Formation and large-scale patterns of filament channels and filaments

2015ASSL..415..355MThe properties and large-scale patterns of filament channels and filaments are considered. Initially, the global formation locations of filament channels and filaments are discussed, along with their hemispheric pattern. Next, observations of the formation of filament channels and filaments are described where two opposing views are considered. Finally, the wide range of models that have been constructed to consider the formation of filament channels and filaments over long time-scales are described, along with the origin of the hemispheric pattern of filaments

Properties of the prominence magnetic field and plasma distributions as obtained from 3D whole-prominence fine structure modeling

Aims. We analyze distributions of the magnetic field strength and prominence plasma (temperature, pressure, plasma beta, and mass) using the 3D whole-prominence fine structure model. Methods. The model combines a 3D magnetic field configuration of an entire prominence, obtained from non-linear force-free field simulations, with a detailed semi-empirically derived description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along multiple fine structures within the 3D magnetic model. Results. We show that in the modeled prominence, the variations of the magnetic field strength and its orientation are insignificant on scales comparable to the smallest dimensions of the observed prominence fine structures. We also show the ability of the 3D whole-prominence fine structure model to reveal the distribution of the prominence plasma, with respect to its temperature within the prominence volume. This provides new insights into the composition of the prominence-corona transition region. We further demonstrate that the values of the plasma beta are small throughout the majority of the modeled prominence when realistic photospheric magnetic flux distributions and prominence plasma parameters are assumed. While this is generally true, we also find that in the region with the deepest magnetic dips, the plasma beta may increase towards unity. Finally, we show that the mass of the modeled prominence plasma is in good agreement with the mass of observed non-eruptive prominences.Publisher PDFPeer reviewe

Simulating the coronal evolution of bipolar active regions to investigate the formation of flux ropes

Funding: S.L.Y. would like to acknowledge STFC for support via the Consolidated Grant SMC1/YST025. D.H.M. would like to thank STFC, the Leverhulme Trust and the ERC under the Synergy Grant: The Whole Sun, grant agreement no. 810218 for financial support. L.M.G. is thankful to the Royal Society for a University Research Fellowship and the Leverhulme Trust.The coronal magnetic field evolution of 20 bipolar active regions (ARs) is simulated from their emergence to decay using the time-dependent nonlinear force-free field method of Mackay, Green, and van Ballegooijen (Astrophys. J. 729, 97, 2011). A time sequence of cleaned photospheric line-of-sight magnetograms, which covers the entire evolution of each AR, is used to drive the simulation. A comparison of the simulated coronal magnetic field with the 171 and 193 Å observations obtained by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), is made for each AR by manual inspection. The results show that it is possible to reproduce the evolution of the main coronal features such as small- and large-scale coronal loops, filaments and sheared structures for 80% of the ARs. Varying the boundary and initial conditions, along with the addition of physical effects such as Ohmic diffusion, hyperdiffusion and a horizontal magnetic field injection at the photosphere, improves the match between the observations and simulated coronal evolution by 20%. The simulations were able to reproduce the build-up to eruption for 50% of the observed eruptions associated with the ARs. The mean unsigned time difference between the eruptions occurring in the observations compared to the time of eruption onset in the simulations was found to be ≈5 hrs. The simulations were particularly successful in capturing the build-up to eruption for all four eruptions that originated from the internal polarity inversion line of the ARs. The technique was less successful in reproducing the onset of eruptions that originated from the periphery of ARs and large-scale coronal structures. For these cases global, rather than local, nonlinear force-free field models must be used. While the technique has shown some success, eruptions that occur in quick succession are difficult to reproduce by this method and future iterations of the model need to address this.Publisher PDFPeer reviewe

