Space Physics and Aeronomy, Ionosphere Dynamics and Applications. Группа авторов
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Название: Space Physics and Aeronomy, Ionosphere Dynamics and Applications
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119815532
isbn:
(from Thomas & Shepherd, 2018; Reproduced with permission of John Wiley and Sons).
During the 1980s, it became clear that convection could be highly dynamic in response to intermittent phenomena such as changes in the IMF and magnetospheric processes such as substorms (e.g., Kamide & Vickery, 1983; Etemadi et al., 1988; Williams et al., 1989; Moses, 1989). A new, more dynamic picture of the Dungey cycle and its driving of ionospheric convection began to emerge, which recognized that the magnetosphere was driven independently by processes at the magnetopause and in the magnetotail (e.g., Russell, 1972; Holzer et al., 1986; Siscoe & Huang, 1985; Freeman & Southwood, 1988; Lockwood et al., 1990; Cowley & Lockwood, 1992; Lockwood & Cowley, 1992). To test these ideas, it was necessary to find techniques to instantaneously observe flows over large regions of the polar ionosphere, and systems such as the Super Dual Auroral Radar Network (SuperDARN) (Greenwald et al., 1995; Chisham et al., 2007) and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) (Richmond & Kamide, 1988) were developed. Together with spacecraft missions to observe the large‐scale morphology of auroras, such as Polar (Acuña et al., 1997) and the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) (Burch, 2000), and FACs, such as the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) (Coxon et al., 2018, and references therein), a new picture of the time‐variability of convection has come about. This is discussed in section 2.4.
2.2 THE MAGNETOSPHERE‐IONOSPHERE SYSTEM
This section provides a tutorial introduction to the morphology of the magnetosphere‐ionosphere system and the plasma physics that determines its dynamics. This will provide the background for sections 2.3 and 2.4.
2.2.1 Morphology of the Magnetosphere‐Ionosphere System
Figure 2.2a presents a schematic of the magnetosphere in the Geocentric Solar Magnetospheric (GSM) Y = 0 plane, with the solar wind approaching from the left. The boundary between the magnetosphere and the solar wind, the “magnetopause,” is shown as a black dashed line. The position of the magnetopause is determined by stress balance between the ram pressure of the solar wind and the magnetic pressure exerted outward by the terrestrial magnetic field. To first order, the solar wind cannot penetrate the magnetopause and is deflected around the magnetosphere, over the poles, and around the flanks. Magnetic field lines are represented as red and blue full lines. Inside the magnetosphere, the field is roughly dipolar, formed by electric currents flowing in the Earth's interior. The field strength is 62,000 nT at the surface of the Earth at the poles, but falls to a few tens of nT at the magnetopause. Outside the magnetosphere, the IMF is weaker, approximately 6 nT. This field is carried by the solar wind from the Sun at an average speed of 450 km s−1, and can point in any direction; indeed, the IMF orientation changes on timescales of minutes and hours. In Figure 2.2a, the IMF has been drawn with southward (IMF BZ < 0) and earthward (IMF BX < 0) components; although the Y‐component is not specified, it is on average the dominant component and, in principle, could point into (IMF BY < 0) or out of (BY > 0) the figure.
Figure 2.2 (a) A schematic of the magnetosphere showing closed (red) and open (blue) magnetic field lines. Green crosses mark the approximate locations of magnetopause and magnetotail reconnection X‐lines for southward IMF. The inset panel shows the relationship between closed field lines and the auroral zone, and open field lines and the polar cap (after Milan et al., 2009). (b) The Dungey cycle of magnetospheric convection. Arrowed lines are magnetic field lines in the solar wind (left and right) and of the magnetosphere (joined to the Earth). Magnetic reconnection occurs in front of and behind the Earth. Arrows show the sense of plasma flow, mainly antisunward, though with sunward flow within the magnetosphere
(from Milan (2009): https://www.ann‐geophys.net/27/659/2009/. Licensed under CC BY 3.0; from Dungey, 1961: Reproduced with permission of American Physical Society).
Field lines colored red in Figure 2.2a, known as “closed” field lines, have both ends on the surface of the Earth. Blue field lines are “open,” interlinking with the interplanetary medium. As first speculated by Dungey (1961, 1963), at interfaces in the plasma where there is a strong shear in the magnetic field, such as at the subsolar magnetopause when IMF BZ < 0 (dayside green cross), “magnetic reconnection” can take place to link the otherwise segregated regions. In Figure 2.2a, IMF field lines are shown that have undergone reconnection with closed field lines at the magnetopause and are in the process of being carried antisunward by the flow of the solar wind to form the “magnetotail.” The open field lines of the tail are known as the “magnetotail lobes.” The erosion of the dayside magnetosphere by reconnection cannot continue indefinitely: eventually magnetic reconnection is initiated between the oppositely directed field lines in the central plane of the magnetotail, the “neutral sheet,” to reclose flux (nightside green cross). As indicated in Figure 2.2b, the opening and closing of flux leads to an antisunward motion of flux and plasma across the high‐latitude regions of the magnetosphere and sunward return flow at low latitudes, known as the “Dungey cycle” (Dungey, 1961).
After reconnection has taken place at the magnetopause, protons and electrons from the solar wind can enter the magnetosphere by moving along the newly opened field lines and across the magnetopause. The volume of the lobes is large, so the plasma is tenuous here (~0.01 cm−3). However, as the field lines convect toward the neutral sheet, the plasma is heated and compressed to a density of ~1 cm−3 to form the “plasma sheet,” roughly coincident with the closed field line regions of Figure 2.2a. The hot plasma at the inner edge of the plasma sheet encircles the Earth forming an electric current system known as the “ring current.”
The inset of Figure 2.2a shows where the magnetospheric field lines have their footprints in the ionosphere, poleward of approximately 60o latitude. The closed field line region and plasma sheet maps to the “auroral zones,” where the majority of auroras are produced. The open field lines of the lobes map to the regions poleward of the auroral zones, known as the “polar caps,” which are largely devoid of auroras. Magnetospheric convection is communicated to the polar ionosphere, and constitutes the high‐latitude convection that is the focus of this review. Closed field lines equatorward of the auroral zones map to the “plasmasphere,” a region of cold, dense (~10–1,000 СКАЧАТЬ