2,290 research outputs found
The Photospheric Poynting Flux and Coronal Heating
Some models of coronal heating suppose that convective motions at the
photosphere shuffle the footpoints of coronal magnetic fields and thereby
inject sufficient magnetic energy upward to account for observed coronal and
chromospheric energy losses in active regions. Using high-resolution
observations of plage magnetic fields made with the Solar Optical Telescope
aboard the Hinode satellite, we investigate this idea by estimating the upward
transport of magnetic energy --- the vertical Poynting flux, S_z --- across the
photosphere in a plage region. To do so, we combine: (i) estimates of
photospheric horizontal velocities, v_h, determined by local correlation
tracking applied to a sequence of line-of-sight magnetic field maps from the
Narrowband Filter Imager, with (ii) a vector magnetic field measurement from
the SpectroPolarimeter. Plage fields are ideal observational targets for
estimating energy injection by convection, because they are: (i) strong enough
to be measured with relatively small uncertainties; (ii) not so strong that
convection is heavily suppressed (as within umbrae); and (iii) unipolar, so S_z
in plage is not influenced by mixed-polarity processes (e.g., flux emergence)
unrelated to heating in stable, active-region fields. In this plage region, we
found that the average S_z varied in space, but was positive (upward) and
sufficient to explain coronal heating, with values near (5 +/- 1) x 10^7
erg/cm^2/s. We find the energy input per unit magnetic flux to be on the order
of 10^5 erg/s/Mx. A comparison of intensity in a Ca II image co-registered with
the this plage shows stronger spatial correlations with both total field, B,
and unsigned vertical field, |B_z|, than either S_z or horizontal field, B_h.
The observed Ca II brightness enhancement, however, probably contains a strong
contribution from a near-photosphere hot-wall effect unrelated to atmospheric
heating.Comment: 30 pages, 11 figures, accepted by Pub. Astron. Soc. Japa
A Comprehensive Method of Estimating Electric Fields from Vector Magnetic Field and Doppler Measurements
Photospheric electric fields, estimated from sequences of vector magnetic
field and Doppler measurements, can be used to estimate the flux of magnetic
energy (the Poynting flux) into the corona and as time-dependent boundary
conditions for dynamic models of the coronal magnetic field. We have modified
and extended an existing method to estimate photospheric electric fields that
combines a poloidal-toroidal (PTD) decomposition of the evolving magnetic field
vector with Doppler and horizontal plasma velocities. Our current, more
comprehensive method, which we dub the "{\bf P}TD-{\bf D}oppler-{\bf F}LCT {\bf
I}deal" (PDFI) technique, can now incorporate Doppler velocities from
non-normal viewing angles. It uses the \texttt{FISHPACK} software package to
solve several two-dimensional Poisson equations, a faster and more robust
approach than our previous implementations. Here, we describe systematic,
quantitative tests of the accuracy and robustness of the PDFI technique using
synthetic data from anelastic MHD (\texttt{ANMHD}) simulations, which have been
used in similar tests in the past. We find that the PDFI method has less than
error in the total Poynting flux and a error in the helicity flux
rate at a normal viewing angle ) and less than and
errors respectively at large viewing angles (). We compare our
results with other inversion methods at zero viewing angle, and find that our
method's estimates of the fluxes of magnetic energy and helicity are comparable
to or more accurate than other methods. We also discuss the limitations of the
PDFI method and its uncertainties.Comment: 56 pages, 10 figures, ApJ (in press
What is the relationship between photospheric flow fields and solar flares?
We estimated photospheric velocities by separately applying the Fourier Local
Correlation Tracking (FLCT) and Differential Affine Velocity Estimator (DAVE)
methods to 2708 co-registered pairs of SOHO/MDI magnetograms, with nominal
96-minute cadence and ~2" pixels, from 46 active regions (ARs) from 1996-1998
over the time interval t45 when each AR was within 45^o of disk center. For
each magnetogram pair, we computed the average estimated radial magnetic field,
B; and each tracking method produced an independently estimated flow field, u.
We then quantitatively characterized these magnetic and flow fields by
computing several extensive and intensive properties of each; extensive
properties scale with AR size, while intensive properties do not depend
directly on AR size. Intensive flow properties included moments of speeds,
horizontal divergences, and radial curls; extensive flow properties included
sums of these properties over each AR, and a crude proxy for the ideal Poynting
flux, the total |u| B^2. Several magnetic quantities were also computed,
including: total unsigned flux; a measure of the amount of unsigned flux near
strong-field polarity inversion lines, R; and the total B^2. Next, using
correlation and discriminant analysis, we investigated the associations between
these properties and flares from the GOES flare catalog, when averaged over
both t45 and shorter time windows, of 6 and 24 hours. We found R and total |u|
B^2 to be most strongly associated with flares; no intensive flow properties
were strongly associated with flares.Comment: 57 pages, 13 figures; revised content; added URL to manuscript with
higher-quality image
GOCE Gradiometer Measurement Disturbances and their Modelling by Means of Ionospheric Dynamics
We examine the presence of residual non-gravitational signatures in gravitational gradients measured by GOCE Electrostatic Gravity Gradiometer. These signatures are observed over the geomagnetic poles during geomagnetically active days and contaminate the trace of the Gravitational Gradient Tensor by up to three to five times the expected noise level of the instrument (11 mE). We investigate these anomalies in the gradiometer measurements along many satellite tracks and examine possible causes by using external datasets, such as Interplanetary Electric Field observations from the ACE (Advanced Composition Explorer) and WIND spacecraft and Poynting flux (vector) estimated from Equivalent Ionospheric Currents derived from Spherical Elementary Current Systems over North America and Greenland. We show that the variations in the east-west and vertical electrical currents and Poynting flux (vector) components at the satellite position are highly correlated with the disturbances observed in the gradiometer measurements. We identify the relation between the ionospheric dynamics and disturbances and develop a data-driven model to reduce the effects of these disturbances. The results presented in this dissertation discover that the cause of the disturbances are due to intense ionospheric dynamics that are enhanced by increased solar activity which causes a dynamic drag environment. Moreover, using external information about the ionospheric dynamics, we successfully model and remove a high percentage of these disturbances for the first time in GOCE literature and promise improved data for future gravitational field models and studies of the Earth's upper atmosphere
Surface Electromagnetic Waves Thermally Excited: Radiative Heat Transfer, Coherence Properties and Casimir Forces Revisited in the Near Field
We review in this article the influence of surface waves on the thermally
excited electromagnetic field. We study in particular the field emitted at
subwalength distances of material surfaces. After reviewing the main properties
of surface waves, we introduce the fluctuation-dissipation theorem that allows
to model the fluctuating electromagnetic fields. We then analyse the
contribution of these waves in a variety of phenomena. They give a leading
contribution to the density of electromagnetic states, they produce both
temporal coherence and spatial coherence in the near field of planar thermal
sources. They can be used to modify radiative properties of surfaces and to
design partially spatially coherent sources. Finally, we discuss the role of
surface waves in the radiative heat transfer and the theory of dispersion
forces at the subwavelength scale.Comment: Redig\'{e} \`{a} la fin de l'ann\'{e}e 2004. Accept\'{e} dans Surface
Science Report
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