Author name code: forbes
ADS astronomy entries on 2022-09-14
author:"Forbes, Terry G."
------------------------------------------------------------------------
Title: Numerical Investigations of Catastrophe in Coronal Magnetic
Configuration Triggered by Newly Emerging Flux
Authors: Chen, Yuhao; Ye, Jing; Mei, Zhixing; Shen, Chengcai; Roussev,
Ilia I.; Forbes, Terry G.; Lin, Jun; Ziegler, Udo
Bibcode: 2022ApJ...933..148C
Altcode:
We performed 2D magnetohydrodynamical numerical experiments to study
the response of the coronal magnetic configuration to the newly emerging
magnetic flux. The configuration includes an electric-current-carrying
flux rope modeling the prominence floating in the corona and the
background magnetic field produced by two separated magnetic dipoles
embedded in the photosphere. Parameters for one dipole are fixed in
space and time to model the quiet background, and those for another one
are time dependent to model the new flux. These numerical experiments
duplicate important results of the analytic solution but also reveal
new results. Unlike previous works, the configuration here possesses
no symmetry, and the flux rope could move in any direction. The
non-force-free environment causes the deviation of the flux rope
equilibrium in the experiments from that determined in the analytic
solution. As the flux rope radius decreases, the equilibrium could
be found, and it evolves quasi-statically until the flux rope reaches
the critical location at which the catastrophe occurs. As the radius
increases, no equilibrium exists at all. During the catastrophe, two
current sheets form in different ways. One forms as the surrounding
closed magnetic field is stretched by the catastrophe, and another one
forms as the flux rope squeezes the magnetic field nearby. Although
reconnection happens in both the current sheets, it erases the first one
quickly and enhances the second simultaneously. These results indicate
the occurrence of the catastrophe in asymmetric and non-force-free
environment, and the non-radial motion of the flux rope following
the catastrophe.
Title: A New Method of 3-D Magnetic Field Reconstruction
Authors: Torbert, R. B.; Dors, I.; Argall, M. R.; Genestreti, K. J.;
Burch, J. L.; Farrugia, C. J.; Forbes, T. G.; Giles, B. L.; Strangeway,
R. J.
Bibcode: 2020GeoRL..4785542T
Altcode: 2019arXiv190911255T
A method is described to model the magnetic field in the vicinity of
three-dimensional constellations of satellites (at least four) using
field and plasma current measurements. This quadratic model matches
the measured values of the magnetic field and its curl (current)
at each spacecraft, with ∇ • B zero everywhere, and thus extends
the linear curlometer method to second order. Near the spacecraft,
it predicts the topology of magnetic structures, such as reconnecting
regions or flux ropes, and allows a tracking of the motion of these
structures relative to the spacecraft constellation. Comparisons to
particle-in-cell simulations estimate the model accuracy. Reconstruction
of two electron diffusion regions definitively confirms the expected
field line structure. The model can be applied to other small-scale
phenomena (e.g., bow shocks) and can also be modified to reconstruct
the electric field, allowing tracing of particle trajectories.
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
Bibcode: 2019arXiv191014022S
Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.
Title: Effects in the Near-Magnetopause Magnetosheath Elicited by
Large-Amplitude Alfvénic Fluctuations Terminating in a Field and
Flow Discontinuity
Authors: Farrugia, C. J.; Cohen, I. J.; Vasquez, B. J.; Lugaz, N.;
Alm, L.; Torbert, R. B.; Argall, M. R.; Paulson, K.; Lavraud, B.;
Gershman, D. J.; Gratton, F. T.; Matsui, H.; Rogers, A.; Forbes,
T. G.; Payne, D.; Ergun, R. E.; Mauk, B.; Burch, J. L.; Russell,
C. T.; Strangeway, R. J.; Shuster, J.; Nakamura, R.; Fuselier, S. A.;
Giles, B. L.; Khotyaintsev, Y. V.; Lindqvist, P. A.; Marklund, G. T.;
Petrinec, S. M.; Pollock, C. J.
Bibcode: 2018JGRA..123.8983F
Altcode:
In this paper we report on a sequence of large-amplitude Alfvénic
fluctuations terminating in a field and flow discontinuity and their
effects on electromagnetic fields and plasmas in the near-magnetopause
magnetosheath. An arc-polarized structure in the magnetic field was
observed by the Time History of Events and Macroscale Interactions
during Substorms-C in the solar wind, indicative of nonlinear Alfvén
waves. It ends with a combined tangential discontinuity/vortex sheet,
which is strongly inclined to the ecliptic plane and at which there is
a sharp rise in the density and a drop in temperature. Several effects
resulting from this structure were observed by the Magnetospheric
Multiscale spacecraft in the magnetosheath close to the subsolar point
(11:30 magnetic local time) and somewhat south of the geomagnetic
equator (-33° magnetic latitude): (i) kinetic Alfvén waves; (ii)
a peaking of the electric and magnetic field strengths where E·J
becomes strong and negative (-1 nW/m3) just prior to an
abrupt dropout of the fields; (iii) evolution in the pitch angle
distribution of energetic (a few tens of kilo-electron-volts) ions
(H+, Hen+, and On+) and electrons
inside a high-density region, which we attribute to gyrosounding of
the tangential discontinuity/vortex sheet structure passing by the
spacecraft; (iv) field-aligned acceleration of ions and electrons that
could be associated with localized magnetosheath reconnection inside
the high-density region; and (v) variable and strong flow changes,
which we argue to be unrelated to reconnection at partial magnetopause
crossings and likely result from deflections of magnetosheath flow by
a locally deformed, oscillating magnetopause.
Title: Reconnection in the Post-impulsive Phase of Solar Flares
Authors: Forbes, Terry G.; Seaton, Daniel B.; Reeves, Katharine K.
Bibcode: 2018ApJ...858...70F
Altcode: 2018arXiv180400324F
Using a recently developed analytical procedure, we determine the rate
of magnetic reconnection in the “standard” model of eruptive solar
flares. During the late phase, the neutral line is located near the
lower tip of the reconnection current sheet, and the upper region of the
current sheet is bifurcated into a pair of Petschek-type shocks. Despite
the presence of these shocks, the reconnection rate remains slow if
the resistivity is uniform and the flow is laminar. Fast reconnection
is achieved only if there is some additional mechanism that can shorten
the length of the diffusion region at the neutral line. Observations of
plasma flows by the X-ray telescope on Hinode imply that the diffusion
region is, in fact, quite short. Two possible mechanisms for reducing
the length of the diffusion region are localized resistivity and
MHD turbulence.
Title: Structure and Dissipation Characteristics of an Electron
Diffusion Region Observed by MMS During a Rapid, Normal-Incidence
Magnetopause Crossing
Authors: Torbert, R. B.; Burch, J. L.; Argall, M. R.; Alm, L.;
Farrugia, C. J.; Forbes, T. G.; Giles, B. L.; Rager, A.; Dorelli, J.;
Strangeway, R. J.; Ergun, R. E.; Wilder, F. D.; Ahmadi, N.; Lindqvist,
P. -A.; Khotyaintsev, Y.
Bibcode: 2017JGRA..12211901T
Altcode:
On 22 October 2016, the Magnetospheric Multiscale (MMS) spacecraft
encountered the electron diffusion region (EDR) when the magnetosheath
field was southward, and there were signatures of fast reconnection,
including flow jets, Hall fields, and large power dissipation. One
rapid, normal-incidence crossing, during which the EDR structure was
almost stationary in the boundary frame, provided an opportunity
to observe the spatial structure for the zero guide field case of
magnetic reconnection. The reconnection electric field was determined
unambiguously to be 2-3 mV/m. There were clear signals of fluctuating
parallel electric fields, up to 6 mV/m on the magnetosphere side of
the diffusion region, associated with a Hall-like parallel current
feature on the electron scale. The width of the main EDR structure was
determined to be 2 km (1.8 de). Although the MMS spacecraft
were in their closest tetrahedral separation of 8 km, the divergences
and curls for these thin current structures could therefore not be
computed in the usual manner. A method is developed to determine these
quantities on a much smaller scale and applied to compute the normal
component of terms in the generalized Ohm's law for the positions
of each individual spacecraft (not a barocentric average). Although
the gradient pressure term has a qualitative dependence that follows
the observed variation of E + Ve × B, the quantitative
magnitude of these terms differs by more than a factor of 2, which is
shown to be greater than the respective errors. Thus, future research
is required to find the manner in which Ohm's law is balanced.
Title: Magnetic Energy Release in Solar Flares
Authors: Forbes, Terry G.
Bibcode: 2017AAS...22930001F
Altcode:
Solar flares are the result of a rapid release of magnetic energy
stored in the solar corona. An ideal-MHD process, such as a loss of
magnetic equilibrium, most likely initiates the flare, but the non-ideal
process of magnetic reconnection quickly becomes the dominant mechanism
by which energy is released. Within the last few years EUV and X-ray
instruments have directly observed the kind of plasma flows and heating
indicative of magnetic reconnection. Relatively cool plasma is observed
moving slowly into the reconnection region where it is transformed
into two high-temperature, high-speed outflow jets moving in opposite
directions. Observations of the flow in these jets suggest that they
are accelerated to the ambient Alfvén speed in a manner that resembles
the reconnection process first proposed by H. E. Petschek in 1964. This
result is somewhat surprising because Petschek-type reconnection does
not occur in most numerical simulations of magnetic reconnection. The
apparent contradiction between the observations and the simulations
can be understood by the fact that most simulations assume a uniform
resistivity model that is unlikely to occur in reality. Recently,
we have developed a theory that shows how the type of reconnection is
related to the plasma resistivity. The theory is based on a form of
the time-dependent, MHD-nozzle equations that incorporate the plasma
resistivity. These equations are very similar to the equations used
to describe magnetized plasma flow in astrophysical jets.
Title: Magnetic Reconnection in Solar Flares
Authors: Forbes, Terry G.
Bibcode: 2016SPD....4720401F
Altcode:
Reconnection has at least three possible roles in solar flares: First,
it may contribute to the build-up of magnetic energy in the solar
corona prior to flare onset; second, it may directly trigger the
onset of the flare; and third, it may allow the release of magnetic
energy by relaxing the magnetic field configuration to a lower energy
state. Although observational support for the first two roles is
somewhat limited, there is now ample support for the third. Within the
last few years EUV and X-ray instruments have directly observed the
kind of plasma flows and heating indicative of reconnection. Continued
improvements in instrumentation will greatly help to determine
the detailed physics of the reconnection process in the solar
atmosphere. Careful measurement of the reconnection outflows will be
especially helpful in this regard. Current observations suggest that in
some flares the jet outflows are accelerated within a short diffusion
region that is more characteristic of Petschek-type reconnection
than Sweet-Parker reconnection. Recent resistive MHD theoretical and
numerical analyses predict that the length of the diffusion region
should be just within the resolution range of current X-ray and EUV
telescopes if the resistivity is uniform. On the other hand, if the
resistivity is not uniform, the length of the diffusion region could
be too short for the outflow acceleration region to be observable.
Title: Draping of the Interstellar Magnetic Field over the Heliopause:
a Passive Field Model
Authors: Isenberg, Philip A.; Forbes, Terry G.; Möbius, Eberhard
Bibcode: 2015ApJ...805..153I
Altcode: 2015arXiv150400585I
As the local interstellar plasma flows past our heliosphere, it is
slowed and deflected around the magnetic obstacle of the heliopause. The
interstellar magnetic field, frozen into this plasma, then becomes
draped around the heliopause in a characteristic manner. We derive the
analytical solution for this draped magnetic field in the limit of weak
field intensity, assuming an ideal potential flow around the heliopause,
which we model as a Rankine half-body. We compare the structure of
the model magnetic field with observed properties of the Interstellar
Boundary Explorer (IBEX) ribbon and with in situ observations at the
Voyager 1 spacecraft. We find reasonable qualitative agreement, given
the idealizations of the model. This agreement lends support to the
secondary ENA model of the IBEX ribbon and to the interpretation that
Voyager 1 has crossed the heliopause. We also predict that the magnetic
field measured by Voyager 2 after it crosses the heliopause will not
be significantly rotated away from the direction of the undisturbed
interstellar field.
Title: A Trio of Confined Flares in AR 11087
Authors: Joshi, Anand D.; Forbes, Terry G.; Park, Sung-Hong; Cho,
Kyung-Suk
Bibcode: 2015ApJ...798...97J
Altcode:
We investigate three flares that occurred in active region, AR 11087,
observed by the Dutch Open Telescope (DOT) on 2010 July 13, in a
span of three hours. The first two flares have soft X-ray class B3,
whereas the third flare has class C3. The third flare not only was the
largest in terms of area and brightness but also showed a very faint
coronal mass ejection (CME) associated with it, while the earlier two
flares had no associated CME. The active region, located at 27° N,
26° E, has a small U-shaped active region filament to the south of
the sunspot, and a quiescent filament is located to its west. Hα
observations from DOT, as well as extreme-ultraviolet images and
magnetograms from the STEREO spacecraft and Solar Dynamics Observatory,
are used to study the dynamics of the active region during the three
flares. Our observations imply that the first two flares are confined
and that some filament material drains to the surface during these
flares. At the onset of the third flare downflows are again observed
within the active region, but a strong upflow is also observed at the
northern end of the adjacent quiescent filament to the west. It is at
the latter location that the CME originates. The temporal evolution of
the flare ribbons and the dynamics of the filaments are both consistent
with the idea that reconnection in a pre-existing current sheet leads
to a loss of equilibrium.
Title: Draping of the Interstellar Magnetic Field Over the Heliosphere
- A Passive Field Model
Authors: Isenberg, P. A.; Forbes, T. G.; Moebius, E.
Bibcode: 2014AGUFMSH14A..05I
Altcode:
We consider the idealized situation of a passive magnetic field, frozen
into an otherwise uniform plasma flow, as the flow encounters a rigid
heliospheric obstacle. We model the heliopause as a Rankine half-body
and obtain an analytical solution for the steady-state field and flow
structure that results. This idealized model provides a qualitative
picture of the draped field structure which complements recent reports
from detailed numerical simulations and can yield a more intuitive
understanding of the conditions in the outer heliosheath. We apply
this formalism to issues raised by observations of the IBEX ribbon
position and by the magnetic field measurements at Voyager 1. Further
implications will also be discussed.
Title: The formation and stability of Petschek reconnection
Authors: Baty, H.; Forbes, T. G.; Priest, E. R.
Bibcode: 2014PhPl...21k2111B
Altcode:
No abstract at ADS
Title: Catastrophe versus Instability for the Eruption of a Toroidal
Solar Magnetic Flux Rope
Authors: Kliem, B.; Lin, J.; Forbes, T. G.; Priest, E. R.; Török, T.
Bibcode: 2014ApJ...789...46K
Altcode: 2014arXiv1404.5922K
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of
force balance governing the underlying equilibria. Using a toroidal
flux rope in an external bipolar or quadrupolar field as a model
for the current-carrying flux, we demonstrate the occurrence of a
fold catastrophe by loss of equilibrium for several representative
evolutionary sequences in the stable domain of parameter space. We
verify that this catastrophe and the torus instability occur at the same
point; they are thus equivalent descriptions for the onset condition
of solar eruptions.
Title: Rapid CME Cavity Formation and Expansion
Authors: Kliem, Bernhard; Forbes, Terry G.; Patsourakos, Spiros;
Vourlidas, Angelos
Bibcode: 2014AAS...22421206K
Altcode:
A cavity is supposed to be a general feature of well-developed CMEs at
the stage they can be imaged by white-light coronagraphs (in the outer
corona and solar wind). The cavity is interpreted as the cross section
of the CME flux rope in the plane of sky. Preexisting cavities are
observed around some quiescent erupting prominences, but usually not in
active regions. Observations of CME cavities in the inner corona, where
most of them appear to form, have become possible only with the STEREO
and SDO missions. These reveal a very rapid formation and expansion of
"EUV cavities" in fast and impulsively commencing eruptions early in the
phase of main CME acceleration and impulsive flare rise. Different from
the white-light observations, the EUV cavity initially appears to be
larger than the CME flux rope. However, it evolves into the white-light
cavity subsequently. MHD simulations of flux rope eruptions conform to
this picture of initially larger cavity but subsequently approaching
cavity and flux rope size. The initial expansion of ambient flux can
be understood as a "reverse pinch effect", driven by decreasing flux
rope current as the rope rises.
Title: Breakout and Tether-Cutting Eruption Models Are Both
Catastrophic (Sometimes)
Authors: Longcope, D. W.; Forbes, T. G.
Bibcode: 2014SoPh..289.2091L
Altcode: 2013arXiv1312.4435L; 2014SoPh..tmp....3L
We present a simplified analytic model of a quadrupolar magnetic field
and flux rope to model coronal mass ejections. The model magnetic field
is two-dimensional, force-free and has current only on the axis of
the flux rope and within two current sheets. It is a generalization of
previous models containing a single current sheet anchored to a bipolar
flux distribution. Our new model can undergo quasi-static evolution
either due to changes at the boundary or due to magnetic reconnection at
either current sheet. We find that all three kinds of evolution can lead
to a catastrophe, known as loss of equilibrium. Some equilibria can be
driven to catastrophic instability either through reconnection at the
lower current sheet, known as tether cutting, or through reconnection
at the upper current sheet, known as breakout. Other equilibria can be
destabilized through only one and not the other. Still others undergo
no instability, but they evolve increasingly rapidly in response
to slow steady driving (ideal or reconnective). One key feature of
every case is a response to reconnection different from that found
in simpler systems. In our two-current-sheet model a reconnection
electric field in one current sheet causes the current in that sheet
to increase rather than decrease. This suggests the possibility for
the microscopic reconnection mechanism to run away.
Title: Magnetic reconnection mediated by hyper-resistive plasmoid
instability
Authors: Huang, Yi-Min; Bhattacharjee, A.; Forbes, Terry G.
Bibcode: 2013PhPl...20h2131H
Altcode: 2013arXiv1308.1871H
Magnetic reconnection mediated by the hyper-resistive plasmoid
instability is studied with both linear analysis and nonlinear
simulations. The linear growth rate is found to scale as
SH1/6 with respect to the hyper-resistive
Lundquist number SH≡L3VA/ηH, where L is the
system size, VA is the Alfvén velocity, and ηH
is the hyper-resistivity. In the nonlinear regime, reconnection rate
becomes nearly independent of SH, the number of plasmoids
scales as SH1/2, and the secondary current sheet
length and width both scale as SH-1/2. These
scalings are consistent with a heuristic argument assuming secondary
current sheets are close to marginal stability. The distribution of
plasmoids as a function of the enclosed flux ψ is found to obey a
ψ-1 power law over an extended range, followed by a rapid
fall off for large plasmoids. These results are compared with those
from resistive magnetohydrodynamic studies.
Title: Breakout and tether-cutting eruption models are both
catastrophic (sometimes)
Authors: Longcope, Dana; Forbes, T. G.
Bibcode: 2013SPD....44...68L
Altcode:
We present a simplified analytic model of a quadrupolar magnetic field
and flux rope to model coronal mass ejections. The model magnetic field
is two-dimensional, force-free and has current only on the axis of the
flux rope and within two currents sheets. It is a generalization of
previous models containing a single current sheet anchored to a bipolar
flux distribution. Our new model can undergo quasi-static evolution
due either to changes at the boundary or to magnetic reconnection at
either current sheet. We find that all three kinds of evolution can lead
to a catastrophe known as loss of equilibrium. Some equilibria can be
driven to catastrophic instability either through reconnection at the
lower current sheet, known as tether cutting, or through reconnection
at the upper current sheet, known as breakout. Other equilibria can
be destabilized through only one and not the other. Still others
undergo no instability, but evolve increasingly rapidly in response
to slow steady driving (ideal or reconnective). One key feature of
every case is a response to reconnection different from that found
in simpler systems. In our two-current sheet model a reconnection
electric field in one current sheet causes the current in that sheet
to increase rather than decrease. This suggests the possibility for
the microscopic reconnection mechanism to run away.
Title: Shrinking Loops Observations for the 2008 April 9 Flare
Authors: Savage, S. L.; McKenzie, D. E.; Reeves, K. K.; Forbes, T. G.
Bibcode: 2012ASPC..454..295S
Altcode:
Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft
X-rays (SXR)), TRACE (extreme ultra-violet (EUV)), SoHO/LASCO (white
light), SoHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics
such as low emissivity and trajectories which slow as they reach the
top of the arcade are consistent with post-reconnection magnetic flux
tubes. The magnetic flux within the tubes provides pressure against
filling with plasma. As with the standard model of reconnection,
the tubes retract from a reconnection site high in the corona until
they reach a more potential magnetic configuration. Viewed from a
perpendicular angle, SADs should appear as shrinking loops rather
than downflowing voids. We will present observations of supra-arcade
downflowing loops (SADLs) and show that their speeds and decelerations
are consistent with those determined for SADs.
Title: The Coronal Physics Investigator (cpi) Experiment For Iss:
A New Vision For Understanding Solar Wind Acceleration
Authors: Raymond, John C.; Janzen, P. H.; Kohl, J. L.; Reisenfeld,
D. B.; Chandran, B. D. G.; Cranmer, S. R.; Forbes, T. G.; Isenberg,
P. A.; Panasyuk, A. V.; van Ballegooijen, A. A.
Bibcode: 2011SPD....42.2406R
Altcode: 2011BAAS..43S.2406R
We propose an Explorer Mission of Opportunity program to develop and
operate a large-aperture ultraviolet coronagraph spectrometer called
the Coronal Physics Investigator (CPI) as an attached International
Space Station (ISS) payload. The primary goal of this program is
to identify and characterize the physical processes that heat and
accelerate the primary and secondary components of the fast and slow
solar wind. Also, CPI can make key measurements needed to understand
CMEs. CPI is dedicated to high spectral resolution measurements of
the off-limb extended corona with far better stray light suppression
than can be achieved by a conventional instrument. UVCS/SOHO allowed
us to identify what additional measurements need to be made to answer
the fundamental questions about how solar wind streams are produced,
and CPI's next-generation capabilities were designed specifically to
make those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by (1)
observing many ions over a large range of charge and mass,(2) providing
simultaneous measurements of proton and electron temperatures to probe
turbulent dissipation mechanisms, and (3) measuring amplitudes of
low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: The Coronal Physics Investigator (CPI) Experiment for ISS:
A New Vision for Understanding Solar Wind Acceleration
Authors: Kohl, J. L.; Cranmer, S. R.; Raymond, J. C.; Norton, T. J.;
Cucchiaro, P. J.; Reisenfeld, D. B.; Janzen, P. H.; Chandran, B. D. G.;
Forbes, T. G.; Isenberg, P. A.; Panasyuk, A. V.; van Ballegooijen,
A. A.
Bibcode: 2011arXiv1104.3817K
Altcode:
In February 2011 we proposed a NASA Explorer Mission of Opportunity
program to develop and operate a large-aperture ultraviolet
coronagraph spectrometer called the Coronal Physics Investigator
(CPI) as an attached International Space Station (ISS) payload. The
primary goal of this program is to identify and characterize the
physical processes that heat and accelerate the primary and secondary
components of the fast and slow solar wind. In addition, CPI can make
key measurements needed to understand CMEs. UVCS/SOHO allowed us to
identify what additional measurements need to be made to answer the
fundamental questions about how solar wind streams are produced, and
CPI's next-generation capabilities were designed specifically to make
those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by
(1) observing many ions over a large range of charge and mass, (2)
providing simultaneous measurements of proton and electron temperatures
to probe turbulent dissipation mechanisms, and (3) measuring amplitudes
of low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East,
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: Simulations of Overexpanding CME Cavities
Authors: Kliem, B.; Forbes, T.; Vourlidas, A.; Patsourakos, S.
