arXiv subjects

Runbin Luo

Publications and source records attributed to Runbin Luo.

5 recordsLinked to original sources

arXiv2026

What Causes the Asymmetry of Conjugate Hard X-Ray Footpoints in Solar Flares?

Hard X-ray (HXR) emission in solar flares critically diagnoses nonthermal electron acceleration, transport, and precipitation. Observations commonly show asymmetric HXR photon fluxes between paired footpoints, whose physical origin remains debated, as the conventional magnetic mirroring mechanism often fails to explain the observed asymmetry. Here we performed rigorous statistical tests on the association between photospheric magnetic parameters within the HXR footpoint regions and the asymmetry of HXR production. We analyzed 103 time intervals taken from around the peaks of HXR bursts in 67 M- and X-class flares with a clear double-ribbon morphology observed by both the Ramaty High Energy Solar Spectroscopic Imager and the Solar Dynamics Observatory. We found that conjugate HXR footpoint sources are asymmetric in photon fluxes, maximum intensities, and sizes, but rather symmetric in mean intensities. The photon flux ratio of the stronger over weaker footpoint source shows a strong linear correlation with the size ratio and a nonlinear correlation with the maximum intensity ratio. The asymmetry of magnetic field strength and flux at the conjugate footpoints shows a positive correlation with the HXR asymmetry, contrary to what magnetic mirroring effects predict. Importantly, the asymmetry of unsigned photospheric vertical electric current (PVEC) exhibits a strong positive correlation with the HXR footpoint asymmetry. PVEC at HXR footpoints most likely maps the footprints of coronal current layers where flaring reconnections occur. This tight linkage suggests that the electric-current-associated physical processes, including reconnection-induced electric fields and current-driven micro-turbulence, are at work to modulate the production and precipitation of nonthermal electrons.

astro-ph.SR↗

arXiv2025

High-Resolution Observation of Solar Prominence Plumes Induced by Enhanced Spicular Activity

Solar prominences are the most prominent large-scale structures observed above the solar limb in emission in chromospheric lines but in absorption in coronal lines. At the bottom of prominences often appears a bubble, with plumes occasionally rising from the prominence-bubble interface. The plumes may potentially play an important role in the mass supply and thermodynamic evolution of prominences, but their nature and generation mechanism are elusive. Here we use the high-resolution H-alpha observations obtained by the New Vacuum Solar Telescope (NVST) to investigate a quiescent prominence with bubbles and plumes on 8 November 2022. Within an interval of about two hours, enhanced spicular activity disturb the prominence-bubble interface, producing bursts of small-scale plumes rising through the prominence. Characterized by clustered spicules jetting at higher speeds (sometimes exceeding the typical chromopsheric Alfven speed) and longer life-time (over 15 minutes), the enhanced spicular activity differs markedly from regular spicules. We hence conjecture that the enhanced spicular activity may drive shock waves, which trigger the magnetic Richtmyer-Meshkov instability at the prominence-bubble interface, leading to the formation of small-scale plumes. These observations provide evidence that the enhanced spicular activity plays a potentially important role in the dynamic evolution of bubbles and plumes, thereby participating in the mass supply of solar prominences.

astro-ph.SR↗

arXiv2025

Falling Threads During Solar Filament Eruptions

Mass drainage is frequently observed in solar filaments. During filament eruptions, falling material most likely flows along magnetic field lines, which may provide important clues for the magnetic structures of filaments. Here we study three filament eruptions exhibiting significant mass draining, often manifested as falling threads at a constant speed ranging between 30--300 km/s. We found that most of the falling material lands onto the hooked segments of flare ribbons, only a small fraction lands inside the hooks, and almost none lands onto the straight segments of ribbons. Based on these observations we surmise that before eruptions most of the filament mass is entrained by field lines threading the quasi-separatrix layers (QSLs), which wrap around the filament field and whose footpoints are mapped by the hooked ribbons, and that the magnetic reconnection involving these field lines is the major cause of the mass drainage during eruptions. Additionally, the light curves of the hooked ribbons suggest that during eruptions the earlier (later) QSL boundary of filaments is threaded by mass-loaded (depleted) field lines. By assuming that the constant-speed motion is due to a drag force balancing the gravity, we proposed a simplified analytical model to estimate the density contrast of the falling material. The estimated density contrast is then fed into a numerical model, in which filament threads fall along vertical magnetic field lines through a gravitationally stratified atmosphere. The resultant falling speeds, however, are short of observed values, which calls for further investigations.

astro-ph.SR↗

arXiv2024

High-resolution Observation of Blowout Jets Regulated by Sunspot Rotation

Coronal jets are believed to be the miniature version of large-scale solar eruptions. In particular, the eruption of a mini-filament inside the base arch is suggested to be the trigger and even driver of blowout jets. Here we propose an alternative triggering mechanism, based on high-resolution H-alpha observations of a blowout jet associated with a mini-filament and an M1.2-class flare. The mini-filament remains largely stationary during the blowout jet, except that it is straddled by flare loops connecting two flare ribbons, indicating that the magnetic arcade embedding the mini-filament has been torn into two parts, with the upper part escaping with the blowout jet. In the wake of the flare, the southern end of the mini-filament fans out like neighboring fibrils, indicative of mass and field exchanges between the mini-filament and the fibrils. The blowout jet is preceded by a standard jet. With H-alpha fibrils moving toward the single-strand spire in a sweeping fashion, the standard jet transitions to the blowout jet. The similar pattern of standard-to-blowout jet transition occurs in an earlier C-class flare before the mini-filament forms. The spiraling morphology and sweeping direction of these fibrils are suggestive of their footpoints being dragged by the leading sunspot that undergoes clockwise rotation for over two days. Soon after the sunspot rotation reaches a peak angular speed as fast as 10 deg/hr, the dormant active region becomes flare-productive, and the mini-filament forms through the interaction of moving magnetic features from the rotating sunspot with satellite spots/pores. Hence, we suggest that the sunspot rotation plays a key role in building up free energy for flares and jets and in triggering blowout jets by inducing sweeping motions of fibrils.

astro-ph.SR↗

arXiv2022

Where and how does a decay-index profile become saddle-like?

The decay index of solar magnetic fields is known as an important parameter in regulating solar eruptions from the standpoint of the torus instability. In particular, a saddle-like profile of decay index, which hosts a local torus-stable regime at higher altitudes than where the decay index first exceeds the instability threshold, is found to be associated with some confined or two-step eruptions. To understand the occurrence of such a profile, we employed dipoles to emulate different kinds of photospheric flux distributions. Corroborated by observations of representative active regions (ARs), our major results are: 1) in bipolar configurations the critical height increases away from the AR center along the polarity inversion line (PIL) and its average is roughly half of the centroid distance between opposite polarities; 2) in quadrupolar configurations saddle-like profiles appear above the PIL when the two dipoles oriented in the same direction are significantly more separated in this direction than in the perpendicular direction, and when the two dipoles are oriented differently or have unequal fluxes; 3) saddle-like profiles in quadrupolar configurations are associated with magnetic skeletons such as a null point or a hyperbolic flux tube, and the role of such profiles in eruptions is anticipated to be double-edged if magnetic reconnection is involved.

astro-ph.SR↗