Electric and Magnetic Fields in the Ionosphere and Magnetospheres

Home , Atmospheric dynamo, Birkeland current, Ionosphere

J. Geomag. Geoelectr., 30, 67-107, 1978

Gordon ROSTOKER

Institute of Earth and Planetary Physics and Department of Physics, University of Alberta, Edmonton, Canada

(Received October 20, 1977)

Recent observations of electric fields in the ionosphere using various measurement techniques have led to important advances in the understanding of magnetosphere-ionosphere coupling. In particular the discovery of evidence for parallel electric fields in the altitude range 2,000-10,000 km has an important impact on the question of auroral acceleration processes. More detailed map- ping of large scale field-aligned current configurations on field lines penetrating the auroral oval are leading to more complete models for the generator processes in the outer magnetosphere which are associated with energy dissipation in the near-earth environment. Recent observations of significant electric fields equatorward of the auroral oval indicate that significant interaction between the magnetosphere and ionosphere takes place outside of the auroral oval. New techniques are presently being developed for more quantitative studies of magnetosphere-ionosphere coupling.

1. Introduction

For many years it has been clear that the study of electric fields and perturbation magnetic fields on field lines penetrating the auroral oval was of great importance in understanding the interaction between the ionosphere and outer magnetosphere. Since 1970, several studies have served to provide us with key observations describing the gross properties of the electric and magnetic fields. In particular, CAUFFMAN and GURNETT (1971) and HEPPNER (1972) presented evidence leading to the definition of the electric field pattern in the high latitude ionosphere, while MOZER and LUCHT (1974) used balloon borne electric field probes to acquire a large body of data from which the diurnal variation of the auroral zone electric field was evaluated. The field-aligned currents which link the magnetosphere to the auroral zone ionosphere were first identified by ZMUDA et al. (1967), and were subsequently studied in more detail by ARMSTRONG

t Presented as a Reporter Review for Division III of IAGA at the IAGA Congress in Seattle, Washington, August 1977.

67 68 G. ROSTOKER and ZMUDA (1970) and ZMDA and ARMSTRONG (1974). Their studies confirmed the model of the auroral electrojet proposed by BOSTROM (1964), and laid the groundwork for present advances in the area of magnetosphere-ionosphere coupling. In this review I shall summarize the developments of the past two years in the studies of electric and magnetic fields, and I shall attempt to relate them to previous work in these areas. This review will not deal with substorms, except inasmuch as they influence the general electric and perturbation magnetic field configurations in the magnetoshere and ionosphere.

2. High Latitude Electric Fields To commence my review of electric fields, I would first like to deal with the component El normal to the magnetic lines of force. The general pattern of electric fields at high latitudes has been described using satellite data, balloon- borne probe data and backscatter radar data. Little has changed in our knowledge of the gross properties of the electric field since the last IAGA symposium. However, some recent studies have yielded more definitive information about small scale spatial variations in El associated with auroral arcs and substorm disturbed regions. Three studies of electric fields associated with auroral arcs carried out by rocket borne probes stand out. The first of these was reported in MAYNARD et al. (1977) and EVANS et al. (1977). In the first study, it was confirmed that energetic electron precipitation and electric field magnitude inside the arc are anti-correlated as was first reported by MAYNARDet al. (1973). Both the NS and EW components of El were reduced inside the arc compared to outside (Fig. 1). The electric field was primarily northward both outside the arc and in the interior of the arc, while there was some rotation towards the west at the poleward and equatorward edges of the arc. Detailed calculations by Evans et al. suggested that polarization electric fields are built up inside the arc so that there is little difference between the current intensity inside the arc formed compared to outside, in agreement with the results of WALLIS et al. (1976). A second study of note is that by CARLSON and KELLEY (1977), which involved a launch into a substorm activated arc system. The rocket crossed the arc eventually entering a polar cap environment. The electric field was inferred from ion flow data and measured using double-probe techniques. In agreement with the results of MAYNARDet al. (1977), they found noteworthy perturbations of the electric field direction at the borders of the arc, suggesting strong shear flow at the arc boundaries (KELLEY and CARLSON, 1977). Their results are not in accord with those of Maynard et al., in that within the visable Electric and Magnetic Fields in the Ionosphere and Magnetosphere 69

Fig. 1. Electric field normal to B reported for a rocket launch to an altitude of 242km by EVANs et al. (1977). The electric field is measured in a frame fixed to the earth; Ex is parallel to the traversed auroral arc and E~ is perpendicular to the arc. Both Ex and Ey decrease abruptly as the rocket crosses the equatorward border of the arc near 150 sec. arc the electric field was correlated with energetic electron flux rather than being anticorrelated. The third study was reported by EDWARDS et al. (1976) involving a Skylark rocket launch into an evening sector discrete arc emanating from a substorm disturbed region to the east. They report a northward electric field on both sides of the arc, and a southward electric field inside the arc. The westward electric field was rather small (ranging between + 7mV/m) across the arc, and there was no simple relationship between the properties of the electric field and electron flux. Part of the conflicting results often obtained on rocket shots can be under- stood by virtue of a definitive study of auroral arcs and their related electric field carried out recently by DE LA BEAUJARDIERE et al. (1977) using data from the backscatter radar located at Chatanika, Alaska. The Chatanika backscatter radar can measure ion line-of-sight velocity and electron density; the electron values are obtained every few seconds for typical arcs where densities are 105el/cm3. The ion line-of-sight velocity is obtained at different altitudes over sample intervals of N 15sec-1 min. While only one component of E can be deduced from line-of-sight values in the F-region, the other component can be deduced from E-region ion velocity measurements and an appropriate model atmosphere with an expected error of 3-8 mV/m. Finally, H and, can be 70 G. ROSTOKER computed using the electron density profile, and was computed from j11= v For the study by de la Beaujardiere et al. the radar beam was fixed and the arc drifted equatorward through the beam. Studies of evening sector arcs yielded a reduced northward El inside the arc compared to outside, and an increased ne inside compared to outside. The reduced northward field can be thought of as due to an added southward field: there is also an added westward field inside the arc. The current flows almost northward inside the arc compared to eastward outside, and there is evidence of upward Birkeland current flow above the arc. Similar results were obtained for an arc observed near the vicinity of the Harang discontinuity except that the NS field inside the arc was actually southward whereas outside the arc it was northward. For morning sector arcs, the NS E-field is southward outside the arc and even stronger southward inside the arc. de la Beaujardiere and colleagues claim that the EW E-field variations originate in the magnetosphere, while the NS E-field changes stem from polarization effects in the ionosphere, as suggested by Bostrom. That is, westward E causes NS drifts resulting in positive charges concentrating at the poleward boundary of the oval and negative charges at the equatorward boundary. This, fundamentally, is the mechanism proposed by C0R0NITI and KENNEL (1972) for driving the westward auroral electrojet. We see it here now provides a good means by which auroral arc current systems can be driven. Finally, the observations of upward field-aligned current in the arcs adds to a growing amount of circumstantial evidence that the downward flux of energetic electrons responsible for the auroral luminosity indeed represents the current carrying particles. One should not leave this topic area without mentioning that some workers have not been able to note any significant excess or reduction in current flow in arcs using ground based magnetometer data (e.g., WALLIS et al., 1976). It is important to recognize that ground based magnetometers are unable to resolve current structures with scale sized less than the height of the ionosphere currents above the earth's surface (OLDENBURG, 1976). The results of de la Beaujardiere and colleagues are of great importance in showing that polarization fields and conductivity changes can combine to alter current densities inside arcs compared to outside. However, as EVANS et al. (1977) showed, under at least some circum- stances the current densities inside the arc can be the same as those outside the arc. Finally, one may use the results of de la Beaujardiere et al. to explain the disagreement on the question of the correlation or anticorrelation of electric field and energetic electron flux within arcs. One can see that, for arcs within Electric and Magnetic Fields in the Ionosphere and Magnetosphere 71 72 G. ROSTOKER the westward electrojet, the electric field should be enhanced inside the arc because the (southward) polarization field is in the same direction as the electric field outside the arc. Within the eastward electrojet the polarization electric field opposes the ambient electric field, and one would then expect an anticorrelation of electric field and precipitating electron flux. Thus, the fact that the results of MAYNARD et al. (1977) were obtained over an eastward electrojet (El no orthward) while those of CARLSON and KELLEY (1977) were obtained overa westward electrojet is of great importance in understanding the opposite con-

