Rayleigh‐Taylor Instability Observed at the Dayside Magnetopause Under Northward Interplanetary Magnetic Field

Under northward interplanetary magnetic field, periodical fluctuations with the period of 85 s have been observed by one of the Time History of Events and Macroscale Interactions during Substorms spacecraft at the dayside magnetopause when the solar wind dynamic pressure suddenly dropped. The observed magnetic field distortions, characterized by the compression of the main component as well as the flapping of the perpendicular components, are in line with the theoretical predictions of the Rayleigh‐Taylor (R‐T) instability excited at the magnetopause. With the convective electric field and the mean electric field removed, the perturbations in the electric field are identified as typical two sinusoidal signals with a 90° phase difference, consistent with previous prediction of the attendant electrostatic field arising from the R‐T instability. The transverse motions of the plasmas resulting from the electric drifting driven by the electrostatic field are observed both in pitch‐angle distributions and in distribution functions. The growth rate of the R‐T instability is about 43 s, implying that the instability has time to grow during the disturbances. Using the Gauss theorem, the calculated net charge density based on the electric field observation is one part of 27 compared to the totally observed plasma density, indicating that only a tiny fraction of the charge separates to set up the electrostatic field, which is due to a collective effect in the plasma. Plain Language Summary The magnetopause, the outmost edge that our planet's intrinsic magnetic field reaches, is always in motion. The macroscopic acceleration at the magnetopause is sometimes as large as a few km/s2. Theoretically, such an acceleration of the boundary can facilitate an instability in the motion of the interface, the R‐T instability. Such an instability can, in turn, cause a transfer of plasma across the magnetopause, from solar wind into the near‐Earth space. If the resulting particles are sufficiently energetic, operations of many satellites in space can be threatened. However, details of how the instability operates at the magnetopause have been neither verified nor observed in spacecraft observations. Recent measurements from NASA's Time History of Events and Macroscale Interactions during Substorms mission indicate that the R‐T instability can be excited at the magnetopause, and many details of the instability, including those of its electric field and plasma transport across the ma