arXiv subjects

Chenglong Shen

Publications and source records attributed to Chenglong Shen.

At least 37 records · Page 2Linked to original sources

arXiv2016

Damped large amplitude oscillations in a solar prominence and a bundle of coronal loops

We investigate the evolutions of two prominences (P1,P2) and two bundles of coronal loops (L1,L2), observed with SDO/AIA near the east solar limb on 2012 September 22. It is found that there were large-amplitude oscillations in P1 and L1, but no detectable motions in P2 and L2. These transverse oscillations were triggered by a large-scale coronal wave, originating from a large flare in a remote active region behind the solar limb. By carefully comparing the locations and heights of these oscillating and non-oscillating structures, we conclude that the propagating height of the wave is between 50 Mm and 130 Mm. The wave energy deposited in the oscillating prominence and coronal loops is at least of the order of $10^{28}$ erg. Furthermore, local magnetic field strength and Alfvén speeds are derived from the oscillating periods and damping time scales, which are extracted from the time series of the oscillations. It is demonstrated that oscillations can be used in not only coronal seismology, but also revealing the properties of the wave.

astro-ph.SR↗

arXiv2016

Thermodynamic Spectrum of Solar Flares Based on SDO/EVE Observations: Techniques and Statistical Results

SDO/EVE provides rich information of the thermodynamic processes of solar activities, particularly of solar flares. Here, we develop a method to construct thermodynamic spectrum (TDS) charts based on the EVE spectral lines. This tool could be potentially useful to the EUV astronomy to learn the eruptive activities on the distant astronomical objects. Through several cases, we illustrate what we can learn from the TDS charts. Furthermore, we apply the TDS method to 74 flares equal to or greater than M5.0-class, and reach the following statistical results. First, EUV peaks are always behind the soft X-ray (SXR) peaks and stronger flares tend to have a faster cooling rate. There is a power law correlation between the peak delay times and the cooling rates, suggesting a coherent cooling process of flares from SXR to EUV emissions. Second, there are two distinct temperature drift patterns, called Type I and Type II. For Type I flares, the enhanced emission drifts from high to low temperature like a quadrilateral, whereas for Type II flares, the drift pattern looks like a triangle. Statistical analysis suggests that Type II flares are more impulsive than Type I flares. Third, for late-phase flares, the peak intensity ratio of the late phase to the main phase is roughly correlated with the flare class, and the flares with a strong late phase are all confined. We believe that the re-deposition of the energy carried by a flux rope, that unsuccessfully erupts out, into thermal emissions is responsible for the strong late phase found in a confined flare. Besides, with some cases we illustrate the signatures of the flare thermodynamic process in the chromosphere and transition region in TDS charts. These results provide new clues to advance our understanding of the thermodynamic processes of solar flares and associated solar eruptions, e.g., coronal mass ejections.

astro-ph.SR↗

arXiv2015

A Solar Coronal Jet Event Triggers A Coronal Mass Ejection

We present the multi-point and multi-wavelength observation and analysis on a solar coronal jet and coronal mass ejection (CME) event in this paper. Employing the GCS model, we obtained the real (three-dimensional) heliocentric distance and direction of the CME and found it propagate in a high speed over 1000 km/s . The jet erupted before and shared the same source region with the CME. The temporal and spacial relation- ship between them guide us the possibility that the jet triggered the CME and became its core. This scenario could promisingly enrich our understanding on the triggering mechanism of coronal mass ejections and their relations with coronal large-scale jets. On the other hand, the magnetic field configuration of the source region observed by the SDO/HMI instrument and the off- limb inverse Y-shaped configuration observed by SDO/AIA 171 A passband, together provide the first detailed observation on the three-dimensional reconnection process of large-scale jets as simulated in Pariat et al. 2009. The erupting process of the jet highlights that filament-like materials are important during the eruption not only of small-scale X-ray jets (Sterling et al. 2015) but also probably of large-scale EUV jets. Based on our observation and analysis, we propose a most possible mechanism for the whole event with a blob structure overlaying the three-dimensional structure of the jet to describe the interaction between the jet and the CME.

astro-ph.SR↗

arXiv2015

Why does the apparent mass of a coronal mass ejection increase?

