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.

Determining the state of the corona prior to 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 finally 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, and 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 R-circle dot, 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 visible 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 in which 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.

We demonstrate the partial expulsion of a three-dimensional magnetic flux rope, in which an upper, escaping rope is separated from a lower, surviving rope by cusped, reconnecting loop field lines. We use the three-dimensional magnetohydrodynamic model recently presented by Fan, extended to examine the erupting rope's end state. As in that work, the modeled flux rope in spherical coordinates erupts when enough twist has emerged to induce a loss of equilibrium. After multiple reconnections at current sheets that form during the eruption, the rope breaks in two, so that only a part of it escapes. We consider the details of how this separation occurs and discuss the observational significance of such a partially expelled flux rope for partially erupting filaments and re-forming X-ray sigmoids.

We present MHD simulations in both 2D axisymmetric and 3D spherical geometries of the evolution of a twisted magnetic flux rope emerging into the low-beta corona previously occupied by a potential arcade field. These simulations show two distinct stages of the evolution. The earlier evolution is quasistatic during which if the flux emergence is stopped, the flux rope settles to a neighboring equilibrium with stored free magnetic energy. Loss of equilibrium and eruption of the flux rope are found for both a 2D axisymmetric flux rope and a 3D line-tied flux rope, when too much twisted flux has been transported into the corona.

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 40degrees-200degrees per leg/sunspot, consistent with observations.

We present the results of MHD simulations in the low-beta 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.

The three‐dimensional (3‐D) density structure of the solar corona is a fundamental boundary condition on the solar wind. Most easily applied models of the global coronal density have been restricted to date to axisymmetric 2‐D cases. We present here a 3‐D model made up of a superposition of multiple streamers, having distinct gaussian widths in longitude and latitude and both longitudinal and latitudinal dependence of the neutral lines implicit beneath the streamer cores. Nonradiality of streamers and solar B‐angle tilt are also explicitly treated. We show how this simple model can capture many of the general properties of coronal white light observations and demonstrate how such a model can assist in the interpretation of the multiple views on coronal structures such as will be provided by the upcoming STEREO mission.

To investigate the dynamic evolution of a 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 preexisting coronal potential magnetic arcade. Our simulation of a twisted flux tube, which when fully emerged contains a twist of 1.875 x 2pi field-line rotation about the axis between the anchored footpoints, leads to a magnetic structure with substantial writhing of the tube axis (with an apex rotation >90degrees) as a result of the nonlinear 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 drive the formation of an intense current layer that displays an inverse-S shape, consistent with the shape of X-ray sigmoids.

There has been much debate lately about whether twisted magnetic flux ropes exist in the corona. When asked for observational evidence of them, the temptation is to show images of apparently twisted structures (e.g. see Figure 1). However, we must be very careful of projection effects in interpreting these observations. Two critical aspects of understanding how we might observe flux ropes axe 1) the 3D nature of the flux rope, and 2) physically, which bits are visible and for what reasons? In this paper we will use a simple but physically reasonable 3D analytic model to address these two issues, and develop techniques that can in future be used on more general models, both analytic and numerical.

During the third Whole Sun Month Campaign (August 18 – September 14, 1999), the evolution of the active region NOAA 8668 was followed during its meridian passage and at the limb (Sigmoid JOP 106), with simultaneous observations with the Solar and Heliospheric Observatory (SOHO), and with other instruments, both satellite and ground-based. On August 21st, a small flare, associated with a brightening of the sigmoidal structure, occurred. SOHO Coronal Diagnostic Spectrometer (CDS) observations of this small flare are presented. Coronal temperatures and densities of the sigmoid are estimated. High transition region densities (in the range 2.5–7 × 1011 cm−3), obtained using O IV, are present in the brightenings associated with the flare. At coronal level, high temperatures of at least 8 MK were reached, as shown by strong Fe XIX emission. After this small flare, relatively strong blue-shifts (⋍ 30 km/s) are observed in coronal lines, located at the two ends of a small loop system associated with the sigmoid.

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. 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 three-dimensional 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 that 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 three-dimensional 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.

