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Coronal rain in magnetic bipolar weak fields  

Chun Xia   Submitted: 2017-06-08 11:57

We intend to investigate the underlying physics for the coronal rain phenomenon in a representative bipolar magnetic field, including the formation and the dynamics of coronal rain blobs. With the MPI-AMRVAC code, we performed three dimensional radiative magnetohydrodynamic (MHD) simulation with strong heating localized on footpoints of magnetic loops after a relaxation to quiet solar atmosphere. Progressive cooling and in-situ condensation starts at the loop top due to radiative thermal instability. The first large-scale condensation on the loop top suffers Rayleigh-Taylor instability and becomes fragmented into smaller blobs. The blobs fall vertically dragging magnetic loops until they reach low beta regions and start to fall along the loops from loop top to loop footpoints. A statistic study of the coronal rain blobs finds that small blobs with masses of less than 1010 g dominate the population. When blobs fall to lower regions along the magnetic loops, they are stretched and develop a non-uniform velocity pattern with an anti-parallel shearing pattern seen to develop along the central axis of the blobs. Synthetic images of simulated coronal rain with Solar Dynamics Observatory Atmospheric Imaging Assembly well resemble real observations presenting dark falling clumps in hot channels and bright rain blobs in a cool channel. We also find density inhomogeneities during a coronal rain "shower", which reflects the observed multi-stranded nature of coronal rain.

Authors: Chun Xia, Rony Keppens, and Xia Fang
Projects: None

Publication Status: accepted for publication
Last Modified: 2017-06-10 19:39
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Formation and plasma circulation of solar prominences  

Chun Xia   Submitted: 2016-03-18 03:15

Solar prominences are long-lived cool and dense plasma curtains in the hot and rarefied outer solar atmosphere or corona. The physical mechanism responsible for their formation and especially for their internal plasma circulation has been uncertain for decades. The observed ubiquitous down flows in quiescent prominences are difficult to interpret as plasma with high conductivity seems to move across horizontal magnetic field lines. Here we present three-dimensional numerical simulations of prominence formation and evolution in an elongated magnetic flux rope as a result of in-situ plasma condensations fueled by continuous plasma evaporation from the solar chromosphere. The prominence is born and maintained in a fragmented, highly dynamic state with continuous reappearance of multiple blobs and thread structures that move mainly downward dragging along mass-loaded field lines. The prominence plasma circulation is characterized by the dynamic balance between the drainage of prominence plasma back to the chromosphere and the formation of prominence plasma via continuous condensation. Plasma evaporates from the chromosphere, condenses into the prominence in the corona, and drains back to the chromosphere, establishing a stable chromosphere-corona plasma cycle. Synthetic images of the modeled prominence with the Solar Dynamics Observatory Atmospheric Imaging Assembly closely resemble actual observations, with many dynamical threads underlying an elliptical coronal cavity.

Authors: Chun Xia and Rony Keppens
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2016-03-21 15:33
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Solar prominences: 'double, double . . . boil and bubble'  

Chun Xia   Submitted: 2015-05-21 01:49

Observations revealed rich dynamics within prominences, the cool (10,000 K), macroscopic (sizes of order 100 Mm) 'clouds' in the million degree solar corona. Even quiescent prominences are continuously perturbed by hot, rising bubbles. Since prominence matter is hundredfold denser than coronal plasma, this bubbling is related to Rayleigh-Taylor instabilities. Here we report on true macroscopic simulations well into this bubbling phase, adopting a magnetohydrodynamic description from chromospheric layers up to 30 Mm height. Our virtual prominences rapidly establish fully non-linear (magneto)convective motions where hot bubbles interplay with falling pillars, with dynamical details including upwelling pillars forming within bubbles. Our simulations show impacting Rayleigh-Taylor fingers reflecting on transition region plasma, ensuring that cool, dense chromospheric material gets mixed with prominence matter up to very large heights. This offers an explanation for the return mass cycle mystery for prominence material. Synthetic views at extreme ultraviolet wavelengths show remarkable agreement with observations, with clear indications of shear-flow induced fragmentations.

Authors: Rony Keppens and Chun Xia
Projects: None

Publication Status: ApJ Letter accepted
Last Modified: 2015-05-25 09:30
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Simulating the in situ condensation process of solar prominences  

Chun Xia   Submitted: 2014-08-21 14:38

Prominences in the solar corona are hundredfold cooler and denser than their surroundings, with a total mass of 1.e13 up to 1.e15 g. Here we report on the first comprehensive simulations of three-dimensional, thermally and gravitationally stratified magnetic flux ropes, where in situ condensation to a prominence happens due to radiative losses. After a gradual thermodynamic adjustment, we witness a phase where runaway cooling happens while counter-streaming shearing flows drain off mass along helical field lines. After this drainage, a prominence-like condensation resides in concave upward field regions, and this prominence retains its overall characteristics for more than two hours. While condensing, the prominence establishes a prominence-corona transition region, where magnetic field-aligned thermal conduction is operative during the runaway cooling. The prominence structure represents a force-balanced state in a helical flux rope. The simulated condensation demonstrates a right-bearing barb, as a remnant of the drainage. Synthetic images at extreme ultraviolet wavelengths follow the onset of the condensation, and confirm the appearance of horns and a three-part structure for the stable prominence state, as often seen in erupting prominences. This naturally explains recent Solar Dynamics Observatory views with the Atmospheric Imaging Assembly on prominences in coronal cavities demonstrating horns.

