EUV Observations Research Articles

Magnetic Structure in Successively Erupting Active Regions: Comparison of Flare-Ribbons With Quasi-Separatrix Layers

This paper studies the magnetic topology of successively erupting active regions (ARs) 11,429 and 12,371. Employing vector magnetic field observations from Helioseismic and Magnetic Imager, the pre-eruptive magnetic structure is reconstructed by a model of non-linear force-free field (NLFFF). For all the five CMEs from these ARs, the pre-eruptive magnetic structure identifies an inverse-S sigmoid consistent with the coronal plasma tracers in EUV observations. In all the eruption cases, the quasi-separatrix layers (QSLs) of large Q values are continuously enclosing core field bipolar regions in which inverse-S shaped flare ribbons are observed. These QSLs essentially represent the large connectivity gradients between the domains of twisted core flux within the inner bipolar region and the surrounding potential like arcade. It is consistent with the observed field structure largely with the sheared arcade. The QSL maps in the chromosphere are compared with the flare-ribbons observed at the peak time of the flares. The flare ribbons are largely inverse-S shape morphology with their continuity of visibility is missing in the observations. For the CMEs in the AR 12371, the QSLs outline the flare ribbons as a combination of two inverse J-shape sections with their straight parts being separated. These QSLs are typical with the weakly twisted flux rope. Similarly, for the CMEs in the AR 11429, the QSLs are co-spatial with the flare ribbons both in the middle of the PIL and in the hook sections. In the frame work of standard model of eruptions, the observed flare ribbons are the characteristic of the pre-eruptive magnetic structure being sigmoid which is reproduced by the NLFFF model with a weakly twisted flux rope at the core.

How to Estimate the Far-Side Open Flux Using STEREO Coronal Holes

Global magnetic field models use as input synoptic data, which usually show “aging effects” as the longitudinal 360^{circ } information is not obtained simultaneously. Especially during times of increased solar activity, the evolution of the magnetic field may yield large uncertainties. A significant source of uncertainty is the Sun’s magnetic field on the side of the Sun invisible to the observer. Various methods have been used to complete the picture: synoptic charts, flux-transport models, and far side helioseismology. In this study, we present a new method to estimate the far-side open flux within coronal holes using STEREO EUV observations. First, we correlate the structure of the photospheric magnetic field as observed with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (HMI/SDO) with features in the transition region. From the 304 Å intensity distribution, which we found to be specific to coronal holes, we derive an empirical estimate for the open flux. Then we use a large sample of 313 SDO coronal hole observations to verify this relation. Finally, we perform a cross-instrument calibration from SDO to STEREO data to enable the estimation of the open flux at solar longitudes not visible from Earth. We find that the properties of strong unipolar magnetic elements in the photosphere, which determine the coronal hole’s open flux, can be approximated by open fields in the transition region. We find that structures below a threshold of 78% (STEREO) or 94% (SDO) of the solar disk median intensity as seen in 304 Å filtergrams are reasonably well correlated with the mean magnetic flux density of coronal holes (cc_{mathrm{sp}} = 0.59). Using the area covered by these structures (A_{mathrm{OF}}) and the area of the coronal hole (A_{mathrm{CH}}), we model the open magnetic flux of a coronal hole as |Phi _{mathrm{CH}}| = 0.25 A_{mathrm{CH}}~mathrm{exp}(0.032 A_{mathrm{OF}}) with an estimated uncertainty of 40 to 60%.

