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Author name code: abbett
ADS astronomy entries on 2021-05-31
author:"Abbett, William P."

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Title: Coupling a Global Heliospheric Magnetohydrodynamic Model to
    a Magnetofrictional Model of the Low Corona
Authors: Hayashi, Keiji; Abbett, William P.; Cheung, Mark C. M.;
   Fisher, George H.
2021ApJS..254....1H    Altcode:
  Recent efforts coupling our Sun-to-Earth magnetohydrodynamics (MHD)
  model and lower-corona magnetofrictional (MF) model are described. Our
  Global Heliospheric MHD (GHM) model uses time-dependent three-component
  magnetic field data from the lower-corona MF model as time-dependent
  boundary values. The MF model uses data-assimilation techniques to
  introduce the vector magnetic field data from the Solar Dynamics
  Observatory/Helioseismic and Magnetic Imager, hence as a whole this
  simulation coupling structure is driven with actual observations. The
  GHM model employs a newly developed interface boundary treatment that
  is based on the concept of characteristics, and it properly treats
  the interface boundary sphere set at a height of the sub-Alfvénic
  lower corona (1.15 R<SUB>⊙</SUB> in this work). The coupled model
  framework numerically produces twisted nonpotential magnetic features
  and consequent eruption events in the solar corona in response to the
  time-dependent boundary values. The combination of our two originally
  independently developed models presented here is a model framework
  toward achieving further capabilities of modeling the nonlinear
  time-dependent nature of magnetic field and plasma, from small-scale
  solar active regions to large-scale solar wind structures. This work is
  a part of the Coronal Global Evolutionary Model project for enhancing
  our understanding of Sun-Earth physics to help improve space weather
  capabilities.

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Title: The Coronal Global Evolutionary Model: Using HMI Vector
    Magnetogram and Doppler Data to Determine Coronal Magnetic Field
    Evolution
Authors: Hoeksema, J. Todd; Abbett, William P.; Bercik, David J.;
   Cheung, Mark C. M.; DeRosa, Marc L.; Fisher, George H.; Hayashi, Keiji;
   Kazachenko, Maria D.; Liu, Yang; Lumme, Erkka; Lynch, Benjamin J.;
   Sun, Xudong; Welsch, Brian T.
2020ApJS..250...28H    Altcode: 2020arXiv200614579H
  The Coronal Global Evolutionary Model (CGEM) provides data-driven
  simulations of the magnetic field in the solar corona to better
  understand the build-up of magnetic energy that leads to eruptive
  events. The CGEM project has developed six capabilities. CGEM modules
  (1) prepare time series of full-disk vector magnetic field observations
  to (2) derive the changing electric field in the solar photosphere over
  active-region scales. This local electric field is (3) incorporated
  into a surface flux transport model that reconstructs a global
  electric field that evolves magnetic flux in a consistent way. These
  electric fields drive a (4) 3D spherical magnetofrictional (SMF) model,
  either at high resolution over a restricted range of solid angles or
  at lower resolution over a global domain to determine the magnetic
  field and current density in the low corona. An SMF-generated initial
  field above an active region and the evolving electric field at the
  photosphere are used to drive (5) detailed magnetohydrodynamic (MHD)
  simulations of active regions in the low corona. SMF or MHD solutions
  are then used to compute emissivity proxies that can be compared
  with coronal observations. Finally, a lower-resolution SMF magnetic
  field is used to initialize (6) a global MHD model that is driven by
  an SMF electric field time series to simulate the outer corona and
  heliosphere, ultimately connecting Sun to Earth. As a demonstration,
  this report features results of CGEM applied to observations of the
  evolution of NOAA Active Region 11158 in 2011 February.

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Title: Modeling a Carrington-scale Stellar Superflare and Coronal
    Mass Ejection from {\kappa }^{1}{Cet}
Authors: Lynch, Benjamin J.; Airapetian, Vladimir S.; DeVore,
   C. Richard; Kazachenko, Maria D.; Lüftinger, Teresa; Kochukhov,
   Oleg; Rosén, Lisa; Abbett, William P.
2019ApJ...880...97L    Altcode: 2019arXiv190603189L
  Observations from the Kepler mission have revealed frequent
  superflares on young and active solar-like stars. Superflares
  result from the large-scale restructuring of stellar magnetic
  fields, and are associated with the eruption of coronal material
  (a coronal mass ejection, or CME) and energy release that can be
  orders of magnitude greater than those observed in the largest solar
  flares. These catastrophic events, if frequent, can significantly
  impact the potential habitability of terrestrial exoplanets through
  atmospheric erosion or intense radiation exposure at the surface. We
  present results from numerical modeling designed to understand how
  an eruptive superflare from a young solar-type star, κ <SUP>1</SUP>
  Cet, could occur and would impact its astrospheric environment. Our
  data-inspired, three-dimensional magnetohydrodynamic modeling shows
  that global-scale shear concentrated near the radial-field polarity
  inversion line can energize the closed-field stellar corona sufficiently
  to power a global, eruptive superflare that releases approximately
  the same energy as the extreme 1859 Carrington event from the Sun. We
  examine proxy measures of synthetic emission during the flare and
  estimate the observational signatures of our CME-driven shock, both
  of which could have extreme space-weather impacts on the habitability
  of any Earth-like exoplanets. We also speculate that the observed
  1986 Robinson-Bopp superflare from κ <SUP>1</SUP> Cet was perhaps as
  extreme for that star as the Carrington flare was for the Sun.

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Title: MHD Simulation of a Superflare and Associated Carrington-Scale
    CME Event From the Young Sun
Authors: Lynch, B. J.; Airapetian, V.; Kazachenko, M.; Lueftinger,
   T.; DeVore, C. R.; Abbett, W. P.
2018AGUFM.P43H3844L    Altcode:
  Recent Kepler observations reveal frequent superflares on young
  active solar-like stars. We present preliminary simulation results
  for a global eruptive flare from the young-Sun analog Kappa-1 Cet. Our
  simulation magnetic field initialization is based on a low-order PFSS
  representation of the observed stellar magnetogram that provides a
  non-trivial dipolar magnetic field configuration with a significantly
  warped helmet streamer belt. We use a standard Parker [1958] isothermal
  solar wind for the coronal atmosphere and energize the closed-field
  stellar corona with idealized shearing flows parallel to the radial
  field polarity inversion line. We examine the energy evolution of the
  global superflare showing a release of 7.1e+33 erg of magnetic free
  energy over the course of 10 hours while the maximum kinetic energy
  increase of the CME eruption reaches 2.8e+33 erg, i.e. approximately
  the strength of the famous 1859 Carrington Event. We use a flare-ribbon
  geometric proxy to calculate a total unsigned flare reconnection flux
  of 2.2e+23 Mx and a peak reconnection rate of 8.0e+18 Mx/s. We examine
  various proxy measures of synthetic emission during the flare and
  discuss the potential for extreme space weather impacts on the early
  Earth associated with the CME-driven shock and the CME/ICME flux rope
  field structure and orientation.

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Title: Initiation of Superflares and Super-CMEs in Active Solar-type
    Stars
Authors: Lynch, B. J.; Airapetian, V. S.; Kazachenko, M. D.;
   Lueftinger, T.; DeVore, C. R.; Abbett, W. P.
2018csc..confE..94L    Altcode:
  Recent Kepler observations reveal frequent superflares on young
  active solar-like stars. We present preliminary simulation results
  for a global eruptive flare from the young-Sun analog Kappa-1 Cet. Our
  simulation magnetic field initialization is based on a low-order PFSS
  representation of the observed stellar magnetogram that provides a
  non-trivial dipolar magnetic field configuration with a significantly
  warped helmet streamer belt. We use a standard Parker [1958] isothermal
  solar wind for the coronal atmosphere and energize the closed-field
  stellar corona with idealized shearing flows parallel to the radial
  field polarity inversion line. We examine the energy evolution of the
  global superflare showing a release of 7.1e+33 erg of magnetic free
  energy over the course of 10 hours while the maximum kinetic energy
  increase of the CME eruption reaches 2.8e+33 erg, i.e. approximately
  the strength of the famous 1859 Carrington Event. We use a flare-ribbon
  geometric proxy to calculate a total unsigned flare reconnection flux
  of 2.2e+23 Mx and a peak reconnection rate of 8.0e+18 Mx/s. We examine
  various proxy measures of synthetic emission during the flare and
  discuss the potential for extreme space weather impacts on the early
  Earth associated with the CME-driven shock and the CME/ICME flux rope
  field structure and orientation.

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Title: Excess Lorentz Force in Major Solar Eruptions
Authors: Sun, Xudong; Lynch, Benjamin; Abbett, William; Li, Yan
2018csc..confE..41S    Altcode:
  The solar active region photospheric magnetic field evolves rapidly
  during major eruptive events, suggesting appreciable feedback from
  the corona. Using high-cadence vector magnetograms, multi-wavelength
  coronal imaging, and numerical simulation, we show how the observed
  photospheric "magnetic imprints" are highly structured in space and
  time, and how it can in principle be used to estimate the impulse of
  the Lorentz force that accelerates the coronal mass ejection (CME)
  plasma. In an archetypical event, the Lorentz force correlates well
  with the CME acceleration, but the total force impulse surprisingly
  exceeds the CME momentum by almost two orders of magnitude. Such a clear
  trend exists in about two thirds of the eruptions in our survey for
  Cycle 24. We propose a "gentle photospheric upwelling" scenario, where
  most of the Lorentz force is trapped in the lower atmosphere layer,
  counter-balanced by gravity of the upwelled mass. This unexpected effect
  dominates the momentum processes, but is negligible for the energy
  budget. We discuss how the upcoming high-sensitivity observations and
  new-generation numerical models may help elucidate the problem.

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Title: Using Convection Zone-to-Corona Models to Understand the
    Physics of the Solar Wind
Authors: Abbett, W. P.
2015AGUFMSH13E..01A    Altcode:
  How magnetic energy and flux emerges from the turbulent convective
  interior of the Sun into the solar atmosphere is of great importance to
  a number of challenging problems in Heliophysics. With the wealth of
  data from space-based and ground-based observatories, it is evident
  that solar magnetic fields span the entirety of the convection
  zone-to-corona system, and do not exist in isolation in a localized
  region, or interact only over a prescribed spatial scale. The challenge
  of modeling this system in its entirety is that the magnetic field not
  only spans multiple scales, but also regions whose physical conditions
  vary dramatically. In this overview, I will summarize recent progress
  in the effort to dynamically model the upper convection zone-to-corona
  system over large spatial scales, and will discuss applications of
  these new models to Solar Probe Plus science.

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Title: The Coronal Global Evolutionary Model: Using HMI Vector
    Magnetogram and Doppler Data to Model the Buildup of Free Magnetic
    Energy in the Solar Corona
Authors: Fisher, G. H.; Abbett, W. P.; Bercik, D. J.; Kazachenko,
   M. D.; Lynch, B. J.; Welsch, B. T.; Hoeksema, J. T.; Hayashi, K.;
   Liu, Y.; Norton, A. A.; Dalda, A. Sainz; Sun, X.; DeRosa, M. L.;
   Cheung, M. C. M.
2015SpWea..13..369F    Altcode: 2015arXiv150506018F
  The most violent space weather events (eruptive solar flares and
  coronal mass ejections) are driven by the release of free magnetic
  energy stored in the solar corona. Energy can build up on timescales
  of hours to days, and then may be suddenly released in the form of a
  magnetic eruption, which then propagates through interplanetary space,
  possibly impacting the Earth's space environment. Can we use the
  observed evolution of the magnetic and velocity fields in the solar
  photosphere to model the evolution of the overlying solar coronal
  field, including the storage and release of magnetic energy in such
  eruptions? The objective of CGEM, the Coronal Global Evolutionary Model,
  funded by the NASA/NSF Space Weather Modeling program, is to develop
  and evaluate such a model for the evolution of the coronal magnetic
  field. The evolving coronal magnetic field can then be used as a
  starting point for magnetohydrodynamic (MHD) models of the corona,
  which can then be used to drive models of heliospheric evolution and
  predictions of magnetic field and plasma density conditions at 1AU.

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Title: Modeling the Convection Zone-to-Corona System over Global
    Spatial Scales
Authors: Abbett, W. P.; Bercik, D. J.; Fisher, G. H.
2014AGUFMSH44A..01A    Altcode:
  How magnetic energy and flux emerges from the turbulent convective
  interior of the Sun into the solar atmosphere is of great importance
  to a number of challenging problems in solar physics. With the wealth
  of data from missions such as SDO, Hinode, and IRIS, it is evident that
  the dynamic interaction of magnetic structures at the photosphere and in
  the solar atmosphere occurs over a vast range of spatial and temporal
  scales. Emerging active regions often develop magnetic connections
  to other regions of activity some distance away on the solar disk,
  and always emerge into a global coronal field whose structural
  complexity is a function of the solar cycle. Yet even small-scale
  dynamic interactions (e.g., processes at granular or supergranular
  scales in the photosphere) can trigger rapid changes in the large-scale
  coronal field sufficient to power eruptive events such as coronal mass
  ejections, or solar flares. The challenge of modeling this system in
  its entirety is that the magnetic field not only spans multiple scales,
  but also regions whose physical conditions vary dramatically. We will
  summarize recent progress in the effort to dynamically model the upper
  convection zone-to-corona system over large spatial scales, and will
  present the latest results from a new, global radiative-MHD model of the
  upper convection zone-to-corona system, RADMHD2S. We will characterize
  the flux of electromagnetic energy into the solar atmosphere as flux
  systems of different scales dynamically interact, and discuss how
  physics-based models of the convection zone-to-corona system can be
  used to guide the development and testing of data-driven models.

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Title: Understanding Measures of Magnetic Activity Using Physics-based
    Models of the Solar Interior and Atmosphere
Authors: Abbett, W. P.; Luhmann, J. G.
2014AGUFMSH13D4140A    Altcode:
  Substantial progress has been made over the past decade in the effort
  to better understand how magnetic flux and energy is generated
  in the convective interior of the Sun, how it emerges into the
  solar atmosphere, and how manifestations of solar magnetic activity
  (such as sunspots, coronal mass ejections, and flares) are connected
  within a dynamic magnetic environment spanning the solar convection
  zone-to-corona system. Here, we present a brief overview of recent
  efforts to model the evolution of active region magnetic fields and
  sunspots over a range of physical conditions and spatial and temporal
  scales. We will focus on how dynamic, physics-based numerical models
  can be used to better understand observed relationships between
  different measures of solar activity as a function of time (e.g.,
  sunspot activity and morphologies, unsigned magnetic flux measured at
  the photosphere, coronal X-ray emissivity). We will determine whether
  local physics-based models of active region evolution can be used
  to better constrain proxies of solar activity such as the sunspot
  number, which remains the only direct record available to trace the
  very long-term influence of the solar dynamo on the earth's environment.

