Regions of strongly enhanced perpendicular electric fields adjacent to auroral arcs

In a joint campaign involving EISCAT, the Cornell University Portable Radar Interferometer (CUPRI), and sounding rockets, we have observed short-lived elevations of E-region electron temperatures, indicating the presence of strong electric fields. The use of a new pulse-code technique has considerably improved our EISCAT data in regions of low ionospheric electron densities. It has been found that strong and apparently short-lived enhancements of electric fields and associated E-region electron temperatures occur more commonly than long-lived ones. However, earlier EISCAT data with simultaneous optical recordings (and also some CUPRI radar data from the ERRRIS campaign) indicate that many of these events are, in fact, not short-lived, but occur in localized regions and are associated with drifting auroral forms. We show that the observed elevations of electron temperatures are created by very intense electric fields which can be found within narrow regions adjacent to auroral arcs. We discuss our observations against the background of models for electric field suppression or enhancement in the vicinity of auroral precipitation.     

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude (E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation (δn/n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.     

The effect of electric field-induced vertical convection on the precipitation E-layer

The effects of perpendicular electric fields on the high-latitude nocturnal precipitation E-layer are studied in terms of model calculations. The deformations of the electron density profiles caused by vertical plasma movements are described assuming various incident electron energies and flux densities, as well as different field intensities and directions. Manifestations of the profile variations in vertical sounding observations and changes in the conductivity profiles are examined. The results show that field-induced vertical convection is important in plasma densities even higher than 10¹¹ m⁻³. Erroneous results may be obtained if the effect of the electric field is neglected when determining the incident electron spectrum from a measured ionospheric density profile.     

Electric field measurements during the MAC/EPSILON campaign

As part of the MAC/EPSILON campaign in northern Norway during October and November 1987, five rocket-borne payloads made three-axis electric field measurements of the middle atmosphere. Flights 31.066 and 31.067 consisted of large multi-experiment packages, while the other three flights (30.036, 30.037 and 30.038) were devoted primarily to electrical measurements. Simultaneous measurements of the horizontal electric field made by flights 31.066 and 30.036 were in general agreement in their limited altitude region of overlap. A simultaneous small temporal feature was observed in both datasets. The relatively more extensive horizontal E-field datasets from 31.066 and 31.067 both exhibited a decreasing mapping function with decreasing altitude, which is an indication of the observation of fields from a local auroral patch. Small-scale variations in the horizontal fields of the flights were similar to observed wave-like variations in the neutral wind field. No unusual features were observed at high altitudes in the measured E₁ field. Two of the payloads observed small vertical layer structures between 40 and 50 km. No electric field structure was observed in association with the presence of a sudden sodium layer.     

Electrical structure in the high-latitude middle atmosphere

Middle atmosphere electrodynamics at high latitudes differs significantly from the normally assumed picture of a passive region through which electric fields of external origin couple. Large Vm ⁻¹ electric fields, both horizontal and vertical, have been observed within bounded regions of the upper stratosphere and lower mesosphere. They seem to occur only in regions where the electrical conductivity is a few times 10⁻¹⁰ S m⁻¹ or less and appear to be current limned. While low conductivity is necessary, it is not a sufficient condition for occurrence. The observed large horizontal electric fields were found to be anticorrelated with the local neutral wind. However, a generation mechanism of these electric fields is as yet unknown but must involve space charge separation rather than dynamo effects. Large variations in the conductivity were also observed to occur with fluctuations in magnetic activity, and these were found to be consistent with measured variations in energy deposition during auroral phenomena. Theoretical concepts of mapping of electric fields downward from the thermosphere along equipotential magnetic field lines were shown to hold qualitatively in the D-region at the mV m⁻¹ level. Perturbations affecting such models were determined to be small.     

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.     

On the structure and circulation of the polar thermosphere under magnetically quiet conditions

A theoretical study is presented bearing on the thermospheric circulation and composition at polar latitudes. The observed motions and density perturbations in N₂, O and He have signatures which may be understood in terms of two different source mechanisms. We consider electric field momentum coupling and Joule heating as well as interactions between both processes. A spectral model in terms of vector spherical harmonics (with magnetic coordinates) is used, delineating the diurnal and mean (time independent) components. The important non-linearities are evaluated in configuration space. The electric field model of Volland and the global average density and temperature variations of Hedin (MSIS) are adopted as input. Our analysis leads to the following conclusions. (1) The vortex type double cell polar circulation (zonal wave number m = 1) is primarily driven by collisional momentum transfer from electric field induced ion convection. (2) Because of the thermospheric low pass filter, a large time independent component (zonal wave number m = 0) is produced by Joule heating; the heavier species (N₂) being concentrated where the lighter ones (O, He) are depleted, and vice versa due to wind induced diffusion (3) The electric field driven vortex circulation redistributes the mass and energy in the time independent density and temperature variations (from Joule heating), producing primarily diurnal variations (m = 1) in the temperature and composition near the pole and at auroral latitudes, again the heavier and lighter species varying out of phase. The above results are in substantial agreement with observations. It is worth noting that momentum rectification associated with the diurnally varying electric field and conductivity induces a weak zonally symmetric (single cell) prograde polar vortex. However, this motion is partially compensated by a retrograde vortex from geostrophic balance due to Joule heating, which dominates near the pole. These motions are small compared with the diurnally varying component in the polar circulation.     

