分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： A magnetic flux rope (FR), hosting hot plasma, is thought to be central to the physics of coronal mass ejection. Such FRs are widely observed with passbands of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), that are sensitive to emission from the hot plasma around 10 MK. In contrast, observations of warmer (around 1 MK) counterparts of FRs are sparse. In this study, we report the failed eruption of a multi-thermal FR, hosting both hot and warm plasma. On 2015 May 1, a hot channel appeared in the AIA high temperature passbands out of the southeastern solar limb to the south of a nearby flare, and then erupted outward. During the eruption, it rotated perpendicular to the erupting direction. The hot channel stopped erupting, and disappeared gradually, showing a failed eruption. During the hot channel eruption, a warm channel appeared sequentially in the AIA low temperature passbands. It underwent the similar evolution, including the failed eruption, rotation, and disappearance, to the hot channel. A bright compression front is formed in front of the warm channel eruption in AIA low temperature images. Under the hot and warm channel eruptions, a small flare occurred, upon which several current sheets, connecting the erupting channels and the underneath flare, formed in the AIA high temperature passbands. Investigating the spatial and temporal relation between the hot and warm channels, we suggest that both channels twist together, constituting the same multi-thermal FR that has plasma with the high and low temperatures.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： A magnetic flux rope (FR), hosting hot plasma, is thought to be central to the physics of coronal mass ejection. Such FRs are widely observed with passbands of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), that are sensitive to emission from the hot plasma around 10 MK. In contrast, observations of warmer (around 1 MK) counterparts of FRs are sparse. In this study, we report the failed eruption of a multi-thermal FR, hosting both hot and warm plasma. On 2015 May 1, a hot channel appeared in the AIA high temperature passbands out of the southeastern solar limb to the south of a nearby flare, and then erupted outward. During the eruption, it rotated perpendicular to the erupting direction. The hot channel stopped erupting, and disappeared gradually, showing a failed eruption. During the hot channel eruption, a warm channel appeared sequentially in the AIA low temperature passbands. It underwent the similar evolution, including the failed eruption, rotation, and disappearance, to the hot channel. A bright compression front is formed in front of the warm channel eruption in AIA low temperature images. Under the hot and warm channel eruptions, a small flare occurred, upon which several current sheets, connecting the erupting channels and the underneath flare, formed in the AIA high temperature passbands. Investigating the spatial and temporal relation between the hot and warm channels, we suggest that both channels twist together, constituting the same multi-thermal FR that has plasma with the high and low temperatures.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Context. Emission lines formed in the transition region and corona show dominantly redshifts and blueshifts, respectively. Aims. We investigate the Doppler shifts in a 3D radiation MHD model of the quiet Sun and compare these to observed properties. We concentrate on Si IV 1394 A originating in the transition region and examine the Doppler shifts of several other spectral lines at different formation temperatures. Methods. We construct a radiation MHD model extending from the upper convection zone to the lower corona using the MURaM code. In this quiet Sun model the magnetic field is self-consistently maintained by the action of a small-scale dynamo. We synthesize the profiles of several optically thin emission lines, formed at temperatures from the transition region into the corona. We investigate the spatial structure and coverage of red- and blueshifts and how this changes with line-formation temperature. Results. The model successfully reproduces the observed change of average net Doppler shifts from red- to blueshifted from the transition region into the corona. In particular, the model shows a clear imbalance of area coverage of red- vs. blueshifts in the transition region of ca. 80% to 20%. We determine that (at least) four processes generate the systematic Doppler shifts in our model, including pressure enhancement in the transition region, transition region brightenings unrelated to coronal emission, boundaries between cold and hot plasma, and siphon-type flows. Conclusions. We show that there is not a single process that is responsible for the observed net Doppler shifts in the transition region and corona. Because current 3D MHD models do not yet fully capture the evolution of spicules, one of the key ingredients of the chromosphere, most probably these have still to be added to the list of processes responsible for the persistent Doppler shifts.