science community, as well as for graduates and upper-level undergraduate students at universities, and engineers and environment officers/operators in government agencies who are interested in space weather applications.
Chaosong Huang Air Force Research Laboratory, Kirtland AFB, New Mexico, USA
Gang Lu National Center for Atmospheric Research, Boulder, Colorado, USA
1 Magnetospheric Energy Input to the Ionosphere
Cheryl Huang
Air Force Research Laboratory Space Vehicles Directorate, Kirtland Air Force Base, Albuquerque, NM, USA
ABSTRACT
The high‐latitude region of the ionosphere‐thermosphere (IT) system responds to solar wind driving via complex physical processes. The goal of global models of IT coupling is to specify and forecast the effect of magnetospheric energy input, in particular, energy dissipation into Joule heating and increase in kinetic energy of neutral and charged particles. Established paradigms for IT coupling have been challenged by recent investigations into the nature of energy input and the locations where energy is deposited. These results have forced a reexamination of our fundamental assumptions, which are captured in a series of empirical and physics‐based models of energy input. An assessment of these models reveals areas of significant discrepancy between model output and observations. This chapter summarizes the current understanding of magnetospheric energy input, and indicates research areas that remain to be explored.
1.1 INTRODUCTION
The paradigm for coupling between the magnetosphere and ionosphere was established in a series of seminal papers in which connection of the Earth’s magnetic field to the interplanetary magnetic field (IMF) was postulated to drive ionospheric convection (Axford & Hines, 1961; Dungey, 1961; Axford, 1969). In this view, based on magnetohydrodynamic (MHD) fluid theory, the solar wind connects to the Earth’s magnetosphere, allowing solar wind momentum to be transferred across the boundary of the magnetosphere to set into motion magnetospheric plasma. Given the Earth’s magnetic field, B, motion of charged particles, V, is possible only if an electric field, E is generated where E = ‐ V x B.
This paradigm explained magnetic activity in which energy enters the high‐latitude ionosphere and is manifest as auroral brightening. Supporting this paradigm were the discovery of energetic particle precipitation in the auroral zones (Frank & Ackerson, 1971), and persistent field‐aligned currents (FACs) in the same latitude range (Zmuda et al., 1970). These discoveries contributed to widespread acceptance that (1) MHD reconnection is the dominant mechanism through which solar wind energy enters the ionosphere‐thermosphere (IT) system; (2) energy input and dissipation is highly localized in the auroral zones under active and quiet conditions. This hypothesis has been widely explored and challenged in recent years.
Ultimately the impact of energy deposition is heating of the ionosphere and thermosphere, commonly referred to as Joule heating. The transition from energy input to Joule heating involves the conductivity of the medium. Limitations on the length of this chapter preclude a full summary of Joule heat and conductivity, both complex topics in themselves, but brief descriptions of the physical processes are included for completeness.
1.2 ENERGY ENTERING THE IONOSPHERE‐THERMOSPHERE (IT) SYSTEM
1.2.1 Electromagnetic and Particle Energies
The magnetosphere‐ionosphere‐thermosphere (MIT) coupled system is strongly driven by the solar wind. In order to specify the IT response to magnetospheric energy input, the sources of forcing and the physical processes by which the system responds must be understood. The magnetosphere couples to the ionosphere through the magnetic field of the Earth so that magnetospheric electric field, E, and magnetic field, B, couple to ionospheric E and B fields. FACs provide the communication between the magnetosphere and the solar wind to the ionosphere. Solar UV radiation energy maintains ionization, and solar wind coupling with the magnetosphere provides additional energy via fluctuations in the E and B fields.
The energy that enter the IT system from the magnetosphere takes two forms, electromagnetic (EM) in the form of Poynting flux, and kinetic, in the form of precipitating particles. The majority of magnetospheric energy maps to the high‐latitude region of the Earth. Thus, both types of energy occur at high latitudes in the polar cap, auroral zones, and subauroral regions.
The Poynting flux vector, S, is written
(1.1)
where μ0 is the permeability of free space. The magnetic field, B, includes the Earth’s magnetic field, the steady state contribution from large‐scale currents, and EM wave fields. Only the perturbation wave field, δB, produces energy that dissipates in the ionosphere (Kelley et al., 1991; Richmond, 2010), and only contributions to S from the perturbation magnetic field are considered relevant to energy input to the IT system.
Poynting’s theorem in differential form is written
where energy, W, in the EM wave is
(1.3)
ε0 is the permittivity of free space, j is the current density, E is the electric field, and j • E is the energy dissipation or conversion rate. It is usually assumed that there is little change in the wave energy with time in comparison with the j.⋅ E term and
S, as defined above, is the perturbation Poynting flux,
Richmond (2010) pointed out that the interpretation of Poynting’s theorem as normally applied for ionospheric energy dissipation requires that the sides of the volume over which the Poynting flux is dissipated be equipotentials. Gary et al. (1994) argued that individual flux tubes can be regarded as the appropriate volume for correct application of Poynting’s theorem.
Figure 1.1, reproduced from Knipp et al. (2004), shows the partition of energy input from 1975 to 2003, based on (1) estimates of power in the solar spectrum from the SOLAR2000 model (Tobiska et al., 2000); (2) estimates of auroral zone particle power based on observations from the Defense Meteorological Satellite Program (DMSP) and NOAA TIROS and Polar‐Orbiting Operational Environmental Satellites (POES); and (3) a combination of the Polar Cap (PC) index (Troshichev et al., 1988) and the Dst index used to parameterize estimates of Joule power. As stated by the authors, the indirect estimates of Joule heat are probably conservative. However Figure 1.1 shows