Multipoint measurements of solar wind magnetic holes
Time: Thu 2025-04-03 13.00
Location: F3 (Flodis), Lindstedtsvägen 26 & 28, Stockholm
Video link: https://kth-se.zoom.us/j/66900145957
Language: English
Subject area: Electrical Engineering
Doctoral student: Henriette Trollvik , Rymd- och plasmafysik
Opponent: Dr. Jan Soucek, Institute of Atmospheric Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic
Supervisor: Professor Tomas Karlsson, Rymd- och plasmafysik
QC 20250312
Abstract
Magnetic holes (MH) are localized depressions of the magnetic field. They are found in various plasma environments, most commonly in the solar wind. The magnetic field direction sometimes varies across the hole, and it is common to divide MH into two categories, linear and rotational, where the first shows little to no rotation of the field. Linear MH are often in pressure balance with the surrounding plasma and can survive for long periods. It is unknown exactly where and how they are formed, but there is strong evidence that they are related to the mirror instability and waves associated with this instability. Rotational MH, on the other hand, appear as current sheet-like structures and are suggested to be related to magnetic reconnection or possibly in connection with wave activity. Both linear and rotational MH have similar properties, such as magnetic field magnitude and temporal scales. In this thesis, we use the Cluster mission, which has four spacecraft moving closely together providing multipoint measurements, to understand the phenomenon of magnetic holes better. Cluster provides over 20 years of data, with varying spacecraft spacing. Sometimes, the spacing of the satellites is such that one satellite is located in the solar wind while another is in the magnetosheath. In these instances, we have identified the same MH in the solar wind and later in the magnetosheath, suggesting they can cross the bowshock (Paper I). Using the Cluster database, we have identified times when the same MH was observed with all four spacecraft. Applying a timing analysis on a subset of these, we have derived their velocity, and conclude that they are convected with the solar wind speed (Paper II). When the same MH is observed by all four satellites in the solar wind, based on the spacecraft separation, one has four measuring points separated by distances comparable to the size of the MH. These observations can be combined with a local coordinate transformation and a model to estimate the scales of the MH and derive information about the three-dimensional morphology of MH (PaperIII). A large number of events were identified, and the results were combined to provide a statistical analysis of the morphology of solar wind magneticholes (Paper IV).