Licentiatseminarium: Multi-spacercraft Studies of Electron Holes in Space Plasmas
- Datum: –12.00
- Plats: Ångströmlaboratoriet Å80101
- Föreläsare: Konrad Steinvall
- Arrangör: Institutionen för fysik och astronomi, avdelningen för astronomi och rymdfysik
- Kontaktperson: Yuri Khotyaintsev
- Telefon: 018-4715929
Magnetic reconnection is a fundamental plasma process in which magnetic energy is explosively converted into plasma thermal and kinetic energy. During magnetic reconnection, the magnetic field topology changes, allowing plasma transport across boundaries. This is important for coupling the Earth’s magnetosphere to the solar wind. Although extensively studied, magnetic reconnection remains poorly understood at the scales of the electron inertial length and gyroradius. In this thesis, we discuss a particular type of plasma wave, electron phase-space holes (EHs), as they are regularly observed during magnetic reconnection, and have been shown to influence the process.
We use data from NASA’s Magnetospheric Multiscale (MMS) mission which allows us to investigate EHs in unprecedented detail thanks to the high-quality instruments and the electron scale spacecraft separation. We investigate electrostatic EHs at the magnetopause and electromagnetic EHs in the plasma sheet boundary layer. By timing the EHs between the four spacecraft we calculate their velocity, parallel length scale, and potential with higher accuracy than previous missions have allowed. Furthermore, we develop and use a method for estimating the perpendicular length scale and the center potential of the EHs. Our results show that the properties of the observed EHs are consistent with previous observations and theory, and by estimating the perpendicular length scale of individual EHs, we find the structures to be oblate. By computing the perpendicular length scale and center potential of electromagnetic EHs, we are able to estimate the magnetic field contribution of three separate mechanisms. We find that for EHs with speeds much smaller than the electron Alfv ́en speed, the measured magnetic field is consistent with a magnetic field generated by electrons E × B drifting around the EH; for faster speeds, the contribution of Cherenkov emitted whistler waves becomes important. We present the first, direct, observational evidence of EHs radiating whistler waves via the Cherenkov mechanism. For the plasma conditions observed, the Cherenkov resonant waves were strongly damped, and consequently localized to the EHs.
In summary, our results allow us to better quantify the electrostatic as well as electromagnetic properties of EHs. Our results also show that the Cherenkov emission of whistler waves from EHs (which is an often neglected generation mechanism) occur in physical space plasmas.