Fig. 3. Doppler shift measured by HALDOUPis and SoFKO (1976) using a 42 MHz CW aurora backscatter system showing reveral in drift velocity in ionosphere from southward to northward across 0300-0500 CST (Central Standard Time). This implies a change in electric field polarity from westward to eastward as one moves towards dawn. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 73

clusions reached regarding correlatians of El with precipitating electron fluxes. A new and powerful tool for inferring ionospheric electric fields comes from measuring the drift velocity of electron density irregularities using Doppler backscatter radars. New and interesting studies utilizing this technique have been carried out by Tsunoda and co-workers using a radar unit at Homer, Alaska and by Greenwald and co-workers using Doppler radar units in Finland and Norway (STARE). Greenwald and colleagues have now put the STARE system into operation and, by combining the radial Doppler velocities of the irregularities observed at the two sites in Finland and Norway, they are able to compute drift velocities and hence E-region electric fields. In Fig. 2 we see a representative example of their data for a case in which a westward electrojet (eastward drifting electrons) penetrates to the north of the evening sector eastward electrojet (westward drifting electrons). The clear ability of this technique to yield quantitative information on the spatial variation of ionospheric electric fields is very im- pressive. Data from the STARE system, taken together with ground magnetometer data taken is Scandanavia should provide a comprehensive picture of the relationship between auroral electric fields and ionospheric currents. Electric fields in the ionosphere near the southern border of the auroral oval have also been inferred from the Doppler spectrum of 42 MHz continuous wave auroral backscatter by HALDOUPIS and SOFKO (1976). They have detected drift motion in the ionosphere which changes from being strongly southward to weakly northward as time proceeds from midnight to dawn (Fig. 3). They infer an electric field transition occurs from westward to eastward part way through the post-midnight quadrant, in agreement with the results of MOZERand LUCHT (1974). SOLVANG et al. (1977) have also studied drifting irregularities in the electrojet regions, using a 2.75 MHz pulsed transmitter and an array of loop antennae, and have measured drift velocities in excess of 1,000 m/sec in a direction consistent with the expected EX B drift in both the eastward electrojet and westward electrojet regions. Now, in our review of electric fields normal to the magnetic field, we deal with some very significant results pertaining to the dayside cleft reported by HEELISet al. (1976). From vector ion measurements aboard Atmospheric Explorer C, they inferred the electric field pattern over the dayside high latitude region. They observed the usual electric field reversal across the auroras oval reported earlier by several authors (e.g., CAUFFMAN and GURNETT, 1971; HEPPNER, 1972), but they also noted that over most of the dayside cleft the electric field polarity change represented a shear reversal, viz, the electric field goes to zero at the point of reversal. Only near local noon, was it inferred that the electric field changed polarity by rotating through the dawn-to-dusk direction. The implication of this result is that most of the magnetopause may be electri- 74 G. ROSTOKER cally equipotential with the exception of a limited region near the noon meridian. This implication, of course, depends on the ability to map ionospheric electric fields up the fields into the magnetosphere. Recent observations of parallel electric fields above the ionosphere which we shall shortly describe may cause some difficulties in this respect. Finally, I should like to draw attention to some preliminary attempts to generate models describing the behaviour of electric fields at high latitudes. This job has proven to be very difficult indeed, with even the relationship between the horizontal component of the electric field and the perturbation magnetic field across the electrojet being very complex (LANGEL, 1975). None- theless, there have been recent attempts to improve the physical basis for the design of electric field configurations by VOLLAND (1975a), KAWASAKI (1975), YASUHARA and AKASOFU (1977), and HEPPNER (1977). Each of the aforemen- tioned studies has concentrated on different aspects of the problem. While Volland dealt extensively with the convection aspects, Kawasaki and Yasuhara and Akasofu concentrated on the role played by the high conductivity channel associated with the auroral oval. Heppner concentrated on the ramifications of the various electric field signatures at different local times noted in the Ogo 6 data supplemented by barium cloud drift data obtained in the polar cap. His treatment of the difierences between electric field configurations in the summer and winter hemispheres is particularily interesting when taken in conjunction with correlation of electric and perturbation magnetic fields (LANGEL,1975). We now come to what promises to be the most exciting and significant developments in our understanding of electric fields and currents over the past two years-namely the observation of phenomena consistent with the existence of field-aligned electric fields. For some years there was considerable controversy regarding the existence of such fields. Several workers claim to have measured persistent parallel electric fields of the order of mV/m on rocket launches whose apogee was below 300km (e.g., MOZER and BRUSTON, 1967; KELLEY et al., 1975; MAHON et al., 1977 among others) while other researchers claim that such electric fields are absent at these altitudes in the auroral regions (e.g., most recently MAYNARD et al., 1977). Because of the difficulties in making in situ measurements of electric fields in the lower ionosphere, the question of the existence of parallel electric fields in the lower ionosphere is still a matter of controversy. What has developed, however, is overwhelming evidence for parallel electric fields in the altitude range 2,000-10,000km. The first strong evidence for the existence of these parallel E-fields was reported by WESCOTT et al. (1976) in which they followed the drift motion of a barium cloud which extended from near 4500 km to near 10,000 km altitude in the dawn sector (Fig. 4). At the time that the projected 100 km intercept of Electric and Magnetic Fields in the Ionosphere and Magnetosphere 75 76 G. ROSTOKER

