The excitation of plasma convection in the high-latitude ionosphere

Recent observations of ionospheric flows by ground-based radars, in particular by the European Incoherent Scatter (EISCAT) facility using the “Polar” experiment, together with previous analyses of the response of geomagnetic disturbance to variations of the interplanetary magnetic field (IMF), suggest that convection in the high-latitude ionosphere should be considered to be the sum of two intrinsically time-dependent patterns, one driven by solar wind-magnetosphere coupling at the dayside magnetopause, the other by the release of energy in the geomagnetic tail (mainly by dayside and nightside reconnection, respectively). The flows driven by dayside coupling are largest on the dayside, where they usually dominate, are associated with an expanding polar cap area, and are excited and decay on ∼10-min time scales following southward and northward turnings of the IMF, respectively. The latter finding indicates that the production of new open flux at the dayside magnetopause excites magnetospheric and ionospheric flow only for a short interval, ∼10 min, such that the flow driven by this source subsequently decays on this time scale unless maintained by the production of more open flux tubes. Correspondingly, the flows excited by the release of energy in the tail, mainly during substorms, are largest on the nightside, are associated with a contracting polar cap boundary, and are excited on ∼1-hour time scales following a southward turn of the IMF. In general, the total ionospheric flow will be the sum of the flows produced by these two sources, such that due to their different response times to changes in the IMF, considerable variations in the flow pattern can occur for a given direction and strength of the IMF. Consequently, the ionospheric electric field cannot generally be regarded as arising from a simple mapping of the solar wind electric field along open flux tubes.

Midday auroral breakup events and related energy and momentum transfer from the magnetosheath

Combined observations by meridian-scanning photometers, all-sky auroral TV camera and the EISCAT radar permitted a detailed analysis of the temporal and spatial development of the midday auroral breakup phenomenon and the related ionospheric ion flow pattern within the 71°–75° invariant latitude radar field of view. The radar data revealed dominating northward and westward ion drifts, of magnitudes close to the corresponding velocities of the discrete, transient auroral forms, during the two different events reported here, characterized by IMF |BY/BZ| < 1 and > 2, respectively (IMF BZ between −8 and −3 nT and BY > 0). The spatial scales of the discrete optical events were ∼50 km in latitude by ∼500 km in longitude, and their lifetimes were less than 10 min. Electric potential enhancements with peak values in the 30–50 kV range are inferred along the discrete arc in the IMF |BY/BZ| < 1 case from the optical data and across the latitudinal extent of the radar field of view in the |BY/BZ| > 2 case. Joule heat dissipation rates in the maximum phase of the discrete structures of ∼ 100 ergs cm−2 s−1 (0.1 W m−2) are estimated from the photometer intensities and the ion drift data. These observations combined with the additional characteristics of the events, documented here and in several recent studies (i.e., their quasi-periodic nature, their motion pattern relative to the persistent cusp or cleft auroral arc, the strong relationship with the interplanetary magnetic field and the associated ion drift/E field events and ground magnetic signatures), are considered to be strong evidence in favour of a transient, intermittent reconnection process at the dayside magnetopause and associated energy and momentum transfer to the ionosphere in the polar cusp and cleft regions. The filamentary spatial structure and the spectral characteristics of the optical signature indicate associated localized ˜1-kV potential drops between the magnetopause and the ionosphere during the most intense auroral events. The duration of the events compares well with the predicted characteristic times of momentum transfer to the ionosphere associated with the flux transfer event-related current tubes. It is suggested that, after this 2–10 min interval, the sheath particles can no longer reach the ionosphere down the open flux tube, due to the subsequent super-Alfvénic flow along the magnetopause, conductivities are lower and much less momentum is extracted from the solar wind by the ionosphere. The recurrence time (3–15 min) and the local time distribution (∼0900–1500 MLT) of the dayside auroral breakup events, combined with the above information, indicate the important roles of transient magnetopause reconnection and the polar cusp and cleft regions in the transfer of momentum and energy between the solar wind and the magnetosphere.

