Comparisons of observed and forward modeled Balmer-α intensities from WACCM-X and MSIS-00

From 2000 to 2001, ground-based geocoronal hydrogen Balmer- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> (H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> ) observations were obtained from Pine Bluff Observatory (PBO) in Wisconsin (43.07 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m3"> <mml:mrow> <mml:mo>°</mml:mo> </mml:mrow> </mml:math> N, 270.33 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m4"> <mml:mrow> <mml:mo>°</mml:mo> </mml:mrow> </mml:math> E) using a high spectral resolution (R <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m5"> <mml:mrow> <mml:mo>≈</mml:mo> </mml:mrow> </mml:math> 80,000) Fabry-Perot interferometer (FPI). In previous work, the PBO March 2000 observations have been compared to forward modeled MSISE-90 H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m6"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> intensities using the radiative transport code, lyao_rt (Bishop, Journal of Qunatitative Spectroscopy and Radiative Transfer, 1999, 61, 473–491). Results indicate that hydrogen column abundances exceed those predicted by MSISE-90 (Bishop et al., Journal of Geophysical Research, 2004, 109, 473–491). Furthermore, the PBO 2000–2001 observations have been compared to NRLMSISE-00 (MSIS-00) H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m7"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> intensities generated by lyao_rt. It was found that the observed dusk-to-dawn intensity variation and MSIS-00 show good agreement near the equinoxes and summer solstice, however, MSIS-00 underestimates the dusk-to-dawn asymmetry near the winter solstice (Gallant et al., Journal of Geophysical Research: Space Physics, 2019, 124, 4525–4538). More recent work has focused on forward modeling Whole Atmosphere Community Climate Model-eXtended (WACCM-X) simulations for three separate observatory locations, including PBO. Here, we investigate variations in upper thermospheric hydrogen by comparing WACCM-X and MSIS-00 forward modeled H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m8"> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:math> intensities for the March equinox with the PBO March 2000 evening observations used in Bishop et al. (Journal of Geophysical Research, 2004, 109, 473–491). WACCM-X underestimates the March 2000 observations by less than a factor of 2.0 rayleighs whereas MSIS-00 underestimates the observations by 2.6 rayleighs. A further analysis suggests that WACCM-X is better at simulating the underlying hydrogen density distribution than MSIS-00. Model-model-data comparisons between the 2000–2001 PBO observations, MSIS-00, and WACCM-X simulations that are representative of the observational conditions are currently underway.