UNIVERSAL TIME, 29 JULY 1976 Fig, 5. DC electric field, AC electric field and energetic electron fluxes during episodes on which large electric fields (hundreds of mV/m) were observed at high altitude as reported by MOZER et al. (1977). The large parallel E fields are particularly interesting. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 77 the barium streak coincided with the poleward boundary of the auroral oval, the cloud brightened noticeably. Simultaneously; the part of the streak above 5,500km split into multiple streaks and moved off rapidly with respect to the streak below 5, 500km. The drift velocities at 9,000 km altitude were 15km/sec, which would correspond to a 200 mV/m field at 100 km; however, the lack of motion of the streak below 5,500 km indicated that E1 did not map to E-region altitudes. Finally, evidence was presented suggesting upward acceleration of barium ions in the region of anomalous E above 5,500 km. Shortly after the study by Wescott et al., a completely independent study by MoZER et al. (1977) yielded in situ measurements of parallel electric fields and anomalously high electric fields normal to the magnetic field in the altitude range 2,000-8,000 km over local times from 0500-2300 on magnetic field lines with L values ranging from 7-20. Evidence for such anomalous electric fields is shown in Fig. 5. The fields vary on time scales less than 10 sec and are very structured; there is a great deal of plasma turbulence in the regions of strong anomalous E. Oxygen and hydrogen ions which must have been accelerated to keV energies below the satellite are observed to emanate from the ionosphere, confirming the results of SHELLEYet al. (1976). The discovery of anomalously high electric fields in the topside ionosphere and above has coincided with intensive theoretical studies which predicted the existence of such fields. Several possible mechanisms for the production of parallel electric fields are being considered, as I shall describe below. The theoretical concept of double layers as applied to the magnetosphere- ionosphere system has been promoted primarily by BLOCK(1972), with the principle being developed both analytically and numerically by KNORR and GOERTZ (1974), and GOERTZ and JOYCE (1975). In particular, Goertz and Joyce have demonstrated a relationship between the thickness of the double layer and the potential drop across it (Fig. 6). BLOCK(1975) has reviewed the status of

Fig. 6. Thickness of a double layer (L) as a function of the potential drop across the layer (C/O) as calculated by GoERTZ and JOYCE (1975). There appears to be a range of potential drops above a curtain threshold for which the thickness of the double layer is relatively constant. 78 G. ROSTOKER double layers, and has suggested that current densities of 2X105A/m2 at 3,400 km would lead to the formation of double layers. Therefore, accord- ing to Block, conditions for double layer production would be favored in the altitude range where large parallel electric fields have been detected. The idea of anomalous resistivity as an agent for the development of parallel electric fields in and above the ionosphere has been considered extensively by several workers and has been reviewed recently by HASEGAWA(1974), BLOCK and FALTHAMMAR (1976), and PAPADOPOULOS (1977). Basically, the concept centers around current limiting instabilities where wave-particle interactions lead to turbulence and eventually potential drops along the magnetic lines of force. In a third approach, BLOCK and FALTHAMMAR (1976) and LENNARTSSON (1976) have reintroduced the concept of generating parallel electric fields by invoking different pitch angle distributions of electrons and protons which was put forward by ALFVEN and FALTHAMMAR (1963). Yet another mechanism was considered by SWIFT(1975) who proposed the existence of current-driven laminar electrostatic shocks oblique to the ambient magnetic field. The mechanism has much in common with double layers, although double layers have a thickness which scales as the Debye length whereas the electrostatic shocks proposed by Swift scale as the ion gyroradius. Nonethe- less, both double layers and electrostatic shocks are current driven, laminar and do not require plasma turbulence to produce resistivity. Insofar as the observations are concerned, MOZER et al. (1977) seem to favor the oblique electrostatic shock model over the classical double layer since the acceleration region appears to have a thickness of the order of the ion gyroradius, which is much larger than the Debye length. On the other hand, BLOCK (1975) seems to regard the oblique electrostatic shock as a double layer phenomenon so that there may only be a problem of semantics to consider. However, it is worth noting that both the double layers and the oblique electrostatic shocks do not invoke plasma turbulence, while the observations by Mozer et al. clearly indicate the presence of turbulence in the region of enhanced electric field. I would expect that the next two years should see intensified activity in both solidifying the data base and resolving the theoretical arguments pertaining to parallel electric fields in the ionosphere and magnetosphere. The final topic which I want the deal with in the area of electric fields related to those electric fields generated by time-varying magnetic fields. While the existence of these induction electric fields has been known on strictly physical grounds for a long time, only within the last decade have they been considered quantitatively in a magnetospheric context. In the early 1970's, J.S. Kim and colleagues began carrying out studies primarily directed towards the induced Electric and Magnetic Fields in the Ionosphere and Magnetosphere 79 electric fields associated with variations in the strength of the ring current. More recently, they have dealt with the manner in which inductive electric fields affect the trapped electron distribution (MURPHY et al., 1975a), the inductive electric field associated with magnetotail currents (MURPHY et al.,1975b) and they have continued their work on ring current effects (MURPHY et al., 1975c). Most recently, HEIKKILA and PELLINEN (1977) have treated the problem of the inductive electric field in the magnetotail associated with substorm disturbances. They contend that large electric fields parallel to magnetic field lines may persist in the tail for some minutes, associated with short circuiting of cross-tail current into the ionosphere in a limited longitudinal sector of the tail. They attribute the anisotropic fluxes of high energy (E200 keY) electrons observed by KIRSCHet al. (1977) to such inductive electric fields. While this short review of recent developments in our knowledge of electric fields in the high latitude magnetosphere and ionosphere is somewhat brief, the reader may refer to reviews by GUREVICH et al. (1976), MOZER (1976b), and STERN (1977) for more detailed discussions.

