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Magnetic nulls and super-radial expansion in the solar corona  

Sarah Gibson   Submitted: 2017-04-29 11:01

Magnetic fields in the sun's outer atmosphere - the corona - control both solar-wind acceleration and the dynamics of solar eruptions. We present the first clear observational evidence of coronal magnetic nulls in off-limb linearly polarized observations of pseudostreamers, taken by the Coronal Multichannel Polarimeter (CoMP) telescope. These nulls represent regions where magnetic reconnection is likely to act as a catalyst for solar activity. CoMP linear-polarization observations also provide an independent, coronal proxy for magnetic expansion into the solar wind, a quantity often used to parameterize and predict the solar wind speed at Earth. We introduce a new method for explicitly calculating expansion factors from CoMP coronal linear-polarization observations, which does not require photospheric extrapolations. We conclude that linearly-polarized light is a powerful new diagnostic of critical coronal magnetic topologies and the expanding magnetic flux tubes that channel the solar wind.

Authors: Sarah E. Gibson, Kevin Dalmasse, Laurel A. Rachmeler, Marc L. De Rosa, Steven Tomczyk, Giuliana de Toma, Joan Burkepile, Michael Galloy
Projects: None

Publication Status: accepted
Last Modified: 2017-05-01 14:34
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Coronal cavities: observations and implications for the magnetic environment of prominences  

Sarah Gibson   Submitted: 2017-04-24 15:53

Dark and elliptical, coronal cavities yield important clues to the magnetic structures that cradle prominences, and to the forces that ultimately lead to their eruption. We review observational analyses of cavity morphology, thermal properties (density and temperature), line-of-sight and plane-of-sky flows, substructure including hot cores and central voids, linear polarization signatures, and observational precursors and predictors of eruption. We discuss a magnetohydrodynamic interpretation of these observations which argues that the cavity is a magnetic flux rope, and pose a set of open questions for further study.

Authors: Sarah E. Gibson
Projects: None

Publication Status: published
Last Modified: 2017-04-26 12:03
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Partially-ejected flux ropes: implications for space weather  

Sarah Gibson   Submitted: 2007-03-07 12:50

The structure and evolution of the sources of solar activity directly affects the nature of space weather disturbances that reach the Earth. We have previously demonstrated that the loss of equilibrium and partial ejection of a coronal magnetic flux rope matches observations of coronal mass ejections (CMEs) and their precursors.In this paper we discuss the significance of such a partially-ejected rope for space weather. We will consider how the evolution and bifurcation of the rope modifies it from its initial, source configuration. In particular, we will consider how reconnections and writhing motions lead to an escaping rope which has an axis rotated counterclockwise from the original rope axis orientation, and which is rooted in transient coronal holes external to the original source region.

Authors: Sarah E. Gibson and Yuhong Fan
Projects: None

Publication Status: Proceedings of the International Astronomical Union, Volume 2, Symposium S233, 319, 2006
Last Modified: 2007-03-07 12:52
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The evolving sigmoid: evidence for magnetic flux ropes in the corona before, during, and after CMEs  

Sarah Gibson   Submitted: 2007-03-07 12:41

It is generally accepted that the energy that drives coronal mass ejections (CMEs) is magnetic in origin. Sheared and twisted coronal fields can store free magnetic energy which ultimately is released in the CME. We explore the possibility of the specific magnetic configuration of a magnetic flux rope of field lines that twist about an axial field line. The flux rope model predicts coronal observables, including heating along forward or inverse S-shaped, or sigmoid, topological surfaces. Therefore, studying the observed evolution of such sigmoids prior to, during, and after the CME gives us crucial insight into the physics of coronal storage and release of magnetic energy. In particular, we consider (1) soft-X-ray sigmoids, both transient and persistent; (2) The formation of a current sheet and cusp-shaped post-flare loops below the CME; (3) Reappearance of sigmoids after CMEs; (4) Partially erupting filaments; (5) Magnetic cloud observations of filament material.

