88 research outputs found
Is dark matter an extra-dimensional effect?
We investigate the possibility that the observed behavior of test particles
outside galaxies, which is usually explained by assuming the presence of dark
matter, is the result of the dynamical evolution of particles in higher
dimensional space-times. Hence, dark matter may be a direct consequence of the
presence of an extra force, generated by the presence of extra-dimensions,
which modifies the dynamic law of motion, but does not change the intrinsic
properties of the particles, like, for example, the mass (inertia). We discuss
in some detail several possible particular forms for the extra force, and the
acceleration law of the particles is derived. Therefore, the constancy of the
galactic rotation curves may be considered as an empirical evidence for the
existence of the extra dimensions.Comment: 11 pages, no figures, accepted for publication in MPLA; references
adde
New Path Equations in Absolute Parallelism Geometry
The Bazanski approach, for deriving the geodesic equations in Riemannian
geometry, is generalized in the absolute parallelism geometry. As a consequence
of this generalization three path equations are obtained. A striking feature in
the derived equations is the appearance of a torsion term with a numerical
coefficients that jumps by a step of one half from equation to another. This is
tempting to speculate that the paths in absolute parallelism geometry might
admit a quantum feature.Comment: 4 pages Latex file Journal Reference: Astrophysics and space science
228, 273, (1995
Fleeting small-scale surface magnetic fields build the quiet-Sun corona
Arch-like loop structures filled with million Kelvin hot plasma form the
building blocks of the quiet-Sun corona. Both high-resolution observations and
magnetoconvection simulations show the ubiquitous presence of magnetic fields
on the solar surface on small spatial scales of 100\,km. However, the
question of how exactly these quiet-Sun coronal loops originate from the
photosphere and how the magnetic energy from the surface is channeled to heat
the overlying atmosphere is a long-standing puzzle. Here we report
high-resolution photospheric magnetic field and coronal data acquired during
the second science perihelion of Solar Orbiter that reveal a highly dynamic
magnetic landscape underlying the observed quiet-Sun corona. We found that
coronal loops often connect to surface regions that harbor fleeting weaker,
mixed-polarity magnetic field patches structured on small spatial scales, and
that coronal disturbances could emerge from these areas. We suggest that weaker
magnetic fields with fluxes as low as \,Mx and or those that evolve on
timescales less than 5\,minutes, are crucial to understand the coronal
structuring and dynamics.Comment: Accepted for publication in The Astrophysical Journal Letter
Magnetic fields inferred by Solar Orbiter: A comparison between SO/PHI-HRT and SDO/HMI
The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic
Imager on board the Solar Orbiter spacecraft (SO/PHI) and the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) both infer
the photospheric magnetic field from polarised light images. SO/PHI is the
first magnetograph to move out of the Sun--Earth line and will provide
unprecedented access to the Sun's poles. This provides excellent opportunities
for new research wherein the magnetic field maps from both instruments are used
simultaneously. We aim to compare the magnetic field maps from these two
instruments and discuss any possible differences between them. We used data
from both instruments obtained during Solar Orbiter's inferior conjunction on 7
March 2022. The HRT data were additionally treated for geometric distortion and
degraded to the same resolution as HMI. The HMI data were re-projected to
correct for the separation between the two observatories.
SO/PHI-HRT and HMI produce remarkably similar line-of-sight magnetograms, with
a slope coefficient of , an offset below G, and a Pearson correlation
coefficient of . However, SO/PHI-HRT infers weaker line-of-sight fields
for the strongest fields. As for the vector magnetic field, SO/PHI-HRT was
compared to both the -second and -second HMI vector magnetic field:
SO/PHI-HRT has a closer alignment with the -second HMI vector. In the weak
signal regime ( G), SO/PHI-HRT measures stronger and more horizontal
fields than HMI, very likely due to the greater noise in the SO/PHI-HRT data.
In the strong field regime ( G), HRT infers lower field strengths
but with similar inclinations (a slope of ) and azimuths (a slope of
). The slope values are from the comparison with the HMI -second
vector.Comment: 10 pages, 5 figures, accepted for publication in A&A; manuscript is a
part of Astronomy & Astrophysics special issue: Solar Orbiter First Results
(Nominal Mission Phase
Wavefront error of PHI/HRT on Solar Orbiter at various heliocentric distances
We use wavefront sensing to characterise the image quality of the the High
Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (SO/PHI)
data products during the second remote sensing window of the Solar Orbiter (SO)
nominal mission phase. Our ultimate aims are to reconstruct the HRT data by
deconvolving with the HRT point spread function (PSF) and to correct for the
effects of optical aberrations on the data. We use a pair of focused--defocused
images to compute the wavefront error and derive the PSF of HRT by means of a
phase diversity (PD) analysis. The wavefront error of HRT depends on the
orbital distance of SO to the Sun. At distances \,au, the wavefront error
is small, and stems dominantly from the inherent optical properties of HRT. At
distances \,au, the thermo-optical effect of the Heat Rejection Entrance
Window (HREW) becomes noticeable. We develop an interpolation scheme for the
wavefront error that depends on the thermal variation of the HREW with the
distance of SO to the Sun. We also introduce a new level of image
reconstruction, termed `aberration correction', which is designed to reduce the
noise caused by image deconvolution while removing the aberrations caused by
the HREW. The computed PSF via phase diversity significantly reduces the
degradation caused by the HREW in the near-perihelion HRT data. In addition,
the aberration correction increases the noise by a factor of only
compared to the factor of increase that results from the usual PD
reconstructions
Geodesic motion in the neighbourhood of submanifolds embedded in warped product spaces
We study the classical geodesic motions of nonzero rest mass test particles
and photons in (3+1+n)- dimensional warped product spaces. An important feature
of these spaces is that they allow a natural decoupling between the motions in
the (3+1)-dimensional spacetime and those in the extra n dimensions. Using this
decoupling and employing phase space analysis we investigate the conditions for
confinement of particles and photons to the (3+1)- spacetime submanifold. In
addition to providing information regarding the motion of photons, we also show
that these motions are not constrained by the value of the extrinsic curvature.
