Speakers are asked to refer to the Notes for Speakers.
Prominences: Formation and plasma properties
Brigitte Schmieder (LESIA, Observatoire de Paris)
Solar prominences are cool structures (7000K) embedded in the hot corona (1 MK). They have been observed during eclipses and/or with ground based telescopes for several decades. With the launch of high temporal and spatial resolution instruments on board various spacecraft (SOHO, Hinode, SDO, STEREO, IRIS), a new era of research is now open and many new questions arise. It is a challenge to understand the formation of prominences, their stability as well as their dynamics. Movies obtained with space imagers are very impressive. Nevertheless we need to look at spectroscopic and polarimetric data to derive the real plasma flows.
Understanding the emission by the cool prominence plasma needs a treatment of radiative transfer. Models in two dimensions are required to understand the fine structures of prominences and compute the plasma properties in the prominence and in the prominence-corona transition region. I will review the state-of-the-art in these areas and suggest where progress needs to be made.
Prominences and the Chromosphere-Corona Mass Cycle
Thomas Berger (National Solar Observatory)
We review recent observational, theoretical, and modeling work implying that prominences are the return flow of a pervasive mass cycle between the chromosphere and corona. Quiescent prominences in particular show evidence of condensation from coronal plasma in "coronal cavities", large-scale non-potential magnetic formations ("flux ropes") in the corona. Similarly, recently observed "funnel prominences" appear to condense from localized dips in long coronal magnetic field lines. Theoretical work shows that the hydromagnetic interior of quiescent prominences is structured on local scales by the interplay between Lorentz forces, gravitational forces, and field-aligned thermal conduction on plasma condensing via a radiative cooling instability. Observations of ubiquitous down-flow threads that cross the apparently horizontal field lines in quiescent prominences support the theoretical model. Mass, magnetic flux, and helicity transport into coronal flux ropes can be attributed at least in part to emerging flux below the structures with rapidly heated, low-density, emerging plasma contacting higher-density overlying prominence plasma to trigger a Rayleigh-Taylor buoyancy instability that structures both the down-flows and the turbulent upflows into the corona. Prominences can thus be understood as the visible indicators of non-potential magnetic field configurations in the corona in which plasma trapping followed by condensation via radiative instability and subsequent gravitational down-flow is favored. They also present interface regions where the down-flowing "chromospheric" plasma contacts rapidly heated emerging flux that gradually and continuously transports "coronal" plasma and magnetic flux and helicity upward into the system until it destabilizes and erupts as a CME.
Other contributing authors: B.C. Low,
Wei Liu, Roberto Cassini, and Andrew Hillier
Synthetic high-resolution prominence observations
Stanislav Gunár (School of Mathematics and Statistics, University of St Andrews)
High-resolution observations of quiescent prominences reveal a large amount of details of the prominence fine structures. Their analysis helps us to understand the structure of the magnetic field supporting the cool prominence material against the gravity and insulating it from the surrounding hot corona.
The whole-prominence magnetic fields are currently studied by several authors employing the linear or non-linear force-free MHD simulations. These are able to produce realistic 3D configurations of the magnetic field which well corresponds to the general structure of the observed prominences/filaments. However, the commonly used technique of visualization of the fine structures located in the magnetic dips present in these magnetic field configurations does not allow for a direct comparison with the filtergram observations such as those by Hinode/SOT.
We use a 3D non-linear force-free field simulation to produce the whole-prominence magnetic field configuration and we fill selected magnetic dips by a realistic prominence plasma. This allows us to populate the simulated prominence with fine structures in accordance with the observed prominences. We than compute the synthetic hydrogen H-alpha radiation emerging from this 3D structure with any given line of sight.
A statistical study of Alfvenic waves in a quiescent prominence
Richard Morton (Northumbria University)
Also contributed: Andrew Hillier (Kyoto University, Japan), Robertus Erdélyi (Sheffield University, UK)
The launch of the Hinode satellite has allowed for seeing-free observations at high-resolution and high-cadence making it well suited to study the dynamics of quiescent prominences. In recent years it has become clear that quiescent prominences support small-amplitude transverse oscillations, however, sample sizes are usually too small for general conclusions to be drawn. We present the results from an initial statistical study of transverse oscillations in vertical prominence threads.
Over a 4 hr period of observations it was possible to measure the properties of 3436 waves, ﬁnding periods from 50 to 6000 s with typical velocity amplitudes ranging between 0.2 and 23 km/s. The large number of observed waves suggests Alfvénic motions, with a large frequency range, are ubiquitous in prominences.
In addition, we present the velocity power spectrum for the Alfvénic waves. For frequencies less than 7 mHz, the frequency dependence of the velocity power is consistent with the velocity power spectra generated from observations of the horizontal motions of magnetic elements in the photosphere, suggesting that the prominence transverse waves are driven by photospheric motions. However, at higher frequencies the two distributions signiﬁcantly diverge, with relatively more power found at higher frequencies in the prominence oscillations. Similar features have also been found in power spectra derived from observations of the transverse motions of chromospheric fibrils (Morton et al., 2014).
We will discuss potential implications of these results with regards to prominence energetics and energy transfer from the photosphere to the prominence.
