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 Pre-flare coronal dimmings Qingmin Zhang   Submitted: 2016-11-27 18:45 Coronal dimmings are regions of decreased extreme-ultravoilet (EUV) and/or X-ray (originally Skylab, then Yohkoh/SXT) intensities, which are often associated with flares and coronal mass ejections (CMEs). The large-scale, impulsive dimmings have substantially been observed and investigated. The pre-flare dimmings prior to the flare impulsive phase, however, have rarely been studied in detail. In this paper, we focus on the pre-flare coronal dimmings. We report our multiwavelength observations of the GOES X1.6 solar flare and the accompanying halo CME produced by the eruption of a sigmoidal magnetic flux rope (MFR) in NOAA active region (AR) 12158 on 2014 September 10. The eruption was observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamic Observatory (SDO). The photospheric line-of-sight magnetograms were observed by the Helioseismic and Magnetic Imager (HMI) aboard SDO. The soft X-ray (SXR) fluxes were recorded by the GOES spacecraft. The halo CME was observed by the white light coronagraphs of the Large Angle Spectroscopic Coronagraph (LASCO) aboard SOHO. About 96 minutes before the onset of flare/CME, narrow pre-flare coronal dimmings appeared at the two ends of the twisted MFR. They extended very slowly with their intensities decreasing with time, while their apparent widths (8-9 Mm) nearly kept constant. During the impulsive and decay phases of flare, typical fanlike twin dimmings appeared and expanded with much larger extent and lower intensities than the pre-flare dimmings. The percentage of 171 Å intensity decrease reaches 40%. The pre-flare dimmings are most striking in 171, 193, and 211 Å with formation temperatures of 0.6-2.5 MK. The northern part of the pre-flare dimmings could also be recognized in 131 and 335 Å. To our knowledge, this is the first detailed study of pre-flare coronal dimmings, which can be explained by the density depletion as a result of the gradual expansion of the coronal loop system surrounding the MFR during the slow rise of the MFR. Authors: Q. M. Zhang, Y. N. Su, and H. S. Ji Projects: SDO-AIA
 Publication Status: to be accepted for publication by A&A Last Modified: 2016-11-30 12:19

 Chromospheric Condensation and Quasi-periodic Pulsations in a Circular-ribbon Flare Qingmin Zhang   Submitted: 2016-09-13 02:44 In this paper, we report our multiwavelength observations of the C3.1 circular-ribbon flare SOL2015-10-16T10:20 in active region (AR) 12434. The flare consisted of a circular flare ribbon (CFR), an inner flare ribbon (IFR) inside, and a pair of short parallel flare ribbons (PFRs). The PFRs located to the north of IFR were most striking in the Interface Region Imaging Spectrograph (IRIS) 1400 Å and 2796 Å images. For the first time, we observed the circular-ribbon flare in the Ca II H line of the Solar Optical Telescope (SOT) aboard Hinode, which has similar shape as observed in the Atmospheric Imaging Assembly (AIA) 1600 Å aboard the Solar Dynamic Observatory (SDO). Photospheric line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO show that the flare was associated with positive polarities and a negative polarity inside. The IFR and CFR were cospatial with the negative polarity and positive polarities, implying the existence of a magnetic null point (B=0) and the dome-like spine-fan topology. During the impulsive phase of the flare, two-step'' raster observations of IRIS with a cadence of 6 s and an exposure time of 2 s show plasma downflow at the CFR in the Si IV \lambda1402.77 line (log T≈4.8), suggesting chromospheric condensation. The downflow speeds first increased rapidly from a few km s-1 to the peak values of 45-52 km s-1, before decreasing gradually to the initial levels. The decay timescales of condensation were 3-4 minutes, indicating ongoing magnetic reconnection. Interestingly, the downflow speeds are positively correlated with logarithm of the Si IV line intensity and time derivative of the GOES soft X-ray (SXR) flux in 1-8 Å. The radio dynamic spectra are characterized by a type III radio burst associated with the flare, which implies that the chromospheric condensation was most probably driven by nonthermal electrons. Using an analytical expression and the peak Doppler velocity, we derived the lower limit of energy flux of the precipitating electrons, i.e., 0.65x1010 erg cm-2 s-1. The Si IV line intensity and SXR derivative show quasi-periodic pulsations with periods of 32-42 s, which are likely caused by intermittent null-point magnetic reconnections modulated by the fast wave propagating along the fan surface loops at a phase speed of 950-1250 km s-1. Periodic accelerations and precipitations of the electrons result in periodic heating observed in the Si IV line and SXR. Authors: Q. M. Zhang, D. Li, and Z. J. Ning Projects: IRIS

