arXiv subjects

Mehdi Yousefzadeh

Publications and source records attributed to Mehdi Yousefzadeh.

9 recordsLinked to original sources

arXiv2026

Plasmoid-Trapped Condensation Associated with a Transient Thermal-Instability-Like Process in Chromospheric Magnetic Reconnection

We perform high-resolution 2.5D single-fluid magnetohydrodynamic (MHD) simulations to explore the development of cool-dense structures within plasmoids during chromospheric magnetic reconnection. The model incorporates temperature-dependent ionization degrees of hydrogen and helium, resulting in improved diffusivities and viscosity, as well as adequate radiative cooling. The numerical results show that plasma is significantly hot with temperatures reaching several tens of thousands of Kelvin in newly developed plasmoids in the low initial plasma-$β$ cases, $β_0$ = 0.05. Later, the localized accumulation of plasma and decreasing temperature result in an explosive much stronger radiative cooling process. Radiative cooling peaks over the temperature range 8000 K$-$20,000 K. The co-spatial density enhancement, substantial radiative cooling, shorter radiative cooling timescales, and a significant temperature decrease support the existence of thermal-instability-like condensation within the plasmoids. Similar cool-dense structures are identified in all low-$β_0$ cases at different chromospheric altitudes, suggesting that this occurrence is not limited to a single chromospheric layer. In contrast, in the high initial plasma-$β$, $β_0$ = 0.5 case, the maximum temperature inside the dense plasmoids is only about 8000 K. Therefore, the radiative cooling has never entered into the explosive increasing stage and the thermal-instability-like condensation does not happen. These findings reveal that plasmoid-trapped condensation may develop in chromospheric reconnection once radiative losses become self-amplifying under low-$β_0$ conditions.

astro-ph.SR↗

arXiv2026

Kinetic Processes to Radio Burst: First Observational-driven Study in Coronal Loops

Understanding the origin of coherent solar radio bursts requires linking macroscopic coronal structures with the kinetic processes responsible for wave generation. We investigate emissions from slowly positively drifting bursts (SPDBs), a specific type of solar radio emission. SPDBs provide observational constraints for modeling beam-plasma interactions in coronal loops, serving as a basis for a multiscale description. We employ a three--stage numerical framework that combines nonlinear force--free field (NLFFF) magnetic extrapolation, guiding--center simulations, and fully kinetic particle--in--cell (PIC) modeling. The background plasma density is described using a hydrostatic model consistent with active--region conditions, producing a plasma--frequency gradient comparable to that inferred from the observed SPDBs spectrum. Energetic electrons injected near the loop top evolve through magnetic mirroring, pitch--angle scattering, turbulence development, and partial precipitation. Evolved velocity distribution functions (EVDFs) are sampled after approximately one bounce period and used in PIC simulations to evaluate emission properties. The results show that the evolved beam distribution of energetic electrons predominantly excites beam--Langmuir waves and fundamental plasma emission along the loop, with the emission intensity gradually decreasing from the loop top toward the footpoint. The modest initial beam velocity of energetic electrons explains the inefficient generation of harmonic plasma emission. The temporal evolution of the modeled emission reproduces key SPDB characteristics, including the ~ 4s duration and frequency drift behavior. These results suggest plasma emission explains the mechanisms behind SPDB generation and demonstrate the feasibility of a unified model connecting coronal magnetic topology, particle transport, and radio emission.

astro-ph.SR↗

arXiv2026

Coronary Artery Segmentation and Vessel-Type Classification in X-Ray Angiography

X-ray coronary angiography (XCA) is the clinical reference standard for assessing coronary artery disease, yet quantitative analysis is limited by the difficulty of robust vessel segmentation in routine data. Low contrast, motion, foreshortening, overlap, and catheter confounding degrade segmentation and contribute to domain shift across centers. Reliable segmentation, together with vessel-type labeling, enables vessel-specific coronary analytics and downstream measurements that depend on anatomical localization. From 670 cine sequences (407 subjects), we select a best frame near peak opacification using a low-intensity histogram criterion and apply joint super-resolution and enhancement. We benchmark classical Meijering, Frangi, and Sato vesselness filters under per-image oracle tuning, a single global mean setting, and per-image parameter prediction via Support Vector Regression (SVR). Neural baselines include U-Net, FPN, and a Swin Transformer, trained with coronary-only and merged coronary+catheter supervision. A second stage assigns vessel identity (LAD, LCX, RCA). External evaluation uses the public DCA1 cohort. SVR per-image tuning improves Dice over global means for all classical filters (e.g., Frangi: 0.759 vs. 0.741). Among deep models, FPN attains 0.914+/-0.007 Dice (coronary-only), and merged coronary+catheter labels further improve to 0.931+/-0.006. On DCA1 as a strict external test, Dice drops to 0.798 (coronary-only) and 0.814 (merged), while light in-domain fine-tuning recovers to 0.881+/-0.014 and 0.882+/-0.015. Vessel-type labeling achieves 98.5% accuracy (Dice 0.844) for RCA, 95.4% (0.786) for LAD, and 96.2% (0.794) for LCX. Learned per-image tuning strengthens classical pipelines, while high-resolution FPN models and merged-label supervision improve stability and external transfer with modest adaptation.

