The solar corona — the tenuous, magnetically structured plasma extending millions of kilometers above the Sun’s surface — is more than a thousand times hotter than the photosphere below it. Maintaining this temperature against radiative and conductive losses demands a continuous energy supply of hundreds of watts per square meter. Identifying and quantifying the mechanisms responsible is one of solar physics’ most celebrated unsolved problems.
The chromosphere, a 2000 km thick partially ionized layer between the photosphere and corona, is where much of this energy transport must occur. It is a collisional plasma where neutral atoms and ions interact frequently, producing a rich set of instabilities that have no analog in the fully ionized corona or laboratory plasmas.
In 2020, Oppenheim, Dimant, Longley, and Fletcher reported in the Astrophysical Journal Letters a previously unknown plasma instability operating in the solar chromosphere: the thermal Farley–Buneman (TFB) instability. This instability arises when electron–neutral and ion–neutral collision rates combine with electron temperature gradients in a magnetized plasma, driving oscillations that grow rapidly and produce turbulence. The discovery drew on our group’s decades of experience studying the same class of instability in Earth’s ionosphere — and opened an entirely new avenue for understanding chromospheric heating.
Subsequent multi-fluid and kinetic PIC simulations by graduate student Samuel Evans and collaborators (2023, 2025, 2026) have confirmed the instability across a broad range of chromospheric conditions, quantified its heating rate, and compared predictions with solar observations from SDO and IRIS.
The TFB instability grows wherever magnetized electrons drift relative to ions and neutrals, which occurs across much of the chromosphere. Our simulations show it generates turbulence on scales of centimeters to tens of meters — far below what current solar telescopes can resolve, but detectable through their integrated heating effect. Multi-fluid simulations (Evans et al. 2023) capture the turbulent energy cascade and heating; fully kinetic PIC simulations (Evans et al. 2025) confirm the fluid results and reveal additional wave-particle physics.
The chromosphere hosts electrons, protons, neutral hydrogen, helium, and heavier species — a genuinely multi-species, partially ionized plasma. Our group develops specialized multi-fluid codes that track each species separately, capturing ambipolar diffusion, ion–neutral chemistry, and differential magnetization. For problems where kinetic effects matter, we deploy our electrostatic and electromagnetic PIC codes, treating each charged particle individually on massively parallel supercomputers.
A key goal is connecting our simulations to solar observations. The TFB instability should produce detectable spectral line broadening, Doppler shifts, and brightness temperature enhancements in chromospheric spectral diagnostics (Ca II, Mg II, Hα). Working with solar observer collaborator Juan Martínez-Sykora (Lockheed Martin Solar and Astrophysics Lab), we compute predicted observational signatures and compare with high-resolution data from IRIS and the upcoming DKIST telescope.
Alongside simulations, Senior Research Scientist Yakov Dimant develops rigorous fluid and kinetic theories of the TFB instability and related phenomena. These analytic results — including unified theories of E×B instabilities in magnetized collisional plasmas — provide the physical intuition that guides simulation design and help explain simulation results in terms of fundamental plasma physics principles.
Collaborative Research: Simulations, Theory and Observations of Plasma Turbulence and Heating in the Solar Chromosphere. Co-investigators include Juan Martínez-Sykora and Yakov Dimant.
Collaborative Research: Heating the Solar Chromosphere through Plasma Turbulence. This earlier grant supported the initial discovery of the TFB instability.
This program offers a rare combination: cutting-edge astrophysics questions, massively parallel computing, analytic theory, and direct comparison with space telescope observations. Graduate students working in this area join a world-leading group and contribute to research that has appeared in the Astrophysical Journal, ApJL, and Physics of Plasmas.
Explore the BU Astronomy PhD program at bu.edu/astronomy/graduate or write to meerso@bu.edu.