Characterizing Unexpectedly Localized Slowing of the Thermospheric Cross‐Polar Jet of Neutral Wind Over Alaska in the Midnight Sector

Abstract

Earth's thermosphere is the region of atmosphere from E  90 km up to E  500-1,000 km altitude depending on solar-cycle conditions. Its lower edge is defined by the transition to a positive vertical temperature gradient above the mesopause, whereas the upper limit is generally taken to be the height at which the mean free path exceeds one scale height. The upper portion of the thermosphere corresponds to the altitude region occupied by low Earth-orbiting spacecraft. Understanding the details of the weather in the thermosphere is thus important for orbital prediction and for space debris collision avoidance. The dominant terms involved in orbital predictions are obtained from the direct application of well-known equations of Newtonian mechanics. However, operational responses to space debris hazards are driven by the uncertainty in these predictions, on the time scales of days to a week or so ahead. The largest contribution to this uncertainty comes from aerodynamic drag effects, due to imperfect knowledge of the (vector) wind and (scalar) mass density fields of the ambient atmosphere. Thus, in order to reduce these uncertainties, an accurate thermospheric model, including wind, is needed.

A Numerical Investigation on the Formation of the Stalling of the Cross‐Polar Jet in Midnight Thermospheric Winds

“…On 1 February 2019, observations show the occurrence of the cross‐polar jet inside the polar cap (Itani & Conde, 2021). On the other hand, as shown in Figure 10b, simulations indicate that equatorward winds were stalled and a high‐temperature region occurred over southeastern Alaska, forcing the neutral winds to turn poleward through the pressure gradient force.…”

“…We use the model results at pressure surface level Z p = 1, which is close to the ∼240 km altitude where the 630.0 nm emission from atomic oxygen is observed by the ground‐based Doppler spectroscopy. We conducted TIEGCM simulations of 12 individual cases, as observed in Itani and Conde (2021), to investigate the driving mechanisms of thermospheric wind stalling and the dates for 12 cases were given in Column 1 of Table 1. All simulations were preconditioned for 20 days to reach a steady state.…”

Large‐Scale Traveling Atmospheric and Ionospheric Disturbances Observed in GUVI With Multi‐Instrument Validations

“…This emission layer itself extends below 200 km and above 300 km in altitude and can vary based on background conditions (Ogawa et al., 2002), thus is most sensitive to long vertical wavelengths. The SDI data consist of 115 different look regions in the sky, retrieving U and V wind components (Conde & Smith, 1995, 1998; Itani & Conde, 2021). For this study of LSTAD perturbations, data from 19 January were used starting at ∼2 UT.…”

Wavelike Oscillations in High Latitude Thermospheric Doppler Temperature and Line-of-Sight Wind Observed Using All-Sky Imaging Fabry-Perot Spectrometers

“…These secondary GWs exhibit horizontal scales that are much larger than those of the primary GWs (Vadas et al, 2018). Itani & Conde (2021) investigated an abrupt stalling of the cross-polar jet in the midnight sector over Alaska. They reported that the characteristic length scale of the stalling could be as short as ∼200 km.…”