 Explosive Chromospheric Evaporation in a Circular-ribbon Flare Qingmin Zhang   Submitted: 2016-05-10 21:13 In this paper, we report our multiwavelength observations of the C4.2 circular-ribbon flare in active region (AR) 12434 on 2015 October 16. The short-lived flare was associated with positive magnetic polarities and a negative polarity inside, as revealed by the photospheric line-of-sight magnetograms. Such magnetic pattern is strongly indicative of a magnetic null point and spine-fan configuration in the corona. The flare was triggered by the eruption of a mini-filament residing in the AR, which produced the inner flare ribbon (IFR) and the southern part of a closed circular flare ribbon (CFR). When the eruptive filament reached the null point, it triggered null point magnetic reconnection with the ambient open field and generated the bright CFR and a blowout jet. Raster observations of the Interface Region Imaging Spectrograph (IRIS) show plasma upflow at speed of 35-120 km s-1 in the Fe XXI 1354.09 Å line (log T≈7.05) and downflow at speed of 10-60 km s-1 in the Si IV 1393.77 Å line (log T≈4.8) at certain locations of the CFR and IFR during the impulsive phase of flare, indicating explosive chromospheric evaporation. Coincidence of the single HXR source at 12-25 keV with the IFR and calculation based on the thick-target model suggest that the explosive evaporation was most probably driven by nonthermal electrons. Authors: Q. M. Zhang, D. Li, Z. J. Ning, Y. N. Su, H. S. Ji and Y. Guo Projects: IRIS

 Observations of multiple blobs in homologous solar coronal jets in closed loops Qingmin Zhang   Submitted: 2016-01-18 20:58 Coronal bright points (CBPs) and jets are ubiquitous small-scale brightenings that are often associated with each other. In this paper, we report our multiwavelength observations of two groups of homologous jets. The first group was observed by the Extreme-Ultraviolet Imager (EUVI) aboard the behind Solar TErrestrial RElations Observatory (STEREO) spacecraft in 171 Å and 304 Å on 2014 September 10, from a location where data from the Solar Dynamic Observatory (SDO) could not observe. The jets (J1-J6) recurred for six times with intervals of 5-15 minutes. They originated from the same primary CBP (BP1) and propagated in the northeast direction along large-scale, closed coronal loops. Two of the jets (J3 and J6) produced sympathetic CBPs (BP2 and BP3) after reaching the remote footpoints of the loops. The time delays between the peak times of BP1 and BP2 (BP3) are 240±75 s (300±75 s). The jets were not coherent. Instead, they were composed of bright and compact blobs. The sizes and apparent velocities of the blobs are 4.5-9 Mm and 140-380 km s-1, respectively. The arrival times of the multiple blobs in the jets at the far-end of the loops indicate that the sympathetic CBPs are caused by jet flows rather than thermal conduction fronts. The second group was observed by the Atmospheric Imaging Assembly aboard SDO in various wavelengths on 2010 August 3. Similar to the first group, the jets originated from a short-lived bright point (BP) at the boundary of active region 11092 and propagated along a small-scale, closed loop before flowing into the active region. Several tiny blobs with sizes of ~1.7 Mm and apparent velocity of ~238 km s-1 were identified in the jets. We carried out the differential emission measure (DEM) inversions to investigate the temperatures of the blobs, finding that the blobs were multithermal with average temperature of 1.8-3.1 MK. The estimated number densities of the blobs were (1.7-2.8)x109 cm-3. Authors: Q. M. Zhang, H. S. Ji, and Y. N. Su Projects: SDO-AIA
 Publication Status: accepted for publication in Solar Physics Last Modified: 2016-01-20 12:25

