Prominence Cavities Research Articles

Toward a Unified Explanation for the Three-part Structure of Solar Coronal Mass Ejections

Coronal mass ejections (CMEs) are associated with the eruption of magnetic flux ropes (MFRs), which usually appear as hot channels in active regions and coronal cavities in quiet-Sun regions. CMEs often exhibit a classical three-part structure in the lower corona when imaged with white-light coronagraphs, including a bright front, dark cavity, and bright core. For several decades, the bright core and dark cavity have been regarded as the erupted prominence and MFR, respectively. However, recent studies have clearly demonstrated that both the prominence and hot-channel MFR can be observed as the CME core. The current research presents a three-part CME resulting from the eruption of a coronal prominence cavity on 2010 October 7, with observations from two vantage perspectives, i.e., edge-on from the Earth and face-on from the Solar Terrestrial Relations Observatory (STEREO). Our observations illustrate two important results: (1) for the first time, the erupting coronal cavity is recorded as a channel-like structure in the extreme-ultraviolet passband, analogous to the hot-channel morphology, and is dubbed as the warm channel; and (2) both the prominence and warm-channel MFR (coronal cavity) in the extreme-ultraviolet passbands evolve into the CME core in the white-light coronagraphs of STEREO-A. The results suggest that we are working toward a unified explanation for the three-part structure of CMEs, in which both prominences and MFRs (hot or warm channels) are responsible for the bright core.

Magnetofrictional Modeling of an Erupting Pseudostreamer

Abstract In this study, we present the magnetic configuration of an erupting pseudostreamer observed on 2015 April 19, on the southwest limb of the Sun, with a prominence cavity embedded inside. The eruption resulted in a partial halo coronal mass ejection. The prominence eruption begins with a slow rise and then evolves to a fast-rise phase. We analyze this erupting pseudostreamer using the flux-rope insertion method and magnetofrictional relaxation to establish a sequence of plausible out-of-equilibrium magnetic configurations. This approach allows the direct incorporation of observations of structures seen in the corona (filament and cavity) to appropriately model the pseudostreamer based on SDO/HMI line-of-sight photospheric magnetograms. We also perform a topological analysis in order to determine the location of quasiseparatrix layers (QSLs) in the models, producing Q-maps to examine how the QSL locations progress in the higher iterations. We found that the axial flux in our best-fit unstable model was a factor of 20 times higher than we found in our marginally stable case. We computed the average magnetic field strength of the prominence and found that the unstable model exhibits twice the average field strength of the stable model. The eruption height from our modeling matches very well with the prominence eruption height measured from the AIA observation. The Q-maps derived from the model reproduce structures observed in LASCO/C2. Thus, the modeling and topological analysis results are fully consistent with the observed morphological features, implying that we have captured the large magnetic structure of the erupting filament in our magnetofrictional simulation.

Using SDO/AIA to Understand the Thermal Evolution of Solar Prominence Formation

Abstract We investigated the thermal properties of prominence formation using time series analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) data. Here, we report the first time-lag measurements derived from SDO/AIA observations of a prominence and its cavity on the solar limb, made possible by AIA's different wave bands and high time resolution. With our time-lag analysis, which tracks the thermal evolution using emission formed at different temperatures, we find that the prominence cavity exhibited a mixture of heating and cooling signatures. This is in contrast to prior time-lag studies of multiple active regions that chiefly identified cooling signatures and very few heating signatures, which is consistent with nanoflare heating. We also computed time lags for the same pairs of SDO/AIA channels using output from a one-dimensional hydrodynamic model of prominence material forming through thermal nonequilibrium (TNE). We demonstrate that the SDO/AIA time lags for flux tubes undergoing TNE are predicted to be highly complex, changing with time and location along the flux tube, and are consistent with the observed time-lag signatures in the cavity surrounding the prominence. Therefore, the time-lag analysis is a sensitive indicator of the heating and cooling processes in different coronal regions. The time lags calculated for the simulated prominence flux tube are consistent with the behavior deduced from the AIA data, thus supporting the TNE model of prominence formation. Future investigations of time lags predicted by other models for the prominence mass could be a valuable method for discriminating among competing physical mechanisms.

