Stresses of importance inside the magnetosphere include ionospheric ion drag, plasma pressure, inertia and viscous stresses. These, or the corresponding currents, must be combined with boundary conditions of plasma inflow, tangential electric fields and either magnetic fields or currents to solve for the internal convection. The convection of dipolar flux tubes is best understood, the weakest point being the boundary conditions. Convection in the plasma sheet is poorly understood since neither the plasma input nor the energy loss mechanism are clearly understood. On open flux tubes, the ionosphere and outer boundary conditions determine the convection. Understanding here is, at best, qualitative. The solar wind interacts with the magnetosphere to twist flux tubes about their axes (producing distributed Birkeland currents) or about each other (producing current sheets). Time constants involved include the untwisting of flux tubes (decay of Birkeland current) by convection antisunward of the connection point of the flux tube to the solar wind (2000 sec), and by convection of the flux tube feet in the ionosphere (a few hours). The presence of tangential discontinuities and also of X lines and separatrices should be indicated by Birkeland current sheets, since both occur at discontinuities in the topological connection of field lines to the outer regions. If, as the IMF rotates in the yz plane, the merging sites move from closed to open field lines as predicted by the antiparallel merging model, then the hierarchy of convection cells: viscous, merging, lobe and reclosure appears to be plausible. The reclosure cell also requires nightside merging and closed field lines at high latitudes. However, these are not the only possibilities and much work on merging models and topology remains to be done.

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We discuss the practical ways to determine the polar cap boundary (PCB, i. e. the boundary between open and closed flux tubes) from observations available at low altitudes. A theory generally predicts a simple topology of PCB (one bundle of the open flux tubes in each polar cap) and the character of its shape and size dependence on IMF but the accurate predictions require to model self-consistently the processes both at the magnetopause and in the magnetotail. The practical ways of PCB indication based on transparent physics and supported by the observations are very few. Among them we discuss: the equatorward boundary of auroral particle precipitation in the cusp proper (in the sector of dayside convection throat); the trapping boundary of the magnetospheric energetic particles; the sharp boundary of the solar electron plateau. The physics of the latter boundary is discussed in more detail. Recent observations indicate that the whole plasma sheet is filled by the solar electrons and that their sharp precipitation boundary originates inside of the closed plasma sheet tubes, indeed, due to the particle scattering (weak non-adiabatic process) in the tail current sheet. The nightside portion of this solar electron plateau boundary generated in this way can be distinguished from its dayside part (which coincides with PCB) by its energy (rigidity) dependent behaviour. In cases of significant north-south hemispherical difference of the solar electron flux the PCB can be accurately determined everywhere. The morphology and physics of the boundaries of the solar electron precipitation as well as their relationship with the patterns of other phenomena (precipitation, convection, FAC etc.) deserve to be studied in much more detail.

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Work in the past has established that a few percent of the time, under northward interplanetary magnetic field and thus magnetically quiet conditions, sun aligned arcs are found in the polar cap with intensities greater than the order of a kilo Rayleigh in the visible. Here we extend this view. We first note that imaging systems with sensitivity down to tens of Rayleighs in the visible find sun aligned arcs in the polar cap far more often, closer to half the time than a few percent. Furthermore, these sun aligned arcs have simple electrodynamics. They mark boundaries between rapid antisunward flow of ionospheric plasma on their dawn side and significantly slower flow, or even sunward flow, on their dusk side. Since the sun aligned arcs are typically the order of 1000km to transpolar in the sun-earth direction, and the order of 100km or less in the dawn-dusk direction, they demarcate lines of strongly anisotropic ionospheric flow shears or convection cells. The very quiet polar cap (strongly northward IMF) is in fact characterized by the presence of sun aligned arcs and multiple highly anisotropic ionospheric flow shears. Sensitive optical images are a valuable diagnostic with which to study polar ionospheric convection under these poorly understood conditions.

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Selected case studies of auroral structure/activity observed at different local times on the dayside are presented and discussed in the context of electrodynamic coupling between the different magnetospheric boundary regions and the ionosphere. The first case addresses the question of the auroral signatures of the two boundary regions referred to as cusp and cleft/LLBL. Combined ground-based and satellite data reveal the different latitudinal zones of auroral forms/particle precipitation/field-aligned current and the relationship with the respective magnetospheric plasma populations, i. e. CPS, BPS, LLBL, and the plasma mantle. Midday auroral breakup events and the related ionospheric ion drift and magnetic observations show many of the features that have been predicted to be ionospheric signatures of flux transfer events. An alternative explanation that has been proposed by others, i. e. ionospheric effect of magnetopause perturbations excited by dynamic pressure pulses in the magnetosheath is also discussed. A critical question in this respect is the location of the breakup event sequence in relation to the polar cap boundary. Does the poleward motion of the events correspond to newly-reconnected flux tubes moving into the polar cap or to an outward radial motion of the magnetopause and the boundary layers, associated with changes in the external dynamic pressure? Periodic auroral forms in the 14-15MLT sector, within 70-70° MLAT, and associated Pc5 magnetic pulsations at lower latitudes are discussed in terms of large-scale waves on the LLBL/PS interface.

