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Magnetic jam in the corona of the Sun  

Feng Chen   Submitted: 2015-08-25 03:46

The outer solar atmosphere, the corona, contains plasma at temperatures of more than a million kelvin-more than 100 times hotter than the solar surface. How this gas is heated is a fundamental question tightly interwoven with the structure of the magnetic field. Together this governs the evolution of coronal loops, the basic building block prominently seen in X-rays and extreme ultraviolet (EUV) images. Here we present numerical experiments accounting for both the evolving three-dimensional structure of the magnetic field and its complex interaction with the plasma. Although the magnetic field continuously expands as new magnetic flux emerges through the solar surface, plasma on successive field lines is heated in succession, giving the illusion that an EUV loop remains roughly at the same place. For each snapshot the EUV images outline the magnetic field. However, in contrast to the traditional view, the temporal evolution of the magnetic field and the EUV loops can be quite different. This indicates that the thermal and the magnetic evolution in the outer atmosphere of a cool star should be treated together, and should not be simply separated as predominantly done so far.

Authors: Feng Chen, Hardi Peter, Sven Bingert, and Mark Cheung
Projects: None

Publication Status: Published in Nature Physics, 2015, 11, 492
Last Modified: 2015-08-25 16:46
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Using coronal seismology to estimate the magnetic field strength in a realistic coronal model  

Feng Chen   Submitted: 2015-08-05 03:03

Coronal seismology is extensively used to estimate properties of the corona, e.g. the coronal magnetic field strength are derived from oscillations observed in coronal loops. We present a three-dimensional coronal simulation including a realistic energy balance in which we observe oscillations of a loop in synthesised coronal emission. We use these results to test the inversions based on coronal seismology. From the simulation of the corona above an active region we synthesise extreme ultraviolet (EUV) emission from the model corona. From this we derive maps of line intensity and Doppler shift providing synthetic data in the same format as obtained from observations. We fit the (Doppler) oscillation of the loop in the same fashion as done for observations to derive the oscillation period and damping time. The loop oscillation seen in our model is similar to imaging and spectroscopic observations of the Sun. The velocity disturbance of the kink oscillation shows an oscillation period of 52.5s and a damping time of 125s, both being consistent with the ranges of periods and damping times found in observation. Using standard coronal seismology techniques, we find an average magnetic field strength of B\rm kink=79G for our loop in the simulation, while in the loop the field strength drops from some 300G at the coronal base to 50G at the apex. Using the data from our simulation we can infer what the average magnetic field derived from coronal seismology actually means. It is close to the magnetic field strength in a constant cross-section flux tube that would give the same wave travel time through the loop. Our model produced not only a realistic looking loop-dominated corona, but also provides realistic information on the oscillation properties that can be used to calibrate and better understand the result from coronal seismology.

Authors: Feng Chen & Hardi Peter
Projects: None

Publication Status: Accepted for publication on A&A
Last Modified: 2015-08-05 10:53
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A model for the formation of the active region corona driven by magnetic flux emergence  

Feng Chen   Submitted: 2014-02-24 10:35

We present the first model that couples the formation of the corona of a solar active region to a model of the emergence of a sunspot pair. This allows us to study when, where, and why active region loops form, and how they evolve. We use a 3D radiation MHD simulation of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a 3D MHD coronal model. The latter accounts for the braiding of the magnetic fieldlines, which induces currents in the corona heating up the plasma. We synthesize the coronal emission for a direct comparison to observations. Starting with a basically field-free atmosphere we follow the filling of the corona with magnetic field and plasma. Numerous individually identifiable hot coronal loops form, and reach temperatures well above 1 MK with densities comparable to observations. The footpoints of these loops are found where small patches of magnetic flux concentrations move into the sunspots. The loop formation is triggered by an increase of upwards-directed Poynting flux at their footpoints in the photosphere. In the synthesized EUV emission these loops develop within a few minutes. The first EUV loop appears as a thin tube, then rises and expands significantly in the horizontal direction. Later, the spatially inhomogeneous heat input leads to a fragmented system of multiple loops or strands in a growing envelope.