 Multiwavelength observations of a partially eruptive filament on 2011 September 8 Qingmin Zhang   Submitted: 2015-03-10 19:25 In this paper, we report our multiwavelength observations of a partial filament eruption event in NOAA active region 11283 on 2011 September 8. A magnetic null point and the corresponding spine and separatrix surface are found in the active region. Beneath the null point, a sheared arcade supports the filament along the highly complex and fragmented polarity inversion line. After being activated, the sigmoidal filament erupted and split into two parts. The major part rose at the speeds of 90-150 km s-1 before reaching the maximum apparent height of ~115 Mm. Afterwards, it returned to the solar surface in a bumpy way at the speeds of 20-80 km s-1. The rising and falling motions were clearly observed in the extreme-ultravoilet (EUV), UV, and Hα wavelengths. The failed eruption of the main part was associated with an M6.7 flare with a single hard X-ray source. The runaway part of the filament, however, separated from and rotated around the major part for ~1 turn at the eastern leg before escaping from the corona, probably along large-scale open magnetic field lines. The ejection of the runaway part resulted in a very faint coronal mass ejection (CME) that propagated at an apparent speed of 214 km s-1 in the outer corona. The filament eruption also triggered transverse kink-mode oscillation of the adjacent coronal loops in the same AR. The amplitude and period of the oscillation were 1.6 Mm and 225 s. Our results are important for understanding the mechanisms of partial filament eruptions and provide new constraints to theoretical models. The multiwavelength observations also shed light on space weather prediction. Authors: Q. M. Zhang, Z. J. Ning, Y. Guo, T. H. Zhou, X. Cheng, H. S. Ji, L. Feng, T. Wiegelmann Projects: SDO-AIA

 Reciprocatory magnetic reconnection in a coronal bright point Qingmin Zhang   Submitted: 2014-06-22 20:21 Coronal bright points (CBPs) are small-scale and long-duration brightenings in the lower solar corona. They are often explained in terms of magnetic reconnection. We aim to study the sub-structures of a CBP and clarify the relationship among the brightenings of different patches inside the CBP. The event was observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft on 2009 August 22-23. The CBP showed repetitive brightenings (or CBP flashes). During each of the two successive CBP flashes, i.e., weak and strong flashes which are separated by ~2 hr, the XRT images revealed that the CBP was composed of two chambers, i.e., patches A and B. During the weak flash, patch A brightened first, and patch B brightened ~2 min later. During the transition, the right leg of a large-scale coronal loop drifted from the right side of the CBP to the left side. During the strong flash, patch B brightened first, and patch A brightened ~2 min later. During the transition, the right leg of the large-scale coronal loop drifted from the left side of the CBP to the right side. In each flash, the rapid change of the connectivity of the large-scale coronal loop is strongly suggestive of the interchange reconnection. For the first time we found reciprocatory reconnection in the CBP, i.e., reconnected loops in the outflow region of the first reconnection process serve as the inflow of the second reconnection process. Authors: Q. M. Zhang, P. F. Chen, M. D. Ding, and H. S. Ji Projects: None

 Blobs in recurring EUV jets Qingmin Zhang   Submitted: 2014-05-20 19:44 Coronal jets are one type of ubiquitous small-scale activities caused by magnetic reconnection in the solar corona. They are often associated with cool surges in the chromosphere. In this paper, we report our discovery of blobs in the recurrent and homologous jets that occurred at the western edge of NOAA active region 11259 on 2011 July 22. The jets were observed in the seven extreme-ultraviolet (EUV) filters of the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). Using the base-difference images of the six filters (94, 131, 171, 211, 193, and 335 Å), we carried out the differential emission measure (DEM) analyses to explore the thermodynamic evolutions of the jets. The jets were accompanied by cool surges observed in the Hα line center of the ground-based telescope in the Big Bear Solar Observatory. The jets that had lifetimes of 20-30 min recurred at the same place for three times with interval of 40-45 min. Interestingly, each of the jets intermittently experienced several upward eruptions at the speed of 120-450 km s-1. After reaching the maximum heights, they returned back to the solar surface, showing near-parabolic trajectories. The falling phases were more evident in the low-T filters than in the high-T filters, indicating that the jets experienced cooling after the onset of eruptions. We identified bright and compact blobs in the jets during their rising phases. The simultaneous presences of blobs in all the EUV filters were consistent with the broad ranges of the DEM profiles of the blobs (5.5≤ log T≤7.5), indicating their multi-thermal nature. The median temperatures of the blobs were ~2.3 MK. The blobs that were ~3 Mm in diameter had lifetimes of 24-60 s. To our knowledge, this is the first report of blobs in coronal jets. We propose that these blobs are plasmoids created by the magnetic reconnection as a result of tearing-mode instability and ejected out along the jets. Authors: Q. M. Zhang and H. S. Ji Projects: None