The role of partial ionisation in the stability of prominence structures
Istvan Ballai (University of Sheffield)
Due to the low temperature of prominences, the plasma is not completely ionised and is made up of protons, electrons and neutrals. In the present talk I will concentrate on the effect of partial ionisation on the stability of the prominences and investigate how the combined effects of plasma flows and the presence of neutrals will determine the dynamical behaviour of such plasmas.
Prominences: Magnetic Field and Heliospheric Connection
Spiro Antiochos (NASA/GSFC)
The magnetic field of a prominence/filament, generally referred to as a “filament channel”, is undoubtedly the most important and most puzzling structure in the Sun’s corona. Filament channels are the only locations in the corona where the field is strongly non-potential. The highly stressed fields in filament channels are the energy sources of the giant magnetic explosions observed as coronal mass ejections (CME)/eruptive flares. Understanding how filament channels form and why they erupt have long been two of the outstanding problems in space physics and astrophysics. The eruption of filament channels clearly has major implications for heliospheric structure and dynamics, but in this talk I will argue that there is an even deeper connection between the heliosphere and the formation of prominence magnetic fields. Results from analytic calculations and numerical simulations will be presented demonstrating that a single process, “helicity condensation”, is responsible for the origin of both prominence magnetic fields and the slow solar wind. The implications of these results for interpreting observations will be discussed.
This work was supported by the LWS and SR&T Programs.
AIA Observations of Fast Counter-streaming Flows along a Solar Filament channel
Jeff Smith (Aberystwyth University)
Counter-steaming flows in the order of 10 kilometers per second are often observed by H-alpha in solar filaments. Such observations are important since they provide an indication of the orientation of the magnetic field in filaments. In this work, we report the observation of fast counter-streaming flows along a U shaped solar filament channel using the AIA/SDO passbands 171, 304 and 193 during a three day period in August 2012. The filament channel experienced a partial eruption during the three days and the morphology of the channel remained unchanged after the partial eruption. The speed of the flows was in the range of 100 ~ 400 km/s. The flows were persistent and ubiquitous during the three day period. The ends of the filament channel were rooted in two active regions. Magnetic flux emergence can be identified in one active region from HMI data. Since emerging magnetic flux often produces magnetic reconnection with the existing magnetic flux, the observed emerging flux is a plausible cause of the observed restless flows along the filament channel shown in the AIA passbands. We will discuss the implications of the study to our understanding of the magnetic structure of filament channels.
Dynamics of descending plasma knots in solar prominences
Ramon Oliver (University of the Balearic Islands, Spain)
Dense plasma knots are often observed falling vertically in quiescent prominences. Their measured speeds lie between 10 and 60 km/s and their acceleration is much smaller than that of gravity (and even zero). The dynamics of these condensations is investigated using a simple model. After the dense knot is formed, the pressure below it undergoes a quick rearrangement that results in the generation of a large pressure gradient that provides an upward force opposing gravity. After several hundreds of seconds, the knot achieves a roughly constant descending velocity. The observed falling speeds require knot to environment density ratios between 10 and 150.
Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament
Manuel Luna (Instituto de Astrofísica de Canarias)
On 20 August 2010 an energetic disturbance triggered large-amplitude longitudinal oscillations in a nearby filament. The triggering mechanism appears to be episodic jets connecting the energetic event with the filament. In the present work we analyze this periodic motion in a large fraction of the filament to characterize the underlying physics of the oscillation as well as the filament properties. The results support our previous theoretical conclusions that the restoring force of large-amplitude longitudinal oscillations is solar gravity, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Based on our previous work, we used the fitted parameters to determine the magnitude and radius of curvature of the dipped magnetic field along the filament, as well as the mass accretion rate onto the filament threads. These derived properties are nearly uniform along the filament, indicating a remarkable degree of cohesiveness throughout the filament channel. Moreover, the estimated mass accretion rate implies that the footpoint heating responsible for the thread formation, according to the thermal nonequilibrium model, agrees with previous coronal heating estimates. We estimate the magnitude of the energy released in the nearby event by studying the dynamic response of the filament threads, and discuss the implications of our study for filament structure and heating.
Magnetic Topology of Bubbles in Quiescent Prominences
Jaroslav Dudík (DAMTP, University of Cambridge)
We present a model of the polar-crown prominence magnetic field in the linear force-free approximation. Weakly twisted flux-tube is perturbed by presence of parasitic polarities within the filament channel. The parasitic polarities create openings in the prominence, which we identify with the observed "bubbles". Cusp-shaped prominences with bubbles similar to observed ones are created by sheared parasitic bipoles. The bubbles are constituted by the arcade-like field lines, as opposed to that of the prominence, which is created by magnetic dips. Interface between these two flux-systems is a separator lying around the intersection of fan planes of two magnetic null-points. Analysis of the observations show that there is virtually no additional coronal emission produced within the bubble.
The Solar Prominence Magnetic Field Measurements
Véronique Bommier ( LESIA, Observatoire de Paris)
The presentation will be a brief review of the prominence magnetic field measurement main results (essentially from the interpretation of the Hanle effect). Attention will be drawn to the ambiguity resolution, the last but not least step of magnetic field measurements.