BOSTON UNIVERSITY
Department of Astronomy
Center for Space Physics

Solar Chromosphere Physics

A newly discovered plasma instability, massively parallel simulations, and analytic theory address a long-standing puzzle: what heats the solar chromosphere, where the temperature mysteriously reverses and climbs back up after falling below 4000 K?

Heating the Solar Chromosphere — and a New Mechanism

Full-disk ultraviolet image of the Sun at 304 angstroms, glowing red-orange, showing the structured chromosphere and a large prominence.
The Sun at 304 Å. This ultraviolet image from NASA’s Solar Dynamics Observatory captures light from roughly 50,000 K plasma in the chromosphere and transition region — the thin, partially ionized layer where we found the thermal Farley–Buneman instability can operate. The filamentary structure and prominences trace magnetic fields threading this turbulent, collisional plasma. Image: NASA/SDO (public domain).

Above the Sun’s visible surface, the temperature of the solar atmosphere does not simply rise — it changes in steps that are still not fully understood. Moving up from the roughly 5800 K photosphere, the temperature first falls through the lower chromosphere over roughly the first 500–2000 km. Then, in the mid-chromosphere, it reverses: the temperature jumps from as low as ~3800 K to over 10,000 K for reasons that remain unknown. It stays roughly constant for a few thousand kilometers, and only then, at the thin transition region, does it climb steeply to more than a million kelvin in the corona above.

Our group focuses on the chromosphere itself — and in particular on that mysterious heating in the mid-chromosphere that reverses the temperature decline. We do not study the separate problem of how the corona reaches its million-degree temperatures. The chromosphere is a collisional plasma, a partially ionized layer where neutral atoms and ions interact frequently, producing a rich set of instabilities with some similarities to the lower ionosphere.

★ Discovery: The Thermal Farley–Buneman Instability in the Sun

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.

Single-fluid temperature across a 2.5D Bifrost simulation of a solar plage region, with black contours outlining where the thermal Farley-Buneman instability can operate.
Where the thermal Farley–Buneman instability can operate in the chromosphere. Single-fluid temperature (TSF) across a 2.5D Bifrost radiative-MHD simulation of a solar plage region, plotted against horizontal position x and height z. Black contours outline the regions where conditions favor the TFB instability. Maps like this let our group predict where TFBI-driven turbulence and heating occur throughout the chromosphere. From Evans et al. (2026), Fig. 1.

Research Topics

Thermal Farley–Buneman Turbulence

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.

Multi-Fluid and Kinetic PIC Modeling

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.

Predicting Observational Signatures

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.

Analytic Theory of Collisional Plasma Instabilities

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.

Selected Publications

Chromospheric Research

This work combines astrophysics questions, massively parallel computing, analytic theory, and comparison with space-telescope observations, with results appearing in the Astrophysical Journal, ApJL, and Physics of Plasmas.

Research in the group runs through the BU Astronomy PhD program: bu.edu/astronomy/graduate.

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