Quiescent prominences in the era of ALMA. II. Kinetic temperature diagnostics

Funding: UK STFC, the Leverhulme Trust, and NASA (D.H.M.)We provide the theoretical background for diagnostics of the thermal properties of solar prominences observed by the Atacama Large Millimeter/submillimeter Array (ALMA). To do this, we employ the 3D Whole-Prominence Fine Structure (WPFS) model that produces synthetic ALMA-like observations of a complex simulated prominence. We use synthetic observations derived at two different submillimeter/millimeter (SMM) wavelengths—one at a wavelength at which the simulated prominence is completely optically thin and another at a wavelength at which a significant portion of the simulated prominence is optically thick—as if these were the actual ALMA observations. This allows us to develop a technique for an analysis of the prominence plasma thermal properties from such a pair of simultaneous high-resolution ALMA observations. The 3D WPFS model also provides detailed information about the distribution of the kinetic temperature and the optical thickness along any line of sight. We can thus assess whether the measure of the kinetic temperature derived from observations accurately represents the actual kinetic temperature properties of the observed plasma. We demonstrate here that in a given pixel the optical thickness at the wavelength at which the prominence plasma is optically thick needs to be above unity or even larger to achieve a sufficient accuracy of the derived information about the kinetic temperature of the analyzed plasma. Information about the optical thickness cannot be directly discerned from observations at the SMM wavelengths alone. However, we show that a criterion that can identify those pixels in which the derived kinetic temperature values correspond well to the actual thermal properties in which the observed prominence can be established.Publisher PDFPeer reviewe

Quiescent prominences in the era of ALMA : simulated observations using 3D whole-prominence fine structure model

We use the detailed 3D whole-prominence fine structure model to produce the first simulated high-resolution ALMA observations of a modeled quiescent solar prominence. The synthetic brightness temperature and optical thickness maps shown in the present paper are produced using a visualization method for the sub-millimeter/millimeter radio continua synthesis. We have obtained the simulated observations of both the prominence at the limb and the filament on the disk at wavelengths covering a broad range which encompasses the full potential of ALMA.We demonstrate here to what extent the small-scale and large-scale prominence and filament structures will be visible in the ALMA observations spanning both the optically thin and thick regimes. We analyze the relationship between the brightness and kinetic temperature of the prominence plasma. We also illustrate the opportunities ALMA will provide for studying the thermal structure of the prominence plasma from the cool prominence fine structure cores to the prominence-corona transition region. In addition, we show that the detailed 3D modeling of entire prominences with their numerous fine structures will be important for the correct interpretation of future ALMA prominence observations.PostprintPeer reviewe

Impact of an L5 magnetograph on nonpotential solar global magnetic field modeling

We present the first theoretical study to consider what improvement could be obtained in global nonpotential modeling of the solar corona if magnetograph data were available from the L5 Lagrange point, in addition to from the direction of Earth. To consider this, we first carry out a "reference Sun" simulation over two solar cycles. An important property of this simulation is that random bipole emergences are allowed across the entire solar surface at any given time (such as can occur on the Sun). Next we construct two "limited data" simulations, where bipoles are only included when they could be seen from (i) an Earth-based magnetograph and (ii) either Earth- or L5 based magnetographs. The improvement in reproducing the reference Sun simulation when an L5 view is available is quantified through considering global quantities in the limited data simulations. These include surface and polar flux, total magnetic energy, volume electric current, open flux and the number of flux ropes. Results show that when an L5 observational viewpoint is included, the accuracy of the global quantities in the limited data simulations can increase by 26-40%. This clearly shows that a magnetograph at the L5 point could significantly increase the accuracy of global nonpotential modeling and with this the accuracy of future space weather forecasts.Publisher PDFPeer reviewe

Modelling and observations : a comparison of the magnetic field properties in a prominence