Bibcode: 2010AGUFMSH51A1661K
Altcode:
Coronal mass ejection (CME) cavities seen in white-light coronagraphs
expand nearly self similarly in the outer corona and inner solar
wind. Little is known about their initial expansion in the inner
corona. A two-phase evolution, consisting of an initial overexpansion
up to a heliocentric front height of about 1.5 solar radii, followed by
nearly self-similar expansion, was recently discovered in STEREO/SECCHI
observations of a fast CME (Patsourakos et al. 2010). The overexpansion
is expressed as a decrease of the cavity aspect ratio (center height
by radius) by at least a factor of 2 during the rise phase of the
main CME acceleration. We present MHD simulations of erupting flux
ropes that show the initial overexpansion of a cavity in line with
the observed evolution. The contributions of ideal-MHD expansion and
of magnetic reconnection to the growth of the flux rope and cavity
in the simulations will be quantified to identify the primary cause
of the overexpansion. This assesses the diagnostic potential of the
overexpansion for the change of flux rope current and the role of
magnetic reconnection in the early evolution of CMEs.
Title: Current Sheet Energetics, Flare Emissions, and Energy Partition
in a Simulated Solar Eruption
Authors: Reeves, Katharine K.; Linker, Jon A.; Mikić, Zoran; Forbes,
Terry G.
Bibcode: 2010ApJ...721.1547R
Altcode:
We investigate coronal energy flow during a simulated coronal mass
ejection (CME). We model the CME in the context of the global corona
using a 2.5D numerical MHD code in spherical coordinates that includes
coronal heating, thermal conduction, and radiative cooling in the energy
equation. The simulation domain extends from 1 to 20 Rs
. To our knowledge, this is the first attempt to apply detailed
energy diagnostics in a flare/CME simulation when these important
terms are considered in the context of the MHD equations. We find
that the energy conservation properties of the code are quite good,
conserving energy to within 4% for the entire simulation (more than 6
days of real time). We examine the energy release in the current sheet
as the eruption takes place, and find, as expected, that the Poynting
flux is the dominant carrier of energy into the current sheet. However,
there is a significant flow of energy out of the sides of the current
sheet into the upstream region due to thermal conduction along field
lines and viscous drag. This energy outflow is spatially partitioned
into three separate components, namely, the energy flux flowing out
the sides of the current sheet, the energy flowing out the lower tip
of the current sheet, and the energy flowing out the upper tip of the
current sheet. The energy flow through the lower tip of the current
sheet is the energy available for heating of the flare loops. We
examine the simulated flare emissions and energetics due to the
modeled CME and find reasonable agreement with flare loop morphologies
and energy partitioning in observed solar eruptions. The simulation
also provides an explanation for coronal dimming during eruptions and
predicts that the structures surrounding the current sheet are visible
in X-ray observations.
Title: Reconnection Outflows and Current Sheet Observed with
Hinode/XRT in the 2008 April 9 "Cartwheel CME" Flare
Authors: Savage, Sabrina L.; McKenzie, David E.; Reeves, Katharine K.;
Forbes, Terry G.; Longcope, Dana W.
Bibcode: 2010ApJ...722..329S
Altcode: 2010arXiv1003.4758S
Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft
X-rays (SXR)), TRACE (extreme ultraviolet (EUV)), SOHO/LASCO (white
light), SOHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics
such as low emissivity and trajectories, which slow as they reach the
top of the arcade, are consistent with post-reconnection magnetic flux
tubes retracting from a reconnection site high in the corona until they
reach a lower-energy magnetic configuration. Viewed from a perpendicular
angle, SADs should appear as shrinking loops rather than downflowing
voids. We present X-ray Telescope (XRT) observations of supra-arcade
downflowing loops (SADLs) following a coronal mass ejection (CME) on
2008 April 9 and show that their speeds and decelerations are consistent
with those determined for SADs. We also present evidence for a possible
current sheet observed during this flare that extends between the
flare arcade and the CME. Additionally, we show a correlation between
reconnection outflows observed with XRT and outgoing flows observed
with LASCO.
Title: Reconnection Outflows and Current Sheet Observed with
Hinode/XRT in the April 9 2008 "Cartwheel CME" Flare
Authors: Savage, Sabrina; McKenzie, D. E.; Reeves, K. K.; Forbes,
T. G.; Longcope, D. W.
Bibcode: 2010AAS...21640423S
Altcode: 2010BAAS...41R.903S
Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft
X-rays (SXR)), TRACE (extreme ultra-violet (EUV)), SoHO/LASCO (white
light), SoHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics
such as low emissivity and trajectories which slow as they reach the
top of the arcade are consistent with post-reconnection magnetic flux
tubes. The magnetic flux within the tubes provides pressure against
filling with plasma. As with the standard model of reconnection,
the tubes then retract from a reconnection site high in the corona
until they reach a more potential magnetic configuration. Viewed from
a perpendicular angle, SADs should appear as shrinking loops rather
than downflowing voids. We will present observations of supra-arcade
downflowing loops (SADLs) following a CME on April 9, 2008 with XRT
and show that their speeds and decelerations are consistent with those
determined for SADs. We will also present evidence for a possible
current sheet observed during this flare that extends between the CME
and the flare arcade. Additionally, we will show a correlation between
reconnection outflows observed with XRT and outgoing flows observed
with LASCO.
Title: Models of coronal mass ejections and flares
Authors: Forbes, Terry
Bibcode: 2010hssr.book..159F
Altcode:
No abstract at ADS
Title: The Triggering of Large-Scale Waves by CME Initiation
Authors: Forbes, Terry
Bibcode: 2010cosp...38.1797F
Altcode: 2010cosp.meet.1797F
Studies of the large-scale waves generated at the onset of a coronal
mass ejection (CME) can provide important information about the
processes in the corona that trigger and drive CMEs. The size of
the region where the waves originate can indicate the location of
the magnetic forces that drive the CME outward, and the rate at
which compressive waves steepen into shocks can provide a measure
of how the driving forces develop in time. However, in practice it
is difficult to separate the effects of wave formation from wave
propagation. The problem is particularly acute for the corona because
of the multiplicity of wave modes (e.g. slow versus fast MHD waves)
and the highly nonuniform structure of the solar atmosphere. At the
present time large-scale numerical simulations provide the best hope for
deconvolving wave propagation and formation effects from one another.
Title: Reconnection outflows and current sheet observed with
Hinode/XRT in the April 9 2008 "Cartwheel CME" flare
Authors: McKenzie, David; Savage, Sabrina; Reeves, Kathy; Forbes,
Terry; Longcope, Dana
Bibcode: 2010cosp...38.1952M
Altcode: 2010cosp.meet.1952M
The "Cartwheel CME" event of April 9, 2008, presents an exemplary
view of motions and structures consistent with the commonly held view
of magnetic reconnection. Following the eruption of the coronal mass
ejection from the southwest limb of the Sun, an X-ray-bright arcade
was observed to form. Hinode/XRT observed coronal loops shrinking into
the top of the arcade, consistent with the reconnection picture. We
present measurements of the speeds and decelerations of these
shrinking loops, and demonstrate that the measurements are consistent
with those determined for supra-arcade downflows in other eruptive
flares. Moreover, we present evidence for a possible current sheet
observed during this flare, extending between the CME and the flare
arcade. The shrinking loops, and also outflows moving radially away
from the Sun, were observed to move along this current sheet. Finally,
we show a correlation between the anti-Sunward reconnection outflows
observed with XRT and identifiable features in the CME observed
with LASCO. This work is supported by NASA contracts NNM07AB07C and
NNX08AG44G, and NSF con-tract ATM-0837841.
Title: Magnetic reconnection
Authors: Forbes, Terry G.
Bibcode: 2009hppl.book..113F
Altcode:
No abstract at ADS
Title: An Analytical Model for Reconnection Outflow Jets Including
Thermal Conduction
Authors: Seaton, Daniel B.; Forbes, Terry G.
Bibcode: 2009ApJ...701..348S
Altcode:
We develop a model that predicts the velocity, density, and temperature
of coronal reconnection jets in the presence of thermal conduction. Our
model is based on the quasi-one-dimensional treatment of reconnecting
current sheets developed by B. Somov and V. S. Titov using the
magnetohydrodynamics nozzle equations. We incorporate thermal conduction
into the Somov-Titov framework using slow-shock jump conditions
modified to include losses due the conduction of thermal energy
along field lines mapping to the chromosphere. We find that thermal
conduction has a significant effect on the fast-mode Mach number of
the reconnection outflow, producing Mach numbers possibly as high as
7 for some solar-flare conditions. This value is three times greater
than previously calculated. We conclude that these termination shocks
are considerably more efficient at producing particle acceleration
than previously thought since the efficiency of particle acceleration
at shocks increases dramatically with Mach number. We compare this
model with numerical simulations by T. Yokoyama and K. Shibata and
find good agreement.
Title: Reconnection in Three-Dimensional Models of Coronal Mass
Ejections
Authors: Forbes, Terry Gene
Bibcode: 2009shin.confE.190F
Altcode:
Present-day models for large solar eruptions are based on the principle
that the energy that drives them comes from the free magnetic energy
associated with electrical current flows in the solar corona. Some of
these models depend on magnetic reconnection to trigger the eruption but
others do not. In both types, however, reconnection plays an essential
role in the release of the stored magnetic energy. The exact nature
of the reconnection process in these three-dimensional models remains
largely unexplored, but there is already evidence that its character is
significantly different from the type of reconnection that occurs in
two-dimensional models. Recently developed analytical and numerical
models show that the reconnection tends to occur along structures
known as quasi-separatrices. In three dimensions reconnection leads to
apparent motions of field lines both perpendicular and parallel to the
quasi-separatrices. This kind of reconnection is some time referred
to as
Title: Current Sheet Energetics, Flare Emissions, and Energy Partition
in a Simulated Solar Eruption
Authors: Reeves, Kathy K.; Linker, Jon; Mikic, Zoran; Forbes, Terry
Bibcode: 2009shin.confE.171R
Altcode:
We investigate coronal energy flow during a simulated coronal mass
ejection (CME). We model the CME using a 2.5D numerical MHD code that
includes coronal heating, thermal conduction and radiative cooling
in the energy equation. We apply energy diagnostics to this numerical
simulation and find that the energy is conserved to within 1% during
the stages leading up to the eruption, and to within 5% during the
eruption, when the current sheet is formed. These errors are most likely
due to the additional numerical dissipation associated with numerical
diffusion. We also examine the energy release in the current sheet as
the eruption takes place. We find that Poynting flux is the dominant
carrier of energy into the current sheet region, although its effect is
mitigated by losses due to conductive flux and viscosity. Additionally,
we examine the simulated emissions due to the flare associated with
the CME and find that the simulation reproduces observed flare loop
morphologies and provides an explanation for coronal dimming during
eruptions.
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikić, Z.;
Linker, J. A.
Bibcode: 2009ApJ...693.1029T
Altcode: 2008arXiv0807.2892T
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method
is based on the field-line mapping technique previously used only
for the analysis of a magnetic structure at a given time. This
technique is extended here so as to analyze the evolution of a
magnetic structure. Such a generalization is made with the help of
new dimensionless quantities called "slip-squashing factors." Their
large values define the surfaces that border the reconnected or
to-be-reconnected magnetic flux tubes for a given period of time
during the magnetic evolution. The proposed method is universal,
since it assumes only that the time sequence of evolving magnetic
field and the tangential boundary flows are known. The application
of the method is illustrated for simple examples, one of which was
considered previously by Hesse and coworkers in the framework of the
general magnetic reconnection theory. The examples help us to compare
these two approaches; it reveals also that, just as for magnetic null
points, hyperbolic and cusp minimum points of a magnetic field serve
as favorable sites for magnetic reconnection. The new method admits a
straightforward numerical implementation and provides a powerful tool
for the diagnostics of magnetic reconnection in numerical models of
solar-flare-like phenomena in space and laboratory plasmas.
Title: Role of Loss of Equilibrium and Magnetic Reconnection in
Coronal Eruptions: Resistive and Hall MHD simulations
Authors: Yang, H.; Bhattacharjee, A.; Forbes, T. G.
Bibcode: 2008AGUFMSH52A..02Y
Altcode:
It has long been suggested that eruptive phenomena such as coronal mass
ejections, prominence eruptions, and large flares might be caused by
a loss of equilibrium in a coronal flux rope (Van Tend and Kuperus,
1978). Forbes et al. (1994) developed an analytical two-dimensional
model in which eruptions occur due to a catastrophic loss of equilibrium
and relaxation to a lower-energy state containing a thin current
sheet. Magnetic reconnection then intervenes dynamically, leading to
the release of magnetic energy and expulsion of a plasmoid. We have
carried out high-Lundquist-number simulations to test the loss-of
equilibrium mechanism, and demonstrated that it does indeed occur
in the quasi-ideal limit. We have studied the subsequent dynamical
evolution of the system in resistive and Hall MHD models for single
as well as multiple arcades. The typical parallel electric fields
are super-Dreicer, which makes it necessary to include collisionless
effects via a generalized Ohm's law. It is shown that the nature of
the local dissipation mechanism has a significant effect on the global
geometry and dynamics of the magnetic configuration. The presence of
Hall currents is shown to alter the length of the current sheet and
the jets emerging from the reconnection site, directed towards the
chromosphere. Furthermore, Hall MHD effects break certain symmetries of
resistive MHD dynamics, and we explore their observational consequences.
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikic, Z.;
Linker, J. A.
Bibcode: 2008AGUFMSM31A1713T
Altcode:
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method is
based on the field-line mapping technique previously used only for
the analysis of magnetic structure at a given time. This technique is
extended here so as to analyze the evolution of magnetic structure. Such
a generalization is made with the help of new dimensionless quantities
called "slip-squashing factors". Their large values define the surfaces
that border the reconnected or to-be-reconnected magnetic flux tubes
for a given period of time during the magnetic evolution. The proposed
method is universal, since it assumes only that the time sequence
of the evolving magnetic field and the tangential boundary flows are
known. The application of the method is illustrated for simple examples,
one of which was considered previously by Hesse and coworkers in the
framework of the general magnetic reconnection theory. The examples
help to compare these two approaches; they reveal also that, just as for
magnetic null points, hyperbolic and cusp minimum points of a magnetic
field may serve as favorable sites for magnetic reconnection. The new
method admits a straightforward numerical implementation and provides
a powerful tool for the diagnostics of magnetic reconnection in
numerical models of solar-flare-like phenomena in space and laboratory
plasmas. Research partially supported by NASA and NSF.
Title: Simulations of the CME-Flare Relationship
Authors: Kliem, B.; Török, T.; Forbes, T. G.
Bibcode: 2008AGUFMSH23B1648K
Altcode:
Observations of coronal mass ejections (CMEs) and solar flares have
revealed a high correlation between the acceleration of the ejecta
and the plasma heating and particle acceleration signified by the
soft and hard X-ray emissions of the associated flare. The latter are
generally thought to result from magnetic reconnection. This finding
has stimulated the discussion of the CME-flare relationship, but at
the same time it has made it difficult to find a conclusive answer as
to whether magnetic reconnection or an ideal MHD instability is the
prime cause of the eruptions. Numerical simulations of unstable flux
ropes will be presented that are in very satisfactory quantitative
agreement with erupting filaments, both, confined to the corona and
ejective (i.e., developing into a CME). Some of these simulations
indeed show a high degree of synchronization between the initial
exponential acceleration of the flux rope, due to the ideal MHD
instability, and the development of reconnection flows. However,
others show a very delayed onset of reconnection, even after the
flux rope's acceleration peak. In addition, the reconnection flows
generally lag behind the motions driven by the ideal instability as
the flux rope rise velocity nears the saturation phase. Comparison of
the simulation results with observations suggests that the ideal MHD
process is the primary driver of the coupled CME-flare phenomenon. The
strong differences in the degree of synchronization, which the simulated
systems show in the main rise phase of the eruption, are related to
the magnetic topology prior to the eruption. Given the observational
result of a high correlation between CME and flare development (Zhang
& Dere 2006), these simulations yield constraints on the topology
and lead us to conclude that a seed for a reconnecting current sheet
must typically be present already at the onset of the eruption.
Title: Observations and analysis of the April 9, 2008 CME using
STEREO, Hinode TRACE and SoHO data
Authors: Reeves, K. K.; Patsourakos, S.; Stenborg, G.; Miralles, M.;
Deluca, E.; Forbes, T.; Golub, L.; Kasper, J.; Landi, E.; McKenzie,
D.; Narukage, N.; Raymond, J.; Savage, S.; Su, Y.; van Ballegooijen,
A.; Vourlidas, A.; Webb, D.
Bibcode: 2008AGUFMSH12A..04R
Altcode:
On April 9, 2008 a CME originating from an active region behind the limb
was well-observed by STEREO, Hinode, TRACE and SoHO. Several interesting
features connected to this eruption were observed. (1) The interaction
of the CME with open field lines from a nearby coronal hole appeared
to cause an abrupt change in the direction of the CME ejecta. (2) The
prominence material was heated, as evidenced by a change from absorption
to emission in the EUV wavelengths. (3) Because the active region was
behind the limb, the X-Ray Telescope on Hinode was able to take long
enough exposure times to observe a faint current- sheet like structure,
and it was able to monitor the dynamics of the plasma surrounding this
structure. This event is also being studied in the context of activity
that occurred during the Whole Heliosphere Interval (WHI).
Title: Predicting the Onset of Solar Eruptions
Authors: Forbes, T. G.
Bibcode: 2008AIPC.1043..245F
Altcode:
Large solar eruptions, including those that produce coronal mass
ejections, are most likely the result of a rapid release of magnetic
energy stored in the corona. A primary puzzle regarding such a
process is the identity of the mechanism that triggers the energy
release and initiates the eruption. One possibility is the onset of
an ideal-MHD instability or, more generally, the loss of an ideal-MHD
equilibrium. A mechanism of this type would easily account for the
fact that large eruptions typically occur on the Alfvén timescale of
the corona. Another possibility is the onset of a resistive instability
that involves magnetic reconnection. Models of both types have recently
been developed. Hybrid models that involve ideal and resistive processes
acting in tandem are also plausible.
Title: Field Line Shrinkage in Flares Observed by the X-Ray Telescope
on Hinode
Authors: Reeves, Katharine K.; Seaton, Daniel B.; Forbes, Terry G.
Bibcode: 2008ApJ...675..868R
Altcode:
The X-Ray Telescope on Hinode has observed individual loops of plasma
moving downward in a manner that is consistent with field line shrinkage
in the aftermath of reconnection at higher altitudes. An on-disk B3.8
flare observed on 2007 May 2 has loops that clearly change in shape from
cusp-shaped to more rounded. In addition, bright loops are observed that
decrease in altitude with a speed of approximately 5 km s-1,
and fainter, higher loop structures shrink with a velocity of 48 km
s-1. A C2.1 flare observed on 2006 December 17 also has
loops that change shape. Many bright features are seen to be moving
downward in this event, and we estimate their speed to be around 2-4
km s-1. We measure the shrinkage in both of these events,
and find that it is 17%-27%, which is consistent with theoretical
predictions.
Title: What Models Tell Us About The Relationship Between CME Dynamics
and Current Sheet Energetics
Authors: Reeves, Kathy; Linker, Jon; Forbes, Terry; Mikic, Zoran
Bibcode: 2008cosp...37.2584R
Altcode: 2008cosp.meet.2584R
Observers have noted correlations between peak CME acceleration and the
rise of soft X-ray flare emission. One reason given for this observation
is that acceleration of the flux rope and the thermal energy release
rate are are both consequences of the reconnection process, and should
thus have similar time profiles. CME models provide useful frameworks
for examining this question in detail. Using an analytical CME model, we
find that the correlation between thermal energy release rate and flux
rope acceleration is good for cases where the background magnetic field
is high and the reconnection rate is fast, but that the correlation is
poor for cases with low background magnetic fields and slow reconnection
rates. In this analytical model, we assume that the Poynting flux into
the current sheet is completely thermalized. We re-examine these results
in light of recent 2.5D numerical simulations aimed at understanding
the energy balance in the current sheet during an eruption.
Title: A Three-dimensional Line-tied Magnetic Field Model for Solar
Eruptions
Authors: Isenberg, Philip A.; Forbes, Terry G.
Bibcode: 2007ApJ...670.1453I
Altcode:
We introduce a three-dimensional analytical model of a coronal flux rope
with its ends embedded in the solar surface. The model allows the flux
rope to move in the corona while maintaining line-tied conditions at
the solar surface. These conditions ensure that the normal component
of the coronal magnetic field at the surface remains fixed during an
eruption and that no magnetic energy enters the corona through the
surface to drive the eruption. The model is based on the magnetic
configuration of Titov & Démoulin, where a toroidal flux rope is
held in equilibrium by an overlying magnetic arcade. We investigate
the stability of this configuration to specific perturbations and
show that it is subject to the torus instability when the flux rope
length exceeds a critical value. A force analysis of the configuration
shows that flux ropes are most likely to erupt in a localized region
near the apex, while the regions near the surface remain relatively
undisturbed. Thus, the flux rope will tend to form an aneurysm-like
structure once it erupts. Our analysis also suggests how the flux rope
rotation seen in some eruptions and simulations may be related to the
observed orientation of the overlying arcade field. This model exhibits
the potential for catastrophic loss of equilibrium as a possible trigger
for eruptions, but further study is required to prove this property.
Title: Theoretical Predictions of X-Ray and Extreme-UV Flare Emissions
Using a Loss-of-Equilibrium Model of Solar Eruptions
Authors: Reeves, Katharine K.; Warren, Harry P.; Forbes, Terry G.
Bibcode: 2007ApJ...668.1210R
Altcode:
In this paper, we present numerical simulations of solar flares that
couple a loss-of-equilibrium solar eruption model with a one-dimensional
hydrodynamic model. In these calculations, the eruption is initiated by
footpoint motions that disrupt the balance of forces acting on a flux
rope. After the eruption begins, a current sheet forms and an arcade
of flare loops is created by reconnecting magnetic fields. Thermal
energy input into the flare loops is found by assuming the complete
thermalization of the Poynting flux swept into the current sheet. This
thermal energy is input into a one-dimensional hydrodynamic code for
each loop formed in the multithreaded flare arcade. We find that a
density enhancement occurs at the loop top when the two evaporating
plasma fronts in each leg of the loop collide there. Simulated flare
images show that these loop-top density enhancements produce ``bars''
of bright emission similar to those observed in the Transition Region
and Coronal Explorer (TRACE) 195 Å bandpass and loop-top ``knots'' of
bright emission seen in flare observations by the Soft X-Ray Telescope
(SXT) on Yohkoh. We also simulate flare spectra from the Bragg Crystal
Spectrometer (BCS) on Yohkoh. We find that during the early stages of
flare initiation, there are significant blueshifts in the Ca XIX line,
but the intensities are too faint to be observed with BCS. In general,
the results of this model simulate observed flare emissions quite well,
indicating that the reconnection model of solar flares is energetically
consistent with observations.
