What determines the intensity of magnetospheric substorms?

One of the central issues in substorm research is what determines the substorm intensity. Through an introduction on what constitutes a magnetospheric substorm, we discuss several parameters which are available to measure the substorm intensity. In terms of ionospheric quantities, we have the auroral electroject indices, the total current in the westward auroral electrojet, the area of bright aurora, the maximum poleward advance of the auroral bulge, and the duration of auroral substorm activities. In terms of magnetospheric quantities, we have the innermost location of the substorm injection boundary and the amount of current reduction in the cross-tail current within the substorm current wedge. A measure reflecting substorm activities in both the ionosphere and the magnetosphere is the total substorm energy dissipation but its drawback lies in the difficulty of assessing it accurately if the energy loss due to plasmoids is to be included. We also discuss the predictability of substorm intensity, which leads us to the issue of whether a substorm is a directly-driven or an unloading process. The recent success in predicting the auroral electrojet index from solar wind parameters with a cross-correlation of ~ 0.9 suggests that substorm activities over a long time scale are primarily directly-driven while those over a short time scale are governed by impulsive unloading processes. This understanding allows us to reconcile the apparently conflicting dual nature of magnetospheric substorms.     

Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.     

Low latitude signatures of substorm activity

Although magnetospheric substorms have their most easily detectable observable consequences in the high latitude auroral oval, most of the early studies of the phenomenon took place using data from low latitude observing sites. Many of these early studies concentrated on magnetic storms, yielding observations and interpretations of the ring current and sudden storm commencements which are valid to this day. Over the years, ground-based observations have been concentrated in the high latitude regions under and immediately adjacent to the auroral oval and studies of storm and substorm phenomena using low latitude data have been few in number. Despite the intensity of research activity in recent years, the physics of the substorm process still remains a matter of controversy. The STEP period represents an excellent opportunity for substorm physicists to solve some of the outstanding problems they still face. Space scientists monitoring the geospace environment at observing sites well equatorward of the auroral oval have an excellent opportunity to contribute to the ultimate solution of the substorm problem during the STEP interval, however they will have to focus on specific observational manifestations of substorms if they are to optimize their probability of success. In this paper I shall try to point out some areas where successful observations and interpretation of substorm phenomena at low latitudes would be extremely useful in helping the substorm community resolve their outstanding scientific problems.     

The auroral distribution and its mapping according to substorm phase

An attempt is made to reconcile two competing views as to where the auroral distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a near-Earth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic 1–10 keV electrons and 0.1–1 keV structured electrons. The ionospheric local time sector between 13 and 18 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an auroral distribution specific to the substorm recovery phase. These auroral observations provide a means of inferring important information concerning magnetospheric topology.     

An alternative interpretation of auroral precipitation and luminosity observations from the DE,DMSP, AUREOL,and Viking satellites in terms of their mapping to the nightside magnetosphere

We present an interpretation, which differs from that commonly accepted, of several published case studies of the patterns of auroral electron precipitation into the high-latitude upper atmosphere in the near-midnight sector based on their mapping to the nightside magnetosphere. In our scheme bright discrete auroral structures of the oval and respective precipitation are considered to be on the field lines of the Central, or Main, Plasma Sheet at distances from 5–10 to 30–50 R_E, depending on activity. This auroral electron precipitation pattern was discussed in detail by Feldstein and Galperin [(1985) Rev. Geophys.23, 217] and Galperin and Feldstein [(1991) Auroral Physics, p. 207. Cambridge University Press. It is applied and shown to be consistent with the results of case studies based on selected transpolar passes of the DE, DMSP, AUREOL-3 and Viking satellites.A diagram summarising the polar precipitation regions and their mapping from the magnetospheric plasma domains is presented. It can be considered as a modification of the Lyons and Nishida (1988) scheme which characterizes the relationship between the gross magnetospheric structure and regions of nightside auroral precipitation. The modification takes into account non-adiabatic ion motions in the tail neutral sheet, so that the ion beams characteristic of the Boundary Plasma Sheet (BPS) originate on closed field lines of the distant Central Plasma Sheet (say, at distances more than ~30 R_E).     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

Electric field sources in the quiet plasmasphere from whistler observations

Existing evidence for the ionospheric dynamo being the source of quiet time electric fields in the plasmasphere is reviewed. Part of a 24 h set of whistler data recorded continuously at Sanae, Antarctica (L = 4), during quiet magnetic (average K_p = 1) is analysed to obtain westward electric fields in the equatorial plane. These electric fields are examined as a function of L-value in order to infer their source. It is found that for periods of outward flow of plasma during the noon-midnight local time period, the electric fields are consistent with the dominant source being the ionospheric dynamo. There is some evidence that during the evening period of inward flow the electric fields are magnetospheric in origin, although this could also be consistent with a refined dynamo model. The observed whistler duct convection patterns do not fit either of two theoretical models, which invoke a magnetospheric field but not a dynamo field.     