The Evolution of Magnetic Rayleigh–Taylor Unstable Plumes and Hybrid KH-RT Instability into a Loop-like Eruptive Prominence

Abstract MRT-unstable plumes are observed in a loop-like eruptive prominence using Solar Dynamic Observatory/Atmospheric Imaging Assembly observations. The small-scale cavities are developed within the prominence, where perturbations trigger dark plumes (P1 and P2) propagating with speeds of 35–46 km s−1. The self-similar plume formation initially shows the growth of a linear MRT-unstable plume (P1), and thereafter the evolution of a nonlinear single-mode MRT-unstable second plume (P2). A differential emission measure analysis shows that plumes are less dense and hotter than the prominence. We have estimated the observational growth rate for both plumes as 1.32 ± 0.29 × 10−3 s−1 and 1.48 ± 0.29 × 10−3 s−1, respectively, which are comparable to the estimated theoretical growth rate (1.95 × 10−3 s−1). The nonlinear phase of an MRT-unstable plume (P2) may collapse via a Kelvin–Helmholtz vortex formation in the downfalling plasma. Later, a plasma thread is evident in the rising segment of this prominence. It may be associated with the tangled field and Rayleigh–Taylor instability. The tangled field initiates shearing at the prominence–cavity boundary. Due to this shear motion, the plasma downfall occurred at the right part of the prominence–cavity boundary. It triggers the characteristic KH unstable vortices and MRT-unstable plasma bubbles propagating at different speeds and merging with each other. The shear motion and lateral plasma downfall may initiate hybrid KH-RT instability there.

The Eruption of a Prominence-carrying Coronal Flux Rope: Forward Synthesis of the Magnetic Field Strength Measurement by the COronal Solar Magnetism Observatory Large Coronagraph

From a magnetohydrodynamic (MHD) simulation of the eruption of a prominence hosting coronal flux rope, we carry out forward synthesis of the circular polarization signal (Stokes V signal) of the FeXIII emission line at 1074.7 nm produced by the MHD model as measured by the proposed COronal Solar Magnetism Observatory (COSMO) Large Coronagraph (LC) and infer the line-of-sight magnetic field BLOS above the limb. With an aperture of 150 cm, integration time of 12 min, and a resolution of 12 arcsec, the LC can measure a significant BLOS with sufficient signal to noise level, from the simulated flux rope viewed nearly along its axis with a peak axial field strength of about 10 G. The measured BLOS is found to relate well with the axial field strength of the flux rope for the height range of the prominence, and can discern the increase with height of the magnetic field strength in that height range that is a definitive signature of the concave upturning dipped field supporting the prominence. The measurement can also detect an outward moving BLOS due to the slow rise of the flux rope as it develops the kink instability, during the phase when its rise speed is still below about 41 km/s and up to a height of about 1.3 solar radii. These results suggest that the COSMO LC has great potential in providing quantitative information about the magnetic field structure of CME precursors (e.g. the prominence cavities) and their early evolution for the onset of eruption.

MHD Simulation of Prominence Eruption

Abstract We carry out magnetohydrodynamic (MHD) simulations of the quasi-static evolution and eruption of a twisted coronal flux rope under a coronal streamer built up by an imposed flux emergence at the lower boundary. The MHD model incorporates simple empirical coronal heating, optically thin radiative cooling, and field-aligned thermal conduction, and thus allows the formation of prominence condensations. We find that during the quasi-static evolution, prominence/filament condensations of an elongated, sigmoid morphology form in the dips of the significantly twisted field lines of the emerged flux rope due to runaway radiative cooling. A prominence cavity also forms around the prominence, which is best observed above the limb with the line of sight nearly along the length of the flux rope, as shown by synthetic SDO/AIA EUV images. The magnetic field supporting the prominence is significantly non-force-free despite the low plasma β. By comparing with a simulation that suppresses prominence formation, we find that the weight of the prominence is dynamically important and can suppress the onset of the kink instability and hold the flux rope in equilibrium for a significantly long time, until draining of the prominence plasma develops and reduces the weight of the prominence. The flux rope eventually develops the kink instability and erupts, producing a prominence eruption. The synthetic AIA 304 Å images show that the prominence is lifted up into an erupting loop, exhibiting helical features along the loop and substantial draining along the loop legs, as is often seen in observations.

Twisting/Swirling Motions during a Prominence Eruption as Seen from SDO/AIA

A quiescent prominence was observed at north-west limb of the Sun using different channels of Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). We report and analyse twisting/swirling motions during and after the prominence eruption. We segregate the observed rotational motions into small and large scale. Small scale rotational motions manifest in the barbs of the prominence while the large scale rotation manifests as the roll motion during the prominence eruption. We noticed that both footpoints of the prominence rotate in the counter-clockwise direction. We propose that similar sense of rotation in both footpoints leads to prominence eruption. The prominence erupted asymmetrically near the southern footpoint which may be due to uneven mass distribution and location of the cavity near southern footpoint. Furthermore, we study the swirling motion of the plasma along different circular paths in the cavity of the prominence after the prominence eruption. The rotational velocities of the plasma moving along different circular paths are estimated to be $\sim$ 9-40 km s$^{-1}$. These swirling motions can be explained in terms of twisted magnetic field lines in the prominence cavity. Finally, we observe the twist built up in the prominence, being carried away by the coronal mass ejection (CME) as seen in the Large Angle Spectrometric Coronagraph (LASCO) onboard Solar and Heliospheric Observatory (SOHO).