cs.CV↗

arXiv2025

GraphDerm: Fusing Imaging, Physical Scale, and Metadata in a Population-Graph Classifier for Dermoscopic Lesions

Introduction. Dermoscopy aids melanoma triage, yet image-only AI often ignores patient metadata (age, sex, site) and the physical scale needed for geometric analysis. We present GraphDerm, a population-graph framework that fuses imaging, millimeter-scale calibration, and metadata for multiclass dermoscopic classification, to the best of our knowledge the first ISIC-scale application of GNNs to dermoscopy. Methods. We curate ISIC 2018/2019, synthesize ruler-embedded images with exact masks, and train U-Nets (SE-ResNet-18) for lesion and ruler segmentation. Pixels-per-millimeter are regressed from the ruler-mask two-point correlation via a lightweight 1D-CNN. From lesion masks we compute real-scale descriptors (area, perimeter, radius of gyration). Node features use EfficientNet-B3; edges encode metadata/geometry similarity (fully weighted or thresholded). A spectral GNN performs semi-supervised node classification; an image-only ANN is the baseline. Results. Ruler and lesion segmentation reach Dice 0.904 and 0.908; scale regression attains MAE 1.5 px (RMSE 6.6). The graph attains AUC 0.9812, with a thresholded variant using about 25% of edges preserving AUC 0.9788 (vs. 0.9440 for the image-only baseline); per-class AUCs typically fall in the 0.97-0.99 range. Conclusion. Unifying calibrated scale, lesion geometry, and metadata in a population graph yields substantial gains over image-only pipelines on ISIC-2019. Sparser graphs retain near-optimal accuracy, suggesting efficient deployment. Scale-aware, graph-based AI is a promising direction for dermoscopic decision support; future work will refine learned edge semantics and evaluate on broader curated benchmarks.

cs.CV↗

arXiv2025

The Efficiency of Harmonic Emissions Excited by Energetic Electrons in Coronal Loops

Magnetic reconnection is a key process that drives the energy release in solar flares. This process can occur at multiple locations along the coronal loop. The reconnection generates energetic electrons capable of exciting wave modes and emissions as they propagate through the loop. In this follow-up study, we investigate the influence of the injection site location of these energetic electrons - either at the looptop (LT) or at the leg of the loop around a footpoint (FP) - on the excitation of wave modes especially the second harmonic emissions (X2) in coronal loops. Our simulations reveal that the injection location significantly impacts the spatial distribution and intensity of excited wave modes. When electrons are injected at the LT, electromagnetic X2, and Z modes dominate along the loop, with minimal excitation of Langmuir waves (Yousefzadeh et al. 2021; 2022). Conversely, the present study reveals that injection close to FP leads to a strong Langmuir wave excitation throughout the loop, particularly as electrons ascend toward the LT. We find that X2 and Z modes are consistently excited at the injection site with different intensities, regardless of the injection location. However, electron injection near the FP scenario creates favorable conditions for significant Langmuir wave generation, potentially leading to plasma emission under specific circumstances. These findings emphasize the importance of electron injection location in determining the properties of the excited and emitted waves in solar coronal loops.

astro-ph.SR↗

arXiv2025

Emission Characteristics of Energetic Electrons with Crescent-shaped Velocity Distributions

Solar flares release magnetic energy through reconnection, accelerating electrons into nonthermal velocity distributions, including crescent-shaped electron populations. These energetic electron distributions are crucial in driving instabilities which can lead to distinct electromagnetic emissions. This study investigates the emission properties of crescent-shaped electron velocity distribution functions (EVDFs) under different frequency ratios ($ω_{pe}/Ω_{ce}$), critical for understanding plasma conditions in various astrophysical environments, by comparing the emissions and intensities of waves among different cases. Here, we study and analyze three distinct frequency ratio conditions (2.2, 10, and 1, designated as cases A, B, and C, respectively). We found that the beam-Langmuir (BL) and upper-hybrid (UH) modes can be efficiently excited, leading to further plasma emissions in different cases. Our study reveals that the fundamental (O/F) emission can reach a maximum value of $\sim$$10^{-4} E_{\mathrm{k}0}$, while the harmonics (H) can extend to $\sim$$1.5 \times 10^{-5} E_{\mathrm{k}0}$ depending on the frequency ratio of the environment. The intensity of the fundamental mode exceeds previous findings for pure-ring, beam, and ring-beam distributions, highlighting the impact of crescent-shaped electron velocity distributions on wave excitation and emission processes. This effect is notably influenced by different frequency ratios, offering new insights into the way that nonthermal electron distributions affect the plasma emission process.