Substorm activity precursors in the dayside magnetic perturbations

Magnetic data from a meridional chain of stations in Greenland and AL-indices of magnetic activity have been used to study the relationship between magnetic perturbations in the dayside cleft region and substorm activity in the night-time auroral zone. The analysis of 14 substorms, isolated and prolonged, has shown that intensification of westward currents in the postnoon sector of the cleft precedes or accompanies substorm development in the night-time auroral zone. Westward currents appear in the northern cleft as substorm precursors even under the adverse influence of the IMF positive B_y component. These currents trend to extend in the prenoon sector. To explain the relationship between the cleft currents and auroral electrojet the connection between neutral layer currents and noon Birkeland currents is proposed. This connection can be realized by means of the source region acting just inside the daytime magnetopause owing to stationary reconnection of geomagnetic field and IMF, the source region flowing downstream to the tail magnetopause.     

Photometric observations of proton aurora at Sanae

Proton aurora was monitored at Sanae (invariant latitude 60°S) by recording Hβ emission with a tilting-filter photometer, and observed for K_p⩾ a 3_. Study of two nights of intense auroral activity during 1978 revealed that the times of onset of proton and electron auroral substorms and magnetic substorms agreed within 5 min. Further, electron aurora occurred poleward or equatorward of proton aurora for a location under the eastward or westward auroral electrojet respectively.     

Auroral radio absorption as an indicator of magnetospheric electrons and of conditions in the disturbed auroral D-region

In a previous paper we demonstrated a method by which the auroral radio absorption measured with a riometer can be predicted from energetic electron measurements at geosynchronous orbit. The present paper enquires to what extent the process can be inverted: what levels of magnetospheric electron flux, and of D-region production rate, electron density and incremental absorption, are predicted by a given measurement of radio absorption and what reliance can be placed on such predictions?Using data from 45 precipitation features recorded with riometers in Scandinavia and at geosynchronous orbit with GEOS-2, it is shown that electron fluxes in the ranges 20–40,40–80 and 80–160 keV increase with increasing absorption and can be predicted to better than 50% for absorption events of 2 dB or greater. Electrons above 160 keV show little or no correlation with absorption. D-region production rates and electron densities can be predicted to within factors of 2 and √2, respectively.It is more difficult to specify the height of the absorbing region because of uncertainly in the profile of the effective recombination coefficient. Having regard to other data, an α_eff profile is proposed which satisfies rocket and incoherent scatter data as well as the present calculations. It is shown that any day-night variation in auroral absorption is associated with a change of spectrum rather than a change of recombination coefficient.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

Electrodynamic structural features of the auroral zone as deduced using data from a meridional chain of ionospheric and magnetic stations

This study has used ionospheric and magnetic observational data obtained at a meridional chain of stations during the high latitude geophysical experiment ‘Taimir-82’ in the winter of 1982–1983. Mean statistical latitude-time distributions of the occurrence probability of various types of E_s, their blanketing frequency and of the amplitude of geomagnetic field H-variations have been constructed. Based on these distributions and taking the E_s properties into account, an analysis is made of the mutual correspondence of large-scale structures of the auroral ionosphere and ionospheric currents.Ionospheric currents flow mainly in the region of high E-layer ionization. With increasing magnetic activity, the zone of currents and the zone of ionization expand simultaneously toward lower latitudes. The evening eastward electrojet and the morning westward electrojet are localized inside the zone of diffuse auroral precipitation which is responsible for the formation of E_s type r. The equatorial part of the midnight westward electrojet is also located in the zone of diffuse precipitation which coincides also with the region of maximum ionization of the E-layer. The polar part of this electrojet, which extends far into the dusk sector, is located in the zone of discrete auroral precipitation (a type E_s). Whereas there exists in the meridional cross-section quite a definite relationship between the Harang discontinuity and ionospheric parameters, such a relationship is not manifested in the zonal cross-section of the Harang discontinuity.     

Model calculation and analysis of magnetosphere-ionosphere-thermosphere coupling

Model calculations are used to analyse the function of the magnetosphere-ionosphere electrodynamic coupling and that of the ionospheric tide-dynamo. The phenomena concerned are discussed.It is shown that for geomagnetic and ionospheric research in low-latitude and equatorial regions, the penetration and shielding of magnetospheric disturbances should be taken into account. On the other hand, the effect of the tidal dynamo on the ionosphere of the auroral region should not be ignored, especially on geomagnetically disturbed days when the conductivity is very high. The features and the relative importance of these two effects are different at different stages of a magnetic storm, and depend on latitude. There is an evident dawn-dusk asymmetry in these effects. The influence of weak geomagnetic disturbances on the low-latitude ionosphere should also be taken into account.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

Magnetospheric disturbance effects on the Equatorial Ionization Anomaly (EIA) : an overview

The Equatorial lonization Anomaly (EIA) development can undergo drastic modification in the form of an anomalous occurrence at local times outside that of its quiet time development and/or inhibition/enhancement at local times of its normal occurrences. This happens for disturbed electrodynamic conditions of the global ionosphere-thermosphere-magnetosphere system, consequent upon the triggering of a magnetospheric storm event. Direct penetration to equatorial latitudes of the magnetospheric electric fields and the thermospheric disturbances involving winds, electric fields and composition changes produce significant alteration in the EIA morphology and dynamics. Results on statistical behaviour based on accumulated ground-based data sets, and those from recent theoretical modelling efforts and from satellite and ground-based observations, are reviewed. Some outstanding problems of the EIA response to magnetospheric disturbances that deserve attention in the coming years are pointed out.     

