Space Physics and Aeronomy, Upper Atmosphere Dynamics and Energetics

Wenbin Wang, Yongliang Zhang, Wenbin Wang

• Éditeur: American Geophysical Union
• Année: 2021
• ISBN:
  9781119815556
• Édition: 1
• Sujet: Sciences physiques
• Sous-sujet: Énergie

A comprehensive overview of the structure and variability of the upper atmosphere

Earth's upper atmosphere is an open system that is strongly influencedby energy and momentum inputs from both above and below. New observation and modeing techniques have provided insights into dynamics, energetics, and chemical processes in the upper atmosphere.

Upper Atmosphere Dynamics and Energetics presents an overview of keyresearch advances in upper atmospheric physics, and measurement and modeling techniques, along with remaining challenges for understanding the state and variability of the upper atmospheric system.

Volume highlights include:

• Insights into the interconnections between different areas of upper atmospheric science
• Appreciation of the dynamics and complexity of the global upper atmospheric system
• Techniques for observing and measuring the upper atmosphere
• Responses of the upper atmosphere to external drivers

The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

Find out more about the Space Physics and Aeronomy collection in this Q&A with the Editors in Chief

Part I
Energetics and Dynamics of the Upper Atmosphere

1
Joule Heating in the Thermosphere

Arthur D. Richmond

High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA

ABSTRACT

High‐latitude Joule heating is an important energy source for thermospheric dynamics and composition. It is influenced by winds, plasma turbulence, variable electric fields, and conductivity modifications by strong electric fields. The height‐integrated heating can be estimated from the Poynting flux above the ionosphere. Most energy is deposited near the morning and afternoon/evening sides of the auroral oval and in the cusp region. Multi‐instrument data assimilation can help quantify complex spatial/temporal variations of Joule heating. Rapid changes of heating launch gravity waves that propagate globally. Within several hours, a global circulation sets up that reduces horizontal variations of the pressure scale height, causing spatial correlation between the temperature and the mean molecular mass. The distributions of temperature and density in the upper thermosphere often show little relation to the distribution of Joule heating. Vertical winds decrease the O/N₂ ratio in regions of heating and increase the ratio in regions of subsidence. The upper thermosphere is affected more strongly by the fraction of Joule heating deposited above 150 km than by the larger amount of Joule heating deposited below 150 km.

1.1. INTRODUCTION

Joule heating, which is the irreversible conversion of electromagnetic energy into heat through ohmic currents, is a significant source of energy for the high‐latitude thermosphere (Cole, 1962; Thayer, 2000; Lu et al., 2016). Unlike heating by solar ultraviolet and extreme ultraviolet radiation, Joule heating occurs over only a small fraction of the Earth, and can drive large vertical velocities that alter the thermospheric circulation, leading to local and global temperature increases and changes in the structure of thermospheric composition, temperature, and density (e.g., Taeusch et al., 1971; Mayr & Volland, 1972, 1973; Mayr et al., 1978; Volland, 1979; Roble et al., 1983; Rees & Fuller‐Rowell, 1989; Rees, 1995; Liu & Lühr, 2005; Sutton et al., 2005; Lei et al., 2010; Liu et al., 2010; Fedrizzi et al., 2012; Fuller‐Rowell, 2013). Thermospheric responses to Joule heating during magnetic storms can be dramatic (e.g., Prölss, 1980, 1995; Rishbeth, 1991; Fuller‐Rowell et al., 1994, 1997; Rees, 1995; Lu et al., 2016; Deng et al., 2018). In addition to temperature increases, which produce large density increases in the upper thermosphere, the upwelling in the high‐latitude region of heating induces a global circulation within several hours (Volland & Mayr, 1971; Mayr & Volland, 1973), accompanied by downwelling at lower latitudes. The circulation dampens the upper‐thermosphere density response at high latitudes and spreads this response globally. The upwelling decreases the O/N₂ ratio at high latitudes (Taeusch et al., 1971; Mayr & Volland, 1972; Lu et al., 2016). Rapid variations of the heating generate thermospheric gravity waves in the lower thermosphere that propagate globally into the upper thermosphere, causing oscillations of wind, temperature, composition, and density as well as large‐scale traveling ionospheric disturbances (e.g., Wright, 1960; Lu et al., 2016). The effects of Joule heating depend not only on its highly variable intensity and its distribution over the polar regions, but also on the altitude distribution of the heating. Effects observed in the upper thermosphere have a complex relation to the heating distribution, such that thermospheric density increases usually do not coincide with regions of maximum heat input, due not only to the presence of gravity waves, but also to the fact that circulation changes rapidly redistribute density (Johnson, 1960). Furthermore, temperature changes are coupled to composition changes, such that the temperature and the thermospheric O/N₂ ratio tend to be inversely correlated in space. This is due to the tendency of the circulation to smooth out horizontal variations of the pressure scale height (Hays et al., 1973). This effect contributes to the fact that horizontal variations of density and composition during magnetic storms can be very different (e.g., Lei et al., 2010).

1.2. PHYSICS OF JOULE HEATING

The physics of thermospheric Joule heating involves collisional interactions among electrons, positive ions, and neutral molecules. These species have differential bulk motions owing to the presence of electric and magnetic fields, so that collisions result in frictional momentum exchange and heating (e.g., Brekke & Kamide, 1996; Thayer & Semeter, 2004; Zhu et al., 2005; Vasyliunas & Song, 2005; Strangeway, 2012). The sum of frictional heating of all species gives the total Joule heating. The frictional heating causes the species to have different temperatures, with the electron and ion temperatures exceeding the neutral temperature (e.g., St. Maurice & Hanson, 1982; Heelis & Coley, 1988; St. Maurice et al., 1999), and additional collisions transfer heat from hotter to cooler species. On timescales longer than...