Global studies of auroral electrons were first conducted by an Australian associate of Van Allen, Brian O'Brien, using the University of Iowa's "Injun 1" satellite, launched in 1961 and named for the "Injun territory" in which Iowa was formed. Later Injun 3 measured the distribution of arrival directions of auroral electrons (pitch angles) and also their total energy flux, and found the latter too high to be explained by the "leaky bucket" model [O'Brien and Taylor, 1964].

Among the many satellite observations of the aurora performed since that time, the most striking ones have been produced by imaging cameras, from which a global view of the entire oval can at times be obtained. The earliest images came from the Canadian Isis 2 spacecraft, launched 1 April 1971 [Lui and Anger, 1973], and they revealed for the first time the true dimensions and significance of the diffuse aurora (below). Scientifically useful results were also obtained from military imagers aboard spacecraft of the U.S. Air Force [Pike and Whalen, 1974], especially those of the DMSP series, which continues to this day; later DMSPs also carried a variety of scientific sensors. The Dynamics Explorer satellite DE-1, launched 3 August 1981 [Hoffman, 1988], carried a particularly successful imager [Frank and Craven, 1988], and more recently, several other satellites employed imagers, in particular the two Swedish spacecraft Viking [Viking Science Team, 1986; Hultqvist, 1987] and Freja, and also the Japanese Akebono [Tsuruda and Oya, 1991].

Different types of aurora may be distinguished. The brightest auroras are discrete arcs and bands. Their structure may include multiple parallel curtains, folds ("striations") and swirls of various sizes [Hallinan, 1976]. A typical electron energy spectrum in the discrete aurora [Boyd, 1975], observed above the atmosphere (Figure 10), has a peak around 5 keV and falls off steeply around 10-15 keV, while below 1 keV a large population of secondary electrons seems to exist. During substorms (sections 12-14 below) the aurora greatly intensifies and the region of such arcs expands equatorwards and intermittently polewards. The poleward expansions are highly dependent on local time and on the phase of the substorm.

Figure 10

The diffuse aurora is fainter (detectable on the ground by photometers but not usually by the eye) and it tends to extend around the entire auroral oval; its significance was only realized after its global configuration was seen by Isis 2 [Lui and Anger, 1973]. It appears to be produced by electrons of the plasma sheet (typical energy, 1 keV) scattered into orbits that intercept the atmosphere. A mid-latitude red aurora [Rees and Roble, 1975], produced by low-energy electrons from the ring current, is usually subvisual and was discovered by Barbier [1958]. A red aurora also appears in the regions linked to the polar cusps [Shepherd, 1979], caused by magnetosheath electrons that reach the ionosphere.

"Sun-aligned arcs" appear during northward IMF and extend from the auroral oval into the polar cap, pointing roughly sunwards. They were observed by Gustafsson [1967], who felt that they were part of the regular pattern at high latitudes, rather than a separate branch; later they were studied by the Isis 2 imager [Ismail et al., 1977] and were found to be associated with low activity and northward IMF [Burch et al., 1979]. Sometimes they stretch completely across the polar cap, forming a "theta aurora" [Frank, 1986], so called because the combined pattern of the auroral oval and the arc across its middle resembles the letter theta. No generally accepted explanation of these phenomena exists and in general, the behavior of the magnetosphere during prolonged northward IMF is still poorly understood, although some interesting convection patterns in the distant tail, during such times, were recently noted by Nishida et al. [1995].

10. Field Aligned Voltage Drops

Before the spaceflight era it was often held that auroral particles came from the Sun (see BH-1). Satellite observations suggested that the acceleration process took place in the magnetosphere, but until 1974-77 it was widely believed that for electrons of auroral arcs, the most conspicuous and energetic type, this happened far from Earth, probably in the plasma sheet. Then Evans [1974, 1976a, b] proposed that the electrons which produced auroral arcs received much of their energy from field aligned voltage drops within 1-2 RE of Earth. Evidence from the S3-3 satellite (below) soon convinced the community that this indeed was the case.

