed at the same heliographic locations
on the solar disk. We find a significant variation in both components of the
horizontal velocity in active regions as compared to quiet regions. The
magnitude is higher in emerging-flux regions than in the decaying-flux region,
in agreement with earlier findings. Further, we clearly see a significant
temporal variation in depth profiles of both zonal and meridional flow
components in AR 10930, with the variation in the zonal component being more
pronounced. We also notice a significant influence of the plasma motion in
areas closest to the rotating sunspot in AR 10930 while areas surrounding the
non-rotating sunspots in all three cases are least affected by the presence of
the active region in their neighborhood.Comment: Solar Physics (in press), includes 11 figure
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
A nilpotent IP polynomial multiple recurrence theorem
We generalize the IP-polynomial Szemer\'edi theorem due to Bergelson and
McCutcheon and the nilpotent Szemer\'edi theorem due to Leibman. Important
tools in our proof include a generalization of Leibman's result that polynomial
mappings into a nilpotent group form a group and a multiparameter version of
the nilpotent Hales-Jewett theorem due to Bergelson and Leibman.Comment: v4: switch to TeXlive 2016 and biblate
Campaigning and Contestation:Comments on politicians’ Facebook pages during the 2011 Danish general election campaign
This article is a critical study of the Facebook pages of politicians as public spheres using Dahlberg’s notion of contestation. A method is implemented inspired by qualitative content analysis and including focus groups in order to study citizen comments on eight main political candidates’ Facebook pages during the 2011 Danish election campaign. An analytical framework is presented that conceptualizes the particular platform as a dinner party, with a dinner table, a host, and the invited guests. The dinner party exhibits the interplay between these elements and how they limit the option of contesting the dominating discourse in favor of a supportive marketing logic
Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots
The power of solar acoustic waves is reduced inside sunspots mainly due to
absorption, emissivity reduction, and local suppression. The coefficients of
these power-reduction mechanisms can be determined by comparing time-distance
cross-covariances obtained from sunspots and from the quiet Sun. By analyzing
47 active regions observed by SOHO/MDI without using signal filters, we have
determined the coefficients of surface absorption, deep absorption, emissivity
reduction, and local suppression. The dissipation in the quiet Sun is derived
as well. All of the cross-covariances are width corrected to offset the effect
of dispersion. We find that absorption is the dominant mechanism of the power
deficit in sunspots for short travel distances, but gradually drops to zero at
travel distances longer than about 6 degrees. The absorption in sunspot
interiors is also significant. The emissivity-reduction coefficient ranges from
about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and
accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total
power reduction. Local suppression is nearly constant as a function of travel
distance with values of 0.80 and 0.665 for umbrae and whole sunspots
respectively, and is the major cause of the power deficit at large travel
distances.Comment: 14 pages, 21 Figure
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
A closure model with plumes I. The solar convection
Oscillations of stellar p modes, excited by turbulent convection, are
investigated. We take into account the asymmetry of the up and downflows
created by turbulent plumes through an adapted closure model. In a companion
paper, we apply it to the formalism of excitation of solar p modes developed by
Samadi & Goupil 2001. Using results from 3D numerical simulations of the upper
most part of the solar convection zone, we show that the two-scale-mass-flux
model (TFM) is valid only for quasi-laminar or highly skewed flows (Gryanik &
Hartmann 2002). We build a generalized-Two-scale-Mass-Flux Model (GTFM) model
which takes into account both the skew introduced by the presence of two flows
and the effects of turbulence in each flow. In order to apply the GTFM to the
solar case, we introduce the plume dynamics as modelled by Rieutord & Zahn
(1995) and construct a Closure Model with Plumes (CMP). When comparing with 3D
simulation results, the CMP improves the agreement for the fourth order
moments, by approximatively a factor of two compared with the use of the
quasi-normal approximation or a skewness computed with the classical TFM. The
asymmetry of turbulent convection in the solar case has an important impact on
the vertical-velocity fourth-order moment which has to be accounted for by
models. The CMP is a significant improvement and is expected to improve the
modelling of solar p-mode excitation.Comment: 13 pages, accepted for publication in A&
Searching for gravitational waves from known pulsars
We present upper limits on the amplitude of gravitational waves from 28
isolated pulsars using data from the second science run of LIGO. The results
are also expressed as a constraint on the pulsars' equatorial ellipticities. We
discuss a new way of presenting such ellipticity upper limits that takes
account of the uncertainties of the pulsar moment of inertia. We also extend
our previous method to search for known pulsars in binary systems, of which
there are about 80 in the sensitive frequency range of LIGO and GEO 600.Comment: Accepted by CQG for the proceeding of GWDAW9, 7 pages, 2 figure
Setting upper limits on the strength of periodic gravitational waves from PSR J1939+2134 using the first science data from the GEO 600 and LIGO detectors
Data collected by the GEO 600 and LIGO interferometric gravitational wave detectors during their first observational science run were searched for continuous gravitational waves from the pulsar J1939+2134 at twice its rotation frequency. Two independent analysis methods were used and are demonstrated in this paper: a frequency domain method and a time domain method. Both achieve consistent null results, placing new upper limits on the strength of the pulsar's gravitational wave emission. A model emission mechanism is used to interpret the limits as a constraint on the pulsar's equatorial ellipticity
- …