1,329 research outputs found
A comparison of measured and simulated solar network contrast
Long-term trends in the solar spectral irradiance are important to determine
the impact on Earth's climate. These long-term changes are thought to be caused
mainly by changes in the surface area covered by small-scale magnetic elements.
The direct measurement of the contrast to determine the impact of these
small-scale magnetic elements is, however, limited to a few wavelengths, and
is, even for space instruments, affected by scattered light and instrument
defocus. In this work we calculate emergent intensities from 3-D simulations of
solar magneto-convection and validate the outcome by comparing with
observations from Hinode/SOT. In this manner we aim to construct the contrast
at wavelengths ranging from the NUV to the FIR.Comment: Proceedings paper, IAU XXVII, Symposium 264, 3 page
Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface
The variation in the radiative output of the Sun, described in terms of solar
irradiance, is important to climatology. A common assumption is that solar
irradiance variability is driven by its surface magnetism. Verifying this
assumption has, however, been hampered by the fact that models of solar
irradiance variability based on solar surface magnetism have to be calibrated
to observed variability. Making use of realistic three-dimensional
magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art
solar magnetograms from the Solar Dynamics Observatory, we present a model of
total solar irradiance (TSI) that does not require any such calibration. In
doing so, the modeled irradiance variability is entirely independent of the
observational record. (The absolute level is calibrated to the TSI record from
the Total Irradiance Monitor.) The model replicates 95% of the observed
variability between April 2010 and July 2016, leaving little scope for
alternative drivers of solar irradiance variability at least over the time
scales examined (days to years).Comment: Supplementary Materials;
https://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.119.091102/supplementary_material_170801.pd
Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
We present a reconstruction of the spectral solar irradiance since 1700 using
the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope
era version 2) model. This model uses as input magnetograms simulated with a
surface flux transport model fed with semi-synthetic records of emerging
sunspot groups. We used statistical relationships between the properties of
sunspot group emergence, such as the latitude, area, and tilt angle, and the
sunspot cycle strength and phase to produce semi-synthetic sunspot group
records starting in the year 1700. The semisynthetic records are fed into a
surface flux transport model to obtain daily simulated magnetograms that map
the distribution of the magnetic flux in active regions (sunspots and faculae)
and their decay products on the solar surface. The magnetic flux emerging in
ephemeral regions is accounted for separately based on the concept of extended
cycles whose length and amplitude are linked to those of the sunspot cycles
through the sunspot number. The magnetic flux in each surface component
(sunspots, faculae and network, and ephemeral regions) was used to compute the
spectral and total solar irradiance between the years 1700 and 2009. This
reconstruction is aimed at timescales of months or longer although the model
returns daily values. We found that SATIRE-T2, besides reproducing other
relevant observations such as the total magnetic flux, reconstructs the total
solar irradiance (TSI) on timescales of months or longer in good agreement with
the PMOD composite of observations, as well as with the reconstruction starting
in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI
of 1.2[+0.2, -0.3] Wm-2 between 1700 and the present. The spectral irradiance
reconstruction is in good agreement with the UARS/SUSIM measurements as well as
the Lyman-alpha composite.Comment: 13 pages, 10 figure
Relativistic Quantum Measurements, Unruh effect and Black Holes
It is shown how the technique of restricted path integrals (RPI) or quantum
corridors (QC) may be applied for the analysis of relativistic measurements.
Then this technique is used to clarify the physical nature of thermal effects
as seen by an accelerated observer in Minkowski space-time (Unruh effect) and
by a far observer in the field of a black hole (Hawking effect). The physical
nature of the "thermal atmosphere" around the observer is analysed in three
cases: a) the Unruh effect, b) an eternal (Kruskal) black hole and c) a black
hole forming in the process of collapse. It is shown that thermal particles are
real only in the case (c). In the case (b) they cannot be distinguished from
real particles but they do not carry away mass of the black hole until some of
these particles are absorbed by the far observer. In the case (a) thermal
particles are virtual.Comment: 24 pages (Latex), 8 EPS figures The text was edited for the new
versio
Cosmological Time in Quantum Supergravity
The version of supergravity formulated by Ogievetsky and Sokatchev is almost
identical to the conventional theory, except that the cosmological
constant appears as a dynamical variable which is constant only by
virtue of the field equations. We consider the canonical quantisation of this
theory, and show that the wave function evolves with respect to a dynamical
variable which can be interpreted as a cosmological time parameter. The square
of the modulus of the wave function obeys a set of simple conservation
equations and can be interpreted as a probability density functional. The usual
problems associated with time in quantum gravity are avoided.Comment: 12 pages, LaTe
Pseudo-Schwarzschild Spherical Accretion as a Classical Black Hole Analogue
We demonstrate that a spherical accretion onto astrophysical black holes,
under the influence of Newtonian or various post-Newtonian pseudo-Schwarzschild
gravitational potentials, may constitute a concrete example of classical
analogue gravity naturally found in the Universe. We analytically calculate the
corresponding analogue Hawking temperature as a function of the minimum number
of physical parameters governing the accretion flow. We study both the
polytropic and the isothermal accretion. We show that unlike in a general
relativistic spherical accretion, analogue white hole solutions can never be
obtained in such post-Newtonian systems. We also show that an isothermal
spherical accretion is a remarkably simple example in which the only one
information--the temperature of the fluid, is sufficient to completely describe
an analogue gravity system. For both types of accretion, the analogue Hawking
temperature may become higher than the usual Hawking temperature. However, the
analogue Hawking temperature for accreting astrophysical black holes is
considerably lower compared with the temperature of the accreting fluid.Comment: Final Version to appear in the journal General Relativity &
Gravitation, Volume 27, Issue 11, 2005. 17 pages, Two colour and one black
and white figures. Typos corrected. Recent reference on analogue effect in
relativistic accretion disc adde
Quantum fields in gravity
We give a brief description of some compelling connections between general
relativity and thermodynamics through i) the semi-classical tunnelling
method(s) and ii) the field-theoretical modelling of Unruh-DeWitt detectors. In
both approaches it is possible to interpret some quantities in a
thermodynamical frame.Comment: 4 pages, no figures, contribution to the proceedings of the
conference "Relativity and Gravitation - 100 years after Einstein in Prague
Fluctuation-dissipation theorem and the Unruh effect of scalar and Dirac fields
We present a simple and systematic method to calculate the Rindler noise,
which is relevant to the analysis of the Unruh effect, by using the
fluctuation-dissipative theorem. To do this, we calculate the dissipative
coefficient explicitly from the equations of motion of the detector and the
field. This method gives not only the correct answer but also a hint as to the
origin of the apparent statistics inversion effect. Moreover, this method is
generalized to the Dirac field, by using the fermionic fluctuation-dissipation
theorem. We can thus confirm that the fermionic fluctuation-dissipation theorem
is working properly.Comment: 26 page
Comment on `Hawking radiation from fluctuating black holes'
Takahashi & Soda (2010 Class. Quantum Grav. v27 p175008, arXiv:1005.0286)
have recently considered the effect (at lowest non-trivial order) of dynamical,
quantized gravitational fluctuations on the spectrum of scalar Hawking
radiation from a collapsing Schwarzschild black hole. However, due to an
unfortunate choice of gauge, the dominant (even divergent) contribution to the
coefficient of the spectrum correction that they identify is a pure gauge
artifact. I summarize the logic of their calculation, comment on the
divergences encountered in its course and comment on how they could be
eliminated, and thus the calculation be completed.Comment: 12 pages, 1 fig; feynmp, amsref
Computing the spectrum of black hole radiation in the presence of high frequency dispersion: an analytical approach
We present a method for computing the spectrum of black hole radiation of a
scalar field satisfying a wave equation with high frequency dispersion. The
method involves a combination of Laplace transform and WKB techniques for
finding approximate solutions to ordinary differential equations. The modified
wave equation is obtained by adding a higher order derivative term suppressed
by powers of a fundamental momentum scale to the ordinary wave equation.
Depending on the sign of this new term, high frequency modes propagate either
superluminally or subluminally. We show that the resulting spectrum of created
particles is thermal at the Hawking temperature, and further that the out-state
is a thermal state at the Hawking temperature, to leading order in , for
either modification.Comment: 26 pages, plain latex, 6 figures included using psfi
- …