Interplanetary magnetic field control of dayside auroral activity and the transfer of momentum across the dayside magnetopause

The orientation of the Interplanetary Magnetic Field (IMF) during transient bursts of ionospheric flow and auroral activity in the dayside auroral ionosphere is studied, using data from the EISCAT radar, meridian-scanning photometers, and an all-sky TV camera, in conjunction with simultaneous observations of the interplanetary medium by the IMP-8 satellite. It is found that the ionospheric flow and auroral burst events occur regularly (mean repetition period equal to 8.3 ± 0.6 min) during an initial period of about 45 min when the IMF is continuously and strongly southward in GSM coordinates, consistent with previous observations of the occurrence of transient dayside auroral activity. However, in the subsequent 1.5 h, the IMF was predominantly northward, and only made brief excursions to a southward orientation. During this period, the mean interval between events increased to 19.2 ± 1.7 min. If it is assumed that changes in the North-South component of the IMF are aligned with the IMF vector in the ecliptic plane, the delays can be estimated between such a change impinging upon IMP-8 and the response in the cleft ionosphere within the radar field-of-view. It is found that, to within the accuracy of this computed lag, each transient ionospheric event during the period of predominantly northward IMF can be associated with a brief, isolated southward excursion of the IMF, as observed by IMP-8. From this limited period of data, we therefore suggest that transient momentum exchange between the magnetosheath and the ionosphere occurs quasi-periodically when the IMF is continuously southward, with a mean period which is strikingly similar to that for Flux Transfer Events (FTEs) at the magnetopause. During periods of otherwise northward IMF, individual momentum transfer events can be triggered by brief swings to southward IMF. Hence under the latter conditions the periodicity of the events can reflect a periodicity in the IMF, but that period will always be larger than the minimum value which occurs when the IMF is strongly and continuously southward.

Analysis of incoherent scatter radar data from non-thermal F-region plasma

A procedure is presented for fitting incoherent scatter radar data from non-thermal F-region ionospheric plasma, using theoretical spectra previously predicted. It is found that values of the shape distortion factor D∗, associated with deviations of the ion velocity distribution from a Maxwellian distribution, and ion temperatures can be deduced (the results being independent of the path of iteration) if the angle between the line-of-sight and the geomagnetic field is larger than about 15–20°. The procedure can be used with one or both of two sets of assumptions. These concern the validity of the adopted model for the line-of-sight ion velocity distribution in the one case or for the full three-dimensional ion velocity distribution function in the other. The distribution function employed was developed to describe the line-of-sight velocity distribution for large aspect angles, but both experimental data and Monte Carlo simulations indicate that the form of the field-perpendicular distribution can also describe the distribution at more general aspect angles. The assumption of this form for the line-of-sight velocity distribution at a general aspect angle enables rigorous derivation of values of the one-dimensional, line-of-sight ion temperature. With some additional assumptions (principally that the field-parallel distribution is always Maxwellian and there is a simple relationship between the ion temperature anisotropy and the distortion of the field-perpendicular distribution from a Maxwellian), fits to data for large aspect angles enable determination of line-of-sight temperatures at all aspect angles and hence, of the average ion temperature and the ion temperature anisotropy. For small aspect angles, the analysis is restricted to the determination of the line-of-sight ion temperature because the theoretical spectrum is insensitive to non-thermal effects when the plasma is viewed along directions almost parallel to the magnetic field. This limitation is expected to apply to any realistic model of the ion velocity distribution function and its consequences are discussed. Fit strategies which allow for mixed ion composition are also considered. Examples of fits to data from various EISCAT observing programmes are presented.