Authors: Chun Xia, Rony Keppens, Patrick Antolin, Oliver Porth
Projects: None

Publication Status: accepted in ApJ Letter
Last Modified: 2014-08-27 12:58
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3D Prominence-hosting Magnetic Configurations: Creating a Helical Magnetic Flux Rope  

Chun Xia   Submitted: 2013-11-22 01:31

The magnetic configuration hosting prominences and their surrounding coro- nal structure is a key research topic in solar physics. Recent theoretical and observational studies strongly suggest that a helical magnetic flux rope is an es- sential ingredient to fulfill most of the theoretical and observational requirements for hosting prominences. To understand flux rope formation details and obtain magnetic configurations suitable for future prominence formation studies, we here report on three-dimensional isothermal magnetohydrodynamic simulations including finite gas pressure and gravity. Starting from a magnetohydrostatic corona with a linear force-free bipolar magnetic field, we follow its evolution when introducing vortex flows around the main polarities and converging flows towards the polarity inversion line near the bottom of the corona. The con- verging flows bring feet of different loops together at the polarity inversion line and magnetic reconnection and flux cancellation happens. Inflow and outflow signatures of the magnetic reconnection process are identified, and the thereby newly formed helical loops wind around pre-existing ones so that a complete flux rope grows and ascends. When a macroscopic flux rope is formed, we switch off the driving flows and find that the system relaxes to a stable state containing a helical magnetic flux rope embedded in an overlying arcade structure. A major part of the formed flux rope is threaded by dipped field lines which can stably support prominence matter, while the total mass of the flux rope is in the order of 4-5.e14 g.

Authors: Chun Xia, Rony Keppens, Yang Guo
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2013-11-22 09:03
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Simulations of Prominence Formation in the Magnetized Solar Corona by Chromospheric Heating  

Chun Xia   Submitted: 2012-02-28 19:13

Starting from a realistically sheared magnetic arcade connectingchromospheric, transition region to coronal plasma, we simulate thein-situformation and sustained growth of a quiescent prominence in the solarcorona.Contrary to previous works, our model captures all phases of theprominenceformation, including the loss of thermal equilibrium, its successivegrowth inheight and width to macroscopic dimensions, and the gradual bending ofthearched loops into dipped loops, as a result of the mass accumulation.Our2.5-dimensional, fully thermodynamically and magnetohydrodynamicallyconsistentmodel mimics the magnetic topology of normal-polarity prominencesabove aphotospheric neutral line, and results in a curtain-like prominenceabove theneutral line through which the ultimately dipped magnetic field linesprotrudeat a finite angle. The formation results from concentrated heating inthechromosphere, followed by plasma evaporation and later rapidcondensation inthe corona due to thermal instability, as verified by linear instabilitycriteria. Concentrated heating in the lower atmosphere evaporatesplasma frombelow to accumulate at the top of coronal loops and supply mass to thelaterprominence constantly. This is the first evaporation-condensationmodel studywhere we can demonstrate how the formed prominence stays in a forcebalancedstate, which can be compared to the Kippenhahn-Schluter typemagnetohydrostaticmodel, all in a finite low-beta corona.

Authors: Chun Xia, P. F. Chen, and Rony Keppens
Projects: None

Publication Status: ApJ Letter (accepted)
Last Modified: 2012-02-29 08:25
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Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating  

Chun Xia   Submitted: 2011-05-31 23:38

It has been established that cold plasma condensations can form in a magneticloop subject to localized heating of the footpoints. In this paper, we usegrid-adaptive numerical simulations of the radiative hydrodynamic equations toparametrically investigate the filament formation process in a pre-shaped loopwith both steady and finite-time chromospheric heating. Compared to previousworks, we consider low-lying loops with shallow dips, and use a more realisticdescription for the radiative losses. We demonstrate for the first time thatthe onset of thermal instability satisfies the linear instability criterion.The onset time of the condensation is roughly sim 2 hr or more after thelocalized heating at the footpoint is effective, and the growth rate of thethread length varies from 800 km hr-1 to 4000 km hr-1, depending on theamplitude and the decay length scale characterizing this localizedchromospheric heating. We show how single or multiple condensation segments mayform in the coronal portion. In the asymmetric heating case, when two segmentsform, they approach and coalesce, and the coalesced condensation later drainsdown into the chromosphere. With a steady heating, this process repeats with aperiodicity of several hours. While our parametric survey confirms and augmentsearlier findings, we also point out that steady heating is not necessary tosustain the condensation. Once the condensation is formed, it can keep growingalso when the localized heating ceases. Finally, we show that the condensationcan survive continuous buffeting by perturbations resulting from thephotospheric p-mode waves.

Authors: C. Xia, P. F. Chen, R. Keppens, A. J. van Marle
Projects:

Publication Status: ApJ(accepted)
Last Modified: 2011-06-01 21:19
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Abstracts by Author
Coronal rain in magnetic bipolar weak fields
Formation and plasma circulation of solar prominences
Solar prominences: 'double, double . . . boil and bubble'
Simulating the in situ condensation process of solar prominences
3D Prominence-hosting Magnetic Configurations: Creating a Helical Magnetic Flux Rope
Simulations of Prominence Formation in the Magnetized Solar Corona by Chromospheric Heating
Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating

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