Previously, many theorists believed that field-aligned voltage drops (a "parallel electric field" E//) were unimportant in magnetospheric physics, because electrons and ions moving along field lines would immediately cancel any electric charges that produced such drops. In many plasma situations, this indeed holds true. However, Alfven [1963] and his student Persson [1963, 1966] argued that E// could exist if it was balanced by the "mirror force" opposing the entry of charged particles into regions of converging field lines. Such a possibility was also known in laboratory plasma physics [Grad,1966] and is behind the operation of plasma containment machines of the tandem mirror type.

The Alfven-Persson solution will not persist in the magnetosphere under static conditions, without a constant input of energy. However, observations indicate a strong correlation between discrete arcs, where acceleration often occurs, and field aligned Birkeland currents (next section). On a distended field line, the bundle of orbits that reaches the ionosphere ("loss cone") may be too small to carry the line's share of the Birkeland current, and under such conditions, the existence of E// widens the loss cone and increases the line's capacity to carry current [Knight, 1973; Chiu and Schulz, 1978]. It is thought that such lines appropriate part of the voltage of the Birkeland circuit to provide them with the necessary E//.

An important feature of the Alfven-Persson theory is that the field aligned potential F is proportional to the intensity B of the magnetic field. A dipole field weakens with distance like r-3, hence B drops by 7/8 of its value within 1 RE of the Earth's surface, and the theory therefore predicts that the main drop of F should also occur close to Earth. As will be seen, the appearance of E// seems to be associated with field aligned currents (further below). Some theorists have also suggested that E// may arise from an "anomalous resistivity" along magnetic field lines, produced by plasma wave instabilities affecting field-aligned currents [Papadopoulos, 1977]; such processes, too, favor low altitudes where such currents have their highest density. Plasma wave instabilities are probably the source of the intense auroral kilometric radiation (AKR), discovered by Gurnett [1974]. AKR was detected before that by the first Radio Astronomy Explorer RAE-1 [Stone, 1969], but its nature and source were not recognized and the only consequence was a decision to place the follow-up satellite RAE-2 in an orbit around the moon, away from the interfering noise.

Evans [1974] proposed that many of the low-energy electrons in discrete arcs were secondaries from collisions, temporarily trapped, unable to reach the dense atmosphere below because of a magnetic mirror and unable to escape along field lines because of E// . Very clear evidence came from the S3-3 spacecraft of the US Air Force, supported by the Office of Naval Research (ONR), which detected beams of O+ ions (the dominant positive ion in the ionosphere) rising upwards, apparently impelled by the same E// which accelerated electrons downwards [Shelley et al., 1976; Johnson, 1979; Mizera et al., 1981]. In addition to the O+ beams, "ion conics" were found; these were events in which the O+ flux peaked at some intermediate angle to the magnetic field direction, suggesting that plasma wave phenomena at some lower altitude had preferrentially accelerated the velocity component v-perp perpendicular to the magnetic field [Sharp et al., 1977]. The observation of beams and conics solved the riddle of O+ ions in the ring current, first detected by Shelley et al. [1972; Sharp et al., 1974].

An alternative acceleration process, promoted by Alfven [Brush, 1990] and by Block [1972, 1978; Goertz, 1979], centered on the existence of a "double layer," an abrupt field-aligned voltage jump of appreciable intensity. Large impulsive electric fields were observed by electric field probes aboard S3-3 [Mozer et al., 1977] and the suggestion was made that they might be the signature of double layers. However, other possible explanations also exist, and no compelling evidence for the existence of such layers in space has surfaced since then.

11. Birkeland currents

Magnetic variations observed on the ground in the auroral zone are much larger than those at middle and low latitudes: swings of 500-1000 nT at auroral latitudes (out of about 60,000 nT) are much more common than 100 nT disturbances at the equator, which would be classified as fair-sized magnetic storms (Rufenach et al. [1992], Figures 12 and 13, respectively). The strong polar disturbances are localized, suggesting that the currents producing them flow nearby, probably in the ionosphere.

Birkeland [1908, 1913; Bostrom, 1968; Stern, 1977; see also BH-1] noted that the direction of the disturbance field in the auroral zone tended to be perpendicular to auroral arcs. He concluded that large electric currents flowed lengthwise along the arcs, and speculated that those currents arrived along magnetic field lines at one end of the arc and returned to space by a similar route at the other end. An overall pattern inferred in this way was later mapped, especially by Silsbee and Vestine [1942], and its currents were named auroral electrojets; they seemed to originate on the day side and to flow towards midnight along both sides of the auroral oval. Sugiura and Davis [1966] combined the readings of about a dozen magnetic observatories around the auroral zone and extracted an "AE (auroral electrojet) index" which gauged the strength of the electrojets. Values of this index are now regularly compiled and often serve as indicators of substorms and of the level of magnetospheric agitation [Rostoker, 1972b; Mayaud,1980].