 A swirling flare-related EUV jet Qingmin Zhang   Submitted: 2013-12-06 03:29 {We report our observations of a swirling flare-related EUV jet on 2011 October 15 at the edge of NOAA active region 11314.} {We utilised the multiwavelength observations in the extreme-ultraviolet (EUV) passbands from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). We extracted a wide slit along the jet axis and 12 thin slits across its axis to investigate the longitudinal motion and transverse rotation. We also used data from the Extreme-Ultraviolet Imager (EUVI) aboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft to investigate the three-dimensional (3D) structure of the jet. Gound-based Hα images from the El Teide as a member of the Global Oscillation Network Group (GONG) provide a good opportunity to explore the relationship between the cool surge and hot jet. Line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO enable us to study the magnetic evolution of the flare/jet event. We carried out potential-field extrapolation to figure out the magnetic configuration associated with the jet.} {The onset of jet eruption coincided with the start time of C1.6 flare impulsive phase. The initial velocity and acceleration of the longitudinal motion were 254pm10 km s-1 and -97pm5 m s-2, respectively. The jet presented helical structure and transverse swirling motion at the beginning of its eruption. The counter-clockwise rotation slowed down from an average velocity of sim122 km s-1 to sim80 km s-1. The interwinding thick threads of the jet untwisted into multiple thin threads during the rotation that lasted for 1 cycle with a period of sim7 min and an amplitude that increases from sim3.2 Mm at the bottom to sim11 Mm at the upper part. Afterwards, the curtain-like leading edge of the jet continued rising without rotation, leaving a dimming region behind before falling back to the solar surface. The appearance/disappearance of dimming corresponded to the longitudinal ascending/descending motions of jet. Cospatial Hα surge and EUV dimming imply that the dimming resulted from the absorption of hot EUV emission by cool surge. The flare/jet event was caused by continuous magnetic cancellation before the start of flare. The jet was associated with the open magnetic fields at the edge of AR 11314.} Authors: Q. M. Zhang and H. S. Ji Projects: None

 Chromospheric evaporation in sympathetic coronal bright points Qingmin Zhang   Submitted: 2013-07-29 19:16 {Chromospheric evaporation is a key process in solar flares that has extensively been investigated using the spectroscopic observations. However, direct soft X-ray (SXR) imaging of the process is rare, especially in remote brightenings associated with the primary flares that have recently attracted dramatic attention.} {We intend to find the evidence for chromospheric evaporation and figure out the cause of the process in sympathetic coronal bright points (CBPs), i.e., remote brightenings induced by the primary CBP.} {We utilise the high-cadence and high-resolution SXR observations of CBPs from the X-ray Telescope (XRT) aboard the Hinode spacecraft on 2009 August 23.} {We discover thermal conduction front propagating from the primary CBP, i.e., BP1, to one of the sympathetic CBPs, i.e., BP2 that is 60arcsec away from BP1. The apparent velocity of the thermal conduction is sim138 km s-1. Afterwards, hot plasma flowed upwards into the loop connecting BP1 and BP2 at a speed of sim76 km s-1, a clear signature of chromospheric evaporation. Similar upflow was also observed in the loop connecting BP1 and the other sympathetic CBP, i.e., BP3 that is 80arcsec away from BP1, though less significant than BP2. The apparent velocity of the upflow is sim47 km s-1. The thermal conduction front propagating from BP1 to BP3 was not well identified except for the jet-like motion also originating from BP1.} {We propose that the gentle chromospheric evaporation in the sympathetic CBPs were caused by thermal conduction originating from the primary CBP.} Authors: Q. M. Zhang and H. S. Ji Projects: None

