The Kristian Birkeland Medal 2017

Dr. Bojan Vršnak

Dr. Bojan Vršnak has worked on various aspects of solar eruptive phenomena and their space weather impact, from observational as well as from theoretical side. These phenomena under study include solar flares, flux ropes and coronal mass ejections (CMEs), the role of magnetic reconnection in flare/CME events as well as the interplanetary propagation of CMEs. He has published in total more than 180 papers in international peer reviewed journals, 60 of them as lead author.

The research of Dr. Vršnak uniquely combines fundamental research of the physics of solar eruptive events, and how to transfer this knowledge to the applicability and development of models to better predict their space weather effects at Earth. This combination is most obvious in the development and improvements of the drag based model of interplanetary propagation of CMEs, of which Dr. Vršnak is one of the fundamental developers. This model is an important space weather tool as it allows to predict CME arrival and to study CME-CME interaction in the heliosphere.

In his early works Dr. Vršnak has focused on the role of flux ropes in solar eruptions and studies of their stability. On the one hand, he studied helical structures in prominences and how we can determine from these observations of twist whether a prominence will eruptive or not. On the other hand, he also analytically modeled the stability of cylindrical flux rope and their twisted fields. These influential studies had been done at a time where the concept of flux ropes and their importance for solar eruptions was by far not established. Today we know that flux ropes are the fundamental building blocks of solar eruptions, and that they are part of any CME model, either as pre-existing structures or being formed during the eruption via magnetic reconnection. Another area of space weather related research to which Dr. Bojan Vršnak has substantially contributed is the role and physics of shock waves in the corona. These have been studied in coronal and interplanetary type II radio bursts, and how their band split can be used to determine the strength of the shock in terms of its Mach number.