Authors: F. Chen, H. Peter, S. Bingert, M. C. M. Cheung
Projects: None

Publication Status: accepted for publication in A&A
Last Modified: 2014-02-24 13:04
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Spectroscopic analysis of interaction between an EIT wave and a coronal upflow region  

Feng Chen   Submitted: 2011-07-27 07:58

We report a spectroscopic analysis of an EIT wave event that occurred inactive region 11081 on 2010 June 12 and was associated with an M2.0 classflare. The wave propagated near circularly. The south-eastern part of the wavefront passed over an upflow region nearby a magnetic bipole. Using EIS rasterobservations for this region, we studied the properties of plasma dynamics inthe wave front, as well as the interaction between the wave and the upflowregion. We found a weak blueshift for the Fe XII {lambda}195.12 and Fe XIII{lambda}202.04 lines in the wave front. The local velocity along the solarsurface, which is deduced from the line of sight velocity in the wave front andthe projection effect, is much lower than the typical propagation speed of thewave. A more interesting finding is that the upflow and non-thermal velocitiesin the upflow region are suddenly diminished after the transit of the wavefront. This implies a significant change of magnetic field orientation when thewave passed. As the lines in the upflow region are redirected, the velocityalong the line of sight is diminished as a result. We suggest that thisscenario is more in accordance with what was proposed in the field-linestretching model of EIT waves.

Authors: F. Chen, M. D. Ding, P. F. Chen and L. K. Harra
Projects: Hinode/EIS

Publication Status: ApJ accepted for publication
Last Modified: 2011-07-29 00:43
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Evidence of Explosive Evaporation in a Microflare Observed by Hinode/EIS  

Feng Chen   Submitted: 2010-09-15 20:37

We present a detailed study of explosive chromospheric evaporation during a microflare which occurred on 2007 December 7 as observed with the EUV Imaging Spectrometer (EIS) onboard Hinode. We find temperature-dependent upflows for lines formed from 1.0 to 2.5 MK and downflows for lines formed from 0.05 to 0.63 MK in the impulsive phase of the flare. Both the line intensity and the nonthermal line width appear enhanced in most of the lines and are temporally correlated with the time when significant evaporation was observed. Our results are consistent with the numerical simulations of flare models, which take into account a strong nonthermal electron beam in producing the explosive chromospheric evaporation. The explosive evaporation observed in this microflare implies that the same dynamic processes may exist in events with very different magnitudes.

Authors: F. Chen and M. D. Ding
Projects: Hinode/EIS

Publication Status: accepted to publish on the Astrophysical Journal
Last Modified: 2010-09-17 08:54
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SPECTROSCOPIC ANALYSIS OF AN EIT WAVE/DIMMING OBSERVED BY HINODE/EIS  

Feng Chen   Submitted: 2010-08-19 21:26

EUV Imaging Telescope (EIT) waves are a wavelike phenomenon propagating outward from the coronal mass ejection source region, with expanding dimmings following behind. We present a spectroscopic study of an EIT wave/dimming event observed by the Hinode/Extreme-ultraviolet Imaging Spectrometer. Although the identification of the wave front is somewhat affected by the pre-existing loop structures, the expanding dimming is well defined. We investigate the line intensity, width, and Doppler velocity for four EUV lines. In addition to the significant blueshift implying plasma outflows in the dimming region as revealed in previous studies, we find that the widths of all four spectral lines increase at the outer edge of the dimmings. We illustrate that this feature can be well explained by the field line stretching model, which claims that EIT waves are apparently moving brightenings that are generated by the successive stretching of the closed field lines.

Authors: F. Chen, M. D. Ding and P. F. Chen
Projects: Hinode/EIS

Publication Status: Published in the ApJ
Last Modified: 2010-08-20 09:50
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Abstracts by Author
Magnetic jam in the corona of the Sun
Using coronal seismology to estimate the magnetic field strength in a realistic coronal model
A model for the formation of the active region corona driven by magnetic flux emergence
Spectroscopic analysis of interaction between an EIT wave and a coronal upflow region
Evidence of Explosive Evaporation in a Microflare Observed by Hinode/EIS
SPECTROSCOPIC ANALYSIS OF AN EIT WAVE/DIMMING OBSERVED BY HINODE/EIS

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