Radio evidence for a shock wave reflected by a coronal hole

We report the first unambiguous observational evidence in the radio range of the reflection of a coronal shock wave at the boundary of a coronal hole. The event occurred above an active region located at the northwest limb of the Sun and was characterized by an eruptive prominence and an extreme-ultraviolet wave steepening into a shock. The EUV observations acquired by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager instrument on board the Solar TErrestrial RElations Observatory were used to track the development of the EUV front in the inner corona. Metric type II radio emission, a distinguishing feature of shock waves propagating in the inner corona, was simultaneously recorded by ground-based radio spectrometers. The radio dynamic spectra displayed an unusual reversal of the type II emission lanes, together with type III-like herringbone emission, indicating shock-accelerated electron beams. Combined analysis of imaging data from the two space-based EUV instruments and the Nançay Radioheliograph evidences that the reverse-drifting type II emission was produced at the intersection of the shock front, reflected at a coronal hole boundary, with an intervening low-Alfvén-speed region characterized by an open field configuration. We also provide an outstanding data-driven reconstruction of the spatiotemporal evolution in the inner corona of the shock-accelerated electron beams produced by the reflected shock.

Observations of the Quiet Sun during the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM: Sub-A-class Microflares outside Active Regions

Abstract Solar flares, with energies ranging over several orders of magnitude, result from impulsive release of energy due to magnetic reconnection in the corona. Barring a handful, almost all microflares observed in X-rays are associated with the solar active regions. Here we present, for the first time, a comprehensive analysis of a large sample of quiet-Sun microflares observed in soft X-rays by the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission during the 2019–2020 solar minimum. A total of 98 microflares having peak flux below GOES A-level were observed by the XSM during observations spanning 76 days. By using the derived plasma temperature and emission measure of these events obtained by fitting the XSM spectra along with volume estimates from concurrent imaging observations in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly, we estimated their thermal energies to be ranging from 3 × 1026 to 6 × 1027 erg. We present the frequency distribution of the quiet-Sun microflares with energy and discuss the implications of these observations of small-scale magnetic reconnection events outside active regions on coronal heating.

Joint Geoeffectiveness and Arrival Time Prediction of CMEs by a Unified Deep Learning Framework

Fast and accurate prediction of the geoeffectiveness of coronal mass ejections (CMEs) and the arrival time of the geoeffective CMEs is urgent, to reduce the harm caused by CMEs. In this paper, we present a new deep learning framework based on time series of satellites’ optical observations that can give both the geoeffectiveness and the arrival time prediction of the CME events. It is the first time combining these two demands in a unified deep learning framework with no requirement of manually feature selection and get results immediately. The only input of the deep learning framework is the time series images from synchronized solar white-light and EUV observations. Our framework first uses the deep residual network embedded with the attention mechanism to extract feature maps for each observation image, then fuses the feature map of each image by the feature map fusion module and determines the geoeffectiveness of CME events. For the geoeffective CME events, we further predict its arrival time by the deep residual regression network based on group convolution. In order to train and evaluate our proposed framework, we collect 2400 partial-/full-halo CME events and its corresponding images from 1996 to 2018. The F1 score and Accuracy of the geoeffectiveness prediction can reach 0.270% and 75.1%, respectively, and the mean absolute error of the arrival time prediction is only 5.8 h, which are both significantly better than well-known deep learning methods and can be comparable to, or even better than, the best performance of traditional methods.

Perspective on Energetic and Thermal Atmospheric Photoelectrons

Atmospheric photoelectrons are central to the production of planetary ionospheres. They are created by photoionization of the neutral planetary atmosphere by solar EUV and soft X-ray irradiance. They provide the energy to heat the thermosphere. Thermalized photoelectrons permeate magnetospheres creating polarization electric fields and plasma waves as they interact with ions to maintain charge neutrality. Energetic photoelectrons (>1 eV) have a distinctive energy spectral shape as first revealed in data from the Atmosphere Explorer satellites. Energetic photoelectrons escaping the ionosphere follow local magnetic fields illuminating the planet's magnetic topology. Current models using state-of-the-art EUV observations accurately capture their production and transport. However, in spite of 60 years of space research the electron thermalization processes occurring below 1 eV at low altitudes in planetary thermospheres are not understood quantitatively. Results from event analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission are not consistent with current models of photoelectron thermalization. The lack of quantitative understanding reflects the complexity of the physics and the lack of a large data base of simultaneous neutral, ion, and electron densities and temperatures in lower planetary thermospheres.

