Abstract The determination of basic plasma parameters in atmospheric pressure discharges is critical to advancing their use in applications. Atmospheric pressure plasma jets have found use in the fields of medicine, agriculture, material modification and others. Atmospheric pressure plasma jets often generate plasma surface ionization waves which interact with and propagate over surfaces. Electrical diagnostics are challenging in surface ionization waves due to high collision frequencies and small scale of the plasma discharge. This work employs a passive optical emission line ratio technique to estimate the mean electron energy in surface ionization waves over planar dielectric substrates. The method uses an intensity ratio of two helium triplet lines: He($3{}^3S$) at 706.5 nm and He($3{}^3D$) at 587.56 nm. A collisional-radiative model is used to correlate line ratio to mean electron energy and determine dependencies on electron density and He/Air gas mixture. Mean electron energies ranging from 3-8 eV are determined in He/Air mixtures and are found to remain constant as the surface wave propagates radially. This work provides a 2D, time-resolved, mean electron energy diagnostic for surface ionization wave propagation and validation of numerical modeling in similar systems.