Recent Research on Black Holes
Generation of large-scale magnetic fields through Kelvin-Helmholtz instability
Magnetic field generation via large-scale unstable shear flows. (a) The large-scale component of the magnetic field quasi-cyclically reverses, analogous to how the Sun’s magnetic field switches polarity every 11 years. (b) The magnetic fields are generated via the non-traditional Upsilon-effect—a mean-field dynamo process postulated in 1990 and validated here for the first time. (c) The Upsilon-effect is driven by nonlinear jets induced by large-scale unstable flow.
Magnetic field generation via large-scale unstable shear flows. (a) The large-scale component of the magnetic field quasi-cyclically reverses, analogous to how the Sun’s magnetic field switches polarity every 11 years. (b) The magnetic fields are generated via the non-traditional Upsilon-effect—a mean-field dynamo process postulated in 1990 and validated here for the first time. (c) The Upsilon-effect is driven by nonlinear jets induced by large-scale unstable flow.
Large-scale dynamos driven by shear-flow-induced jets, B. Tripathi, A. E. Fraser, P. W. Terry, E. G. Zweibel, M. J. Pueschel, R. Fan, 2026
The origin of cosmic magnetic fields remains an open problem in astrophysics. In 1955, Eugene Parker introduced mean-field dynamo theory by parameterizing the effects of small-scale turbulence. Although this framework successfully reproduces observed large-scale magnetic fields, it relies on parameters that are difficult to constrain from first principles.
A recent article published in Nature, led by Bindesh Tripathi (Columbia) in collaboration with Ellen Zweibel (UW–Madison), addresses the generation of large-scale magnetic fields in unstable shear flows. The authors develop an analytic theory and perform advanced three-dimensional simulations of magnetohydrodynamic turbulence with resolutions up to 4,096 × 4,096 × 8,192 grid points. Their results demonstrate the ab initio emergence of quasi-periodic, large-scale magnetic fields. The mechanism operates through the mean-vorticity effect—an additional mean-field dynamo process proposed in 1990—and critically depends on the formation of large-scale, three-dimensional nonlinear jets.
Predictions from the jet-driven dynamo are confirmed using data from a shear-driven laboratory dynamo experiment. Researchers are currently investigating implications of this discovery to a variety of astrophysical systems, including accretion flows and mergers of compact objects.
Kinetically Informed Reconnection in Black Hole Magnetospheres
Left: The reconnection electric field, indicating a reconnection rate of 0.1 for the new resistivity model (top half), and instead 0.03 for uniform resistivity (bottom half). Right: A zoom into the resistive electric field in the X-point where reconnection occurs (top for the new resistivity model, bottom for uniform) as indicated by the silver rectangle on the left. White lines show magnetic field lines.
Left: The reconnection electric field, indicating a reconnection rate of 0.1 for the new resistivity model (top half), and instead 0.03 for uniform resistivity (bottom half). Right: A zoom into the resistive electric field in the X-point where reconnection occurs (top for the new resistivity model, bottom for uniform) as indicated by the silver rectangle on the left. White lines show magnetic field lines.
Magnetic reconnection with a 0.1 rate: Effective resistivity in general relativistic magnetohydrodynamics, B. Ripperda, M. P. Grehan, A. Moran, Selvi, L. Sironi, A. Philippov, A. Bransgrove, and O. Porth, 2026
Relativistic magnetic reconnection powers high-energy emission from neutron stars and black holes. Standard magnetohydrodynamics models, however, underestimate energy conversion rates when using a small uniform resistivity, compared to first principles kinetic (PIC) simulations.
This work led by Bart Ripperda and Michael Grehan (CITA) in collaboration with Lorenzo Sironi (Columbia), Alexander Philippov (UMD), and other collaborators, introduces an effective resistivity model, inspired by first-principles kinetic simulations, that links the reconnection electric field to charge-starved current density. With this approach, resistive relativistic MHD reproduces the fast reconnection rates seen in kinetic models — both in local current sheets and in global systems.
In simulations of a current sheet in a black hole magnetosphere, the new model yields reconnection rates ≳0.1 (in agreement with general relativistic PIC results, and an order of magnitude faster than with a uniform resistivity), which can power fast flares and ejection of magnetic flux.
This framework enables realistic global 3D resistive magnetohydrodynamic simulations of black hole and neutron star magnetospheres, jets, and accretion flows, while capturing key kinetic reconnection physics that can influence the flow dynamics without requiring a fully kinetic simulation.