Large-amplitude, Wideband, Doppler-shifted, Ion Acoustic Waves Observed on the Parker Solar Probe

<p>Electric field spectra measured on the\&nbsp;Parker\&nbsp;Solar\&nbsp;Probe\&nbsp;typically contain upwards of 1000 large-amplitude (similar to 15 mV m(-1)), wideband (similar to 100-15,000 Hz), few-second-duration, electric field waveforms per day. The satellite also collected about 85 three-second bursts of electric field waveforms per day at a data rate of similar to 150,000 samples per second. Eight such bursts caught these waves, all of which were located in switchbacks of the magnetic field. A wave burst on 2019 September 7, when the spacecraft was at an altitude of 55\&nbsp;solar\&nbsp;radii, is described. It contained Doppler-shifted ion acoustic waves that propagated in the direction opposite to the local magnetic field at all rest-frame frequencies from 60 Hz to nearly the proton plasma frequency of 2200 Hz, while no other wave modes were present. The eight bursts all contained ion acoustic waves whose individual net potentials were \&lt;= 1 V. A second burst, analyzed in conjunction with ion plasma measurements, showed that the ion acoustic waves were associated with broadened, plateaued, ion spectra containing unheated ions. Because the ion acoustic waves had phase velocities that varied from 140 to 90 km s(-1), ions interacting with these waves via the Landau resonance had a chance to diffuse in parallel velocity space from 90 to 140 km s(-1)in the plasma rest frame. The most likely generation mechanism of the ion acoustic waves is the ion-ion acoustic instability, while the electron-ion instability is a less likely candidate.</p>
Year of Publication
The Astrophysical Journal
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Date Published