We obtain the general conditions for the confinement of geodesics in the case
of pseudo-Riemannian manifolds as well as establishing the conditions for the
stability of such confinement. These results also generalise a recent result of
the authors concerning the embeddings of hypersurfaces with codimension one.Comment: 8 pages, 1 figure. To appear in General Relativity and Gravitation as
a contributed paper to Mashhoon Festschrif
Stereoscopic disambiguation of vector magnetograms: first applications to SO/PHI-HRT data
Spectropolarimetric reconstructions of the photospheric vector magnetic field
are intrinsically limited by the 180-ambiguity in the orientation of
the transverse component. So far, the removal of such an ambiguity has required
assumptions about the properties of the photospheric field, which makes
disambiguation methods model-dependent. The basic idea is that the unambiguous
line-of-sight component of the field measured from one vantage point will
generally have a non-zero projection on the ambiguous transverse component
measured by the second telescope, thereby determining the ``true'' orientation
of the transverse field. Such an idea was developed and implemented in the
Stereoscopic Disambiguation Method (SDM), which was recently tested using
numerical simulations. In this work we present a first application of the SDM
to data obtained by the High Resolution Telescope (HRT) onboard Solar Orbiter
during the March 2022 campaign, when the angle with Earth was 27 degrees. The
method is successfully applied to remove the ambiguity in the transverse
component of the vector magnetogram solely using observations (from HRT and
from the Helioseismic and Magnetic Imager), for the first time. The SDM is
proven to provide observation-only disambiguated vector magnetograms that are
spatially homogeneous and consistent. A discussion about the sources of error
that may limit the accuracy of the method, and of the strategies to remove them
in future applications, is also presented.Comment: 32 pages, 12 figures, accepted in A&A on 09/07/202
Intensity contrast of solar network and faculae close to the solar limb, observed from two vantage points
The brightness of faculae and network depends on the angle at which they are
observed and the magnetic flux density. Close to the limb, assessment of this
relationship has until now been hindered by the increasingly lower signal in
magnetograms. This preliminary study aims at highlighting the potential of
using simultaneous observations from different vantage points to better
determine the properties of faculae close to the limb. We use data from the
Solar Orbiter/Polarimetric and Helioseismic Imager (SO/PHI), and the Solar
Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI), recorded at
angular separation of their lines of sight at the Sun. We use
continuum intensity observed close to the limb by SO/PHI and complement it with
the co-observed from SDO/HMI, originating closer to disc centre
(as seen by SDO/HMI), thus avoiding the degradation of the magnetic field
signal near the limb. We derived the dependence of facular brightness in the
continuum on disc position and magnetic flux density from the combined
observations of SO/PHI and SDO/HMI. Compared with a single point of view, we
were able to obtain contrast values reaching closer to the limb and to lower
field strengths. We find the general dependence of the limb distance at which
the contrast is maximum on the flux density to be at large in line with single
viewpoint observations, in that the higher the flux density is, the closer the
turning point lies to the limb. There is a tendency, however, for the maximum
to be reached closer to the limb when determined from two vantage points. We
note that due to the preliminary nature of this study, these results must be
taken with caution. Our analysis shows that studies involving two viewpoints
can significantly improve the detection of faculae near the solar limb and the
determination of their brightness contrast relative to the quiet Sun
The ratio of horizontal to vertical displacement in solar oscillations estimated from combined SO/PHI and SDO/HMI observations
In order to make accurate inferences about the solar interior using
helioseismology, it is essential to understand all the relevant physical
effects on the observations. One effect to understand is the (complex-valued)
ratio of the horizontal to vertical displacement of the p- and f-modes at the
height at which they are observed. Unfortunately, it is impossible to measure
this ratio directly from a single vantage point, and it has been difficult to
disentangle observationally from other effects. In this paper we attempt to
measure the ratio directly using 7.5 hours of simultaneous observations from
the Polarimetric and Helioseismic Imager on board Solar Orbiter and the
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. While
image geometry problems make it difficult to determine the exact ratio, it
appears to agree well with that expected from adiabatic oscillations in a
standard solar model. On the other hand it does not agree with a commonly used
approximation, indicating that this approximation should not be used in
helioseismic analyses. In addition, the ratio appears to be real-valued.Comment: Accepted for publication in Astronomy & Astrophysics. 8 pages, 8
figure
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