3. Field-Aligned Currents and Ionospheric Electrojet Currents in the Auroral Oval Prior to the last IAGA meeting, important investigations regarding the char- acter of field-aligned currents penetrating the auroral oval had been carried out by Zmuda and colleagues using magnetometer data from polar orbiter satellites. This work culminated in the realization that field-aligned currents flowed in pairs of east-west aligned antiparallel sheets as predicted by BOSTROM (1964). In the afternoon sector current was into the ionosphere in the equatorward portion of the auroral oval and out of the ionoshere in the poleward portion: the current flow directions are reversed in the morning sector oval. By 1975, the first hints that the current sheets did not provide balanced flow when integrated along a meridian were coming to light. These results are summarized in a study by SUGIURA (1976a). SUGIURA (1975) and YA5UHARA et al. (1975) both utilized polar orbiter magnetometer data to show that there was an excess of current into the ionosphere in the pre-noon quadrant and an excess of current out of the ionosphere in the dusk sector. Since that time there have been well documented definitive studies carried out by SUGIURA and PoTEMRA (1976), and IIJIMA and POTEMRA (1976a, b) using magnetometer data from the Triad satellite which have been very useful in defining the patterns of field- aligned current flow in more detail. A statistical summary of their findings is shown in Fig. 7. The intrusion of morning sector flow pattern into the evening sector, also noted by ROSTOKER et al. (1975) for substorm events is quite notewor- 80 G. ROSTOKER. thy. As well, the pattern is very complicated near local magnetic noon, with cleft currents in the high latitude regions being particularly complex. Peak unbalanced current flow was found to be into the ionosphere 0700-0800 LMT and out of the ionosphere. 1500-1600 LMT. KAMIDE et al. (1976b) have shown that the westward auroral electrojet spans both inward and outward current flow in the morning sector, while ROSTOKER et al. (1975) found that the eastward electrojet in the evening sector spans both downward and some upward current. Most recently, KAMIDE and AKASOFU (1976b), and KAMIDE and ROSTOKER (1977) have shown that upward current appears to be carried by downward moving energetic electrons in discrete auroral forms. Thus, the upward current in the poleward portion of the eastward electrojet is found in discrete arcs imbedded in the diffuse aurora and at the poleward border of the diffuse auroral oval (where the electric field suffers a transition in polarity). As far as the morning

Currents into Ionosphere Currents Away from Ionosphere Fig. 7. Composite diagram (courtesy Dr. T.A. Potemra) showing statistical distribution of regions of inward and outward field aligned current flow as inferred from Triad data. Some correlative features are also shown on the diagram. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 81 sector is concerned, KAMIDEet al. (1976b) have shown that the field-aligned currents flow entirely within the region of the westward electrojet with downward current flowing into the poleward portion and upward current flowing out of the equatorward portion of the electrojet. KAMIDE and ROSTOKER (1977) have shown that the downward current is likely carried by thermal electrons streaming upward from the ionosphere. While the Triad magnetometer has been instrumental in generating the data suites on which most of our detailed knowledge of field-aligned currents is now based, McDiarmid and colleagues using the ISIS 2 magnetometer have recently been able to provide interesting and important new information on field-aligned currents particularly in the polar cap. The advantage of the ISIS 2 data is that the spacecraft also carries a full complement of particle detectors and scanning photometers. In addition, the tracking facilities permitted data to be acquired across the entire polar cap along with the auroral oval, whereas

Fig. 8a. East-west magnetic field perturbation and energetic electron parameters measured on a dusk-to-dawn pass at 1,400 km by the ISIS 2 satellite and reported by MCDIARMID et al. (1977). Current flow directions (vertical arrows) are in agreement with results of ZMUDA and ARMSTRONG (1974). 82 G. ROSTOKER the Triad data was always cut off near the geomagnetic pole. MCDIARMID et al. (1977) have shown that, while most passes exhibit normal field-aligned current configurations in agreement with the results of ZMUDA and ARMSTRONG (1974) (Fig. 8a), some morning passes show anomalous flow in the polar cap (Fig. 8b). Another interesting result stemming from analysis of the ISIS 2 data was reported by KLUMPAR et al. (1976) who claimed that, in the morning sector, downward current is carried by structured fluxes of low energy electrons moving upwards from the ionosphere. A similar conclusion has been reached by KAMIDE and RoSTOKER (1977) based on coordinated Triad, DMSP and ground based data. It should be pointed out that, at the present time, it is usual to consider spatial gradients in the measured east-west component of the magnetic field above the ionosphere as evidence of in situ field-aligned current flow from an east-west aligned sheet. However, such simple models imply the current sheets have infinite east-west extent which is, of course, not realistic. KISABETH (1977)

Fig. 8b. Same as Fig. 8a except field-aligned current directions as the morning sector appears to be reversed compared to the statistical pattern presented by ZMUDA and ARMSTRONG (1974). Electric and Magnetic Fields in the Ionosphere and Magnetosphere 83 has shown that a satellite passing off the edge of a Birkeland current loop configuration will see edge effects which might incorrectly be interpreted as in situ current flow (Fig. 9a). In addition, the finite east-west extent of the current flow results in a level change in the perturbation magnetic field across the region of field-aligned current flow which is not inductive of net current flow but merely of the geometry of the current system. Thus, many level changes which have been interpreted in terms of net field-aligned current flow may be, in part,

Fig. 9a. Latitude profiles of magnetic perturbation calculated for a model Birkeland current system with a longitudinal extent at 90 reported by KABETH (1977). Each profile is taken at a different position relative to the central meridian C. The current is concentrated at the poleward and equatorward edges of the system. Note that gradients in B do not necessarily imply in situ field-aligned current flow, but reflect edge effects. 84 G. ROSTOKER

