Research

In my research, I primarily investigate the plasma physics of the solar wind and the solar corona. The solar wind is a tenuous magnetised plasma. A plasma is a gas in which the ions and electrons are separated from each other and do not form neutral atoms like in a regular gas. The moving charged plasma particles modify and react to electric and magnetic fields. This leads to a behaviour that is very different from the behaviour of neutral gases like the air. Many of these processes also apply to other plasma environments in the Universe, reaching from accretion disks around black holes to the intra-cluster medium, which fills the space between galaxies in galaxy clusters.

Under certain conditions, a plasma can be described like a fluid (for example with the equations of magnetohydrodynamics), but often a description based on kinetic physics is necessary. I work on the kinetic aspects of the plasma behaviour in the solar wind. I am especially interested in wave-particle interactions, kinetic waves and instabilities, turbulence, plasma heating, and shock waves.

One of my current major projects is the application of our computer code ALPS, which stands for Arbitrary Linear Plasma Solver. ALPS is a numerical solver for the full hot-plasma dispersion relation in a plasma with arbitrary background distribution functions. The numerical code determines the behaviour of kinetic plasma waves including relativistic effects. More details can be found on http://www.alps.space.

Our solver NHDS (The New Hampshire Dispersion Relation Solver) calculates the linear hot-plasma dispersion relation in a plasma consisting of drifting bi-Maxwellians. The code is publicly available at https://github.com/danielver02/NHDS, and the code paper can be found here.

HolmMHD is a versatile numerical code that solves the nonlinear set of equations of isotropic, polytropic, ideal magnetohydrodynamics (MHD) on a Cartesian grid in three dimensions. It uses a hybrid spatial discretisation based on a fourth-order central scheme and the Rusanov scheme, which are mixed through a min-mod flux limiter.
The code performs well on a range of benchmark problems such as a spherical hydro-blast wave, the Orszag-Tang vortex, Alfvén waves in one and three dimensions, the MHD aligned rotator, an isotropic three-dimensional decaying-turbulence setup, and the Kelvin-Helmholtz instability (shown on the right-hand side). The code is publicly available at https://github.com/danielver02/HolmMHD, and the code paper can be found here.

I am the Mission Co-PI Science for the Debye mission. This mission concept underwent the review process for ESA’s F-class programme and is now the backup for the Comet Interceptor mission. If selected, Debye will answer the science question “How are electrons heated in astrophysical plasmas?”. It will consist of one main spacecraft that measures electrons with very high cadence and resolution, electric fields, magnetic fields, and protons. Then it will have three smaller deployable spacecraft that measure high-frequency fluctuations in the magnetic field. The spacecraft separation will vary from a few hundred metres to a few thousand kilometres during the mission lifetime. In this way, Debye will study the thermodynamics of electrons on small plasma scales in the solar wind. In addition, we have submitted the White Paper “A Case for Electron-Astrophysics” to ESA’s Voyage 2050 programme. In this White Paper, we argue for the importance of electron-scale physics to understand the global plasma evolution in the solar wind and throughout the Universe.

I am a Co-Investigator for the Solar Wind Plasma Analyser (SWA) instrument on board ESA’s Solar Orbiter spacecraft. Solar Orbiter will launch in 2020 and explore the inner heliosphere and its connection with the Sun in great detail. The SWA instrument suite will measure the in-situ properties of protons, electrons, alpha particles, and heavy ions with unprecedented resolution and quality.

Image Credit: NASA’s WIND spacecraft from nasa.gov

Our research on the comparison between kinetic and fluid models of compressive fluctuations in the solar wind has been featured in a UNH press release. We compared predictions for the three lowest velocity moments from gyrokinetic theory with magnetohydrodynamics (MHD) and solar-wind observations from the WIND spacecraft. Surprisingly, the MHD predictions agree better with the observations than the kinetic predictions. Here is the link to the press release:
https://www.unh.edu/unhtoday/news/release/2017/05/15/unh-researcher-identifies-key-differences-solar-wind-models

This movie shows an isosurface of the proton velocity distribution function in the solar wind. It was observed by the Helios spacecraft in the 1970s. The distribution function fully describes the kinetic state of the plasma particles. The little bump on the upper side of this isosurface represents the “proton beam”, which is a bunch of faster protons that stream through the core protons. The upcoming missions Solar Orbiter and Parker Solar Probe will measure distribution functions for protons, electrons, and heavy ions with unprecedented resolution.