3,587 research outputs found
Super-diffusion versus competitive advection: a simulation
Magnetic element tracking is often used to study the transport and diffusion
of the magnetic field on the solar photosphere. From the analysis of the
displacement spectrum of these tracers, it has been recently agreed that a
regime of super-diffusivity dominates the solar surface. Quite habitually this
result is discussed in the framework of fully developed turbulence. But the
debate whether the super-diffusivity is generated by a turbulent dispersion
process, by the advection due to the convective pattern, or by even another
process, is still open, as is the question about the amount of diffusivity at
the scales relevant to the local dynamo process. To understand how such
peculiar diffusion in the solar atmosphere takes places, we compared the
results from two different data-sets (ground-based and space-borne) and
developed a simulation of passive tracers advection by the deformation of a
Voronoi network. The displacement spectra of the magnetic elements obtained by
the data-sets are consistent in retrieving a super-diffusive regime for the
solar photosphere, but the simulation also shows a super-diffusive displacement
spectrum: its competitive advection process can reproduce the signature of
super-diffusion. Therefore, it is not necessary to hypothesize a totally
developed turbulence regime to explain the motion of the magnetic elements on
the solar surface
Occurrence and persistence of magnetic elements in the quiet Sun
Turbulent convection efficiently transports energy up to the solar
photosphere, but its multi-scale nature and dynamic properties are still not
fully understood. Several works in the literature have investigated the
emergence of patterns of convective and magnetic nature in the quiet Sun at
spatial and temporal scales from granular to global. Aims. To shed light on the
scales of organisation at which turbulent convection operates, and its
relationship with the magnetic flux therein, we studied characteristic spatial
and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to
an unprecedented data set entirely enclosing a supergranule, occurrence and
persistence analysis of magnetogram time series were used to detect spatial and
long-lived temporal correlations in the quiet Sun and to investigate their
nature. Results. A relation between occurrence and persistence representative
for the quiet Sun was found. In particular, highly recurrent and persistent
patterns were detected especially in the boundary of the supergranular cell.
These are due to moving magnetic elements undergoing motion that behaves like a
random walk together with longer decorrelations ( h) with respect to
regions inside the supergranule. In the vertices of the supegranular cell the
maximum observed occurrence is not associated with the maximum persistence,
suggesting that there are different dynamic regimes affecting the magnetic
elements
Imaging Spectropolarimetry with IBIS: Evolution of Bright Points in the Quiet Sun
We present the results from first spectropolarimetric observations of the
solar photosphere acquired at the Dunn Solar Telescope with the Interferometric
Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I
630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions
for 53 minutes, with a Stokes V noise of 0.003 the continuum intensity level.
The dataset allows us to study the evolution of several magnetic features
associated with G-band bright points in the quiet Sun. Here we focus on the
analysis of three distinct processes, namely the coalescence, fragmentation and
cancellation of G-band bright points. Our analysis is based on a SIR inversion
of the Stokes I and V profiles of both Fe I lines. The high spatial resolution
of the G-band images combined with the inversion results helps to interpret the
undergoing physical processes. The appearance (dissolution) of high-contrast
G-band bright points is found to be related to the local increase (decrease) of
the magnetic filling factor, without appreciable changes in the field strength.
The cancellation of opposite-polarity bright points can be the signature of
either magnetic reconnection or the emergence/submergence of magnetic loops.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
Chromospheric heating by acoustic waves compared to radiative cooling
Acoustic and magnetoacoustic waves are among the possible candidate
mechanisms that heat the upper layers of solar atmosphere. A weak chromospheric
plage near a large solar pore NOAA 11005 was observed on October 15, 2008 in
the lines Fe I 617.3 nm and Ca II 853.2 nm with the Interferometric
Bidimemsional Spectrometer (IBIS) attached to the Dunn Solar Telescope.
Analyzing the Ca II observations with spatial and temporal resolutions of 0.4"
and 52 s, the energy deposited by acoustic waves is compared with that released
by radiative losses. The deposited acoustic flux is estimated from power
spectra of Doppler oscillations measured in the Ca II line core. The radiative
losses are calculated using a grid of seven 1D hydrostatic semi-empirical model
atmospheres. The comparison shows that the spatial correlation of maps of
radiative losses and acoustic flux is 72 %. In quiet chromosphere, the
contribution of acoustic energy flux to radiative losses is small, only of
about 15 %. In active areas with photospheric magnetic field strength between
300 G and 1300 G and inclination of 20-60 degrees, the contribution increases
from 23 % (chromospheric network) to 54 % (a plage). However, these values have
to be considered as lower limits and it might be possible that the acoustic
energy flux is the main contributor to the heating of bright chromospheric
network and plages.Comment: 9 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
Dynamics of the solar atmosphere above a pore with a light bridge
Context: Solar pores are small sunspots lacking a penumbra that have a
prevailing vertical magnetic field component. They can include light bridges at
places with locally reduced magnetic field. Like sunspots, they exhibit a wide
range of oscillatory phenomena.
Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to
obtain characteristics of a chromospheric filamentary structure around the
pore, to analyse oscillations and waves in and around the pore, and to
understand the structure and brightness of the light bridge.
Methods: Spectral imaging observations in the line Ca II 854.2 nm and
complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS
spectrometer and HINODE/SOT spectropolarimeter, were used to measure
photospheric and chromospheric velocity fields, oscillations, waves, the
magnetic field in the photosphere, and acoustic energy flux and radiative
losses in the chromosphere.
Results: The chromospheric filamentary structure around the pore has all
important characteristics of a superpenumbra: it shows an inverse Evershed
effect and running waves, and has a similar morphology and oscillation
character. The granular structure of the light bridge in the upper photosphere
can be explained by radiative heating. Acoustic waves leaking up from the
photosphere along the inclined magnetic field in the light bridge transfer
enough energy flux to balance the total radiative losses of the light-bridge
chromosphere.
Conclusions: The presence of a penumbra is not a necessary condition for the
formation of a superpenumbra. The light bridge is heated by radiation in the
photosphere and by acoustic waves in the chromosphere.Comment: 14 pages, 14 figures, 3 tables, accepted for publication in
Astrononomy & Astrophysic
Pair separation of magnetic elements in the quiet Sun
The dynamic properties of the quiet Sun photosphere can be investigated by
analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic
elements).
By using hr-long Hinode magnetograms at high spatial resolution
(), we tracked magnetic element pairs within a supergranular
cell near the disk center.
The computed pair separation spectrum, calculated on the whole set of
particle pairs independently of their initial separation, points out what is
known as a super-diffusive regime with spectral index , in
agreement with the most recent literature, but extended to unprecedented
spatial and temporal scales (from granular to supergranular). Furthermore, for
the first time, we investigated here the spectrum of the mean square
displacement of pairs of magnetic elements, depending on their initial
separation . We found that there is a typical initial distance above
(below) which the pair separation is faster (slower) than the average. A
possible physical interpretation of such a typical spatial scale is also
provided
The ambitions and challenges of SROI.
With the growing interest in measuring the social impact of third sector activities, there have been a range of approaches developed. One of these, social return on investment (SROI) has received particular attention and is being promoted by third sector organisations, as well as public and private bodies. This paper examines this approach in detail and identifies a series of issues that require further investigation. These include technical and methodological issues related to this adjusted cost-benefit analysis such as quantifying the value of social benefits, and attribution; the judgement involved in setting indicators; whether projects deemed successful based on an SROI analysis can provide the basis for replicability and scaling up; and the ways in which SROI is being used by stakeholders. Through examining these challenges in detail, the approaches to measuring social impact can be strengthened, standardised and made more rigorous. While the issues raised here are essential to developing SROI further, they are also valid for more general discussions regarding the proving and improving of the value added by the UK third sector
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