The Earth's Magnetosphere

by Stanley W. H. Cowley


Earth in Space Vol. 8, No. 7, March 1996, p.9. © 1996 American Geophysical Union. Permission is hereby granted to journalists to use this material so long as credit is given, and to teachers to use this material in classrooms.
Contents

The magnetosphere is the region of space to which the Earth's magnetic field is confined by the solar wind plasma blowing outward from the Sun, extending to distances in excess of 60,000 kilometers from Earth. Much has been learned about this dynamic plasma region over the past 40 years, since the first direct measurements were made by the early Sputnik and Explorer spacecraft.

The Earth's magnetosphere is formed from two essential ingredients. The first is the Earth's magnetic field, generated by currents flowing in the Earth's core. Outside the Earth this field has the same form as that of a bar magnet, a dipole field, aligned approximately with the Earth's spin axis. The second ingredient is the solar wind, a fully ionized hydrogen/helium plasma that streams continuously outward from the Sun into the solar system at speeds of about 300–800 kilometers per second. This wind is therefore composed of protons and alpha particles, together with sufficient electrons that it is electrically neutral overall. The solar wind is also pervaded by a large-scale interplanetary magnetic field, the solar magnetic field transported outward into the solar system by the solar wind plasma. There is a third ingredient that also plays an important role: the Earth's ionosphere. The upper atmosphere is partially ionized by far-ultraviolet and X rays from the Sun above altitudes of about 100 km. The resulting ionosphere forms a second source of plasma for the magnetosphere, mainly of protons, singly charged helium and oxygen, and the requisite number of electrons for electric charge neutrality.

The Chapman-Ferraro Magnetosphere

The basic nature of the interaction between the solar wind and the Earth's magnetic field, leading to the concept of the Earth's magnetosphere, was first deduced by Sydney Chapman and his student Vincenzo Ferraro in the early 1930s. It is based on two theoretical principles. The first concerns the way in which plasmas and magnetic fields interact; they behave, approximately, as if they are "frozen" together. As a result of this freezing together, magnetic fields are transported by flowing plasmas; the field lines are bent and twisted as the flow bends and twists. An important example is the interplanetary magnetic field. It is wound into a large spiral structure by the Sun's rotation, and near the Earth it has a strength of about 5 nanoteslas (nT). This is a rather weak field, about one ten thousandth of the field at the Earth's surface, but nevertheless, it plays a crucial role in the Earth's interaction with the solar wind. The second principle concerns the force that the magnetic field exerts on the plasma, which usually opposes the bending and twisting of the field, or its compression, in the frozen-in flow. There are two components of this force. First, the field exerts an effective pressure on the plasma proportional to the square of the field strength. This force resists compressions or rarefactions of the magnetic field. Second, bent field lines exert a tension force on the plasma like stretched rubber bands. This force resists the bending and twisting of field lines.

Applying the first of these ideas to the interaction between the solar wind and the Earth, we recognize that the solar wind plasma is frozen to the interplanetary magnetic field, and the Earth's plasma (for example, from the ionosphere) to the Earth's field. When these plasmas meet each other, therefore, they do not mix, but instead they form distinct regions separated by a thin boundary. The solar wind thus confines the Earth's magnetic field to a cavity surrounding the planet, forming the Earth's magnetosphere, shown in Figure 1. The size of the cavity is then determined by pressure balance at the boundary between the solar wind on one side, and the magnetic pressure of the planetary field on the other. Given a planetary "bar magnet" field that produces a field strength of about 30,000 nT at the Earth's surface at the equator, estimates place the boundary, called the magnetopause, at a geocentric distance of about 10 Earth radii on the upstream (day) side, and this is where it is generally observed. (The Earth's radius is about 6400 km.) On the downstream (night) side the cavity extends into a long magnetic tail. Across the magnetopause the magnetic field usually undergoes a sharp change in both strength and direction. Ampere's law then tells us that a sheet of electrical current must flow in the plasma in this interface, called the Chapman-Ferraro current. A shock wave also stands in the flow upstream of the cavity (in Figure 1), which forms because the speed of the solar wind relative to the Earth is much faster than that of wave propagation within it. Across the shock the flow is slowed, compressed, and heated, forming a layer of turbulent plasma outside the magnetopause called the magnetosheath. Inside the cavity, in this picture, the terrestrial plasma will approximately rotate with the Earth (see Figure 1a). This will occur because the Earth's field lines are frozen into the ionospheric plasma. There corotation is enforced, in the absence of other driving processes, by collisions between ions and atmospheric neutrals at heights of about 120–140 kilometers.