Fig. 9b. Latitude profile across a region of Birkeland current flow finite longitudinal extent. The current distribution is indicated by the vertical hatching. Upward current is equal to downward current integrated over the latitudinal extent of the Birkeland current system. The apparent level change is not due to net field- aligned current, but to edge effects. simply due to edge effects of the finite sized Birkeland current configuration (Fig. 9b). The above observations of Birkeland current flow have, combined with ground based magnetometer results, permitted some more comprehensive attempts to model high latitude current flow. YASUHARA et al. (1975) and YASUHARA and AKASOFU (1977) have produced a comprehensive model current system involving field-aligned, electrojet and cross-polar cap ionospheric currents. They used vertical currents to represent field-aligned currents and utilized integrated Hall to Pederson conductivity ratios based on results from the Chatanika backscatter radar. In Fig. 10, we show their calculated ionospheric current pattern for net field-aligned current into the ionosphere at dawn and out of the ionosphere at dusk. A second comprehensive current distribution has been produced by HUGHES and ROSTOKER (1977a, b) and is shown in Fig. 11. This model is based on a synthesis of ground based meridian line magnetometer data and polar orbiter space craft data, and is consistent with the electric field distribution in the high latitude ionosphere reported by MOZER and LUCHT (1974). The major concept centers around net current flow into the noon and pre-noon sector with the field-aligned current diverging into the ionosphere as the eastward and westward electrojets. Part of the electrojet current flows up the field lines at the Electric and Magnetic Fields in the Ionosphere and Magnetosphere 85 dawn and dusk terminators where there is a significant ionospheric conductivity discontinuity. Continuing our review of field-aligned currents, we note that a great deal of work has been carried out over the last two years using backscatter and Doppler radar systems to infer the gross properties of ionospheric and field-

ionospheric Current Pattern

(a)

(b)

Fig.. 14. High latitude ionospheric and field-aligned current system inferred from Triad data by YASUHARAet al. (1975). The dashed line in part (a) of the figure shows the Harang discontinuity. 86 G. ROSTOKER aligned current flow in the auroral oval region. KAMIDE et al. (1976a) have used the height-integrated ionospheric current densities deduced from data from the Chatanika backscatter facility and simultaneous ground based magnetometer data from the Alaska meridian line to infer the existence of net upward field- aligned current flow on the poleward side of the auroral oval in the evening sector; this observation was subsequently incorporated into a more formal phe- nomenological model by KAMIDE and AKASOFU (1976a). A definitive study of the relationship of radar aurora, visual aurora, and the auroral electrojets was carried out by TsUNODA et al. (1976a) using a 398 MHz phased array radar at Homer, Alaska. They found that diffuse auroral echoes at 398 MHz occurred in the region of the eastward electrojet, while discrete auroral echoes were associated with discrete auroral arcs close to the poleward boundary of the

eastward jet. Similar data were used by TsUNODA et al. (1976b) to relate radar aurora echoes to field-aligned current flow in the evening sector. They con- cluded that downward field-aligned current was collocated with diffuse radar aurora and the eastward electrojet, and that upward field-aligned current was

Fig. 11. High latitude ionospheric and field-aligned current inferred from ground based and Triad data by HUGHES and ROSTOKER (1977b). Field-aligned current directions are indicated by x and. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 87 associated with discrete arcs. They also noted a poleward electric field between the two current sheets, suggesting closure by Pedersen current flow. Recently, RINGet al. (1977) have developed techniques for inferring horizontal ionospheric current densities at different altitudes from the line-of-sight ion velocity measured using backscatter radar. They were also able to compute neutral wind velocities in the E-region and find that while ion drag is a significant factor in establishing night-time neutral wind patterns, there are other unidentified driving forces competing directly with ion drag forces. This is consistent with the theoretical calculations of MAEDA(1976) who found that neutral winds of electric field origin are of the same order of magnitude as those of thermal origin. There has been a significant amount of work carried out on the development of models dealing with the physics of field-aligned current flow. The various mechanisms have been reviewed in some detail by RoEDERER (1977). Basically they can be grouped as follows: 1) Generator is on field lines connected to the interplanetary medium or entry layer. 2) Plasma in tail gives energy to the electrical circuit through MHD generator processes. 3) Poleward and equatorward current sheets are decoupled. A recent example of mechanism (1) is found in EASTMAN et al. (1976) who call on MHD generator processes in the boundary layer, where leakage of the polarization charge along magnetic field lines to the ionosphere results and field-aligned currents. ROSTOKER and BOSTROM (1976) have developed a mechanism (2) in which energy is extracted from plasma as it drifts towards the flanks of the magnetotail. This energy supplies the requirements for an electric circuit which involves currents transverse to magnetotail field lines being connected to ionospheric Pedersen currents by the field-aligned current sheets. The third mechanism has been proposed by IIJIMA and POTEMRA (1976a), who feel that the difference in behaviour of the two antiparallel current sheets warrants the conclusion that they are driven by different sources. SUGIURA (1975) has suggested that the equatorward current sheet connects the ionospheric current to the cross-tail current sheet. Another model which has been developed involving electrojet and field- aligned current flow has been presented by PUDOVKIN and UVAROV (1976) who considered the effects where one hemisphere is in darkness and the other one is illuminated. They reached the conclusion that the auroral electrojet in the evening sector is partially fed by current from the summer hemisphere. They further contend that the current configurations in the summer and winter hemispheres are, geometrically, quite different. 88 G. ROSTOKER

Fukushima has carried out several studies of the magnetic perturbation patterns generated by the combination of field-aligned currents and the ionospheric currents to which they connect (FUKUSHIMA, 1974, 1975a, b, c, d, 1976; FUKUSHIMA and KAWASAKI, 1974; KAWASAKI and FUKUSHIMA, 1975). These works deal with analytic treatment of charge accumulation at boundaries, realistic conductivity distributions and current distributions; particular emphasis is laid on the ability of ionospheric and field-aligned currents to produce off- setting magnetic perturbations at the earth's surface. Thus, ground based magnetometers may detect little or no perturbation, although substantial ionospheric and field-aligned current may flowing. SATO (1976) has also treated analytically the problem of the relationship between field-aligned current, the electric field and electron density gradients in the polar cap region. He finds self-consistent solutions which are morphologically acceptable if there is an electric field component along the auroral belt pointing from noon to midnight. We now shall look at the recent work carried out on field-aligned currents associated with auroral arcs. H.R. Anderson and P.A. Cloutier and colleagues have carried out detailed studies of rocket launch data involving passage through a discrete auroral arc in the evening sector (ANDERSON and CLOUTIER, 1975; PAZICH and ANDERSON, 1975; SPIGER and ANDERSON, 1975; CASSERLY and CLOUTIER, 1975). The arc lay near the poleward edge of an eastward electrojet. It was found that there were oppositely directed Birkeland current sheets associated with the arc, with upward current flow (2.7X10-5 Aim) coinciding with peak auroral luminosity and maximum energetic electron precipitation. The downward current was equatorward of the upward current. The upward current deduced from the. particle detectors (0.5

Fig. 12a. Schematic of an auroral arc of north- south extent W and infinite east-wast extent proposed by VoNDRAK (1975). Current flows into the ionosphere on both sides of the arc, and out of the ionosphere within the arc. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 89