Low-altitude signatures of the cusp and flux transfer events

The usual interpretation of a flux transfer event (FTE) at the magnetopause, in terms of time-dependent and possibly patchy reconnection, demands that it generate an ionospheric signature. Recent ground-based observations have revealed that auroral transients in the cusp/cleft region have all the characteristics required of FTE effects. However, signatures in the major available dataset, namely that from low-altitude polar-orbiting satellites, have not yet been identified. In this paper, we consider a cusp pass of the DE-2 spacecraft during strongly southward IMF. The particle detectors show magnetosheath ion injection signatures. However, the satellite motion and convection are opposed, and we discuss how the observed falling energy dispersion of the precipitating ions can have arisen from a static, moving or growing source. The spatial scale of the source is typical of an FTE. A simple model of the ionospheric signature of an FTE reproduces the observed electric and magnetic field perturbations. Precipitating electrons of peak energy ∼100eV are found to lie on the predicted boundary of the newly-opened tube, very similar to those found on the edges of FTEs at the magnetopause. The injected ions are within this boundary and their dispersion is consistent with its growth as reconnection proceeds. The reconnection potential and the potential of the induced ionospheric motion are found to be the same (≃25kV). The scanning imager on DE-1 shows a localised transient auroral feature around DE-2 at this time, similar to the recent optical/radar observations of FTEs.

June 1987 GISMOS experiment: Preliminary report on high time resolution, multi-radar measurements

Data collected by ground magnetometers and high latitude radars during a small isolated substorm are discussed in terms of the global changes in convection during the substorm. This substorm was observed during the international GISMOS (Global Ionospheric Simultaneous Measurements of Substorms) Experiment of 1 – 5 June 1987 and the array of observations discussed here span the night sector from approximately dusk to dawn. The substorm, observed by the Sondrestrom radar and auroral and midlatitude magnetometers is associated with a polar cap contraction observed near dusk by the EISCAT radar.

Analysis of incoherent scatter spectra from non-Maxwellian plasma

Incoherent scatter data from non-thermal F-region ionospheric plasma are analysed, using theoretical spectra predicted by Raman et al. It is found that values of the semi-empirical drift parameter D∗, associated with deviations of the ion velocity distribution from a Maxwellian, and the plasma temperatures can be rigorously deduced (the results being independent of the path of iteration) if the angle between the line-of-sight and the geomagnetic field is larger than about 15–20 degrees. For small aspect angles, the deduced value of the average (or 3-D) ion temperature remains ambiguous and the analysis is restricted to the determination of the line-of-sight temperature because the theoretical spectrum is insensitive to non-thermal effects when the plasma is viewed along directions almost parallel to the magnetic field. This limitation is expected to apply to any realistic model of the ion velocity distribution, and its consequences are discussed. Fit strategies which allow for mixed ion composition are also considered. Examples of fits to data from various EISCAT observing programmes are presented.

Pressure-driven magnetopause motions and attendant response on the ground

The terrestrial magnetopause suffered considerable sudden changes in its location on 9–10 September 1978. These magnetopause motions were accompanied by disturbances of the geomagnetic field on the ground. We present a study of the magnetopause motions and the ground magnetic signatures using, for the latter, 10 s averaged data from 14 high latitude ground magnetometer stations. Observations in the solar wind (from IMP 8) are employed and the motions of the magnetopause are monitored directly by the spacecraft ISEE 1 and 2. With these coordinated observations we are able to show that it is the sudden changes in the solar wind dynamic pressure that are responsible for the disturbances seen on the ground. At some ground stations we see evidence of a “ringing” of the magnetospheric cavity, while at others only the initial impulse is evident. We note that at some stations field perturbations closely match the hypothesized ground signatures of flux transfer events. In accordance with more recent work in the area (e.g. Potemra et al., 1989, J. geophys. Res., in press), we argue that causes other than impulsive reeonnection may produce the twin ionospheric flow vortex originally proposed as a flux transfer even signature.

Dayside auroral activity and magnetic flux transfer from the solar wind

Combined observations by meridian-scanning photometers and the EISCAT radar show that the "midday-auroral breakup" phenomenon is associated with major increases in ionospheric flow. A sequence of nine events is observed in the early afternoon MLT sector during a period when the IMF is strongly southward with a large positive By component. Each auroral structure is seen at both 630 and 557.7nm and initially moves westward, accompanied by an increase in potential of 30-60kV across the north-south dimension of the EISCAT field-of-view. After a few minutes the arc (or arc fragment) moves into the polar cap and fades, and the velocities observed by the radar swing from westward toward northward. We conclude that dayside auroral breakup is closely associated with momentum transfer across the magnetopause which occurs in a series of events 5-15 minutes apart. The largest of the observed events has dimensions of about 300km (in the direction of westward motion) by 700km, is bounded on its poleward edge by a 5kR arc and is associated with a potential of at least 80kV.

