Funding for this research was provided by:
European Space Agency (contract NO. 4000109587/13/I-NB)
Deutsche Forschungsgemeinschaft (project number 273453909)
Swarm DISC (contract NO. SW-CO-DTU-GS-112)
Received: 30 May 2020
Accepted: 30 September 2020
First Online: 16 October 2020
: The authors declare that they have no competing interests.