On 1999 August 26, a coronal jet occurred at the northwest limb near a sigmoid active region (AR 8668) that was the target for a joint observation plan (SOHO joint observing program 106) during the third Whole Sun Month Campaign. This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory CHIP and PICS) and at 1.64 R☉ (SOHO/UVCS). At the limb, this jet event displayed both low- and high-temperature components. Both high- and low-temperature components were evident during the early phase (first 20 minutes) of the event. However, the low-temperature component is maintained for ~1 hr after the higher temperature component is gone. There is a second brightening (a possible second jet) seen by EIT and TRACE about 50 minutes after the onset of the first jet. The line-of-sight motion at the limb began with a 300 km s-1 redshift and evolved to a 200 km s-1 blueshift. At 1.64 R☉, the intensities of Lyα and Lyβ in the jet increased by a factor of several hundred compared with the background corona. The C III λ977 line also brightened significantly. This indicates low-temperature [~(1-2) × 105 K] emission in the jet, while the intensities of O VI λ1032 and O VI λ1037 increased by as much as a factor of 8. The UVCS data show evidence of heating at the early phase of the event. The Doppler shift in the lines indicates that the line-of-sight (LOS) velocity in the jet started from ~150 km s-1 in blueshift and ended at ~100 km s-1 in redshift. This LOS motion seen at 1.64 R☉ was apparently opposite to what was observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the LOS speed. Interestingly, UVCS observations at 2.33 and 2.66 R☉ show no trace of the jet and SOHO/LASCO observations also yield no firm detection. We find that a simple ballistic model can explain most of the dynamical properties of this jet, while the morphology and the thermal properties agree well with reconnection-driven X-ray jet models.

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 (3‐D) 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.

Empirical determinations of outflow velocities in the solar corona provide a much needed constraint, along with density and temperature determinations, of the acceleration and heating mechanisms in the extended corona. Much progress has been made on density determinations from white light polarized brightness observations but outflow velocities have been more difficult to determine. We present the first determinations of outflow velocities versus height and latitude based on a three‐dimensional (3‐D) reconstruction of the O VI 1032 and 1037 À emissivities. The Doppler dimming (and pumping) of the local emissivities give true localized outflow velocities at the selected locations in the extended corona from ∼1.75 to 2.75 solar radii. The velocities are based on an empirical model of the corona which is constrained by the reconstructed O VI emissivities derived from the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) synoptic observations and by electron density determinations based on white light measurements from the SOHO Large Angle Spectroscopic Coronagraph (LASCO) and Mauna Loa Solar Observatory (MLSO) Mk III coronameter.

We model electron densities of the simplest, most symmetric solar minimum streamer structure observed during the Whole Sun Month (WSM) campaign, using coronal observations of both visible white light and extreme ultraviolet (EUV) emission. Using white light data from the SOHO/LASCO/C2 and HAO/ Mauna Loa Mark 3 coronagraphs, we determine electron densities by way of a Van de Hulst inversion. We compare the white light densities to those determined from the density sensitive EUV line ratios of Si IX 350/342 Angstrom observed by the SOHO/coronal diagnostic spectrometer (CDS). Moreover, from the white light density profiles we calculate hydrostatic temperature profiles and compare to temperatures derived from the Si XII/Mg X line ratio. We find the white light and spectral analysis produce consistent density and temperature information.

The quiet corona at times close to solar minimum shows a striking north-south asymmetry which suggests that neither dipole-like nor octupole-like fields are sufficient to describe the global coronal magnetic field. We believe that such phenomena reflect the asymmetry of the intrinsic magnetic held of the sun as a star; this weak field is usually obscured by active regions. Empirical models for spherical corona (at solar maximum) and for ellipsoidal corona (at solar minimum) have been established. We extend the existing classification to include an empirical model for the quiet solar corona with strong north-south asymmetry. We show examples of such asymmetric corona in the green line for three different solar minima and evolution of corona from almost ellipsoidal type to corona with strong north-south asymmetry "bald man with double beard"). A theoretical model (1984) of Osherovich et al. (1) relates such asymmetry to the existence of a significant quadrupole term in the global magnetic field of the sun. According to this model, the size of northern and southern polar coronal holes is affected differently by a quadrupole term which creates asymmetry in the magnetic and thermodynamic parameters as well as in the velocity of the outflow from the two polar regions.

