Relating the Solar Wind Turbulence Spectral Break at the Dissipation Range with an Upstream Spectral Bump at Planetary Bow Shocks

Abstract

At scales much larger than the ion inertial scale and the gyro-radius of thermal protons, magnetohydrodynamic (MHD) theory is well equipped to describe the nature of solar wind turbulence. The turbulent spectrum itself is defined by a power-law manifesting the energy cascading process. A break in the turbulence spectrum develops near ion scales, signaling the onset of energy dissipation. The exact mechanism for the spectral break is still a matter of debate. In this work, we use the 20 Hz \textit{MESSENGER} magnetic field data during four planetary flybys at different heliocentric distances to examine the nature of the spectral break in the solar wind. %By carefully selecting the spacecraft trajectory, We relate the spectral break frequencies of the solar wind MHD turbulence, found in the range of $0.3$ to $0.7$ Hz, with the well-known characteristic spectral bump at frequencies $\sim 1$ Hz upstream of planetary bow shocks. Spectral breaks and spectral bumps during three planetary flybys are identified from the \textit{MESSENGER} observations, with heliocentric distances in the range of $0.3$ to $0.7$ au. The \textit{MESSENGER} observations are complemented by one \textit{MMS} observation made at 1 au. We find that the ratio of the spectral bump frequency to the spectral break frequency appears to be $r$- and $B$- independent. From this, we postulate that the wavenumber of the spectral break and the frequency of the spectral bump have the same dependence on the magnetic field strength $|B|$. The implication of our work on the nature of the break scale is discussed.