arXiv subjects

Jun Cui

Publications and source records attributed to Jun Cui.

43 records · Page 3Linked to original sources

arXiv2019

Extreme-ultraviolet Late Phase Caused by Magnetic Reconnection over Quadrupolar Magnetic Configuration in a Solar Flare

A second emission enhancement in warm coronal extreme-ultraviolet (EUV) lines (about 2-7 MK) during some solar flares is known as the EUV late phase. Imaging observations confirm that the late phase emission originates from a set of longer or higher loops than the main flare loops. Nevertheless, some questions remain controversial: What is the relationship between these two loop systems? What is the heating source of late phase emission, a heating accompany the main phase heating or occuring quite later? In this paper, we present clear evidence for heating source in a late-phase solar flare: magnetic reconnection of overlying field in a quadrupolar magnetic configuration. The event is triggered by an erupted core structure that eventually leads to a coronal mass ejection (CME). Cusp feature and its shrinkage motion high in the late-phase emission region are the manifestation of the later phase reconnection following the main flare reconnection. Using the enthalpy-based thermal evolution of loops (EBTEL) model, we reasonably reproduce the late-phase emissions in some EUV lines. We suggest that a continuous additional heating is responsible for the appearance of the elongated EUV late phase.

astro-ph.SR↗

arXiv2019

Why torus-unstable solar filaments experience failed eruption?

To investigate the factors that control the success and/or failure of solar eruptions, we study the magnetic field and 3-Dimensional (3D) configuration of 16 filament eruptions during 2010 July - 2013 February. All these events, i.e., erupted but failed to be ejected to become a coronal mass ejection (CME), are failed eruptions with the filament maximum height exceeding $100 Mm$. The magnetic field of filament source regions is approximated by a potential field extrapolation method. The filament 3D configuration is reconstructed from three vantage points by the observations of STEREO Ahead/Behind and SDO spacecraft. We calculate the decay index at the apex of these failed filaments and find that in 7 cases, their apex decay indexes exceed the theoretical threshold ($n_{crit} = 1.5$) of the torus instability. We further determine the orientation change or rotation angle of each filament top during the eruption. Finally, the distribution of these events in the parameter space of rotation angle versus decay index is established. Four distinct regimes in the parameter space are empirically identified. We find that, all the torus-unstable cases (decay index $n > 1.5$), have a large rotation angles ranging from $50^\circ - 130^\circ$. The possible mechanisms leading to the rotation and failed eruption are discussed. These results imply that, besides the torus instability, the rotation motion during the eruption may also play a significant role in solar eruptions.

astro-ph.SR↗

arXiv2018

A comparative study between a failed and a successful eruption initiated from the same polarity inversion line in AR 11387

In this paper, we analyzed a failed and a successful eruption that initiated from the same polarity inversion line within NOAA AR 11387 on December 25, 2011. They both started from a reconnection between sheared arcades, having distinct pre-eruption conditions and eruption details: before the failed one, the magnetic fields of the core region had a weaker non-potentiality; the external fields had a similar critical height for torus instability, a similar local torus-stable region, but a larger magnetic flux ratio (of low corona and near-surface region) as compared to the successful one. During the failed eruption, a smaller Lorentz force impulse was exerted on the outward ejecta; the ejecta had a much slower rising speed. Factors that might lead to the initiation of the failed eruption are identified: 1) a weaker non-potentiality of the core region, and a smaller Lorentz force impulse gave the ejecta a small momentum; 2) the large flux ratio, and the local torus-stable region in the corona provided strong confinements that made the erupting structure regain an equilibrium state.

astro-ph.SR↗

arXiv2018

Ultra-low-field magneto-elastocaloric cooling in a multiferroic composite device

The advent of caloric materials for magnetocaloric, elastocaloric, and electrocaloric cooling is changing the landscape of solid state cooling technologies with potentials for high-efficiency and environmentally-friendly residential and commercial cooling as well as heating applications. Given that caloric materials are ferroic materials which undergo first (or second) order transitions near room temperature, they open up intriguing possibilities for novel multiferroic devices with hitherto unexplored functionalities coupling their thermal properties with different fields (magnetic, electric, and stress) through composite configurations. Here, we demonstrate a composite magneto-elastocaloric effect with ultra-low magnetic field (0.16 T) in a compact geometry to generate a cooling temperature change as large as 4 K using a magnetostriction/superelastic alloy composite. Such composite systems can be used to circumvent shortcomings of existing technologies such as the need for high-stress actuation mechanism for elastocaloric materials and the high magnetic-field requirement of magnetocaloric materials, while enabling new applications such as compact remote cooling devices.

