RAMON E. LOPEZ MAGNETOSPHERIC SUBSTORMS The Earth's magnetic tail acts as a reservoir for the energy that is extracted by the interaction between the solar wind and the Earth's magnetosphere. Occasionally, a portion of that energy is released through a violent process known as a magnetospheric substorm. The sub storm is one of the most important mag­ netospheric phenomena, and it is the subject of extensive research. Recent work utilizing data collected by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer satellite, built at APL, has contributed markedly to our understanding of substorms. INTRODUCTION A plasma is an ionized gas, and it is the most preva­ where much of the power is dissipated. Thus, Birkeland lent form of matter in the universe. Within our own solar currents are a fundamental element of energy transport system, the Sun produces a tenuous, but rapidly flowing in the magnetosphere. plasma called the solar wind, which fills interplanetary The interplanetary magnetic field (IMF) carried by the space and carries the solar magnetic field imbedded into solar wind also plays a crucial role in the transfer of ener­ it. In the neighborhood of the Earth's orbit, the average gy to the magnetosphere. Both the magnitude and the solar wind stream has a velocity of 400 km/s, a number orientation of the IMF are quite variable, and, in particu­ density of 5 cm - 3, a temperature of 64,000 K, and a lar, there is no preferred z orientation of the field (where magnetic field strength of 5 nT. Each of these quantities x points along the Earth-Sun line, z is positive north­ varies considerably over time scales ranging from minutes ward, and y completes the right-handed coordinate sys­ to years. tem). The Earth's magnetic field points primarily north­ When the solar wind strikes a magnetized body such ward, so when the IMF contains a southward component as the Earth, a complicated interaction occurs. Plasmas (-Bz), a complicated process called magnetic reconnec­ and magnetic fields exert forces on each other. Thus, tion occurs. on the dayside of the Earth, the progress of the solar Reconnection is a process by which magnetic energy wind is checked when the pressure exerted by the solar is converted into plasma energy. As the name implies, wind matches that exerted by the Earth's magnetic field. reconnection (in its most commonly understood form) The solar wind flows around the dayside to the night­ involves the interconnection of oppositely directed mag­ side, and there the Earth's magnetic field is drawn out netic fields. This process was first postulated in the con­ into a long magnetic tail. This magnetotail lines up in text of solar flares and was applied to the magnetosphere the direction of the solar wind flow like a wind sock and by Dungey. 1 An illustration of the Dungey-type, or extends at least 3 times the distance from the Earth to open, magnetosphere is shown in Figure 2. When the the Moon. The resulting comet-shaped magnetic cavity IMF has a southward component, reconnection occurs is known as the magnetosphere, and it is illustrated in on the dayside, and the magnetic lines of force in the Figure 1. solar wind become connected with the lines of force from Although it is convenient to speak in terms of mag­ the Earth. The place where this interconnection occurs netic pressure balancing plasma pressure, in reality the is called an X-line because of the magnetic geometry in distortion of the Earth's magnetic field is effected by that region. The reconnected magnetic field lines are enormous electrical currents. The interaction between the dragged antisunward by the solar wind flow and stretch solar wind and the Earth's magnetosphere acts as a cos­ out to form the lobes of the magnetotail. The field lines mic dynamo that supplies the power to drive the currents. connected to the solar wind are referred to as "open." These currents flow through space across and along lines In the distant magnetotail, another X-line forms, which of magnetic force. One portion of this current system severs the open-lobe field lines. The reconnected field flows in a sheet across the center of the magnetotail, lines now have both ends attached to the Earth and are dividing the tail into two lobes of oppositely directed referred to as "closed." The closed lines of magnetic magnetic field. This current is known as the cross-tail flux contain a relatively dense plasma that forms a sheet current, and, as we shall see, it plays a central role in down the center of the magnetotail called, naturally substorms. The currents that flow along the magnetic enough, the plasma sheet. Thus, in this model, the mag­ field are known as Birkeland currents, named after the netic field is visualized to undergo a grand circulation Norwegian