Title: The Origin of Solar Eruptions
Authors: Forbes, T. G.
Bibcode: 2007AIPC..934...13F
Altcode:
The occurrence of eruptions in the solar atmosphere has been known for
nearly a 150 years, yet the underlying mechanism that creates them
remains obscure. Most present-day models are based on the principle
that the energy that drives an eruption comes from the magnetic energy
associated with stressed magnetic fields in the solar corona. However,
there is no general agreement in the solar research community as to
what triggers the release of this energy. One possibility is that it
is caused by a combination of ideal (e.g. loss of equilibrium) and
non-ideal (e.g. magnetic reconnection) processes. The first process
can explain the rapid onset of the eruption, but the second is needed
to explain the large scale of the energy release.
Title: An Analytical Three-Dimensional Field Model for Coronal
Mass Ejections
Authors: Forbes, T. G.; Isenberg, P. A.
Bibcode: 2007AGUSMSH32A..05F
Altcode:
We have obtained an explicit expression for the magnetic field of
an upward moving 3D flux rope whose feet are anchored in the solar
surface. The magnetic field in this model is rigorously line-tied. The
new field model allows us to investigate the equilibrium and stability
properties of line-tied flux ropes. We find that long flux ropes tend to
be more unstable than short ones, and that the shape and orientation of
the flux rope are likely to be significantly altered upon eruption. We
are also using this model to examine the reconnection process in
a situation where there is no topological distinction between field
lines. Our analysis indicates that reconnection can significantly alter
the appearance of the flux-rope footprint on the surface. This work
is supported by NSF National Space Weather Program grants ATM0518218
and ATM0519249 to the University of New Hampshire and Helio Research.
Title: A Cross Comparison of Physical Processes in the Genesis of
Coronal Mass Ejections and Plasmoids in the Earth's Magnetic Tail
Authors: Forbes, T. G.; Nakamura, R.
Bibcode: 2007AGUSMSH51D..01F
Altcode:
Both coronal mass ejections (CMEs) and plasmoid formation in the
geomagnetic tail have been characterized in terms of models that involve
a slow build-up of magnetic energy followed by its sudden release. In
each case the model invokes a growth phase during which the magnetic
energy is stored, a trigger mechanism which releases this stored energy,
and a recovery phase during which the field relaxes to a relatively low
energy state. Because of these shared principles, researchers working
on these two phenomena often ask similar questions. For example, "How
long must the growth phase be in order to store sufficient energy?,"
"What is the physics of the trigger mechanism?," and "What is the role
of magnetic reconnection during each phase?". There are, however, some
fundamental differences between models of CMEs and plasmoid formation
that prevent a one-model-fits- all approach. The field configurations
prior to onset are quite different as are the properties of the plasmas
in which the two phenomena occur. We assess the commonalities and
differences within each phenomenon and try to determine the areas of
research where a direct comparison is useful.
Title: Features and Properties of Coronal Mass Ejection/Flare
Current Sheets
Authors: Lin, J.; Li, J.; Forbes, T. G.; Ko, Y. -K.; Raymond, J. C.;
Vourlidas, A.
Bibcode: 2007ApJ...658L.123L
Altcode:
Solar eruptions occur when magnetic energy is suddenly converted into
heat and kinetic energy by magnetic reconnection in a current sheet
(CS). It is often assumed that CSs are too thin to be observable
because the electric resistivity ηe in CSs is taken
to be very small. In this work, we show the implications for the
CS thickness d estimated from observations of three eruptions by
the UVCS and the LASCO experiments on SOHO. We infer the effective
ηe causing the rapid reconnection, which predicts much
faster reconnection in a thick CS than that caused by the classical
and anomalous resistivities. We find that in these events CSs are
observable and have extremely large values of d and ηe,
implying that large-scale turbulence is operating within CSs. We also
discuss the properties of the so-called hyperresistivity caused by
the tearing mode and the relation to our results.
Title: Magnetic Reconnection
Authors: Priest, Eric; Forbes, Terry
Bibcode: 2007mare.book.....P
Altcode:
Preface; 1. Introduction; 2. Current-sheet formation; 3. Magnetic
annihilation; 4. Steady reconnection: the classical solutions;
5. Steady reconnection: new generation of fast regimes; 6. Unsteady
reconnection: the tearing mode; 7. Unsteady reconnection: other
approaches; 8. Reconnection in three dimensions; 9. Laboratory
applications; 10. Magnetospheric applications; 11. Solar applications;
12. Astrophysical applications; 13. Particle acceleration; References;
Appendices; Index.
Title: 2006 LWS TR & T Solar Wind Focused Science Topic Team:
The Beginnings
Authors: Miralles, M. P.; Bhattacharjee, A.; Landi, E.; Markovskii,
S.; Cranmer, S. R.; Doschek, G. A.; Forbes, T. G.; Isenberg, P. A.;
Kohl, J. L.; Ng, C.; Raymond, J. C.; Vasquez, B. J.
Bibcode: 2006AGUFMSH11A0371M
Altcode:
The Solar Wind Focused Science Topic (FST) team was created to
apply a combination of theoretical studies, numerical simulations,
and observations to the understanding of how the fast and slow solar
wind are heated and accelerated. Four proposals were selected for this
FST team. They will investigate the role of energy sources and kinetic
mechanisms responsible for the heating and acceleration of the solar
wind. In particular, the FST team will examine magnetic reconnection
and turbulence as possible heating mechanisms. Plasma properties and
their evolution over the solar cycle, determined from the analysis of
remote and in situ measurements, will be used to put firm constraints
on the models. The work of the Solar Wind FST team is in its initial
stages. The organization, planning, and findings resulting from the
first FST team meeting will be reported.
Title: Using a Loss-of-Equilibrium CME Model to Predict X-Ray and
EUV Emissions Resulting From Solar Flares
Authors: Reeves, K. K.; Warren, H. P.; Forbes, T. G.
Bibcode: 2006IAUJD...3E..68R
Altcode:
In this study, we use a loss-of-equilibrium model for solar eruptions
to calculate the thermal energy input into a system of flare loops. In
this model, the flare consists of a system of reconnecting loops
below a current sheet that connects the flare to an erupting flux
rope. The thermal energy is calculated by assuming that all of the
Poynting flux into the current sheet is thermalized. The density,
temperature and velocity of the plasma in each reconnected loop
are then calculated using a 1D hydrodynamic code. These parameters
are coupled with the instrument response functions of various solar
instruments to calculate flare emissions. We simulate spectra from the
Bragg Crystal Spectrometer (BCS) on Yohkoh, and find that the strong
blueshifts that should be present due to chromospheric evaporation
during flare initiation are difficult to observe with BCS, but may be
better observed with a more sensitive instrument. We also find that a
density enhancement occurs at the top of a loop when evaporating plasma
fronts in each loop leg collide there. This enhancement gives rise to
bright loop-top intensities in simulated Transition Region and Coronal
Explorer (TRACE) and Yohkoh Soft X-ray Telescope (SXT) images. These
loop-top features have been observed in TRACE and SXT images, and are
not explained by single-loop flare models. The Atmospheric Imaging
Assembly (AIA) on the Solar Dynamics Observatory should be able to
observe these features in detail, and we will use this model to help
develop flare observing programs for AIA.
Title: Stability and Dynamic Evolution of Three-dimensional Flux Ropes
Authors: Birn, Joachim; Forbes, Terry G.; Hesse, Michael
Bibcode: 2006ApJ...645..732B
Altcode:
A crucial problem in the study of coronal mass ejections (CMEs)
and solar flares is the identification of initial configurations
and boundary conditions that can produce an eruption of the field
configuration. In this paper we use (ideal) magnetohydrodynamic (MHD)
simulations to investigate the stability and dynamic evolution
of two (approximate) equilibrium configurations. The initial
models investigated are derived within a general framework for the
construction of series of suitable coronal states. They consist of
twisted flux ropes, connected to the photosphere and anchored in
the corona by an overlying arcade, embedded in a helmet streamer
type configuration. The two models studied differ by the magnitude
of the toroidal field and, correspondingly, the degree of twist and
the amount of plasma pressure. The model with the least twist remains
stable and settles into an equilibrium that differs only slightly from
the initial state. In contrast, the more strongly twisted flux rope
becomes unstable. Some portion of it breaks out in a kinklike fashion
and moves rapidly outward, while another portion remains below. The
evolved stage is characterized by the formation of a thin current
sheet below an outward-moving rope.
Title: Investigations of the Reconnecting Current Sheets in Solar
Eruptions
Authors: Lin, Jun; Li, J.; Forbes, T. G.; Ko, Y.; Raymond, J. C.;
van Ballegooijen, A. A.; Vourlidas, A.
Bibcode: 2006SPD....37.0826L
Altcode: 2006BAAS...38..235L
Observations and theories continuously provide evidence of formation
and development of the reconnecting current sheets during major
eruptions. Because the high electric conductivity and the force-free
environment of the solar corona, the current sheet is usually
confined in a small volume compared to the other structures involved
in the eruption in order to allow the energy conversion or magnetic
reconnection to take place at a plausible rate. The work investigating
the sizes, especially thickness, of the current sheet developed by the
eruption in reality was never conducted before since it is believed
that the current sheet is too thin to be observable. It has often been
stated that the thickness is limited either by the Lamor radius of
particles, which is about tens of meters in the coronal environment, or
by the mean-free-path of particles in the region where the interaction
between particles and ion-acoustic waves occurs. In the latter case,
the particle mean-free-path is a few kilometers. Here we use a set
of unique observations provided by the UVCS and LASCO experiments
on SOHO to determine the thickness, d, and furthermore the electric
resistivity, etae, of the current sheets for three eruptive
events. We find that d ranges from 1.0 x 104 km to 6.0
x 105 km, and etae from 1.0 x 105
ohm m to 4.0 x 106 ohm m. These values of etae
are 12-14 orders of magnitude greater than the classical collisional
resistivity, 4-6 orders of magnitude greater than estimates of anomalous
resistivity, and even 1-3 orders greater than those often used for solar
flares. The existence of such large values for d and etae
suggests that large scale turbulent processes are operating within
the current sheet. Understanding how a high level of turbulence can
develop so rapidly is a challenging goal for future research.
Title: Three-Dimensional Line-Tied Configurations with Eruptive
Behavior
Authors: Forbes, Terry G.; Isenberg, P. A.
Bibcode: 2006SPD....37.2204F
Altcode: 2006BAAS...38..249F
We have analytically determined equilibrium and stability properties
of a large class of three-dimensional line-tied configurations which
include both sheared arcades and flux ropes. Our analysis shows which
types of configurations are likely to produce CMEs and which are likely
to lead to "failed" CMEs. The analysis also predicts which type of
configurations are likely to show a strong "aneurysm-effect" in which
only the middle portion of a pre-existing flux rope erupts. This work
is supported by NSF National Space Weather Program grants ATM0518218
and ATM0519249 to the University of New Hampshire and Helio Research.
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena.
Report of Working Group F
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
Bibcode: 2006SSRv..123..341P
Altcode: 2006SSRv..tmp...60P
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since CMEs
originate in the low solar corona, understanding the physical processes
that generate them is strongly dependant on coordinated multi-wavelength
observations. CMEs display a large diversity in morphology and kinematic
properties, but there is presently no statistical evidence that those
properties may serve to group them into different classes. When a CME
takes place, the coronal magnetic field undergoes restructuring. Much
of the current research is focused on understanding how the corona
sustains the stresses that allow the magnetic energy to build up and
how, later on, this magnetic energy is released during eruptive flares
and CMEs. Multi-wavelength observations have confirmed that reconnection
plays a key role during the development of CMEs. Frequently, CMEs
display a rather simple shape, exhibiting a well known three-part
structure (bright leading edge, dark cavity and bright knot). These
types of events have led to the proposal of the ‘`standard model’'
of the development of a CME, a model which predicts the formation
of current sheets. A few recent coronal observations provide some
evidence for such sheets. Other more complex events correspond to
multiple eruptions taking place on a time scale much shorter than the
cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. “Complex type III-like events,”which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
Bibcode: 2006SSRv..123..251F
Altcode: 2006SSRv..tmp...59F
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
Title: Properties of the Post-CME Current Sheets in Solar Eruptions
Authors: Lin, J.; Li, J.; Forbes, T. G.; Ko, Y. -K.; Raymond, J. C.;
van Ballegooijen, A. A.; Vourlidas, A.
Bibcode: 2006cosp...36..198L
Altcode: 2006cosp.meet..198L
Solar eruptions constitute the most violent energy release process in
the solar system They are are created when magnetic energy is suddenly
converted into heat and kinetic energy by magnetic reconnection in
a field reversal region or current sheet The effective electrical
resistivity eta e of the sheet plasma plays a crucial role in energy
conversion Here we present the results for the current sheet thickness
d and eta e determined by analyzing a set of unique data for three
eruptions observed by the UVCS and the LASCO experiments on SOHO Such a
work was never conducted before since it is believed that the current
sheet is too thin to be observable The extremely large values of d
and eta e obtained in this work suggest that the current sheet in
solar eruptions is observable in some circumstances and large-scale
turbulence is operating within the current sheet This constitutes a
challenging goal for future research on the magnetic energy conversion
occurring in both space and laboratory
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
Bibcode: 2006cme..book..341P
Altcode:
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since
CMEs originate in the low solar corona, understanding the physical
processes that generate them is strongly dependant on coordinated
multi-wavelength observations. CMEs display a large diversity in
morphology and kinematic properties, but there is presently no
statistical evidence that those properties may serve to group them
into different classes. When a CME takes place, the coronal magnetic
field undergoes restructuring. Much of the current research is focused
on understanding how the corona sustains the stresses that allow the
magnetic energy to build up and how, later on, this magnetic energy is
released during eruptive flares and CMEs. Multiwavelength observations
have confirmed that reconnection plays a key role during the development
of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a
well known three-part structure (bright leading edge, dark cavity and
bright knot). These types of events have led to the proposal of the
"standard model" of the development of a CME, a model which predicts
the formation current sheets. A few recent coronal observations provide
some evidence for such sheets. Other more complex events correspond
to multiple eruptions taking place on a time scale much shorter than
the cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. "Complex type III-like events," which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
Bibcode: 2006cme..book..251F
Altcode:
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
Title: Resistive and Hall MHD Dynamics of a Flux Rope Model of
Eruptive Flares: Simulations and Observations
Authors: Ma, Z.; Bhattacharjee, A.; Forbes, T.
Bibcode: 2005AGUFMSH11C..04M
Altcode:
During the last ten years, an eruptive flare model based on a loss
of equilibrium in a coronal flux rope has been developed (Forbes and
Priest 1995, Lin and Forbes 2000) and has been tested by observations
from SOHO and RHESSI. We present fully self-consistent resistive
and Hall MHD simulations of the model beginning from an equilibrium
solution of the Grad-Shafranov equation in which a flux rope is nested
in an arcade. The system is then driven by photospheric converging
flows. It is found that the system evolves into a configuration with
a thin current sheet which grows progressively thinner and longer,
driving the flux rope upward. Magnetic reconnection in the thin current
sheet eventually leads to the accelerated expulsion of the flux rope,
as anticipated qualitatively by earlier theoretical calculations. In the
resistive MHD model, the reconnection rate as well as the acceleration
is controlled by the resistivity. In the Hall MHD model, the current
sheet becomes thinner and more dynamic, the acceleration is faster and
much more weakly dependent on the resistivity (at high values of the
Lundquist number). Comparisons will be made with an eruptive event on
2003 November 18.
Title: On the Relation between Reconnected Magnetic Flux and Parallel
Electric Fields in the Solar Corona
Authors: Hesse, Michael; Forbes, Terry G.; Birn, Joachim
Bibcode: 2005ApJ...631.1227H
Altcode:
Analytical theory and kinematic models are applied to magnetic
reconnection in the solar corona. The emphasis of the present
investigation is on the relation between the reconnection electric
field, the reconnection rate, and the change in magnetic connectivity
among photospheric magnetic footpoints. The results are not tied
to the presence or absence of specific topological features of the
magnetic field, such as a separatrix layer or separators. It is shown
that the critical element in determining the location of ribbon-like
bright features on the solar surface may be the parallel electric
field, integrated along magnetic field lines. A general relation
between the change of the reconnected magnetic flux and the integrated
parallel electric field is derived. The results of this analysis are
applied to the solar coronal case by means of two kinematic models,
one of which affords a fully analytical treatment. The results show
that reconnection-induced changes of magnetic connectivity on the
corona-photosphere interface are directly related to the maximum value
of the field line-integrated parallel electric field.
Title: Predicted Light Curves for a Model of Solar Eruptions
Authors: Reeves, Katharine K.; Forbes, Terry G.
Bibcode: 2005ApJ...630.1133R
Altcode:
We determine the thermal radiation generated by a loss-of-equilibrium
model for CMEs and eruptive solar flares. The magnetic configuration
of the model consists of an outward-moving flux rope with a vertical
current sheet below it. Reconnection at the sheet releases magnetic
energy, some of which is converted into thermal energy that drives
chromospheric evaporation along the newly connected field lines exiting
the current sheet. The thermal energy release is calculated by assuming
that all of the Poynting flux flowing into the reconnection region
is eventually thermalized. We find that the fraction of the released
magnetic energy that goes into thermal energy depends on the inflow
Alfvén Mach number. The evolution of the temperatures and densities
resulting from chromospheric evaporation is calculated using a simple
evaporative cooling model. Using these temperatures and densities, we
calculate simulated flare light curves for TRACE, the SXT on Yohkoh, and
GOES. We find that when the background magnetic field strength is weak,
the radiation emitted by the reconnected X-ray loops beneath a CME is
faint. Additionally, it is possible to have two CMEs with nearly the
same trajectories and speeds that have a significant difference in the
peak intensities of their light curves. We also examine the relationship
between the thermal energy release rate and the derivative of the soft
X-ray light curve and discuss the implications for the Neupert effect.
Title: Magnetic Reconnection and the Deduced Properties of Plasma
inside the CME/Flare Current Sheet
Authors: Lin, J.; Li, J.; Forbes, T. G.; Ko, Y.; Raymond, J. C.;
van Ballegooijen, A. A.
Bibcode: 2005AGUSMSH51A..04L
Altcode:
In the present work, we display our results of studying and analyzing
the observational data from UVCS and other remote sensing instruments
for three CME/flare events that obviously developed a long current
sheet during the eruptions. These results include the thickness of the
current sheets, magnetic diffusivities and electrical conductivities
(resistivities) of the plasma inside the current sheets. This is
the first time that the electrical conductivity (resistivity) within
magnetic reconnection region during the real eruptive processes has
been deduced since the theory of magnetic reconnection was applied to
the solar eruptions about 6 decades ago. The thickness of the current
sheet developed during the January 8, 2002 event varies from 7 ×
104 km to 2.2 × 105 km for altitudes between 2.5 R⊙ and 5.5 R⊙,
with the average thickness of 1.4× 105 km, and the speed of magnetic
reconnection inflow near the current sheet is about 10 km s-1. These
results suggest a magnetic diffusivity of the plasma inside the current
sheet to be 0.7 × 1012 m2 s-1 (compared to the classical value for
the quiet corona of 1 m2 s-1, and to the corresponding value for the
"turbulent plasma" of 3.4× 106 m2 s-1). For the event occurring on
November 18, 2003, the data from UVCS indicate that the upper limit
of the current sheet thickness at altitude of 1.7 R⊙ is about 2.8
× 104 km, and that the velocity of magnetic reconnection inflow near
the current sheet ranges from 10.5 km s-1 to 106 km s-1. Combining
these results yields a range of magnetic diffusivity from 1.4× 1011
m2 s-1 to 1.4× 1012 m2 s-1. During the event observed on March 23,
1998, the upper limit of the thickness of the current sheet in the
wake of a CME is about 105 km according to data from UVCS. No data
for the velocity of the magnetic reconnection near the current sheet
in this event were obtained. Considering the fact that this event was
more gradual than the other two cases, we assume the inflow speed in
this event to be 5 km s-1. So, we obtain that the magnetic diffusivity
of the plasma inside the current sheet has an upper limit of 2.5×
1011 m2 s-1. We notice that values of magnetic diffusivity deduced
for three different events are within the range of magnitude.
Title: The Effects of Gravity of an Loss-of-Equilibrium CME Initiation
Model
Authors: Reeves, Katharine K.; Forbes, Terry G.
Bibcode: 2005IAUS..226..250R
Altcode:
We include gravity in a loss of equilibrium model for the initiation of
coronal mass ejections (CMEs). We examine equilibria for both normal and
inverse polarity and neglect the effects of current sheets. Although
equilibria exist for normal polarities, in the absence of current
sheets, the equilibria are unstable to horizontal perturbations. For
the inverse polarity configuration, we find that gravity generally
has a negligible effect if the magnetic field is strong ( >50 G)
but that it can have a significant effect if the magnetic field is
weak. Specifically, if the characteristic magnetic field is less than
about 6 G, no eruption occurs if the CME mass is on the order of 2 ×
1016 gm.
Title: The Connections Between CME Models and Observations
Authors: Forbes, Terry
Bibcode: 2005IAUS..226..289F
Altcode:
SCHWENN: About the spiral pattern in the U.Michigan animation: We saw
such unwinding spirals. Question: what are they?
Title: On the Relation Between Reconnected Flux and Parallel Electric
Fields in the Solar Corona
Authors: Johnson, J.; Hesse, M.; Forbes, T.; Birn, J.
Bibcode: 2004AGUFMSH13A1150J
Altcode:
The complicated magnetic topology in the solar corona often does not
lend itself to the ready differentiation of topologically different
regions. Instead, magnetic reconnection often happens in coronal
fields that do not vanish, and for which no separator or separatrix is
readily identifiable. For this, rather generic, situation, the question
of how to relate observed magnetic flux changes to parallel electric
fields remains unanswered. In order to shed light on this question, we
apply to this problem the theory of General Magnetic Reconnection. We
prove that a unique relation exists between the time evolution of the
reconnected magnetic flux on one hand, and the integral along magnetic
field lines of the parallel electric field on the other. Furthermore,
we will use two explicit examples to illustrate how parallel electric
fields are related to the formation of closed loops by the reconnection
process, and how flux rope-like topologies result from reconnection
if photospheric electric fields are negligible on flare time scales.
Title: A New Field Line Advection Model for Solar Particle
Acceleration
Authors: Sokolov, I. V.; Roussev, I. I.; Gombosi, T. I.; Lee, M. A.;
Kóta, J.; Forbes, T. G.; Manchester, W. B.; Sakai, J. I.
Bibcode: 2004ApJ...616L.171S
Altcode:
The diffusive acceleration of solar protons at a shock wave driven by
a realistic coronal mass ejection is modeled using a new field line
advection model for particle acceleration coupled with a global MHD
code. The new model described in this Letter includes effects important
for the particle acceleration and transport, by means of diffusive shock
acceleration, and employs Lagrangian meshes. We performed a frequent
dynamical coupling between two numerical codes in order to account for
the time-dependent history of an evolving shock wave driven by a solar
eruption. The numerical results discussed here demonstrate that this
mechanism can account for the production of high-energy solar protons
observed during the early stages of gradual events.