Because the ionosphere conducts electricity, the existence of a dawn-to-dusk polar electric field (Figure 5b, contours viewed as electric equipotentials) suggests that an electric current flows across the polar cap; the current might enter on the morning side of the polar cap and exit in symmetric fashion on the evening side, like the current in Figure 8. The pattern of E, however, also has fringe fields that extend equatorward of the oval, to field lines that are shorter and therefore thread parts of the magnetosphere closer to Earth. In a static electric field E = -grad V, if E// is negligible, it follows from (1) that B.grad V= 0 and hence that the electric potential V is constant along field lines. The fringe pattern then maps F and E to the near-earth magnetosphere.

Schield et al. [1969] deduced from this an important new effect. The earthward flow in the tail predicted by both convection theories (Axford-Hines and Dungey) is associated (by (1)) with a dawn-to-dusk electric field E across the tail, which then maps along field lines to the polar cap, and the polar fringe pattern extends this E to nightside equatorial regions closer to the Earth. When convecting ions and electrons arrive near Earth, appreciable guiding-center drifts caused by the dipole-like internal field are added to their convective flow. These deflect the flow around the inner part of the magnetosphere, as was assumed by Axford and Hines [1961] and as was claimed even earlier by Alfven [1939; see BH-1] .

However, the magnetic drifts move positive ions and electrons in opposite directions. Schield et al. [1969] showed that as a result, if such drifts are added to the convective flow, the plasma no longer stays electrically neutral. This cannot be allowed to happen, because even a relatively tiny deviation from strict neutrality produces huge electric fields. The process may be halted in one of two ways: either E is modified in a way that keeps the plasma flow out of the region of strong magnetic drifts, or else electric currents arise along magnetic field lines (the easy flow direction in a plasma) and drain away the excess of electric charge. Both processes seem to occur.

The modification of E takes the form of "shielding," of an exclusion of E from the vicinity of the Earth, making the fringe-pattern in Figure (5b) narrower than what a calculation based solely on ionospheric conductivity would give. This was first calculated by Vasyliunas [1970] and also by Jaggi and Wolf [1973], who devised a way of simulating the process on a computer. The method was later expanded by Wolf and his group at Rice University in Houston with the "Rice Convection Model" (RCM) which simulates both the shielding and the neutralizing currents [Spiro and Wolf, 1984].

The existence of neutralizing field-aligned currents was predicted by Schield et al. [1969]. Because of Birkeland's early ideas on field-aligned currents flowing in and out of the ionosphere, they named all such currents (including the primary ones) "Birkeland currents." The neutralizing currents flow in the opposite direction from the primary currents--out of the ionosphere on the morning side and into it on the evening side, and they were expected to intersect the ionosphere somewhat equatorward of the primary currents (Figure 11). This qualitative theory was given a mathematical expression by Vasyliunas [1972].

Figure 11

Current patterns like those predicted by Schield et al. [1969] were ultimately discovered by Zmuda and Armstrong [1974], using a magnetometer aboard a low-altitude satellite in polar orbit, and this, rather than flow across the pole, turned out to be the main mode by which the currents closed. Two factors delayed that discovery. The OGO-2, OGO-4 and OGO-6 missions, launched in 1965, 1967 and 1969, respectively, followed low-altitude polar orbits and carried precise magnetometers, but these instruments were intended for a survey of the Earth's internal field [e.g. Langel, 1974], and they only returned the intensity |B|, much easier to obtain accurately than the direction of B. Unfortunately, the signature of Birkeland currents is a rotation of the observed vector of B when the satellite crosses the current sheet, accompanied by practically no change in intensity. Thus the polar OGOs ("POGOs") failed to detect any field-aligned currents; later Sugiura [1975] deduced the currents' existence by observing B on near-earth passes of OGO-5, but his work appeared after the article of Zmuda and Armstrong. Some earlier observations [e.g. Zmuda et al., 1966] were also tentatively identified as signatures of field-aligned curents [Cummings and Dessler, 1967], but no global pattern was deduced.