APPEARANCES AND STATISTICS OF CORONAL CAVITIES DURING THE ASCENDING PHASE OF SOLAR CYCLE 24

We present a survey of 429 coronal prominence cavities found between 2010 May and 2015 February using the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly limb synoptic maps. We examined correlations between each cavity's height, width, and length. Our findings showed that around 38% of the cavities were prolate, 27% oblate, and 35% circular in shape. The lengths of the cavities ranged from 0.06 to 2.9 . When a cavity is longer than 1.5 , it has a narrower height range (0.1–0.3 ), whereas when the cavity was shorter than 1.5 , it had a wider height range (0.07–0.5 ). We find that the overall three-dimensional topology of the long, stable cavities can be characterized as a long tube with an elliptical cross section. We also noted that the circular and oblate cavities are longer in length than the prolate cavities. We also studied the physical mechanisms behind the cavity drift toward the pole and found it to be tied to the meridional flow. Finally, by observing the evolution of the cavity regions using SDO/Helioseismic Magnetic Imager (HMI) surface magnetic field observations, we found that the cavities formed a belt near the polar coronal hole boundary; we call this the cavity belt. Our results showed that the cavity belt migrated toward higher latitude over time and the cavity belt disappeared after the polar magnetic field reversal. This result shows that cavity evolution provides new insight into the solar cycle.

DIRECT EVIDENCE OF AN ERUPTIVE, FILAMENT-HOSTING MAGNETIC FLUX ROPE LEADING TO A FAST SOLAR CORONAL MASS EJECTION

Magnetic flux ropes (MFRs) are believed to be at the heart of solar coronal mass ejections (CMEs). A well-known example is the prominence cavity in the low corona that sometimes makes up a three-part white-light (WL) CME upon its eruption. Such a system, which is usually observed in quiet-Sun regions, has long been suggested to be the manifestation of an MFR with relatively cool filament material collecting near its bottom. However, observational evidence of eruptive, filament-hosting MFR systems has been elusive for those originating in active regions. By utilizing multi-passband extreme-ultra-violet (EUV) observations from SDO/AIA, we present direct evidence of an eruptive MFR in the low corona that exhibits a hot envelope and a cooler core; the latter is likely the upper part of a filament that undergoes a partial eruption, which is later observed in the upper corona as the coiled kernel of a fast, WL CME. This MFR-like structure exists more than one hour prior to its eruption, and displays successive stages of dynamical evolution, in which both ideal and non-ideal physical processes may be involved. The timing of the MFR kinematics is found to be well correlated with the energy release of the associated long-duration C1.9 flare. We suggest that the long-duration flare is the result of prolonged energy release associated with the vertical current sheet induced by the erupting MFR.

Coronal Cavity Survey: Morphological Clues to Eruptive Magnetic Topologies

We present a survey on coronal prominence cavities conducted using 19 months of data from the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO) satellite. Coronal cavities are elliptical regions of rarefied density lying above and around prominences. They can be long-lived (weeks to months) but are often observed to eventually erupt as part of a coronal mass ejection (CME). We determine morphological properties of the cavities both by qualitatively assessing their shape, and quantitatively fitting them with ellipses. We demonstrate consistency between these two approaches, and find that fitted ellipses are taller than they are wide for almost all cavities studied, in agreement with an earlier analysis of white-light cavities. We examine correlations between cavity shape, aspect ratio, and propensity for eruption. We find that cavities with a teardrop-shaped morphology are more likely to erupt, and we discuss the implications of this morphology for magnetic topologies associated with CME models. We provide the full details of the survey for broad scientific use as supplemental material.

