Chapter 6 Source and Loss Processes in the Magnetotail

Chapter 6 Source and loss processes in the magnetotail 6.1. Introduction There are two main sources of plasma for Earth’s magnetosphere and more specif- ically for its magnetotail, the solar wind and the ionosphere. Either one is capable of supplying the observed magnetospheric plasma over a broad energy spectrum. Plasma is transported into the magnetosphere from these sources through a va- riety of mechanisms which we can associate with different locations and whose efficiency is affected by solar wind conditions, magnetospheric state and history. The ionosphere supplies plasma through the polar wind, the cleft ion fountain, the auroral regions, the cusp ionospheric footpoint and the plasmasphere. The solar wind sources are the high altitude cusp, the plasma mantle, and the low latitude boundary layer. The number of processes in operation and their variability render even a simple quantitative comparison among the two main sources less than straightforward. Contemporary instrumentation has provided direct evidence for the source of magnetotail ions. By measuring their composition, charge state and spectral energy shape, recent studies have shown that the two main sources are generally both operational and result in populations that are well mixed throughout the magnetotail. Particles of solar wind origin dominate below energies of 15 keV during > magnetic quiet times, and at all energies at distances Ê 30 RE (Williams, 1997, and references therein). However, the ionosphere remains an important contributor to the near-Earth magnetotail plasma, especially during active times. Furthermore, when ionospheric particles can be identified in the composition measurements, their relative abundance and their spectral shape can help outline the magnetic flux tube’s prior history and the acceleration mechanisms that have acted on its populations have undergone. For these reasons, it is essential that we develop a data-bound interpretative model of ionospheric and solar wind particle transport to the magnetotail that can be expanded upon by using the multiple platform observations from the ISTP programme. To pursue the problem of geomagnetic tail formation and behavior it has proven fruitful to develop realistic computer models of particle transport in dy- 285 286 CHAPTER 6 namic topological configurations. Due to the non-linear character of particle tra- jectories in some regions (Speiser, 1965a), kinetic equations of motion must be used in these codes, requiring significant resources in computational speed and memory. Modern computing facilities allow researchers to perform such tasks, and thus motivate thinking of the magnetospheric populations in a time-dependent way, which is closer to reality. Such models can track the ions to their source locations outlined above (e.g., auroral region, LLBL, PM) and deconvolve the observed, often structured ion distribution into components attributable to different sources. This technique can also unravel the source geometries and relative strengths required to duplicate the measured distribution function. In the cases run to date, both main plasma sources contribute to the observed distribution function, but at well defined regions of particle phase space. The source geometries recovered by this technique are generally complex, possibly as a result of statistical averaging that is involved in region definition. When additional cases are run it may be possible to bin the observed and modelled ion distribution types into classes expected under different states of the solar wind and characterise the relative source-location contribution to those classes. Such an effort would result in a map of ‘source-location efficiency’ under different external conditions. Similar calculations for energetic ions have yet to be performed. Even with ad-hoc (although not unreasonable) source geometries, today’s models are making predictions about a tail structure that appears to be radically different from that envisioned based on ensemble averages from single spacecraft data sets. These models when put to test against multiple-platform observations from the ISTP fleet promise to open a new window to the dynamical processes that are responsible for plasma circulation throughout the magnetosphere. Model/data comparisons should first achieve closure in the area of source location geome- try and efficiency. This will immediately provide researchers with a quantitative tool with which to investigate theoretical transport paths for a given time-varying magnetospheric topology. This will allow us to further understand energization and loss processes. This chapter presents a perspective on the state of this process today. In Sec- tion 6.2 an overview of the magnetotail configuration and the source and loss processes is presented. In Section 6.3 the theoretical basis is discussed for the present understanding of the magnetotail with emphasis on plasma sources, transport and losses. Particular attention is given to models and simulation results that can be quantitatively evaluated against data. The observations of plasmas and fields in the magnetotail are discussed in detail in Section 6.4 while in Section 6.5 the question is addressed of how well the models and simulations of Section 6.3 actually reproduce the observed properties of the magnetotail. Much of our understanding of the solar wind source in the magnetotail is based on a reconnecting magnetosphere, however, Section 6.6 considers alternate scenarios. Section 6.7 MAGNETOTAIL 287 summarises the current state of our understanding of plasma sources and losses and discuss some unanswered questions and new directions for research. 