Alexa J. Halford
NASA GSFC and Dept. of Physics and Astronomy, Dartmouth College
The Little Mission that could: BARREL observations of a solar storm
We often find ourselves focusing on very specific topics as scientists. This can be a fantastic way to advance the field’s knowledge. However, every once in awhile an event comes along which forces us to zoom out and see the grandeur and interconnectedness of the universe, in this case the Sun - Earth system. I would have thought that this experience would come for me in the form of another Carrington event [Carrington, R. C. (1859), Monthly Notices of the Royal Astronomical Society] (the largest solar and geomagnetic storm in modern times), or something like the Feb. 10 - 11 1958 solar and geomagnetic storm discussed by Winckler et al [1959 JGR vol.64] (another large storm and a fantastic read). Instead it was a very small event, if you are interested in geomagnetic activity, or one of the largest of the current solar cycle, if solar energetic particle events are more your thing. However you classify it, one of the largest sunspots of this solar cycle AR1944, produced an active region that allowed us to take a closer look at how the entire solar system is intertwined.
The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission of opportunity working in tandem with the Van Allen Probes was designed to study the loss of radiation belt electrons to the upper atmosphere. However BARREL is able to see X-rays from a multitude of sources. During the second campaign, the Sun produced, and BARREL observed, an X-class flare. This was followed by observations of X-rays, gamma-rays, and directly injected protons from the solar energetic particle event associated with the eruption from the Sun. Two days later the shock generated by the interplanetary coronal mass ejection (ICME-shock) hit the Earth while BARREL was in an amazing conjunction with the Van Allen Probes. We here at Earth only received a glancing blow of the iCME-shock and thus did not produce a large geomagnetic storm, but the compression led to the formation of ultra low frequency (ULF) waves, electromagnetic ion cyclotron (EMIC) waves, and very low frequency (VLF) whistler mode waves. The combination of these waves and the enhancement of the local particle population led to precipitation of electrons remotely observed by BARREL. Others have gone on to study the effects of this CME at Mars as the red planet was directly in the path of this CME. Although we do not discuss this here, a storm like this would have large implications for a manned mission to Mars.
This was not a Halloween, Bastille Day, or one of the now many St. Patricks Day storms. In fact it’s unlikely that it will ever get it’s own holiday nickname (almost a tradition in the field of space physics). But unlike those larger geomagnetic events, this one was less complicated allowing us to, among other things, directly test the relative importance of multiple loss mechanisms, see how waves are generated, and by and large gain a more complete understanding of how the system interacts and how quiet times can affect radiation belt dynamics and space weather. It was this large and yet small heliospheric storm which gave me the opportunity to zoom out and see how our heliospheric system is intertwined.