Estimation of initial auroral proton energy fluxes from Doppler profiles

An energetic auroral proton entering the atmosphere will, by charge exchange in collisions with atmospheric constituents, alternate between being a proton H⁺ and a neutral hydrogen atom H. This study provides a procedure to evaluate the auroral Doppler shifted and broadened hydrogen Balmer profile as a function of initial energy, flux, pitch angle and view angle relative to the geomagnetic field. The differential proton energy flux entering the atmosphere is deduced using ground-based measurements of H_α and H_β from Nordlysstasjonen in Adventdalen, Longyearbyen. The main assumptions are that the geomagnetic field lines are: parallel and vertical, and that the pitch angle of the H/H⁺-particle is preserved in collisions with atmospheric constituents before being thermalized. This numerical method estimates the fate of the auroral H/H⁺-particle in the atmosphere, and from measured Doppler profiles the corresponding incoming particle flux can be deduced. Optimization of the method will continue through extensive use of observational data.     

The generation and propagation of atmospheric gravity waves from activity in the auroral electrojet

EISCAT measurements of the electric field in the auroral electrojet are compared with the signature of TIDs propagating equatorward as observed by an HF-Doppler network. At night-time the onset of auroral activity is usually followed by the arrival of a TID at lower latitude. Cross-correlation of the time variations of the electric field measured by EISCAT and the frequency offset recorded by the HF-Doppler system confirms a relationship between the auroral activity and the gravity wave, indicating both the travel time and the periodicity of the wave. The relationship is especially close under quiet conditions when the cross-correlation coefficient is typically 60%, significant at 0.1%. When the observed electric field is used as input to a thermosphere-ionosphere coupled global model it predicts the time signature of the observed HF-Doppler variation reasonably well but seriously underestimates the amplitude of the disturbance. Examination of this discrepancy may lead to a better understanding of the mechanisms involved in the generation and propagation of atmospheric gravity waves.     

What determines the latitudinal extent of the auroral acceleration region?

Characteristic scales associated with auroral precipitation are investigated on the basis of quasistatic magnetotail models, resistive MHD simulations of magnetotail dynamics, and a general relation between parallel electric fields and velocity shear. Since the inverted-V precipitation region of discrete auroras (on the dusk side) is associated with upward flowing, region 1, currents, we investigate the distribution of these currents first. The overall distribution of region 1 type field-aligned currents and their dynamic changes can be explained by characteristic scales in the magnetotail and their mapping to the ionosphere. The quiet time region 1 currents are associated with the decrease of tail flaring. Their overall extent in the north-south direction is closely related to the scale height of the cross-tail current. Dynamic region 1 currents are related to the velocity shear of earthward flow, which can be generated by a tail instability. In that case the peaks of the enhanced region 1 currents are found to map closer to midnight and to lower latitudes than the quiet region 1 currents, consistent with average observations [Iijima and Potemra (1976a), J. geophys. Res.81, 2165]. On the basis of a general relation between parallel electric fields and ‘slippage’ in the plasma transport [Hesse and Schindler (1988), J. geophys. Res.93, 5559; Schindleret al. (1991), Astrophys. J.380, 293], we make estimates of the spatial extent of nonideal regions, where parallel electric fields may exist. For a plausible model of substorm reconfiguration, we find a latitudinal extent of about 7 km for a time scale of 1 min and a integrated parallel electric field of 5 kV. The length scale is proportional to the time scale. The sign of this parallel electric field is consistent with downward acceleration of electrons on the dusk side. The spatial extent of the parallel electric field region is independent of the microscopic generation mechanism if the time scale and the characteristic parallel potential difference (i.e. the integrated parallel electric field) are independent of this mechanism.     

Simultaneous measurements of the ionospheric electric field by probes and radar methods

Ionospheric electric field values are presented, obtained simultaneously by the double probe technique on board a rocket and by two incoherent backscatter radar installations. The measurements were performed during auroral activity over northern Scandinavia. The spatial distribution of the field reveals pronounced local variations.     

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.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

A model of the lower ionosphere above Red Lake,Canada, during the 26 February 1979 solar eclipse

Measurements of ionization sources, ionization profiles and minor atmospheric constituents were conducted during the 26 February 1979 solar eclipse above Red Lake, Canada. A model of the lower thermosphere has developed to describe the D- and E-regions of the ionosphere for this case with the model being guided by the measurements. During the eclipse a rather intense particle precipitation event was in progress. For this reason, an auroral deposition code was coupled to a chemical-kinetics code to calculate degraded primary and secondary electron fluxes, ionization rates, positive ion and electron densities. The model was calibrated with the experimental measurements of electron flux below 100 km and electron density between 70 and 150 km. This calculation not only satisfactorily described the ionization in the E-region but also the gross electron density characteristics of the D-region. Bursts in the observed electron flux were also simulated with the model to give electron density profiles that were remarkably consistent with small perturbations seen in the electron density measurements.     