Figure 12

Another delaying factor was the fact that the current flows expected from the convection pattern of Figure (5b) were quite different from the auroral electrojets inferred from ground data. One reason for the discrepancy was found by Fukushima [1969] who pointed out that when a current (Figure 12) flowed into an infinite plane conducting sheet through a perpendicular straight wire, and flowed out again by a similar wire at another point, no magnetic effect existed on the other side of the sheet. The result was later extended to a spherical geometry [Fukushima, 1976], and while the actual structure of the Earth's magnetosphere differs from these ideal cases, these results strongly suggested that Birkeland currents flowing into the ionosphere from space, across it and then out again, produced only small magnetic effects on the ground and were virtually invisible from the ground. The disturbance on the ground is almost entirely due to the auroral electrojets (further below).

The magnetometer used by Alfred Zmuda and James Armstrong, of the Johns Hopkins U. Applied Physics Lab, was a relatively crude instrument (resolution 12 nT), flown as an additional "piggyback" payload aboard the Navy's navigational satellite Triad. The magnetometer had no boom to keep it away from interference and it used no tape recorder, while the satellite itself--a long structure, three parts linked by long booms--swung back and forth like a pendulum.

Yet Triad observed very clearly the predicted rotations of B. Zmuda and Armstrong found two parallel current sheets following the morningside auroral oval for almost its full length, with the polar sheet flowing into the ionosphere and the equatorial one out of it; two similar sheets, but with opposite flow directions, were found along the eveningside oval. At the suggestion of Masahisa Sugiura, Iijima and Potemra [1976a, b] later named the polar sheets "region 1" and the equatorward ones "region 2" (Figure 13). Tragically, by the time their work was published [Zmuda and Armstrong, 1974], both authors had died--Zmuda of an untimely heart attack, Armstrong by suicide.

Figure 13

Figure 13. A map of the polar ionosphere, showing the average configuration of Birkeland (field-aligned) currents there. Regions where the current enters the ionosphere have dark shading, regions where it flows away from Earth and into space have light shading. From Iijima and Potemra, [1976b]; the origin is at the magnetic pole and the Sun's direction is on top.

The pattern of Figure 13 was derived from Triad observations by Iijima and Potemra [1976a, b] and is marked by lines of magnetic local time (MLT), measured around the magnetic pole with noon in the Sun's direction. Note that the transition between the ingoing and outgoing portions of the pattern is centered not at midnight but around 2200 MLT: a similar rotation of the electric field pattern should be added in Figure (5b) and could be due to the Hall conductivity [Vasyliunas, 1970]. The crossing-over beginning near 2200 MLT coincides with the region of changes in auroral and magnetic activity known as the Harang discontinuity Heppner [1972b; Fukushima, 1994, appendix D]. Overlaps exist at midnight and additional currents are observed near noon, possibly associated with the cusps; some have named them "region zero.".

It should be stressed that Figure 13 is a statistical average and that actual sheets are much more fragmented and irregular (e.g. Plate 2 of Bythrow et al., [1984]). The greatest intensity of region 1 currents occurs on the day side, in agreement with the observation [Heelis et al., 1976] that E, too, is strongest in a "throat region" near noon. During substorms region 1 is reinforced by a "wedge current" diverting part of the cross-tail current earthward [McPherron et al., 1973]; during northward IMF Bz, the system may weaken and almost disappear [Rich and Gussenhoven, 1987] but characteristic "NBZ currents" may then be observed on the day side, strongly dependent on IMF By and possibly related to the Svalgaard effect.

The flow of region 1 currents far from Earth is still being debated. While some currents near noon may flow down directly along open field lines in the manner of circuit ABCD in Figure 8, the tracing of polar field lines using data-based models of the magnetic field suggests that most of them flow on closed field lines [Stern, 1992] and may therefore be connected to the cross-tail current and to its sunward extensions (Atkinson [1978], Figure 3). Most recently Tsyganenko et al. [1993] found in-situ statistical evidence suggesting that a significant part of the nightside region 1 flow, on the nightside, originates in the plasma sheet, at distances of 10-30 RE, with very little coming from greater distances.