Evidence of Sub-proton-scale Magnetic Holes in the Venusian Magnetosheath
Résumé
We present the first evidence of sub-proton-scale magnetic holes in the Venusian magnetosheath. Depressions in magnetic field strength, commonly referred to as magnetic holes, are observed ubiquitously in space plasmas. Magnetic holes with spatial scales larger than the local proton gyroradius (ρ) are usually attributed to the mirror instability. Those smaller than or on the order of ρ, termed "sub-proton-scale" magnetic holes, are likely supported by electron currents vortices, rotating perpendicular to the ambient magnetic field. While there are numerous accounts of sub-proton-scale magnetic holes within the Earth's magnetosphere, there are no reported observations in other space plasma environments. During Parker Solar Probe's first Venus Gravity Assist, the spacecraft crossed the planet's bow shock and subsequently observed the Venusian magnetosheath. The FIELDS instrument suite onboard the spacecraft achieved magnetic and electric field measurements of magnetic hole structures. The electric field associated with magnetic depressions are consistent with electron current vortices with amplitudes on the order of 1μA/m2. This observation suggests sub-proton-scale magnetic holes are signatures of a universal process in space plasmas.