Authors: S. E. Gibson, Y. Fan, T. Toeroek, and B. Kliem
Projects: None

Publication Status: Space Science Reviews, 124, 131, 2007
Last Modified: 2007-03-07 12:50
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Coronal prominence structure and dynamics: A magnetic flux rope interpretation  

Sarah Gibson   Submitted: 2006-12-13 11:40

The solar prominence is an example of a space physics phenomenon that can be modeled as a twisted magnetic flux tube or magnetic flux ?rope.? In such models the prominence is one observable part of a larger magnetic structure capable of storing magnetic energy to drive eruptions. We show how a flux rope model explains a range of observations of prominences and associated structures such as cavities and soft X-ray sigmoids and discuss in particular the observational and dynamic consequences of three-dimensional reconnections in and around the evolving magnetic flux rope. We demonstrate that the flux rope model can describe the prominence's preeruption structure and dynamics, loss of equilibrium, and behavior during and after an eruption in which part of the flux rope is expelled from the corona.

Authors: S. E. Gibson and Y. Fan
Projects: None

Publication Status: Journal of Geophysical Research, 111, A12103, 10.1029/2006JA011871
Last Modified: 2006-12-13 12:01
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On the nature of the X-ray bright core in a stable filament channel  

Sarah Gibson   Submitted: 2006-12-13 11:36

In a search for the cause of the intense heating revealed by X-ray emission in filament channels, we have simulated the evolution of a twisted toroidal flux rope emerging quasi-statically into the corona. Initially, the simulated flux rope remains confined in equilibrium as the stored magnetic energy increases. With enough twist buildup, there is a sudden catastrophic loss of equilibrium and total expulsion of the flux rope. We focused on the quasi-static phase in which a current sheet forms within the flux rope cavity, along the so-called bald-patch separatrix surface (BPSS). This comprises an envelope of field lines that graze the anchoring lower boundary, enclosing the detached helical field that supports the prominence. Significant magnetic energy dissipation and heating are expected to center around such current sheets. The heating that should result provides a plausible explanation for the hot X-ray sources, although they appear to be colocated with cool material. If our physical picture is correct, then the development of X-ray ``bright cores'' or ``sigmoids'' in a filament channel suggests the presence of a BPSS separating the helical field of a twisted flux rope in stable confinement from the surrounding untwisted fields.

Authors: Y. Fan and S. E. Gibson
Projects: Yohkoh-SXT

Publication Status: Astrophysical Journal Letters, 641, L149, 2006
Last Modified: 2006-12-13 12:01
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NUMERICAL SIMULATIONS OF THREE-DIMENSIONAL CORONAL MAGNETIC FIELDS  

Sarah Gibson   Submitted: 2006-01-11 10:11

We present the results of MHD simulations in the low- regime of the evolution of the three-dimensional coronal magnetic field as an arched, twisted magnetic flux tube emerges into a preexisting coronal potential magnetic arcade. We find that the line-tied emerging flux tube becomes kink-unstable when a sufficient amount of twist is transported into the corona. For an emerging flux tube with a left-handed twist (which is the preferred sense of twist for active region flux tubes in the northern hemisphere), the kink motion of the tube and its interaction with the ambient coronal magnetic field lead to the formation of an intense current layer that displays an inverse-S shape, consistent with the X-ray sigmoid morphology preferentially seen in the northern hemisphere. The position of the current layer in relation to the lower boundary magnetic field of the emerging flux tube is also in good agreement with the observed spatial relations between the X-ray sigmoids and their associated photospheric bipolar magnetic regions. We argue that the inverse-S–shaped current layer formed is consistent with being a magnetic tangential discontinuity limited by numerical resolution and thus may result in the magnetic reconnection and significant heating that causes X-ray sigmoid brightenings.