Funding: UK STFC, ERC, and Leverhulme Trust (DHM).Context. Direct magnetic field measurements in solar prominences occur infrequently and are difficult to make and interpret. As a consequence, alternative methods are needed to derive the main properties of the magnetic field that supports the prominence mass. This is important for our understanding of solar prominences, but also for understanding how eruptive prominences may affect space weather. Aims. We present the first direct comparison of the magnetic field strength derived from spectro-polarimetric observations of a solar prominence, with corresponding results from a theoretical flux rope model constructed from on-disc normal component magnetograms. Methods. We first used spectro-polarimetric observations of a prominence obtained with the magnetograph THEMIS operating in the Canary Islands to derive the magnetic field of the observed prominence by inverting the Stokes parameters measured in the He D3 line. Next, we constructed two data-constrained non-linear force-free field (NLFFF) models of the same prominence. In one model we assumed a strongly twisted flux rope solution, and in the other a weakly twisted flux rope solution. Results. The physical extent of the prominence at the limb (height and length) is best reproduced with the strongly twisted flux rope solution. The line-of-sight average of the magnetic field for the strongly twisted solution results in a magnetic field that has a magnitude of within a factor of 1−2 of the observed magnetic field strength. For the peak field strength along the line of sight, an agreement to within 20% of the observations is obtained for the strongly twisted solution. The weakly twisted solution produces significantly lower magnetic field strengths and gives a poor agreement with the observations. Conclusions. The results of this first comparison are promising. We found that the flux rope insertion method of producing a NLFFF is able to deduce the overall properties of the magnetic field in an observed prominence.Publisher PDFPeer reviewe

Determining the source and eruption dynamics of a stealth CME using NLFFF modelling and MHD simulations

Funding: S.L.Y. would like to acknowledge STFC for support via the consolidated grant SMC1/YST037 and alsoNERC for funding via the SWIMMR Aviation Risk Modelling (SWARM) Project, grant number NE/V002899/1. P.P.would like to thank the ERC for support via grant No. 647214. D.H.M. would like to thank the STFC for support via consolidated grant ST/N000609/1 and, the Leverhulme trust, and the ERC under the Synergy Grant: The Whole Sun (grant agreement no. 810218) for financial support. P.P. and D.H.M. would like to thank STFC for IAA funding under grant number SMC1-XAS012. L.A.U. was supported by the NSF Atmospheric and Geospace Sciences Postdoctoral Research Fellowship Program (Award AGS-1624438).Context. Coronal mass ejections (CMEs) that exhibit weak or no eruption signatures in the low corona, known as stealth CMEs, are problematic as upon arrival at Earth they can lead to geomagnetic disturbances that were not predicted by space weather forecasters. Aims. We investigate the origin and eruption of a stealth event that occurred on 2015 January 3 that was responsible for a strong geomagnetic storm upon its arrival at Earth. Methods. To simulate the coronal magnetic field and plasma parameters of the eruption we use a coupled approach. This approach combines an evolutionary nonlinear force-free field model of the global corona with a MHD simulation. Results. The combined simulation approach accurately reproduces the stealth event and suggests that sympathetic eruptions occur. In the combined simulation we found that three flux ropes form and then erupt. The first two flux ropes, which are connected to a large AR complex behind the east limb, erupt first producing two near-simultaneous CMEs. These CMEs are closely followed by a third, weaker flux rope eruption in the simulation that originated between the periphery of AR 12252 and the southern polar coronal hole. The third eruption coincides with a faint coronal dimming, which appears in the SDO/AIA 211 Å observations, that is attributed as the source responsible for the stealth event and later the geomagnetic disturbance at 1 AU. The incorrect interpretation of the stealth event being linked to the occurrence of a single partial halo CME observed by LASCO/C2 is mainly due to the lack of STEREO observations being available at the time of the CMEs. The simulation also shows that the LASCO CME is not a single event but rather two near-simultaneous CMEs. Conclusions. These results show the significance of the coupled data-driven simulation approach in interpreting the eruption and that an operational L5 mission is crucial for space weather forecastingPostprintPeer reviewe

Eruptions from coronal hole bright points : observations and non-potential modeling

