Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signatures

We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause ([X(GSM), Y(GSM), Z(GSM)] approximate to [2, 7, 9] R(E)). The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is well-defined by two separate studies: (1) the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events cor-relate well with swings of the IMF clock angle (in GSM) to near 90degrees. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10-15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Event (FTE), with allowance for magnetospheric ion reflection at Alfvenic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 R(E) in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary (when the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.

Coordinated Cluster, ground-based instrumentation and low-altitude satellite observations of transient poleward-moving events in the ionosphere and in the tail lobe

During the interval between 8:00-9:30 on 14 January 2001, the four Cluster spacecraft were moving from the central magnetospheric lobe, through the dusk sector mantle, on their way towards intersecting the magnetopause near 15:00 MLT and 15:00 UT. Throughout this interval, the EIS-CAT Svalbard Radar (ESR) at Longyearbyen observed a series of poleward-moving transient events of enhanced F-region plasma concentration ("polar cap patches"), with a repetition period of the order of 10 min. Allowing for the estimated solar wind propagation delay of 75 ( 5) min, the interplanetary magnetic field (IMF) had a southward component during most of the interval. The magnetic footprint of the Cluster spacecraft, mapped to the ionosphere using the Tsyganenko T96 model (with input conditions prevailing during this event), was to the east of the ESR beams. Around 09:05 UT, the DMSP-F12 satellite flew over the ESR and showed a sawtooth cusp ion dispersion signature that also extended into the electrons on the equatorward edge of the cusp, revealing a pulsed magnetopause reconnection. The consequent enhanced ionospheric flow events were imaged by the SuperDARN HF backscatter radars. The average convection patterns (derived using the AMIE technique on data from the magnetometers, the EISCAT and SuperDARN radars, and the DMSP satellites) show that the associated poleward-moving events also convected over the predicted footprint of the Cluster spacecraft. Cluster observed enhancements in the fluxes of both electrons and ions. These events were found to be essentially identical at all four spacecraft, indicating that they had a much larger spatial scale than the satellite separation of the order of 600 km. Some of the events show a correspondence between the lowest energy magnetosheath electrons detected by the PEACE instrument on Cluster (10-20 eV) and the topside ionospheric enhancements seen by the ESR (at 400-700 km). We suggest that a potential barrier at the magnetopause, which prevents the lowest energy electrons from entering the magnetosphere, is reduced when and where the boundary-normal magnetic field is enhanced and that the observed polar cap patches are produced by the consequent enhanced precipitation of the lowest energy electrons, making them and the low energy electron precipitation fossil remnants of the magnetopause reconnection rate pulses.

Cusp ion steps, field-aligned currents and poleward moving auroral forms

We predict the field-aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a B-Y component that is large in magnitude. For B-Y > 0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for B-Y < 0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field-aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field-aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF B-Y, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field-aligned current directions will be reversed. Observations by the Polar spacecraft on August 26,1998, show the predicted upward current sheets at steps seen in the mantle region for IMF B-Y > 0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field-aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.

Ionospheric ion and electron heating at the poleward boundary of a poleward expanding substorm-disturbed region

We present observations of a poleward propagating substorm-disturbed region which was observed by the European Incoherent SCATter (EISCAT) radar and the Svalbard International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers in the postmidnight sector. The expansion of the disturbance was launched by a substorm intensification which started similar to 25 min after the initial onset, and similar to 10 min before the disturbance arrived over Svalbard. In association with the magnetic disturbance, a poleward expanding enduring enhancement in the F region electron temperature was observed, indicative of soft electron precipitation, with a narrow band of enhanced ion temperature straddling its poleward edge, indicative of fast ion flows and ion-neutral collisional heating. This electron temperature boundary was coincident with the poleward propagating electrojet current system detected by the high-latitude IMAGE magnetometer stations and is taken to be a proxy for the observation of a substorm auroral bulge. The electron temperature boundary is inferred to have a width comparable or less than one radar range gate (similar to 60 km transverse to the magnetic field), while the region of high ion temperature was found to be approximately three gates wide, extending approximately two gates (similar to 120 km) poleward of the electron temperature boundary, and approximately one gate (similar to 60 km) equatorward. The two-beam radar line-of-sight velocity data are found to be consistent with the existence of a layer of high-speed flow in the boundary, peaking at values similar to1.5-3 km s(-1), roughly consistent with the ion temperature data. The flow is directed either east or west along the boundary depending on the direction of the flow in the poleward region. We infer that the flow is deflected along and around the boundary of the substorm-disturbed region due to the high conductivity of the latter. Variations in the flow poleward of the boundary produced no discernible magnetic effects on the ground, confirming the low conductivity of the preboundary ionosphere.