Authors: Y. Fan and S. E. Gibson
Projects:

Publication Status: ApJ, 609, 1123, 2004
Last Modified: 2006-01-12 09:41
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OBSERVATIONAL CONSEQUENCES OF A MAGNETIC FLUX ROPE EMERGING INTO THE CORONA  

Sarah Gibson   Submitted: 2006-01-11 10:04

We show that a numerical simulation of a magnetic flux rope emerging into a coronal magnetic field predicts solar structures and dynamics consistent with observations. We first consider the structure, evolution, and relative location and orientation of S-shaped, or sigmoid, active regions and filaments. The basic assumptions are that (1) X-ray sigmoids appear at the regions of the flux rope known as ‘‘bald-patch–associated separatrix surfaces (BPSSs), where, under dynamic forcing, current sheets can form, leading to reconnection and localized heating, and that (2) filaments are regions of enhanced density contained within dips in the magnetic flux rope. We demonstrate that the shapes and relative orientations and locations of the BPSS and dipped field are consistent with observations of X-ray sigmoids and their associated filaments. Moreover, we show that current layers indeed form along the sigmoidal BPSS as the flux rope is driven by the kink instability. Finally, we consider how apparent horizontal motions of magnetic elements at the photosphere caused by the emerging flux rope might be interpreted. In particular, we show that local correlation tracking analysis of a time series of magnetograms for our simulation leads to an underestimate of the amount of magnetic helicity transported into the corona by the flux rope, largely because of undetectable twisting motions along the magnetic flux surfaces. Observations of rotating sunspots may provide better information about such rotational motions, and we show that if we consider the separated flux rope legs as proxies for fully formed sunspots, the amount of rotation that would be observed before the region becomes kink unstable would be in the range 40 –200 per leg/sunspot, consistent with observations.

Authors: S. E. Gibson, Y. Fan, C. Mandrini, G. Fisher, and P. Demoulin
Projects:

Publication Status: ApJ, 617, 600, 2004
Last Modified: 2006-01-12 09:40
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The calm before the storm: the link between quiescent cavities and CMEs  

Sarah Gibson   Submitted: 2006-01-11 09:55

Determining the state of the corona prior to coronal mass ejections (CMEs) is crucial to understanding and ultimately predicting solar eruptions. A common and compelling feature of CMEs is their three-part morphology as seen in white light observations of a bright expanding loop, followed by a relatively dark cavity, and lastly a bright core associated with an erupting prominence/filament. This morphology is an important constraint on CME models. It is also quite common for a three-part structure of loop/cavity/prominence-core to exist quiescently in the corona, and this is equivalently an important constraint on models of CME-precursor magnetic structure. These quiescent structures exist in the low corona, primarily below approximately 1.6 Rsun, and so are currently observable in white light during solar eclipses, or else by the Mauna Loa Solar Observatory Mk4 coronameter. We present the first comprehensive, quantitative analysis of white light quiescent cavities as observed by the Mk4 coronameter. We find that such cavities are ubiquitous, as they are the coronal limb counterparts to filament channels observed on the solar disk. We consider examples that range from extremely long-lived, longitudinally extended polar-crown-filament-related cavities, to smaller cavities associated with filaments near or within active regions. The former are often stable for days and even weeks at a time, and can be identified as long-lived cavities that survive for months. We quantify cavity morphology and intensity contrast properties, and consider correlations between these properties. We find multiple cases where quiescent cavities directly erupt into CMEs, and consider how morphological and intensity contrast properties of these cases differ from the general population of cavities. Finally, we discuss the implications that these observations may have for the state of the corona just prior to a CME, and more generally for the nature of coronal MHD equilibria.

Authors: S. E. Gibson, D. Foster, J. Burkepile, G. de Toma, A. Stanger
Projects: Soho-EIT

Publication Status: In press: ApJ, 641, 2006 (10 April)
Last Modified: 2006-01-11 09:55
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The emergence of a twisted magnetic flux tube into a preexisting coronal arcade  

Sarah Gibson   Submitted: 2003-11-11 09:53

To investigate the dynamic evolution of coronal magnetic field in response to the emergence of significantly twisted magnetic structures, we perform MHD simulations in the low-beta regime of the emergence of a twisted magnetic flux tube into a pre-existing coronal potential magnetic arcade. Our simulation of a twisted flux tube, which when fully emerged, contains a twist of 1.875 imes 2 pi field-line rotation about the axis between the anchored footpoints, leads to a magnetic structure with substantial writhing of the tube axis (apex rotation > 90 degrees) as a result of the non-linear evolution of the kink instability. For an emerging tube with a left-handed twist (which is the preferred sense of twist for active regions in the northern hemisphere), the writhing of the tube is also left-handed, producing a forward S-shape for the tube axis as viewed from the top, which is opposite to the inverse S-shaped X-ray sigmoid structures preferentially seen in the northern hemisphere. However we find that the writhing motion of the tube and its interaction with the ambient coronal magnetic field also drives the formation of an intense current layer which displays an inverse S-shape, consistent with the shape of X-ray sigmoids.