A large number of synoptic maps from a variety of instruments are used to show the general morphology of the Sun at the time of the First Whole Sun Month Campaign. The campaign was conducted from August 10 to September 8, 1996. The synoptic maps cover the period from Carrington rotation 1912/253 degrees to Carrington rotation 1913/45 degrees. The synoptic maps encompass both on-disk data and limb data from several heights in the solar atmosphere. The maps are used to illustrate which wavelengths and data sets show particular features, such as active regions and coronal holes. Of particular interest is the equatorial coronal hole known as the "elephant's trunk," which is clearly evident in the synoptic maps of on-disk data. The elephant's trunk is similar in appearance to the Shylab-era, "Boot of Italy," equatorial coronal hole. The general appearance of the limb maps is explained as well. The limb maps also show evidence for equatorial coronal holes.

Until recently [Guhathakurta and Fisher, 1998], inference of electron density distribution in the solar corona was limited by the field of view of white-light coronagraphs (typically out to 6 R-s). Now, for the first time we have a series of white-light coronagraphs (SOHO/LASCO) whose combined field of view extends from 1.1-30 R-s. Quantitative information on electron density distribution of coronal hole and coronal plumes/rays are estimated by using white-light, polarized brightness (pB) observations from the SOHO/LASCO/C2, and C3 and HAO/Mauna Loa Mark III coronagraphs from 1.15 to 8.0 R-s. Morphological information on the boundary of the polar coronal hole and streamer interface is determined from the white-light observations in a manner similar to the Skylab polar coronal hole boundary estimate [Guhathakurta and Holzer, 1994], The average coronal hole electron density in the region 1-1.15 R-s is estimated from the density-sensitive EUV line ratios of Si IX 350/342 Angstrom observed by the SOHO/coronal diagnostic spectrometer (CDS), We combine these numbers with the estimate from white-light (WL) observations to obtain a density profile from 1 to 8 R-s for the plumes and the polar coronal hole. We find that white light and spectral analysis produce consistent density information. Extrapolated densities inferred from SOHO observations are compared to Ulysses in situ observations of density. Like the density inferred from the Spartan 201-03 coronagraph, the current SOHO density profiles suggest that the acceleration of the fast solar wind takes place very close to the Sun, within 10-15 R-s. The density information is used to put constraints on solar wind flow velocities and effective temperatures. Finally, these results are compared to the recent analysis of the Spartan 201-03 white-light observations.

The Whole Sun Month campaign (August 10 to September 8, 1996) brought together a wide range of space-based and ground-based observations of the Sun and the interplanetary medium during solar minimum. The wealth of data collected provides a unique opportunity for testing coronal models. We develop a three-dimensional magnetohydrodynamic (MHD) model of the solar corona (from 1 to 30 solar radii) applicable to the WSM time period, using measurements of the photospheric magnetic field as boundary conditions for the calculation. We compare results from the computation with daily and synoptic white-light and emission images obtained from ground-based observations and the SOHO spacecraft and with solar wind measurements from the Ulysses and WIND spacecraft. The results from the MHD computation show good overall agreement with coronal and interplanetary structures, including the position and shape of the streamer belt, coronal hole boundaries, and the heliospheric current sheet. From the model, we can infer the source locations of solar wind properties measured in interplanetary space. We find that the slow solar wind typically maps back to near the coronal hole boundary, while the fast solar wind maps to regions deeper within the coronal holes. Quantitative disagreements between the MHD model and observations for individual features observed during Whole Sun Month give insights into possible improvements to the model.

Because coronal mass ejections (CMEs) are viewed in projection, it is difficult to determine their three-dimensional nature. We use an analytic model of CMEs as an example of a fully three-dimensional magnetic field structure in MHD force balance with an emerging CME. We present the CME magnetic field and its associated density structure, seen projected at the limb from two viewing angles perpendicular to the plane of the sky, and emerging from disk center representing "earth-directed" CME events. The range of CME structures thus produced compares well to existing CME white-light coronagraph and full disk EW and X-ray observations. In particular, we find that both 3-part "front-cavity-core" and "U-shaped" white light CMEs, as well as the twin dimmings (also referred to as transient coronal holes) observed in X-ray and EUV, can successfully be reproduced by the CME model. All of these structures are a direct consequence of a single three-dimensional magnetic field topology, viewed from different directions.