ESR and EISCAT observations of the response of the cusp and cleft to IMF orientation changes

We report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Tromso and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Alesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened held lines for the observed negative IMF B-y. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward B-z, and explain their morphology in the context of previous theoretical work.

A multipoint study of a substorm occurring on 7 December, 1992, and its theoretical implications

On 7 December 1992, a moderate substorm was observed by a variety of satellites and ground-based instruments. Ionospheric flows were monitored near dusk by the Goose Bay HF radar and near midnight by the EISCAT radar. The observed flows are compared here with magnetometer observations by the IMAGE array in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, the SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF B-z component. The data presented show that the substorm onset electrojet at midnight was confined to closed field lines equatorward of the preexisting convection reversal boundaries observed in the dusk and midnight regions. No evidence of substantial closure of open flux was detected following this substorm onset. Indeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward IMF, such that growth and expansion phase features were simultaneously present. Clear indications of closure of open flux were not observed until a subsequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well poleward of the pre-existing convection reversal boundary, and strong flow perturbations were observed by the Goose Bay radar, indicative of flows driven by reconnection in the tail.

Temporal and spatial variability of auroral forms in the 10–14 MLT sector: Relationship to plasma convection and solar wind-magnetosphere coupling

Ground-based observations of dayside auroral forms and magnetic perturbations in the arctic sectors of Svalbard and Greenland, in combination with the high-resolution measurements of ionospheric ion drift and temperature by the EISCAT radar, are used to study temporal/spatial structures of cusp-type auroral forms in relation to convection. Large-scale patterns of equivalent convection in the dayside polar ionosphere are derived from the magnetic observations in Greenland and Svalbard. This information is used to estimate the ionospheric convection pattern in the vicinity of the cusp/cleft aurora. The reported observations, covering the period 0700-1130 UT, on January 11, 1993, are separated into four intervals according to the observed characteristics of the aurora and ionospheric convection. The morphology and intensity of the aurora are very different in quiet and disturbed intervals. A latitudinally narrow zone of intense and dynamical 630.0 nm emission equatorward of 75 degrees MLAT, was observed during periods of enhanced antisunward convection in the cusp region. This (type 1 cusp aurora) is considered to be the signature of plasma entry via magnetopause reconnection at low magnetopause latitudes, i.e. the low-latitude boundary layer (LLB I,). Another zone of weak 630.0 nm emission (type 2 cusp aurora) was observed to extend up to high latitudes (similar to 79 degrees MLAT) during relatively quiet magnetic conditions, when indications of reverse (sunward) convection was observed in the dayside polar cap. This is postulated to be a signature of merging between a northward directed IMF (B-z > 0) and the geomagnetic field poleward of the cusp. The coexistence of type 1 and 2 auroras was observed under intermediate circumstances. The optical observations from Svalbard and Greenland were also used to determine the temporal and spatial evolution of type 1 auroral forms, i.e. poleward-moving auroral events occurring in the vicinity of a rotational convection reversal in the early post-noon sector. Each event appeared as a local brightening at the equatorward boundary of the pre-existing type 1 cusp aurora, followed by poleward and eastward expansions of luminosity. The auroral events were associated with poleward-moving surges of enhanced ionospheric convection and F-layer ion temperature as observed by the EISCAT radar in Tromso. The EISCAT ion flow data in combination with the auroral observations show strong evidence for plasma flow across the open/closed field line boundary.