Thermal-nonthermal energy partition in solar flares derived from X-ray, EUV, and bolometric observations

Context.In solar flares, energy is released impulsively and is partly converted into thermal energy of hot plasmas and kinetic energy of accelerated nonthermal particles. It is crucial to constrain the partition of these two energy components to understand energy release and transport as well as particle acceleration in solar flares. Despite numerous efforts, no consensus on quantifying this energy balance has yet been reached.Aims.We aim to understand the reasons for the contradicting results on energy partition obtained by various recent studies. The overarching question we address is whether there is sufficient energy in nonthermal particles to account for the thermal flare component.Methods.We considered five recent studies that address the thermal-nonthermal energy partition in solar flares. Their results are reviewed, and their methods are compared and discussed in detail.Results.The main uncertainties in deriving the energy partition are identified as (a) the derivation of the differential emission measure distribution and (b) the role of the conductive energy loss for the thermal component, as well as (c) the determination of the low-energy cutoff for the injected electrons. The bolometric radiated energy, as a proxy for the total energy released in the flare, is a useful independent constraint on both thermal and nonthermal energetics. In most of the cases, the derived energetics are consistent with this constraint. There are indications that the thermal-nonthermal energy partition changes with flare strength: in weak flares, there appears to be a deficit of energetic electrons, while the injected nonthermal energy is sufficient to account for the thermal component in strong flares. This behavior is identified as the main cause of the dissimilar results in the studies we considered. The changing partition has two important consequences: (a) an additional direct (i.e. non-beam) heating mechanism has to be present, and (b) considering that the bolometric emission originates mainly from deeper atmospheric layers, conduction or waves are required as additional energy transport mechanisms.

Dynamics Evolution of a Solar Active-region Filament from a Quasi-static State to Eruption: Rolling Motion, Untwisting Motion, Material Transfer, and Chirality

Abstract To better understand the magnetic structure and eruptive process of solar filaments, a solar active-region filament (labeled F2) eruption associated with a B-class flare was investigated by using high-resolution Hα data from the 1 m New Vacuum Solar Telescope (NVST), combined with EUV observations of the Solar Dynamics Observatory (SDO). The filament F2 was disturbed by another filament (labeled F1) eruption that experienced a whip-like motion. Before the filament F2 eruption, the Dopplergrams show that the southern and the northern parts of the filament F2 body exhibit blueshift and redshift along the filament spine, simultaneously. This implies that the filament F2 was rolling from one side to the other. During the filament F2 eruption, the Doppler shifts of the filament body are opposite of those before its eruption. This demonstrates that the filament body exhibits an untwisting motion, which can also be identified by tracing the movement of the eruptive filament threads. Moreover, it is found that the material of the filament F2 was transferred to the surrounding magnetic field loops, which is caused by magnetic reconnection between the filament F2 and the surrounding magnetic loops. According to the right-bearing threads of the filament F2 before its eruption, it can be deduced that the filament F2 is initially supported by a sheared arcade. The following observations reveal that the twisted magnetic structure of filament F2 formed in the eruption phase.

Investigation of the Middle Corona with SWAP and a Data-Driven Non-Potential Coronal Magnetic Field Model

The large field-of-view of the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) instrument onboard the PRoject for Onboard Autonomy 2 (PROBA2) spacecraft provides a unique opportunity to study extended coronal structures observed in the EUV in conjunction with global coronal magnetic field simulations. A global non-potential magnetic field model is used to simulate the evolution of the global corona from 1 September 2014 to 31 March 2015, driven by newly emerging bipolar active regions determined from Helioseismic and Magnetic Imager (HMI) magnetograms. We compare the large-scale structure of the simulated magnetic field with structures seen off-limb in SWAP EUV observations. In particular, we investigate how successful the model is in reproducing regions of closed and open structures, the scale of structures, and compare the evolution of a coronal fan observed over several rotations. The model is found to accurately reproduce observed large-scale, off-limb structures. When discrepancies do arise they mainly occur off the east solar limb due to active regions emerging on the far side of the Sun, which cannot be incorporated into the model until they are observed on the Earth-facing side. When such “late” active region emergences are incorporated into the model, we find that the simulated corona self-corrects within a few days, so that simulated structures off the west limb more closely match what is observed. Where the model is less successful, we consider how this may be addressed, through model developments or additional observational products.