Title: Chandra observation of an unusually long and intense X-ray
flare from a young solar-like star in M78
Authors: Grosso, N.; Montmerle, T.; Feigelson, E. D.; Forbes, T. G.
Bibcode: 2004sf2a.conf..293G
Altcode: 2004sf2a.confE.313G
LkHA312 has been observed serendipitously with the ACIS-I detector on
board Chandra with 26h continuous exposure. This Halpha emission line
star belongs to the star-forming region M78 (NGC2068). From the optical
and NIR data, we show that it is a pre-main sequence (PMS) low-mass
star with a weak NIR excess. This genuine T Tauri star displayed an
X-ray flare with an unusual long rise phase (~8h). The X-ray emission
was nearly constant during the first 18h of the observation, and
then increased by a factor of 13 during a fast rise phase (~2h), and
reached a factor of 16 above the quiescent X-ray level at the end of a
gradual phase (~6h) showing a slower rise. To our knowledge this flare,
with ~0.4-~0.5 cts/s, has the highest count rate observed so far with
Chandra from a PMS low-mass star. By chance, the source position, 8.2'
off-axis, protected this observation from pile-up. We make a spectral
analysis of the X-ray emission versus time, showing that the plasma
temperature of the quiescent phase and the flare peak reaches 29MK
and 88MK, respectively. The quiescent and flare luminosities in the
energy range 0.5--8keV corrected from absorption (NH~1.7E21
cm-2) are 6E30erg/s and ~1E32erg/s, respectively. The ratio
of the quiescent X-ray luminosity on the LkHA312 bolometric luminosity
is very high with log(LX/Lbol)= -2.9, implying
that the corona of LkHA312 reached the `saturation' level. The X-ray
luminosity of the flare peak reaches ~2% of the stellar bolometric
luminosity. The different phases of this flare are finally discussed in
the framework of solar flares, which leads to the magnetic loop height
from 3.1E10 to 1E11 cm (0.2-0.5 R*, i.e., 0.5-1.3 Rsun).
Title: Energy partition in two solar flare/CME events
Authors: Emslie, A. G.; Kucharek, H.; Dennis, B. R.; Gopalswamy, N.;
Holman, G. D.; Share, G. H.; Vourlidas, A.; Forbes, T. G.; Gallagher,
P. T.; Mason, G. M.; Metcalf, T. R.; Mewaldt, R. A.; Murphy, R. J.;
Schwartz, R. A.; Zurbuchen, T. H.
Bibcode: 2004JGRA..10910104E
Altcode:
Using coordinated observations from instruments on the Advanced
Composition Explorer (ACE), the Solar and Heliospheric Observatory
(SOHO), and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI),
we have evaluated the energetics of two well-observed flare/CME events
on 21 April 2002 and 23 July 2002. For each event, we have estimated
the energy contents (and the likely uncertainties) of (1) the coronal
mass ejection, (2) the thermal plasma at the Sun, (3) the hard X-ray
producing accelerated electrons, (4) the gamma-ray producing ions,
and (5) the solar energetic particles. The results are assimilated
and discussed relative to the probable amount of nonpotential magnetic
energy available in a large active region.
Title: Chandra observation of an unusually long and intense X-ray
flare from a young solar-like star in M 78
Authors: Grosso, N.; Montmerle, T.; Feigelson, E. D.; Forbes, T. G.
Bibcode: 2004A&A...419..653G
Altcode: 2004astro.ph..2672G
LkHα 312 has been observed serendipitously with
the ACIS-I detector on board the Chandra X-ray Observatory with
a 26 h continuous exposure. This Hα emission line star belongs
to M 78 (NGC 2068), one of
the star-forming regions of the Orion B giant molecular cloud at
a distance of 400 pc. From the optical and the near-infrared (NIR)
data, we show that LkHα 312 is a pre-main sequence (PMS) low-mass
star with a weak NIR excess. This genuine T Tauri star displayed an
X-ray flare with an unusually long rise phase (∼8 h). The X-ray
emission was nearly constant during the first 18 h of the observation,
and then increased by a factor of 13 during a fast rise phase (∼2 h),
and reached a factor of 16 above the quiescent X-ray level at the end
of a gradual phase (∼6 h) showing a slower rise. To our knowledge
this flare, with ∼0.4-0.5 cts s-1, has the highest count
rate observed so far with Chandra from a PMS low-mass star. By chance,
the source position, 8.2 arcmin off-axis, protected this observation
from pile-up. We make a spectral analysis of the X-ray emission versus
time, showing that the plasma temperature of the quiescent phase and
the flare peak reaches 29 MK and 88 MK, respectively. The quiescent
and flare luminosities in the energy range 0.5-8 keV corrected from
absorption (NH ≈ 1.7× 1021 cm-2)
are 6× 1030 erg s-1 and ∼1032
erg s-1, respectively. The ratio of the quiescent X-ray
luminosity on the LkHα 312 bolometric luminosity is very high with
log (LX/Lbol)= -2.9, implying that the corona
of LkHα 312 reached the ``saturation'' level. The X-ray luminosity of
the flare peak reaches ∼2% of the stellar bolometric luminosity. The
different phases of this flare are finally discussed in the framework
of solar flares, which leads to the magnetic loop height from 3.1×
1010 to 1011 cm (0.2-0.5 Rstar, i.e.,
0.5-1.3 R⊙).
Title: A Numerical Model of CME Initiation and Shock Development for
the 1998 May 2 Event: Implications for the Acceleration of GeV Protons
Authors: Roussev, I. I.; Sokolov, I. V.; Forbes, T. G.; Gombosi,
T. I.; Lee, M. A.
Bibcode: 2004AAS...204.6704R
Altcode: 2004BAAS...36..783R
We present modeling results on the initiation and evolution of the
coronal mass ejection which occurred on 1998 May 2 in NOAA AR8210. This
is done within the framework of a global model of the solar magnetic
field as it was observed by the Wilcox Solar Observatory. Our
calculations are fully three-dimensional and involve compressible
magnetohydrodynamics. We begin by first producing a steady-state solar
wind for Carrington Rotation 1935/6. The solar eruption is initiated
by slowly evolving the boundary conditions until a critical point is
reached where the configuration loses mechanical equilibrium. As this
point, the field erupts, and a flux rope is ejected away from the Sun,
reaching a maximum speed in excess of 1,000 km/s. The shock that forms
in front of the rope reaches a fast-mode Mach number in excess of 4
and a compression ratio greater than 3 by the time it has traveled a
distance of 5 solar radii from the surface. Thus, by constructing a
fully three-dimensional numerical model, which incorporates magnetic
field data and a loss-of-equilibrium mechanism, we have been able to
demonstrate that a shock can develop close to the Sun sufficiently
strong to account for the energization of solar particles. For this
event, diffusive-shock-acceleration theory predicts a distribution of
solar energetic protons with a cut-off energy of about 10 GeV.
Title: A Numerical Model of a Coronal Mass Ejection: Shock Development
with Implications for the Acceleration of GeV Protons
Authors: Roussev, I. I.; Sokolov, I. V.; Forbes, T. G.; Gombosi,
T. I.; Lee, M. A.; Sakai, J. I.
Bibcode: 2004ApJ...605L..73R
Altcode:
The initiation and evolution of the coronal mass ejection, which
occurred on 1998 May 2 in NOAA Active Region 8210, are modeled using a
fully three-dimensional, global MHD code. The initial magnetic field
for the model is based on magnetogram data from the Wilcox Solar
Observatory, and the solar eruption is initiated by slowly evolving
the boundary conditions until a critical point is reached where the
configuration loses equilibrium. At this time, the field erupts, and
a flux rope is ejected that achieves a maximum speed in excess of 1000
km s-1. The shock that forms in front of the rope reaches a
fast-mode Mach number in excess of 4 and a compression ratio greater
than 3 by the time it has traveled a distance of 5 Rsolar
from the surface. For such values, diffusive shock acceleration theory
predicts a distribution of solar energetic protons with a cutoff
energy of about 10 GeV. For this event, there appears to be no need
to introduce an additional acceleration mechanism to account for solar
energetic protons with energies below 10 GeV.
Title: Overview of energy release and particle acceleration at the Sun
Authors: Forbes, T. G.
Bibcode: 2004cosp...35.1620F
Altcode: 2004cosp.meet.1620F
The magnetic energy associated with currents flowing in the corona is
the most likely source of energy for the acceleration of particles
in flares, coronal mass ejections, and other explosive solar
phenomena. Therefore, to understand particle acceleration fully
one needs to understand both the mechanism of the magnetic energy
release, as well as the processes which transfer this energy to the
particles. Models of the energy release mechanism based on an ideal-MHD
loss of equilibrium or stability can readily account for the rapid
time scale of the energy release observed in explosive events, but
such models cannot alone account for the acceleration of the energetic
particles and the heating of the plasma. Non-ideal processes, such as
magnetic reconnection and shock formation, must be invoked to account
for these aspects. Radiation produced by energetic particles provides
unique information about the location and growth of these non-ideal
processes. Since these processes vary considerably from one model to
another, observations of this radiation can, at least in principle,
be used to discriminate between the various models.
Title: 3D MHD simulations of the May 2, 1998 halo CME: Comparison
of CME initiation models and their characteristics at L1
Authors: Manchester, W. B.; Roussev, I. I.; Gombosi, T. I.; Sokolov,
I. V.; Forbes, T. G.
Bibcode: 2004cosp...35...77M
Altcode: 2004cosp.meet...77M
We present the results of two numerical models of the partial-halo CME
event associated with NOAA AR8210 on May 2, 1998. Our simulations are
fully three-dimensional and involve compressible magnetohydrodynamics
with turbulent energy transport. We begin by first producing a
steady-state solar wind for Carrington Rotation 1935/6, following the
methodology described in Roussev et al. (2003). For the first model, we
superpose the Gibbson-Low magnetic flux rope into the helmet streamer of
AR8210. In the second newer model, instead, we impose shearing motions
along the polarity inversion line of AR8210, followed by converging
motions, both via the modification of the boundary conditions at the
Sun's surface. In the first model, a magnetic flux rope exists in the
corona prior to the eruption, whereas in the second model, a flux rope
forms from reconnection within the sheared arcade during the CME. In
either case, flux ropes are expelled from the Sun, manifesting a
partial-halo CME through a highly structured, ambient solar wind. We
follow the ejected plasma flows from the corona to the Earth's orbit
and compare the time evolution of the solar wind parameters predicted
by the two models with satellite observations at the L1 point. With
such a comparison, we hope to address much debated issue of whether
magnetic flux ropes are a component of the pre-event corona.
Title: 3D MHD Simulations of the May 2, 1998 halo CME: Shock formation
and SEP acceleration
Authors: Sokolov, I. V.; Roussev, I. I.; Gombosi, T. I.; Forbes,
T. G.; Lee, M. A.
Bibcode: 2004cosp...35...76S
Altcode: 2004cosp.meet...76S
We present the results of two numerical models of the partial-halo CME
event associated with NOAA AR8210 on May 2, 1998. Our simulations are
fully three-dimensional and involve compressible magnetohydrodynamics
with turbulent energy transport. We begin by first producing a
steady-state solar wind for Carrington Rotation 1935/6, following the
methodology described in Roussev et al. (2003). We impose shearing
motions along the polarity inversion line of AR8210, followed by
converging motions, both via the modification of the boundary conditions
at the Sun's surface. As a consequence, a flux rope forms within the
sheared arcade during the CME. The flux rope gradually accelerates,
leaving behind the remnants of a flare loop system that results
from ongoing magnetic reconnection in the naturally formed current
sheet. The flux rope leaves the Sun, forming a CME emerging through
a highly structured, ambient solar wind. A shock wave forms in front
of the ejected matter. Estimates for the spectral index and cutoff
energy for the diffusive solar energetic particle shock acceleration
mechanism show that the protons can be efficiently accelerated up to
energies 0.1-10 GeV.
Title: Pre-Eruptive and Eruptive Magnetic Structures in the Solar
Corona
Authors: Forbes, T. G.
Bibcode: 2003AGUFMSH41A..01F
Altcode:
One of the greatest difficulties in the development of models of
coronal mass ejections (CMEs) and flares is the lack of information
about the coronal magnetic field prior to its eruption. The situation is
exacerbated by the fact that only the component of the magnetic field
generated by currents flowing in the corona contributes to the energy
release. The amount of energy released during a CME or flare implies
that net change in the total magnetic field in the corona is no more
than about 10 percent on average. However, there may be localized
regions where the changes are significantly greater, perhaps on the
order of 100 percent. The pattern of these localized changes can be
used to distinguish between various models. For example, flux rope
models of CMEs and eruptive flares imply that the transverse field
at the base of the corona can reverse sign in the region immediately
below the flux rope, but that the transverse field further away
increases. Recent observational studies suggest that such CME and flare
associated changes in the coronal magnetic field will not penetrate to
the level of the photosphere, but will be confined to regions above an
altitude of 400 km or higher. Thus, observations of the chromospheric
and coronal fields are needed to make progress in understanding the
mechanism of solar eruptions.
Title: Observational evidence of new current sheets trailing coronal
mass ejections
Authors: Webb, D. F.; Burkepile, J.; Forbes, T. G.; Riley, P.
Bibcode: 2003JGRA..108.1440W
Altcode:
Field line reconnection in the wake of coronal mass ejections (CMEs)
is a fundamental aspect of some magnetically driven eruptive flare/CME
models, e.g., the standard reconnection model [cf. Svestka and Cliver,
1992]. This model features a growing hot loop arcade beneath a rising
X-type neutral point that is connected to the retreating CME. In models
invoking reconnection the rising CME and neutral point are connected
by a stretched current sheet. Two recent models, Lin and Forbes [2000]
and Linker et al. [2003], predict that an extended, long-lived current
sheet must be formed for any physically plausible reconnection rate. Lin
and Forbes derive estimates for heights or lengths of current sheets
and the energy input as functions of time. In a previous observational
study of SMM CMEs observed from 1984-1989 having candidate magnetic
disconnection features, primarily transient concave-outward bright
regions following the CME leading edge, we found that about half were
followed by coaxial, bright rays suggestive of newly formed current
sheets. The rays appeared relatively suddenly several hours after
the main CME had left the field of view. In this paper we present the
results of analysis of these structures, including their heights and
lengths, widths, alignments, and motions, all as functions of time,
and show that they are consistent with the existence of current sheets
lasting for several hours and extending more than five solar radii
into the outer corona.
Title: A 3D Line-Tied Model of Flux Rope Eruptions
Authors: Isenberg, P. A.; Forbes, T. G.
Bibcode: 2003AGUFMSH21A..03I
Altcode:
We consider the 3D force-free flux rope configuration which was proposed
by Titov and Demoulin [Astron. Astrophys., 351, 707, 1999] as an
approximation to the observed topology of coronal structures leading
to eruptive flares. This configuration consists of a toroidal flux
rope, oriented vertically, which intersects the photospheric surface
at some distance above the center of the torus, along with several
other photospheric sources which make up the potential background
field in the corona. We extend the analysis of this configuration to
correctly implement the effects of photospheric line-tying. We then
investigate the equilibrium properties of this configuration and the
conditions which yield an ideal loss of equilibrium. We follow the
subsequent eruption of the flux rope under the assumption that the
coronal portion retains a circular arc structure. Ultimately, this
model will provide a semi-analytic 3D description of the evolution of
coronal magnetic structures during an eruptive flare.
Title: On Flux Rope in the 1997 May 12 Event
Authors: Liu, Y.; Forbes, T. G.
Bibcode: 2003AGUFMSH42B0521L
Altcode:
The solar source of this event was reported to be an C1.3/1N flare with
a duration of more than 8 hours, occurring at 04:42 UT on 12 May 1997
in the active region AR8038 at N21W08, which was the only active region
in solar disk. The magnetic configuration of this active region is very
simple -- a dominant leader sunspot with positive polarity surrounded
by weak negative magnetic field patches. Associated with this flare
were eruption of a filament and a complete halo CME with a speed of
\( 250 km\dot s^{-1} \) (Plunkett, et al., 1999). SOHO also observed
double dimmings and EIT wave (Thompson, et al., 1999). This flare
appears to be a classic two-ribbon flare: With eruption of the filament,
two bright ribbons showed up and moved apart, seen in Hα observation;
sigmoidal bright patterns shown in YOHKOH/SXT and SOHO/EIT images became
post-flare arcades; and magnetic shear computed from observed vector
magnetic field significantly decreased. These observations provide an
opportunity for us to investigate the possible flux rope in this event,
as illuminated in the two-ribbon flare model. We took three approaches
to estimate the properties of the flux rope in this event. We found
that the twist from these methods is strikingly different. We will
discuss possible errors in our computation. This research was supported
by NASA under contract NAS5-30386, MURIs of UC Berkeley and Michigan
University, and the CISM of Boston University. SOHO is a joint project
between the ESA and NASA.
Title: Models of Three-dimensional Flux Ropes and Their Stability
Authors: Schindler, K.; Birn, J.; Forbes, T. G.
Bibcode: 2003AGUFMSH41A..05S
Altcode:
A crucial problem in the study of coronal mass ejections is the
understanding of the evolution prior to the eruption and the development
of initial configurations and boundary conditions that can produce
an eruption of the field configuration and the identification of
observable features from these configurations. We present a general
framework to derive approximate equilibrium configurations suitable
to describe pre-eruption states, using an approach originally
developed for the Earth's magnetotail but including magnetic shear
and gravity. We illustrate the approach by deriving configurations for
both force-free and non-force-free states. The variable models contain
a twisted flux rope, connected to the photosphere and anchored in the
corona by an overlying arcade. The models may also include a magnetic
configuration above the flux rope typical for helmet streamers. Using
three-dimensional MHD simulations, we also ivestigate the models for
their stability.
Title: Solar flare theory and light curves
Authors: Forbes, T. G.
Bibcode: 2003AdSpR..32.1043F
Altcode:
During the last few years several models have been developed to explain
the onset and energy release in large, eruptive solar flares. Although
there is no consensus yet about the precise mechanism which triggers the
onset of an eruption, there is a consensus that the mechanism involves
the release of magnetic energy stored in the solar corona. Several of
the proposed models use an ideal-MHD loss of equilibrium to trigger
the eruption, but these models still require magnetic reconnection
to extract the bulk of the magnetic energy. Coupling the ideal
loss-of-equilibrium process with the non-ideal reconnection processes,
and incorporating thermal conduction, chromospheric evaporation,
and radiative cooling, shows great promise as a way to account for
the light curves of both solar and stellar flares.
Title: Models of Three-dimensional Flux Ropes
Authors: Birn, Joachim; Forbes, Terry G.; Schindler, Karl
Bibcode: 2003ApJ...588..578B
Altcode:
A crucial problem in the study of coronal mass ejections is
understanding the evolution prior to the eruption and the development
of initial configurations and boundary conditions that can produce
an eruption of the field configuration and the identification of
observable features from these configurations. In this paper we present
a general framework to derive approximate equilibrium configurations
suitable to describe pre-eruption states, using an approach originally
developed for the Earth's magnetotail but including magnetic shear
and gravity. The solutions hold for configurations that vary most
strongly in one spatial direction perpendicular to the direction of
gravity. We illustrate the approach by deriving configurations for
both force-free and non-force-free states. The variable models contain
a twisted flux rope connected to the photosphere and anchored in the
corona by an overlying arcade. The models may also include a magnetic
configuration above the flux rope typical for helmet streamers.
Title: A Three-dimensional Flux Rope Model for Coronal Mass Ejections
Based on a Loss of Equilibrium
Authors: Roussev, Ilia I.; Forbes, Terry G.; Gombosi, Tamas I.;
Sokolov, Igor V.; DeZeeuw, Darren L.; Birn, Joachim
Bibcode: 2003ApJ...588L..45R
Altcode:
A series of simulation runs are carried out to investigate the loss of
equilibrium of the three-dimensional flux rope configuration of Titov
& Démoulin as a suitable mechanism for the initiation of coronal
mass ejections. By means of these simulations, we are able to determine
the conditions for which stable equilibria no longer exist. Our results
imply that it is possible to achieve a loss of equilibrium even though
the ends of the flux rope are anchored to the solar surface. However,
in order to have the flux rope escape, it is necessary to modify the
configuration by eliminating the arcade field.
Title: Evolution of a semicircular flux rope with two ends anchored
in the photosphere
Authors: Lin, J.; van Ballegooijen, A. A.; Forbes, T. G.
Bibcode: 2002JGRA..107.1438L
Altcode:
We investigated a coronal magnetic configuration including a
semicircular flux rope with two ends anchored in the photosphere. The
background field is produced by two source regions on the
photosphere. We study the evolution of this configuration in response
to the gradual change in the background field, which is modeled by
varying either the strength of the source or the distance between
the source regions on the photosphere. Our results indicate that the
evolution due to the change in source strength shows the likelihood of
catastrophic loss of equilibrium, and that the evolution due to the
change in the distance is smooth and does not manifest any tendency
to lose equilibrium. In the former case, the current sheet starts
developing fairly early; it forms even before the evolution reaches
the maximum current state. We notice that the effect of the curvature
of flux rope on the evolution of the system is significant, such that
the equilibrium curve does not form a cusp-catastrophic structure but
a simple fold-catastrophic structure even if the evolution is ideal
MHD and a current sheet attached to the boundary surface occurs. The
curvature strengthens the magnetic compression between the flux rope
and the photosphere and makes the loss of equilibrium easier. However,
the question of how the system behaves after the loss of equilibrium
is still open since the flux rope is not likely to remain semicircular
at the stage of fast evolution.
Title: Numerical Test of a Three-Dimensional Flux Rope Model for
Coronal Mass Ejections Based on Ideal MHD Processes
Authors: Roussev, I. I.; Forbes, T. G.; Gombosi, T.; Sokolov, I.
Bibcode: 2002AGUFMSH21A0476R
Altcode:
A series of simulation runs in Cartesian coordinates are carried out,
using the BATS-R-US code, to investigate the loss of equilibrium of
the 3D flux rope configuration of Titov and Démoulin (1999) as a
potential CME initiation mechanism. All numerical experiments are
fully 3D and involve ideal magnetohydrodynamics. Our results show
that the criterion R > √ {2} L, derived in the earlier study
by neglecting the effects of line-tying of the poloidal field, may
be a necessary condition for a loss of equilibrium, but it is not a
sufficient one. The line-tying of the ends of flux rope leads to a
much more stringent condition for an eruption to occur. The question
remains whether it is possible to get a loss of equilibrium with the
Titov and Démoulin model which will lead to a CME-like eruption. We do
find evidence for a loss of equilibrium, but the resulting evolution
of the system more closely resembles an impulsive-type flare rather
than a CME. Whether there is a region of the model's parameter space
where CME-like eruptions can occur remains to be determined.
Title: Theory of CMEs: Eruption, Reconnection, and FLux Ropes
Authors: Forbes, T. G.