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Title: RADMHD2S: A Global 3D Radiative-MHD Model of the Upper
    Convection Zone-to-Corona System
Authors: Abbett, William P.; Bercik, David J
2014AAS...22412347A    Altcode:
  We present the latest results from a new, global radiative-MHD
  model of the upper convection zone-to-corona system, RADMHD2S. The
  numerical methods build upon those of the RADMHD model of Abbett
  (2007) and Abbett &amp; Fisher (2012), and significantly extend the
  capabilities of that code to allow for large-scale, sufficiently
  resolved, global calculations over a non-uniform, 3D curvilinear
  (spherical) mesh. RADMHD2S utilizes a high-order, non-dimensionally
  split, semi-implicit finite volume formalism to update the
  system of conservation equations in a way that properly propagates
  discontinuities in off-axis directions, while simultaneously preserving
  the 3D solenoidal constraint on the magnetic field. In addition, we
  will discuss improvements in the treatment of energetics, radiative
  transport, and cross-field diffusion that allow for more realistic
  data-driven modeling of the model's photosphere and chromosphere.

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Title: Buildup of Magnetic Shear and Free Energy during Flux Emergence
    and Cancellation
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
   der Holst, Bart
2012ApJ...754...15F    Altcode: 2012arXiv1205.3764F
  We examine a simulation of flux emergence and cancellation, which shows
  a complex sequence of processes that accumulate free magnetic energy
  in the solar corona essential for the eruptive events such as coronal
  mass ejections, filament eruptions, and flares. The flow velocity at
  the surface and in the corona shows a consistent shearing pattern along
  the polarity inversion line (PIL), which together with the rotation of
  the magnetic polarities, builds up the magnetic shear. Tether-cutting
  reconnection above the PIL then produces longer sheared magnetic field
  lines that extend higher into the corona, where a sigmoidal structure
  forms. Most significantly, reconnection and upward-energy-flux transfer
  are found to occur even as magnetic flux is submerging and appears
  to cancel at the photosphere. A comparison of the simulated coronal
  field with the corresponding coronal potential field graphically shows
  the development of non-potential fields during the emergence of the
  magnetic flux and formation of sunspots.

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Title: Buildup of Free Energy for Eruptive Events during Flux
    Emergence
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
   der Holst, Bart
2012shin.confE..39F    Altcode:
  In a simulation of magnetic flux emergence from the convection zone,
  a complex sequence of processes accumulate free magnetic energy in the
  solar corona essential for the eruptive events such as coronal mass
  ejections (CMEs), filament eruptions and flares. The flow velocity at
  the surface and in the corona shows a consistent shearing pattern along
  the polarity inversion line (PIL), which together with the rotation of
  the magnetic polarities, builds up the magnetic shear. Tether-cutting
  reconnection above the PIL then produces longer sheared magnetic field
  lines that extend higher into the corona, where a sigmoidal structure
  forms. Most significantly, reconnection and upward energy-flux transfer
  are found to occur even as magnetic flux is submerging and appears
  to cancel at the photosphere. A comparison of the simulated coronal
  field with the corresponding coronal potential field graphically shows
  the development of non-potential fields during the emergence of the
  magnetic flux and formation of sunspots.

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Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
   T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
   S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
   C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
   DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
   S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
   K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
   Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
   Z.; Riley, P.; Wu, S. T.
2012ASPC..455..337L    Altcode:
  The Helioseismic and Magnetic Imager (HMI; Scherrer &amp; Schou 2011)
  is one of the three instruments aboard the Solar Dynamics Observatory
  (SDO) that was launched on February 11, 2010 from Cape Canaveral,
  Florida. The instrument began to acquire science data on March 24. The
  regular operations started on May 1. HMI measures the Doppler velocity
  and line-of-sight magnetic field in the photosphere at a cadence of
  45 seconds, and the vector magnetic field at a 135-second cadence,
  with a 4096× 4096 pixels full disk coverage. The vector magnetic
  field data is usually averaged over 720 seconds to suppress the p-modes
  and increase the signal-to-noise ratio. The spatial sampling is about
  0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
  has a Landé factor of 2.5. These data are further used to produce
  higher level data products through the pipeline at the HMI-AIA Joint
  Science Operations Center (JSOC) - Science Data Processing (Scherrer et
  al. 2011) at Stanford University. In this paper, we briefly describe the
  data products, and demonstrate the performance of the HMI instrument. We
  conclude that the HMI is working extremely well.

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Title: An Improved 3D Radiative-MHD Model of the Convection
    Zone-to-Corona System
Authors: Abbett, William P.; Bercik, D. J.; Kazachenko, M.
2012AAS...22020507A    Altcode:
  We present the latest results from an improved radiative-MHD model of
  the convection zone-to-corona system. The numerical methods of the
  RADMHD model of Abbett &amp; Fisher (2012) have been significantly
  updated so that the underlying finite volume scheme is (1) no longer
  dimensionally split along coordinate axes; (2) of much higher order
  accuracy using a three-dimensional 27-point stencil; and (3) capable
  of performing much larger scale calculations in both spherical polar
  coordinates and Cartesian coordinates. We will describe the improvements
  of the underlying scheme in detail, present a 3D dynamic convection
  zone-to-corona quiet Sun model using the new formalism, and compare
  the latest results with previous models.

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Title: Generation of electric currents via neutral-ion drag in the
    chromosphere and ionosphere
Authors: Krasnoselskikh, V.; Abbett, W. P.; Hudson, H.; Vekstein,
   G.; Bale, S. D.
2012AIPC.1439...42K    Altcode:
  We consider the generation of electric currents in the solar
  chromosphere. The ionization level in this region is generally supposed
  to be low. We show that the ambient electrons are magnetized even
  for weak magnetic fields (30 G), i.e. their gyrofrequency is larger
  than the collision frequency; ion motions continue to be dominated by
  ion-neutral collisions in this region. Under such conditions the ions
  are dragged by neutrals. As a result, the dynamics of magnetic field
  resembles frozen-in motion of the field with the neutral gas. On the
  other hand magnetized electrons drift under the action of the electric
  and magnetic fields induced in the reference frame of ions moving with
  the neutral gas. This relative motion of electrons and ions results in
  the generation of quite intense electric currents. The dissipation of
  these currents leads to the resistive electron heating and efficient
  gas ionization. Ionization by electron-neutral impact does not alter
  the dynamics of the heavy particles; thus the gas turbulent motions
  persist even when the plasma becomes fully ionized and the resistive
  current dissipation continues to heat electrons and ions. This heating
  process is so efficient that it can result in typical temperature
  increases with altitude as large as 0.1-0.3 eV/km. We conclude that this
  process can play a major role in the heating of the chromosphere and
  corona. We show that the physical conditions in the solar chromosphere,
  in particular the neutral and ion density dependencies upon altitude,
  are very similar to those in the lower ionosphere of the Earth. A
  very similar process of current generation occurs in the ionosphere
  after strong earthquakes, resulting in the generation of strong
  perturbations in the ionosphere. We then present well-known results of
  the observations of such perturbations, which allow an evaluation of the
  increment of the growth of the perturbations with altitude, making use
  of ionospheric sounding. These results are in perfect agreement with
  estimates obtained making use a model similar to ours. We consider
  that these observations clearly show the efficiency of the physical
  mechanisms discussed, and thus provide strong support for our ideas.

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Title: The Impact of the Chromosphere on Numerical Models of the
    Convection Zone-to-Corona System
Authors: Abbett, W. P.
2012decs.confE..52A    Altcode:
  This review will provide an overview of recent progress toward
  simulating the magnetic and energetic connection between the convection
  zone and corona with a particular emphasis on the effect of the
  chromosphere on the coupled system. We will discuss the challenges
  inherent in modeling the dynamics and energetics of the chromosphere,
  then review what 3D MHD simulations of the atmosphere are able to tell
  us about about the transport of magnetic flux and energy from below the
  visible surface into the low atmosphere and corona. We will explore how
  the dynamic chromosphere affects the structure and non-potentiality
  of the overlying coronal field, and what implications this may have
  to force-free models based on photospheric magnetograms.

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Title: Momentum Distribution in Solar Flare Processes
Authors: Hudson, H. S.; Fletcher, L.; Fisher, G. H.; Abbett, W. P.;
   Russell, A.
2012SoPh..277...77H    Altcode:
  We discuss the consequences of momentum conservation in processes
  related to solar flares and coronal mass ejections (CMEs), in particular
  describing the relative importance of vertical impulses that could
  contribute to the excitation of seismic waves ("sunquakes"). The
  initial impulse associated with the primary flare energy transport
  in the impulsive phase contains sufficient momentum, as do the
  impulses associated with the acceleration of the evaporation flow (the
  chromospheric shock) or the CME itself. We note that the deceleration
  of the evaporative flow, as coronal closed fields arrest it, will tend
  to produce an opposite impulse, reducing the energy coupling into
  the interior. The actual mechanism of the coupling remains unclear
  at present.

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Title: Electric Fields and Poynting Fluxes from Vector Magnetograms
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2012decs.confE..75F    Altcode:
  The availability of vector-magnetogram sequences with sufficient
  accuracy and cadence to estimate the temporal derivative of the magnetic
  field allows us to use Faraday's law to find an approximate solution
  for the electric field in the photosphere, using a Poloidal-Toroidal
  Decomposition (PTD) of the magnetic field and its partial time
  derivative. Without additional information, however, the electric
  field found from this technique is under-determined - Faraday's law
  provides no information about the electric field that can be derived
  from the gradient of a scalar potential. Here, we show how additional
  information in the form of line-of-sight Doppler-flow measurements,
  and motions transverse to the line-of-sight determined with ad-hoc
  methods such as local correlation tracking, can be combined with the
  PTD solutions to provide much more accurate solutions for the solar
  electric field, and therefore the Poynting flux of electromagnetic
  energy in the solar photosphere. Reliable, accurate maps of the Poynting
  flux are essential for quantitative studies of the buildup of magnetic
  energy before flares and coronal mass ejections.

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Title: Radiative Cooling in MHD Models of the Quiet Sun Convection
    Zone and Corona
Authors: Abbett, W. P.; Fisher, G. H.
2012SoPh..277....3A    Altcode: 2011arXiv1102.1035A
  We present a series of numerical simulations of the quiet-Sun plasma
  threaded by magnetic fields that extend from the upper convection zone
  into the low corona. We discuss an efficient, simplified approximation
  to the physics of optically thick radiative transport through the
  surface layers, and investigate the effects of convective turbulence
  on the magnetic structure of the Sun's atmosphere in an initially
  unipolar (open field) region. We find that the net Poynting flux below
  the surface is on average directed toward the interior, while in the
  photosphere and chromosphere the net flow of electromagnetic energy is
  outward into the solar corona. Overturning convective motions between
  these layers driven by rapid radiative cooling appears to be the source
  of energy for the oppositely directed fluxes of electromagnetic energy.

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Title: Can We Determine Electric Fields and Poynting Fluxes from
    Vector Magnetograms and Doppler Measurements?
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2012SoPh..277..153F    Altcode: 2011arXiv1101.4086F
  The availability of vector-magnetogram sequences with sufficient
  accuracy and cadence to estimate the temporal derivative of the magnetic
  field allows us to use Faraday's law to find an approximate solution
  for the electric field in the photosphere, using a Poloidal-Toroidal
  Decomposition (PTD) of the magnetic field and its partial time
  derivative. Without additional information, however, the electric
  field found from this technique is under-determined - Faraday's law
  provides no information about the electric field that can be derived
  from the gradient of a scalar potential. Here, we show how additional
  information in the form of line-of-sight Doppler-flow measurements,
  and motions transverse to the line-of-sight determined with ad-hoc
  methods such as local correlation tracking, can be combined with the
  PTD solutions to provide much more accurate solutions for the solar
  electric field, and therefore the Poynting flux of electromagnetic
  energy in the solar photosphere. Reliable, accurate maps of the Poynting
  flux are essential for quantitative studies of the buildup of magnetic
  energy before flares and coronal mass ejections.

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Title: Dynamic Coupling of Convective Flows and Magnetic Field during
    Flux Emergence
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
   der Holst, Bart
2012ApJ...745...37F    Altcode: 2011arXiv1111.1679F
  We simulate the buoyant rise of a magnetic flux rope from the solar
  convection zone into the corona to better understand the energetic
  coupling of the solar interior to the corona. The magnetohydrodynamic
  model addresses the physics of radiative cooling, coronal heating,
  and ionization, which allow us to produce a more realistic model
  of the solar atmosphere. The simulation illustrates the process by
  which magnetic flux emerges at the photosphere and coalesces to form
  two large concentrations of opposite polarities. We find that the
  large-scale convective motion in the convection zone is critical to
  form and maintain sunspots, while the horizontal converging flows
  in the near-surface layer prevent the concentrated polarities from
  separating. The footpoints of the sunspots in the convection zone
  exhibit a coherent rotation motion, resulting in the increasing
  helicity of the coronal field. Here, the local configuration of the
  convection causes the convergence of opposite polarities of magnetic
  flux with a shearing flow along the polarity inversion line. During
  the rising of the flux rope, the magnetic energy is first injected
  through the photosphere by the emergence, followed by energy transport
  by horizontal flows, after which the energy is subducted back to the
  convection zone by the submerging flows.