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Context. Recent observations by the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have characterized prevalent small-scale transient brightenings in the corona above the quiet Sun termed campfires. Aims. In this study we search for comparable brightenings in a numerical model and then investigate their relation to the magnetic field and the processes that drive these events. Methods. We use the MURaM code to solve the 3D radiation MHD equations in a box that stretches from the upper convection zone to the corona. The model self-consistently produces a supergranular network of the magnetic field and a hot corona above this quiet Sun. For the comparison with the model we synthesize the coronal emission as seen by EUI in its 174 {\AA} channel, isolate the seven strongest transient brightenings, and investigate (the changes of) the magnetic field in and around these in detail. Results. The transients we isolate have a lifetime of about 2 minutes and are elongated loop-like features with lengths around 1Mm to 4 Mm. They tend to occur at heights of about 2Mm to 5Mm above the photosphere a bit offset from magnetic concentrations that mark the bright chromospheric network and they reach temperatures of above 1 MK. With this they very much resemble the (larger) campfires found in observations. In our model most events are energised by component reconnection between (bundles of) field lines that interact at coronal heights. In one case we find that untwisting of a highly twisted flux rope initiates the heating. Conclusions. Based on our study we propose that the majority of campfire events found by EUI are driven by component reconnection and our model suggests that this process contributes significantly to the heating of the corona above the quiet Sun.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Solar coronal rain is classified generally into two categories: flare-driven and quiescent coronal rain. The latter is observed to form along both closed and open magnetic field structures. Recently, we proposed that some of the quiescent coronal rain events, detected in the transition region and chromospheric diagnostics, along loop-like paths could be explained by the formation mechanism for quiescent coronal rain facilitated by interchange magnetic reconnection between open and closed field lines. In this study, we revisited 38 coronal rain reports from the literature. From these earlier works, we picked 15 quiescent coronal rain events out of the solar limb, mostly suggested to occur in active region closed loops due to thermal nonequilibrium, to scrutinize their formation mechanism. Employing the extreme ultraviolet images and line-of-sight magnetograms, the evolution of the quiescent coronal rain events and their magnetic fields and context coronal structures is examined. We find that 6, comprising 40%, of the 15 quiescent coronal rain events could be totally or partially interpreted by the formation mechanism for quiescent coronal rain along open structures facilitated by interchange reconnection. The results suggest that the quiescent coronal rain facilitated by interchange reconnection between open and closed field lines deserves more attention.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In solar filament formation mechanisms, magnetic reconnection between two sets of sheared arcades forms helical structures of the filament with numerous magnetic dips, and cooling and condensation of plasma trapped inside the helical structures supply mass to the filament. Although each of these processes, namely, magnetic reconnection and coronal condensation have been separately reported, observations that show the whole process of filament formation are rare. In this Letter, we present the formation of a sigmoid via reconnection between two sets of coronal loops, and the subsequent formation of a filament through cooling and condensation of plasma inside the newly formed sigmoid. On 2014 August 27, a set of loops in the active region 12151 reconnected with another set of loops that are located to the east. A longer twisted sigmoidal structure and a set of shorter lower-lying loops then formed. The observations coincide well with the tether-cutting model. The newly formed sigmoid remains stable and does not erupt as a coronal mass ejection. From the eastern endpoint, signatures of injection of material into the sigmoid (as brightenings) are detected, which closely outline the features of increasing emission measure at these locations. This may indicate the chromospheric evaporation caused by reconnection, supplying heated plasma into the sigmoid. In the sigmoid, thermal instability occurs, and rapid cooling and condensation of plasma take place, forming a filament. The condensations then flow bi-directionally to the filament endpoints. Our results provide a clear observational evidence of the filament formation via magnetic reconnection and coronal condensation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： On the Sun, Doppler shifts of bidirectional outflows from the magnetic-reconnection site have been found only in confined regions through spectroscopic observations. Without spatially resolved spectroscopic observations across an extended region, the distribution of reconnection and its outflows in the solar atmosphere cannot be made clear. Magnetic reconnection is thought to cause the splitting of filament structures, but unambiguous evidence has been elusive. Here we report spectroscopic and imaging analysis of a magnetic-reconnection event on the Sun, using high-resolution data from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. Our findings reveal that the reconnection region extends to an unprecedented length of no less than 14,000 km. The reconnection splits a filament structure into two branches, and the upper branch erupts eventually. Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which decelerate beyond the reconnection site. Differential-emission-measure analysis reveals that in the reconnection region the temperature reaches over 10 MK and the thermal energy is much larger than the kinetic energy. This Letter provides definite spectroscopic evidence for the splitting of a solar filament by magnetic reconnection in an extended region.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： On the Sun, Doppler shifts of bidirectional outflows from the magnetic-reconnection site have been found only in confined regions through spectroscopic observations. Without spatially resolved spectroscopic observations across an extended region, the distribution of reconnection and its outflows in the solar atmosphere cannot be made clear. Magnetic reconnection is thought to cause the splitting of filament structures, but unambiguous evidence has been elusive. Here we report spectroscopic and imaging analysis of a magnetic-reconnection event on the Sun, using high-resolution data from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. Our findings reveal that the reconnection region extends to an unprecedented length of no less than 14,000 km. The reconnection splits a filament structure into two branches, and the upper branch erupts eventually. Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which decelerate beyond the reconnection site. Differential-emission-measure analysis reveals that in the reconnection region the temperature reaches over 10 MK and the thermal energy is much larger than the kinetic energy. This Letter provides definite spectroscopic evidence for the splitting of a solar filament by magnetic reconnection in an extended region.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Sudip Mandal
Hardi Peter
Lakshmi Pradeep Chitta
Regina A. Cuadrado
Udo Schühle
Luca Teriaca
Sami K. Solanki
Louise Harra
David Berghmans
Frédéric Auchère
Susanna Parenti
Andrei N. Zhukov
Éric Buchlin
Cis Verbeeck
Emil Kraaikamp
Luciano Rodriguez
David M. Long
Conrad Schwanitz
Krzysztof Barczynski
Gabriel Pelouze
摘要： The solar chromosphere hosts a wide variety of transients, including dynamic fibrils (DFs) that are characterised as elongated, jet-like features seen in active regions, often through H$\alpha$ diagnostics. So far, these features have been difficult to identify in coronal images primarily due to their small size and the lower spatial resolution of the current EUV imagers. Here we present the first unambiguous signatures of DFs in coronal EUV data using high-resolution images from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. Using the data acquired with the 174~{\AA} High Resolution Imager (HRI$_{EUV}$) of EUI, we find many bright dot-like features (of size 0.3-0.5 Mm) that move up and down (often repeatedly) in the core of an active region. In a space-time map, these features produce parabolic tracks akin to the chromospheric observations of DFs. Properties such as their speeds (14 km~s$^{-1}$), lifetime (332~s), deceleration (82 m~s$^{-2}$) and lengths (1293~km) are also reminiscent of the chromospheric DFs. The EUI data strongly suggest that these EUV bright dots are basically the hot tips (of the cooler chromospheric DFs) that could not be identified unambiguously before because of a lack of spatial resolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Both observations and simulations suggest that the solar filament eruption is closely related to magnetic flux emergence. It is thought that the eruption is triggered by magnetic reconnection between the filament and the emerging flux. However, the details of such a reconnection are rarely presented. In this study, we report the detailed reconnection between a filament and its nearby emerging fields, that led to the reconfiguration and subsequent partial eruption of the filament located over the polarity inversion line of active region 12816. Before the reconnection, we observed repeated brightenings in the filament at a location that overlies a site of magnetic flux cancellation. Plasmoids form at this brightening region, and propagate bi-directionally along the filament. These indicate the tether-cutting reconnection that results in the formation and eruption of a flux rope. To the northwest of the filament, magnetic fields emerge, and reconnect with the context ones, resulting in repeated jets. Afterwards, another magnetic fields emerge near the northwestern filament endpoints, and reconnect with the filament, forming the newly reconnected filament and loops. Current sheet repeatedly occurs at the interface, with the mean temperature and emission measure of 1.7 MK and 1.1$\times$10$^{28}$ cm$^{-5}$. Plasmoids form in the current sheet, and propagate along it and further along the newly reconnected filament and loops. The newly reconnected filament then erupts, while the unreconnected filament remains stable. We propose that besides the orientation of emerging fields, some other parameters, such as the position, distance, strength, and area, are also crucial for triggering the filament eruption.
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