Bibcode: 2002AAS...200.6505F
Altcode: 2002BAAS...34..751F
The mechanism which causes sudden eruptions in the solar atmosphere
remains controversial. However, there is some consensus that the most
energetic eruptions are powered by the sudden release of magnetic
energy associated with currents flowing in the corona. Of the various
mechanism which have been suggested, most involve the disappearance
of a stable equilibrium as a result of the slow evolution of the
photospheric magnetic field. This disappearance may be due to a loss
of ideal-MHD equilibrium or stability such as occurs in the kink mode,
or to a loss of resistive-MHD equilibrium as a result of magnetic
reconnection. Mechanisms involving gravitational processes have
also been considered. Many of the proposed mechanisms are based on
configurations containing a flux rope imbedded in a magnetic arcade, or
a highly sheared arcade which has flux-rope-like properties. Application
of the MHD virial theorem to such configurations implies that the key
property which allows them to lose equilibrium, or stability, is the
inverse polarity of the magnetic field created by the presence of the
flux rope.
Title: Theory of solar flares
Authors: Forbes, T.
Bibcode: 2002cosp...34E.775F
Altcode: 2002cosp.meetE.775F
The calculation of solar flare emissions from first principles requires
consideration of the coronal reconnection process which releases
magnetic energy in the corona, the transport process which conveys
this energy to the chromosphere, and the evaporation process which
injects mass back into the corona to form the flare loops. Although
numerous calculations of this kind have been carried out on the latter
two processes, very few have been done on the first one, that is how
reconnection heats and accelerates plasma over the course of the
flare. The main reason so few calculations have been done is that
the way the reconnection process converts magnetic energy into heat
and flows depends critically on the mechanism which initiates the
flare, and the nature of this mechanism is poorly understood at the
present time. Here we present a theoretical light curve for a flare
based on the assumption that the initiating mechanism is an ideal-MHD
instability which drives reconnection at a current sheet that forms
after flare onset.
Title: Flares & Reconnection in flares: where we stand?
Authors: Forbes, Terry
Bibcode: 2002ocnd.confE..11F
Altcode:
No abstract at ADS
Title: The magnetic nature of solar flares
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 2002A&ARv..10..313P
Altcode:
The main challenge for the theory of solar eruptions has been to
understand two basic aspects of large flares. These are the cause of
the flare itself and the nature of the morphological features which
form during its evolution. Such features include separating ribbons of
Hα emission joined by a rising arcade of soft x-ray loops, with hard
x-ray emission at their summits and at their feet. Two major advances
in our understanding of the theory of solar flares have recently
occurred. The first is the realisation that a magnetohydrodynamic (MHD)
catastrophe is probably responsible for the basic eruption and the
second is that the eruption is likely to drive a reconnection process
in the field lines stretched out by the eruption. The reconnection is
responsible for the ribbons and the set of rising soft x-ray loops,
and such a process is well supported by numerical experiments and
detailed observations from the Japanese satellite Yohkoh. Magnetic
energy conversion by reconnection in two dimensions is relatively well
understood, but in three dimensions we are only starting to understand
the complexity of the magnetic topology and the MHD dynamics which are
involved. How the dynamics lead to particle acceleration is even less
well understood. Particle acceleration in flares may in principle occur
in a variety of ways, such as stochastic acceleration by MHD turbulence,
acceleration by direct electric fields at the reconnection site, or
diffusive shock acceleration at the different kinds of MHD shock waves
that are produced during the flare. However, which of these processes
is most important for producing the energetic particles that strike
the solar surface remains a mystery.
Title: Prominence eruptions and coronal mass ejections triggered by
newly emerging flux
Authors: Lin, J.; Forbes, T. G.; Isenberg, P. A.
Bibcode: 2001JGR...10625053L
Altcode:
Using a simple model for the onset of solar eruptions, we investigate
how an existing magnetic configuration containing a flux rope evolves
in response to new emerging flux. Our results show that the emergence
of new flux can cause a loss of ideal MHD equilibrium under certain
circumstances, but the circumstances which lead to eruption are much
richer and more complicated than one might expect given the simplicity
of the model. The model results suggest that the actual circumstances
leading to an eruption are sensitive not only to the polarity of
the emerging region, but also to several other parameters, such as
the strength, distance, and area of the emerging region. It has been
suggested by various researchers that the emergence of new flux with
an orientation which allows reconnection with the preexisting flux (a
process sometimes referred to as tether cutting) will generally lead to
destabilization of the coronal or prominence magnetic field. Although
our results can replicate such behavior for certain restricted classes
of boundary conditions, we find that, in general, there is no simple,
universal relation between the orientation of the emerging flux and
the likelihood of an eruption.
Title: The nature of Petschek-type reconnection
Authors: Forbes, T. G.
Bibcode: 2001EP&S...53..423F
Altcode:
It is not always appreciated that Petschek's reconnection mechanism
is a particular solution of the MHD equations which applies only
when special conditions are met. Specifically, it requires that the
flow into the reconnection region be set up spontaneously without
external forcing. This condition is satisfied when reconnection in
a simple current sheet is initiated by enhancing the resistivity in
a localized region. Such a process disrupts the current sheet and
launches slow-mode waves which steepen into nearly switch-off shocks
of the type predicted by Petschek. As these shocks propagate outwards,
the current sheet reforms at the original point of the disturbance, and
a quasi-steady Petschek-like configuration is set up. Syrovatskii-like
configurations which force reconnection by driving a flow toward an
initially current-free, orthogonal x-point are less likely to satisfy
the conditions required for Petschek-type reconnection.
Title: Observational Evidence of New Current Sheets Following CMEs
Authors: Webb, D. F.; Burkepile, J.; Forbes, T. G.
Bibcode: 2001AGUSM..SH51C03W
Altcode:
Some degree of disconnection of the magnetic flux associated with
CMEs appears to be required to prevent a continuous increase in the
net interplanetary magnetic flux, which is not observed. Field line
reconnection in the wake of CMEs is also a fundamental aspect of some
magnetically driven eruptive flare/CME models, e.g., the "standard"
CHSKP model. This model features a growing hot loop arcade beneath a
rising X-type neutral point that is connected to the retreating CME
by a current sheet. Some recent MHD models invoking reconnection in
such a current sheet, e.g., Lin and Forbes (2000), make predictions
of the height or length of the current sheet and the energy input
as functions of time. In a previous study of SMM CMEs observed from
1984-1989 having candidate magnetic disconnection features, primarily
transient concave-outward bright regions following the CME leading edge,
we found that about half were followed by coaxial, bright, narrow rays
suggestive of newly formed current sheets. The rays appeared relatively
suddenly several hours after the CME leading edge had left the field
of view. We present the results of analysis of these structures,
including their heights and lengths, brightness variations, and widths
all as functions of time, and compare the results with CME/flux rope
reconnection models which involve current sheets.
Title: The Role of the Photosphere During Solar Eruptions
Authors: Forbes, T. G.
Bibcode: 2001AGUSM..SH41C03F
Altcode:
From time to time various researchers have proposed that solar flares or
coronal mass ejections (CMEs) are driven by a surge of electromagnetic
energy flowing either from the convection zone or the photosphere into
the corona. However, this point of view is difficult to reconcile with
the extremely tranquil conditions that exist in the photosphere during
flares and CMEs. Models based on the storage of magnetic energy prior to
an eruption also transfer energy from the convective zone to the corona,
but this process occurs over a long time period on the order of hours to
days prior to the CME. The photospheric motions are directly observed,
while the build-up of current which results from them can be inferred
from vector magnetograms and changes in field-aligned plasma structures.
Title: Magnetohydrodynamics
Authors: Priest, E.; Forbes, T.
Bibcode: 2000eaa..bookE1983P
Altcode:
Magnetohydrodynamics (or MHD for short) is the study of the interaction
between a magnetic field and a plasma treated as a continuous medium
(e.g. Cowling 1957, Roberts 1967, Priest 1982, 1994). Most of the
universe is not a normal gas but is instead a plasma. We are all
familiar on Earth with the three states of matter (solid, liquid and
gas). You change from one state to another (such as ice to...
Title: What can we learn about reconnection from coronal mass
ejections?
Authors: Forbes, T. G.; Lin, J.
Bibcode: 2000JASTP..62.1499F
Altcode: 2000JATP...62.1499F
It may be possible to calculate the rate of reconnection in the corona
by measuring the rate at which the temporary coronal hole formed by a
coronal mass ejection (CME) disappears. This calculation is possible
if the disappearance of the hole is caused by the same reconnection
process which creates the giant X-ray arches associated with CMEs. These
arches form just below the vertical current sheet that is created as
the CME drags magnetic field lines out into interplanetary space, and
they are similar in form to `post'-flare loops, except that they often
have an upward motion that is different. Instead of continually slowing
with time as `post'-flare loops do, they move upwards at a rate which
increases, or remains nearly constant, with time. This difference has
raised doubts about the relevance of reconnection to the formation and
propagation of the arches. Using a two-dimensional flux rope model to
calculate the size and location of the current sheet as a function of
time, we find that the difference between the motion of `post'-flare
loops and giant arches can be explained simply by the variation of
the coronal Alfvén speed with height.
Title: Solar Flare Models
Authors: Forbes, T.
Bibcode: 2000eaa..bookE2295F
Altcode:
Even though FLARES have been observed on the Sun for nearly 150
years, their origin remains a mystery. At the present time there is
no generally accepted model which explains why they occur, but there
do exist models which successfully explain certain limited aspects
such as the formation of flare loops and ribbons. Before discussing
particular models, we review the constraints imposed on models ...
Title: Simulations of Three-Dimensional Reconnection in the Solar
Corona
Authors: Birn, Joachim; Gosling, John T.; Hesse, Michael; Forbes,
Terry G.; Priest, Eric R.
Bibcode: 2000ApJ...541.1078B
Altcode:
Using ideal and resistive MHD, we investigate the stability
and dynamic evolution of three-dimensional magnetic field
configurations, representing stretched arcade structures above a dipolar
``photospheric'' magnetic field. Two types of configurations are studied
that differ by the amount of divergence (``fanning'') of the initial
field lines as viewed in the horizontal direction perpendicular to the
photospheric magnetic neutral line and, correspondingly, by the radial
decrease of magnetic field strength and current density. The two sets
of configurations are found to differ in their stability behavior. The
strongly fanning fields, associated with a rapid radial decrease of
the field strength, current density, and plasma pressure, are more
stable. A stability difference is found also when the configurations
are first subjected to a converging motion of photospheric footpoints
toward the neutral line, which leads to the buildup of thin current
sheets in the region above. This current sheet formation is more
pronounced for the weakly fanning fields. For similar current density
enhancements, the occurrence of anomalous dissipation (resistivity)
initiates magnetic reconnection in either configuration. However,
the effects are much more drastic in magnitude and spread in the
weakly diverging field structure. In the unstable cases, a strongly
localized electric field parallel to the magnetic field develops,
which results in integrated voltages with maximum values of the
order of a few hundred MeV, both on open and closed field lines. For
comparison, we studied both low-beta, force-free, and high-beta initial
states. The weakly fanning high-beta configurations tend to show more
drastic instability effects than the corresponding low-beta fields,
but the stabilization of the strongly fanning fields pertains to both
low-beta and high-beta fields. The three-dimensional reconnection in
the unstable cases generates a region of intertwined magnetic flux
tubes with different topologies that lie below a region of closed flux
ropes not affected by reconnection. The topological changes could be
the source of open flux tubes that are occasionally observed within
coronal mass ejections, as recently discussed by Gosling et al. The
fast outward flow generated in these simulations affects only the
regions of changing topology but does not cause the above-lying closed
flux ropes to move (within the times considered). This may be seen
as an indication that reconnection may be associated with the onset
of a flare, initiated after the eruption of a coronal mass ejection,
but is not the driver of the coronal mass ejection itself.
Title: A review on the genesis of coronal mass ejections
Authors: Forbes, T. G.
Bibcode: 2000JGR...10523153F
Altcode:
This paper provides a short review of some of the basic concepts related
to the origin of coronal mass ejections (CMEs). The various ideas which
have been put forward to explain the initiation of CMEs are categorized
in terms of whether they are force-free or non-force-free and ideal or
nonideal. A few representative models of each category are examined
to illustrate the principles involved. At the present time there is
no model which is sufficiently developed to aid forecasters in their
efforts to predict CMEs, but given the current pace of research,
this situation could improve dramatically in the near future.
Title: Magnetic Reconnection
Authors: Priest, Eric; Forbes, Terry
Bibcode: 2000mare.book.....P
Altcode:
Magnetic reconnection is at the core of many dynamic phenomena in
the universe, such as solar flares, geomagnetic substorms and tokamak
disruptions. Written by two world leaders on the subject, this volume
provides a comprehensive overview of this fundamental process. Coverage
gives both a pedagogical account of the basic theory and a wide-ranging
review of the physical phenomena created by reconnection--from
laboratory machines, the Earth's magnetosphere, and the Sun's atmosphere
to flare stars and astrophysical accretion disks. It also includes a
succinct account of particle acceleration by electric fields, stochastic
fields and shock waves, and how reconnection can be important in these
mechanisms. Clearly written and highly accessible, this volume serves
as an essential introduction for graduate students in solar physics,
astrophysics, plasma physics and space science. Researchers in these
fields also will find Magnetic Reconnection an authoritative reference.
Title: A Method for Determining Coronal Reconnection Rates in
Eruptive Events
Authors: Forbes, T. G.
Bibcode: 2000SPD....31.0280F
Altcode: 2000BAAS...32..825F
By measuring the rate at which the magnetic flux opened by a coronal
mass ejection, or an eruptive flare, disappears, one can determine
the global rate of reconnection in Webers per second (or Maxwells
per second in CGS units) as a function of time. No specific model of
the coronal magnetic field is needed using this approach. All that is
required is the line-of-sight component of the field as obtained from
a standard magnetogram and measurements of the surface area on the Sun
corresponding to newly closed field lines. The latter can be determined
by observing the motion of the chromospheric ribbons which lie at the
feet of the X-ray loops formed in the aftermath of an eruptive event.
Title: What Causes Deceleration of Coronal Mass Ejection
Authors: Lin, J.; Forbes, T. G.
Bibcode: 2000SPD....31.0907L
Altcode: 2000BAAS...32Q.842L
More and more observational data show evidence of deceleration in
some coronal mass ejections (CMEs) (e.g. Sheeley et al. 1999). Sheeley
et al. (1999) classified CMEs into two categories: (1) Gradual CMEs,
which have speeds of 400-600 km/s, usually form when prominences erupt;
and (2) impulsive CMEs, which have speeds of 500-1000 km/s, are often
associated with flares. CMEs in the second category typically undergo
a period of deceleration after their initial acceleration, while CMEs
in the first category do not. What could cause the deceleration of the
fast CMEs? After ruling out the effects of projection and gravity,
Sheeley et al. (1999) suggested that the mass swept up by the shock
accompanying the ejection may be the reason. Here we suggest another
possibility, namely the rapid formation of an extensive current sheet
below the CME. Such a current sheet tends to form for both slow and
fast CMEs, but in the case of a fast CME the growth of the sheet is
much more rapid than its dissipation by magnetic reconnection. In the
case of a slow CME the growth and reconnection processes are more evenly
balanced, and thus the current sheet is not as extensive. Consequently,
deceleration is less likely to occur in a slow CME.
Title: Effects of reconnection on the coronal mass ejection process
Authors: Lin, J.; Forbes, T. G.
Bibcode: 2000JGR...105.2375L
Altcode:
This work investigates how magnetic reconnection affects the
acceleration of coronal mass ejections (CMEs) and how the acceleration
in turn affects the reconnection process. To model the CME process,
we use a two-dimensional flux rope model, which drives the ejection
by means of a catastrophic loss of mechanical equilibrium. Our model
provides a method for relating the motion of the ejected material to
the reconnection rate in the current sheet created by the erupting
field. In the complete absence of reconnection the tension force
associated with the current sheet is always strong enough to prevent
the flux rope from escaping from the Sun. However, our results imply
that even a fairly small reconnection rate is sufficient to allow
the flux rope to escape. Specifically, for a coronal density model
that decreases exponentially with height we find that average Alfvén
Mach number MA for the inflow into the reconnection site
can be as small as MA=0.005 and still be fast enough to
give a plausible eruption. The best fit to observation is obtained by
assuming an inflow rate on the order of MA~0.1. With this
value the energy output matches the temporal behavior inferred for
the long duration events often associated with CMEs. The model also
suggests an explanation for the peculiar motion of giant X-ray arches
reported by Svestka et al. [1995, 1997]. X-ray arches are the large
loops associated with CMEs which are similar in form to ``post''-flare
loops, but they have an upward motion that is often different. Instead
of continually slowing with time, the arches move upward at a rate
that remains nearly constant or may even increase with time. Here we
show how the difference can be explained by reversal of the gradient
of the coronal Alfvén speed with height.
Title: Solar and stellar flares
Authors: Forbes, T. G.
Bibcode: 2000RSPTA.358..711F
Altcode:
New observations by space telescopes during the last ten years have
led to significant advances in understanding the nature of solar
flares. X-ray and UV imaging of flare emissions have confirmed that
flares are powered by the sudden release of magnetic energy associated
with currents flowing in the solar atmosphere. Although many different
processes have been suggested as possible trigger mechanisms, the
one which seems to fit the observations best is a loss of ideal-MHD
equilibrium, or stability, combined with magnetic reconnection. An
ideal-MHD process by itself has the drawback that it releases a
very small amount of magnetic energy, but when it is coupled with
magnetic reconnection, this drawback is eliminated. Stellar flares are
very likely to be fundamentally similar to solar flares in that they
involve the sudden release of magnetic energy associated with currents
flowing in their coronae. However, it is unlikely that they all involve
exactly the same type of field configuration or the same type of trigger
mechanism. What these mechanisms might be will be difficult to determine
without further information on the structure of stellar magnetic fields.
Title: Magnetic reconnection : MHD theory and applications
Authors: Priest, Eric; Forbes, Terry
Bibcode: 2000mrmt.conf.....P
Altcode:
No abstract at ADS
Title: Reconnection Theory in Three Dimensions
Authors: Forbes, T. G.
Bibcode: 2000AdSpR..26..549F
Altcode:
One of the most interesting aspects of magnetic reconnection theory
in three-dimensions, as opposed to two, is that electric fields are
generated with a component parallel to the magnetic field. When the
magnetic field has null points, the parallel electric field occurs on
the separator field line joining the null points, but if null points are
absent, then the region of parallel electric field typically extends
over a much broader region. The exact distribution of the parallel
electric field is determined by the way the plasma flow drives the
reconnection. Topologically, there are two basic types of reconnection,
known as spine reconnection and fan reconnection, but it is not yet
clear how these two types are related to actual reconnection processes
occurring in the Sun. Several important advances have been made recently
in applying reconnection theory to three-dimensional configurations
such as flares and X-ray bright points where magnetic reconnection
is thought to be important. However, our knowledge of the generation
of electric fields in these configurations still remains incomplete,
and this makes it difficult to understand the relationship between
magnetic reconnection and the acceleration of energetic particles
Title: TRACE and Yohkoh Observations of High-Temperature Plasma in
a Two-Ribbon Limb Flare
Authors: Warren, H. P.; Bookbinder, J. A.; Forbes, T. G.; Golub, L.;
Hudson, H. S.; Reeves, K.; Warshall, A.
Bibcode: 1999ApJ...527L.121W
Altcode:
The ability of the Transition Region and Coronal Explorer
(TRACE) to image solar plasma over a wide range of temperatures
(Te~104-107 K) at high spatial
resolution (0.5" pixels) makes it a unique instrument for observing
solar flares. We present TRACE and Yohkoh observations of an M2.4
two-ribbon flare that began on 1999 July 25 at about 13:08 UT. We
observe impulsive footpoint brightenings that are followed by the
formation of high-temperature plasma (Te>~10 MK)
in the corona. After an interval of about 1300 s, cooler loops
(Te<2 MK) form below the hot plasma. Thus, the
evolution of the event supports the qualitative aspects of the standard
reconnection model of solar flares. The TRACE and Yohkoh data show that
the bulk of the flare emission is at or below 10 MK. The TRACE data
are also consistent with the Yohkoh observations of hotter plasma
(Te~15-20 MK) existing at the top of the arcade. The
cooling time inferred from these observations is consistent with a
hybrid cooling time based on thermal conduction and radiative cooling.
Title: Relationship of `Post'-Flare Loops to Giant Arches
Authors: Lin, J.; Forbes, T. G.
Bibcode: 1999AAS...19410109L
Altcode: 1999BAAS...31..999L
Recently, using data from Yohkoh, Svestka and his co-workers
have analyzed giant X-ray arches that appear to be very similar to
`post'-flare loops except that they are much larger and have a different
pattern of growth. Instead of growing upwards at a rate which decreases
with time, the giant arches move upwards at rate which increases with
time. Because of this difference, Svestka has suggested that the giant
arches may be fundamentally different from `post'-flare loops. Here
we show theoretically that this need not be the case. By introducing
reconnection with a finite rate into a flux-rope model of coronal
mass ejections (CMEs), we show that an increasing rate of upward
motion can occur in a loop system if it reaches sufficiently high
altitudes. Our two-dimensional model contains a magnetic flux-rope
which loses equilibrium when the photospheric sources of the magnetic
field are moved together. Initially there is no current sheet, but
once the flux rope loses equilibrium and starts to move upwards,
a current sheet appears. As the current sheet grows, reconnection
tends to dissipate it. During the early phase of the eruption,
the current sheet grows too quickly for the reconnection to have a
major effect, but during the very late phase, reconnection eventually
starts to dominate and this leads to a rapid shrinkage of the current
sheet. During the early phase, the velocity of the lower tip of the
current sheet decreases with time, but during the late phase, the
opposite is true. Our model shows that the upward motion of the lower
tip of the current sheet is sensitive to the vertical gradient of the
Alfven speed in the corona. Because the Alfven speed increases with
height, reconnection is more effective at high altitudes than at lower
altitudes in dissipating the current sheet. We estimate that a loop
system associated with the lower tip of the current sheet will start to
speed up a few hours after onset if heights in excess of around 10(5)
km are reached. Thus, we conclude that the giant arches are essentially
the same phenomenon as post-flare loops and show a different pattern
of motion only because of the extreme heights at which they occur.
Title: The Effect of Curvature on Flux-Rope Models of Coronal Mass
Ejections
Authors: Lin, J.; Forbes, T. G.; Isenberg, P. A.; Démoulin, P.
Bibcode: 1998ApJ...504.1006L
Altcode:
The large-scale curvature of a flux rope can help propel it outward
from the Sun. Here we extend previous two-dimensional flux-rope models
of coronal mass ejections to include the curvature force. To obtain
analytical results, we assume axial symmetry and model the flux rope as
a torus that encircles the Sun. Initially, the flux rope is suspended
in the corona by a balance between magnetic tension, compression,
and curvature forces, but this balance is lost if the photospheric
sources of the coronal field slowly decay with time. The evolution of
the system shows catastrophic behavior as occurred in previous models,
but, unlike the previous models, flux ropes with large radii are more
likely to erupt than ones with small radii. The maximum total magnetic
energy that can be stored before equilibrium is lost is 1.53 times the
energy of the potential field, and this value is less than the limiting
value of 1.662 for the fully opened field. As a consequence, the loss
of ideal MHD equilibrium that occurs in the model cannot completely
open the magnetic field. However, the loss of equilibrium does lead
to the sudden formation of a current sheet, and if rapid reconnection
occurs in this sheet, then the flux rope can escape from the Sun. We
also find that the field can gradually become opened without suffering
any loss of equilibrium if the photospheric field strength falls
below a critical value. This behavior is analogous to the opening of
a spherically symmetric arcade in response to a finite amount of shear.