 Parametric survey of longitudinal prominence oscillation simulations Qingmin Zhang   Submitted: 2013-04-16 02:58 {Longitudinal filament oscillations recently attracted more and more attention, while the restoring force and the damping mechanisms are still elusive.} {In this paper, we intend to investigate the underlying physics for coherent longitudinal oscillations of the entire filament body, including their triggering mechanism, dominant restoring force, and damping mechanisms.} {With the MPI-AMRVAC code, we carry out radiative hydrodynamic numerical simulations of the longitudinal prominence oscillations. Two types of perturbations, i.e., impulsive heating at one leg of the loop and an impulsive momentum deposition are introduced to the prominence, which then starts to oscillate. We study the resulting oscillations for a large parameter scan, including the chromospheric heating duration, initial velocity of the prominence, and field line geometry.} {It is found that both microflare-sized impulsive heating at one leg of the loop and a suddenly imposed velocity perturbation can propel the prominence to oscillate along the magnetic dip. An extensive parameter survey results in a scaling law, showing that the period of the oscillation, which weakly depends on the length and height of the prominence, and the amplitude of the perturbations, scales with sqrt{R/g_odot}, where R represents the curvature radius of the dip, and g_odot is the gravitational acceleration of the Sun. This is consistent with the linear theory of a pendulum, which implies that the field-aligned component of gravity is the main restoring force for the prominence longitudinal oscillations, as confirmed by the force analysis. However, the gas pressure gradient becomes non-negligible for short prominences. The oscillation damps with time in the presence of non-adiabatic processes. Compared to heat conduction, the radiative cooling is the dominant factor leading to the damping. A scaling law for the damping timescale is derived, i.e., ausim l1.63 D0.66w-1.21v0-0.30, showing strong dependence on the prominence length l, the geometry of the magnetic dip (characterized by the depth D and the width w), and the velocity perturbation amplitude v0. The larger the amplitude, the faster the oscillation damps. It is also found that mass drainage significantly reduces the damping timescale when the perturbation is too strong.} Authors: Q. M. Zhang, P .F. Chen, C. Xia, R. Keppens, H. S. Ji Projects: None

 Observation and Simulation of Longitudinal Oscillations of an Active Region Prominence Qingmin Zhang   Submitted: 2012-04-17 08:35 {Filament longitudinal oscillations have been observed on the solar disk in Hα .} {We intend to find an example of the longitudinal oscillations of a prominence, where the magnetic dip can be seen directly, and examine what is the restoring force of such kind of oscillations.} {We carry out a multiwavelength data analysis of the active region prominence oscillations above the western limb on 2007 February 8. Besides, we perform a one-dimensional hydrodynamic simulation of the longitudinal oscillations.} {The high-resolution observations by Hinode/SOT indicate that the prominence, seen as a concave-inward shape in lower-resolution Extreme Ultraviolet (EUV) images, actually consists of many concave-outward threads, which is indicative of the existence of magnetic dips. After being injected into the dip region, a bulk of prominence material started to oscillate for more than 3.5 hours, with the period being 52 min. The oscillation decayed with time, with the decay timescale being 133 min. Our hydrodynamic simulation can well reproduce the oscillation period, but the damping timescale in the simulation is 1.5 times as long as the observations.} {The results clearly show the prominence longitudinal oscillations around the dip of the prominence and our study suggests that the restoring force of the longitudinal oscillations might be the gravity. Radiation and heat conduction are insufficient to explain the decay of the oscillations. Other mechanisms, such as wave leakage and mass accretion, have to be considered. The possible relation between the longitudinal oscillations and the later eruption of a prominence thread, as well as a coronal mass ejection (CME), is also discussed.} Authors: Qingmin Zhang, Pengfei Chen, Chun Xia, Rony Keppens Projects: Hinode/SOT