Simultaneous ground-based observations of polar cap arcs and spacecraft measurements of particle precipitation

In order to investigate the particles which produce the polar cap aurora at the Vostok station in Antarctica, charged particle data obtained by the DMSP satellites for some days in a period from April to August 1985 were surveyed. Due to the satellite orbit the local time range in which the data were available was the morning sector. For all the events when sun-aligned arcs were observed on the ground the simultaneous DMSP measurements on almost the same field line showed an increased integral number flux J. > 10⁸ (cm⁸/s/sr)⁻¹ of the precipitating electrons with energy E_e > 200 eV. The electron spectra with double peaks are typical of intense electron precipitation in the polar cap arcs. The most noticeable feature of ion spectra in the polar cap arcs is the prominent minimum in ion flux in the energy range 0.1 < E_i < 1 keV in contrast with the oval precipitation ; this feature gives the possibility to separate the polar arcs from the aurora in the oval. In some events the satellite crossed the system of two widely separated arcs ; one of them was a sun-aligned arc whereas the other was circular at constant latitude according to the Vostok data. The analysis of the DMSP electron and ion precipitation data has shown that in these events the latitude-oriented arcs are located in the polar cap and not in the auroral oval.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—III. Comparison of observed ΔH values with those estimated from measured electric fields

From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field (E_y) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D_st) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.     

Auroral radar observations at Siple Station,Antarctica

A dual coherent 50 MHz auroral radar installed at Siple Station, Antarctica, continuously monitored echo intensity and mean-Doppler velocity from 29 December 1976 until late March 1978. We describe here the experimental technique and present some statistical results including yearly averaged echooccurrence patterns and irregularity drift velocity characteristics. Our results show that:

• 1.(1) the irregularity drifts are approximately westward (poleward electric fields) in the evening, and eastward (equatorward electric fields) in the morning following the electric field reversal in the region of the Harang discontinuity;
• 2.(2) the Harang discontinuity under disturbed conditions (average K_p = 5₀) as seen by both radars is located around 2100–2300 MLT;
• 3.(3) the relationship between the irregularity drift velocity and the actual electron drift velocity is strongly dependent on the angle between the radar beam and the earth's magnetic field, as predicted by linear theory.

     

Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8, and Søndrestrøm incoherent scatter radar observations

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft and the Søndrestrøm incoherent scatter radar coincide in space. Such coincidence offers a unique opportunity to study temporal vs spatial variations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP satellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distribution and line-of-sight plasma velocities. We combine these data sets to construct a two-dimensional map of a possible auroral pattern above Søndrestrøm. It is characterized by the following properties. No difference is seen between the gross precipitation patterns measured along the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic east-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells. However, F6 and F8 observed minor differences in the small-scale structures. More significant differences are found between small-scale features in the DMSP precipitation measurements and in radar observations of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4°, in magnetic longitude, and 0–40 s in time along the spacecraft trajectories (varying with magnetic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide. The combined data suggest that the plasma flow reversal delineates the polar cap boundary, that is, the boundary between precipitation characteristic for the plasma mantle and for the plasma sheet boundary layer.     

Electron temperature enhancements in the polar E-region measured with EISCAT

We have found electron temperature enhancements up to 1000K at 110 km altitude using the EISCAT multipulse method which allows high spatial resolution measurements within E-region. This electron temperature enhancement which is closely related to the d.c. electric field strength, is in good agreement with theoretical estimates based on wave heating. The results of the measurements are presented together with a discussion of the electron gas heating, its height variations and its difference in the eastward and westward electrojet.     

Energy input and spectral variations of precipitated particles during three nights of the Energy Budget Campaign

Observations were carried out during the Energy Budget Campaign with the aim of measuring keV particle precipitation in the ionosphere over a region covering the rocket range. These measurements are important as particle precipitation constitutes one of the major energy inputs into the ionosphere during an auroral substorm. Estimates of electron precipitation have been made in three different regions of the energy spectrum. Two optical emissions were measured that are proportional to electron precipitation of a few hundred eV and a few keV, respectively, and cosmic noise absorption related to electrons of a few tens of keV. The relative variation between the signals may be interpreted as changes of the energy spectrum of the particles. Measurements from the ground have been compared with satellite measurements to obtain the relation between the two methods. The ground-based observations, which are continuous in time and cover a large area, may be used to separate the spatial and temporal variations and to give the large scale substorm frame of reference for the rocket data. The results may also be used in the interpretation of other measurements on board rockets where a knowledge of the precipitated particles is required.