Title: Magnetic energy release in solar flares (abstract)
Authors: Forbes, T. G.
Bibcode: 1998PAICz..88..139F
Altcode:
No abstract at ADS
Title: The Energetics of Flux-Rope Prominence Models in Axially
Symmetric Systems
Authors: Lin, J.; Forbes, T. G.; Isenberg, P. A.; Demoulin, P.
Bibcode: 1998ASPC..150..350L
Altcode: 1998npsp.conf..350L; 1998IAUCo.167..350L
No abstract at ADS
Title: Unsolved problems in prominence research.
Authors: Forbes, T. G.
Bibcode: 1997ASIC..494..149F
Altcode: 1997topr.conf..149F
Understanding the magnetic field structure of prominences is a challenge
to both observers and theorists. A model for the magnetic field of a
prominence must be able to fulfill several basic functions. The model
must explain why prominences erupt and prescribe the conditions that
lead to eruption. Additionally, the pre-eruption field configuration
must be consistent with the observed structure of quiescent prominences
and account for their gravitational and thermal stability. So far,
there are no models which can satisfy all these conditions.
Title: A Strong Limitation on the Rapidity of Flux-Pile-Up
Reconnection
Authors: Litvinenko, Y. E.; Forbes, T. G.; Priest, E. R.
Bibcode: 1996SoPh..167..445L
Altcode:
The reconnection rate which can be achieved in the steady-state
flux-pile-up regime is severely limited by the gas pressure of the
plasma. Using the family of solutions obtained previously by Priest and
Forbes, we show that the Alfvén Mach number of the plasma flowing
towards the reconnection site cannot exceed πβe/(8
ln Rme), where βe and Rme are
the plasma β and magnetic Reynolds numbers at large distance. This
limit corresponds to a very weak flux-pile-up, and it is a factor of
βe slower than the maximum Petschek rate. Thus the maximum
flux-pile-up reconnection rate in the corona is at least two orders
of magnitude smaller than the rate implied by flare observations.
Title: Reconnection dynamics in cusp-shaped flare loops
Authors: Forbes, T. G.
Bibcode: 1996AIPC..374..275F
Altcode: 1996hesp.conf..275F
The soft X-ray telescope on Yohkoh has observed plasma structures at the
top of flare loops which are suggestive of reconnection jets. However,
these structures are relatively cool compared to the surrounding plasma,
and their location within the dense flare loops is not consistent with
the location expected of a reconnection jet. Numerical simulations
of field line reconnection in a radiative plasma suggest that the
observed structures are instead condensations which are formed below
a reconnection jet lying at higher altitude. In the simulations the
condensation is caused by the increase in density downstream of the
fast-mode shock which terminates the reconnection jet. The increased
density locally enhances the radiative cooling and causes the top of
the loop to cool faster than the legs.
Title: Reconnection and Field Line Shrinkage in Solar Flares
Authors: Forbes, T. G.; Acton, L. W.
Bibcode: 1996ApJ...459..330F
Altcode:
We use images of flare loops taken by the Soft X-ray Telescope (SXT)
on Yohkoh to estimate the decrease in height that open field lines
undergo after they have reconnected to form closed loops. Following
previous practice, we refer to this decrease as field line shrinkage
and assume that intensity structures trace out the field lines. For
this study, we examine two long-duration events near the limb which
have flare loops that continually grow with time. The shrinkage is
determined by comparing the height of a field line when it lies at the
outermost edge of the flare loop system with the height it has later on
when it lies at the innermost edge. We find that the field lines shrink
by about 20% of their initial height in one flare and by about 32%
in the other. These values are within 5% of the shrinkage predicted by
a simple model of the reconnecting field which assumes that the field
is potential everywhere except for a current sheet extending upward
from the top of the loops. Numerical integration of the model density
along the line of sight implies that most of the discrepancy between
the observations and the theory is due to projection effects which
occur when an arcade of loops is viewed at an arbitrary angle. Both
flares have bright regions at the top of the loops, but in one flare
the lower part of the region is cooler and denser than the rest of
the loop, while in the other flare it is not. Consideration of the
mapping of the bright regions to the footpoint of the loops implies
that the cool region is formed by a thermal instability downstream of
a reconnection outflow in the uppermost part of the loop. The absence
of a cool, dense region in the other flare may be caused by the fact
that it is a very weak event with temperatures and densities too low
to trigger a thermal instability.
Title: Reconnection Theory for Flares (Invited)
Authors: Forbes, T. G.
Bibcode: 1996ASPC..111..259F
Altcode: 1997ASPC..111..259F
The principal evidence for reconnection in solar flares comes from
observations of flare loops and ribbons during the gradual phase of
large-scale events. The motion of these features implies that the rate
of reconnection during the gradual phase is relatively fast despite the
large value of the magnetic Reynolds number in the corona. This result
suggests that current sheets in the lower corona start to dissipate
by means of reconnection as soon as they are formed.
Title: Arcade Flare Models
Authors: Forbes, T. G.
Bibcode: 1996mpsa.conf..287F
Altcode: 1996IAUCo.153..287F
No abstract at ADS
Title: Models for the Motions of Flare Loops and Ribbons
Authors: Lin, J.; Forbes, T. G.; Priest, E. R.; Bungey, T. N.
Bibcode: 1995SoPh..159..275L
Altcode:
We have found a conformal mapping which is valid for any magnetic
boundary condition at the photosphere and which can be used to determine
the evolution of an open, two-dimensional magnetic field configuration
as it relaxes to a closed one. Solutions obtained with this mapping
are in quasi-static equilibrium, and they contain a vertical current
sheet and have line-tied boundary conditions. As a specific example,
we determine the solution for a boundary condition corresponding to a
submerged, two-dimensional dipole below the photosphere. We assume that
the outer edges of the hottest X-ray loops correspond to field lines
mapping from the outer edges of the Hα ribbon to the lower tip of
the current sheet where field lines reconnect at aY-type neutral line
which rises with time. The cooler Hα loops are assumed to lie along
the field lines mapping to the inner edges of the flare ribbons. With
this correspondence between the plasma structures and the magnetic
field we determine the shrinkage that field lines are observed to
undergo as they are disconnected from the neutral line. During the
early phase of the flare, we predict that shrinkage inferred from
the height of the Hα and X-ray loops is close to 100% of the loop
height. However, the shrinkage should rapidly decrease with time to
values on the order of 20% by the late phase. We also predict that
the shrinkage in very large loops obeys a universal scaling law which
is independent of the boundary condition, provided that the field
becomes self-similar (i.e., all field lines have the same shape) at
large distances. Specifically, for any self-similar field containing
aY-type neutral line, the observed shrinkage at large distances should
decrease as (ΔX/XR)−2/3, where ΔX is the
ribbon width andXRis the ribbon separation. Finally, we
discuss the relation between the electric field at the neutral line
and the motions of the flare loops and ribbons.
Title: Photospheric Magnetic Field Evolution and Eruptive Flares
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1995ApJ...446..377F
Altcode:
Using an eruptive flare model based on a loss of equilibrium in a
coronal flux rope, we show that the average horizontal component
of the photospheric field does not necessarily become more like
a potential (current-free) field immediately after the eruption
begins. Therefore, recent observations showing that the average
horizontal field becomes less potential during the impulsive phase of
a flare do not necessarily imply that the magnetic energy in the corona
has increased as has sometimes been assumed. The flux-rope model which
we use also has relevance to coronal mass ejections. It differs from
previous models because eruption is triggered solely by the converging
motion of two photospheric field sources that lie below the coronal
flux rope. Because this boundary condition is much simpler than that
used in previous formulations, this particular version of the model
is especially well-suited for two-dimensional numerical simulations
with translational symmetry.
Title: How Does Fast Reconnection Work?
Authors: Forbes, T. G.
Bibcode: 1995LNP...462..319F
Altcode: 1995ssst.conf..319F
Considerable controversy exists as to whether Petschek's solution
for fast reconnection actually occurs in nature or even whether it
is a valid solution of the MHD equations. This paper argues that
Petschek's mechanism is a mathematically consistent solution, though
not necessarily a stable one. Furthermore, Petschek's mechanism is
not the only type of fast reconnection which can occur, and it may
very well be that it does not typically occur in nature. Other types
of reconnection such as flux-pile-up, where the field is strongly
compressed by an external flow, may be more common. Even the slow type
of reconnection originally proposed by Sweet and Parker may be fast
if the effective electrical resistivity is determined by turbulent
processes rather than laminar ones.
Title: Material Ejection
Authors: Webb, David F.; Forbes, Terry G.; Aurass, Henry; Chen, James;
Martens, Piet; Rompolt, Bogdan; Rusin, Vojtech; Martin, Sara F.
Bibcode: 1994SoPh..153...73W
Altcode:
This paper reviews the major discussions and conclusions of the Flares
22 Workshop concerning the physical processes involved in mass ejecta
events, with an emphasis on large-scale phenomena, especially Coronal
Mass Ejections (CMEs). New insights have been gained from recent
data obtained from the SMM andYohkoh spacecraft and from several new
ground-based radio and optical instruments, as well as from theoretical
advances concerning the origins, driving mechanisms and long-term
evolution of CMEs.
Title: 'Fireflies' on the Sun
Authors: Forbes, T. G.
Bibcode: 1994Natur.369..278F
Altcode:
No abstract at ADS
Title: On the Maximum Energy Release in Flux Rope Models of Eruptive
Flares
Authors: Forbes, T. G.; Priest, E. R.; Isenberg, P. A.
Bibcode: 1994SoPh..150..245F
Altcode:
We determine the photospheric boundary conditions which maximize
the magnetic energy released by a loss of ideal-MHD equilibrium in
two-dimensional flux-rope models. In these models a loss of equilibrium
causes a transition of the flux rope to a lower magnetic energy state at
a higher altitude. During the transition a vertical current sheet forms
below the flux rope, and reconnection in this current sheet releases
additional energy. Here we compute how much energy is released by the
loss of equilibrium relative to the total energy release. When the
flux-rope radius is small compared to its height, it is possible to
obtain general solutions of the Grad-Shafranov equation for a wide
range of boundary conditions. Variational principles can then be
used to find the particular boundary condition which maximizes the
magnetic energy released for a given class of conditions. We apply
this procedure to a class of models known as cusp-type catastrophes,
and we find that the maximum energy released by the loss of equilibrium
is 20.8% of the total energy release for any model in this class. If
the additional restriction is imposed that the photospheric magnetic
field forms a simple arcade in the absence of coronal currents, then
the maximum energy release reduces to 8.6%.
Title: On Current Sheet Approximations in Models of Eruptive Flares
Authors: Bungey, T. N.; Forbes, T. G.
Bibcode: 1994SoPh..149..205B
Altcode:
We consider an approximation sometimes used for current sheets in
flux-rope models of eruptive flares. This approximation is based on a
linear expansion of the background field in the vicinity of the current
sheet, and it is valid when the length of the current sheet is small
compared to the scale length of the coronal magnetic field. However,
we find that flux-rope models which use this approximation predict
the occurrence of an eruption due to a loss of ideal-MHD equilibrium
even when the corresponding exact solution shows that no such eruption
occurs. Determination of whether a loss of equilibrium exists can only
be obtained by including higher order terms in the expansion of the
field or by using the exact solution.
Title: Motions of Loops and Ribbons of Two-Ribbon Flares
Authors: Lin, J.; Forbes, T. G.; Priest, E. R.
Bibcode: 1994ASPC...68..202L
Altcode: 1994sare.conf..202L
No abstract at ADS
Title: Catastrophic Evolution of a Force-free Flux Rope: A Model
for Eruptive Flares
Authors: Isenberg, P. A.; Forbes, T. G.; Demoulin, P.
Bibcode: 1993ApJ...417..368I
Altcode:
We present a self-consistent, two-dimensional, magnetohydrodynamic
model of an eruptive flare based on an ideal-MHD coronal magnetic
field configuration which is line-tied at the photosphere and contains
a forcefree flux rope. If the flux rope is not too large, the gradual
disappearance of the photospheric field causes the flux rope to lose
equilibrium catastrophically and jump to a higher altitude, releasing
magnetic energy in the process. During the jump, an extended current
sheet forms below the flux rope, and subsequent reconnection of this
current sheet allows the flux rope to escape into the outer corona. A
critical flux-rope radius, which depends on the form of the photospheric
field, divides configurations which undergo a catastrophic loss of
equilibrium from those which do not. For a photospheric field equivalent
to that produced by a submerged, two-dimensional magnetic quadrupole,
the critical radius is 0.23 times the length scale of the photospheric
field. This result shows that catastrophic eruptions can occur for
flux ropes having plausible solar values. We identify the catastrophic
loss of equilibrium with the impulsive phase of eruptive flares and
the subsequent reconnection of the current sheet with the gradual phase.
Title: Upper Limits for Impulsive Energy Release in Flux-Rope Models
of Eruptive Flares
Authors: Forbes, T. G.; Isenberg, P. A.; Priest, E. R.
Bibcode: 1993BAAS...25.1198F
Altcode:
No abstract at ADS
Title: Model Prediction for Magnetic Shear Changes During Solar Flares
(Invited)
Authors: Forbes, T. G.
Bibcode: 1993ASPC...46..415F
Altcode: 1993mvfs.conf..415F; 1993IAUCo.141..415F
No abstract at ADS
Title: Does fast magnetic reconnection exist?
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1992JGR....9716757P
Altcode:
The classical Petschek model of fast, steady state reconnection
has been generalized in two families of reconnection regimes. The
first family, which we refer to as ``almost uniform,'' models the
reconnection of nearly uniform, antiparallel magnetic fields, and
it includes Petschek's model as a special case. The second family,
which we refer to as nonuniform, models the reconnection of strongly
curved magnetic fields, and it includes separatrix jets and reversed
current spikes at the ends of the diffusion region. In general,
both families contain regimes having fast reconnection rates,
but we show here that these fast reconnection regimes do not occur
when the boundary conditions often used in numerical experiments
are adopted. In 1986, D. Biskamp carried out a series of numerical
experiments to check Petschek's prediction that the maximum reconnection
rate should scale with the magnetic Reynolds number, Rme,
as [ln(Rme)]-1. Biskamp found that the maximum
reconnection rate in his experiments did not scale in this way but
instead as Rme-1/2. Because this corresponds to
the scaling predicted by the slow reconnection theory of Sweet (1958)
and Parker (1957), Biskamp has argued that his numerical experiments
show that fast reconnection does not exist at high magnetic Reynolds
numbers. However, by applying boundary conditions similar to Biskamp's
to the ``nonuniform'' family of reconnection regimes, we are able
to explain Biskamp's scaling results and to explain why he did not
achieve fast reconnection in his numerical experiments. Therefore,
we conclude that numerical experiments with suitably designed boundary
conditions are highly likely to exhibit fast reconnection and that such
reconnection is a common process in astrophysical and space plasmas.
Title: The Structure of Radiative Slow-Mode Shocks
Authors: Xu, P.; Forbes, T. G.
Bibcode: 1992SoPh..139..315X
Altcode:
We investigate the structure of slow-mode MHD shocks in a plasma
where both radiation and thermal conduction are important. In such
a plasma a slow shock dissociates into an extended foreshock, an
isothermal subshock, and a downstream radiative cooling region. Our
analysis, which is both numerical and analytical, focuses on the
nearly switch-off shocks which are generated by magnetic reconnection
in a strong magnetic field. These shocks convert magnetic energy into
kinetic energy and heat, and we find that for typical flare conditions
about f of the conversion occurs in the subshock while the remaining
1/3 occurs in the foreshock. We also find that no stable, steady-state
solutions exist for radiative slow shocks unless the temperature in the
radiative region downstream of the subshock falls below 105
K. These results suggest that about 2/3 of the magnetic energy released
in flare loops is released at the top of the loop, while the remaining
1/3 is released in the legs of the loop.
Title: The Generation of Thermal X-Rays in Solar Flares by Magnetic
Reconnection
Authors: Forbes, T. G.
Bibcode: 1992AAS...180.3013F
Altcode: 1992BAAS...24..777F
No abstract at ADS
Title: Weighted Current Sheets Supported in Normal and Inverse
Configurations: A Model for Prominence Observations
Authors: Demoulin, P.; Forbes, T. G.
Bibcode: 1992ApJ...387..394D
Altcode:
A technique which incorporates both photospheric and prominence
magnetic field observations is used to analyze the magnetic support
of solar prominences in two dimensions. The prominence is modeled
by a mass-loaded current sheet which is supported against gravity
by magnetic fields from a bipolar source in the photosphere and a
massless line current in the corona. It is found that prominence
support can be achieved in three different kinds of configurations:
an arcade topology with a normal polarity; a helical topology with a
normal polarity; and a helical topology with an inverse polarity. In
all cases the important parameter is the variation of the horizontal
component of the prominence field with height. Adding a line current
external to the prominence eliminates the nonsupport problem which
plagues virtually all previous prominence models with inverse polarity.
Title: Magnetic Flipping: Reconnection in Three Dimensions Without
Null Points
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1992JGR....97.1521P
Altcode:
In three dimensions, magnetic reconnection may take place in a sheared
magnetic field at any singular field line, where the nearby field
has X-type topology in planes perpendicular to the field line and
where an electric field is present parallel to the field line. In
the ideal region around the singular line there will, in general,
be singularities in the plasma flow and electric field, both at the
singular line and at ``magnetic flipping layers,'' which are remnants
of local magnetic separatrices. In the absence of a three-dimensional
magnetic neutral point or null point, reconnection of field lines can
still occur by a process of magnetic flipping, in which the plasma
crosses the flipping layers but the field lines rapidly flip along
them by magnetic diffusion. Depending on the boundary conditions,
there may be two or four flipping layers which converge on the singular
line. A boundary layer analysis of a flipping layer is given in which
the magnetic field parallel to the layer decreases as one crosses it
while the plasma pressure (or magnetic pressure associated with the
field along the singular line) increases. The width of the flipping
layer decreases with distance from the singular line.
Title: Field Opening and Reconnection
Authors: Forbes, T. G.
Bibcode: 1992LNP...399...79F
Altcode: 1992esf..coll...79F; 1992IAUCo.133...79F
During an eruptive flare a large magnetic loop or plasmoid is ejected
into interplanetary space, and the closed magnetic field structure which
exists prior to the flare becomes opened. One of the requirements of
flare models is to explain how the field can be opened while decreasing
the overall magnetic energy of the system. After the field is opened,
reconnection must occur in order to restore the field to its pre-flare
configuration. The strongest evidence for reconnection in flares comes
from observations of the chromospheric ribbons and the coronal loops
which form after the onset of a large flare. The ribbons and loops
appear to propagate through the chromosphere and corona during the
flare, but Doppler-shift measurements show conclusively that these
apparent motions are not due to mass motions of the solar plasma. The
motions can only be explained by the upward propagation of an energy
source in the corona, and in the 1VIHD-reconnection model of flares, the
propagating energy source is an x-line accompanied by slow-mode shocks.
Title: Why magnetic reconnection?
Authors: Forbes, T. G.
Bibcode: 1992mrpa.work...19F
Altcode:
Interest in magnetic reconnection has continually grown since it was
first proposed in the 1940's. The reasons for this growth are varied,
but the most important is that reconnection is necessary for the
efficient release of energy stored in stellar and planetary magnetic
fields. This paper reviews the recent work on steady-state theory and
the x-type collapse.
Title: Reconnection in solar and space plasmas.
Authors: Forbes, T. G.
Bibcode: 1992mrpa.work..165F
Altcode:
This paper reviews the evidence for the existence of magnetic
reconnection in the solar atmosphere and draws comparisons with
reconnection phenomenon in other space plasma regimes.
Title: Magnetic reconnection in solar flares
Authors: Forbes, T. G.
Bibcode: 1991GApFD..62...16F
Altcode:
The magnetic energy stored in the corona is the only plausible
source for the energy released during large solar flares. During
the last 20 years most theoretical work has concentrated on models
which store magnetic energy in the corona in the form of electrical
currents, and a major goal of present day research is to understand
how these currents are created, and then later dissipated during
a flare. Another important goal is to find a flare model which can
eject magnetic flux into interplanetary space. Although many flares do
not eject magnetic flux, those which do are of special importance for
solar-terrestrial relations since the ejected flux can have dramatic
effects if it hits the Earth's magnetosphere. Three flare models which
have been extensively investigated are the emerging-flux model, the
sheared-arcade model, and the magnetic-flux-rope model. All of these
models can store and release magnetic energy efficiently provided that
rapid magnetic reconnection occurs. However, only the magnetic-flux-rope
model appears to provide a plausible mechanism for ejecting magnetic
flux into interplanetary space.
Title: Magnetic reconnection in solar flares
Authors: Forbes, T. G.
Bibcode: 1991GApFD..62...15F
Altcode:
The magnetic energy stored in the corona is the only plausible
source for the energy released during large solar flares. During
the last 20 years most theoretical work has concentrated on models
which store magnetic energy in the corona in the form of electrical
currents, and a major goal of present day research is to understand
how these currents are created, and then later dissipated during
a flare. Another important goal is to find a flare model which can
eject magnetic flux into interplanetary space. Although many flares do
not eject magnetic flux, those which do are of special importance for
solar-terrestrial relations since the ejected flux can have dramatic
effects if it hits the Earth's magnetosphere. Three flare models which
have been extensively investigated are the emerging-flux model, the
sheared-arcade model, and the magnetic-flux-rope model. All of these
models can store and release magnetic energy efficiently provided that
rapid magnetic reconnection occurs. However, only the magnetic-flux-rope
model appears to provide a plausible mechanism for ejecting magnetic
flux into interplanetary space.
Title: A numerical simulation of magnetic reconnection and radiative
cooling in line-tied current sheets
Authors: Forbes, T. G.; Malherbe, J. M.
Bibcode: 1991SoPh..135..361F
Altcode:
We have used the radiative MHD equations for an optically thin
plasma to carry out a numerical experiment related to the formation of
`post'-flare loops. The numerical experiment starts with a current sheet
that is in mechanical and thermal equilibrium, but which is unstable
to both tearing-mode and thermal-condensation instabilities. The
current sheet is line-tied at one end to a photospheric-like boundary
and evolves asymmetrically. The effects of thermal conduction,
resistivity variation, and gravity are ignored. In general, we find that
reconnection in the nonlinear stage of the tearing-mode instability can
strongly affect the onset of condensations unless the radiative cooling
time scale is much smaller than the tearing-mode time scale. When the
ambient plasma β is less than 0.2, the reconnection enters a regime
where the outflow from the reconnection region is supermagnetosonic with
respect to the fast-mode wave speed. In the supermagnetosonic regime
the most rapidly condensing regions occur downstream of a fast-mode
shock that forms where the outflow impinges on closed loops attached
to the photospheric-like boundary. A similar shock-induced condensation
might occur during the formation of `post'-flare loops.
Title: A Catastrophe Mechanism for Coronal Mass Ejections
Authors: Forbes, T. G.; Isenberg, P. A.