Funding: DHM would like to thank the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement No. 647214).Context. We report on the third part of a series of studies on eruptions associated with small-scale loop complexes named coronal bright points (CBPs). Aims. A single case study of a CBP in an equatorial coronal hole with an exceptionally large size is investigated to extend our understanding of the formation of mini-filaments (MFs), their destabilisation and the origin of the eruption triggering the formation of jet-like features recorded in the extreme-ultraviolet (EUV) and X-ray emission. We aim to explore the nature of the so called micro-flares in CBPs associated with jets in coronal holes and mini coronal mass ejections in the quiet Sun. Methods. Co-observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO), and GONG Halpha images are used together with a Non-Linear Force Free Field (NLFFF) relaxation approach, where the latter is based on a time series of HMI line-of-sight magnetograms. Results. A mini-filament (MF) that formed beneath the CBP arcade around 3–4 h before the eruption is seen in the Halpha and EUV AIA images to lift up and erupt triggering the formation of an X-ray jet. No significant photospheric magnetic flux concentration displacement (convergence) is observed and neither is magnetic flux cancellation between the two main magnetic polarities forming the CBP in the time period leading to the MF liftoff. The CBP micro-flare is associated with three flare kernels that formed shortly after the MF liftoff. No observational signature is found for reconnection beneath the erupting MF. The applied NLFFF modeling successfully reproduces both the CBP loop complex as well as the magnetic flux rope that hosts the MF. Conclusions. The applied NLFFF modellng is able to clearly show that an initial potential field can be evolved into a non-potential magnetic field configuration that contains free magnetic energy in the region that observationally hosts the eruption. The comparison of the magnetic field structure shows that the magnetic NLFFF model contains many of the features that can explain the dfferent observational signatures found in the evolution and eruption of the CBP. In future it may eventually indicate the location of destabilisation that results in the eruptions of flux ropes.Publisher PDFPeer reviewe

Eruptions from coronal bright points : a spectroscopic view by IRIS of a mini-filament eruption, QSL reconnection, and reconnection-driven outflows

Funding: The authors thank very much the referee for the very important comments and suggestions. MM and TW acknowledge DFG-grant WI3211/8-1. D.H.M. would like to acknowledge STFC for support via the Consolidated Grant SMC1/YST037. Open Access funding provided by the Max Planck Society.Context. Our study investigates a mini-filament eruption associated with cancelling magnetic fluxes. The eruption originates from a small-scale loop complex commonly known as a coronal bright point (CBP). The event is uniquely recorded in both the imaging and spectroscopic data taken with the Interface Region Imaging Spectrograph (IRIS). Aims. The investigation aims to gain a better understanding of the physical processes driving these ubiquitous small-scale eruptions. Methods. We analysed IRIS spectroscopic and slit-jaw imaging observations as well as images taken in the extreme-ultraviolet channels of the Atmospheric Imaging Assembly (AIA) and line-of-sight magnetic-field data from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory. As the observations can only indicate the possible physical processes at play, we also employed a non-linear force-free field (NLFFF) relaxation approach based on the HMI magnetogram time series. This allowed us to further investigate the evolution of the magnetic-field structures involved in the eruption process. Results. We identified a strong small-scale brightening as a micro-flare in a CBP, recorded in emission from chromospheric to flaring plasmas. The mini-eruption is manifested via the ejection of hot (CBP loops) and cool (mini-filament) plasma recorded in both the imaging and spectroscopic data. The micro-flare is preceded by the appearance of an elongated bright feature in the IRIS slit-jaw 1400 Å images, located above the polarity inversion line. The micro-flare starts with an IRIS pixel size brightening and propagates bi-directionally along the elongated feature. We detected, in both the spectral and imaging IRIS data and AIA data, strong flows along and at the edges of the elongated feature; we believe that these represent reconnection outflows. Both edges of the elongated feature that wrap around the edges of the erupting MF evolve into a J-type shape, creating a sigmoid appearance. A quasi-separatrix layer (QSL) is identified in the vicinity of the polarity inversion line by computing the squashing factor, Q, in different horizontal planes of the NLFFF model.  Conclusions. This CBP spectro-imaging study provides further evidence that CBPs represent downscaled active regions and, as such, they may make a significant contribution to the mass and energy balance of the solar atmosphere. They are the sources of all range of typical active-region features, including magnetic reconnection along QSLs, (mini-)filament eruptions, (micro-)flaring, reconnection outflows, etc. The QSL reconnection site has the same spectral appearance as the so-called explosive events identified by strong blue- and red-shifted emission, thus providing an answer to an outstanding question regarding the true nature of this spectral phenomenon.Publisher PDFPeer reviewe