Apparent Solar Tornado-Like Prominences

Recent high-resolution observations from the Solar Dynamics Observatory (SDO) have reawakened interest in the old and fascinating phenomenon of solar tornado-like prominences. This class of prominences was first introduced by Pettit (Astrophys. J. 76, 9, 1932), who studied them over many years. Observations of tornado prominences similar to the ones seen by SDO had already been documented by Secchi (Le Soleil, 1877). High-resolution and high-cadence multiwavelength data obtained by SDO reveal that the tornado-like appearance of these prominences is mainly an illusion due to projection effects. We discuss two different cases where prominences on the limb might appear to have a tornado-like behavior. One case of apparent vortical motions in prominence spines and barbs arises from the (mostly) 2D counterstreaming plasma motion along the prominence spine and barbs together with oscillations along individual threads. The other case of apparent rotational motion is observed in a prominence cavity and results from the 3D plasma motion along the writhed magnetic fields inside and along the prominence cavity as seen projected on the limb. Thus, the “tornado” impression results either from counterstreaming and oscillations or from the projection on the plane of the sky of plasma motion along magnetic-field lines, rather than from a true vortical motion around an (apparent) vertical or horizontal axis. We discuss the link between tornado-like prominences, filament barbs, and photospheric vortices at their base.

Polarimetric Properties of Flux Ropes and Sheared Arcades in Coronal Prominence Cavities

The coronal magnetic field is the primary driver of solar dynamic events. Linear and circular polarization signals of certain infrared coronal emission lines contain information about the magnetic field, and to access this information, either a forward or an inversion method must be used. We study three coronal magnetic configurations that are applicable to polar-crown filament cavities by doing forward calculations to produce synthetic polarization data. We analyze these forward data to determine the distinguishing characteristics of each model. We conclude that it is possible to distinguish between cylindrical flux ropes, spheromak flux ropes, and sheared arcades using coronal polarization measurements. If one of these models is found to be consistent with observational measurements, it will mean positive identification of the magnetic morphology that surrounds certain quiescent filaments, which will lead to a greater understanding of how they form and why they erupt.

THE MAGNETIC STRUCTURE OF SOLAR PROMINENCE CAVITIES: NEW OBSERVATIONAL SIGNATURE REVEALED BY CORONAL MAGNETOMETRY

The Coronal Multi-channel Polarimeter (CoMP) obtains daily full-Sun above-the-limb coronal observations in linear polarization, allowing for the first time a diagnostic of the coronal magnetic field direction in quiescent prominence cavities. We find that these cavities consistently possess a characteristic "lagomorphic" signature in linear polarization indicating twist or shear extending up into the cavity above the neutral line. We demonstrate that such a signature may be explained by a magnetic flux-rope model, a topology with implications for solar eruptions. We find corroborating evidence for a flux rope structure in the pattern of concentric rings within cavities seen in CoMP line-of-sight velocity.

The spatial relation between EUV cavities and linear polarization signatures

AbstractSolar coronal cavities are regions of rarefied density and elliptical cross-section. The Coronal Multi-channel Polarimeter (CoMP) obtains daily full-Sun coronal observations in linear polarization, allowing a systematic analysis of the coronal magnetic field in polar-crown prominence cavities. These cavities commonly possess a characteristic “lagomorphic” signature in linear polarization that may be explained by a magnetic flux-rope model. We analyze the spatial relation between the EUV cavity and the CoMP linear polarization signature.

A solar tornado triggered by flares?

Solar tornados are dynamical, conspicuously helical magnetic structures mainly observed as a prominence activity. We investigate and propose a triggering mechanism for the solar tornado observed in a prominence cavity by SDO/AIA on September 25, 2011. High-cadence EUV images from the SDO/AIA and the Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the neighbouring active-region (NOAA 11303), and their EUV waves, with the dynamical developments of the tornado. The timings of the flares and EUV waves observed on-disk in 195\AA\ are analyzed in relation to the tornado activities observed at the limb in 171\AA. Each of the three flares and its related EUV wave occurred within 10 hours of the onset of the tornado. They have an observed causal relationship with the commencement of activity in the prominence where the tornado develops. Tornado-like rotations along the side of the prominence start after the second flare. The prominence cavity expands with acceleration of tornado motion after the third flare. Flares in the neighbouring active region may have affected the cavity prominence system and triggered the solar tornado. A plausible mechanism is that the active-region coronal field contracted by the `Hudson effect' due to the loss of magnetic energy as flares. Subsequently the cavity expanded by its magnetic pressure to fill the surrounding low corona. We suggest that the tornado is the dynamical response of the helical prominence field to the cavity expansion.

Prominence Cavity Regions Observed Using SWAP 174 Å Filtergrams and Simultaneous Eclipse Flash Spectra

SWAP images from PROBA2 taken at 174 Å in the Fe ix/x lines are compared with simultaneous slitless flash spectra obtained during the solar total eclipse of 11 July 2010. Myriad faint low-excitation emission lines together with the He i and He ii Paschen α chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We analyzed a deep flash spectrum obtained by summing 80 individual spectra to evaluate the intensity modulations of the continuum. Intensity deficits are observed and measured at the prominences boundaries in both eclipse and SWAP images. The prominence cavities interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences, and some intense heating occurring in these regions, are discussed. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.

