arXiv subjects

Ryan Golant

Publications and source records attributed to Ryan Golant.

3 recordsLinked to original sources

arXiv2025

Intermittency in Collisionless Large-Amplitude Turbulence

Large-amplitude turbulence -- characterized by a fluctuating magnetic field component, $\delta B$, that is stronger than the mean component, $B_0$ -- is generically intermittent, populated with intense localized structures such as sharp field-line bends and rapid field reversals. Recent MHD simulations suggest that these structures play an important role in particle transport and acceleration; however, MHD is inapplicable in most of our Universe, where the plasma is so hot or diffuse that Coulomb collisions are negligible. Therefore, in this paper, we analyze the intermittent properties of collisionless large-amplitude turbulence in electron-positron plasmas via fully kinetic 3D simulations, exploring a wide range of $\delta B / B_0$ and scale separations between the turbulence driving scale, $L$, and kinetic scales, $c/\omega_{\rm p}$. The steady-state collisionless turbulence in our simulations broadly resembles that of MHD, but the development of pressure anisotropy steepens the scaling between magnetic field strength, $B$, and scalar field-line curvature, $K_\parallel$ -- yielding $B \propto K_\parallel^{-3/4}$ -- and consequently modifies the power-law slope of the probability density function of $K_\parallel$; this slope hardens from $K_\parallel^{-2.5}$ to $K_\parallel^{-2.0}$ as $\delta B / B_0$ increases from 4 to 140. Pressure anisotropy also triggers mirror and firehose instabilities, with the volume-filling fractions of these fluctuations increasing with $\delta B / B_0$; for our largest $\delta B / B_0$, $20\%$ of the volume is mirror-unstable and $6\%$ is firehose-unstable. Both the curvature and the Larmor-scale fluctuations in collisionless large-amplitude turbulence are expected to significantly influence cosmic ray transport and acceleration in the interstellar medium of our Galaxy and the intracluster medium of galaxy clusters.

astro-ph.HE↗

arXiv2024

Generation of Large-scale Magnetic Fields Upstream of Gamma-Ray Burst Afterglow Shocks

The origins of the magnetic fields that power gamma-ray burst (GRB) afterglow emission are not fully understood. One possible channel for generating these fields involves the pre-conditioning of the circumburst medium: in the early afterglow phase, prompt photons streaming ahead of the GRB external shock can pair produce, seeding the upstream with drifting electron-positron pairs and triggering electromagnetic microinstabilities. To study this process, we employ 2D periodic particle-in-cell simulations in which a cold electron-proton plasma is gradually enriched with warm electron-positron pairs injected at mildly relativistic speeds. We find that continuous pair injection drives the growth of large-scale magnetic fields via filamentation-like instabilities; the temporal evolution of the field is self-similar and depends on a single parameter, $\left[\alpha/(t_{\rm f} \omega_{\rm pi})\right]^{1/2} t\omega_{\rm pi}$, where $\alpha$ is the ratio of final pair beam density to background plasma density, $t_{\rm f}$ is the duration of pair injection, and $\omega_{\rm pi}$ is the plasma frequency of background protons. Extrapolating our results to parameter regimes realistic for long GRBs, we find that upstream pair enrichment generates weak magnetic fields on scales much larger than the proton skin depth; for bright bursts, the extrapolated coherence scale at a shock radius of $R \sim 10^{17} \, {\rm cm}$ is $\left<\lambda_y\right> \sim 100 \; c/\omega_{\rm pi}$ and the corresponding magnetization is $\sigma \sim 10^{-8}$ for typical circumburst parameters. These results may help explain the persistence of magnetic fields at large distances behind GRB shocks.

astro-ph.HE↗

arXiv2023

Improving Undergraduate Astronomy Students' Skills with Research Literature via Accessible Summaries: An Exploratory Case Study with Astrobites-based Reading Assignments

Undergraduate physics and astronomy students are expected to engage with scientific literature as they begin their research careers, but reading comprehension skills are rarely explicitly taught in major courses. We seek to determine the efficacy of a reading assignment designed to improve undergraduate astronomy (or related) majors' perceived ability to engage with research literature by using accessible summaries of current research written by experts in the field. During the 2022-2023 academic year, faculty members from six institutions incorporated reading assignments using accessible summaries from Astrobites into their undergraduate astronomy major courses, surveyed their students before and after the activities, and participated in follow-up interviews with our research team. Quantitative and qualitative survey data from 52 students show that students' perceptions of their abilities with jargon and identifying main takeaways of a paper significantly improved with use of the tested assignment template. Additionally, students report increased confidence of their abilities within astronomy after exposure to these assignments, and instructors valued a ready-to-use resource to incorporate reading comprehension in their pedagogy. This exploratory case study with Astrobites-based assignments suggests that incorporating current research in the undergraduate classroom through accessible literature summaries may increase students' confidence and ability to engage with research literature, assisting in their preparation for participation in research careers.

physics.ed-ph↗