Fig. 1. Sketches of the structure of the Earth's magnetosphere in the noon-midnight meridian plane, showing a) the Chapman-Ferraro closed magnetosphere based on strict application of the frozen-in approximation, and b) the Dungey open magnetosphere, in which there is an essential breakdown of frozen-in flow at the dayside magnetopause and in the tail leading to the occurrence of reconnection. The arrowed solid lines indicate magnetic field lines, the arrowed dashed lines plasma streamlines, and the heavy long-dashed lines the principal boundaries (the bow shock and magnetopause). The circled dots marked E in b) indicate the electric field associated with the flow, which is perpendicular to the flow and the field, and given in magnitude by their product.

Dungey's Open Magnetosphere

In Figure 1a, the interplanetary magnetic field is compressed against the magnetopause and draped over it by the flow, but ultimately the field lines slip around the "sides" of the magnetosphere, frozen into the magnetosheath plasma. However, the "frozen-in" picture is only an approximation, and under some circumstances it will break down. One of those circumstances occurs when high current densities are present in the plasma, and we have seen that large currents will occur at the magnetopause boundary. James Dungey, in the early 1960s, was the first to recognize the importance of this breakdown and to study its consequences.

When the frozen-in condition is relaxed, the field will diffuse relative to the plasma in the magnetopause, allowing the interplanetary and terrestrial field lines to connect through the boundary, as shown in Figure 1b. Dungey called this process magnetic reconnection. The distended loops of "open" magnetic flux formed by reconnection exert a magnetic tension force that accelerates the plasma in the boundary north and south away from the site where reconnection takes place, thus causing the open tubes to contract over the magnetopause toward the poles. This flow was first observed by the International Sun-Earth Explorer-1 and -2 satellites in 1978. The open tubes are then carried downstream by the magnetosheath flow, and stretched into a long cylindrical tail. Eventually, the open tubes close again by reconnecting in the center of the tail. This process forms distended closed flux tubes on one side of the tail reconnection site, which contract back toward the Earth and eventually flow to the dayside where the process can repeat. On the other side, "disconnected" field lines accelerate the tail plasma back into the solar wind. The key feature of the "open" magnetosphere is therefore the cyclical flow excited in the interior by these reconnection processes.

The ionospheric image of the magnetospheric flow is shown in Figure 2, which shows a view looking down onto the north pole of the Earth. It consists of twin flow vortices with open field lines that flow away from the Sun over the polar cap and closed field lines that flow toward the Sun at lower latitudes. Typical ionospheric flow speeds are several hundred meters per second. The overall flow cycle time is about 12 hours. From this the length of the tail can be estimated to be about 1000 Earth radii.

Fig. 2. View looking down on the northern high-latitude ionosphere showing the plasma flow streamlines associated with the Dungey flow cycle (arrowed solid lines) and the main zones where plasma particles of differing characteristics flow down the Earth's magnetic field lines from the magnetosphere into the upper atmosphere and ionosphere. The interior heavy dashed circle shows the boundary between open field lines at high latitudes and closed field lines at lower latitudes (compare with the noon-midnight cross-section in Figure 1b).

The most important feature of the magnetosphere-ionosphere flow, however, is that its strength is modulated by variations in the direction and strength of the interplanetary magnetic field. The dayside reconnection rate, and hence the magnetic flux throughput in the magnetosphere, is strong when the interplanetary field points south, opposite to the equatorial field of the Earth (see Figure 1b). When the interplanetary field points north, however, equatorial reconnection cannot occur, and the cyclical flow described above dies away. This dependence of the flow on the direction of the interplanetary field distinguishes Dungey's open magnetosphere from other possibilities (for example, flow excited in a closed Chapman-Ferraro system by "viscous" forces at the magnetopause), and has been demonstrated by many studies over the past 30 years. It is important to realize, however, that the magnetic flux throughput in the system, even at its strongest, amounts to no more than about 20% of the interplanetary magnetic flux brought up to the dayside magnetosphere by the solar wind. Most of the interplanetary flux is indeed deflected around the magnetosphere as deduced by Chapman and Ferraro. However, it is the breakdown of this picture at the 10–20% level due to reconnection at the magnetopause that is critical to the Earth's magnetospheric dynamics. The contribution to the flux transport by other nonreconnection mechanisms appears to be much smaller than this.

