Spacecraft that take measurements of plasmas in the solar system—like the solar wind—or in Earth’s magnetic field are designed to avoid potential interference from the spacecraft itself. Because a spacecraft’s metal body is a conducting surface, currents of ions and electrons flowing through it can produce electric and magnetic disturbances as the spacecraft plows through space plasmas, like a boat producing a sheath of waves around it and a wake directly behind it.
Usually, spacecraft designers avoid these effects by placing instruments on the ends of meters-long booms that extend outside a spacecraft’s sheath and wake. But now Miyake et al. have identified another form of spacecraft interference that they have dubbed electron wings—features that are broader and not as easily avoided as previously known sources of interference.
The researchers’ computer simulations, run using the electromagnetic spacecraft environment simulator (EMSES) software, show that these wings are generated when a few conditions are met. First, the spacecraft must be negatively charged. Second, it must be traveling nearly perpendicular to the direction of the local magnetic field. Third, the spacecraft must be as large as or larger than the average electron gyroradius (the radius of electrons’ circular motion).
Under these conditions, a negatively charged satellite will repulse incoming electrons, pushing them off to its sides if the spacecraft is traveling fast enough through plasma. The electrons will be directed along magnetic field lines in jetlike flows that resemble wings in the researchers’ models, extending tens or even hundreds of meters to the sides of the spacecraft.
These expansive wings are hard to avoid even with booms: Spacecraft are often placed in slow spins to stabilize them, so booms would pass through the electron wings twice per rotation, creating data artifacts. Indeed, the team identified anomalies in electric field readings from the Freja spacecraft that correspond to their predictions.
The authors wrote that this previously unidentified effect may occur commonly at Earth’s magnetic poles, where the magnetic field lines are approximately vertical. The magnetic poles are an area of great interest for both orbiting satellites and suborbital sounding rockets, so researchers should take steps to identify and eliminate this interference in their data analysis, they noted. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA027379, 2019)
—Mark Zastrow, Freelance Writer