03/15/2022
By Lynne Schaufenbil
Short bio: Dag Evensberget is a Norwegian residing in Australia where he recently submitted his Ph.D. thesis at the University of Southern Queensland. Dag’s Ph.D. project “the winds of young Suns” has combined Solar and space physics, astronomy, and numerical simulations. Dag has a background in physics and numerical mathematics and has previously worked as a scientific programmer for the European Space Agency. In his spare time Dag likes skiing, surfing, and reading fiction.
Title: Constraining the history of the Solar wind from stellar magnetic field observations
Abstract: Stars produce stellar wind – a tenuous flow of magnetized plasma extending deep into circumstellar space. It is thought that forceful stellar wind can erode the atmospheres of planets lacking strong magnetic fields, and thus reduce the planets' potential habitability. The gradually slowing rotation of Solar-type stars with increasing age is also caused by the stellar wind.
We present stellar wind maps of a selection of young, Solar-type stars with well characterized ages that have had their surface magnetic field mapped with Zeeman-Doppler imaging. By driving the state-of-the-art Solar wind modelling code SWMF/AWSoM with the stellar surface magnetic fields, we have obtained three-dimensional steady-state wind maps of 25 stars aged between 0.05 and 0.6 billion years.
Unlike later epochs, this age range is characterized by the absence of single valued relations between stellar age, stellar rate of rotation, and measures of stellar magnetic activity. The resulting collection of wind maps is the largest created to date and permits the application of robust statistics.
Based on our wind models, we find that the Earth's magnetic field has protected the Earth from atmospheric erosion by strong Solar winds for at least 4 billion years despite the more powerful winds of the young Sun.
For Solar-type stars younger than about 1/4 billion years, we find that the stellar spin-down resulting from our wind models is insufficient to explain the observed spin-down of stars with increasing age. This suggests that transient phenomena such as coronal mass ejections may account for a large part of these stars' spin-down.
This talk will also touch upon the background and limitations inherent in Zeeman-Doppler imaging, issues when comparing stellar magnetograms and Solar magnetograms, numerical modelling issues encountered with SWMF/AWSoM, and the validity of using Solar-calibrated parameters for progressively younger Solar-type stars.
If you are interested in attending, please contact Lynne_Schaufenbil@uml.edu for the Zoom link.