  Hydrostatic equilibrium is an excellent approximation for the dense layers of
planetary atmospheres where it has been canonically used to interpret
transmission spectra of exoplanets. Here we exploit the ability of
high-resolution spectrographs to probe tenuous layers of sodium and potassium
gas due to their formidable absorption cross-sections. We present an
atmosphere-exosphere degeneracy between optically thick and optically thin
mediums, raising the question of whether hydrostatic equilibrium is appropriate
for Na I lines observed at exoplanets. To this end we simulate three
non-hydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and
(iii) torus to examine their imprint on an alkaline exosphere in transmission.
By analyzing an evaporative curve of growth we find that equivalent widths of
$W_{\mathrm{Na D2}} \sim 1- 10$ mA are naturally driven by evaporation rates
$\sim 10^3 - 10^5$ kg/s of pure atomic Na. To break the degeneracy between
atmospheric and exospheric absorption, we suggest that if the line ratio is
$\mathrm{D2/D1} \gtrsim 1.2$ the gas is optically thin on average and roughly
indicating a non-hydrostatic structure of the atmosphere/exosphere. We show
this is the case for Na I observations at hot Jupiters WASP-49b and HD189733b
and also simulate their K I spectra. Lastly, motivated by the slew of metal
detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b
and WASP-121b is consistent with the Na I data at present.