---------------------------------------------------------
Title: Can we Determine Electric Fields and Poynting Fluxes from
    Vector Magnetograms and Doppler Measurements?
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2011AGUFMSH33C..07F    Altcode:
  The availability of vector magnetogram sequences with sufficient
  accuracy and cadence to estimate the time derivative of the magnetic
  field allows us to use Faraday's law to find an approximate solution
  for the electric field in the photosphere, using a Poloidal-Toroidal
  Decomposition (PTD) of the magnetic field and its partial time
  derivative. Without additional information, however, the electric
  field found from this technique is under-determined -- Faraday's law
  provides no information about the electric field that can be derived the
  gradient of a scalar potential. Here, we show how additional information
  in the form of line-of-sight Doppler flow measurements, and motions
  transverse to the line-of-sight determined with ad-hoc methods such as
  local correlation tracking, can be combined with the PTD solutions to
  provide much more accurate solutions for the solar electric field,
  and therefore the Poynting flux of electromagnetic energy in the
  solar photosphere. Reliable, accurate maps of the Poynting flux are
  essential for quantitative studies of the buildup of magnetic energy
  before flares and coronal mass ejections. This work was supported by the
  NASA Heliophysics Theory Program, the NASA Living-With-a-Star Program,
  and the NSF Geosciences Directorate

---------------------------------------------------------
Title: The Effect of Subsurface Flows during Flux Emergence
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst, B.
2011AGUFMSH54A..06F    Altcode:
  Here we carry out magnetohydrodynamic simulations on the emergence of
  a buoyant magnetic flux rope through a realistic convection zone that
  extends 21 Mm below the photosphere and 21 Mm up into the corona, with
  solar thermodynamic processes taken into account. The total maximum
  magnetic flux at the photosphere reaches 6.85±10<SUP>20</SUP> Mx,
  of the same order of magnitude of solar pores. The main aim of the
  simulations is to study the mechanism of the energy and magnetic flux
  transfer during the interaction between the subphotospheric flows and
  the rising magnetic flux rope. The magnetic flux emerges as bipoles
  on the photospheric and subphotospheric layers, then the bipoles
  are quickly pulled apart by the horizontal flows and concentrate
  in downdrafts. The coalescence of the small-scale bipoles and
  convective collapse in the near surface layers form the large-scale
  concentrated magnetic flux, i.e. solar pores. The horizontal flow
  also exhibits a coherent pattern of rotation, which extends into the
  convection zone. Vertical flow in the convection zone pushes down
  the endpoints of the flux rope and maintains the bipolar pores during
  the emergence. Analysis of the Poynting energy fluxes associated with
  vertical and horizontal flows shows that horizontal flow is the main
  contributor to the energy transfer from the convection into the corona,
  with a value of 6.78±10<SUP>31</SUP> ergs at the photosphere within
  8 hours.

---------------------------------------------------------
Title: Roles for Data Assimilation in Studying Solar Flares &amp; CMEs
Authors: Welsch, B. T.; Abbett, W. P.; Fisher, G. H.
2011AGUFMSH54A..05W    Altcode:
  Solar flares and coronal mass ejections (CMEs) are driven by the
  sudden release of free magnetic energy stored in electric currents
  the solar corona. While there is a consensus that free energy enters
  the corona from the solar interior, there is ongoing debate about the
  physical processes primarily responsible for transporting free energy
  into the corona and / or triggering its release once there. Since
  direct measurements of the coronal vector magnetic field, necessary
  to quantify coronal currents, are currently not feasible, it is hoped
  that modeling of the coronal field can improve our understanding
  of processes that drive the corona to flare and produce CMEs. Many
  coronal modeling efforts employ spectropolarimetric observations
  of the photosphere, which can be used to infer magnetic fields and
  flows there; the model then relates these photospheric measurements
  to coronal currents. Observations of coronal emission structures
  might also usefully inform coronal field models. Here, I will discuss
  different approaches to modeling the coronal magnetic field, using
  both photospheric and other data sets, and possible roles for data
  assimilation.

---------------------------------------------------------
Title: Observational Analysis of Photospheric Magnetic Field
    Restructuring During Energetic Solar Flares
Authors: Alvarado, J. D.; Buitrago, J. C.; Martinez Oliveros, J.;
   Lindsey, C. A.; Abbett, W. P.; Fisher, G. H.
2011AGUFMSH13B1944A    Altcode:
  The magnetic field has proven to be the main driver in the behavior,
  dynamics and evolution of several solar atmospheric phenomena including
  sunspots, plages, faculae, CME's and flares. Observational evidence of
  photospheric magnetic field restructuring during energetic flares have
  shown an enhancement of the transversal field component suggesting
  an apparent relation between this process with the generation of
  “sunquakes”, expanding ripples on the solar photosphere as a
  result of the momentum-energy transfer into the solar photosphere
  and subphotosphere. In this work we present a doppler and magnetic
  observational study of some recent energetic flaring events (X and
  M type of the 24th solar cycle) trying to find possible acoustic
  signatures and make a characterization of the photospheric magnetic
  field evolution during those flares, being this the observational
  basis of a future numerical modeling of the field restructuring during
  this phenomenon.

---------------------------------------------------------
Title: Coupling of Convective Flows and Emerging Magnetic Fields
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
   der Holst, Bart
2011sdmi.confE..31F    Altcode:
  We carry out radiative MHD simulations on the rising process of a
  buoyant magnetic flux rope inside the convection zone and its further
  emergence into the upper atmosphere. Our model takes into account
  of the radiative cooling, coronal heating and the ionization. The
  emergence of the flux rope is accompanied by turbulent surrounding
  plasma flows. Analysis on the magnetic fluxes shows that the convective
  downflows play an important role in formation of the concentrated
  polarities in the convection zone. During the rising of the flux
  rope, the magnetic energy is first injected through the photosphere
  by the emergence, followed by energy transport by horizontal flows,
  after which the energy is subducted back to the convection zone by
  the submerging flows.

---------------------------------------------------------
Title: Radiative Cooling in MHD Models of the Quiet Sun Convection
    Zone and Corona
Authors: Abbett, William; Fisher, George
2011shin.confE..10A    Altcode:
  We present a series of numerical simulations of the quiet Sun plasma
  threaded by magnetic fields that extend from the upper convection zone
  into the low corona. We discuss an efficient, simplified approximation
  to the physics of optically thick radiative transport through the
  surface layers, and investigate the effects of convective turbulence
  on the magnetic structure of the Sun's atmosphere in an initially
  unipolar (open field) region. We find that the net Poynting flux below
  the surface is on average directed toward the interior, while in the
  photosphere and chromosphere the net flow of electromagnetic energy is
  outward into the solar corona. Overturning convective motions between
  these layers driven by rapid radiative cooling appears to be the source
  of energy for the oppositely directed fluxes of electromagnetic energy.

---------------------------------------------------------
Title: Modeling the Physical Connection Between the Solar Convection
    Zone and Corona
Authors: Abbett, William P.
2011SPD....42.0101A    Altcode: 2011BAAS..43S.0101A
  How magnetic energy and flux emerges from below the surface into the
  solar atmosphere is a topic ripe for theoretical and observational
  investigation, particularly in the SDO era. Data from this mission
  is showing us that magnetic fields from the interior emerge through
  the surface, and energize the dynamic chromosphere and corona over
  a wide range of spatial and temporal scales. The interplay between
  granular-scale magnetic features, and large-scale structures from
  decaying active regions, for example, are seen to affect the large-scale
  solar magnetic field in complex ways. Being able to model these
  interactions in a way that captures the disparate spatial and temporal
  scales of the convection zone-to-corona system while simultaneously
  allowing for direct comparison with observations would be of enormous
  value in the effort to better understand the physics of coronal heating,
  the energetics of the solar wind, and the onset of magnetic eruptions
  (among other phenomena). In this lecture, I will summarize current
  progress in the effort to model the magnetic and energetic connection
  between the solar interior and atmosphere, and will describe the
  limitations and challenges inherent to this holistic approach.

---------------------------------------------------------
Title: Can We Determine Electric Fields and Poynting Fluxes from
    Vector Magnetograms and Doppler Measurements?
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.
2011SPD....42.1717F    Altcode: 2011BAAS..43S.1717F
  The availability of vector-magnetogram sequences with sufficient
  accuracy and cadence to estimate the temporal derivative of the magnetic
  field allows us to use Faraday's law to find an approximate solution
  for the electric field in the photosphere, using a Poloidal--Toroidal
  Decomposition (PTD) of the magnetic field and its partial time
  derivative. Without additional information, however, the electric
  field found from this technique is under-determined -- Faraday's law
  provides no information about the electric field that can be derived
  the gradient of a scalar potential. Here, we show how additional
  information in the form of line-of-sight Doppler-flow measurements,
  and motions transverse to the line-of-sight determined with ad-hoc
  methods such as local correlation tracking, can be combined with the
  PTD solutions to provide much more accurate solutions for the solar
  electric field, and therefore the Poynting flux of electromagnetic
  energy in the solar photosphere. Reliable, accurate maps of the Poynting
  flux are essential for quantitative studies of the buildup of magnetic
  energy before flares and coronal mass ejections.

---------------------------------------------------------
Title: Simulation of Flux Emergence in Solar Active Regions
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst,
   B.; Schrijver, C. J.
2010AGUFMSH31A1781F    Altcode:
  We present results of magnetohydrodynamic (MHD) simulations of
  magnetic flux emergence from the convection zone into the solar
  corona using BATSRUS. The MHD equations are modified to take account
  of the radiative terms, coronal heating and heat conduction. The
  implementation of non-ideal equation of state describes the partially
  ionized plasma in the convection zone. The simulations are carried out
  on a domain of active-region size of 30×30×40 Mm3, extending 20 Mm
  down into the convection zone. The magnetic fields are coupled with
  the convective motion during the emerging process, and concentrates
  in the downflow regions. A coherent shear pattern is formed in the
  lower corona during the rising. We also compare our model results
  at the photosphere with SDO/HMI vector magnetograms and illustrate
  the mechanism of flux emergence that give rise to complexity of the
  structures in active regions.

---------------------------------------------------------
Title: Generation of electric currents in the chromosphere via
    neutral-ion drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S.;
   Abbett, W. P.
2010AGUFMSH31C1810K    Altcode:
  We consider the generation of electric currents in the solar
  chromosphere. The ionization level in this region is generally supposed
  to be low. We show that the ambient electrons become magnetized even for
  weak magnetic fields (30 G), i.e. their gyrofrequency becomes larger
  than the collision frequency; ion motions continue to be dominated by
  ion-neutral collisions in this region. Under such conditions the ions
  are dragged by neutrals and magnetic field dynamics resembles frozen-in
  motion of the field with the neutral gas. On the other hand magnetized
  electrons drift under the action of the electric and magnetic fields
  induced in the reference frame of ions moving with the neutral gas. This
  relative motion of electrons and ions results in the generation of quite
  intense electric currents. The dissipation of these currents leads to
  the resistive electron heating and efficient gas ionization. Ionization
  by electron-neutral impact does not alter the dynamics of the heavy
  particles; thus the gas turbulent motions continue even when the plasma
  becomes fully ionized and the resistive current dissipation continues
  to heat electrons and ions. This heating process is so efficient that
  it can result in typical temperature increases with altitude as large
  as 0.1-0.3 eV/km. We conclude that this process can play a major role
  in the heating of the chromosphere and corona.

---------------------------------------------------------
Title: Generation of Electric Currents in the Chromosphere via
    Neutral-Ion Drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S. D.;
   Abbett, W. P.
2010ApJ...724.1542K    Altcode: 2010arXiv1011.5834K
  We consider the generation of electric currents in the solar
  chromosphere where the ionization level is typically low. We show that
  ambient electrons become magnetized even for weak magnetic fields (30
  G); that is, their gyrofrequency becomes larger than the collision
  frequency while ion motions continue to be dominated by ion-neutral
  collisions. Under such conditions, ions are dragged by neutrals,
  and the magnetic field acts as if it is frozen-in to the dynamics of
  the neutral gas. However, magnetized electrons drift under the action
  of the electric and magnetic fields induced in the reference frame of
  ions moving with the neutral gas. We find that this relative motion of
  electrons and ions results in the generation of quite intense electric
  currents. The dissipation of these currents leads to resistive electron
  heating and efficient gas ionization. Ionization by electron-neutral
  impact does not alter the dynamics of the heavy particles; thus, the
  gas turbulent motions continue even when the plasma becomes fully
  ionized, and resistive dissipation continues to heat electrons and
  ions. This heating process is so efficient that it can result in
  typical temperature increases with altitude as large as 0.1-0.3 eV
  km<SUP>-1</SUP>. We conclude that this process can play a major role
  in the heating of the chromosphere and corona.

---------------------------------------------------------
Title: A Simplified Treatment of Radiative Transfer in Large-scale
    Convection Zone-to-Corona Models
Authors: Abbett, William P.; Fisher, G. H.
2010shin.confE...6A    Altcode:
  We present the latest in a series of numerical simulations of quiet
  Sun magnetic fields that extend from the upper convection zone into
  the low corona. We apply an efficient, simplified treatment of the
  physics of optically-thick radiative transfer throughout the surface
  layers, and investigate the effects of convective turbulence on the
  magnetic structure of the Sun's upper atmosphere in an initially
  unipolar (open-field) region. We then compare these results with
  earlier simulations that use an ad-hoc, parameterized treatment of
  surface cooling.

---------------------------------------------------------
Title: Assimilating Measurements of the Photospheric Magnetic Field
    into MHD Models of the Solar Atmosphere
Authors: Abbett, William P.; Fisher, G. H.; Welsch, B. T.; Bercik,
   D. J.
2010AAS...21640502A    Altcode: 2010BAAS...41..889A
  We introduce a rudimentary assimilative technique that allows a time
  series of vector magnetograms to be directly incorporated into the
  active cells of a RADMHD model of the solar atmosphere. We apply this
  technique to a simplified large-scale model of NOAA AR-8210, a flare
  and CME-producing active region. We begin by relaxing an initial
  magnetic configuration based on the first in a series of IVM vector
  magnetograms from the Mees Solar Observatory at Haleakala HI. This
  low-beta, near force-free configuration is achieved by solving the
  MHD system in the presence of a time-dependent artificial damping
  that is reduced as the configuration relaxes. Once the magnetic
  and thermodynamic initial state is achieved, we advance the system
  using our assimilative technique applied to a 4 hour sequence of IVM
  magnetograms. In addition, we present a simple 3D MHD simulation of
  the response of the initial AR-8210 pre-flare model corona to flare
  energy deposited in the upper chromosphere near a sheared neutral line.

---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2010AAS...21640113F    Altcode: 2010BAAS...41..859F
  Determining the electric 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
  electric field also provides valuable input for data-driven models of
  the solar atmosphere and the Sun-Earth system. We show how observed
  vector magnetogram time series can be used to estimate the photospheric
  electric field. Our method uses a "poloidal-toroidal decomposition"
  (PTD) of the time derivative of the vector magnetic field. These
  solutions provide an electric field whose curl obeys all three
  components of Faraday's Law. The PTD solutions are not unique; the
  gradient of a scalar potential can be added to the PTD electric field
  without affecting consistency with Faraday's Law. We then present an
  iterative technique to determine a potential function consistent with
  ideal MHD evolution; but this 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 electric field,
  a generalization of Longcope's "Minimum Energy Fit". The PTD technique,
  the iterative technique, and the variational technique are used to
  estimate electric fields from a pair of synthetic vector magnetograms
  taken from an MHD simulation; and these fields are compared with the
  simulation's known electric fields. The PTD and iteration techniques
  compare favorably to results from existing velocity inversion
  techniques. 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.