Crete arc structures traversed by the rocket. Sheet thicknesses ranged from 20 to 60km while current densities varied from 10-45 pA/m. The upward current occurred in the region of discrete aurora, in agreement with the results of SPIDER and ANDERSON (1975). Based on rocket observations in auroral arcs, VONDRAK (1975) and CARLSON and KELLEY(1977) have constructed models of the field-aligned current flow in which the vertical currents are driven by variations in the horizontal currents arising from electric field gradients (as opposed to conductivity gradients). Vondrak's model would appear to demand downward current flow at both edges of the arc (Fig. 12a) while Carlson and Kelley consider downward current on one side of the arc only (Fig. 12b). Vondrak attributes the observations of downward flow being only on one side of the arc to factors as yet unknown. It is, however, interesting to note that BURCHet al. (1976b), using AE-C ion drift measurements, have developed a model similar to that of Vondrak to explain electric field observations in inverted V structures (which are supposed to be associated with discrete auroral arcs). Their model (shown in Fig. 12c) suggests significant differences in the electric field patterns from low altitudes (E-region) to higher altitudes (top-side ionosphere). Finally, we note the work of KAMIDE and AKASOFU (1975) who have used DMSP and ground based magnetometer data to relate the position of the auroral electrojets to the auroral features (luminosity>2kR) definable by the DMSP

POLARIZATION FIELD AND ELECTROJET

Fig. 12b. Parameters of a model auroral arc suggested by CARLsON and KELLEY (1977). Downward current is only on one side of the arc, while the current flow is upward inside the arc. 90 G. ROSTOKER

Fig. 12c. Parameters of a model auroral arc suggested by BURCH et al. (1976b). Current flow direction are the same as those suggested by VoNDRAK (1975), but the E-field changes markedly as a function of altitude. imagery. They find that the westward jet flows along the diffuse aurora in the morning and midnight sectors, while the eastward jet flows mainly equatorward of the discrete arcs in the evening sector.

4. Polar Cap Effects We now come to work which deals with polar cap effects inferred from ground based magnetometer data. We shall first of all deal with effects related to the north-south component of the IMF after which we shall treat effects associated with the azimuthal component of the IMF. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 91

One of the major pieces of work dealing with polar cap magnetic variations was carried out by MAEZAWA (1976), who investigated the forms of the equivalent current system associated with purely northward and southward IMF Bz. He used regression techniques to subtract the effects introduced by the azimuthal component of the IMF By, and considered only absolute values of Bx>1nT. The equivalent current systems in the polor cap for southward and northward IMF conditions are shown in Fig. 13a and 13b as convection vector diagrams. The outstanding feature is the reversal in the direction of the equivalent current flow between 80 and 85 near local magnetic noon. The equivalent current for northward IMF is not merely reduced in magnitude compared to southward IMF situations--it is actually reversed in direction. Such a possible current configuration was pointed out for quiet times by IwASAKI(1971). A similar result has been reported by (1977) and REZHENOV and FELDSTEIN (1977). It is interesting to note that the major perturbations occur in the region of the noon polar cleft for the northward IMF case. Of course, in any study of this nature, the question arises whether or not the equivalent currents represent real ionospheric currents or the distant effects

a b Fig. 13. Equivalent current vectors with arrow directions reversed to indicate the direction of cross polar cap convective flow as computed by MAEZAWA (1976). Part (a) pertains to southward IMF and part (b) to northward IMF. The scale factor at the bottom of each figure relates polar cap magnetic field perturbation strength to north-south IMF (the ratio being 10: 1). Note the remarkable reversal of convective flow from (a) to (b) in the dayside cleft region. 92 G. ROSTOKER or field-aligned currents (or both these effects). The problem stems from the fact that a winter polar ionosphere has a very low conductivity-thus currents flowing within the electrojets towards the pole would encounter a major conductivity discontinuity and would tend to diverge up the field lines rather than flow into the polar cap ionosphere. The situation would, of course, be different for a summer ionosphere when the polar cap ionospheric conductivity might be expected to be rather larger than for the winter. This question has been attacked by PRIMDAHL and SPANGSLEV (1977) who have shown that magnetic variations attributable to polar cap ionospheric flow are higher by a factor of 10 in the summer months compared to the winter months. In addition, they have pointed out the importance of the dawn and dusk terminators, in that ionospheric current will diverge into the magnetosphere at these conductivity discontinuities. This effect has also been noted by ROSTOKERet al. (1977) as being important as the auroral oval regions. Further work on polar cap magnetic variations has been carried out by GIZLER et al. (1976) following on from the studies of TROSHICHEV et al. (1974) who separated the high latitude current configuration into three parts: DP2which is a two-vortex current system at latitudes >60 without any jet currents in the auroral zones; DP12which is a two-vortex system in the polar regions with two auroral electrojets; DP11which is a single current system with one westward jet associated with the substorm expansive phase. Gizler et al. claim that DP12disturbances are associated with pure ionospheric currents, while DP11stems from a three-dimensional Birkeland current loop. KUZNETSOv and TROSHICHEv (1977) have also studied polar cap magnetic variations and find reversed equivalent current flow in the day-side high latitude regions in agreement with MAEZAWA (1976); they term this type of pattern DP3. They also claim that disturbances near the pole associated with the development of southward IMF are reliable substorm precursors, and they have designed an index PCL to quantify the precursor signals. Perhaps the problem regarding polar cap disturbances which has been attacked most intensively over the past two years has been that of the effect of the azimuthal component of the IMF, B. After the initial proposition of the effect independently by SvALGAARD (1968) and MANSUROv (1969), and the definitive study by SvALGAARD (1973) a great deal of work has been done particularly in the Soviet Union. We first turn our attention to an important publication by FELDSTEIN(1976) which reviews previous work in the area of the influence of B on polar cap magnetic disturbances, as well as introducing some new material. Particularly interesting is a discussion of polar cap conductivities under illuminated and unilluminated conditions. In this treatment, Feldstein finds that DPC dis- Electric and Magnetic Fields in the Ionosphere and Magnetosphere 93 turbances (associated with the IMF B component) drop dramatically in ampli tude across the terminator separating the sunlit and dark ionosphere. However, the size of the drop in amplitude of the magnetic disturbances is far greater than would be expected on the basis of conductivity considerations alone (Fig, 14). Feldstein concludes that the UT dependence of the DPC effect (SVALGAARD, 1973) may, in large measure, account for the discrepancy discussed above. A second major review of the influence of the IMF B component on polar cap magnetic disturbances has been presented by MISHIN(1977). He suggests that the magnetic variations in the polar cap associated with the B5 component of the IMF are generated in the polar cleft, supporting the earlier contentions of fORGENSEN et al. (1972), STERN (1973), LEONTJEV and LYATSKY (1974), and MATVEEV and MISHIN (1975). More recently, MISHIN et al. (1977) have presented evidence that it is not permissable to neglect the effect of the IMF Bx component, in that it causes a rotational orientation of the cross-polar cap equivalent current flow. This result is not in accord with earlier conclusions of FRIIS- CHRISTENSEN et al. (1972) and SUMARUK et al. (1974) who contend that the IMF Bx component has no readily identifiable polar cap magnetic signature. FRIIS-CHRISTENSEN and WILHJELM (1975) have continued their studies of the relationship between polar cap magnetic fields and the component of the IMF in the yz-plane, and have noted that for northward IMF there is a poleward shift of the currents associated with the IMF B5 component; they point out a marked reduction in By related polar magnetic variations from summer to winter

