Electron temperature and electron concentration in the active zone of a miniaturized radio frequency (RF) non-thermal atmospheric pressure plasma jet in argon have been determined using two independent approaches: the spectroscopic measurement of the broadening of Balmer H_β and H_γ lines and a time-dependent, spatially two-dimensional fluid model of a single discharge filament. The plasma source has been configured as a capacitively coupled RF jet (27.12 MHz, 8 W generator output power) with two outer ring electrodes around a quartz capillary with diameter of 4.0 mm between which Ar flows at typical rates of 0.3 slm. The discharge has been operated in a self-organized mode, where equidistant, stationary filaments rotate regularly with a constant frequency at the inner wall of the outer capillary. For the purpose of calculating the spectral line broadening different models applicable at higher electron concentration have been evaluated. Resulting electron concentrations are between 2.2 and 3.3 × 1014 cm-3. The calculation according to the line broadening model provides electron temperatures between 20 000 and 30 000 K which is in agreement with the results of the fluid model calculations. Here, a broad radial profile with a maximal value of about 22 000 K in the centre of the column and an electron concentration of about 7 × 1013 cm-3 have been obtained. Moreover, the results of the model calculations reveal a structural change of the filament from the dielectric surface through the sheath to the column. The axially inhomogeneous region has an extension of about 0.5 mm. In the column a concentration of about 1013 cm-3 has been found for the excited argon atoms, whose collisions with electrons represent the most important ionization channel there.