Helium’s Departure from Diffusive Equilibrium in the Thermosphere

Abstract:

As a minor and inert species in most of the thermosphere, helium’s distribution often departs from major-species behavior in the upper thermosphere – the winter helium bulge is a clear demonstration. This behavior indicates processes, other than molecular diffusion, are effective in modifying helium’s distribution. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) V2.0 has included helium as an additional species in its composition solver and allows for investigation of such processes. In this talk, we will demonstrate simulations that produce regions of hyper- and hypo-diffusive states of helium within the middle thermosphere. Hyper-diffusive states are regions in the middle thermosphere where the helium scale height is greater than its molecular-diffusive scale height resulting in enhanced regions of helium concentration in the upper thermosphere. Hypo-diffusive states are regions where the helium scale height is less than its molecular-diffusive scale height resulting in depleted regions of helium concentration in the upper thermosphere. Upon identifying these regions, the TIEGCM 3-D circulation field is evaluated to investigate the role of vertical advection, horizontal divergence, and vertical wind gradients in producing these regions. It is found that a combination of convergent/divergent motion and vertical advection behavior in the thermospheric column produce the resultant hyper- and hypo-diffusive states. This behavior in 3-D circulation is altitude dependent and highlights the need to better observe the middle thermosphere where strong gradients in winds and composition can significantly alter the way upper thermosphere composition is distributed.