---------------------------------------------------------
Title: Determing Flow Fields Consistent with Vector Magnetic Evolution
Authors: Welsch, Brian; Fisher, G. H.; Abbett, W. P.; Bercik, D. J.
2010AAS...21640112W    Altcode: 2010BAAS...41..858W
  Sequences of photospheric vector magnetograms can be used to drive
  time-dependent models of magnetic evolution in the overlying atmosphere,
  as well as to investigate dynamics in the atmospheric layer imaged
  in the magnetograms. While several methods of estimating electric
  fields consistent with the observed evolution of the magnetic field
  normal to the magnetogram surface have been developed, these do not
  explicitly employ evolution of the horizontal field components in
  deriving electric fields. The recently developed poloidal- toroidal
  decomposition (PTD) method (Fisher et al. 2010) does use this extra
  information; PTD electric fields, however, are generally not ideal,
  so ideality must be imposed post facto. Here, we present formalism
  for deriving ideal electric fields consistent with vector magnetic
  evolution, assuming that the induction equation in the MHD approximation
  governs the magnetic evolution; accordingly, we term the approach
  "inductive vector driving", or IVD. This formalism can incorporate
  explicit resistive terms. Moreover, IVD allows direct inclusion of
  results from tracking methods, which can provide additional information
  regarding photospheric evolution. This work is supported by NASA's
  Heliophysics Theory Program and NSF's SHINE program.

---------------------------------------------------------
Title: Simulation of Flux Emergence from the Convection Zone to
    the Corona
Authors: Fang, Fang; Manchester, Ward; Abbett, William P.; van der
   Holst, Bart
2010ApJ...714.1649F    Altcode: 2010arXiv1003.6118F
  Here, we present numerical simulations of magnetic flux buoyantly
  rising from a granular convection zone into the low corona. We study
  the complex interaction of the magnetic field with the turbulent
  plasma. The model includes the radiative loss terms, non-ideal
  equations of state, and empirical corona heating. We find that
  the convection plays a crucial role in shaping the morphology and
  evolution of the emerging structure. The emergence of magnetic fields
  can disrupt the convection pattern as the field strength increases,
  and form an ephemeral region-like structure, while weak magnetic flux
  emerges and quickly becomes concentrated in the intergranular lanes,
  i.e., downflow regions. As the flux rises, a coherent shear pattern
  in the low corona is observed in the simulation. In the photosphere,
  both magnetic shearing and velocity shearing occur at a very sharp
  polarity inversion line. In a case of U-loop magnetic field structure,
  the field above the surface is highly sheared while below it is relaxed.

---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2010ApJ...715..242F    Altcode: 2009arXiv0912.4916F
  Determining the electric 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
  electric field also provides valuable input for data-driven models of
  the solar atmosphere and the Sun-Earth system. We show how observed
  vector magnetogram time series can be used to estimate the photospheric
  electric field. Our method uses a "poloidal-toroidal decomposition"
  (PTD) of the time derivative of the vector magnetic field. These
  solutions provide an electric field whose curl obeys all three
  components of Faraday's Law. The PTD solutions are not unique; the
  gradient of a scalar potential can be added to the PTD electric field
  without affecting consistency with Faraday's Law. We then present an
  iterative technique to determine a potential function consistent with
  ideal MHD evolution; but this 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 electric field,
  a generalization of Longcope's "Minimum Energy Fit." The PTD technique,
  the iterative technique, and the variational technique are used to
  estimate electric fields from a pair of synthetic vector magnetograms
  taken from an MHD simulation; and these fields are compared with the
  simulation's known electric fields. The PTD and iteration techniques
  compare favorably to results from existing velocity inversion
  techniques. 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. Careful examination of the results from all
  three methods indicates that evolution of the magnetic vector by itself
  does not provide enough information to determine the true electric field
  in the photosphere. Either more information from other measurements,
  or physical constraints other than those considered here are necessary
  to find the true electric field. However, we show it is possible to
  construct physically reasonable electric field distributions whose curl
  matches the evolution of all three components of B. We also show that
  the horizontal and vertical Poynting flux patterns derived from the
  three techniques are similar to one another for the cases investigated.

---------------------------------------------------------
Title: Improving large-scale convection-zone-to-corona models.
Authors: Abbett, W. P.; Fisher, G. H.
2010MmSAI..81..721A    Altcode: 2010arXiv1005.0641A
  We introduce two new methods that are designed to improve the realism
  and utility of large, active region-scale 3D MHD models of the solar
  atmosphere. We apply these methods to RADMHD, a code capable of modeling
  the Sun's upper convection zone, photosphere, chromosphere, transition
  region, and corona within a single computational volume. We first
  present a way to approximate the physics of optically-thick radiative
  transfer without having to take the computationally expensive step
  of solving the radiative transfer equation in detail. We then briefly
  describe a rudimentary assimilative technique that allows a time series
  of vector magnetograms to be directly incorporated into the MHD system.

---------------------------------------------------------
Title: 3D simulation of flux emergence from convective zone to corona
    with BATSRUS
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst, B.
2009AGUFMSH41B1656F    Altcode:
  To study the interaction between magnetic field and convective
  motion, we present a 3d simulation of the emergence of magnetic flux
  ropes from the convective zone into the corona, applying radiation
  terms and non-ideal equation of state table to BATSRUS code. To
  perform this simulation, we first generate a solar atmosphere,
  whose physical properties are comparable with Bercik(2002) data,
  with a turbulent convective zone. The upgoing convective motion is
  cooled down and stopped by the surface loss and sharp temperature
  decrease at photosphere. The magnetic flux is observed to concentrate
  at the intergranular lanes with downflowing plasma and decrease in
  the granules. We then heat up the corona to temperature of above 1MK,
  using an empirical relationship between heating and unsigned magnetic
  flux. In the high-temperature, low-density upper atmosphere, radiative
  loss term is approximated with optically thin limit and the radiative
  cooling curve is obtained from CHIANTI database. The Field aligned heat
  conduction is applied to channel heat flux along the magnetic field
  lines in corona and form a more realistic transition region. After
  producing a superadiabatic atmosphere matching the observed properties,
  we introduce a buoyant magnetic flux rope below the photosphere. The
  flux rope shows shear flow with velocity of 8km/s at the photosphere
  where it emerges. We then compare our results with previous simulations
  without convection (Manchester et al. 2004).

---------------------------------------------------------
Title: Incorporating Magnetogram Data into Time-Dependent Coronal
    Field Models
Authors: Fisher, G. H.; Abbett, W. P.; Bercik, D. J.; McTiernan,
   J. M.; Welsch, B. T.
2009AGUFMSM51A1340F    Altcode:
  We briefly review our efforts to incorporate sequences of photospheric
  vector magnetograms into MHD simulations of coronal evolution, in an
  effort to create data-driven models of the coronal magnetic field. Such
  models should improve our understanding of flares and coronal mass
  ejections (CMEs), and might eventually lead to predictive capabilities.

---------------------------------------------------------
Title: Estimating Electric Fields from Sequences of Vector
    Magnetograms
Authors: Fisher, George H.; Welsch, Brian T.; Abbett, William P.;
   Bercik, David J.
2009shin.confE..10F    Altcode:
  We describe a new technique for estimating the three-dimensional vector
  electric field in the solar atmosphere by using a time-sequence
  of vector magnetograms to find an electric field distribution
  that obeys all 3 components of Faraday's law. The technique uses a
  "poloidal-toroidal" decomposition (PTD) to describe the electric field
  in terms of two scalar functions. The "inductive" PTD solutions to
  Faraday's Law are not unique, however, since additional contributions to
  the electric field from a potential function have no effect on Faraday's
  law. <P />We then describe how estimates for the total electric field
  including both the inductive and potential components can be made
  by using variational techniques. The variational approach we develop
  is similar to Longcope's "Minimum Energy Fit" technique, in that the
  electric field obeys the vertical component of the magnetic induction
  equation, while also minimizing a positive definite functional. The
  purely potential part of the electric field can then be recovered by
  subtracting the PTD electric field from the total field.

---------------------------------------------------------
Title: The Dynamic Evolution of Quiet Sun Magnetic Fields
Authors: Abbett, William P.; Fisher, G. H.
2009SPD....40.0903A    Altcode:
  We present the latest in a series of numerical simulations of
  quiet Sun magnetic fields. The upper convection zone, photosphere,
  chromosphere, transition region, and corona are all included within a
  single computational domain that is sufficiently large to encompass
  a typical active region. We introduce a simplified treatment of the
  physics of optically-thick radiative transfer throughout the surface
  layers, and compare these results with an earlier, non-physics based
  parameterized treatment of radiative cooling.

---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2009SPD....40.0605F    Altcode:
  We describe a new technique for estimating the three-dimensional vector
  electric field in the solar atmosphere by using a time-sequence
  of vector magnetograms to find an electric field distribution
  that obeys all 3 components of Faraday's law. The technique uses a
  “poloidal-toroidal” decomposition (PTD) to describe the electric field
  in terms of two scalar functions. The “inductive” PTD solutions to
  Faraday's Law are not unique, however, since additional contributions to
  the electric field from a potential function have no effect on Faraday's
  law. <P />We then describe how estimates for the total electric field
  including both the inductive and potential components can be made by
  using variational techniques. The variational approach we develop is
  similar to Longcope's “Minimum Energy Fit” technique, in that the
  electric field obeys the vertical component of the magnetic induction
  equation, while also minimizing a positive definite functional. The
  purely potential part of the electric field can then be recovered by
  subtracting the PTD electric field from the total field.

---------------------------------------------------------
Title: Erratum: "Tests and Comparisons of Velocity-Inversion
    Techniques" (ApJ, 670, 1434 [2007])
Authors: Welsch, B. T.; Abbett, W. P.; DeRosa, M. L.; Fisher, G. H.;
   Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
   P. W.
2008ApJ...680..827W    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Using Ideal Electric Fields Estimated from Vector Magnetogram
    Sequences to Drive Coronal MHD Simulations
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.; Bercik, D. J.
2008AGUSMSH54A..04W    Altcode:
  Dynamic models of the coronal magnetic field show promise as space
  weather forecasting tools. Such models should be driven by electric
  fields derived from sequences of photospheric vector magnetograms,
  the only routine measurements of the solar magnetic field currently
  available. Previous studies derived flows --- or, equivalently, ideal
  electric fields --- consistent with evolution of the normal photospheric
  field, which could be used in "component driving" of an MHD model,
  i.e., enforcing consistent evolution of the observed and modeled normal
  magnetic fields. In this extension of the component-driving approach,
  we demonstrate how to derive ideal electric fields consistent with the
  observed evolution of both the normal and horizontal magnetic field,
  useful for "vector driving," i.e., enforcing consistency between all
  three components of the observed and model photospheric magnetic
  vectors. To drive an MHD model, this "ideal vector driving" (IVD)
  approach amount to specification of both the velocity (perpendicular
  the magnetic field) and its vertical derivative at the model's bottom
  boundary. The IVD method can incorporate results from local/ tracking
  methods (e.g., LCT or DAVE) and/or results from global methods (e.g.,
  MEF or poloidal-toroidal decomposition [PTD]). We have applied this new
  approach to "synthetic magnetograms" extracted from MHD simulations
  (where the magnetic and electric fields are exactly known), as well
  as to a four-hour sequence of vector magnetograms from NOAA AR 8210,
  on 01 May 1998, just prior to an M-class flare and geoeffective CME.

---------------------------------------------------------
Title: The Dynamic Evolution of Active Region Magnetic Fields in
    the Solar Atmosphere
Authors: Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Bercik, D. J.
2008AGUSMSH31A..08A    Altcode:
  We present the latest results from a series of three-dimensional MHD
  simulations of active region magnetic fields. The computational domain
  extends from the upper convection zone out into the corona, and includes
  the highly-stratified layers of the photosphere, chromosphere, and
  transition region. We characterize the effects of convective turbulence
  on large-scale magnetic structures, the magnetic connectivity between
  sub-surface and coronal fields, and the energetics of the low atmosphere
  and corona.

---------------------------------------------------------
Title: Inferring Photospheric Velocity Fields Using a Combination of
    Minimum Energy Fit, Local Correlation Tracking, and Doppler Velocity
Authors: Ravindra, B.; Longcope, D. W.; Abbett, W. P.
2008ApJ...677..751R    Altcode:
  The minimum energy fit (MEF), a velocity inversion technique, infers
  all components of the photospheric velocity that are consistent with
  the induction equation. From the set of consistent velocity fields,
  it selects the smallest overall flow speed by minimizing a kinetic
  energy functional. If partial velocity information is available from
  other measurements, it can be incorporated into the MEF methodology
  by minimizing the squared difference from that data. We incorporate
  the partial velocity information provided by local correlation
  tracking (LCT) technique and Doppler velocity measurements. We
  test the incorporation of these auxiliary velocity fields using the
  simulated magnetograms and velocitygrams. To the known velocity field
  we compare the results obtained from the MEF alone, the MEF with LCT
  constraints, and the MEF with LCT and Doppler information. We find
  that the combination of MEF with LCT and vertical velocity yields
  the best agreement. We also apply these three methods to actual vector
  magnetograms of AR 8210 obtained by the Imaging Vector Magnetograph. The
  results suggest that in this active region the helicity and energy
  fluxes are dominated by the horizontal rather than the vertical
  components of the velocity.

---------------------------------------------------------
Title: Connecting the Quiet-Sun Convection Zone and Corona
Authors: Abbett, W. P.
2008ASPC..383..327A    Altcode:
  We present the first results of a new numerical model designed to
  simultaneously evolve the upper convection zone and low-corona within
  a single computational domain. We characterize (1) the properties of
  a quiet-Sun model atmosphere that forms as a result of the action of a
  convective dynamo; (2) the efficacy of parameterized cooling as a means
  of approximating the physics of optically-thick radiative transfer in
  the model chromosphere; (3) the magnetic and thermodynamic properties
  of the quiet-Sun atmosphere, and the magnetic connectivity between
  the turbulent sub-surface layers and corona; and (4) the properties
  of horizontally-directed magnetic fields in the low atmosphere.

---------------------------------------------------------
Title: Tests and Comparisons of Velocity-Inversion Techniques
Authors: Welsch, B. T.; Abbett, W. P.; De Rosa, M. L.; Fisher, G. H.;
   Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
   P. W.
2007ApJ...670.1434W    Altcode:
  Recently, several methods that measure the velocity of magnetized
  plasma from time series of photospheric vector magnetograms have been
  developed. Velocity fields derived using such techniques can be used
  both to determine the fluxes of magnetic energy and helicity into the
  corona, which have important consequences for understanding solar
  flares, coronal mass ejections, and the solar dynamo, and to drive
  time-dependent numerical models of coronal magnetic fields. To date,
  these methods have not been rigorously tested against realistic,
  simulated data sets, in which the magnetic field evolution and
  velocities are known. Here we present the results of such tests
  using several velocity-inversion techniques applied to synthetic
  magnetogram data sets, generated from anelastic MHD simulations of
  the upper convection zone with the ANMHD code, in which the velocity
  field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and
  ILCT algorithms performed comparably in many categories. While DAVE
  estimated the magnitude and direction of velocities slightly more
  accurately than the other methods, MEF's estimates of the fluxes of
  magnetic energy and helicity were far more accurate than any other
  method's. Overall, therefore, the MEF algorithm performed best in
  tests using the ANMHD data set. We note that ANMHD data simulate
  fully relaxed convection in a high-β plasma, and therefore do not
  realistically model photospheric evolution.

---------------------------------------------------------
Title: The Magnetic Connection between the Convection Zone and Corona
    in the Quiet Sun
Authors: Abbett, W. P.
2007ApJ...665.1469A    Altcode:
  To understand the dynamic, magnetic, and energetic connection
  between the convectively unstable layers below the visible surface
  of the Sun and the overlying solar corona, we have developed
  a new three-dimensional magnetohydrodynamic code capable of
  simultaneously evolving a model convection zone and corona within a
  single computational volume. As a first application of this numerical
  model, we present a series of simulations of the quiet Sun in a domain
  that encompasses both the upper convection zone and low corona. We
  investigate whether the magnetic field generated by a convective
  surface dynamo can account for some of the observed properties of
  the quiet-Sun atmosphere. We find that (1) it is possible to heat
  a model corona to X-ray-emitting temperatures with the magnetic
  fields generated from a convective dynamo and an empirically based
  heating mechanism consistent with the observed relationship between
  X-ray emission and magnetic flux observed at the visible surface;
  (2) within the limitations of our numerical models of the quiet Sun,
  resistive and viscous dissipation alone are insufficient to maintain
  a hot corona; (3) the quiet-Sun model chromosphere is a dynamic,
  non-force-free layer that exhibits a temperature reversal in the
  convective pattern in the relatively low density layers above the
  photosphere; (4) the majority of the unsigned magnetic flux lies
  below the model photosphere in the convectively unstable portion of
  the domain; (5) horizontally directed magnetic structures thread the
  low atmosphere, often connecting relatively distant concentrations
  of magnetic flux observed at the surface; and (6) low-resolution
  photospheric magnetograms can significantly underestimate the amount
  of unsigned magnetic flux threading the quiet-Sun photosphere.

---------------------------------------------------------
Title: The Dynamic Evolution of Quiet Sun Magnetic Fields in the
    Solar Atmosphere
Authors: Abbett, William P.
2007AAS...210.9109A    Altcode: 2007BAAS...39..205A
  We present the latest results from a series of three-dimensional
  MHD simulations of the Quiet Sun magnetic field. The computational
  domain extends from the upper convection zone out into the corona, and
  includes the highly-stratified layers of the photosphere, chromosphere,
  and transition region. Our study focuses on the following questions:
  Can the magnetic field generated by a convective surface dynamo heat a
  model corona to soft X-ray emitting temperatures? Do physical processes
  such as resistive and viscous dissipation supply the necessary heating
  throughout the model atmosphere, or is an additional empirically-based
  heating mechanism required? What is the magnetic connection between
  the flux threading the model photosphere, and that filling the model
  corona? Can the magnetic field in the dynamic models be successfully
  reproduced by static extrapolations?

---------------------------------------------------------
Title: Active Region Magnetic Fields in the Solar Interior
Authors: Abbett, W. P.; Fisher, G. H.
2006ASPC..354..135A    Altcode:
  We present a brief review of recent efforts to understand the life-cycle
  of active region magnetic fields with an emphasis on what photospheric
  observations can tell us about the evolution of large-scale magnetic
  structures deep in the convective interior. A critical component of
  these efforts is to understand the dynamic connection between magnetic
  fields (at both large and small scales) observed threading the solar
  atmosphere and their sub-surface counterparts. We conclude our survey
  by presenting early results from a new numerical model capable of
  self-consistently incorporating both sub-photospheric layers and the
  low solar corona into a single computational domain.

---------------------------------------------------------
Title: Are Convective Dynamos Responsible for the Minimum X-ray
    Fluxes Observed in the Sun and Late-Type Main Sequence Stars?
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, C. M.; Abbett,
   W. P.; Lundquist, L. L.
2006ASPC..354..127B    Altcode:
  We extend the investigation of tet{Bercik05} to the case of a
  non-rotating solar-type reference star. Using three-dimensional
  numerical simulations of a turbulent dynamo driven by convection and
  the empirical relationship of tet{Pevtsov03}, we predict the level of
  X-ray emission from such a convective turbulent dynamo, and find that
  our results are consistent with quiet Sun observations. This implies
  that it is plausible that the Sun may have a rotation-independent
  convective dynamo working together with the large-scale dynamo believed
  to be responsible for the solar cycle.

---------------------------------------------------------
Title: Simulations of the Quiet Sun Magnetic Field: From the Upper
    Convection Zone into the Corona
Authors: Abbett, William P.
2006SPD....37.0701A    Altcode: 2006BAAS...38..227A
  We present the latest in a series of simulationsdesigned to directly
  investigate whether the magneticfield generated by a convective dynamo
  in the upperconvection zone can account for the observedproperties
  of the Quiet Sun magnetic field andatmosphere. The simulations are
  performed usinga new numerical code capable of evolving a modelsolar
  atmosphere that extends from the upper convectionzone into the
  low corona. This code is a parallel,semi-implicit solver capable
  of accomodating thespatial and temporal disparities intrinsic to
  thiscombined system.

---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Optical and Ultraviolet
    Emission from M Dwarf Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
   Carlsson, Mats
2006ApJ...644..484A    Altcode: 2006astro.ph..3195A
  We report on radiative hydrodynamic simulations of M dwarf stellar
  flares and compare the model predictions to observations of several
  flares. The flares were simulated by calculating the hydrodynamic
  response of a model M dwarf atmosphere to a beam of nonthermal
  electrons. Radiative back-warming through numerous soft X-ray,
  extreme-ultraviolet, and ultraviolet transitions are also included. The
  equations of radiative transfer and statistical equilibrium are treated
  in non-LTE for many transitions of hydrogen, helium, and the Ca II
  ion, allowing the calculation of detailed line profiles and continuum
  radiation. Two simulations were carried out, with electron beam fluxes
  corresponding to moderate and strong beam heating. In both cases we
  find that the dynamics can be naturally divided into two phases: an
  initial gentle phase in which hydrogen and helium radiate away much
  of the beam energy and an explosive phase characterized by large
  hydrodynamic waves. During the initial phase, lower chromospheric
  material is evaporated into higher regions of the atmosphere, causing
  many lines and continua to brighten dramatically. The He II 304 line
  is especially enhanced, becoming the brightest line in the flaring
  spectrum. The hydrogen Balmer lines also become much brighter and show
  very broad line widths, in agreement with observations. We compare
  our predicted Balmer decrements to decrements calculated for several
  flare observations and find the predictions to be in general agreement
  with the observations. During the explosive phase both condensation and
  evaporation waves are produced. The moderate flare simulation predicts
  a peak evaporation wave of ~130 km s<SUP>-1</SUP> and a condensation
  wave of ~30 km s<SUP>-1</SUP>. The velocity of the condensation wave
  matches velocities observed in several transition region lines. The
  optical continuum also greatly intensifies, reaching a peak increase
  of 130% (at 6000 Å) for the strong flare, but does not match observed
  white-light spectra.

---------------------------------------------------------
Title: Radiative Hydrodynamic Models of the Optical and Ultraviolet
    Emission from Solar Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
   Carlsson, Mats
2005ApJ...630..573A    Altcode: 2005astro.ph..7335A
  We report on radiative hydrodynamic simulations of moderate and strong
  solar flares. The flares were simulated by calculating the atmospheric
  response to a beam of nonthermal electrons injected at the apex of a
  one-dimensional closed coronal loop and include heating from thermal
  soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The
  equations of radiative transfer and statistical equilibrium were
  treated in non-LTE and solved for numerous transitions of hydrogen,
  helium, and Ca II, allowing the calculation of detailed line profiles
  and continuum emission. This work improves on previous simulations
  by incorporating more realistic nonthermal electron beam models and
  includes a more rigorous model of thermal XEUV heating. We find that
  XEUV back-warming contributes less than 10% of the heating, even in
  strong flares. The simulations show elevated coronal and transition
  region densities resulting in dramatic increases in line and continuum
  emission in both the UV and optical regions. The optical continuum
  reaches a peak increase of several percent, which is consistent with
  enhancements observed in solar white-light flares. For a moderate flare
  (~M class), the dynamics are characterized by a long gentle phase of
  near balance between flare heating and radiative cooling, followed
  by an explosive phase with beam heating dominating over cooling and
  characterized by strong hydrodynamic waves. For a strong flare (~X
  class), the gentle phase is much shorter, and we speculate that for even
  stronger flares the gentle phase may be essentially nonexistent. During
  the explosive phase, synthetic profiles for lines formed in the upper
  chromosphere and transition region show blueshifts corresponding to
  a plasma velocity of ~120 km s<SUP>-1</SUP>, and lines formed in the
  lower chromosphere show redshifts of ~40 km s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Convective Dynamos and the Minimum X-Ray Flux in Main-Sequence
    Stars
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, Christopher M.;
   Abbett, W. P.
2005ApJ...631..529B    Altcode: 2005astro.ph..6027B
  The objective of this paper is to investigate whether a convective
  dynamo can account quantitatively for the observed lower limit of X-ray
  surface flux in solar-type main-sequence stars. Our approach is to use
  three-dimensional numerical simulations of a turbulent dynamo driven
  by convection to characterize the dynamic behavior, magnetic field
  strengths, and filling factors in a nonrotating stratified medium and
  to predict these magnetic properties at the surface of cool stars. We
  use simple applications of stellar structure theory for the convective
  envelopes of main-sequence stars to scale our simulations to the
  outer layers of stars in the F0-M0 spectral range, which allows us
  to estimate the unsigned magnetic flux on the surface of nonrotating
  reference stars. We find agreement between our G0 star calculations
  and the observed unsigned magnetic flux density in the quiet Sun. With
  these magnetic flux estimates we use the recent results of Pevtsov
  et al. to predict the level of X-ray emission from such a turbulent
  dynamo and find that our results compare well with observed lower
  limits of surface X-ray flux. If we scale our predicted X-ray fluxes
  to Mg II fluxes, we also find good agreement with the observed lower
  limit of chromospheric emission in K dwarfs. This suggests that dynamo
  action from a convecting, nonrotating plasma is a viable alternative to
  acoustic heating models as an explanation for the basal emission level
  seen in chromospheric, transition-region, and coronal diagnostics from
  late-type stars.

---------------------------------------------------------
Title: Turbulent Dynamos and the Minimum X-ray Flux in Solar-Type
    Main Sequence Stars
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, C. M.; Abbett,
   W. P.
2005AGUSMSP43B..01B    Altcode:
  We investigate whether a small-scale turbulent dynamo can account
  quantitatively for the observed lower limit of X-ray surface flux in
  solar-type main sequence stars. Our approach is to use 3D numerical
  simulations of a turbulent dynamo driven by convection to characterize
  the dynamic behavior, magnetic field strengths, and filling factors in a
  non-rotating stratified medium, and to predict these magnetic properties
  at the surface of cool stars. We use simple applications of stellar
  structure theory for the convective envelopes of main-sequence stars
  to scale our simulations to the outer layers of stars in the F0--M0
  spectral range, which allows us to estimate the unsigned magnetic flux
  on the surface of non-rotating reference stars. With these estimates
  we use the observed magnetic flux--X-ray flux correlation of Pevtsov et
  al. (2003) to predict the level of X-ray emission from such a turbulent
  dynamo, and find that our results compare well with observed lower
  limits of surface X-ray flux. This suggests that dynamo action from a
  convecting, non-rotating plasma is a viable alternative to acoustic
  heating models as an explanation for the basal emission level seen
  in chromospheric, transition region, and coronal diagnostics from
  late-type stars.

---------------------------------------------------------
Title: 3D MHD Simulations of Magnetic Flux Emergence in Active Regions
Authors: Abbett, W. P.
2005AGUSMSP41A..11A    Altcode:
  We report on the progress of 3D simulations of active region magnetic
  flux emergence (and decay) through the stratified, sub-photospheric
  layers of the upper convection zone into the solar atmosphere and low
  corona. We use a recently-developed 3D semi-implicit MHD code (with a
  non-uniform, adaptive mesh) to address the inherent stiffness of the
  system of equations, and will compare our results with similar studies
  using second-order accurate, fully explicit numerical schemes.

---------------------------------------------------------
Title: The photospheric boundary of Sun-to-Earth coupled models
Authors: Abbett, W. P.; Mikić, Z.; Linker, J. A.; McTiernan, J. M.;
   Magara, T.; Fisher, G. H.
2004JASTP..66.1257A    Altcode: 2004JATP...66.1257A
  The least understood component of the Sun-to-Earth coupled system
  is the solar atmosphere—the visible layers of the Sun that
  encompass the photosphere, chromosphere, transition region and
  low corona. Coronal mass ejections (CMEs), principal drivers of
  space weather, are magnetically driven phenomena that are thought to
  originate in the low solar corona. Their initiation mechanism, however,
  is still a topic of great debate. If we are to develop physics-based
  models with true predictive capability, we must progress beyond
  simulations of highly idealized magnetic configurations, and develop
  the techniques necessary to incorporate observations of the vector
  magnetic field at the solar photosphere into numerical models of the
  solar corona. As a first step toward this goal, we drive the SAIC
  coronal model with the complex magnetic fields and flows that result
  from a sub-photospheric MHD simulation of an emerging active region. In
  particular, we successfully emerge a twisted Ω-loop into a pre-existing
  coronal arcade. <P />To date, it is not possible to directly measure
  the magnetic field in the solar corona. Instead, we must rely on
  non-potential extrapolations to generate the twisted, pre-eruptive
  coronal topologies necessary to initiate data-driven MHD simulations
  of CMEs. We therefore investigate whether a non-constant-α force-free
  extrapolation can successfully reproduce the magnetic features of a
  self-consistent MHD simulation of flux emergence through a stratified
  model atmosphere. We generate force-free equilibria from simulated
  photospheric and chromospheric vector magnetograms, and compare these
  results to the MHD calculation. We then apply these techniques to an IVM
  (Mees Solar Observatory) vector magnetogram of NOAA active-region 8210,
  a source of a number of eruptive events on the Sun.

---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Magnetic Flux Tubes in a
    Three-dimensional Convecting Flow. II. Turbulent Pumping and the
    Cohesion of Ω-Loops
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.; Bercik, D. J.
2004ApJ...612..557A    Altcode:
  We present a set of three-dimensional MHD simulations using the
  anelastic approximation of active region-scale flux ropes embedded
  in a turbulent, stratified model convection zone. We simulate the
  evolution of Ω-loops and other magnetic structures of varying field
  strengths, helicities, and morphologies in both rotating and nonrotating
  background states. We show that if the magnetic energy of a flux tube
  is weak relative to the kinetic energy density of strong downdrafts,
  convective flows dominate the evolution, flux tubes of any shape
  rapidly lose cohesion, and the magnetic field redistributes itself
  throughout the domain over timescales characteristic of convective
  turnover. We determine the conditions under which magnetic tension
  resulting from field line twist can provide the force necessary to
  prevent a relatively weak flux tube from losing cohesion during its
  ascent through the turbulent convection zone. Our simulations show
  that there is no initial tendency for a horizontal magnetic flux tube
  or layer to be preferentially transported in one vertical direction
  over the other solely as a result of the presence of an asymmetric
  vertical flow field. However, as the simulations progress, there is
  a transient net transport of magnetic flux into the lower half of the
  computational domain as the distribution of the magnetic field changes
  and flux is expelled from cell centers into converging downflows and
  intergranular lanes. This pumping mechanism is weak and uncorrelated
  with the degree of vertical flow asymmetry. We find that the strong
  turbulent pumping evident in simulations of penetrative convection-the
  efficient transport of magnetic flux to the base of the convection
  zone over several local turnover times-does not manifest itself in a
  closed domain in the absence of a convective overshoot layer. Thus,
  we suggest that this rapid redistribution of flux is primarily due to
  the penetration of magnetic flux into the stable layer where it remains
  over a timescale that far exceeds that of convective turnover. We also
  find that different treatments of the viscosity of a Newtonian fluid-in
  which the coefficient of either kinematic or dynamic viscosity is
  held constant throughout the domain-do not affect the global average
  evolution of embedded magnetic structures, although the details of
  the evolution may differ between models.

---------------------------------------------------------
Title: ILCT: Recovering Photospheric Velocities from Magnetograms
    by Combining the Induction Equation with Local Correlation Tracking
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.; Regnier, S.
2004ApJ...610.1148W    Altcode:
  We present three methods for deriving the velocity field in magnetized
  regions of the Sun's photosphere. As a preliminary step, we introduce
  a Fourier-based local correlation tracking (LCT) routine that we term
  “FLCT.” By explicitly employing the observation made by Démoulin
  &amp; Berger, that results determined by LCT applied to magnetograms
  involve a combination of all components of the velocity and magnetic
  fields, we show that a three-component velocity field can be derived, in
  a method we term algebraic decomposition, or ADC. Finally, we introduce
  ILCT, a method that enforces consistency between the normal component
  of the induction equation and results obtained from LCT. When used with
  photospheric vector magnetograms, ILCT determines a three-component
  photospheric velocity field suitable for use with time sequences of
  such magnetograms to drive boundary conditions for MHD simulations
  of the solar corona. We present results from these methods applied to
  vector magnetograms of NOAA AR 8210 on 1998 May 1.

---------------------------------------------------------
Title: ILCT: Combining Local Correlation Tracking with the Magnetic
    Induction Equation
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.; Regnier, S.
2004AAS...204.8805F    Altcode: 2004BAAS...36..820F
  In order to use sequences of vector magnetogram data as input to MHD
  simulations of the solar atmosphere, one must ensure that the data is
  consistent with the MHD induction equation. We describe a new technique,
  ILCT, that uses local correlation tracking to determine a 3-D flow
  field that is consistent with the ideal MHD induction equation. The
  flow fields are thus suitable for incorporation into the photospheric
  boundary of an MHD simulation of the solar atmosphere.

---------------------------------------------------------
Title: Radiative Hydrodynamic Simulations of Solar and Stellar Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.
2004AAS...204.0305A    Altcode: 2004BAAS...36..671A
  We have constructed radiative hydrodynamic simulations of the effects
  of flare heating on model solar and dMe stellar atmospheres. The
  heating is assumed to be driven by a beam of non-thermal electrons
  originating in the corona and impacting on the lower transition region
  and chromosphere. We use thick target bremsstrahlung fits to RHESSI
  hard X-ray observations of the July 23, 2002 and February 26, 2002
  flares to model the electron beam. Our simulations include detailed
  calculations of numerous bound-bound and bound-free transitions which
  we compare with line profiles measured during flares on the Sun and on
  the dMe star AD Leo. We also investigate the possibility that the 511
  keV emission line is produced from a significant amount of material
  at transition region temperatures.

---------------------------------------------------------
Title: HST, EUVE and Ground-Based Observations of Flares on AD Leo
Authors: Allred, J. C.; Hawley, S. L.; Johns-Krull, C. M.; Fisher,
   G. H.; Abbett, W. P.; Avgoloupis, S. I.; Seiradakis, J. H.
2004IAUS..219..829A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Turbulent Magnetic Field Generation in Rotating Stars
Authors: Bercik, D. J.; Abbett, W. P.; Fisher, G. H.; Fan, Y.
2003AGUFMSH42B0536B    Altcode:
  Observationally, it has been found that magnetic activity is a
  strong function of rotation rate. The connection between rotation and
  dynamo-generated fields is not well understood, however. The typical
  interface dynamo theory applied to the Sun to describe its activity
  cycle assumes the existence of a velocity shear layer. Such a model
  is inappropriate for fully convective stars that are nevertheless
  active, such as late-type M and L stars and pre-main sequence T Tauri
  stars; in these stars a turbulent dynamo is generally believed to
  be the mechanism of magnetic field generation. We investigate the
  connection between observed activity behavior and magnetic field
  generation in fully convective stars through a series of simulations
  of the turbulent dynamo. The simulations were performed in a Cartesian
  domain using ANMHD, a 3D MHD anelastic code. We compare the resulting
  magnetic topologies for a series of Rossby numbers and comment on the
  implications for the sizes of coronal loops and activity levels.

---------------------------------------------------------
Title: Incorporating Vector Magnetic Field Measurements into MHD
    Models of the Solar Atmosphere
Authors: Abbett, W. P.
2003AGUFMSM11A..03A    Altcode:
  We report on our efforts to incorporate high cadence vector magnetic
  field measurements of the CME and flare producing active region NOAA
  8210 (observed from April 28 to May 2 1998) into the photospheric
  boundary layers of our 3D MHD models of the solar atmosphere. We
  find that it is is essential to be able to specify an initial model
  atmosphere that is both consistent with soft X-ray observations of
  the corona and with observed vector magnetic field measurements at
  the photosphere. Further, MHD codes require that certain components of
  the flowfield be specified at the lower boundary in such a way as to
  self-consistently update the model photosphere between each successive
  magnetogram. We will present the results of our application of several
  techniques to infer the velocity field of magnetized plasma in the
  photosphere using a time-series of IVM vector magnetograms of NOAA
  8210, and will present the results from our latest attempt to model
  this complex active region.

---------------------------------------------------------
Title: Temperature, Density, and Magnetic Field Reconstructions of
    Active Region Coronae
Authors: Lundquist, L. L.; Fisher, G. H.; Régnier, S.; Liu, Y.;
   Abbett, W. P.
2003AGUFMSH42B0509L    Altcode:
  We present simulated coronal emission pictures of some case-study solar
  active regions, including NOAA-designated regions 8210 and 8038. The
  simulated emissions are calculated from a 3-d temperature, density,
  and magnetic field model of the corona based on first principles. The
  method involves a static energy balance along individual coronal loops,
  with the heating term taken from a given coronal heating theory. The
  predicted emissions can be compared with observed X-ray and UV satellite
  images. By comparing the predictions of various heating theories with
  observations, we can determine constraints on the probable mechanisms
  of coronal heating. The model is also useful for a variety of other
  applications, such as testing of coronal magnetic field extrapolation
  techniques, calculations of wave propagation and shock phenomena, and
  testing assumptions about the spatial distribution of heating along
  loops. This work was supported by a DoD/AFOSR MURI grant, "Understanding
  Magnetic Eruptions and their Interplanetary Consequences."

---------------------------------------------------------
Title: I+LCT: A Method for Determining Photospheric Flows from
    Magnetograms
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.
2003AGUFMSH22A0177W    Altcode:
  Coronal mass ejections (CME's) are magnetically-driven reconfigurations
  of plasma in the low-β solar corona; they are the primary drivers of
  space weather. One way to investigate coronal field evolution before
  and during such events involves driving MHD simulations of the coronal
  magnetic field using data from time series of vector magnetograms. Doing
  so requires specification of a three-component velocity field at the
  simulated photosphere, consistent with the observed magnetic field
  evolution in that layer. Unfortunately, such velocity data are not
  generally available. We have developed a method that finds photospheric
  plasma velocities consistent with both the flows derived by the familiar
  techinique of local correlation tracking (LCT), and the field evolution
  described by z-component of the induction equation. We present results
  obtained by applying this “I+LCT” technique to vector magnetograms,
  and to “false” magnetograms obtained from MHD simulations of
  photospheric field evolution.

---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Solar White Light Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.;
   Hudson, H. S.; Metcalf, T. R.
2003AGUFMSH22A0175A    Altcode:
  We report on theoretical radiative hydrodynamic simulations of solar
  white light flares. The solar atmosphere is modeled in detail from
  the transition region to the photosphere. The coronal pressure and
  X-ray backheating are included self-consistently. Flare heating is
  assumed to be from an electron beam which is modeled for several
  white light flares using data from RHESSI, TRACE and Yohkoh. We also
  investigate the possibility that the 511 keV line width is produced
  from a significant column depth of atmosphere at transition region
  temperatures. We compare our new solar flare models to previous results,
  and to models of M dwarf stellar flares.

---------------------------------------------------------
Title: Multiwavelength Observations of Flares on AD Leonis
Authors: Hawley, Suzanne L.; Allred, Joel C.; Johns-Krull, Christopher
   M.; Fisher, George H.; Abbett, William P.; Alekseev, Ilya; Avgoloupis,
   Stavros I.; Deustua, Susana E.; Gunn, Alastair; Seiradakis, John H.;
   Sirk, Martin M.; Valenti, Jeff A.
2003ApJ...597..535H    Altcode:
  We report results from a multiwavelength observing campaign conducted
  during 2000 March on the flare star AD Leo. Simultaneous data were
  obtained from several ground- and space-based observatories, including
  observations of eight sizable flares. We discuss the correlation of
  line and continuum emission in the optical and ultraviolet wavelength
  regimes, as well as the flare energy budget, and we find that the
  emission properties are remarkably similar even for flares of very
  different evolutionary morphology. This suggests a common heating
  mechanism and atmospheric structure that are independent of the detailed
  evolution of individual flares. We also discuss the Neupert effect,
  chromospheric line broadening, and velocity fields observed in several
  transition region emission lines. The latter show significant downflows
  during and shortly after the flare impulsive phase. Our observations are
  broadly consistent with the solar model of chromospheric evaporation
  and condensation following impulsive heating by a flux of nonthermal
  electrons. These data place strong constraints on the next generation
  of radiative hydrodynamic models of stellar flares.

---------------------------------------------------------
Title: The March 2000 AD Leo Flare Campaign
Authors: Hawley, S. L.; Johns-Krull, C. M.; Fisher, G. H.; Abbett,
   W. P.; Seiradakis, J. H.; Avgoloupis, S. I.
2003csss...12..975H    Altcode:
  Flares are by their nature random and unpredictable events and flare
  observations are often the serendipitous result of programs designed for
  other scientific endeavors. Thus, few observations of flares covering
  multiple wavelength regimes, with both spectroscopic and photometric
  information, are available to test stellar flare models. Occasionally,
  a bold and reckless team will put together a flare campaign, employing
  suitable statistical arguments to convince the relevant telescope
  allocation committees that such a campaign will prove fruitful, while
  hoping desperately for the combination of clear weather, working
  instruments and cooperative star necessary to warrant the herculean
  organizational effort. We report here on one such campaign, conducted
  during March, 2000 on the dM3e flare star AD Leo.

---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Luhmann, J. G.; Abbett, W. P.
2003AIPC..679..323L    Altcode:
  The Wang-Sheeley relationship relates the solar wind speed at the
  Earth to the divergence rate of open magnetic flux tubes in the solar
  corona. This relationship is based on a statistically significant
  correlation between the flux tube divergence parameter “fs” derived
  from a photospheric field-based potential field source surface model,
  and satellite observations of the solar wind speed. The fast solar wind
  emanates from regions of small magnetic divergence, while slow solar
  wind comes from regions of high magnetic divergence. Arge and Pizzo
  [2] improved the reliability of the method by relating the coronal
  flux tube expansion factor to the solar wind speed at the source
  surface instead of the satellite. We use a three-dimensional MHD model
  of the solar corona to further investigate the implications of the
  Wang-Sheeley relationship for solar wind acceleration. The results
  suggest what additional heating and momentum inputs may be necessary
  in an MHD model to obtain the observed relationship between flux tube
  divergence and solar wind speed.

---------------------------------------------------------
Title: Can Simulations of Active Region Magnetic Fields Lead to a
    Simplified Model of Turbulent Pumping?
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.; Bercik, D. J.
2003SPD....34.1905A    Altcode: 2003BAAS...35..842A
  We present a series of 3-D MHD simulations in the anelastic
  approximation of active region scale magnetic flux ropes embedded in
  a highly stratified, turbulent model convection zone. The numerical
  calculations are carried out over long time scales (of order a solar
  rotation time) at high magnetic Reynolds numbers and suggest that
  the process of “turbulent pumping” --- the tendency for magnetic
  flux to be efficiently transported from surface layers to the base of
  the convection zone --- does not manifest itself in the absence of
  a convective overshoot layer. If the overshoot layer is present, we
  suggest a simple, statistical model (similar to a 1-D, depth-dependent
  eddy-diffusivity treatment with a characteristic time-scale of
  supergranulation) that describes the average properties of the flux
  storage process.

---------------------------------------------------------
Title: Modeling of the Turbulent Dynamo
Authors: Bercik, D. J.; Fisher, G. H.; Abbett, W. P.
2003SPD....34.1904B    Altcode: 2003BAAS...35R.842B
  We report on the development progress of a 3-D spherical anelastic
  MHD code (SANMHD) that will be used to study the turbulent dynamo in
  solar and stellar interiors. SANMHD is a complement to the existing
  Cartesian anelastic MHD code, ANMHD; to model the interaction between
  penetrative turbulent convection and magnetic field deep in the interior
  of stars, it becomes necessary to consider a gravitationally stratified
  atmosphere in a spherical geometry. We discuss issues regarding the
  creation of a highly modular code that is portable across multiple
  platforms, code structuring to allow the flexibility to investigate
  a variety of physical scenarios and testing strategies.

---------------------------------------------------------
Title: Comparison of the Coronal Magnetic Field Derived from PFSS
    and MHD Models
Authors: Ledvina, S. A.; Luhmann, J. G.; Li, Y.; Abbett, W. P.
2003SPD....34.0601L    Altcode: 2003BAAS...35..817L
  The coronal magnetic field determines many properties of the solar
  corona such as the location of the heliospheric current sheet and
  regions of high and low speed solar wind. Thus understanding the
  structure of the coronal magnetic field is crucial to the understanding
  of space weather. Several models use a synoptic map to derive the
  structure of the coronal field out to several solar radii. One such
  model is the potential field source surface model (PFSS). This model
  neglects electric currents between the photosphere and a "source
  surface" (typically 2.5 Rs). At the source surface the field lines
  are forced to be radial in order to mimic the effects of the solar
  wind. In contrast MHD models try to self-consistently derive the coronal
  field and the plasma properties of the corona. We compare the coronal
  magnetic field structures derived by the PFSS and MHD models in order to
  understand what role dynamical effects may have on the field structure.

---------------------------------------------------------
Title: A Temperature and Density Model of the Solar Corona
Authors: Lundquist, L. L.; Regnier, S.; Abbett, W. P.; Fisher, G. H.
2003SPD....34.0404L    Altcode: 2003BAAS...35..811L
  We have developed the foundations of a 3-d global temperature and
  density model of the solar corona based on first principles. The method
  involves a static energy balance along individual coronal loops, with
  the heating term taken from a given coronal heating theory. We use
  the model to create synthetic emission images of active regions for
  comparison with observed X-ray and UV satellite images. The technique
  will enable us to perform a statistical study of active region heating
  with Yohkoh data from the last decade, comparing observations with the
  predicted emission measures and X-ray morphologies for different heating
  theories. The model is also useful for a variety of other applications,
  such as calculations of wave propagation and shock phenomena, testing of
  coronal magnetic field extrapolation techniques such as the potential
  and FFF models, and testing assumptions about the spatial distribution
  of heating along loops. <P />We have applied the technique to two
  cases: a simulated emerged active region, and NOAA active region
  8210. These cases employ a heating term derived from the empirical
  relationship of Pevtsov et al. (2003) relating soft X-ray luminosity
  to total unsigned magnetic flux for a wide range of solar and stellar
  magnetic features. We present results from these two cases, including a
  comparison of the synthetic emission images of AR 8210 with Yohkoh SXT
  data. This work was supported by a DoD/AFOSR MURI grant, "Understanding
  Magnetic Eruptions and their Interplanetary Consequences."

---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Magnetic Flux Tubes in a
    Three-dimensional Convecting Flow. I. Uniformly Buoyant Horizontal
    Tubes
Authors: Fan, Y.; Abbett, W. P.; Fisher, G. H.
2003ApJ...582.1206F    Altcode:
  We present three-dimensional numerical simulations of the dynamic
  evolution of uniformly buoyant, twisted horizontal magnetic flux
  tubes in a three-dimensional stratified convective velocity field. Our
  calculations are relevant to understanding how stratified convection in
  the deep solar convection zone may affect the rise and the structure
  of buoyant flux tubes that are responsible for the emergence of solar
  active regions. We find that in order for the magnetic buoyancy force
  of the tube to dominate the hydrodynamic force due to the convective
  downflows, the field strength B of the flux tube needs to be greater
  than (H<SUB>p</SUB>/a)<SUP>1/2</SUP>B<SUB>eq</SUB>~3B<SUB>eq</SUB>,
  where H<SUB>p</SUB> is the pressure scale height, a is the tube
  radius, and B<SUB>eq</SUB> is the field strength in equipartition
  with the kinetic energy density of the strong downdrafts. For tubes
  of equipartition field strength (B=B<SUB>eq</SUB>), the dynamic
  evolution depends sensitively on the local condition of the convective
  flow. Sections of the tube in the paths of strong downdrafts are
  pinned down to the bottom despite their buoyancy, while the rise
  speed of sections within upflow regions is significantly boosted;
  Ω-shaped emerging tubes can form between downdrafts. Although flux
  tubes with B=B<SUB>eq</SUB> are found to be severely distorted by
  convection, the degree of distortion obtained from our simulations
  is not severe enough to clearly rule out the Ω-tubes that are able
  to emerge between downdrafts as possible progenitors of solar active
  regions. As the initial field strength of the tube becomes higher than
  the critical value of ~(H<SUB>p</SUB>/a)<SUP>1/2</SUP>B<SUB>eq</SUB>
  given above, the dynamic evolution converges toward the results of
  previous simulations of the buoyant rise of magnetic flux tubes in a
  static, adiabatically stratified model solar convection zone. Tubes
  with 10 times the equipartition field strength are found to rise
  unimpeded by the downdrafts and are not significantly distorted by
  the three-dimensional convective flow.

---------------------------------------------------------
Title: A Coupled Model for the Emergence of Active Region Magnetic
    Flux into the Solar Corona
Authors: Abbett, W. P.; Fisher, G. H.
2003ApJ...582..475A    Altcode:
  We present a set of numerical simulations that model the emergence
  of active region magnetic flux into an initially field-free model
  corona. We simulate the buoyant rise of twisted magnetic flux tubes
  initially positioned near the base of a stable stratified model
  convection zone and use the results of these calculations to drive a
  three-dimensional magnetohydrodynamic model corona. The simulations show
  that time-dependent subsurface flows are an important component of the
  dynamic evolution and subsequent morphology of an emerging magnetic
  structure. During the initial stages of the flux emergence process,
  the overlying magnetic field differs significantly from a force-free
  state. However, as the runs progress and boundary flows adjust, most
  of the coronal field-with the exception of those structures located
  relatively close to the model photosphere-relaxes to a more force-free
  configuration. Potential field extrapolations do not adequately
  represent the magnetic structure when emerging active region fields
  are twisted. In the dynamic models, if arched flux ropes emerge with
  nonzero helicity, the overlying field readily forms sigmoid-shaped
  structures. However, the chirality of the sigmoid and other details of
  its structure depend on the observer's vantage point and the location
  within a given loop of emitting plasma. Thus, sigmoids may be an
  unreliable signature of the sign and magnitude of magnetic twist.

---------------------------------------------------------
Title: Comparison of the Coronal Magnetic Field Derived from PFSS
    and MHD Models
Authors: Ledvina, S. A.; Luhmann, J. G.; Li, Y.; Abbett, W. P.
2002AGUFMSH52A0456L    Altcode:
  The coronal magnetic field determines many properties of the solar
  corona such as the location of the heliospheric current sheet and
  regions of high and low speed solar wind. Thus understanding the
  structure of the coronal magnetic field is crucial to the understanding
  of space weather. Several models use a synoptic map to derive the
  structure of the coronal field out to several solar radii. One such
  model is the potential field source surface model (PFSS). This model
  neglects electric currents between the photosphere and a "source
  surface" (typically 2.5 Rs). At the source surface the field lines
  are forced to be radial in order to mimic the effects of the solar
  wind. In contrast MHD models try to self-consistently derive the coronal
  field and the plasma properties of the corona. We compare the coronal
  magnetic field structures derived by the PFSS and MHD models in order to
  understand what role dynamical effects may have on the field structure.

---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Omega-loops in a 3-D
    Convecting Flow
Authors: Abbett, W. P.; Fan, Y.; Fisher, G. H.
2002AGUFMSH52A0474A    Altcode:
  We present the latest results from 3D MHD simulations (in the anelastic
  approximation) of buoyant magnetic flux tubes interacting with turbulent
  convection in the solar interior. We focus our study on active region
  scale flux ropes and Omega-loops, and perform a large parameter space
  study of the effects of not only initial field strength, but twist and
  loop geometry on the morphology and dynamics of sub-surface magnetic
  structures. We also investigate the effects of different numerical
  treatments of viscosity, and quantify the amount of magnetic field in
  each simulation that succumbs to the effects of turbulent pumping.

---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Abbett, W. P.; Luhmann, J. G.
2002AAS...200.5714L    Altcode: 2002BAAS...34Q.739L
  The Wang-Sheeley empirical model enables the calculation of the
  solar wind speed at Earth from the divergence of magnetic flux
  tubes in a synoptic map-based potential field source surface model
  of the coronal magnetic field. The formula used for this purpose was
  derived from observations of the solar wind speed at 1 AU. According
  to the model, fast solar wind emanates from regions of small magnetic
  field divergence, while slow solar wind comes from high divergence
  regions. Arge and Pizzo (2000) recently improved the Wang-Sheeley
  formula by taking the stream interaction effects between the Sun
  and 1 AU into account. We use a three-dimensional MHD model of the
  solar corona to test the assumptions of the Wang- Sheeley model
  and the improvements made by Arge and Pizzo for various magnetic
  configurations. These results provide insight into what additional
  coronal heating may be implied for different flux tube geometries in
  order to obtain the observed relationship with solar wind speed.

---------------------------------------------------------
Title: The Rise of Twisted Horizontal Flux Tubes in a 3D Convecting
    Flow
Authors: Fan, Y.; Abbett, W. P.; Fisher, G. H.
2002AAS...200.0306F    Altcode: 2002BAAS...34..642F
  We present 3D numerical simulations of the dynamic evolution of twisted
  horizontal magnetic flux tubes in a stratified convecting convection
  zone. We investigate how the trajectory, rise velocity, and cohesion of
  the buoyant flux tubes are affected by the 3D stratified convection. It
  is found that the field strength of the magnetic flux tube needs to
  be significantly above the value of equipartition with the kinetic
  energy of convection in order for the flux tube to rise cohesively
  to the top of the stratified domain. These simulations add further
  support to the strong toroidal field strength ( ~ 5 x 10<SUP>4</SUP> G
  to 10<SUP>5</SUP> G) at the base of the solar convection zone, suggested
  by previous thin flux tube calculations of emerging flux tubes through
  the solar convective envelope. NCAR is sponsored by the National Science
  Foundation. Part of this work was carried out while the authors were
  participating in the solar magnetic field program held at ITP, UCSB.

---------------------------------------------------------
Title: Numerical Simulations of Magnetic Flux Emergence in Active
    Regions
Authors: Abbett, W. P.
2002AAS...200.7906A    Altcode: 2002BAAS...34..780A
  Understanding the sub-photospheric structure and dynamics of emerging
  active region magnetic fields, and how these fields are coupled to
  structures observed above the photosphere, is important to a variety of
  ongoing research projects in both the solar physics and space science
  communities (for example, the effort to predict the onset of intense
  episodes of solar activity such as CMEs and flares). Over the past
  decade, much progress has been made by using 2-D MHD codes and the 1-D
  “thin flux tube” approximation to describe the evolution of buoyant
  magnetic flux tubes in the solar interior. However, in recent years,
  the rapid evolution of computer technology, coupled with advances
  in computational algorithms, have made it possible to use physically
  self-consistent, 3-D MHD numerical simulations to model the evolution
  of strong magnetic fields through stratified model convection zones
  without the restrictive assumptions of earlier models. This review will
  summarize efforts to use modern 3-D codes as tools to test predictions
  of earlier theoretical models and to interpret observational data. The
  emphasis will be on the progress made in modeling emerging magnetic flux
  in the solar interior; however, a brief overview of recent efforts to
  couple sub-photospheric simulations to models of the solar atmosphere
  and corona will also be presented.

---------------------------------------------------------
Title: New coupled models of emerging magnetic flux in active regions
Authors: Abbett, W.; Ledvina, S.; Fisher, G.
2002ocnd.confE..22A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Abbett, W. P.; Li, Y.; Luhmann, J. G.
2001AGUFMSH31A0695L    Altcode:
  The Wang-Sheeley empirical model relates the solar wind speed observed
  at the Earth with the divergence rate of magnetic flux tubes expanding
  in the solar corona. This model is based on a statistically significant
  correlation between an open flux tube divergence parameter "fs" derived
  from photospheric field synoptic maps, and satellite observations of
  the solar wind speed. They found that the fast solar wind emanates
  from regions of small magnetic divergence, while slow solar wind comes
  from regions of high magnetic divergence. Arge and Pizzo (2000) have
  since improved the reliability of the Wang-Sheeley model by including
  an empirical function that relates the magnetic expansion factor to
  the solar wind speed at the source surface, and a scheme to account
  for stream interactions as the solar wind propagates outward. We use
  a three-dimensional MHD model of the solar corona to empirically
  test the Wang-Sheely model and the improvements made by Arge and
  Pizzo. These results may provide insight into what additional heating
  may be necessary in different flux tube geometries in order to obtain
  to the observed relationship with solar

---------------------------------------------------------
Title: New Coupled Models of Magnetic Flux in Active Regions
Authors: Abbett, W. P.; Ledvina, S. A.; Fisher, G. H.; MacNeice, P.
2001AGUFMSH11C0727A    Altcode:
  We report progress in our efforts to use the publicly available domain
  decomposition and adaptive mesh refinement framework “PARAMESH” to
  couple our 3D anelastic MHD (ANMHD) model of active region magnetic
  flux in the solar convection zone with a simple, fully compressible
  ZEUS3D MHD model of the photosphere, transition region, and low corona.

---------------------------------------------------------
Title: How do emerging magnetic fields affect the solar coronal
    field configuration?
Authors: LI, Y.; Luhmann, J. G.; Abbett, W.; Linker, J.; Lionello,
   R.; Mikic, Z.
2001AGUFMSH11C0719L    Altcode:
  Experiments are carried out to study the coronal field response to an
  emerging active region into a simple background global magnetic field
  using potential field source surface models. The emerging active region
  used is the radial component of the magnetic field of an emerging flux
  rope from an ANMHD simulation. When the active region is emerging
  into a dipole field, it introduces polar coronal hole extensions,
  warps the source surface neutral lines, and changes the field line
  connections. The active region internal field line connections are also
  changed to be different from an isolated active region. The relative
  strength of the background and active region affect the extent of the
  changes that occur. The field distribution of the background global
  field is important, and different background with the same emerging
  active region may result in different coronal features. A few examples
  of different background fields with the emerging active region will be
  presented and compared. A global MHD simulation is also in preparation
  using the same global magnetic field with the emerging active region
  as the boundary condition.

---------------------------------------------------------
Title: Flux-loss of buoyant ropes interacting with convective flows
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Abbett, W. P.; Nordlund, Å.
2001A&A...380..734D    Altcode: 2001astro.ph.10205D
  We present 3-d numerical magneto-hydrodynamic simulations of a buoyant,
  twisted magnetic flux rope embedded in a stratified, solar-like model
  convection zone. The flux rope is given an initial twist such that it
  neither kinks nor fragments during its ascent. Moreover, its magnetic
  energy content with respect to convection is chosen so that the flux
  rope retains its basic geometry while being deflected from a purely
  vertical ascent by convective flows. The simulations show that magnetic
  flux is advected away from the core of the flux rope as it interacts
  with the convection. The results thus support the idea that the amount
  of toroidal flux stored at or near the bottom of the solar convection
  zone may currently be underestimated.

---------------------------------------------------------
Title: 3-D MHD Simulations of Flux Tube Emergence
Authors: Abbett, W. P.; Fisher, G. H.
2001AGUSM..SH41A10A    Altcode:
  We present initial results from coupled 3-D MHD simulations of twisted
  magnetic flux tubes that have risen through a model solar convection
  zone and emerged into the lower corona. We use the anelastic code
  “ANMHD” to simulate the rise of buoyant flux tubes through the
  solar convection zone, and use this data to generate a photospheric
  boundary that drives a simple simulation of the solar transition region
  and corona using a modified version of the publicly available code
  “ZEUS3D”

---------------------------------------------------------
Title: Initial Behavior of a Buoyant Magnetic Flux Tube Imbedded in
    a Rotating Medium
Authors: Fisher, G. H.; Abbett, W. P.
2001AGUSM..SP51B11F    Altcode:
  In a non-rotating medium with gravity, an initially stationary, buoyant,
  untwisted magnetic flux tube will generate 2 counter-rotating vortices
  as it begins rising. In a 2-D geometry, these vortices ultimately split
  the flux tube into two fragments, which then repel one another. The
  trajectory of the flux tube fragments can be predicted extremely
  well by using a simple analytical treatment based on the initial
  behavior of the flux tube (see e.g. Longcope, Fisher, and Arendt 1996,
  Ap.J. 464, 999). Numerical simulations in both 2-D and 3-D geometries
  show that rotation dramatically changes this behavior, acting to
  strongly suppress magnetic flux tube fragmentation (see e.g. Wissink et
  al. 2000, Ap.J. 536, 982 and Abbett, Fisher, &amp; Fan 2001, Ap. J. 546,
  1194). Coriolis forces deflect the motions that otherwise would result
  in strong circulation around the flux tube fragments. In the same
  spirit as the analytical treatment of Longcope, Fisher &amp; Arendt,
  we derive equations that describe the initial flow pattern for a 2-D
  buoyant, untwisted magnetic flux tube rising in a rotating medium,
  and compare these results to those from numerical simulations.

---------------------------------------------------------
Title: The Emergence of Magnetic Flux in Active Regions
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2001IAUS..203..225A    Altcode:
  Over the past decade, “thin flux tube” models have proven successful
  in explaining many properties of active regions in terms of magnetic
  flux tube dynamics in the solar interior. Unfortunately, recent,
  more sophisticated two-dimensional MHD simulations of the emergence
  of magnetic flux have shown that many of the assumptions adopted in
  the thin flux tube approximation are invalid. For example, unless
  the flux tubes exhibit a large amount of initial field line twist ---
  and observations of emerging active regions suggest they do not ---
  they will fragment (break apart) before they are able to emerge through
  the surface. We attempt to resolve this paradox using a number of 3-D
  MHD simulations (in the anelastic approximation) that describe the
  rise and fragmentation of twisted magnetic flux tubes. We find that
  the degree of fragmentation of an evolving Omega-loop depends strongly
  on the three-dimensional geometry of the tube --- the greater the apex
  curvature, the lesser the degree of fragmentation for a fixed amount of
  initial twist. We also find that the Coriolis force plays a dynamically
  important role in the evolution and emergence of magnetic flux. We are
  able to infer general observational characteristics of the emerging
  flux, and compare our theoretical data with recent observations.

---------------------------------------------------------
Title: The Effects of Rotation on the Evolution of Rising Omega
    Loops in a Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2001ApJ...546.1194A    Altcode: 2000astro.ph..8501A
  We present three-dimensional MHD simulations of buoyant magnetic flux
  tubes that rise through a stratified model convection zone in the
  presence of solar rotation. The equations of MHD are solved in the
  anelastic approximation, and the results are used to determine the
  effects of solar rotation on the dynamic evolution of an Ω-loop. We
  find that the Coriolis force significantly suppresses the degree
  of fragmentation at the apex of the loop during its ascent toward
  the photosphere. If the initial axial field strength of the tube is
  reduced, then, in the absence of forces due to convective motions,
  the degree of apex fragmentation is also reduced. Our simulations
  confirm the results of thin flux-tube calculations that show the
  leading polarity of an emerging active region positioned closer to
  the equator than the trailing polarity and the trailing leg of the
  loop oriented more vertically than the leading leg. We show that the
  Coriolis force slows the rise of the tube and induces a retrograde
  flow in both the magnetized and unmagnetized plasma of an emerging
  active region. Observationally, we predict that this flow will appear
  to originate at the leading polarity and will terminate at the trailing
  polarity.

---------------------------------------------------------
Title: Erratum: The Three-dimensional Evolution of Rising, Twisted
    Magnetic Flux Tubes in a Gravitationally Stratified Model Convection
    Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2000ApJ...542.1119A    Altcode:
  In the article “The Three-dimensional Evolution of Rising, Twisted
  Magnetic Flux Tubes in a Gravitationally Stratified Model Convection
  Zone” by W. P. Abbett, G. H. Fisher, and Y. Fan (ApJ, 540, 548
  [2000]), an error was introduced into one of the equations during
  the production process. A cross product symbol was mistakenly
  removed from equation (2). The corrected equation is as follows:
  ρ<SUB>0</SUB>(∂v/∂t+v̇∇v)= -∇p<SUB>1</SUB>+ρ<SUB>1</SUB>g +
  1/4π (∇XB)XB+∇̇Π. The Press sincerely apologizes for this error.

---------------------------------------------------------
Title: The Three-dimensional Evolution of Rising, Twisted Magnetic
    Flux Tubes in a Gravitationally Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2000ApJ...540..548A    Altcode: 2000astro.ph..4031A
  We present three-dimensional numerical simulations of the rise and
  fragmentation of twisted, initially horizontal magnetic flux tubes that
  evolve into emerging Ω-loops. The flux tubes rise buoyantly through an
  adiabatically stratified plasma that represents the solar convection
  zone. The MHD equations are solved in the anelastic approximation,
  and the results are compared with studies of flux-tube fragmentation
  in two dimensions. We find that if the initial amount of field line
  twist is below a critical value, the degree of fragmentation at the
  apex of a rising Ω-loop depends on its three-dimensional geometry: the
  greater the apex curvature of a given Ω-loop, the lesser the degree
  of fragmentation of the loop as it approaches the photosphere. Thus,
  the amount of initial twist necessary for the loop to retain its
  cohesion can be reduced substantially from the two-dimensional
  limit. The simulations also suggest that, as a fragmented flux
  tube emerges through a relatively quiet portion of the solar disk,
  extended crescent-shaped magnetic features of opposite polarity should
  form and steadily recede from one another. These features eventually
  coalesce after the fragmented portion of the Ω-loop emerges through
  the photosphere.

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Title: 3D MHD Simulation of Flux Tube Dynamics: Comparison with Thin
    Flux Tube Models
Authors: Fisher, G. H.; Abbett, W. P.; Fan, Y.
2000SPD....31.0135F    Altcode: 2000BAAS...32..807F
  We have used the anelastic 3D MHD code “ANMHD” to perform simulations
  of emerging magnetic flux tubes moving in a gravitationally stratified
  background model representing the solar convection zone. The MHD
  model is computed within a local Cartesian geometry, with Coriolis
  forces included, using the f-plane approximation. The evolution of
  flux tubes computed with the code will be compared and contrasted with
  results computed with the thin flux tube approximation. This work was
  supported by NASA and NSF.

---------------------------------------------------------
Title: The Cohesion of 3-D Magnetic Flux Tubes in a Rotating,
    Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G.; Fan, Y.
2000SPD....31.0136A    Altcode: 2000BAAS...32..807A
  We present the latest results from a series of 3-D MHD simulations in
  the anelastic approximation that describe the rise of magnetic flux
  tubes through an adiabatically stratified model convection zone. The
  effects of solar rotation and the Coriolis force are included in
  the models. The simulations begin with initially horizontal magnetic
  flux tubes which subsequently evolve into Omega-loops. We find that
  the degree of “fragmentation” at the apex of a rising Omega-loop
  depends strongly on both the three-dimensional geometry of the loop,
  and on the field strength along the axis of the initial tube. Loops
  with a relatively high degree of apex curvature, and of moderate to low
  initial axial field strength retain their cohesion throughout their
  rise toward the photosphere --- even in the absence of initial field
  line twist. We are able to infer general observational characteristics
  of the emerging flux, and compare our theoretical data with recent
  observations of active regions. This work was funded by NSF grants
  AST 98-19727 and ATM 98-96316, and by NASA grant NAGS-8468. The
  computations were partially supported by the National Center for
  Atmospheric Research, and the National Computational Science Alliance.

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Title: Magnetic flux tubes inside the sun
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
   Abbett, W. P.
2000PhPl....7.2173F    Altcode:
  Bipolar magnetic active regions are the largest concentrations of
  magnetic flux on the Sun. In this paper, the properties of active
  regions are investigated in terms of the dynamics of magnetic flux
  tubes which emerge from the base of the solar convection zone, where
  the solar cycle dynamo is believed to operate, to the photosphere. Flux
  tube dynamics are computed with the “thin flux tube” approximation,
  and by using magnetohydrodynamics simulation. Simulations of active
  region emergence and evolution, when compared with the known observed
  properties of active regions, have yielded the following results: (1)
  The magnetic field at the base of the convection zone is confined to
  an approximately toroidal geometry with a field strength in the range
  3-10×10<SUP>4</SUP> G. The latitude distribution of the toroidal
  field at the base of the convection zone is more or less mirrored by
  the observed active latitudes; there is not a large poleward drift of
  active regions as they emerge. The time scale for emergence of an active
  region from the base of the convection zone to the surface is typically
  2-4 months. (2) The tilt of active regions is due primarily to the
  Coriolis force acting to twist the diverging flows of the rising flux
  loops. The dispersion in tilts is caused primarily by the buffeting of
  flux tubes by convective motions as they rise through the interior. (3)
  Coriolis forces also bend active region flux tube shapes toward the
  following (i.e., antirotational) direction, resulting in a steeper
  leg on the following side as compared to the leading side of an active
  region. When the active region emerges through the photosphere, this
  results in a more rapid separation of the leading spots away from the
  magnetic neutral line as compared to the following spots. This bending
  motion also results in the neutral line being closer to the following
  magnetic polarity. (4) The properties of the strongly sheared, flare
  productive δ-spot active regions can be accounted for by the dynamics
  of highly twisted Ω loops that succumb to the helical kink instability
  as they emerge through the solar interior.

---------------------------------------------------------
Title: Dynamic Models of Optical Emission in Impulsive Solar Flares
Authors: Abbett, William P.; Hawley, Suzanne L.
1999ApJ...521..906A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Dynamic Models of Optical Emission During Solar Flares
Authors: Abbett, W. P.
1999AAS...194.2206A    Altcode: 1999BAAS...31..860A
  The results from a non-LTE radiative-hydrodynamic model of a flare
  loop, from its apex in the corona to its footpoints in the photosphere,
  are presented. The effects of non-thermal heating of the lower solar
  atmosphere by accelerated electrons during the impulsive phase, and the
  subsequent effects of soft X-ray irradiation of the chromosphere from
  the flare-heated transition region and corona during the beginning of
  the gradual phase are investigated. During the impulsive phase, the
  models show a significant continuum (or “white light”) brightening
  resulting from increased hydrogen recombination radiation in the upper
  chromosphere at the point where the accelerated electrons deposit the
  bulk of their energy. Additionally, the models produce a measurable
  time lag between the brightening of the near wings of H-alpha and the
  brightening of the Paschen continuum. This work was funded in part
  by NSF grants AST 96-16886 and AST 94-57455. The computations were
  partially supported by the National Computational Science Alliance,
  and utilized the NCSA SGI/CRAY Power Challenge Array.

---------------------------------------------------------
Title: Dynamical Solar Flare Model Atmospheres
Authors: Abbett, W. P.; Hawley, S. L.
1999ASPC..158..212A    Altcode: 1999ssa..conf..212A
  No abstract at ADS

---------------------------------------------------------
Title: A Theoretical Investigation of Optical Emission in Solar Flares
Authors: Abbett, William Paul
1998PhDT.........1A    Altcode: 1998PhDT........91A; 1998PhDT........87A
  A dynamic theoretical model of a flare loop from its footpoints in the
  photosphere to its apex in the corona is presented, and the effects of
  non-thermal heating of the lower atmosphere by accelerated electrons
  and soft X-ray irradiation from the flare heated transition region and
  corona are investigated. Important transitions of hydrogen, helium,
  and singly ionized calcium and magnesium are treated in non-LTE. Three
  main conclusions are drawn from the models. First, even the strongest of
  impulsive events can be described as having two phases: a gentle phase
  characterized by a state of near equilibrium, and an explosive phase
  characterized by large material flows, and strong hydrodynamic waves
  and shocks. During the gentle phase, one or possibly two temperature
  'plateaus' form in the upper chromosphere. The line emission generated
  in these regions produces profiles that are generally symmetric and
  undistorted, in contrast to emission produced during the explosive
  phase, where large velocity gradients that occur in the upper atmosphere
  produce line profiles that are highly asymmetric and show large emission
  peaks and troughs. Second, a significant continuum (or 'white light')
  brightening results from increased hydrogen recombination radiation
  in the upper chromosphere at the point where the accelerated electrons
  deposit the bulk of their energy. Third, there exists a measurable time
  lag between the brightening of the near wings of Hα and the brightening
  of the Paschen continuum. This delay is controlled by the amount of time
  it takes for electron densities in the upper chromosphere to become
  high enough, and the densities of hydrogen atoms in high energy bound
  states to become low enough, to allow the number of recombinations to
  dominate the number of photoionizations in the region.

---------------------------------------------------------
Title: Non-LTE Radiative-hydrodynamic Models of Solar Flares
Authors: Abbett, William P.; Hawley, Suzanne L.
1997BAAS...29Q1120A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar Convection: Comparison of Numerical Simulations and
    Mixing-Length Theory
Authors: Abbett, William P.; Beaver, Michelle; Davids, Barry;
   Georgobiani, Dali; Rathbun, Pamela; Stein, Robert F.
1997ApJ...480..395A    Altcode:
  We compare the results of realistic numerical simulations of convection
  in the superadiabatic layer near the solar surface with the predictions
  of mixing-length theory. We find that the peak values of such quantities
  as the temperature gradient, the temperature fluctuations, and the
  velocity fluctuations, as well as the entropy jump in the simulation,
  can be reproduced by mixing-length theory for a ratio of mixing length
  to pressure scale height α ~ 1.5. However, local mixing-length theory
  neither reproduces the profiles of these variables with depth nor allows
  penetration of convective motions into the overlying stable photosphere.