Bibcode: 1991ApJ...373..294F
Altcode:
The ideal-MHD equations are used to show that a coronal current
filament can suddenly lose equilibrium if its magnetic energy exceeds
a critical value. The loss of equilibrium in the configuration results
from an imbalance between magnetic tension and compression, and this
imbalance ejects the filament upwards. Near the critical value, the
equilibrium configuration develops a vertical current sheet attached
to the photosphere at the point directly below the filament. When
equilibrium is lost, field lines anchored to the photosphere are
stretched upwards, and the current sheet rapidly grows longer. Without
reconnection in the current sheet, the filament travels only a short
distance before reaching a new equilibrium, and the net magnetic energy
released is less than 1 percent of the stored magnetic energy. However,
with reconnection, the filament travels upwards indefinitely, and all
of the stored energy is released.
Title: The Structure of Radiative MHD Shocks in the Corona
Authors: Xu, P.; Forbes, T. G.
Bibcode: 1991BAAS...23.1042X
Altcode:
No abstract at ADS
Title: A Catastrophe Model for Eruptive Flares and Prominences
Authors: Isenberg, P. A.; Forbes, T. G.
Bibcode: 1991BAAS...23.1036I
Altcode:
No abstract at ADS
Title: The Evolution of Coronal Magnetic Fields
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1990SoPh..130..399P
Altcode:
Slow photospheric motions can produce flow speeds in the corona
which are fast enough to violate quasi-static evolution. Therefore,
high-speed flows observed in the corona are not necessarily due to a
loss of equilibrium or stability. In this letter we present an example
where the flow speed increases indefinitely with, height, while the
coronal magnetic energy increases quadratically with time.
Title: Numerical simulation of a catastrophe model for coronal
mass ejections
Authors: Forbes, T. G.
Bibcode: 1990JGR....9511919F
Altcode:
In 1978, W. Van Tend and M. Kuperus proposed a simple catastrophe model
for magnetically driving coronal mass ejections, prominence eruptions,
and two-ribbon flares. Their model, which is based on simple circuit
concepts, suggests that a stable configuration containing a current
filament will lose equilibrium when the filament current exceeds a
critical value. Here we use a two-dimensional numerical simulation to
test how the Van Tend-Kuperus model works in an ideal MHD fluid. The
simulation exhibits the expected loss of mechanical equilibrium near the
predicted critical value, but the current filament moves only a short
distance upward before coming to rest at a new equilibrium. However,
this new equilibrium contains a current sheet which is resistively
unstable to magnetic reconnection, and if magnetic reconnection occurs
rapidly, the filament can continue to move upward at Alfvénic speeds.
Title: Evaporation in the Transition Region during the Gradual Phase
of Flares
Authors: Schmieder, B.; Malherbe, J. M.; Simnett, G. M.; Forbes,
T. G.; Tandberg-Hanssen, E.
Bibcode: 1990ApJ...356..720S
Altcode:
Previous observations have revealed that small, but sustained,
H-alpha blueshifts occur in flare ribbons during the gradual, or late,
phase of flares. These blueshifts suggest that there is a gentle
evaporation of chromospheric material throughout the late phase of
flares, but ambiguities in the interpretation of H-alpha leave open
the possibility that these blueshifts are caused by downflowing,
rather than upflowing, material. Using both C IV and soft (3.5-8 keV)
X-ray data from SMM observations, evidence is found which supports the
interpretation of the H-alpha blueshifts as upflows in the range from
4-12 km/s. The blueshifts are interpreted in terms of the reconnection
model proposed by Carmichael (1964). The model produces a sustained
energy release through-out the late phase which accounts for the
prolonged soft X-ray emission after a flare. This energy release comes
from the reconnecting magnetic field above the flare site, and some of
the energy is transported along field lines mapping to the chromosphere
where it drives chromospheric evaporation.
Title: Magnetic Field Evolution during Prominence Eruptions and
Two-Ribbon Flares
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1990SoPh..126..319P
Altcode:
Simple models for the MHD eruption of a solar prominence are presented,
in which the prominence is treated as a twisted magnetic flux tube
that is being repelled from the solar surface by magnetic pressure
forces. The effects of different physical assumptions to deal with
this magneto-hydrodynamically complex phenomenon are evaluated,
such as holding constant the prominence current, radius, flux or
twist or modelling the prominence as a current sheet. Including a
background magnetic field allows the prominence to be in equilibrium
initially with an Inverse Polarity and then to erupt due to magnetic
non-equilibrium when the background magnetic field is too small or the
prominence twist is too great. The electric field at the neutral point
below the prominence rapidly increases to a maximum value and then
declines. Including the effect of gravity also allows an equilibrium
with Normal Polarity to exist. Finally, an ideal MHD solution is
found which incorporates self-consistently a current sheet below the
prominence and which implies that a prominence will still erupt and
form a current sheet even if no reconnection occurs. When reconnection
is allowed it is, therefore, driven by the eruption.
Title: An MHD Model for the Onset of Coronal Mass Ejections
Authors: Forbes, T. G.; Isenberg, P. A.
Bibcode: 1990BAAS...22..814F
Altcode:
No abstract at ADS
Title: Numerical Simulation of a Catastrophe Model for Prominence
Eruptions
Authors: Forbes, T. G.
Bibcode: 1990LNP...363..287F
Altcode: 1990doqp.coll..287F; 1990IAUCo.117..287F
No abstract at ADS
Title: Basic properties and models of solar prominences
Authors: Forbes, T. G.
Bibcode: 1990GMS....58..295F
Altcode:
Prominences are relatively cool (10,000 K) and dense plasma clouds which
may persist for 100 days or more in the midst of the much hotter (1
million K) and more tenuous (10 exp 9/cu cm) corona. Many observations
imply that the magnetic field in and around prominences is responsible
both for isolating prominences from the corona and for supporting
them against gravity. It is not at all obvious how the magnetic field
can do both these tasks, but the limited theoretical models that are
available suggest that a magnetic-flux rope is involved. Using a new
analytical model, it is argued that the flux rope could also play a
key role in the eruption of a prominence by supplying the magnetic
energy necessary to drive the prominence outwards.
Title: The Role of Magnetic Reconnection in Flares and Prominence
Equations
Authors: Forbes, T. G.
Bibcode: 1990IAUS..142..293F
Altcode:
Magnetic Reconnection is often invoked as the primary mechanism for
driving a flare or a prominence eruption. This paper argues that a
catastrophic loss of mechanical equilibrium, rather than reconnection,
is probably the primary mechanism for driving these phenomena. However,
reconnection is still essential in order for any significant amount
of energy to be released. To illustrate this idea, some recent results
are presented from an MHD simulation based on a catastrophe mechanism
first proposed by Van Tend and Kuperus. In order for this mechanism to
be effective, a substantial amount of reconnection must occur within
a few Alfven-scale times. Such rapid reconnection is plausible since
the loss of mechanical equilibrium can generate flows which drive the
reconnection at a rapid rate.
Title: The Formation of Flare Loops by Magnetic Reconnection and
Chromospheric Ablation
Authors: Forbes, T. G.; Malherbe, J. M.; Priest, E. R.
Bibcode: 1989SoPh..120..285F
Altcode:
Slow-mode shocks produced by reconnection in the corona can provide the
thermal energy necessary to sustain flare loops for many hours. These
slow shocks have a complex structure because strong thermal conduction
along field lines dissociates the shocks into conduction fronts and
isothermal subshocks. Heat conducted along field lines mapping from
the subshocks to the chromosphere ablates chromospheric plasma and
thereby creates the hot flare loops and associated flare ribbons. Here
we combine a non-coplanar compressible reconnection theory with simple
scaling arguments for ablation and radiative cooling, and predict
average properties of hot and cool flare loops as a function of the
coronal vector magnetic field. For a coronal field strength of 100 G the
temperature of the hot flare loops decreases from 1.2 × 107
K to 4.0 × 106 K as the component of the coronal magnetic
field perpendicular to the plane of the loops increases from 0% to 86%
of the total field. When the perpendicular component exceeds 86% of
the total field or when the altitude of the reconnection site exceeds
106km, flare loops no longer occur. Shock enhanced radiative
cooling triggers the formation of cool Hα flare loops with predicted
densities of ≈ 1013 cm−3, and a small gap
of ≈ 103 km is predicted to exist between the footpoints
of the cool flare loops and the inner edges of the flare ribbons.
Title: Steady Magnetic Reconnection in Three Dimensions
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1989SoPh..119..211P
Altcode:
The concepts of magnetic reconnection that have been developed
in two dimensions need to be generalised to three-dimensional
configurations. Reconnection may be defined to occur when there is
an electric field (E∥) parallel to field lines (known as
potential singular lines) which are potential reconnection locations and
near which the field has an X-type topology in a plane normal to that
field line. In general there is a continuum of neighbouring potential
singular lines, and which one supports reconnection depends on the
imposed flow or electric field. For steady reconnection the nearby
flow and electric field are severely constrained in the ideal region
by the condition that E∥ = 0 there. Potential singular
lines may occur in twisted prominence fields or in the complex magnetic
configuration above sources of mixed polarity of an active region or a
supergranulation cell. When reconnection occurs there is dynamic MHD
behaviour with current concentration and strong plasma jetting along
the singular line and the singular surfaces which map onto them.
Title: Onset of Coronal Mass Ejections
Authors: Forbes, T. G.
Bibcode: 1989BAAS...21..856F
Altcode:
No abstract at ADS
Title: Preflare activity.
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. J.; Schmahl, E. J.;
Webb, D. F.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson,
R. S.; Chapman, G. A.; Deloach, A. C.; Gary, G. A.; Jones, H. P.;
Karpen, J. T.; Martres, M. -J.; Porter, J. G.; Schmieder, B.; Smith,
J. B., Jr.; Toomre, J.; Woodgate, B.; Waggett, P.; Bentley, R.;
Hurford, G.; Schadee, A.; Schrijver, J.; Harrison, R.; Martens, P.
Bibcode: 1989epos.conf....1P
Altcode:
Contents: 1. Introduction. 2. Magnetohydrodynamic
instability. 3. Preflare magnetic and velocity fields. 4. Coronal
manifestations of preflare activity.
Title: Shocks Produced by Impulsively Driven Reconnection
Authors: Forbes, T. G.
Bibcode: 1988SoPh..117...97F
Altcode:
Shock waves produced by impulsively driven reconnection may be
important during flares or during the emergence of magnetic flux
from the photosphere into the corona. Here we investigate such shock
waves by carrying out numerical experiments using two-dimensional
magneto-hydrodynamics. The results of the numerical experiments imply
that there are three different categories of shocks associated with
impulsively driven reconnection: (1) fast-mode, blast waves which
rapidly propagate away from the reconnection site; (2) slow-mode,
Petschek shocks which are attached to the reconnection site; and
(3) fast-mode, termination shocks which terminate the plasma jets
flowing out from the reconnection site. Fast-mode blast waves are
a common feature of many flare models, but the Petschek shocks and
jet termination shocks are specific to reconnection models. These two
different types of reconnection shocks might contribute to chromospheric
ablation and energetic particle acceleration in flares.
Title: Magnetic reconnection models of flares.
Authors: Forbes, T. G.
Bibcode: 1988ASSL..143..115F
Altcode: 1988acse.conf..115F
The most feasible energy source for solar and stellar flares is
the energy stored in coronal magnetic fields. Recent numerical and
analytical models of solar flares suggest that the magnetic energy
released by reconnection drives chromospheric ablation in the flare
ribbons. Simple theoretical arguments based on compressible reconnection
theory predict that the temperature of the ablated plasma should be
about 1.03×106B0.62K where B is the coronal
magnetic field strength in Gauss.
Title: Post-flare loops: formation and velocity
Authors: Schmieder, B.; Mein, P.; Malherbe, J. -M.; Forbes, T. G.
Bibcode: 1988AdSpR...8k.145S
Altcode: 1988AdSpR...8..145S
Post-flare loops are generally observed between two ribbon flares. The
formation of post flare loops and active region or plage filaments
has been explained in a model based on magnetic reconnection and
chromospheric ablation /1,2/. This model uses a magnetic topology
which is relevant to solar flares /3/ : a large flare or an instability
opens the magnetic lines of a coronal arcade (or arch) and a vertical
current sheet forms. Then the reconnection of the magnetic field
follows according to the scenario of Kopp and Pneuman /4/. In this
magnetic configuration, we show that the formation of condensations
or dense loops is induced by reconnection shocks. We give in
this communication a new method of diagnostic to derive from Hα
profiles physical parameters, i.e. source function, optical depth and
velocity. This method called ``differential cloud method'' is very
promising for future observations.
Title: Magnetic reconnection and solar flare loops.
Authors: Forbes, T. G.
Bibcode: 1987ESASP.275....3F
Altcode: 1987sspp.symp....3F
Reconnection models of the main phase of large solar flares are
used to explain the energetics and the motions of the large flare
loops that occur during this phase. Correct predictions for the
density and temperature of the X-ray emitting loops are obtained by
coupling magnetic reconnection with chromospheric ablation. In the
reconnection models the ablation is driven by the thermal conduction
of heat along magnetic field lines connecting the reconnection shocks
in the corona with the flare ribbons in the chromosphere. Combining
the compressible reconnection theory of Soward and Priest (1982) with
the magnetohydrodynamic (MHD) subshock criteria of Coroniti (1970)
shows that the Petschek-type slow-mode shocks in the vicinity of the
x-line always dissociate into pairs of isothermal slow-mode subshocks
and thermal conduction fronts. The rate of expansion of the loops
is a function of the reconnection rate, and loops can be evolving
self-similarly in time with their height increasing as sq root t and
the reconnection rate decreasing as t to the minus 1.
Title: Evidence for Gentle Chromospheric Evaporation during the
Gradual Phase of Large Solar Flares
Authors: Schmieder, B.; Forbes, T. G.; Malherbe, J. M.; Machado, M. E.
Bibcode: 1987ApJ...317..956S
Altcode:
The Multichannel Subtractive Double Pass Spectrograph of the Meudon
solar tower is used to obtain high spatial resolution H-alpha line
profiles during the gradual phase of three solar flares. In all cases,
small blueshifts lasting for several hours are observed in the flare
ribbons. By contrast, the region between the two ribbons exhibits
large redshifts that are typical of H-alpha post flare loops. The
blueshifts in the ribbons is interpreted as upward chromospheric flows
of 0.5-10 km/s, and the possible ambiguities of the interpretation
are discussed. A preliminary analysis indicates that such upflows are
sufficient to supply the greater than 10 to the 16th g of mass needed
to maintain a dense H-alpha postflare loop system in the corona.
Title: Magnetohydrodynamic instability
Authors: Priest, E. R.; Cargill, P.; Forbes, T. G.; Hood, A. W.;
Steinolfson, R. S.
Bibcode: 1986epos.conf..1.3P
Altcode: 1986epos.confA...3P
There have been major advances in the theory of magnetic reconnection
and of magnetic instability, with important implications for the
observations, as follows: (1) Fast and slow magnetic shock waves are
produced by the magnetohydrodynamics of reconnection and are potential
particle accelerators. (2) The impulsive bursty regime of reconnection
gives a rapid release of magnetic energy in a series of bursts. (3)
The radiative tearing mode creates cool filamentary structures in the
reconnection process. (4) The stability analyses imply that an arcade
can become unstable when either its height or twist of plasma pressure
become too great.
Title: On the thermal durability of solar prominences, or how to
evaporate aprominence?
Authors: Malherbe, J. M.; Forbes, T. G.
Bibcode: 1986NASCP2442..225M
Altcode: 1986copp.nasa..225M
The authors investigate the thermal disappearance of solar prominences
under strong perturbations due to wave heating, Ohmic heating, viscous
heating or conduction. Specifically, they calculate how large a thermal
perturbation is needed to destroy a stable thermal equilibrium,
and find that the prominence plasma appears to be thermally very
rugged. Its cold equilibrium may most likely be destroyed by either
strong magnetic heating or conduction in a range of parameters which
is relevant to flares.
Title: Formation and support of prominences.
Authors: Forbes, T. G.
Bibcode: 1986NASCP2442...21F
Altcode: 1986copp.nasa...21F
A short introduction is given to the concepts discussed by the group on
the formation and support of prominences. Only quiescent and long-lived
active region prominences were considered, since transient prominence
phenomena, such as sprays, surges, H alpha flare-loops, and coronal
rain, are dynamically distinct from long-lived, prominences. Stable
prominences (which are often referred to as filaments when seen against
the disk) can be subdivided into three categories, namely active region
prominences, quiescent prominences and polar crown prominences. The
third category is closely related to the second since a quiescent
prominence will eventually evolve into a polar crown prominence if
it lasts long enough. The distinction between the first and second
categories is not sharp either since intermediates exist here as well
(Martin, 1973).
Title: Can prominences form in current sheets?
Authors: Malherbe, J. M.; Forbes, T. G.
Bibcode: 1986NASCP2442...33M
Altcode: 1986copp.nasa...33M
Two-dimensional numerical simulations of the formation of cold
condensations in a vertical current sheet have been performed using the
radiative, resistive MHD equations with line-tied boundary conditions
at one end of the sheet. Prominence-like condensations are observed
to appear above and below an X-line produced by the onset of the
tearing-mode instability. Cooling in the sheet is initiated by Ohmic
decay, with the densest condensations occurring in the region downstream
of a fast-mode shock. This shock, which is due to the line-tied boundary
conditions, terminates one of the two supermagnetosonic reconnection
jets that develop when the tearing is fully developed. This paper
emphasizes the condensation properties of shock waves, which may trigger
or considerably enhance the conditions for thermal condensations.
Title: Fast-Shock Formation in Line-tied Magnetic Reconnection Models
of Solar Flares
Authors: Forbes, T. G.
Bibcode: 1986ApJ...305..553F
Altcode:
In a previous study by the author, an approximately stationary fast
shock was tentatively identified in a numerical experiment designed
to study line-tied magnetic reconnection. Here the evidence for
the occurrence of a stationary fast shock is reexamined, and the
previous identification is confirmed. In the numerical experiment,
line-tied reconnection is modeled by a configuration which produces
two supermagnetosonic outflow jets - one directed upward, away from
the photosphere, and one directed downward, toward an arcade of closed
magnetic loops tied to the photosphere. The fast shock occurs when
the downward-directed jet encounters the obstacle formed by the closed
loops. Although the existence of a stationary, or nearly stationary,
fast shock is confirmed, the transition from the supermagnetosonic flow
region upstream of the shock to the nearly static region downstream of
the shock is more complicated than was previously thought. Immediately
downstream of the shock, there exists a deflection sheath in which
the submagnetosonic flow coming out of the shock is diverted around
the region of static closed loops. The MHD jump conditions are used
to investigate the characteristics of the fast shock and to show that
a stationary shock cannot exist unless accompanied by a deflection
sheath. Analysis of the shock's location and dimensions suggests that
such fast shocks may contribute to particle acceleration and to thermal
condensation in flares.
Title: New models for fast steady state magnetic reconnection
Authors: Priest, E. R.; Forbes, T. G.
Bibcode: 1986JGR....91.5579P
Altcode:
A new unified family of models for incompressible, steady state
magnetic reconnection in a finite region is presented. They are
obtained by expanding in powers of the Alfvén Mach number and may
be used to elucidate some of the puzzling properties of numerical
experiments on reconnection which are not present in the classical
models. The conditions imposed on the inflow boundary of the finite
region determine which member of the family occurs. Petscheklike and
Sonneruplike solutions are particular members. The Sonneruplike regime
is a special case of a weak slow mode expansion in the inflow region,
and it separates two classes of members with reversed currents. These
are the hybrid regime with a mixture of strong fast mode and slow mode
expansions and the flux pileup regime with a stong slow mode expansion,
in which the magnetic field strength increases as it approaches the
diffusion region and the flow diverges. The Petscheklike regime is a
singular case of a weak fast mode expansion, and it separates the hybrid
regime from a regime of slow mode compressions. The hybrid expansions
are fast mode in character in the center of the inflow and slow mode
near the edges of the region, while the flux pileup expansions possess
long thin diffusion regions and no maximum reconnection rate. The
maximum rate is calculated for the other solutions as a function of the
magnetic Reynolds number and compared with the classical Sweet-Parker
and Petschek rates. For the flux pileup and hybrid regimes, reconnection
can be ;much faster than the maximum Petschek rate. Care should be
taken in deciding which type of reconnection is operating in a numerical
experiment. Indeed, no experiment to date has used boundary conditions
appropriate for demonstrating steady state Petschek reconnection.
Title: A Shock Condensation Mechanism for Loop Prominences
Authors: Forbes, T. G.; Malherbe, J. M.
Bibcode: 1986ApJ...302L..67F
Altcode:
Self-consistent, numerical solutions of the resistive MHD equations
in two dimensions show that a quasi-stationary, fast-mode shock is a
characteristic feature of the reconnection dynamics of the Kopp-Pneuman
model of two-ribbon flares. A preliminary analysis of the effects of
radiative cooling and thermal conduction suggests that the fast shock
can help trigger a thermal condensation (i.e., a loop prominence)
if the reconnecting magnetic fields are sufficiently strong.
Title: Preflare activity.
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. J.; Schmahl, E. J.;
Webb, D. F.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson,
R. S.; Chapman, G. A.; Deloach, A. C.; Gary, G. A.; Jones, H. P.;
Karpen, J. T.; Martres, M. -J.; Porter, J. G.; Schmieder, B.; Smith,
J. B., Jr.; Toomre, J.; Woodgate, B.; Waggett, P.; Bentley, R.;
Hurford, G.; Schadee, A.; Schrijver, J.; Harrison, R.; Martens, P.
Bibcode: 1986NASCP2439....1P
Altcode:
Contents: 1. Introduction: the preflare state - a review of previous
results. 2. Magnetohydrodynamic instability: magnetic reconnection,
nonlinear tearing, nonlinear reconnection experiments, emerging flux and
moving satellite sunspots, main phase reconnection in two-ribbon flares,
magnetic instability responsible for filament eruption in two-ribbon
flares. 3. Preflare magnetic and velocity fields: general morphology of
the preflare magnetic field, magnetic field shear, electric currents in
the preflare active region, characterization of the preflare velocity
field, emerging flux. 4. Coronal manifestations of preflare activity:
defining the preflare regime, specific illustrative events, comparison
of preflare X-rays and ultraviolet, preflare microwave intensity and
polarization changes, non-thermal precursors, precursors of coronal
mass ejections, short-lived and long-lived HXIS sources as possible
precursors.
Title: A reconnection mechanism for coronal condensations in
two-ribbon flares.
Authors: Forbes, T. G.; Malherbe, J. M.
Bibcode: 1986lasf.conf..443F
Altcode: 1986lasf.symp..443F
During the last few years the authors have used a series of
self-consistent two-dimensional, MHD calculations to explore the
reconnection dynamics implied by the two-ribbon flare model of Kopp
and Pneuman (1976). The calculations show, that in addition to the
standard slow-mode MHD shocks generated by reconnection, there also
exists a standing fast-mode MHD shock. Because of thermal conduction,
the slow shocks generate an evaporative upflow of chromospheric plasma
into the reconnection region. Analysis of the effects of radiative and
conductive cooling suggests that at least some of this evaporated plasma
will undergo thermal condensation when it passes through the fast shock.
Title: Corrigendum
Authors: Forbes, T. G.; Priest, E. R.; Hood, A. W.
Bibcode: 1985JPlPh..34..481F
Altcode:
Numerical solutions were obtained by Forbes, Priest & Hood (1982)
for the resistive decay of a current sheet in an MHD fluid. To check
the accuracy of the numerical solutions, a linear, analytical solution
was also deived for the regime where diffusion is dominant. In a
subsequent reinvestigation of this problem an error in the linear,
analytical solution has been discovered. For the parameter values used
in the numerical solution this error is too small ( 2%) to produce
any significant change in the previous test comparison between the
numerical and analytical solutions. However, for parameter values much
different from those used in the numerical solution, the error in the
linear solution can be significant.
Title: Current evolution in a numerical emerging-magnetic-flux model
Authors: Forbes, T.
Bibcode: 1985svmf.nasa...86F
Altcode:
The resistive-MHD equations are numerically solved in two-dimensions
for an initial-boundary-value problem which models the emergence of
magnetic flux from the photosphere into the corona. As the emergence
begins a current sheet forms around the emerging region which separates
the emerging region from the overlying coronal magnetic field. This
current sheet is the source of the free-magnetic energy in the
system, and in the limit of zero resistivity it is a simple tangential
discontinuity. However, when the resistivity is finite, reconnection
between the magnetic field in the emerging region and the overlying
coronal magnetic field ensures, and the subsequent evolution of the
enveloping current sheet becomes complex. The overall time history of
the current evolution is suggestive of the expected current evolution
for the pre-flare, impulsive, and main phases of flares.
Title: Current evolution in a numerical emerging-magnetic-flux model.
Authors: Forbes, T. G.
Bibcode: 1985NASCP2374...86F
Altcode:
The resistive-MHD equations are numerically solved in two-dimensions
for an initial-boundary-value problem which models the emergence of
magnetic flux from the photosphere into the corona.
Title: Astrophysical Plasmas. (Book Reviews: Magnetic Reconnection
in Space and Laboratory Plasmas)
Authors: Forbes, T. G.
Bibcode: 1984Sci...226.1311F
Altcode:
No abstract at ADS
Title: Astrophysical Plasmas. (Book Reviews: Magnetic Reconnection
in Space and Laboratory Plasmas)
Authors: Forbes, T. G.
Bibcode: 1984Sci...226.1311H
Altcode:
No abstract at ADS
Title: A numerical simulation of the formation of solar prominences.
Authors: Malherbe, J. M.; Forbes, T. G.; Priest, E. R.
Bibcode: 1984ESASP.220..119M
Altcode: 1984ESPM....4..119M
The radiative-resistive MHD equations are numerically solved in
two-dimensions for a magnetic field configuration that starts with
a vertical current sheet which is line-tied at its base and is in
mechanical, but not radiative, equilibrium. The aim of the present
study is to determine whether this initial configuration can achieve
a prominence-like equilibrium in the presence of magnetic reconnection
and tearing in the current-sheet.
Title: Numerical Simulation of Reconnection in an Emerging Magnetic
Flux Region
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1984SoPh...94..315F
Altcode:
The resistive MHD equations are numerically solved in two dimensions for
an initial-boundary-value problem which simulates reconnection between
an emerging magnetic flux region and an overlying coronal magnetic
field. The emerging region is modelled by a cylindrical flux tube with
a poloidal magnetic field lying in the same plane as the external,
coronal field. The plasma betas of the emerging and coronal regions
are 1.0 and 0.1, respectively, and the magnetic Reynolds number for the
system is 2 × 103. At the beginning of the simulation the
tube starts to emerge through the base of the rectangular computational
domain, and, when the tube is halfway into the computational domain, its
position is held fixed so that no more flux of plasma enters through
the base. Because the time-scale of the emergence is slower than
the Alfvén time-scale, but faster than the reconnection time-scale,
a region of closed loops forms at the base. These loops are gradually
opened and reconnected with the overlying, external magnetic field as
time proceeds.
Title: Reconnection of magnetic fields.
Authors: Sonnerup, B. U. Ö.; Baum, P. J.; Birn, J.; Cowley, S. W. H.;
Forbes, T. G.; Hassam, A. B.; Kahler, S. W.; Matthaeus, W. H.; Park,
W.; Paschmann, G.; Priest, E. R.; Russell, C. T.; Spicer, D. S.;
Stenzel, R.
Bibcode: 1984NASRP1120....1S
Altcode:
No abstract at ADS
Title: Current sheet models for solar prominences. II - Energetics
and condensation process
Authors: Malherbe, J. M.; Priest, E. R.; Forbes, T. G.; Heyvaerts, J.
Bibcode: 1983A&A...127..153M
Altcode:
A steady state dynamic model for solar prominences of the Kuperus and
Raadu type was previously proposed by Malherbe and Priest (1983), but
only the motion through series of quasi-static states was investigated
there. The mechanisms for formation, condensation and cooling of
plasma in this model are studied. As hot coronal material approaches
the filament sheet, it is expected to cool and condense. Cold material
is then carried up through the prominence by rising magnetic field
lines due to converging photospheric motions below the filament. Two
possible ways are suggested of triggering a thermal instability and so
producing such a stationary condensation process: a larger pressure in
the sheet, or a smaller wave heating in the reconnected field than in
the surrounding corona. This paper presents a simple model to simulate
the plasma condensation: the thermodynamics of the cooling process,
as well as the dynamics of new material entering the prominence sheet,
are described in detail.
Title: Mass upflows in `post'-flare loops
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1983SoPh...88..211F
Altcode:
A self-consistent numerical model of a reconnecting magnetic field
configuration similar to that occurring during the main-phase
of two-ribbon flares is used to estimate the upflow caused by the
fast-mode expansion of the magnetic field moving into the reconnection
region. Such an expansion creates a field-aligned pressure gradient
which accelerates plasma upward from the chromospheric base of magnetic
field lines in the region external to the loops. The numerical results
imply that the amount of mass sucked up in this way is even smaller
than was previously estimated by Kopp and Pneuman who used a kinematic
model. Therefore, some indirect mechanism (such as evaporation),
which would probably derive its motive power from the thermal energy
generated by the reconnection, is required to explain the large mass
upflows inferred from observations.
Title: A Numerical Experiment Relevant to Line-Tied Reconnection in
Two-Ribbon Flares
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1983SoPh...84..169F
Altcode:
The nonlinear evolution of a reconnecting magnetic field configuration
similar to that occurring just before the onset of `post'-flare loops
in two-ribbon flares is determined. The evolution, which is obtained
by numerically solving the resistive MHD equations, shows two new
features that have not yet been incorporated into contemporary models of
`post'-flare loops. The first of these new features is the formation
of a nearly stationary fast-mode shock above the region corresponding
to the top of the loops. This fast-mode shock occurs just below the
magnetic neutral line and between the slow-mode shocks associated with
fast magnetic reconnection at the neutral line. The second new feature
is the creation and annihilation of large-scale magnetic islands in the
current sheet above the loops. The annihilation of the islands occurs
very rapidly and appears to be a manifestation of the coalescence
instability. The creation and annihilation of magnetic islands could
be important in understanding the energetics of `post'-flare loops
since the coalescence instability can produce an intermittent energy
release more than an order of magnitude faster than that predicted by
steady-state reconnection theories.
Title: On reconnection and plasmoids in the geomagnetic tail
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1983JGR....88..863F
Altcode:
The nonlinear evolution of the collisional tearing mode is numerically
determined for a two-dimensional current sheet configuration whose
magnetic field lines are tied at one end to a stationary surface. The
configuration is analogous to that occurring in the geomagnetic tail
at the start of a substorm. The numerical results suggest that the
formation of a near-earth neutral line at substorm onset is due to
the asymmetric tearing that occurs because the field lines in the
geomagnetic tail are partly line-tied, or anchored, by the earth's
ionosphere. The results also suggest that during substorm recovery
the neutral line in the tail moves away from the earth at a speed on
the order of the speed of the plasma flowing into the neutral line
region. Overall, the solution is consistent with proposed reconnection
models of the substorm but suggests that the recovery phase may be more
complex than previously expected, due to the growth and coalescence
of multiple magnetic islands (i.e., plasmoids).
Title: Numerical Study of Line-Tied Magnetic Reconnection
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1982SoPh...81..303F
Altcode:
A two-dimensional configuration, analogous to that at the start of
the main phase in two-ribbon flares, is modelled numerically by
self-consistently solving the time-dependent MHD equations. The
initial state consists of a vertical current sheet with an
external plasma beta value of 0.1 and a magnetic Reynolds number of
10−3. Although the model does not yet include gravity or
a full energy equation, many of the principal dynamical features of
the main phase in a flare are present. In particular, the numerical
results confirm the earlier prediction of the kinematic Kopp-Pneuman
(1976) model that a neutral line forms at the base of the corona and
rises upwards as open, extended field lines close back down to form
loops (i.e., `post'-flare loops). By the end of the computation a
state of nonlinear reconnection containing slow shocks has developed,
and the velocity of the plasma flowing into the neutral line region is
approximately 0.06 times the corresponding inflow Alfvén velocity -
a value consistent with the steady-state nonlinear reconnection theory
of Soward and Priest (1977). The speed at which the neutral line rises
in the numerical simulation varies from an initial value of ≲ 0.02
to a final value of ∼- 0.12 times the inflow Alfvén speed.
Title: Neutral line motion due to reconnection in two-ribbon solar
flares and magnetospheric substorms
Authors: Forbes, T. G.; Priest, E. R.
Bibcode: 1982P&SS...30.1183F
Altcode:
Two kinematic models of line-tied reconnection are considered which
describe the motion of a magnetic neutral line (NL) during the main
phase of a two-ribbon solar flare and during the recovery phase of
a magnetospheric substorm in the geomagnetic tail. The models are
kinematic in that they use only the magnetic induction equation, which
suffices to determine the position and velocity of the NL as functions
of time if the rate of reconnection is prescribed. The solar flare model
shows that the observed large decrease in the rate at which "post"-flare
loops rise upward from the photosphere during the main phase does not
require a corresponding decrease in the rate of reconnection. Instead
it is found that a constant rate of reconnection can account for the
motion of the loops for almost the entire period during which they
are observed. By contrast, application of the same procedures to the
recovery phase of the magnetospheric substorm in the tail predicts a
slightly increasing speed of NL motion if the rate of reconnection is
constant. Furthermore, it is found that the motion of the NL relative
to the ambient medium may account for much of the observed asymmetry
in the magnetic field in the plasma sheet during recovery. Due to this
motion, the plasma sheet thickness may be up to 4 times smaller and
the normal magnetic field component up to 2 times weaker in the region
tailward of the NL than in the corresponding region earthward of the NL.
Title: Implosion of a uniform current sheet in a low-beta plasma
Authors: Forbes, T. G.
Bibcode: 1982JPlPh..27..491F
Altcode:
Using ideal, one-dimensional MHD equations, numerical and analytic
solutions are presented which describe the nonlinear behaviour of
an imploding current sheet in a low-fl plasma. Initially the current
density is uniformly distributed in asheet of finite thickness, and
the Lorentz force tending to pinch the plasma together is unopposed
by any fluid pressure force. As the implosion develops the current
density in the sheet is concentrated into a thin layer at the centre of
the sheet, and both the current density and the current in this layer
become infinite in a finite time if β = 0. At the moment this occurs,
fast-mode shocks are produced which propagate outward from the centre
of the current sheet, and as the shocks move away an infinitely thin
current sheet is left behind. Although the solutions are related to
electric discharges, they are also closely related to a problem posed
by Dungey concerning the evolution of a uniformly distributed current
in the vicinity of an X-type magnetic neutral line. The implications
of these solutions for Dungey's problemare discussed.
Title: Evolution of current sheets following the onset of enhanced
resistivity
Authors: Forbes, T. G.; Priest, E. R.; Hood, A. W.
Bibcode: 1982JPlPh..27..157F
Altcode:
An important aspect of pre-flare current sheets in the solar atmosphere
is the sudden enhancement of the effective electrical resistivity in
the sheet due to the onset of a plasma micro-instability. Numerical
and analytical solutions to the isothermal MHD equations are here
presented that describe the evolution of a current sheet subsequent to
such an enhancement in the resistivity. The solutions show that, if the
initial width of the current sheet is less than the acoustic-diffusion
length obtained by dividing the resistivity by the sound speed,
then isomagnetic shocks are formed. These shocks propagate outward
from the the centre of the current sheet and are transformed into
fast-mode magneto-acoustic waves when they reach the edges of the
current sheet. The fast-mode waves thus formed continue to propagate
outward beyond the confines of the current sheet. In contrast to a
previous study by Cheng, the present solutions demonstrate that flow
speeds several times greater than the local fast-mode wave speed can
be produced if the plasma beta parameter and the initial sheet width
are sufficiently small. The results may be relevant to the triggering
of a solar flare, as in the emerging flux model of flares.
Title: On the velocity distribution of ion jets during substorm
recovery
Authors: Birn, J.; Forbes, T. G.; Hones, E. W., Jr.; Bame, S. J.;
Paschmann, G.
Bibcode: 1981JGR....86.9001B
Altcode:
The velocity distribution of earthward jetting ions that are
observed principally during substorm recovered by satellites at
~15-35 RE in the magnetotail is compared quantitatively
with two different theoretical models, the 'adiabatic deformation'
of an initially flowing Maxwellian moving into higher magnetic field
strength (model A) and the field-aligned electrostatic acceleration
of an initially nonflowing isotropic Maxwellian including adiabatic
deformation effects (model B). It is assumed that the ions are protons
or, more generally, consist of only one species. Both models can explain
the often observed concave-convex shape of isodensity contours of the
distribution function. Model A, however, gives a somewhat better overall
quantitative agreement with the observed distributions, leading us to
conclude that field-aligned electrostatic acceleration may not play
the fundamental role in the energizing of the jetting ions that was
suggested earlier (Frank et al., 1978; DeCoster and Frank, 1979). The
two models imply different plasma conditions in the tailward source
region. Model A requires a source in the far tail with plasma of kT~1
keV and density less than that at the observing satellite. Model B
requires a source, probably closer to the satellite position, with
plasma of kt~2-5 keV and density greater than that at the observing
satellite. The conditions at the source region implied by model A are
consistent with the plasma's having originated in the tail lobes and
having been accelerated by a reconnection process in the far tail.
Title: Explorer 34 magnetic field measurements near the tail current
sheet and auroral activity.
Authors: Speiser, T. W.; Forbes, T. G.
Bibcode: 1981Ap&SS..77..409S
Altcode:
Explorer 34 (Imp 4) 2.56 s magnetic data during 131 traversals of
the tail current sheet are presented along with simultaneous 2.5
min auroral electrojet indices AE and AL. The normal magnetic field,B
⊥, satellite crossing times and positions are tabulated for
these 131 crossings.B ⊥ is defined in the center of the
sheet: it is the vector magnetic field at the time of field minimum
during the crossing (B x component changes sign). It is
remarkable that the only normal components too large in magnitude to
be classified as ‘fine structure’ occur near the time of onset
of an ‘AE event’. Cases are discussed where the normal component,
defined near the plasma sheet edges, has the opposite sign compared to
the normal component defined at the sheet center. For quiet times, the
current sheet may be only about 1000 km thick within a 3R e
(Earth-radii) plasma sheet, and may carry some 10 15% of the total
tail current.
Title: Substorm-related plasma sheet motions as determined from
differential timing of plasma changes at the Isee satellites
Authors: Forbes, T. G.; Hones, E. W., Jr.; Bame, S. J.; Asbridge,
J. R.; Paschmann, G.; Sckopke, N.; Russell, C. T.
Bibcode: 1981JGR....86.3459F
Altcode:
Comparisons of the arrivals and disappearances of plasma sheet electrons
observed by the Los Alamos Scientific Laboratory/Max-Planck-Institut
analyzers on board the Isee 1 and 2 satellites during the course of
a magnetic substorm often show relative time delays between the two
satellites on the order of a few seconds. Time decay measurements have
been used to determine the motion of the northern plasma sheet boundary
during substorms for the period February 5 to May 25, 1978. Near
substorm onset the average velocity of the surface of the plasma sheet
is 23+/-18 km/s toward the center of the sheet. And for the same times
the average component of the plasma flow based upon moment integrations
of the proton distribution is 20+/-8 km/s in the same direction. During
substorm recovery the plasma sheet reappears, its surface moving
outward from the center with an average velocity of 133+/-31 km/s as
determined by timing its arrival at the two satellites. However, the
corresponding proton flow speed is only 3+/-7 km/s. At the time of the
recovery precursory appearances of the plasma sheet often occur. The
time delays associated with these precursors imply that the surface
of the plasma sheet moves both upwards and downwards with speeds ~100
km/s. Both the appearance of the precursors and the large difference
between the velocity of the plasma sheet surface and the velocity of
the flow normal to the surface can be explained by the occurrence of
surface waves at the boundary of the plasma sheet which propagate past
the satellites at a velocity of 200-600 km/s in the direction of the
convective flow. Estimating the tilt of the wave surfaces as +/-15°, we
deduce an average wavelength of 1-5 RE and a wave amplitude
of ~600-1400 km. Taking into consideration the wave structure at the
time of a recovery, we compute a velocity of ~30 km/s for the net
outward motion of the plasma sheet boundaries during substorm recovery.
Title: Structure of the low-latitude boundary layer
Authors: Sckopke, N.; Paschmann, G.; Haerendel, G.; Sonnerup,
B. U. Oe.; Bame, S. J.; Forbes, T. G.; Hones, E. W., Jr.; Russell,
C. T.
Bibcode: 1981JGR....86.2099S
Altcode:
Observations at high temporal resolution of the frontside magnetopause
and plasma boundary layer, made with the Los Alamos Scientific
Laboratory/Max-Planck-Institut, Institut für Extraterrestrische
Physik, fast plasma analyzer on board the Isee 1 and 2 spacecraft, have
revealed a complex quasi-periodic structure of some of the observed
boundary layers: cool tailward streaming boundary layer plasma is seen
intermittently, with intervening periods of hot tenuous plasma which has
properties similar to the magnetospheric population. While individual
encounters with the boundary layer plasma last only a few minutes,
the total observation time may extend over 1 hour or more. One such
crossing, at 0800 hours local time and 40° northern GSM latitude, is
examined in detail, including a quantitative comparison of the boundary
layer entry and exit times of the two spacecraft. The data are found
to be compatible with a boundary layer that is always attached to the
magnetopause but where the layer thickness has a large-scale spatial
modulation pattern which travels tailward past the spacecraft. Included
are periods when the thickness is essentially zero and others when it is
of the order of 1 RE. The duration of these periods is highly
variable but is typically in the range of 2-5 min, corresponding to a
distance along the magnetopause of the order of 3-8 RE. The
observed boundary layer features include a steep density gradient at
the magnetopause, with an approximately constant boundary layer plasma
density amounting to about 25% of the magnetosheath density, and a
second abrupt density decrease at the inner edge of the layer. It
also appears that the purely magnetospheric plasma is ocassionally
separated from the boundary layer by a halo region in which the
plasma density is somewhat higher, and the temperature somewhat lower,
than in the magnetosphere. A tentative model is proposed in which the
variable boundary layer thickness is produced by the Kelvin-Helmholtz
instability of the inner edge of the layer and in which eddy motion
provides effective mixing within the layer.
Title: Evidence for the tailward retreat of a magnetic neutral line
in the magnetotail during substorm recovery
Authors: Forbes, T. G.; Hones, E. W.; Bame, S. J.; Asbridge, J. R.;
Paschmann, G.; Sckopke, N.; Russell, C. T.
Bibcode: 1981GeoRL...8..261F
Altcode:
On several occasions during the recovery phase of substorms the Los
Alamos/MPI analyzers on the ISEE 1 and 2 satellites have observed
protons counter-streaming along the magnetic field within about 1
Re of the northern (upper) surface of the plasma sheet. We
present data for one of these periods (04:00-04:30, March 1, 1978)
which strongly suggest that the counter-streaming of the protons
results from the mirroring of particles moving Earthward along
the magnetic field. Only the Earthward flow is observed when the
satellites first encounter the plasma sheet. But about 2 minutes
after the entry into the plasma sheet the return tail ward flow
appears while the Earthward flow continues. The speeds of both the
Earthward and tailward moving populations thereafter show a systematic
decrease with time with the tailward speed being persistently ∼200
km/sec greater than the earthward speed. Analysis of the correlation
between the speeds of the two populations shows that the decrease in
speed results from the relative motion of the satellites with respect
to a spatial gradient in the velocity at the boundary of the plasma
sheet. We interpret the time delay between the onset of the Earthward
and tailward streaming protons and the persistent difference between
their speeds as evidence that the source of the Earthward flow is
moving tailwards onto magnetic field lines mapping to progressively
higher polar latitudes as substorm recovery proceeds and we suggest
that the source is the region of magnetic reconnection associated with
a tailward retreating neutral line.
Title: Temporal evolution of magnetic recomiexion in the vicinity
of a magnetic neutral line
Authors: Forbes, T. G.; Speiser, T. W.
Bibcode: 1979JPlPh..21..107F
Altcode:
Following Dungey's original magnetohydrodynamic formulation,
a solution is obtained for the nonlinear evolution of a current
discharge in the vicinity of a magnetic neutral line. For an
ideal gas with constant conductivity and uniform mass density we
obtain a particular exact solution in the limit of an initial,
nearly sheet-like configuration. This particular solution implies
special boundary conditions for the pressure and electric field at
the surface of the conductor. If These conditions are not met, then
the solution eventually breaks down before the current density becomes
infinite. The time required for complete breakdown is determined by the
wave propagation times from the surface of the fluid to the neutral
line and by the diffusion time for the magnetic field through the
fluid. For large conductivity and a small sound speed, the maximum
current density achieved at the time of the solution's complete
breakdown depends upon the ratio of the characteristic diffusion
time to the Alfvén wave propagation time. In the limit of infinite
conductivity or infinite extension of the fluid, the current density
along the neutral line becomes infinite at π/23/2 times
the Alfvénic scale time. At this same time the inflow Alfvenic Mach
number approaches 21/2i while the outflow Alfvenic Mach
number approaches infinity.
Title: Energetic proton fluxes and magnetic field orientation in
the quiet tail magnetosheath
Authors: Forbes, T. G.
Bibcode: 1978PhDT.........2F
Altcode: 1978PhDT........81F
Quiet time magnetosheath data from the Energetic Particle Experiment
(EPE) on Explorer 47 reveals the occurrence of detailed correlations
between the magnetic field orientation and proton flux above 52 kev. A
Compton-Getting analysis of combined EPE, bulk flow, and magnetometer
data shows that the correlation is caused by a variation in the
differential intensity at 52 kev, and a flow of energetic protons
along magnetic field lines. The computed flow velocities along the
magnetic field are 2.0 to 4.1 times greater than the component of the
low energy convective magnetosheath flow along the magnetic field.
Title: Energetic proton fluxes and magnetic field orientation in
the quiet tail magnetosheath
Authors: Forbes, Terry Gene
Bibcode: 1978PhDT.......117F
Altcode:
No abstract at ADS
Title: Mathematical models of the open magnetosphere: Application
to dayside auroras.
Authors: Forbes, T. G.; Speiser, T. W.
Bibcode: 1971JGR....76.7541F
Altcode:
No abstract at ADS