Probing discrete auroral arcs by ionospheric tomography

Optical observations of 557.7 nm and 630.0 nm emissions from discrete auroral arcs in the post-noon sector have been related to localised field-aligned enhancements in the spatial distribution of E- and F-layer electron density respectively seen in images reconstructed by ionospheric tomography. Results from two case studies are presented in which meridian scanning photometer and all-sky camera observations on Svalbard have been compared to electron-density structures found by tomographic inversion of measurements made by reception of radio signals at a chain of four stations at high latitude. The F-layer features are long-lived and show exact correspondence to the red-line emissions. Transient arcs in green-line intensity result in E-region structures that are resolved in one case, but not in another where the dynamic auroral forms are separated by less than one degree of latitude. The signature of an inverted-V precipitation event is clearly evident in one example.     

SuperDARN studies of the ionospheric convection response to a northward turning of the interplanetary magnetic field

The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative B_z and positive B_y, to positive B_z and small B_y, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within 2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of 7 and 14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further 6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.     

Heating of beam ions by ion acoustic waves

Satellite measurements show that ion beams above the auroral acceleration region are heated to hundreds of eV in a direction perpendicular to the magnetic field. We show that ion acoustic waves may be responsible for much of this heating. Even in the absence of a positive slope in the velocity distribution of the beam ions, ion acoustic waves can be generated by a fan instability. We present analytical estimates of the wave growth rate and ion beam heating rate. These estimates, which are confirmed by particle simulations, indicate that the perpendicular temperature of the beam ions will increase by 30 eV/s, or by 1 eV in 20–25 km. From the simulations we also conclude that the heating saturates at a perpendicular temperature around 200 eV, which is consistent with observations.     

A case study of electron precipitation in the late substorm growth phase on and nearby a preonset arc

We follow the electron precipitation characteristics on and nearby a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 min before onset up to 1 min before onset. The arc is the most equatorward one (around MLAT 62°) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, inverted-V type precipitation dominates. Poleward of the arc we also find some precipitation regions, and here there is systematically a cold electron population superposed with a warm population. Using single and double Maxwellian fits to the measured electron spectra we find the ionosphere-magnetosphere coupling parameters (field-aligned conductance K and the parallel potential drop V) as well as the effective source plasma properties (density and temperature) during the event. Compared to typical expansion phase features, the preonset parallel potential drop is smaller by a factor of ten, the electron temperature is smaller by a factor of at least five, and the field-aligned conductance is about the same or larger. The fact that there are two isotropic superposed electron populations on the poleward side of the preonset arc suggests that the distance between warm trapped electrons on dipolar field lines and colder electrons on open field lines has become so small near the onset that mixing e.g. due to finite electron Larmor radius effects can take place.     

Plasma convection at high latitudes using the EISCAT VHF and ESR incoherent scatter radars

The recent availability of substantial data sets taken by the EISCAT Svalbard Radar allows several important tests to be made on the determination of convection patterns from incoherent scatter radar results. During one 30-h period, the Svalbard Radar made 15 min scans combining local field aligned observations with two, low elevation positions selected to intersect the two beams of the Common Programme Four experiment being simultaneously conducted by the EISCAT VHF radar at Troms. The common volume results from the two radars are compared. The plasma convection velocities determined independently by the two radars are shown to agree very closely and the combined three-dimensional velocity data used to test the common assumption of negligible field-aligned flow in this regime.     

A survey of magnetopause FTEs and associated flow bursts in the polar ionosphere

Using the Equator-S spacecraft and SuperDARN HF radars an extensive survey of bursty reconnection at the magnetopause and associated flows in the polar ionosphere has been conducted. Flux transfer event (FTE) signatures were identified in the Equator-S magnetometer data during periods of magnetopause contact in January and February 1998. Assuming the effects of the FTEs propagate to the polar ionosphere as geomagnetic field-aligned-currents and associated Alfveén-waves, appropriate field mappings to the fields-of-view of SuperDARN radars were performed. The radars observed discrete ionospheric flow channel events (FCEs) of the type previously assumed to be related to pulse reconnection. Such FCEs were associated with 80% of the FTEs and the two signatures are shown to be statistically associated with greater than 99% confidence. Exemplary case studies highlight the nature of the ionospheric flows and their relation to the high latitude convection pattern, the association methodology, and the problems caused by instrument limitations.     

The ionospheric footprint of antiparallel merging regions on the dayside magnetopause

The antiparallel merging hypothesis states that reconnection takes place on the dayside magnetopause where the solar and geomagnetic fields are oppositely directed. With this criterion, we have mapped the predicted merging regions to the ionosphere using the Tsyganenko 96 magnetic field model, distinguishing between regions of sub-Alfvénic and super-Alfvénic magnetosheath flow, and identifying the day-night terminator. We present the resulting shape, width and latitude of the ionospheric dayside merging regions in both hemispheres, showing their dependence on the Earths dipole tilt. The resulting seasonal variation of the longitudinal width is consistent with the conjugate electric fields in the northern and southern cusps, as measured by the SuperDARN HF radars, for example. We also find a seasonal shift in latitude similar to that observed in satellite cusp data.     

Simultaneous optical and radar signatures of poleward-moving auroral forms

Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations.     

The dawn and dusk electrojet response to substorm onset

We have investigated the time delay between substorm onset and related reactions in the dawn and dusk ionospheric electrojets, clearly separated from the nightside located substorm current wedge by several hours in MLT. We looked for substorm onsets occurring over Greenland, where the onset was identified by a LANL satellite and DMI magnetometers located on Greenland. With this setup the MARIA magnetometer network was located at dusk, monitoring the eastward electrojet, and the IMAGE chain at dawn, for the westward jet. In the first few minutes following substorm onset, sudden enhancements of the electrojets were identified by looking for rapid changes in magnetograms. These results show that the speed of information transfer between the region of onset and the dawn and dusk ionosphere is very high. A number of events where the reaction seemed to preceed the onset were explained by either unfavorable instrument locations, preventing proper onset timing, or by the inner magnetospheres reaction to the Earthward fast flows from the near-Earth neutral line model. Case studies with ionospheric coherent (SuperDARN) and incoherent (EISCAT) radars have been performed to see whether a convection-induced electric field or enhanced conductivity is the main agent for the reactions in the electrojets. The results indicate an imposed electric field enhancement.     

CUTLASS HF radar observations of high-latitude azimuthally propagating vortical currents in the nightside ionosphere during magnetospheric substorms

High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (800 m s^–1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (1 km s^–1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested.