47 research outputs found
The Photospheric Imprints of Coronal Electric Currents
Flares and coronal mass ejections are powered by magnetic energy stored in
coronal electric currents. Here, we explore the nature of coronal currents in
observed and model active region (ARs) by studying manifestations of these
currents in photospheric vector magnetograms. We employ Gauss's separation
method, recently introduced to the solar physics literature, to partition the
photospheric field into three distinct components, each arising from a separate
source: (i) currents passing through the photosphere, (ii) currents flowing
below it, and (iii) currents flowing above it. We refer to component (iii) as
the photospheric imprint of coronal currents. In both AR 10930 and AR 11158,
photospheric imprints exhibit large-scale, spatially coherent structures along
these regions' central, sheared polarity inversion lines (PILs) that are
consistent with coronal currents flowing horizontally above these PILs, similar
to recent findings in AR 12673 by Schuck et al. (2022). We find similar
photospheric imprints in a simple model of a non-potential AR with known
currents. We find that flare-associated changes in photospheric imprints in AR
11158 accord with earlier reports that near-PIL fields become more horizontal,
consistent with the "implosion" scenario. We hypothesize that this evolution
effectively shortens, in an overall sense, current-carrying coronal fields,
leading to decreased inductive energy (DIE) in the coronal field. We further
hypothesize that, in the hours prior to flares, parts of the coronal field
slowly expand, in a process we deem coronal inflation (CI) -- essentially, the
inverse of the implosion process. Both of these hypotheses are testable with
non-potential coronal field extrapolations.Comment: 28 pages, 10 figures, to be submitted to ApJ. Addition of co-authors
is expecte
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
Photospheric Electric Fields and Energy Fluxes in the Eruptive Active Region NOAA 11158
How much electromagnetic energy crosses the photosphere in evolving solar
active regions? With the advent of high-cadence vector magnetic field
observations, addressing this fundamental question has become tractable. In
this paper, we apply the "PTD-Doppler-FLCT-Ideal" (PDFI) electric field
inversion technique of Kazachenko et al. (2014) to a 6-day HMI/SDO vector
magnetogram and Doppler velocity sequence, to find the electric field and
Poynting flux evolution in active region NOAA 11158, which produced an X2.2
flare early on 2011 February 15. We find photospheric electric fields ranging
up to V/cm. The Poynting fluxes range from to
ergscms, mostly positive, with the largest contribution to
the energy budget in the range of -
ergscms. Integrating the instantaneous energy flux over
space and time, we find that the total magnetic energy accumulated above the
photosphere from the initial emergence to the moment before the X2.2 flare to
be ergs, which is partitioned as and
ergs, respectively, between free and potential energies.
Those estimates are consistent with estimates from preflare non-linear
force-free field (NLFFF) extrapolations and the Minimum Current Corona
estimates (MCC), in spite of our very different approach. This study of
photospheric electric fields demonstrates the potential of the PDFI approach
for estimating Poynting fluxes and opens the door to more quantitative studies
of the solar photosphere and more realistic data-driven simulations of coronal
magnetic field evolution.Comment: 51 pages, 10 figures, accepted by ApJ on August 11, 201
Solar Magnetic Tracking. IV. The Death of Magnetic Features
The removal of magnetic flux from the quiet-sun photosphere is important for
maintaining the statistical steady-state of the magnetic field there, for
determining the magnetic flux budget of the Sun, and for estimating the rate of
energy injected into the upper solar atmosphere. Magnetic feature death is a
measurable proxy for the removal of detectable flux. We used the SWAMIS feature
tracking code to understand how nearly 20000 detected magnetic features die in
an hour-long sequence of Hinode/SOT/NFI magnetograms of a region of quiet Sun.
Of the feature deaths that remove visible magnetic flux from the photosphere,
the vast majority do so by a process that merely disperses the
previously-detected flux so that it is too small and too weak to be detected.
The behavior of the ensemble average of these dispersals is not consistent with
a model of simple planar diffusion, suggesting that the dispersal is
constrained by the evolving photospheric velocity field. We introduce the
concept of the partial lifetime of magnetic features, and show that the partial
lifetime due to Cancellation of magnetic flux, 22 h, is 3 times slower than
previous measurements of the flux turnover time. This indicates that prior
feature-based estimates of the flux replacement time may be too short, in
contrast with the tendency for this quantity to decrease as resolution and
instrumentation have improved. This suggests that dispersal of flux to smaller
scales is more important for the replacement of magnetic fields in the quiet
Sun than observed bipolar cancellation. We conclude that processes on spatial
scales smaller than those visible to Hinode dominate the processes of flux
emergence and cancellation, and therefore also the quantity of magnetic flux
that threads the photosphere.Comment: Accepted by Ap