P. F. Chen

First Evidence of Coexisting EIT Wave and Coronal Moreton Wave from SDO/AIA Observations

“EIT waves” are a globally propagating wavelike phenomenon. They were often interpreted as fast-mode magnetoacoustic waves in the corona, despite various discrepancies between the fast-mode wave model and observations. To reconcile these discrepancies, we suggested that “EIT waves” are the apparent propagation of the plasma compression due to successive stretching of the magnetic field lines pushed by the erupting flux rope. According to this model, an EIT wave should be preceded by a fast-mode wave, which, however, had rarely been observed. With the unprecedented high cadence and sensitivity of the Solar Dynamics Observatory observations, we discern a fast-moving wave front with a speed of 560 km s −1 ahead of an EIT wave, which had a velocity of ∼190 km s −1 , in the “EIT wave” event on 2010 July 27. The results, suggesting that “EIT waves” are not fast-mode waves, confirm the prediction of our field-line stretching model for an EIT wave. In particular, it is found that the coronal Moreton wave was ∼3 times faster than the EIT wave, as predicted.

A New View of Coronal Waves from STEREO

On 2007 December 7, there was an eruption from AR 10977, which also hosted a sigmoid. An EUV Imaging Telescope (EIT) wave associated with this eruption was observed by EUVI on board the Solar Terrestrial Relations Observatory (STEREO). Using EUVI images in the 171 A and the 195 A passbands from both STEREO A and B, we study the morphology and kinematics of this EIT wave. In the early stages, images of the EIT wave from the two STEREO spacecrafts differ markedly. We determine that the EUV fronts observed at the very beginning of the eruption likely include some intensity contribution from the associated coronal mass ejection (CME). Additionally, our velocity measurements suggest that the EIT wave front may propagate at nearly constant velocity. Both results offer constraints on current models and understanding of EIT waves.

Simulations of Prominence Formation in the Magnetized Solar Corona by Chromospheric Heating

Starting from a realistically sheared magnetic arcade connecting the chromospheric, transition region to coronal plasma, we simulate the in situ formation and sustained growth of a quiescent prominence in the solar corona. Contrary to previous works, our model captures all phases of the prominence formation, including the loss of thermal equilibrium, its successive growth in height and width to macroscopic dimensions, and the gradual bending of the arched loops into dipped loops, as a result of the mass accumulation. Our 2.5 dimensional, fully thermodynamically and magnetohydrodynamically consistent model mimics the magnetic topology of normal-polarity prominences above a photospheric neutral line, and results in a curtain-like prominence above the neutral line through which the ultimately dipped magnetic field lines protrude at a finite angle. The formation results from concentrated heating in the chromosphere, followed by plasma evaporation and later rapid condensation in the corona due to thermal instability, as verified by linear instability criteria. Concentrated heating in the lower atmosphere evaporates plasma from below to accumulate at the top of coronal loops and supply mass to the later prominence constantly. This is the first evaporation-condensation model study where we can demonstrate how the formed prominence stays in a force balanced state, which can be compared to the Kippenhahn-Schluter type magnetohydrostatic model, all in a finite low-beta corona.

FORMATION OF SOLAR FILAMENTS BY STEADY AND NONSTEADY CHROMOSPHERIC HEATING

It has been established that cold plasma condensations can form in a magnetic loop subject to localized heating of its footpoints. In this paper, we use grid-adaptive numerical simulations of the radiative hydrodynamic equations to investigate the filament formation process in a pre-shaped loop with both steady and finite-time chromospheric heating. Compared to previous works, we consider low-lying loops with shallow dips and use a more realistic description for radiative losses. We demonstrate for the first time that the onset of thermal instability satisfies the linear instability criterion. The onset time of the condensation is roughly ~2 hr or more after the localized heating at the footpoint is effective, and the growth rate of the thread length varies from 800 km hr?1 to 4000 km hr?1, depending on the amplitude and the decay length scale characterizing this localized chromospheric heating. We show how single or multiple condensation segments may form in the coronal portion. In the asymmetric heating case, when two segments form, they approach and coalesce, and the coalesced condensation later drains down into the chromosphere. With steady heating, this process repeats with a periodicity of several hours. While our parametric survey confirms and augments earlier findings, we also point out that steady heating is not necessary to sustain the condensation. Once the condensation is formed, it keeps growing even after the localized heating ceases. In such a finite-heating case, the condensation instability is maintained by chromospheric plasma that gets continuously siphoned into the filament thread due to the reduced gas pressure in the corona. Finally, we show that the condensation can survive the continuous buffeting of perturbations from photospheric p-mode waves.