The cyclical flow driven by the solar wind is not, however, the only source of flow in the Earth's magnetosphere. We mentioned above that in the absence of other effects (like the Dungey flow cycle) the plasma would tend to rotate around with the Earth. When the flows caused by the Dungey cycle are added to the corotation flow, it is found that the Dungey cycle flows dominate in the outer part of the magnetosphere, while the corotation flow is confined to a small central core of dipole flux tubes, which typically extend in the equatorial plane to 4–6 Earth radii. This latter region is filled to relatively high densities with cold hydrogen/helium plasma from the ionosphere, forming a region called the plasmasphere. Outside this region the cold plasma density is much lower because of heating and loss of the ionospheric plasma during each cycle of the Dungey flow, and the medium is instead characterized by the presence of hot and tenuous plasma that is accelerated in the tail on closed field lines downstream from the tail reconnection site, and is then transported inward toward the Earth.

The size of the plasmasphere is not constant, however. The corotating region extends further from Earth than the average during intervals of weak Dungey cycle convection (northward interplanetary fields), and the ionosphere fills the outer flux tubes of this region toward equilibrium values while this condition persists. The corotating region shrinks when the Dungey cycle flow increases again (southward interplanetary fields), and the cold plasma that accumulated in the outer region is stripped away and flows to the dayside magnetopause, where much of it is heated and flows out into the solar wind. It is replaced by hot plasma flowing in from the tail.

Magnetospheric Substorms

The variability of the magnetospheric flow on timescales of minutes and hours that is associated with changes in the direction of the interplanetary magnetic field has been mentioned above as a key feature. However, observations show that when dayside reconnection is enhanced, for example by a southward turn of the interplanetary field, the magnetosphere generally does not evolve smoothly toward a new steady state of enhanced convection. Instead, the system, particularly the tail, undergoes a characteristic evolution on a 1-2 hour timescale, called a magnetospheric substorm.

Suppose the magnetosphere is initially in a state of low flow during an interval of northward interplanetary field, and that the interplanetary field then turns south. Reconnection starts at the magnetopause, stripping magnetic flux off the dayside and adding it to the tail, so that the dayside magnetopause moves inward (up to an Earth radius), while the tail magnetopause moves out (and the field strength inside increases). These changes are accompanied by an excitation of the large-scale flow as the system adjusts to the altered field configuration. During this interval, called the "growth phase" of the substorm, the field lines in the tail become very distended and nondipolar, as shown in Figure 3a. This development of the tail continues steadily over periods of several tens of minutes. Then, on a timescale of just a minute or two, these distended field lines collapse inward toward the Earth near the equator, and outward at higher latitudes, to a more dipolar form, as shown in Figure 3b. Why the field suddenly reconfigures in this manner is at present unknown, and is the subject of much research worldwide.

Fig. 3. Sketches showing the development of the nightside tail magnetic field during the growth and expansion phases of a substorm. Sketch a) shows the development of distended nondipolar field lines in the near-Earth tail during the growth phase, b) the onset of the field disruption at the beginning of the expansion phase and the rapid return of the inner tail field to a more dipolar form, while c) shows the down-tail propagation of this disruption and the subsequent induction of tail reconnection and plasmoid formation at larger distances from Earth.

As the tail field lines collapse, the plasma that they contain is strongly heated and compressed. This produces a sudden intense flux of electrons at the top of the atmosphere at the feet of the field lines concerned, producing a brilliant auroral display as the atmospheric atoms are excited by collisions with the electrons at altitudes above 100 km, and then radiate. The upper atmosphere also becomes much more strongly ionized at these altitudes than before, and hence more electrically conducting, and strong electric currents flow whose magnetic effects can be observed on the ground.