cond-mat.mtrl-sci↗

arXiv2015

Transition metals doped CuAlSe2 for promising intermediate band materials

Introducing an isolated intermediate band (IB) into a wide band gap semiconductor can potentially improve the optical absorption of the material beyond the Shockley-Queisser limitation for solar cells. Here, we present a systematic study of the thermodynamic stability, electronic structures, and optical properties of transition metals (M = Ti, V, and Fe) doped CuAlSe2 for potential IB thin film solar cells, by adopting the first-principles calculation based on the hybrid functional method. We found from chemical potential analysis that for all dopants considered, the stable doped phase only exits when the Al atom is substituted. More importantly, with this substitution, the IB feature is determined by $3d$ electronic nature of M^{3+} ion, and the electronic configuration of 3d^1 can drive a optimum IB that possesses half-filled character and suitable subbandgap from valence band or conduction band. We further show that Ti-doped CuAlSe2 is the more promising candidate for IB materials since the resulted IB in it is half filled and extra absorption peaks occurs in the optical spectrum accompanied with a largely enhanced light absorption intensity. The result offers a understanding for IB induced by transition metals into CuAlSe2 and is significant to fabricate the related IB materials.

cond-mat.mtrl-sci↗

arXiv2006

Elemental Abundance Measurements in Low-redshift Damped Lyman Alpha Absorbers

We present elemental abundance measurements for 9 damped Ly-alpha systems (DLAs) and 1 sub-DLA at 0.1 < z < 1.5 from recent observations with the Multiple Mirror Telescope. Most of these absorbers are found to be metal-poor, while 2 are found to have ~ 30-50 solar metallicities. Combining our data with other data from the literature, we find that the systems with higher [Zn/H] also have stronger depletion as measured by [Cr/Zn] and [Fe/Zn]. The relationship between the metallicity and H I column density is also investigated. Together with our previous MMT survey (Khare et al. 2004) we have discovered 2 of the 4 known absorbers at z < 1.5 that lie above (although near) the "obscuration threshold". This appears to be a result of selecting absorbers with strong metal lines in our sample. It would be interesting to find other similar systems by observing a larger sample and study how much such systems contribute to the cosmic budget of metals. Finally, an analysis of the N(H I)-weighted mean metallicity vs. redshift for our sample combined with data from the literature supports previous conclusions that the N(H I)-weighted mean global DLA metallicity rises slowly at best and falls short of solar levels by a factor of > 4 even at z=0.

astro-ph↗

arXiv2005

Molecular Hydrogen in the Damped Ly alpha Absorber of Q1331+170

We used HST/STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly$α$ system at $z_{\rm abs}=1.7765$ toward the quasar Q1331+170 at $z_{\rm em}=2.084$. Strong ${\rm H}_2$ absorption was detected, with a total ${\rm H}_2$ column density of $N({\rm H}_2)=(4.45\pm 0.36)\times 10^{19} {\rm cm^{-2}}$.The molecular hydrogen fraction is $f_{{\rm H}_2}=\frac{2N_{\rm H_2}}{N_{\rm HI}+2N_{\rm H_2}}=(5.6\pm 0.7)%$, which is the greatest value reported so far in any redshifted damped Ly$α$ system. This results from the combined effect of a relatively high dust-to-gas ratio, a low gas temperature, and an extremely low ambient UV radiation field. Based on the observed population of $J$ states, we estimate the photo-absorption rate to be $R_{\rm abs}=(7.6\pm 2.4)\times 10^{-13} {\rm s^{-1}}$, corresponding to a local UV radiation field of $J(1000{\rm Å})\approx 2.1\times 10^{-3} J_{1000{\rm Å},\odot}$, where $J_{1000{\rm Å},\odot}$ is the UV intensity at $1000 Å$ in the solar neighborhood. This is comparable with the metagalactic UV background intensity at this redshift, and implies an extremely low star formation rate in the absorber's environment. The observed CO-to-H$_2$ column density ratio is $\frac{N_{\rm CO}}{N_{\rm H_2}}<2.5\times 10^{-7}$, which is similar to the value measured for diffuse molecular clouds in the Galactic ISM. Finally, applying the inferred physical conditions to the observed C I fine structure excitation (Songaila {\it et al.} 1994), we estimate the cosmic microwave background temperature to be $T_{\rm CMB}=(7.2\pm 0.8) {\rm K}$ at $z=1.77654$, consistent with the predicted value of $7.566 {\rm K}$ from the standard cosmology.

astro-ph↗