First Interplanetary Shock Found

The solar wind typically comes in two types — slow and fast. When a fast stream of solar wind overtakes a slower stream, it creates a shock wave, just like a boat moving through a river creates a wave. The wave then spreads out across the solar system. On Jan. 8, 2018, MMS was in just the right spot to see one interplanetary shock as it rolled by.

Catching the Shock

MMS was able to measure the shock thanks to its unprecedentedly fast and high-resolution instruments. One of the instruments aboard MMS is the Fast Plasma Investigation. This suite of instruments can measure ions and electrons around the spacecraft at up to 6 times per second. Since the speeding shock waves can pass the spacecraft in just half a second, this high-speed sampling is essential to catching the shock.

Looking at the data from Jan. 8, the scientists noticed a clump of ions from the solar wind. Shortly after, they saw a second clump of ions, created by ions already in the area that had bounced off the shock as it passed by. Analyzing this second population, the scientists found evidence to support a theory of energy transfer first posed in the 1980s.

MMS consists of four identical spacecraft, which fly in a tight formation that allows for the 3D mapping of space. Since the four MMS spacecraft were separated by only 12 miles at the time of the shock (not hundreds of kilometers as previous spacecraft had been), the scientists could also see small-scale irregular patterns in the shock. The event and results were recently published in the Journal of Geophysical Research.

Data from the Fast Plasma Investigation aboard MMS shows the shock and reflected ions as they washed over MMS. The colors represent the amount of ions seen with warmer colors indicating higher numbers of ions. The reflected ions (yellow band that appears just above the middle of the figure) show up midway through the animation, and can be seen increasing in intensity (warmer colors) as they pass MMS, shown as a white dot.

Credits: Ian Cohen

Going Back for More

Due to timing of the orbit and instruments, MMS is only in place to see interplanetary shocks about once a week, but the scientists are confident that they’ll find more. Particularly now, after seeing a strong interplanetary shock, MMS scientists are hoping to be able to spot weaker ones that are much rarer and less well understood. Finding a weaker event could help open up a new regime of shock physics.
Contacts and sources:
Karen C. FoxNASA’s Goddard Space Flight Center

Citation: High‐Resolution Measurements of the Cross‐Shock Potential, Ion Reflection, and Electron Heating at an Interplanetary Shock by MMS
Ian J. Cohen Steven J. Schwartz Katherine A. Goodrich Narges Ahmadi Robert E. Ergun Stephen A. Fuselier Mihir I. Desai Eric R. Christian David J. McComas Gary P. Zank Jason R. Shuster Sarah K. Vines Barry H. Mauk Robert B. Decker Brian J. Anderson Joseph H. Westlake Olivier Le Contel Hugo Breuillard Barbara L. Giles Roy B. Torbert James L. Burch
First published: 18 June 2019

http://dx.doi.org/10.1029/2018JA026197