Tomographic Estimation of Exospheric Hydrogen Density Distributions

Abstract

For the past decade, the Lyman-alpha detectors on board National Aeronautics and Space Administration's Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission have obtained routine measurements of solar Lyman-photons (121.6 nm) resonantly scattered by atomic hydrogen (H) in the terrestrial exosphere. These data have been used to derive global three-dimensional (3-D) models of exospheric H density beyond 3 R E , which are needed to understand various aspects of aeronomy and heliophysics, such as atmospheric chemistry and energetics, magnetospheric energy dissipation, ion-neutral coupling, and atmospheric evolution through gravitational escape. These empirical distributions are obtained through parametric fitting of assumed functional forms that have little observational justification, thus limiting confidence in conclusions drawn from analysis of the resulting exospheric structure. In this work, we present a new means of global 3-D reconstruction of exospheric H density through tomographic inversion of the scattered H Lyman-emission. Our approach avoids the conventional dependence on ad hoc parametric formulations and, based on the case studies reported here, appears to enable a more accurate characterization of the global structure of the H density in the outer exosphere. We evaluate the bounds of technique feasibility using simulated TWINS data and report new geophysical insights gained from applying this promising new approach to an example of actual TWINS data.

Time‐Dependent Response of the Terrestrial Exosphere to a Geomagnetic Storm

“…At all distances reported, the static H density reconstruction under quiet, prestorm conditions on 12 June is very similar to the static reconstruction on the previous day (11 June) that was reported in Cucho‐Padin and Waldrop (). A detailed discussion of the nature and possible origin of this and other observed discrepancies between our static reconstructions, those derived using alternative analyses of TWINS LAD data, and physics‐based Monte Carlo model simulations, is presented in Cucho‐Padin and Waldrop () and not repeated here.…”

“…Our approach is built on our recent development of a technique to reconstruct static estimates of global, 3‐D exospheric H density via tomographic inversion of an ensemble of optically thin Ly‐ α emission radiance measurements, I , along common‐volume LOSs (see Cucho‐Padin & Waldrop, for thorough details). Such radiance data (in units of Rayleighs, R =10 6 photons·cm −2 ·s −1 , below) is linearly proportional to the LOS column‐integrated H density, n H , as follows: where r denotes the Earth‐centered position of the spacecraft and the LOS integration over l is performed from the spacecraft vantage ( l =0) to the outer exospheric boundary, typically taken to be a sphere of radius 30 R E (see Figure 1 in Cucho‐Padin & Waldrop, ). Measured emission radiance is also proportional to Ψ( β ) describing the angular dependence of the scattered Ly‐ α photon direction, β (Brandt & Chamberlian, ) and to g * , the local scattering g factor (Meier, ).…”

“…where r denotes the Earth-centered position of the spacecraft and the LOS integration over l is performed from the spacecraft vantage (l=0) to the outer exospheric boundary, typically taken to be a sphere of radius 30 R E (see Figure 1 in Cucho-Padin & Waldrop, 2018). Measured emission radiance is also proportional to Ψ(β) describing the angular dependence of the scattered Ly-α photon direction, β (Brandt & Chamberlian, 1959) and to g * , the local scattering g factor (Meier, 1991).…”

Nonparametric H Density Estimation Based on Regularized Nonlinear Inversion of the Lyman Alpha Emission in Planetary Atmospheres

“…Estimation of H density from integrated Line‐Of‐Sight (LOS) measurements of the coronal Ly‐ α emission is a highly nonlinear inverse problem, which typically requires the enforcement of additional constraints in order to obtain a unique and physical solution. In the optically thin regions (i.e., the outer exosphere), the inverse problem can be simplified to be a linear inverse problem for the application of regularization methods, as shown by a recent study of Cucho‐Padin and Waldrop (). However, when the optically thick region is included, nonlinear inversion methods need to be applied.…”

Quantification of the Vertical Transport and Escape of Atomic Hydrogen in the Terrestrial Upper Atmosphere

“…Exospheric structure is expected to exhibit spatial asymmetries associated with these processes, such as polar depletions and enhanced densities near the magnetic equator (Hodges, ; Thomas & Vidal‐Madjar, ; Tinsley et al, ; Vidal‐Madjar & Bertaux, ; Vidal‐Madjar & Thomas, ). However, to date, reported observations of the global, three‐dimensional, and time‐varying terrestrial H density distribution are notoriously sparse and of insufficient accuracy to allow reliable quantification of the partitioning between thermal and nonthermal escape pathways (Carruthers et al, ; Cucho‐Padin & Waldrop, ; Kameda et al, ).…”