The tail field collapse usually starts in a restricted sector near the midnight meridian in the near-Earth end of the tail, and then propagates both across and down the tail, sometimes in a series of steps. In the upper atmosphere the area of bright auroras and strong electric currents therefore expands to the east and west as well as poleward, such that this interval is called the "expansion phase" of the substorm. This field collapse often induces the onset of reconnection in the tail at distances of 20–40 Earth radii as it propagates downtail. When this happens, closed tail field lines containing hot plasma are "pinched off" to form a closed-loop "plasmoid" that propagates downtail and into the solar wind at speeds of 400–800 kilometers per second, as depicted in Figure 3c. Continued reconnection in the near-Earth tail then closes open tail field lines. After a few tens of minutes, however, the reconnection rate slackens and the reconnection region moves back down the tail, signaling the end of expansion. The subsequent "recovery phase" of the substorm typically lasts for many tens of minutes. During this period the system responds and adjusts to the input of magnetic flux and hot plasma on the nightside during the expansion phase.

The above sequence of events typically follows a southward turning of the interplanetary magnetic field that lasts for at least a few tens of minutes. Several such intervals and several substorms can occur in one day. If the southward field persists for significantly longer, however, the system often evolves through a series of substorm cycles each lasting about an hour. The hot tail plasma is then driven deep into the inner magnetosphere by the enhanced convection, this constituting the main characteristic of a magnetic storm proper. The positively charged ions in this plasma drift westward around the Earth, and the negative electrons drift eastward, such that both contribute to a westward-directed ring of current that encircles the Earth at distances, typically, of several Earth radii. The magnetic field produced by this current causes a worldwide decrease in the field strength on the ground at low and middle latitudes.

Future Study

Most of the "geography" of the complex system that is the Earth's magnetosphere has been mapped out in the past 40 years of experimental study, but much remains to be learned about the underlying physical processes that are at work. We may mention in particular the fundamentally important unsolved problems of what determines the rate at which reconnection takes place between the interplanetary magnetic field and the Earth's field at the magnetopause, and the physics of the tail collapse that happens at the onset of the expansion phase of a substorm. While much has been learned, therefore, much remains to be done. It is nevertheless clear that the questions which we are now asking have improved considerably.

Source: Eos, Dec. 19, 1995, p. 525.

Further Reading

An introductory discussion of the physics of the Sun-Earth system may be found in Sun and Earth by H. Friedman (Scientific American Books, Inc, 1986).

Introductory but more mathematical treatments of these topics may be found in Introduction to Space Physics edited by M. G. Kivelson and C. T. Russell (Cambridge University Press, 1995), and in "The Solar-Terrestrial Environment" by J. K. Hargreaves (Cambridge University Press, 1992).

GLOSSARY

alpha particles
the nucleus of a helium atom, consisting of two protons and two neutrons
compression
regions where the pressure is more than the average due to reductions in volume of the gas
downstream (night) side
the region lying further away from the Sun than the Earth, where the solar wind is downstream from the Earth
ionosphere
the region of the atmosphere roughly 100–1000 km in altitude that contains a significant concentration of electrons and ions produced by the ionizing action of the Sun's radiation (ultraviolet and X rays) on atmospheric particles
magnetic flux
the magnetic flux (webers) threading a given area is the integral of the magnetic induction over the area
magnetic flux throughput
the amount of magnetic flux (in webers) transferred every second by solar wind coupling at the magnetopause from the dayside magnetosphere to the tail; in a steady state this is equal to the flux transferred every second from the tail back toward the dayside through the central part of the magnetosphere
magnetopause
the outer boundary of the magnetosphere, where the field changes direction from that of the Earth's field, to that of the interplanetary (Sun's) field
magnetosphere
the cavity surrounding the Earth containing the Earth's magnetic field, formed by the interaction between the field and the solar wind
plasma
gaseous state of matter consisting of freely moving ions and electrons which is very nearly electrically neutral overall
rarefactions
regions where the pressure is less than the average due to increases in volume of the gas
solar wind
continuous outflow of plasma from the solar atmosphere (corona) into the solar system
upstream (day) side
the region lying nearer the Sun than the Earth, where the solar wind has yet to encounter (is upstream from) the Earth
vortices
flow structures in which the flow streamlines form closed loops