407 research outputs found
Search for short-period acoustic waves with high resolution 2D-spectra
The data for this contributionw ere takeninAugust 2000 with the “G¨ottingen” two-dimensional spectrometer in the VTT on Tenerife. Our spectrometer is based on two scanning
Fabry-Perot Interferometers (FPIs). The non-magnetic Fe I 5434˚A line was observed in the quiet Sun at disk center. Time sequences of 41 min duration and with 25 s cadence were taken. Strictly simultaneously with the narrow-band FPI images (32m˚A FWHM), broad-band images were recorded. The latter were reconstructed with speckle methods. With the known “true” broad-band object we are able to restore the narrow-band images as well. Our aim is to find
propagating acoustic waves at short periods and small scales. The results obtained so far are presented and the prospects are discussed
With a rebel yell: Video gamers’ responses to mass shooting moral panics
When a moral panic happens, society believes that a group of people and/or their behaviour is responsible for a threat to society – without any evidentiary basis. How does the target group respond? In the video game context, gamers may fear that their pastime will be blamed for mass shootings leading to social stigma. Group members so threatened are hypothesized to react to protect their group identity. This leads to increased engagement in the activity under threat. In contrast, disasters that do not threaten the group would not affect the amount of video game play. We test these hypotheses by relating the amount of game play to incidents of mass shootings and non-shooting disasters for a large sample of individuals (N = 170,000). Incidents of mass shootings that threaten the gamer community lead to increases in game playing while incidents of other disasters unrelated to gaming divert time away from gaming
Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results
An approximate method for solving the rate equations for the hydrogen
populations was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with
six energy levels and fixed radiative rates. It has been tested extensively in
one-dimensional simulations. The extended method has been used to create a
three-dimensional model that extends from the upper convection zone to the
chromosphere. The ionisation degree of hydrogen in our time-dependent
simulation is comparable to the corresponding equilibrium value up to 500 km
above optical depth unity. Above this height, the non-equilibrium ionisation
degree is fairly constant over time and space, and tends to be at a value set
by hot propagating shock waves. The hydrogen level populations and electron
density are much more constant than the corresponding values for statistical
equilibrium, too. In contrast, the equilibrium ionisation degree varies by more
than 20 orders of magnitude between hot, shocked regions and cool, non-shocked
regions. The simulation shows for the first time in 3D that the chromospheric
hydrogen ionisation degree and electron density cannot be calculated in
equilibrium. Our simulation can provide realistic values of those quantities
for detailed radiative transfer computations.Comment: 8 pages, 7 figure
Channeling 5-min photospheric oscillations into the solar outer atmosphere through small-scale vertical magnetic flux tubes
We report two-dimensional MHD simulations which demonstrate that photospheric
5-min oscillations can leak into the chromosphere inside small-scale vertical
magnetic flux tubes. The results of our numerical experiments are compatible
with those inferred from simultaneous spectropolarimetric observations of the
photosphere and chromosphere obtained with the Tenerife Infrared Polarimeter
(TIP) at 10830 A. We conclude that the efficiency of energy exchange by
radiation in the solar photosphere can lead to a significant reduction of the
cut-off frequency and may allow for the propagation of the 5 minutes waves
vertically into the chromosphere.Comment: accepted by ApJ
Urban Influencers: An Analysis of Urban Identity in YouTube Content of Local Social Media Influencers in a Super-Diverse City
Influencers belong to the daily media diet of many adolescents. As role models, they
have the potential to play a crucial role in the identity construction of their viewers. In
the age of social media, such role models may now be found more locally – in the same
city – and perhaps with more diverse backgrounds. This may be particularly valuable to
adolescents growing up in super-diverse cities, as they are surrounded by a multitude of
groups and identities during a life phase in which they have to make sense of who they
are and where they belong. Despite the heterogeneity of these identities, there is one
thing all have in common: the city they live in. With the city as a common framework,
local influencers may be important role models for these adolescents, particularly in
negotiating their urban identity. This paper aims toward mapping the ways in which
social media can play a role in the negotiation of urban identity among youngsters by
investigating how YouTube influencers from a super-diverse city are related to each
other online, and how their content relates to the (super-diverse) city of Rotterdam.
Findings show that in their videos and on their channel pages, influencers mainly affiliate
themselves with the city through having the city as the background and context of the
videos, through their involvement with cultural trends (e.g., soccer, hip-hop) that link to
the city, and through their affiliation with other local influencers. We argue that influencers
may therefore provide their viewers with
Magnetic properties of photospheric regions having very low magnetic flux
The magnetic properties of the quiet Sun are investigated using a novel
inversion code, FATIMA, based on the Principal Component Analysis of the
observed Stokes profiles. The stability and relatively low noise sensitivity of
this inversion procedure allows for the systematic inversion of large data sets
with very weak polarization signal. Its application to quiet Sun observations
of network and internetwork regions reveals that a significant fraction of the
quiet Sun contains kilogauss fields (usually with very small filling factors)
and confirms that the pixels with weak polarization account for most of the
magnetic flux. Mixed polarities in the resolution element are also found to
occur more likely as the polarization weakens.Comment: To apapear in ApJ. 39 pages, 12 figures (2 of them are color figures
Dissipative Dynamics of an Open Bose Einstein Condensate
As an atomic Bose Einstein condensate (BEC) is coupled to a source of
uncondensed atoms at the same temperature and to a sink (extraction towards an
atom laser) the idealized description in terms of a Gross-Pitaevsky equation
(GP) no longer holds. Under suitable physical assumptions we show that the
dissipative BEC obeys a Complex Ginzburg Landau equation (CGL) and for some
parameter range it undergoes a space time patterning. As a consequence, the
density of BEC atoms within the trap displays non trivial space time
correlations, which can be detected by monitoring the density profile of the
outgoing atom laser. The patterning condition requires a negative scattering
length, as e.g. in Li. In such a case we expect a many domain collapsed
regime, rather than a single one as reported for a closed BEC.Comment: 13 pages, 5 figures, submitt. to Optics Comm., 18th Aug. 99 (special
issue Scully Festschrift
Radiative transfer in moving media II. Solution of the radiative transfer equation in axial symmetry
A new method for the formal solution of the 2D radiative transfer equation in
axial symmetry in the presence of arbitrary velocity fields is presented. The
combination of long and short characteristics methods is used to solve the
radiative transfer equation. We include the velocity field in detail using the
Local Lorentz Transformation. This allows us to obtain a significantly better
description of the photospheric region, where the gradient of the global
velocity is too small for the Sobolev approximation to be valid. Sample test
calculations for the case of a stellar wind and a rotating atmosphere are
presented.Comment: 11 pages, 19 figures. accepted by Astronomy and Astrophysic
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