Authors: Y. Fan and S. Gibson
Projects:

Publication Status: ApJ, 589, L105, 2003
Last Modified: 2003-11-14 09:40
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The structure and evolution of a sigmoidal active region  

Sarah Gibson   Submitted: 2002-10-04 14:43

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections (CMEs). Sigmoids, or ``S''-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's 3-d structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third ``Whole Sun Month'' campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's 3-d density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Authors: S. E. Gibson, L. Fletcher, G. Del Zanna, C. D. Pike, H. E. Mason,C. H. Mandrini, P. Demoulin, H. Gilbert, J. Burkepile, T. Holzer, D. Alexander, Y. Liu, N. Nitta, J. Qiu, B. Schmieder, and B. J. Thompson
Projects: None

Publication Status: ApJ, 574, 1021, 2002
Last Modified: 2006-01-12 09:43
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3-d and twisted: an MHD interpretation of on-disk observational characteristics of CMEs  

Sarah Gibson   Submitted: 2000-01-07 12:03

A physical interpretation of observed coronal ``on-disk'' manifestations of an earth-directed coronal mass ejection (CME) is presented. The fundamental question of how the CME's magnetic field and its plasma distribution are related is largely unanswered, because a crucial piece of the puzzle, that is the three-dimensional morphology of the CME, remains difficult to ascertain so long as coronal observations are limited to projections onto a single plane of the sky. In order to understand the relationship between observations of CMEs projected at the solar limb and those projected on the solar disk, some sort of model of the 3-d CME is required. In this paper we address both the question of the 3-d morphology of the CME and the more fundamental question of the nature of the plasma/magnetic field relationship, by comparing the limb and on-disk CME representations of an analytic 3-d MHD model based on a spheromak-type flux rope magnetic field configuration. In particular, we show that the morphology of twin dimmings (also referred to as transient coronal holes) observed in X-ray and EUV can be reproduced by the CME model as the on-disk projection of the prominence cavity modeled for limb CMEs. Moreover, the bright core of a limb CME, generally corresponding to the material in an erupting prominence, may be interpreted to be the S-shaped central core of the modeled on-disk CME, splitting the cavity into twin dimmings when observed head-on without obstruction. The magnetic field structure of this central core exhibits many of a filament's magnetic field features required to match observations. Finally, we consider the nature of S-shaped filaments and X-ray ``sigmoids'' in the context of the model, in terms of localized heating and cooling acting on the modeled CME magnetic field structure.

Authors: S. E. Gibson and B. C. Low
Projects: None

Publication Status: JGR, accepted Dec. 1999
Last Modified: 2006-01-11 10:15
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Abstracts by Author
Magnetic nulls and super-radial expansion in the solar corona
Coronal cavities: observations and implications for the magnetic environment of prominences
Partially-ejected flux ropes: implications for space weather
The evolving sigmoid: evidence for magnetic flux ropes in the corona before, during, and after CMEs
Coronal prominence structure and dynamics: A magnetic flux rope interpretation
On the nature of the X-ray bright core in a stable filament channel
NUMERICAL SIMULATIONS OF THREE-DIMENSIONAL CORONAL MAGNETIC FIELDS
OBSERVATIONAL CONSEQUENCES OF A MAGNETIC FLUX ROPE EMERGING INTO THE CORONA
The calm before the storm: the link between quiescent cavities and CMEs
The emergence of a twisted magnetic flux tube into a preexisting coronal arcade
The structure and evolution of a sigmoidal active region
3-d and twisted: an MHD interpretation of on-disk observational characteristics of CMEs

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