Imaging and Spectroscopic Observations of a Filament Channel and the Implications for the Nature of Counter-streamings

The dynamics of a filament channel are observed with imaging and spectroscopic telescopes before and during the filament eruption on 2011 January 29. The extreme ultraviolet (EUV) spectral observations reveal that there are no EUV counterparts of the Hα counter-streamings in the filament channel, implying that the ubiquitous Hα counter-streamings found by previous research are mainly due to longitudinal oscillations of filament threads, which are not in phase between each other. However, there exist larger-scale patchy counter-streamings in EUV along the filament channel from one polarity to the other, implying that there is another component of unidirectional flow (in the range of ±10 km s–1) inside each filament thread in addition to the implied longitudinal oscillation. Our results suggest that the flow direction of the larger-scale patchy counter-streaming plasma in the EUV is related to the intensity of the plage or active network, with the upflows being located at brighter areas of the plage and downflows at the weaker areas. We propose a new method to determine the chirality of an erupting filament on the basis of the skewness of the conjugate filament drainage sites. This method suggests that the right-skewed drainage corresponds to sinistral chirality, whereas the left-skewed drainage corresponds to dextral chirality.

EVOLUTION OF OPTICAL PENUMBRAL AND SHEAR FLOWS ASSOCIATED WITH THE X3.4 FLARE OF 2006 DECEMBER 13

The rapid and irreversible decay of penumbrae related to X-class flares has been found in a number of studies. Since the optical penumbral flows are closely associated with the morphology of sunspot penumbra, we use state-of-the-art Hinode data to track penumbral flows in flaring active regions as well as shear flows close to the flaring neutral line. This paper concentrates on AR 10930 around the time of an X3.4 flare on 2006 December 13. We utilize the seeing-free solar optical telescope G-band data as a tracer to obtain the horizontal component of the penumbral and shear flows by local correlation tracking, and Stokes-V data to register positive and negative magnetic elements along the magnetic neutral line. We find that: (1) an obvious penumbral decay appears in this active region intimately associated with the X3.4 flare; (2) the mean magnitude of the horizontal speeds of the penumbral flows within the penumbral decay areas temporally and spatially varies from 0.6 to 1.1 km s?1; (3) the penumbral flow decreases before the flare eruption in two of the four penumbral decay areas; (4) the mean shear flows along the magnetic neutral line of this ?-sunspot started to decrease before the flare and continue to decrease for another hour after the flare. The magnitude of this flow apparently dropped from 0.6 to 0.3 km s?1. We propose that the decays of the penumbra and the penumbral flow are related to the magnetic rearrangement involved in the coronal mass ejection/flare events.

Parametric survey of longitudinal prominence oscillation simulations

Context. Longitudinal filament oscillations recently attracted increasing attention, while the restoring force and the damping mechanisms are still elusive.

SPECTROSCOPIC ANALYSIS OF AN EIT WAVE/DIMMING OBSERVED BY HINODE/EIS

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.

Extreme ultraviolet imaging of three-dimensional magnetic reconnection in a solar eruption

Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively, and it is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is studied in three dimensions using the combined perspectives of two spacecraft. The sequence of extreme ultraviolet images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other, forming a separator or quasi-separator and then reconnecting. The plasma near the reconnection site is subsequently heated from ∼1 to ≥5 MK. Shortly afterwards, warm flare loops (∼3 MK) appear underneath the hot plasma. Other observational signatures of reconnection, including plasma inflows and downflows, are unambiguously revealed and quantitatively measured. These observations provide direct evidence of magnetic reconnection in a three-dimensional configuration and reveal its origin.

QUADRATURE OBSERVATIONS OF WAVE AND NON-WAVE COMPONENTS AND THEIR DECOUPLING IN AN EXTREME-ULTRAVIOLET WAVE EVENT

We report quadrature observations of an extreme-ultraviolet (EUV) wave event on 2011 January 27 obtained by the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory, and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Two components are revealed in the EUV wave event. A primary front is launched with an initial speed of ~440 km s–1. It appears that significant emission enhancement occurs in the hotter channel while deep emission reduction occurs in the cooler channel. When the primary front encounters a large coronal loop system and slows down, a secondary, much fainter, front emanates from the primary front with a relatively higher starting speed of ~550 km s–1. Afterward, the two fronts propagate independently with increasing separation. The primary front finally stops at a magnetic separatrix, while the secondary front travels farther until it fades out. In addition, upon the arrival of the secondary front, transverse oscillations of a prominence are triggered. We suggest that the two components are of different natures. The primary front belongs to a non-wave coronal mass ejection (CME) component, which can be reasonably explained with the field-line stretching model. The multi-temperature behavior may be caused by considerable heating due to nonlinear adiabatic compression on the CME frontal loop. As for the secondary front, it is most likely a linear fast-mode magnetohydrodynamic wave that propagates through a medium of the typical coronal temperature. X-ray and radio data provide us with complementary evidence in support of the above scenario.