Large-scale TIDs and upper atmospheric gravity waves

Excitation of the guided acoustic-gravity waves in the upper thermosphere in response to enhanced auroral electrojets is calculated in the absence of dissipation under a fully ducted condition. It is shown that a model atmosphere terminated with an isothermal half-space supports a long-period, high-speed mode, which is the interface mode guided along the half-space termination of the atmosphere. The dispersion properties and the vertical distributions of the kinetic energy density of this mode are similar to those of the so called ‘gravity pseudomode’. The excitation of this mode is computed to show how the wave generation depends on the source mechanism (the Lorentz force and joule heating) and on the source altitude. Joule heating can generate the waves with appreciable amplitudes. On the other hand, the Lorentz force prevailing in the lower region cannot excite the waves with any observable amplitudes. The waves are intensified with increasing the heat source altitude. The gross features of the calculated waves indicate that the ducted thermospheric gravity waves are capable of producing observable thermospheric waves. It is therefore suggested that further examination of the excitation of the ducted acoustic-gravity waves undergoing partial reflections due to viscosity and thermal conduction should be useful for the theory of large-scale travelling ionospheric disturbances.     

The generation and propagation of atmospheric gravity waves observed during the Worldwide Atmospheric Gravity-wave Study (WAGS)

During the Worldwide Atmospheric Gravity-wave Study (WAGS) in October 1985, the EISCAT incoherent scatter radar was used to observe the generation of atmospheric gravity waves in the auroral zone in conjunction with a network of magnetometers and riometers. At the same time a chain of five ionosondes, an HF-Doppler system, a meteor radar and a radio telescope array were used to monitor any waves propagating southwards over the U.K.The EISCAT measurements indicated that in the evening sector both Joule heating and Lorentz forcing were sufficiently strong to generate waves, and both frequently showed an intrinsic periodicity caused by periodic variation in the magnetospheric electric field.Two occasions have been examined in detail where the onset of a source with intrinsic periodicity was followed by a propagating wave of the same period which was detected about an hour later, travelling southwards at speeds of over 300 m s⁻¹, by the ionosondes and the HF-Doppler radar. In both cases the delay in arrival was consistent with the observed velocity, which suggests a direct relationship between a source in the auroral zone and a wave observed at mid-latitude.     

On the problem of a connection between ozone dynamics at high latitudes and magnetospheric processes

Two data sets of ozone density measurements over a wide latitudinal range in the Arctic during summer and winter seasons arc presented. It is shown that geophysical effects manifest themselves in the O₃ dynamics in the high latitude region under various circumstances. Therefore a type of total ozone content diurnal variation is a distinctive feature in the auroral oval as well as the polar cap and must be taken into account in any full model of ozone dynamics in the polar regions.     

Fine structure and dynamics of the Harang discontinuity: An event study

Two rockets were launched simultaneously from the Andöya Rocket Range in northern Norway into and through a Harang discontinuity region during a small isolated substorm. Each was equipped with an array of instruments for monitoring ionospheric electric fields and auroral particles. Different sized rockets allowed both to traverse a range of 300 km while introducing altitude separations of up to 440 km, and separations between the feet of the magnetic field lines intersecting the payloads of up to 100 km. The data sets, coupled with multi-station ground-based observations, provide information on the structure and dynamics of the Harang discontinuity region. Two boundaries were encountered. The first electric field reversal was observed simultaneously by both payloads in a region of weak precipitation, while the second reversal was associated with an intense auroral band. Since an unambiguous interpretation of these data is not possible, we present two alternative scenarios. In one the discontinuity becomes realigned during the flight following a decay in the activity. The second involves a triple cell convection system, possibly the result of an eddy in the flow.     

The day to night absorption ratio in auroral and subauroral zone riometer measurements during auroral absorption

The day to night ratio of auroral absorption has been studied using data from auroral and subauroral latitudes and by application of different kinds of statistical analyses. Ratios between 0.5 and 3.0 are obtained, depending on the criteria applied to the selection of data. Previous studies obtained similar ratios, but reached different conclusions about the effective solar control of auroral absorption. It is concluded here that evidence of solar control of the day to night ratio of auroral absorption, or the lack thereof, cannot be extracted by these statistical analyses.