The impact of uncertain precipitation data on insurance loss estimates using a flood catastrophe model

Catastrophe risk models used by the insurance industry are likely subject to significant uncertainty, but due to their proprietary nature and strict licensing conditions they are not available for experimentation. In addition, even if such experiments were conducted, these would not be repeatable by other researchers because commercial confidentiality issues prevent the details of proprietary catastrophe model structures from being described in public domain documents. However, such experimentation is urgently required to improve decision making in both insurance and reinsurance markets. In this paper we therefore construct our own catastrophe risk model for flooding in Dublin, Ireland, in order to assess the impact of typical precipitation data uncertainty on loss predictions. As we consider only a city region rather than a whole territory and have access to detailed data and computing resources typically unavailable to industry modellers, our model is significantly more detailed than most commercial products. The model consists of four components, a stochastic rainfall module, a hydrological and hydraulic flood hazard module, a vulnerability module, and a financial loss module. Using these we undertake a series of simulations to test the impact of driving the stochastic event generator with four different rainfall data sets: ground gauge data, gauge-corrected rainfall radar, meteorological reanalysis data (European Centre for Medium-Range Weather Forecasts Reanalysis-Interim; ERA-Interim) and a satellite rainfall product (The Climate Prediction Center morphing method; CMORPH). Catastrophe models are unusual because they use the upper three components of the modelling chain to generate a large synthetic database of unobserved and severe loss-driving events for which estimated losses are calculated. We find the loss estimates to be more sensitive to uncertainties propagated from the driving precipitation data sets than to other uncertainties in the hazard and vulnerability modules, suggesting that the range of uncertainty within catastrophe model structures may be greater than commonly believed.

Predicted signatures of pulsed reconnection in ESR data

Early in 1996, the latest of the European incoherent-scatter (EISCAT) radars came into operation on the Svalbard islands. The EISCAT Svalbard Radar (ESR) has been built in order to study the ionosphere in the northern polar cap and in particular, the dayside cusp. Conditions in the upper atmosphere in the cusp region are complex, with magnetosheath plasma cascading freely into the atmosphere along open magnetic field lines as a result of magnetic reconnection at the dayside magnetopause. A model has been developed to predict the effects of pulsed reconnection and the subsequent cusp precipitation in the ionosphere. Using this model we have successfully recreated some of the major features seen in photometer and satellite data within the cusp. In this paper, the work is extended to predict the signatures of pulsed reconnection in ESR data when the radar is pointed along the magnetic field. It is expected that enhancements in both electron concentration and electron temperature will be observed. Whether these enhancements are continuous in time or occur as a series of separate events is shown to depend critically on where the open/closed field-line boundary is with respect to the radar. This is shown to be particularly true when reconnection pulses are superposed on a steady background rate.

EISCAT/CRRES observations: nightside ionospheric ion outflow and oxygen-rich substorm injections

We present combined observations made near midnight by the EISCAT radar, all-sky cameras and the combined released and radiation efects satellite (CRRES) shortly before and during a substorm. In particular, we study a discrete, equatorward-drifting auroral arc, seen several degrees poleward of the onset region. The arc passes through the field-aligned beam of the EISCAT radar and is seen to be associated with a considerable upflow of ionospheric plasma. During the substorm, the CRRES satellite observed two major injections, 17 min apart, the second of which was dominated by O+ ions. We show that the observed are was in a suitable location in both latitude and MLT to have fed O+ ions into the second injection and that the upward flux of ions associated with it was sufficient to explain the observed injection. We interpret these data as showing that arcs in the nightside plasma-sheet boundary layer could be the source of O+ ions energised by a dipolarisation of the mid- and near-Earth tail, as opposed to ions ejected from the dayside ionosphere in the cleft ion fountain.