Mass is one of the most fundamental parameters characterizing the dynamics of a coronal mass ejection (CME). It has been found that CME apparent mass measured from the brightness enhancement in coronagraph images shows an increasing trend during its evolution in the corona. However, the physics behind it is not clear. Does the apparent mass gain come from the mass outflow from the dimming regions in the low corona, or from the pileup of the solar wind plasma around the CME when it propagates outwards from the Sun? We analyzed the mass evolution of six CME events. Their mass can increase by a factor of 1.6 to 3.2 from 4 to 15 Rs in the field of view (FOV) of the coronagraph on board the Solar Terrestrial Relations Observatory (STEREO). Over the distance about 7 to 15 Rs, where the coronagraph occulting effect can be negligible, the mass can increase by a factor of 1.3 to 1.7. We adopted the `snow-plough' model to calculate the mass contribution of the piled-up solar wind in the height range from about 7 to 15 Rs. For 2/3 of the events, the solar wind pileup is not sufficient to explain the measured mass increase. In the height range from about 7 to 15 Rs, the ratio of the modeled to the measured mass increase is roughly larger than 0.55. Although the ratios are believed to be overestimated, the result gives evidence that the solar wind pileup probably makes a non-negligible contribution to the mass increase. It is not clear yet whether the solar wind pileup is a major contributor to the final mass derived from coronagraph observations. However, our study suggests that the solar wind pileup plays increasingly important role in the mass increase as a CME moves further away from the Sun.

astro-ph.SR↗

arXiv2015

Why Is the Great Solar Active Region 12192 Flare-Rich But CME-Poor?

Solar active region (AR) 12192 of October 2014 hosts the largest sunspot group in 24 years. It is the most prolific flaring site of Cycle 24, but surprisingly produced no coronal mass ejection (CME) from the core region during its disk passage. Here, we study the magnetic conditions that prevented eruption and the consequences that ensued. We find AR 12192 to be "big but mild"; its core region exhibits weaker non-potentiality, stronger overlying field, and smaller flare-related field changes compared to two other major flare-CME-productive ARs (11429 and 11158). These differences are present in the intensive-type indices (e.g., means) but generally not the extensive ones (e.g., totals). AR 12192's large amount of magnetic free energy does not translate into CME productivity. The unexpected behavior suggests that AR eruptiveness is limited by some relative measure of magnetic non-potentiality over the restriction of background field, and that confined flares may leave weaker photospheric and coronal imprints compared to their eruptive counterparts.

astro-ph.SR↗

arXiv2015

Satistical Study of the Interplanetary Coronal Mass Ejections from 1996 to 2014

In this work, we establish an ICME list from 1996 to 2014 based on the in-situ observations from the WIND and ACE satellites. Based on this ICME list, we extend the statistical analysis of the ICMEs to the solar maximum phase of solar cycle 24th. The analysis of the annual variations of the properties of ICMEs show that the number of ICMEs, the number of shocks, the percentage of ICMEs drove shocks, the magnetic field and plasma properties of ICMEs are well correlated with the solar cycle variation. The number of MCs do not show any correlation with sunspot number. But, the percentage of the MCs in ICMEs show good anti-correlation with the sunspot number. By comparison the parameters of MCs with None-MC ICMEs, we found that the MCs are stronger than the None-MC ICMEs . In addition, we compare the parameters of ICMEs with and without shocks. It is found that the ICMEs with shocks are much stronger than the ICME without shocks. Meanwhile, we discuss the distribution of the magnetic field and solar wind plasmas parameters of the sheath regions of ICMEs at first time. We find that the magnetic field and solar wind velocity in the sheath region are higher than them in the ejecta of ICMEs from statistical point of view.

astro-ph.SR↗

arXiv2015

Extremely Large EUV Late Phase of Solar Flares

The second peak in the Fe XVI 33.5 nm line irradiance observed during solar flares by Extreme ultraviolet Variability Experiment (EVE) is known as Extreme UltraViolet (EUV) late phase. Our previous paper (Liu et al. 2013) found that the main emissions in the late phase are originated from large-scale loop arcades that are closely connected to but different from the post flare loops (PFLs), and we also proposed that a long cooling process without additional heating could explain the late phase. In this paper, we define the extremely large late phase because it not only has a bigger peak in the warm 33.5 irradiance profile, but also releases more EUV radiative energy than the main phase. Through detailedly inspecting the EUV images from three point-of-view, it is found that, besides the later phase loop arcades, the more contribution of the extremely large late phase is from a hot structure that fails to erupt. This hot structure is identified as a flux rope, which is quickly energized by the flare reconnection and later on continuously produces the thermal energy during the gradual phase. Together with the late-phase loop arcades, the fail to erupt flux rope with the additional heating create the extremely large EUV late phase.

astro-ph.SR↗

arXiv2015

Investigating plasma motion of magnetic clouds at 1 AU through a velocity-modified cylindrical force-free flux rope model

Magnetic clouds (MCs) are the interplanetary counterparts of coronal mass ejections (CMEs), and usually modeled by a flux rope. By assuming the quasi-steady evolution and self-similar expansion, we introduce three types of global motion into a cylindrical force-free flux rope model, and developed a new velocity-modified model for MCs. The three types of the global motion are the linear propagating motion away from the Sun, the expanding and the poloidal motion with respect to the axis of the MC. The model is applied to 72 MCs observed by Wind spacecraft to investigate the properties of the plasma motion of MCs. First, we find that some MCs had a significant propagation velocity perpendicular to the radial direction, suggesting the direct evidence of the CME's deflected propagation and/or rotation in interplanetary space. Second, we confirm the previous results that the expansion speed is correlated with the radial propagation speed and most MCs did not expand self-similarly at 1 AU. In our statistics, about 62\%/17\% of MCs underwent a under/over-expansion at 1 AU and the expansion rate is about 0.6 on average. Third, most interestingly, we find that a significant poloidal motion did exist in some MCs. Three speculations about the cause of the poloidal motion are therefore proposed. These findings advance our understanding of the MC's properties at 1 AU as well as the dynamic evolution of CMEs from the Sun to interplanetary space.

astro-ph.SR↗

arXiv2015

Evolution of the 2012 July 12 CME from the Sun to the Earth: Data-Constrained Three-Dimensional MHD Simulations

The dynamic process of coronal mass ejections (CMEs) in the heliosphere provides us the key information for evaluating CMEs' geo-effectiveness and improving the accurate prediction of CME induced Shock Arrival Time (SAT) at the Earth. We present a data constrained three dimensional (3D) magnetohydrodynamic (MHD) simulation of the evolution of the CME in a realistic ambient solar wind for the July 12-16, 2012 event by using the 3D COIN-TVD MHD code. A detailed comparison of the kinematic evolution of the CME between the observations and the simulation is carried out, including the usage of the time-elongation maps from the perspectives of both Stereo A and Stereo B. In this case study, we find that our 3D COIN-TVD MHD model, with the magnetized plasma blob as the driver, is able to re-produce relatively well the real 3D nature of the CME in morphology and their evolution from the Sun to Earth. The simulation also provides a relatively satisfactory comparison with the in-situ plasma data from the Wind spacecraft.

astro-ph.SR↗

arXiv2014

Could the collision of CMEs in the heliosphere be super-elastic? --- Validation through three-dimensional simulations

Though coronal mass ejections (CMEs) are magnetized fully-ionized gases, a recent observational study of a CME collision event in 2008 November has suggested that their behavior in the heliosphere is like elastic balls, and their collision is probably super-elastic \citep{Shen_etal_2012}. If this is true, this finding has an obvious impact on the space weather forecasting because the direction and veliocity of CMEs may change. To verify it, we numerically study the event through three-dimensional MHD simulations. The nature of CMEs' collision is examined by comparing two cases. In one case the two CMEs collide as observed, but in the other, they do not. Results show that the collision leads to extra kinetic energy gain by 3%--4% of the initial kinetic energy of the two CMEs. It firmly proves that the collision of CMEs could be super-elastic.

astro-ph.SR↗

arXiv2014

Super-elastic Collision of Large-scale Magnetized Plasmoids in The Heliosphere

Super-elastic collision is an abnormal collisional process, in which some particular mechanisms cause the kinetic energy of the system increasing. Most studies in this aspect focus on solid-like objects, but they rarely consider gases or liquids, as the collision of the latter is primarily a mixing process. With cross-field diffusion being effectively prohibited, magnetized plasmoids are different from ordinary gases. But it remains unclear how they act during a collision. Here we present the global picture of a unique collision between two coronal mass ejections in the heliosphere, which are the largest magnetized plasmoids erupting from the Sun. Our analysis for the first time reveals that these two magnetized plasmoids collided like solid-like objects with a 73% likelihood of being super-elastic. Their total kinetic energy surprisingly increased by about 6.6% through the collision, which significantly influenced the dynamics of the plasmoids.

astro-ph.SR↗

arXiv2014

Early Evolution of an Energetic Coronal Mass Ejection and Its Relation to EUV Waves

We study a coronal mass ejection (CME) associated with an X-class flare, whose initiation is clearly observed in low corona with high-cadence, high-resolution EUV images, providing us a rare opportunity to witness the early evolution of an energetic CME in detail. The eruption starts with a slow expansion of cool overlying loops (~1 MK) following a jet-like event in the periphery of the active region. Underneath the expanding loop system a reverse S-shaped dimming is seen immediately above the brightening active region in hot EUV passbands. The dimming is associated with a rising diffuse arch (~6 MK), which we interpret as a preexistent, high-lying flux rope. This is followed by the arising of a double hot channel (~10 MK) from the core of the active region. The higher structures rise earlier and faster than lower ones, with the leading front undergoing extremely rapid acceleration up to 35 km s$^2$. This suggests that the torus instability is the major eruption mechanism and that it is the high-lying flux rope rather than the hot channels that drives the eruption. The compression of coronal plasmas skirting and overlying the expanding loop system, whose aspect ratio $h/r$ increases with time as a result of the rapid upward acceleration, plays a significant role in driving an outward-propagating global EUV wave and a sunward-propagating local EUV wave, respectively.

astro-ph.SR↗

arXiv2014

Deflected propagation of a coronal mass ejection from the corona to interplanetary space

Among various factors affecting the space weather effects of a coronal mass ejection (CME), its propagation trajectory in the interplanetary space is an important one determining whether and when the CME will hit the Earth. Many direct observations have revealed that a CME may not propagate along a straight trajectory in the corona, but whether or not a CME also experiences a deflected propagation in the interplanetary space is a question, which has never been fully answered. Here by investigating the propagation process of an isolated CME from the corona to interplanetary space during 2008 September 12 -- 19, we present solid evidence that the CME was deflected not only in the corona but also in the interplanetary space. The deflection angle in the interplanetary space is more than 20 degrees toward the west, resulting a significant change in the probability the CME encounters the Earth. A further modeling and simulation-based analysis suggest that the cause of the deflection in the interplanetary space is the interaction between the CME and the solar wind, which is different from that happening in the corona.

physics.space-ph↗

arXiv2014

Full Halo Coronal Mass Ejections: Arrival at the Earth

A geomagnetic storm is mainly caused by a front-side coronal mass ejection (CME) hitting the Earth and then interacting with the magnetosphere. However, not all front-side CMEs can hit the Earth. Thus, which CMEs hit the Earth and when they do so are important issues in the study and forecasting of space weather. In our previous work (Shen et al., 2013), the de-projected parameters of the full-halo coronal mass ejections (FHCMEs) that occurred from 2007 March 1 to 2012 May 31 were estimated, and there are 39 front-side events could be fitted by the GCS model. In this work, we continue to study whether and when these front-side FHCMEs (FFHCMEs) hit the Earth. It is found that 59\% of these FFHCMEs hit the Earth, and for central events, whose deviation angles $ε$, which are the angles between the propagation direction and the Sun-Earth line, are smaller than 45 degrees, the fraction increases to 75\%. After checking the deprojected angular widths of the CMEs, we found that all of the Earth-encountered CMEs satisfy a simple criterion that the angular width ($ω$) is larger than twice the deviation angle ($ε$). This result suggests that some simple criteria can be used to forecast whether a CME could hit the Earth. Furthermore, for Earth-encountered CMEs, the transit time is found to be roughly anti-correlated with the de-projected velocity, but some events significantly deviate from the linearity. For CMEs with similar velocities, the differences of their transit times can be up to several days. Such deviation is further demonstrated to be mainly caused by the CME geometry and propagation direction, which are essential in the forecasting of CME arrival.

astro-ph.SR↗

arXiv2014

A Prominence Eruption Driven by Flux Feeding from Chromospheric Fibrils

We present multi-wavelength observations of a prominence eruption originating from a quadrupolar field configuration, in which the prominence was embedded in a side-arcade. Within the two-day period prior to its eruption on 2012 October 22, the prominence was perturbed three times by chromospheric fibrils underneath, which rose upward, became brightened, and merged into the prominence, resulting in horizontal flows along the prominence axis, suggesting that the fluxes carried by the fibrils were incorporated into the magnetic field of the prominence. These perturbations caused the prominence to oscillate and to rise faster than before. The absence of intense heating within the first two hours after the onset of the prominence eruption, which followed an exponential increase in height, indicates that ideal instability played a crucial role. The eruption involved interactions with the other side-arcade, leading up to a twin coronal mass ejection, which was accompanied by transient surface brightenings in the central arcade, followed by transient dimmings and brightenings in the two side-arcades. We suggest that flux feeding from chromospheric fibrils might be an important mechanism to trigger coronal eruptions.

astro-ph.SR↗

arXiv2014

When and how does a prominence-like jet gain kinetic energy?

A jet is a considerable amount of plasma being ejected from the chromosphere or lower corona into the higher corona and is a common phenomenon. Usually, a jet is triggered by a brightening or a flare, which provides the first driving force to push plasma upward. In this process, magnetic reconnection is thought to be the mechanism to convert magnetic energy into thermal, nonthermal, and kinetic energies. However, most jets could reach an unusual high altitude and end much later than the end of its associated flare. This fact implies that there is another way to continuously transfer magnetic energy into kinetic energy even after the reconnection. The picture described above is well known in the community, but how and how much magnetic energy is released through a way other than reconnection is still unclear. By studying a prominence-like jet observed by SDO/AIA and STEREO-A/EUVI, we find that the continuous relaxation of the post-reconnection magnetic field structure is an important process for a jet to climb up higher than it could through only reconnection. The kinetic energy of the jet gained through the relaxation is 1.6 times that gained from the reconnection. The resultant energy flux is hundreds of times larger than the flux required for the local coronal heating, suggesting that such jets are a possible source to keep the corona hot. Furthermore, rotational motions appear all the time during the jet. Our analysis suggests that torsional Alfvén waves induced during reconnection could not be the only mechanism to release magnetic energy and drive jets.

astro-ph.SR↗

arXiv2013

Full halo coronal mass ejections: Do we need to correct the projection effect in terms of velocity?

The projection effect is one of the biggest obstacles in learning the real properties of coronal mass ejections (CMEs) and forecasting their geoeffectiveness. To evaluate the projection effect, 86 full halo CMEs (FHCMEs) listed in the CDAW CME catalog from 2007 March 1 to 2012 May 31 are investigated. By applying the Graduated Cylindrical Shell (GCS) model, we obtain the de-projected values of the propagation velocity, direction and angular width of these FHCMEs, and compare them with the projected values measured in the plane-of-sky. Although these CMEs look full halo in the view angle of SOHO, it is found that their propagation directions and angular widths could vary in a large range, implying projection effect is a major reason causing a CME being halo, but not the only one. Furthermore, the comparison of the de-projected and projected velocities reveals that most FHCMEs originating within 45$^\circ$ of the Sun-Earth line with a projected speed slower than 900 km s$^{-1}$ suffer from large projection effect, while the FHCMEs originating far from the vicinity of solar disk center or moving faster than 900 km s$^{-1}$ have small projection effect. The results suggest that not all of FHCMEs need to correct projection effect for their velocities.

astro-ph.SR↗

arXiv2013

Waiting Times of Quasi-homologous Coronal Mass Ejections from Super Active Regions

Why and how may some active regions (ARs) frequently produce coronal mass ejections (CMEs)? It is one of the key questions to deepen our understanding of the mechanisms and processes of energy accumulation and sudden release in ARs and to improve our capability of space weather prediction. Although some case studies have been made, the question is still far from fully answered. This issue is now being tried to address statistically through an investigation of waiting times of quasi-homologous CMEs from super ARs in solar cycle 23. It is found that the waiting times of quasi-homologous CMEs have a two-component distribution with a separation at about 18 hours. The first component is a Gaussian-like distribution with a peak at about 7 hours, which indicates a tight physical connection between these quasi-homologous CMEs. The likelihood of occurrences of two or more CMEs faster than 1200 km s-1 from the same AR within 18 hours is about 20%. Furthermore, the correlation analysis among CME waiting times, CME speeds and CME occurrence rates reveals that these quantities are independent to each other, suggesting that the perturbation by preceding CMEs rather than free energy input be the direct cause of quasi-homologous CMEs. The peak waiting time of 7 hours probably characterize the time scale of the growth of instabilities triggered by preceding CMEs. This study uncovers more clues from a statistical perspective for us to understand quasi-homologous CMEs as well as CME-rich ARs.

astro-ph.SR↗