A statistical study of the long-term evolution of coronal hole properties as observed by SDO

Context.Understanding the evolution of coronal holes is especially important when studying the high-speed solar wind streams that emanate from them. Slow- and high-speed stream interaction regions may deliver large amounts of energy into the Earth’s magnetosphere-ionosphere system, cause geomagnetic storms, and shape interplanetary space.Aims.By statistically investigating the long-term evolution of well-observed coronal holes we aim to reveal processes that drive the observed changes in the coronal hole parameters. By analyzing 16 long-living coronal holes observed by the Solar Dynamic Observatory, we focus on coronal, morphological, and underlying photospheric magnetic field characteristics, and investigate the evolution of the associated high-speed streams.Methods.We use the Collection of Analysis Tools for Coronal Holes to extract and analyze coronal holes using 193 Å EUV observations taken by the Atmospheric Imaging Assembly as well as line–of–sight magnetograms observed by the Helioseismic and Magnetic Imager. We derive changes in the coronal hole properties and look for correlations with coronal hole evolution. Further, we analyze the properties of the high–speed stream signatures near 1AU from OMNI data by manually extracting the peak bulk velocity of the solar wind plasma.Results.We find that the area evolution of coronal holes shows a general trend of growing to a maximum followed by a decay. We did not find any correlation between the area evolution and the evolution of the signed magnetic flux or signed magnetic flux density enclosed in the projected coronal hole area. From this we conclude that the magnetic flux within the extracted coronal hole boundaries is not the main cause for its area evolution. We derive coronal hole area change rates (growth and decay) of (14.2 ± 15.0)×108km2per day showing a reasonable anti-correlation (ccPearson = −0.48) to the solar activity, approximated by the sunspot number. The change rates of the signed mean magnetic flux density (27.3 ± 32.2 mG day−1) and the signed magnetic flux (30.3 ± 31.5 1018Mx day−1) were also found to be dependent on solar activity (ccPearson = 0.50 andccPearson = 0.69 respectively) rather than on the individual coronal hole evolutions. Further we find that the relation between coronal hole area and high-speed stream peak velocity is valid for each coronal hole over its evolution, but we see significant variations in the slopes of the regression lines.

Spatiotemporal Energy Partitioning in a Nonthermally Dominated Two-loop Solar Flare

Abstract Solar flares show remarkable variety in the energy partitioning between thermal and nonthermal components. Those with a prominent nonthermal component but only a modest thermal one are particularly well suited for study of the direct effect of the nonthermal electrons on plasma heating. Here, we analyze such a well-observed, impulsive single-spike nonthermal event, a solar flare SOL2013-11-05T035054, where the plasma heating can be entirely attributed to the energy losses of these impulsively accelerated electrons. Evolution of the energy budget of thermal and nonthermal components during the flare is analyzed using X-ray, microwave, and EUV observations and three-dimensional modeling. The results suggest that (i) the flare geometry is consistent with a two-loop morphology and the magnetic energy is likely released due to interaction between these two loops; (ii) the released magnetic energy is converted to the nonthermal energy of accelerated electrons only, which is subsequently converted to the thermal energy of the plasma; (iii) the energy is partitioned in these two flaring loops in comparable amounts; (iv) one of these flaring loops remained relatively tenuous but rather hot, while the other remained relatively cool but denser than the first. Therefore, this solar flare demonstrates an extreme efficiency of conversion of the free magnetic energy to the nonthermal energy of particle acceleration and the flow of energy into two loops from the nonthermal component to the thermal one with negligible direct heating.

