2,249 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
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
Estimating Electric Fields from Vector Magnetogram Sequences
Determining the electric field (E-field) distribution on the Sun's
photosphere is essential for quantitative studies of how energy flows from the
Sun's photosphere, through the corona, and into the heliosphere. This E-field
also provides valuable input for data-driven models of the solar atmosphere and
the Sun-Earth system. We show how Faraday's Law can be used with observed
vector magnetogram time series to estimate the photospheric E-field, an
ill-posed inversion problem. Our method uses a "poloidal-toroidal
decomposition" (PTD) of the time derivative of the vector magnetic field. The
PTD solutions are not unique; the gradient of a scalar potential can be added
to the PTD E-field without affecting consistency with Faraday's Law. We present
an iterative technique to determine a potential function consistent with ideal
MHD evolution; but this E-field is also not a unique solution to Faraday's Law.
Finally, we explore a variational approach that minimizes an energy functional
to determine a unique E-field, similar to Longcope's "Minimum Energy Fit". The
PTD technique, the iterative technique, and the variational technique are used
to estimate E-fields from a pair of synthetic vector magnetograms taken from an
MHD simulation; and these E-fields are compared with the simulation's known
electric fields. These three techniques are then applied to a pair of vector
magnetograms of solar active region NOAA AR8210, to demonstrate the methods
with real data.Comment: 41 pages, 10 figure
- …