Observations of nonthermal plasmas at different aspect angles

Data are presented from the EISCAT (European Incoherent Scatter (Facility)) CP-3-E experiment which show large increases in the auroral zone convection velocities (>2 km s−1) over a wide range of latitudes. These are larger than the estimated neutral thermal speed and allow a study of the plasma in a nonthermal state over a range of observing angles. Spectra are presented which show a well-defined central peak, consistent with an ion velocity distribution function which significantly departs from a Maxwellian form. As the aspect angle decreases, the central peak becomes less obvious. Simulated spectra, derived using theoretical expressions for the O+ ion velocity distribution function based on the generalized relaxation collision model, are compared with the observations and show good first-order, qualitative agreement. It is shown that ion temperatures derived from the observations, with the assumption of a Maxwellian distribution function, are an overestimate of the true ion temperature at large aspect angles and an underestimate at low aspect angles. The theoretical distribution functions have been included in the “standard” incoherent scatter radar analysis procedure, and attempts have been made to derive realistic ionospheric parameters from nonthermal plasma observations. If the expressions for the distribution function are extended to include mixed ion composition, a significant improvement is found in fitting some of the observed spectra, and estimates of the ion composition can be made. The non-Maxwellian analysis of the data revealed that the spectral shape distortion parameter, D*, was significantly higher in this case for molecular ions than for atomic ions in a thin height slab roughly 40 km thick. This would seem unlikely if the main molecular ions present were NO+. We therefore suggest that N2+ formed a significant proportion of the molecular ions present during these observations.

Observations at the magnetopause and in the auroral ionosphere of momentum transfer from the solar wind

Recent radar studies of field-perpendicular flows in the auroral ionosphere, in conjunction with observations of the interplanetary medium immediately upstream of the Earth's bow shock, have revealed direct control of dayside convection by the Bz component of the interplanetary magnetic field (IMF). The ionospheric flows begin to respond to both northward and southward turnings of the IMF impinging upon the magnetopause after a delay of only a few minutes in the early afternoon sector, rising to about 15 minutes nearer dawn and dusk. In both the polar cap and the auroral oval, the subsequent rise and decay times are of order 5–10 minutes. We conclude there is very little convection “flywheel” effect in the dayside polar ionosphere and that only newly-opened flux tubes impart significant momentum to the ionosphere, in a relatively narrow region immediately poleward of the cusp. These findings concerning the effects of quasi-steady reconnection have important implications for any ionospheric signatures of transient reconnection which should be considerably shorter-lived than thought hitherto. In order to demonstrate the difficulty of uniquely identifying a Flux Transfer Event (FTE) in ground-based magnetometer data, we present observations of an impulsive signature, identical with that expected for an FTE if data from only one station is studied, following an observed magnetopause compression when the IMF was purely northward. We also report new radar observations of a viscous-like interaction, consistent with an origin on the flanks of the magnetotail and contributing an estimated 15–30kV to the total cross-cap potential during quiet periods.

Response time of the high-latitude dayside ionosphere to sudden changes in the north-south component of the IMF

The time scale of the response of the high-latitude dayside ionospheric flow to changes in the North-South component of the interplanetary magnetic field (IMF) has been investigated by examining the time delays between corresponding sudden changes. Approximately 40 h of simultaneous IMF and ionospheric flow data have been examined, obtained by the AMPTE-UKS and -IRM spacecraft and the EISCAT “Polar” experiment, respectively, in which 20 corresponding sudden changes have been identified. Ten of these changes were associated with southward turnings of the IMF, and 10 with northward turnings. It has been found that the corresponding flow changes occurred simultaneously over the whole of the “Polar” field-of-view, extending more than 2° in invariant latitude, and that the ionospheric response delay following northward turnings is the same as that following southward turnings, though the form of the response is different in the two cases. The shortest response time, 5.5 ± 3.2 min, is found in the early- to mid-afternoon sector, increasing to 9.5 ± 3.0 min in the mid-morning sector, and to 9.5 ± 3.1 min near to dusk. These times represent the delays in the appearance of perturbed flows in the “Polar” field-of-view following the arrival of IMF changes at the subsolar magnetopause. Overall, the results agree very well with those derived by Etemadi et al. (1988, Planet. Space Sci.36, 471) from a general cross-correlation analysis of the IMF Bz and “Polar” beam-swinging vector flow data.