Continuous Null-point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar AR 12673

Abstract Two X-class solar flares occurred on 2017 September 6 from active region NOAA 12673: the first one is a confined X2.2 flare, and it is followed only ∼3 hr later by the second one, which is the strongest flare in solar cycle 24, reaching X9.3 class and accompanied by a coronal mass ejection. Why did these two X-class flares occur in the same position with similar magnetic configurations, but one is eruptive while the other is not? Here we track the coronal magnetic field evolution via nonlinear force-free field extrapolations from a time sequence of vector magnetograms with high cadence. A detailed analysis of the magnetic field shows that a magnetic flux rope (MFR) forms and grows gradually before the first flare, and shortly afterward, the MFR’s growth is significantly enhanced with a much faster rise in height, from far below the threshold of torus instability to above it, while the magnetic twist only increases mildly. Combining EUV observations and the magnetic field extrapolation, we found that overlying the MFR is a null-point magnetic topology, where recurrent brightening is seen after the first flare. We thus suggest a scenario to interpret the occurrence of the two flares. The first flare occurred since the MFR reached a high enough height to activate the null point, and its continuous expansion forces the null-point reconnection recurrently. Such reconnection weakens the overlying field, allowing the MFR to rise faster, which eventually crosses the threshold of torus instability and triggers the second, eruptive flare.

Triggering Mechanism and Material Transfer of a Failed Solar Filament Eruption

Abstract Solar filament eruptions are often associated with solar flares and coronal mass ejections, which have the greatest impact on space weather. However, the fine structures and the trigger mechanisms of solar filaments are still unclear. To address these issues, we studied a failed solar active-region filament eruption associated with a C-class flare by using high-resolution Hα images from the New Vacuum Solar Telescope, supplemented by EUV observations from the Solar Dynamics Observatory. Before the filament eruption, a small bipolar magnetic field emerged below the filament. Then magnetic reconnection between the filament and the emerging bipolar magnetic field triggered the filament eruption. During the filament eruption, the untwisting motion of the filament can be clearly traced by the eruptive threads. Moreover, the footpoints of the eruptive threads are determined by tracing the descending filament material. Note that the twisted structure of the filament and the right part of the eruptive filament threads cannot be seen before the filament eruption. These eruptive threads in the right part of the filament are found to be rooting in the weak negative polarities near the main negative sunspot. Moreover, a new filament formed in the filament channel due to material injection from the eruptive filament. The above observations and the potential field extrapolations are inclined to support the idea that the filament materials were transferred into the overlying magnetic loops and the nearby filament channel by magnetic reconnection. These observations improve our understanding of the complexity of filament eruptions.

Drifting Pulsation Structure at the Very Beginning of the 2017 September 10 Limb Flare

Abstract Drifting pulsation structures (DPSs) are important radio fine structures usually observed at the beginning of eruptive solar flares. It has been suggested that DPSs carry important information on the energy release processes in solar flares. We study DPS observed in an X8.2-class flare on 2017 September 10 in the context of spatial and spectral diagnostics provided by microwave, EUV, and X-ray observations. We describe DPS and its substructures that were observed for the first time. We use a new wavelet technique to reveal characteristic periods in DPS and their frequency bands. Comparing the periods of pulsations found in this DPS with those in previous DPSs, we found new very short periods in the 0.09–0.15 s range. We present Expanded Owens Valley Solar Array images and spectra of microwave sources observed during the DPS. This DPS at its very beginning has pulsations in two frequency bands (1000–1300 MHz and 1600–1800 MHz), which are interconnected by fast drifting bursts. We show that these double-band pulsations started just at the moment when the ejected filament splits apart in a tearing motion at the location where a signature of the flare current sheet later appeared. Using the standard flare model and previous observations of DPSs, we interpret these double-band pulsations as a radio signature of superthermal electrons trapped in the rising magnetic rope and flare arcade at the moment when the flare magnetic reconnection starts. The results are discussed in a scenario with the plasmoid in the rising magnetic rope.

Reconstructing Coronal Hole Areas With EUHFORIA and Adapted WSA Model: Optimizing the Model Parameters

AbstractThe adopted Wang‐Sheeley‐Arge (WSA) model embedded in EUHFORIA (EUropean Heliospheric FORecasting Information Asset) is compared to EUV observations. According to the standard paradigm, coronal holes are sources of open flux; thus, we use remote sensing EUV observations and CATCH (Collection of Analysis Tools for Coronal Holes) to extract CH areas and compare them to the open flux areas modeled by EUHFORIA. From the adopted WSA model we employ only the Potential Field Source Surface (PFSS) model for the inner corona and the Schatten Current Sheet (SCS) model for the outer (PFSS+SCS). The height, Rss, of the outer boundary of the PFSS, known as the source surface, and the height, Ri, of the inner boundary of the SCS are important parameters affecting the modeled CH areas. We investigate the impact the two model parameters can have in the modeled results. We vary Rss within the interval [1.4, 3.2]R⊙ with a step of 0.1R⊙, and Ri within the interval [1.3, 2.8]R⊙ with the same step, and the condition that Ri<Rss. This way we have a set of 184 initial parameters to the model, and we assess the model results for all these possible height pairs. We conclude that the default heights used so far fail in modeling accurately CH areas and lower heights need to be considered.

The Eruption of Outer Spine-like Loops Leading to a Double-stage Circular-ribbon Flare

Abstract Circular-ribbon flares occur in a confined magnetic structure, but can also be associated with coronal mass ejections (CMEs) when a filament embedded under the fan erupts. Here we study an M8.7 circular-ribbon flare (SOL2014-12-17T04:51), which is accompanied by a CME yet without a clear indication of filament eruption. Using a nonlinear force-free field model, we find that the outer spine-like loops form a magnetic flux rope (FR1) rooted at the edge of the fan, and that there is another flux rope (FR2) at the main magnetic polarity inversion line (PIL) under a fan-like flux rope FR3. We divide the event evolution into two stages by combining modeling results with EUV observations. The onset stage is featured with bidirectional jets that occurred between a filament and FR1, immediately followed by an upward motion of the latter. During this first stage, the inner/outer spine-related ribbons and the circular ribbon begin to brighten up. After about 10 minutes, another ejection stems from the main PIL region. In this second stage, all ribbons are significantly enhanced, and the twist of FR2 footpoints is decreased. We discuss these results in favor of a scenario where the initial reconnection between the filament and FR1 activates the latter to reconnect with FR3 with opposite twist. This produces larger scale erupting loops and consequently causes a weakening of FR3, which induces another eruption of FR2 from below. This event thus represents a new type of eruptive circular-ribbon flare caused by unstable outer spine-like loops.

Joint X-Ray, EUV, and UV Observations of a Small Microflare

Abstract We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations allow us to study the hot coronal and cooler chromospheric/transition region emission from the microflare. This small microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR, SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible in the SDO/AIA Fe xviii emission, which matches a similar structure lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The NuSTAR emission in both 2.5–4 and 4–6 keV is located in a source at this loop location. The IRIS slit was over the microflaring loop, and fits show little change in Mg ii but do show intensity increases, slight width enhancements, and redshifts in Si iv and O iv, indicating that this microflare had most significance in and above the upper chromosphere. The NuSTAR microflare spectrum is well fitted by a thermal component of 5.1 MK and 6.2 × 1044 cm−3, which corresponds to a thermal energy of 1.5 × 1026 erg, making it considerably smaller than previously studied active region microflares. No non-thermal emission was detected but this could be due to the limited effective exposure time of the observation. This observation shows that even ordinary features seen in UV can remarkably have a higher-energy component that is clear in X-rays.