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Observations of dayside aurora from Svalbard recorded by multichannel meridian scanning photometers and all-sky video cameras from four days in December 1988 are presented along with magnetometer data from the Svalbard-North Norway chain of stations. These examples indicate that the midday aurora over Svalbard can be separated in different latitudinal regions, possibly as signatures of the magnetic cusp and cleft. The cusp auroral signature is characterized by stable 630.0nm emissions and weak 557.7nm emissions, although transient discrete forms may occur in this region. Within the more extended cleft, discrete auroral structures are more common. Video camera observations reveal eastward drift of 4km/s in one of these sporadic events, associated with a magnetic disturbance propagating at the same speed. The activity level of long period (several minutes) irregular magnetic pulsations increases as the dayside aurora approaches the zenith or during intensity enhancements.

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The characteristics of polar cap arc occurrence are reviewed to show that the assumption of a closed magnetospheric magnetic field topology at very high latitudes when the IMF B_z is strongly northward is difficult to reconcile with a wide variety of observational and theoretical considerations. In particular, we consider the implications of observations of particle entry for high and low energy electrons, magnetic flux conservation between the near and far tail, the time sequencing in polar cap arcs events, and the hemispherical differences in polar cap arc observations. These points can be explained either by excluding the need for a major topological magnetic field change from explanations of polar cap arc dynamics, or by assuming a long-tailed magnetosphere for all IMF orientations in which magnetic field lines eventually merge with solar wind field lines in either a smooth or a patchy fashion.

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Measurements of electron and ion beams and ion conics from the polar orbiting Viking satellite have been obtained in the cusp-cleft region. Frequencies of occurrence and relative frequencies of occurrence have been calculated in this region for studies of the distributions in MLT, Inv. Lat. and altitude during the period of study (March-June 1986). It was observed that the electron beams were more frequent in the dawn sector than in the dusk sector. These observations were similar and correlated to the ion conics and are compared in this study. On the other hand, an anticorrelation between electron- and ion-beams was also observed. The altitude dependence of the ion conics and electron beams showed a steep increase in frequency above 10000km up to the satellite apogee at about 13500km.

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Propagation experiments providing scintillation, total electron content and drift data in the field of view of an all-sky imager near the magnetic pole in Greenland are utilized to investigate the manner in which ionospheric plasma becomes structured within the polar cap. It is found that under IMF B_z southward conditions, large scale ionization patches which are convected through the dayside cusp into the polar cap get continually structured. The structuring occurs through the E×B gradient drift instability process which operates through an interaction between the antisunward plasma convection in the neutral rest frame and large scale plasma density gradients that exist at the edges of the ionization patches. It is shown that with the increase of solar activity the strength of the irregularities integrated through the ionosphere is greatly increased. Under the IMF B_z northward conditions, the plasma structuring occurs around the polar cap arcs in the presence of inhomogeneous electric field or disordered plasma convection. In that case, the irregularity generation is caused by the competing processes of non-linear Kelvin-Helmholtz instability driven by sheared plasma flows and the gradient drift instability process which operates in the presence of dawn-dusk motion of arc structures. The integrated strength of this class of irregularities also exhibits marked increase with increasing solar activity presumably because the ambient plasma density over the polar cap is enhanced.

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Scintillation and other measurements from the HILAT program were used to study the zone at the edge of the polar cap where the most intense average scintillations are found. The maximum of the zone was found to be horseshoe shaped and located on the dayside. This scintillation zone shows only small variations with season and magnetic activity. On individual HILAT passes discrete onsets of intense scintillations are observed. Averaging of these onsets gives the high average scintillation intensities. The onsets appear to occur where irregularities are being created. These same locations show an increase in ionospheric electron content and an increase in the low energy electron flux. After creation the irregularities are converted towards the polar cap. The irregularity source region is also the region known as the “dayside high-latitude auroral region” where the precipitating soft electron flux produces 630nm auroras.

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The coupling of momentum and energy from the solar wind to the ionosphere remains one of the central questions of solar terrestrial physics. Here we review coupling phenomena with particular attention to effects taking place on a time scale of minutes rather than hours. On this time scale, which is comparable to the travel time for transient phenomena like Alfvén waves across the system, the links in the coupling of the system become more evident as lags are detectable. The search for the low altitude counterpart of magnetopause phenomena and transitory reconnection, in particular has led to the discovery of unexpected new effects, vortices, field aligned current systems and disturbances associated with solar wind pressure changes.

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