astro-ph.SR↗

arXiv2022

Harmonic Electron Cyclotron Maser Emission along the Coronal Loop

Efficient radiation at second and/or higher harmonics of Wce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission mechanism when it is applied to solar radio bursts, such as spikes. In our earlier study, we developed a three-step numerical scheme to connect the dynamics of energetic electrons within a large-scale coronal loop structure with the microscale kinetic instability energized by the obtained nonthermal velocity distribution and found that direct and efficient harmonic X-mode (X2 for short) emission can be achieved due to the strip-like features of the distribution. That study only considered the radiation from the loop top at a specific time. Here we present the emission properties along the loop at different locations and timings. We found that, in accordance with our earlier results, few to several strip-like features can appear in all cases, and the first two strips play the major role in exciting X2 and Z (i.e., the slow extraordinary mode) that propagate quasi-perpendicularly. For the four sections along the loop, significant excitation of X2 is observed from the upper two sections, and the strongest emission is from the top section. In addition, significant excitation of Z is observed for all loop sections, while there is no significant emission of the fundamental X mode. The study provides new insight into coherent maser emission along the coronal loop structure during solar flares.

astro-ph.SR↗

arXiv2021

Harmonic maser emissions from electrons with loss-cone distribution in solar active regions

Electron cyclotron maser emission (ECME) is regarded as a plausible source for the coherent radio radiations from solar active regions (e.g., solar radio spikes). In this Letter, we present a 2D3V fully kinetic electromagnetic particle-in-cell (PIC) simulation to investigate the wave excitations and subsequent nonlinear processes induced by the energetic electrons in the loss-cone distribution. The ratio of the plasma frequency to the electron gyrofrequency $ω_{pe}/Ω_{ce}$ is set to 0.25, adequate for solar active region conditions. As a main result, we obtain strong emissions at the second-harmonic X mode (X2). While the fundamental X mode (X1) and the Z mode are amplified directly via the electron cyclotron maser instability, the X2 emissions can be produced by the nonlinear coalescence between two Z modes and between Z and X1 modes. This represents a novel generation mechanism for the harmonic emissions in plasmas with a low value of $ω_{pe}/Ω_{ce}$, which may resolve the escaping difficulty of explaining solar radio emissions with the ECME mechanism.

astro-ph.SR↗

arXiv2021

Harmonic elctron-cyclotron maser emissions driven by energetic electrons of the horseshoe distribution with application to solar radio spikes

Content. Electron-cyclotron maser emission (ECME) is the favored mechanism for solar radio spikes and has been investigated extensively since the 1980s. Most studies relevant to solar spikes employ a loss-cone-type distribution of energetic electrons, generating waves mainly in the fundamental X/O mode (X1/O1), with a ratio of plasma oscillation frequency to electron gyrofrequency ($ω_ {pe}/Ω_{ce}$) lower than 1. Despite the great progress made in this theory, one major problem is how the fundamental emissions pass through the second-harmonic absorption layer in the corona and escape. This is generally known as the escaping difficulty of the theory. Aims. We study the harmonic emissions generated by ECME driven by energetic electrons with the horseshoe distribution to solve the escaping difficulty of ECME for solar spikes. Methods. We performed a fully kinetic electromagnetic PIC simulation with $ω_ {pe}/Ω_{ce}$ = 0.1, corresponding to the strongly magnetized plasma conditions in the flare region, with energetic electrons characterized by the horseshoe distribution. We also varied the density ratio of energetic electrons to total electrons ($n_e/n_0$) in the simulation. Results. We obtain efficient amplification of waves in Z and X2 modes, with a relatively weak growth of O1 and X3. With a higher-density ratio, the X2 emission becomes more intense, and the rate of energy conversion from energetic electrons into X2 modes can reach $\sim$0.06% and 0.17%, with $n_e/n_0$= 5% and 10%, respectively. Conclusions. We find that the horseshoe-driven ECME can lead to an efficient excitation of X2 and X3 with a low value of $ω_ {pe}/Ω_{ce}$, providing novel means for resolving the escaping difficulty of ECME when applied to solar radio spikes. The simultaneous growth of X2 and X3 can be used to explain some harmonic structures observed in solar spikes.

astro-ph.SR↗