This Letter focuses on the discharge mechanisms of an atmospheric pressure micro-plasma jet optimized for endoscopic applications in biology and medicine. This capacitively coupled plasma (CCP) features a concentric double flow allowing for shielding the Helium or Neon plasma gas with carbon dioxide from the humid ambient air. High-resolution optical emission spectroscopy allows for the analyses of the Stark effect of the He I 492.19 nm and the Hydrogen Hβ lines to determine the electric field (EF) and the electron density spatially resolved along the discharge expansion outside the source. EF in Neon at atmospheric pressure was reliably determined with the Stark shift measurement of the weak Ne I line at 515.196 nm. In both gases, the EF diagnostic revealed a steep transition from CCP afterglow to streamer discharge with a magnitude up to 30 kV/cm. This research is a significant step forward in the field of plasma medicine with a plasma source capable of delivering a reactive chemistry with or without an intense EF to the target.