Fig. 14. Diurnal variation of the relative values of the Z component magnetic disturbances and maximum E-layer ioni- zation at Resolute Bay reported by FELDSTEIN (1976); open circles given maximum ne(nem)normalized to the peak value of nemover the day while closed circles give the Z. component magnetic perturbation normalized to the peak values of AZ over the day. Note the relatively large changes in (4Z/AZmax) compared to those in (nem/nemmag);conductivity changes alone cannot explain this behaviour. 94 G. ROSTOKER compared to the behaviour of Bz related disturbances, indicating B and Bz related magnetic perturbations involve completely different current systems. The same conclusion was reached by KAWASAKIet al. (1973) for short period magnetic perturbations on the polar cap related to the IMF Bx and B components. SVALGAARD(1972) presented a subjective technique for utilizing polar cap magnetograms to determine whether the IMF has an away or toward polarity (the so-called A/C sector). Subsequently, SVALGAARD(1975) developed a more objective technique for inferring IMF polarity using the H-component of God- havn magnetograms. He contends that only when the IMF Bz has a constant sign for extended time intervals are systematic errors introduced. BERTHELIER and GUERIN(1975) have pointed out that the A/C index appears to break down before 1963 as an index which can be relied on a day-to-day basis. As well, RUSSELL et al. (1975b) used data from 1972 to show that there was a higher success rate on predicting toward days compared to away days using the A/C index. From a theoretical viewpoint, VoLLAND (1975b) has attempted to explain the Svalgaard-Mansurov effect by considering the possibility of differential motion of magnetospheric plasma with respect to the earth. He contends that penetration of the interplanetary electric field in the region of the polar cleft will cause drift motion in opposite directions in the northern and southern high latitude regions leading to the asymmetric behaviour of the polar cap electric field reported by HEPPNER (1972).

5. Magnetic and Electric Fields and Current Flow at Middle and Low Latitudes One of the most interesting new results in recent times has been reported by BURGH et al. (1976b). Using ion drift measurements from the AE-C polar orbiting satellite, they have found evidence of remarkably strong electric fields in the region of the electron trough well equatorward of the auroral oval (Fig. 15). These fields result in strong sunward convective flow at low latitudes, and appear to occur in association with substorm activity. CARPENTER and SEELEY (1976) have used whistler data to infer cross-L motions of magnetospheric plasma and have thence inferred the direction of the magnetospheric electric field over the range 3.5

LUCHT (1974) and HALDOUPIS and SOFKO (1976).) After noon, the drift direction reverses again and there is sustained inward drift corresponding to a westward electric field of O.15mV/m. BLANC et al. (1977) have used incoherent scatter data from a low latitude (44.5N geographic) facility at Saint-Santin to measure ion drift velocity and thence to infer low latitude electric field strengths at ionospheric heights. Their results suggest that for quiet times, the contributions of dynamo and magnetospheric electric fields are comparable; however, under conditions of stronger magnetospheric activity, changes in the magnetospheric contribution have significant effects. These results are consistent with those of CARPENTER and KIRCHHOFF (1975) who noted a strong correlation between ionospheric electric fields inferred from ion coherent scatter data at Chatanika (65N) and Mil- lstone Hill (43N). For some time it has been known that dynamo effects in the middle and low latitude ionosphere would lead to the production of electric fields and currents. In a recent study, VoLLAND (1976) has used electrical circuit analogies to deal with the effects of the diurnal and semi-diurnal tidal modes. He points out that conventional dynamo theories neglect the internal resistance of the atmospheric electric circuit, which results in errors in the amplitude and phase relationships between heat input on one hand and wind and electric current on the other hand. RICHMOND et al. (1976) have also re-examined the dynamo theory to see if it can be reconciled with present day observations of ionospheric winds and electric fields. They find that the first negative diurnal tidal mode (which is present in the upper E-region and lower F-region) is capable of ac- counting for most of the Sq currents and a substantial part of the observed low latitude electric field distribution. They suggest that electric fields with their source in the magnetosphere are more important at nighttime than in the day- time at low latitudes. Insofar as low latitude ionospheric current flow is concerned, the many slow-varying current systems have been reviewed recently by MATSUSHITA (1975a) and by KANE (1976). Two new results pertaining to mid-latitude ionospheric current flow are worth noting. BURROWS(1976) found from magnetometer data obtained aboard a rocket payload which reached an altitude of 210 km at middle latitudes, that during a weak magnetic storm there was no evidence for ionospheric current flow over and above the normal Sq contribution. This result again emphasized that most auroral zone ionospheric currents diverge into the magnetosphere as field-aligned currents rather than closing in the ionosphere through low latitude return currents. In another study, MATSUSHITA (1975b) has pointed out that the Svalgaard-Mansurov effect is also detectable at low latitudes. He notes that the focus of the Sq current Electric and Magnetic Fields in the Ionosphere and Magnetosphere 97 system (near 30N) is about 4 further north for away sectors of the IMF than for toward sectors (Fig. 16). Finally, we come to the equator and note several recent studies of the equatorial electrojet. GAGNEPAIN et al. (1976) have developed a theory for longitudinal gradients in the equatorial electrojet which may stem from conductivity gradients, ionospheric winds, or magnetospheric electric fields. They show how electric fields produced at various latitudes are attenuated between the source latitude and the equator, and thus they show that electric fields with small horizontal scale lengths must be generated close to the equator for them to have any significant effects on the equatorial electrojet. The results of a major study of the equatorial electrojet by FAMBITAKOYE (1976) have been presented in a series of papers which deal with the influence of the equatorial electrojet on the regular daily variation SR. FAMBITAKOYE and Mf the equatorial electrojet on the regular daily variation SR. FAMBITAKOYE any the electrojet component S of the regular daily variation and have noted the existence of a counter electrojet as a regular feature in the morning sector and the occasional appearance of the counter electrojet in the afternoon sector. FAMBITAKOYE and MAYAUD (1976b) found that, near noon, the center of the equatorial electrojet coincides with the dip equator while for the morning counter electrojet, the center is about 40 km north of the dip equator. FAMBITAKOYE and MAYAUD (1976c) have gone on to demonstrate the variability