On the longitudinal extent of magnetopause reconnection pulses

The open magnetosphere model of cusp ion injection, acceleration and precipitation is used to predict the dispersion characteristics for fully pulsed magnetic reconnection at a low-latitude magnetopause X-line. The resulting steps, as would be seen by a satellite moving meridionally and normal to the ionospheric projection of the X-line, are compared with those seen by satellites moving longitudinally, along the open/closed boundary. It is shown that two observed cases can be explained by similar magnetosheath and reconnection characteristics, and that the major differences between them are well explained by the different satellite paths through the events. Both cases were observed in association with poleward-moving transient events seen by ground-based radar, as also predicted by the theory. The results show that the reconnection is pulsed but strongly imply it cannot also be spatially patchy, in the sense of isolated X-lines which independently are intermittently active. Furthermore they show that the reconnection pulses responsible for the poleward-moving events and the cusp ion steps, must cover at least 3 h of magnetic local time, although propagation of the active reconnection region may mean that it does not extend this far at any one instant of time.

Earth's magnetospheric cusps

Earth's cusps are magnetic field features in the magnetosphere associated with regions through which plasma from the Sun can have direct access to the upper atmosphere. Recently, new ground-based observations, combined with in situ satellite measurements, have led the way in reinterpreting cusp signatures. These observations, combined with theoretical advances, have stimulated new interest in the solar wind-magnetosphere-ionosphere coupling chain. This coupling process is important because it causes both momentum and energy from the solar wind to enter into the near-Earth region. Here we describe the current ideas concerning the cusps and the supporting observational evidence which have evolved over the past 30 years. We include discussion on the plasma entry process, particle motion between the magnetopause and ionosphere, ground optical and radar measurements, and transient events. We also review the important questions that remain to be answered.

Variability of dayside high latitude convection associated with a sequence of auroral transients

10 second resolution ionospheric convection data covering the invariant latitude range from 71° to 76°, obtained by using the EISCAT UHF and VHF radars, are combined with optical data from Ny Ålesund during a sequence of auroral transients in the post-noon sector (∼ 15 MLT). Satellite observations of polar cap convection patterns suggest negative BZ and BY components of the interplanetary magnetic field. Burst-like enhancements of westward (sunward) post-noon convection were accompanied by eastward moving auroral forms at higher latitudes, above the convection reversal boundary. In this case the background convection was weak, whereas the integrated potential drop across the radar field-of-view associated with the westward flow bursts was typically ∼ 20-35 kV. The auroral phenomenon consists of a series of similar events with a mean repetition period of 8 min. A close correlation between the auroral activity and convection enhancements in the cleft ionosphere is demonstrated.

Events of enhanced convection and related dayside auroral activity

In this paper we study the high-latitude plasma flow variations associated with a periodic (∼8 min) sequence of auroral forms moving along the polar cap boundary, which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field. Satellite data on auroral particle precipitation and ionospheric plasma drifts from DMSP F10 and F11 are combined with ground-based optical and ion flow measurements for January 7, 1992. Ionospheric flow measurements of 10-s resolution over the range of invariant latitudes from 71° to 76° were obtained by operating both the European incoherent scatter (EISCAT) UHF and VHF radars simultaneously. The optical site (Ny Ålesund, Svalbard) and the EISCAT radar field of view were located in the postnoon sector during the actual observations. The West Greenland magnetometers provided information about temporal variations of high-latitude convection in the prenoon sector. Satellite observations of polar cap convection in the northern and southern hemispheres show a standard two-cell pattern consistent with a prevailing negative By component of the interplanetary magnetic field. The 630.0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the ∼1440–1540 MLT sector were observed to coincide with magnetosheath-like particle precipitation and a secondary population of higher energy ions, and they propagated eastward/tailward at speeds comparable with the convection velocity. It is shown that these optical events were accompanied by bursts of sunward (return) flow at lower latitudes in both the morning and the afternoon sectors, consistent with a modulation of Dungey cell convection. The background level of convection was low in this case (Kp =2+). The variability of the high-latitude convection may be explained as resulting from time-varying reconnection at the magnetopause. In that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection.