Bart Ripperda
Institute for Advanced Study
School of Natural Sciences
1 Einstein Drive, Princeton, NJ 08540, USA
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
IAS homepage:
https://www.ias.edu/scholars/bart-ripperda
Flatiron homepage:
https://www.simonsfoundation.org/team/bart-ripperda/
Black Hole Accretion Code:
https://bhac.science
Git repository:
https://gitlab.com/BHAC-Developers/bhac
MPI-AMRVAC:
http://amrvac.org
Git repository:
https://github.com/amrvac/amrvac
Event Horizon Telescope:
https://eventhorizontelescope.org
Hello! I am a NASA Hubble/Einstein fellow at the Institute for Advanced Study in Princeton, and next year I will start as an assistant professor at the Canadian Institute for Theoretical Astrophysics at the University of Toronto. I am interested in theoretical and computational astrophysics and fundamental plasma physics with applications in black holes and neutron stars. I am a developer of both the general relativistic magnetohydrodynamics code BHAC (Black Hole Accretion Code) and its Newtonian basis MPI-AMRVAC. During my PhD (for which I won the EPS PHD prize in plasma physics) in Leuven with Rony Keppens I implemented a general relativistic resistive magnetohydrodynamics algorithm, as well as a general relativistic particle method. With this code I try to simulate how a turbulent gas of charged particles, called a plasma, behaves around a black hole, as you can see in the image below or instead you can listen to my recent talk at Harvard or to my talk at the AAS HEAD seminar! You can read more about my research interests here and if you are really interested you can find a list of publications here. Below I briefly describe some projects I am currently working on, using BHAC.
I am also a member of the Event Horizon Telescope (EHT) collaboration. On April 10th 2019 we unveiled the first direct visual evidence of a supermassive black hole and its shadow, that you can see here below! We recently won the breakthrough prize in Fundamental Physics and the Einstein Medal (click here for a small interview I did) for this image! I am mainly interested in comparing results of my simulations to observations of the EHT.
I am mostly interested in adding more physics to our models (for example radiation, quantum electrodynamics effects, or kinetic physics). I am always trying to improve my code to describe ever-more complicated astrophysical systems or fundamental plasma physics problems. The latest addition to BHAC is a coupled particle integrator, dynamically switching to a guiding center approximation in appropriate regimes where gyration is negligible. Another novel idea we are exploring is to couple resistive magnetohydrodynamics as a description of dissipation zones, with a force-free approximation in the highly magnetized ambient. Together with the adaptive mesh refinement abilities of BHAC we are able to accurately resolve plasmoid formation in a highly magnetised magnetosphere. In the example below the adaptive mesh refines (left) around the current sheet that formed around a black hole in an external Wald field. A large part of the domain is highly magnetised (the red regions on the right side), such that a force-free description is appropriate here and the more expensive resistive magnetohydrodynamics evolution is restricted to the current sheet.
I am also interested in fundamental problems in plasma physics. Such problems can help to figure out which theories, for example magnetohydrodynamics or kinetic physics, are applicable in which regimes. My main goal is to learn from first-principle descriptions how we can incorporate kinetic effects in global magnetohydrodynamics simulations. One nice example where both magnetohydrodynamics and kinetic effects are important is the formation of plasmoids due to the tearing instability in a current sheet (see the extreme resolution resistive magnetohydrodynamics simulation in the image below). By comparing the results of kinetic simulations to magnetohydrodynamics results we can determine how to resolve the discrepancies between the two descriptions. Plasmoid-unstable current sheets are ubiquitous in black hole accretion disks (see the first image, above), jets, and neutron star magnetospheres. By learning from fundamental plasma physics we can improve our understanding of the microphysics in such high-energy astrophysical systems.
In real astrophysical systems dynamics obviously occur in three dimensions. And in 3D the plasmoids that form in a current sheet become elongated flux tubes, that can grow, interact, and merge. This chaotic process can result into turbulence and subsequent particle acceleration by the turbulent dissipation. In the image below you can see how a current sheet looks like in a full 3D simulation.
Flux tubes themselves can become kink-unstable, and curl up, as you can see in the image above. Zooming in on such a flux tube, we can study the evolution of the kink instability, and the development of current sheets at the edges of the tube. In the image below you see a simulation of a single kink-unstable flux tube, that can serve as a model for a black hole jet, or for typical magnetic structures forming in black hole coronae.
Bart Ripperda 