TOWARD AWAY

MAY & JUNE 1965 Fig. 16. Equivalent current flow for toward and away sectors showing the shift in the position of the focus of Sq with IMF polarity reported by MATSVSHITA (1975b). 98 G. ROSTOKER from day-to-day of the regular daily variation SR, which is attributed to the presence of the counter electrojet and to neutral winds in the equatorial ionosphere. FAMBITAKOYE et al. (1976) have demonstrated that east-west winds can create current flow opposed to the primary electrojet, and that these winds augment the planetary component of SR around midday over values produced purely by magnetospheric electric fields. Finally, we note that MAYAUD (1976c) has failed to find evidence of regular variations of the magnetic field at ground level which OLSON (1970) predicted should result from magnetopause, neutral sheet and ring current effects and should be measurable at quiet times.

6. Long Term Geomagnetic Trends Due to External Sources Many of the modern geomagnetic indices can be used to yield magnificent new information regarding major and minor ionospheric and magnetospheric current systems. For many years it has been recognized that the Kp index has, contained within it, biases due to a lack of coverage in the eastern European and Asian sectors (MICHEL, 1964). Recently SVALGAARD (1976) has recalibrated the indices Kp and ap by using the am index (MAYAUD, 1967) to recover the UT time variations lost in the Kp index. Using the new index Am,* Svalgaard was able to confirm the observations of RUSSELL et al. (1975a), MISHIN and SHELOMENTSOV (1975), and BERTHELIER and GUERIN (1975) that intervals of toward polarity were considerably more active than intervals of away polarity prior to 1963. BERTHELIER (1976) has utilized the Am and AE indices to study the annual variation of geomagnetic activity and its variation with UT as a function of the polarity of the IMF Bz component. For the case of Am, she confirms the MCINTOSH (1959) hypothesis that magnetic activity is enhanced when the angle between the earth's magnetic axis and the earth-sun line is close to 90. On the other hand, for AE, neither the semi-annual nor the diurnal variations predicted by the Mclntosh can be observed. In a definitive study of high latitude magnetic activity using the AE, AU and AL indices, ALLEN and KROEHL (1975) have studied the diurnal behaviour of the auroral electrojets. They showed that, during moderately disturbed times, the region of maximum, AL coincides with the expected position of the westward auroral electrojet and with the auroral oval while the region of maximum AU tends to coincide with the expected position of the eastward electrojet, but equatorward of the auroral oval, the peak electrojet strengths for the eastward and west electrojets are found at 1730 IMT and 0315 IMT respectively. Electric and Magnetic Fields in the Ionosphere and Magnetosphere 99

At quiet times a distinct region of AU was noted between 6O-65 latitude on the dawn section while an equally distinct region of AL was noted in the same latitude range across local magnetic noon. SUGIURA (1976b) has utilized Dst to demonstrate the existence of a quasi- biennial geomagnetic variation whose strength is dependent on the level of geomagnetic activity. He contends that this phenomenon is of direct solar origin (as opposed to solar wind origin) and is caused by a secular variation in solar activity. DELouls and MAYAUD (1975) have employed 103 years of values of the 3-hour as index and the K indices from the two antipodal observatories from which as is derived to study long term variations. They find the semiannual and annual variations to be quite stable, while the 11-year cycle and 90-year cycle are more or less stable. Over a time series of this length, the so-called 27-day recurrence is smeared out to the point of being unidentifiable and no influence of the moon was detectable. Full details of this study appear in three detailed studies by MAYAUD (1975, 1976a,b).

7. Mathematical Representations of Magnetospheric and Ionospheric Mag- netic Fields In this final section, I should like to point out some new developments in quantitative modelling of ionospheric currents and magnetospheric currents and field configurations. Recently, KISABETH and ROSTOKER (1977) have published the details of their analytic treatment of three-dimensional current systems. The techniques described involve computation of magnetic perturbations from the two types of Birkeland current systems where earth induction is taken into account. Following on this work, OLDENBURG (1976) has utilized the linear inverse theory of BACKUS and GILBERT (1970) to estimate the distribution of ionospheric current across the auroral electrojets using meridian line magnetometer data. In a subsequent paper, OLDENBURG (1978) has further developed techniques for producing specific height-integrated current distributions which reproduced observed magnetic perturbation patterns to within a certain specified accuracy. Another important paper within the last two years has been published by STERN (1976) describing in detail the several mathematical techniques availa- ble to describe magnetic fields in space. These are: (1) Representations using current density j, which yields B through Biot- Savart's law. (2) Representation of B by the gradient of a scalar potential. (3) Representation of B by orthogonal vectors. (4) Representation of B by Euler potentials. 100 G. ROSTOKER

(5) Local representations where B is expanded around its value at some reference point.

8. Conclusions The last two years have seen important advances in our understanding of the solar-terrestrial interaction. Much of the work done is of a nature which puts our understanding of the phenomenology of the magnetosphere on firmer ground by confirming earlier suggestions and adding to the details of areas which have been less comprehensively explored in the past. Some results are very exciting as they put constraints on our models of the solar-terrestrial interaction and magnetosphere-ionosphere coupling which we could not impose before. Perhaps the most exciting discovery of the past two years has been the presen- tation of in situ measurements confirming the existence of parallel electric fields not far above the aurora! zone ionosphere in the height range 2,000-10,000 km. However, two other sets of observations will have an important bearing in the near future as their impact becomes felt. The observations of HEELISet al. (1976) of the fact that the electric field transition over most of the day-side aurora oval is a shear reversal has important implications regarding the question of whether the magnetopause is, to a large extent, an equipotential surface. Finally, the observations by BURCHet al. (1976b) of large convective flows equatorward of the aurora oval is extremely significant. The region between the auroral oval and the plasmapause (often called the trough) has not been thoroughly studied in the past. The fact that magnetospheric convection may be significant on field lines penetrating this trough region during substorm activity should generate further interest in sub-aurora zone phenomena.

This work was supported by the National Research Council of Canada.

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