THERMAL SIGNATURES OF TETHER-CUTTING RECONNECTIONS IN PRE-ERUPTION CORONAL FLUX ROPES: HOT CENTRAL VOIDS IN CORONAL CAVITIES

Using a 3D MHD simulation, we model the quasi-static evolution and the onset of eruption of a coronal flux rope. The simulation begins with a twisted flux rope emerging at the lower boundary and pushing into a pre-existing coronal potential arcade field. At a chosen time the emergence is stopped with the lower boundary taken to be rigid. Then the coronal flux rope settles into a quasi-static rise phase during which an underlying, central sigmoid-shaped current layer forms along the so called hyperbolic flux tube (HFT), a generalization of the X-line configuration. Reconnections in the dissipating current layer effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically, even though the magnetic energy is decreasing as the system relaxes. We examine the thermal features produced by the current layer formation and the associated "tether-cutting" reconnections as a result of heating and field aligned thermal conduction. It is found that a central hot, low-density channel containing reconnected, twisted flux threading under the flux rope axis forms on top of the central current layer. When viewed in the line of sight roughly aligned with the hot channel (which is roughly along the neutral line), the central current layer appears as a high-density vertical column with upward extensions as a "U" shaped dense shell enclosing a central hot, low-density void. Such thermal features have been observed within coronal prominence cavities. Our MHD simulation suggests that they are the signatures of the development of the HFT topology and the associated tether-cutting reconnections, and that the central void grows and rises with the reconnections, until the flux rope reaches the critical height for the onset of the torus instability and dynamic eruption ensues.

TEMPERATURE AND EXTREME-ULTRAVIOLET INTENSITY IN A CORONAL PROMINENCE CAVITY AND STREAMER

We analyze the temperature and EUV line emission of a coronal cavity and surrounding streamer in terms of a morphological forward model. We use a series of iron line ratios observed with the Hinode Extreme-ultraviolet Imaging Spectrograph (EIS) on 2007 August 9 to constrain temperature as a function of altitude in a morphological forward model of the streamer and cavity. We also compare model predictions to the EIS EUV line intensities and polarized brightness (pB) data from the Mauna Loa Solar Observatory (MLSO) Mark 4 K-coronameter. This work builds on earlier analysis using the same model to determine geometry of and density in the same cavity and streamer. The fit to the data with altitude-dependent temperature profiles indicates that both the streamer and cavity have temperatures in the range 1.4‐1.7 MK. However, the cavity exhibits substantial substructure such that the altitude-dependent temperature profile is not sufficient to completely model conditions in the cavity. Coronal prominence cavities are structured by magnetism so clues to this structure are to be found in their plasma properties. These temperature substructures are likely related to structures in the cavity magnetic field. Furthermore, we find that the model overestimates the EUV line intensities by a factor of 4‐10, without overestimating pB. We discuss this difference in terms of filling factors and uncertainties in density diagnostics and elemental abundances.

Prominence-cavity regions observed in 2010 eclipse flash spectra and SWAP images

Images from the SWAP (Proba 2 mission) taken in the Fe IX/X coronal line emission are compared to simultaneous slitless flash spectra taken during the total solar eclipse of July, 11th 2010. Many faint low excitation emission lines together with the HeI and the HeII Paschen α (P α ) chromospheric lines are recorded in the regions of limb prominences observed in space. We consider a deep flash spectrum obtained by summing 80 individual spectra to show the intensity modulations of the continuum between HeI and HeII lines. Intensity depressions are observed around the prominences in both eclipse and SWAP images. The prominence cavities are interpreted as a relative depression of plasma density produced inside the corona surrounding the prominences, and some evidence of intense heating occuring in the region around the prominence is discussed.

FORWARD MODELING CAVITY DENSITY: A MULTI-INSTRUMENT DIAGNOSTIC

The thermodynamic properties of coronal prominence cavities present a unique probe into the energy and mass budget of prominences. Using a three-dimensional morphological model, we forward model the polarization brightness and extreme-ultraviolet (EUV) emission of a cavity and its surrounding streamer. Using a genetic algorithm, we find the best-fit density model by comparing the models to Mauna Loa Solar Observatory MK4 and Hinode EUV Imaging Spectrometer data. The effect of temperature variations on the derived density is also measured. We have measured the density inside a cavity down to 1.05 R ☉ with height-dependent error bars. Our forward modeling technique compensates for optically thin projection effects. This method provides a complementary technique to traditional line ratio diagnostics that is useful for diffuse off-limb coronal structures.