Large plasma velocities along the magnetic field line in the auroral zone

In the auroral zone, ionospheric plasma often moves horizontally at more than 1 km s−1, driven by magnetospheric electric fields, but it has usually been assumed that vertical velocities are much smaller. On occasions, however, plasma has been seen to move upwards along the magnetic field line at several hundred m s−1. These upward velocities are associated with electric fields strong enough to heat the ion population and drive it into a non-thermal state1,2. Here we report observations of substantial upwards acceleration of plasma, to velocities as high as 500 m s−1. An initial upthrust was provided by a combined upwelling of the neutral atmosphere and ionosphere but the continued acceleration at greater heights is explained by a combination of enhanced plasma pressure and the 'hydrodynamic mirror force'3. This acceleration marks an important stage in the transport of plasma from the ionosphere into the magnetosphere.

Flow in the high latitude ionosphere: measurements at 15s resolution made using the EISCAT ‘Polar’ experiment

We present a first overview of flows in the high latitude ionosphere observed at 15 s resolution using the U.K.-Polar EISCAT experiment. Data are described from experiments conducted on two days, 27 October 1984 and 29 August 1985, which together span the local times between about 0200 and 2130MLT and cover five different regions of ionospheric flow. With increasing local time, these are: the dawn auroral zone flow cell, the dayside region of low background flows equatorward of the flow cells, the dusk auroral zone flow cell, the boundary region between the dusk auroral zone and the polar cap, and the evening polar cap. Flows in both the equatorward and poleward portions of the auroral zone cells appear to be relatively smooth, while in the central region of high speed flow considerable variations are generally present. These have the form of irregular fluctuations on a wide range of time scales in the early morning dawn cell, and impulsive wave-like variations with periods of a few minutes in the afternoon dusk cell. In the dayside region between the flow cells, the ionosphere is often essentially stagnant for long intervals, but low amplitude ULF waves with a period of about 5 min can also occur and persist for many cycles. These conditions are punctuated at one to two hour intervals by sudden ‘flow burst’ events with impulsively generated damped wave trains. Initial burst flows are generally directed poleward and can peak at line-of-sight speeds in excess of 1 km s^{−1} after perhaps 45 s. Flows in the polar cap are reasonably smooth on time scales of a few minutes and show no evidence for the presence of ULF waves. Under most, but not all, of the above conditions, the beam-swinging algorithm used to determine background vector flows should produce meaningful results. Comparison of these flow data with simultaneous plasma and magnetic field measurements in the solar wind, made by the AMPTE IRM and UKS spacecraft, emphasizes the strong control exerted on high latitude flows by the north-south component of the IMF.

Non-Maxwellian ion velocity distributions observed using EISCAT

Recent observations from the EISCAT incoherent scatter radar have revealed bursts of poleward ion flow in the dayside auroral ionosphere which are consistent with the ionospheric signature of flux transfer events at the magnetopause. These bursts frequently contain ion drifts which exceed the neutral thermal speed and, because the neutral thermospheric wind is incapable of responding sufficiently rapidly, toroidal, non-Maxwellian ion velocity distributions are expected. The EISCAT observations are made with high time resolution (15 seconds) and at a large angle to the geomagnetic field (73.5°), allowing the non-Maxwellian nature of the distribution to be observed remotely for the first time. The observed features are also strongly suggestive of a toroidal distribution: characteristic spectral shape, increased scattered power (both consistent with reduced Landau damping and enhanced electric field fluctuations) and excessively high line-of-sight ion temperatures deduced if a Maxwellian distribution is assumed. These remote sensing observations allow the evolution of the distributions to be observed. They are found to be non-Maxwellian whenever the ion drift exceeds the neutral thermal speed, indicating that such distributions can exist over the time scale of the flow burst events (several minutes).