6.2. Magnetotail Overview 6.2.1. PLASMA OBSERVATIONS IN THE MAGNETOTAIL In this section, we offer the observational framework for the basic topology and dynamics of the magnetotail. First we present its spatial regimes, and the plasmas and magnetic fields associated with them. Magnetospheric plasma circulation is primarily driven by magnetic reconnection (merging) between Earth’s and the interplanetary magnetic field (IMF) on the dayside magnetopause, at least for southward IMF. Magnetotail convection is affected, if not driven, by nightside reconnection in the tail. We thus also review the elements of the reconnection process at the dayside and at the nightside as it pertains to particle circulation. While it is difficult to accurately quantify the amount of plasma which any mechanism supplies to the magnetotail, we can at least compare the model predictions to average observations to ascertain if this, or any currently proposed mechanism, is the most likely candidate. Shape and Structure The magnetosphere in Earth’s vicinity is shaped like a paraboloid of revolution with the apex towards the Sun. This is because the solar wind compresses the dayside magnetosphere to a small geocentric distance of 10 RE at the subsolar point and to Ê 15 RE at the terminators (see also Figure 1.2 in Chapter 1). In Earth’s nightside wake region, magnetic field lines emanating from polar latitudes are stretched away from the Sun’s direction to form a long ( > 1000 RE) cylindrical volume of field lines connected to the paraboloid of revolution near Earth. This region constitutes the magnetotail. In terms of volume, the magnetotail dominates Earth’s magnetosphere. In terms of mass content, it is second to the plasmasphere. In the near-Earth region, the tail radius increases with downstream distance, a property called ‘tail flaring’. The tail diameter is 50 RE at a downstream distance of30–50RE. Tail flaring ceases quickly so that in the distant-tail ( Ü<-100 RE), jÜj the tail radius is 50 – 60 RR, not much different from that at 30 RE.The more distant magnetotail ÝÞ -cross-section has been inferred to be circular (e.g., Maezawa et al., 1997) elongated in the Ý dimension (e.g., Sibeck et al., 1985) or elongated in the Þ dimension (e.g., Tsurutani et al., 1984) using different method- ologies. Current observations are based on incomplete statistical sampling of the magnetotail boundary due to orbital biases that are inherent in any data set of in situ measurements of that boundary. Moreover, the magnetotail probably deforms dynamically and twists in response to solar wind stresses and IMF By,which makes the statistical determination of its shape more difficult. 288 CHAPTER 6 Figure 6.1. Schematic drawing of magnetotail (ÝÞ)-cross section showing the basic plasma regimes (from Christon et al. 1998). Early in situ observations revealed the internal structure of the magnetotail. It contains two regions, the northern and southern ‘lobes’, where the magnetic pressure dominates the particle pressure. The high intensity magnetic field points sunward in the northern lobe, and connects to Earth’s northern polar ionosphere. By definition it maps magnetically to the Earth’s northern polar cap. The southern lobe contains antisunward field lines, connecting to the southern polar cap. The magnetotail lobes are separated by a region of weaker, and more variable magnetic field and hotter plasma, called the plasma sheet (Ness, 1965). Across this high- ¬ plasma regime, the magnetic field undergoes a transition from earthward to tailward. This directional change takes place within a section of the plasma sheet limited in north-south extent where a sheet current, known as the cross-tail current, flows in the dawn to dusk direction. The region in the centre of the plasma sheet, B where Ü changes sign, is frequently called the ‘neutral sheet’. The plasma sheet and the embedded current-sheet extend to the moon’s orbit ( 60 RE) (Behannon, 1968). They are robust magnetotail features even 200 RE downtail (Tsurutani et al., 1984). Different plasma regions have been identified in the tail. Figure 6.1 is a schematic of the regions encountered within a yz-cross-section of the magnetotail. Moving from the solar wind inward, the bow shock (BS) is the boundary in interplanetary space separating the shocked solar wind plasma and magnetic field in the magnetosheath (MS) and the unshocked solar wind (SW). The magnetopause (MP), to first order, separates the magnetosheath from the magnetosphere.

Load more