Magnetosonic (MS) waves, also known as equatorial noise, are electromagnetic emissions occurring near the magnetic equator. They get their energy by interacting with protons trapped in Earth’s magnetic field, spiraling around magnetic field lines. Historically, the frequencies of MS waves were believed to be “temporally continuous”—that is, varying smoothly, like a trombone player sliding from one note to the next. This indicated a simple linear interaction between MS waves and protons. Fu et al. report a possible complication to this picture: a sharp rising tone in their spectrogram, like a flute player performing a series of runs and trills.
A rising tone suggests a more complicated, nonlinear series of interactions between the MS waves and protons. Scientists have observed rising-tone features in other kinds of plasma waves, including chorus and electromagnetic ion cyclotron (EMIC) waves. In those waves, the web of forces between the particles creates currents that boost the wave’s frequency. However, scientists had never seen this behavior in MS waves.
The team used data collected from two specific events recorded by NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The THEMIS spacecraft orbits in the magnetosphere near Earth’s magnetic equator and collects data from magnetic storms, the boundary of the magnetosphere on the dayside, and Earth’s radiation belts.
The first event, observed in June 2010, revealed electromagnetic emissions of rising-tone MS waves. The authors studied a second rising-tone MS wave event, observed in August 2010, to eliminate the possibility that the previous rising-tone MS wave emissions were a one-time event. On the basis of the rising-tone feature of MS waves, the scientists concluded that MS waves were possibly generated from nonlinear interactions. (Listen to a sample of the rising-tone MS waves in the sound clip below.)
The team found that chorus and EMIC waves were more powerful than the MS waves, indicating that particles interact more efficiently with those waves than with MS waves. (Geophysical Research Letters, doi:10.1002/2014GL061867, 2014)
—Catherine Minnehan, Freelance Writer
Citation: Minnehan, C. (2015), Spacecraft records rising-tone magnetosonic waves, Eos, 96, doi:10.1o29/2015EO035261. Published on 11 September 2015.