Nonequilibrium Ionization Effects on Solar EUV and X-Ray Imaging Observations

Abstract During transient events such as major solar eruptions, the plasma can be far from the equilibrium ionization state because of rapid heating or cooling. Nonequilibrium ionization (NEI) is important in rapidly evolving systems where the thermodynamical timescale is shorter than the ionization or recombination timescales. We investigate the effects of NEI on EUV and X-ray observations by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory and X-ray Telescope (XRT) on board Hinode. Our model assumes that the plasma is initially in ionization equilibrium at low temperature, and it is heated rapidly by a shock or magnetic reconnection. We tabulate the responses of the AIA and XRT passbands as functions of temperature and a characteristic timescale, n e t. We find that most of the ions reach equilibrium at n e t ≤ 1012 cm−3 s. Comparing ratios of the responses between different passbands allows us to determine whether a combination of plasmas at temperatures in ionization equilibrium can account for a given AIA and XRT observation. It also expresses how far the observed plasma is from equilibrium ionization. We apply the ratios to a supra-arcade plasma sheet on 2012 January 27. We find that the closer the plasma is to the arcade, the closer it is to a single-temperature plasma in ionization equilibrium. We also utilize the set of responses to estimate the temperature and density for shocked plasma associated with a coronal mass ejection on 2010 June 13. The temperature and density ranges we obtain are in reasonable agreement with previous works.

Observations of a Flare-Generated Blast Wave in a Pseudo Coronal Mass Ejection Event

We present an analysis of the event near the east limb, SOL2014-03-06T09:23, in which a pseudo coronal mass ejection (CME) was detected by the Large Angle and Spectrometric Coronagraph (LASCO) C2 instrument and indicated as “Poor Event; Only C2” in the Solar and Heliospheric Observatory (SOHO) LASCO CME Catalog. The analysis was performed based on two main methods: 1) investigation of the difference brightness profiles along specific directions in the solar corona using the EUV observations by the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO); 2) investigation of the spatially-resolved observations of the type II radio bursts made with the Nancay Radioheliograph. Based on the analysis performed we argue that the observed pseudo-CME could be a blast wave caused by impulsive flare energy release in the low corona. We also argue that, in the limited height range of ${\approx}\,0.2 \mathrm{R_{\odot }}\,\mbox{--}\,0.5 \mathrm{R}_{\odot}$, the front of this blast wave could steepen into a shock front.

Velocity Structure and Temperature Dependence of an Extreme-Ultraviolet Jet Observed by Hinode

The acceleration mechanism of EUV/X-ray jets is still unclear. For the most part, there are two candidates for the mechanism. One is magnetic reconnection, and the other is chromospheric evaporation. We observed a relatively compact X-ray jet that occurred between 10:50 - 11:10 UT on February 18, 2011 by using the Solar Dynamics Observatory/Atmospheric Imaging Assembly, and the X-ray Telescope, Solar Optical Telescope, and EUV Imaging Spectrometer aboard Hinode. Our results are as follows: 1) The EUV and X-ray observations show the general characteristics of X-ray jets, such as an arch structure straddling a polarity inversion line, a jet bright point shown at one side of the arch leg, and a spire above the arch. 2) The multi-wavelength observations and Ca II H-band image show the existence of a low-temperature (~10 000K) plasma (i.e., filament) at the center of the jet. 3) In the magnetogram and Ca II H-band image, the filament exists over the polarity inversion line and arch structure is also straddling it. In addition, magnetic cancellation occurs around the jet a few hours before and after the jet is observed. 4) The temperature distribution of the accelerated plasma, which was estimated from Doppler velocity maps, the calculated differential emission measure, and synthetic spectra show that there is no clear dependence between the plasma velocity and its temperature. For the third result above, observational results suggest that magnetic cancellation is probably related to the occurrence of the jet and filament formation. This result suggests that the trigger of the jet is magnetic cancellation rather than an emerging magnetic arch flux. The fourth result indicates that acceleration of the plasma accompanied by an X-ray jet seems to be caused by magnetic reconnection rather than chromospheric evaporation.