scientist Kristian Birkeland, who rust postu­ in which reconnection and the formation of X-lines play lated their existence. They provide the crucial link be­ a central role. Several different processes have been tween the magnetospheric dynamo and the ionosphere, shown to depend on the sign of Bz , and most scientists 264 fohns Hopkins APL Technical Digest, Volume 11, Numbers 3 and 4 (1990) Solar wind Figure 1. A cutaway view of the magnetosphere. The magnetotail is composed of two lobes (north and south) and a plasma sheet at the center of the tail. The cross-tail current runs across the center of the plasma sheet. Reconnection transfers magnetic flux into the mag­ netotail, which is equivalent to storing energy in the mag­ 3 Solar netotail since the energy density (in J/m ) of a given wind amount of magnetic flux density is B 2/21-'0, where B is ~ 2 the magnetic flux density (in Wb/m ) and 1-'0 is the per­ ~ meability of free space. At some point, this energy must ~ be released, and the process by which it is released is ~2 called a substorm. In the following sections, we will dis­ ~ cuss the phenomenology of substorms, the history of ~ substorm research, three of the physical models used to explain them, and the contributions made by the Active Magnetospheric Particle Tracer Explorers/Charge Com­ position Explorer (AMPTE/ CCE) program to understand­ Figure 2. The Dungey-type, or open, model of the magneto­ ing this important phenomenon. sphere. Closed dayside field lines (1) reconnect with south­ ward-pointing interplanetary field lines (2) at a reconnection region called an X-line (3). The newly reconnected field lines PHENOMENOLOGY OF SUBSTORMS (4) are swept back by the solar wind to form the tail lobes (5). A substorm is composed of three main phases: the The open-lobe field lines reconnect at a distant X-line (6) to growth phase, the expansion phase, and the recovery form closed plasma sheet field lines (7). (Adapted from Ref. 1.) phase. Before the onset of auroral activity, during the growth phase, the magnetotail stores energy it extracts invoke reconnection with the IMF to explain such from the solar wind, and as it does so, the cross-tail cur­ phenomena. rent intensifies. This intensification causes the magnetic Johns Hopkins APL Technical Digest, Volume 11, Numbers 3 and 4 (1990) 265 R. E. Lopez field in the lobes to increase, and the magnetotail be­ ery stage. This sequence is illustrated in Figure 3, which comes more stressed as the field lines become stretched shows a series of ultraviolet images taken of the aurora into a more tail-like configuration. The increased stress during a substorm by the polar-orbiting Dynamics Ex­ causes the near-Earth ($15 RE, where RE denotes plorer 1 satellite. Earth radii) plasma sheet to narrow in the north-south In the near-Earth magnetotail at substorm onset, the dimension; this process is referred to as plasma-sheet magnetic field suddenly relaxes from the stressed tail­ thinning. Since the maximum particle flux is found at like configuration to a more dipolar configuration, as the center of the plasma sheet, a satellite located in the if the cross-tail current, which produces the stress, has plasma sheet will observe a decrease in the flux as the suddenly been reduced. 3 This phenomenon is known as plasma sheet thins. It will also observe the increasingly a dipolarization. Attending the dipolarization is an ex­ tail-like configuration of the magnetic field. pansion of the plasma sheet, which is observed to have The onset of the expansion phase of a substorm begins been heated. As the satellite enters the plasma sheet, it when a discrete auroral arc, generally located in the mid­ first encounters earthward-streaming ion beams, fol­ night sector, suddenly brightens and expands poleward. 2 lowed by a general increase in the flux of plasma called In the vicinity of the brightening arc, a local intensifica­ an injection. In contrast, in the more distant magneto­ tion of the westward electrojet occurs (the westward elec­ tail, the plasma sheet thins at substorm onset and ex­ trojet is a current that flows in the ionosphere from east pands only later, during the recovery phase of the sub­ to west). This intensification produces a localized south­ storm. Another observed feature is an increase in the ward perturbation in the magnetic field that can be easily northward component of the magnetic field, which is detected by ground magnetometers located in the auroral correlated with the similar perturbation seen at mid-lati­ zone (the auroral zone consists of several degrees of mag­ tudes. Figure 4 is a schematic illustration of the mag­ 0 netic latitude centered at ~ 67 where auroras are most netic field geometry and plasma observations made by often observed).