Ionospheric Electric Field Measurements Research Articles

Impact of seasonal variations on low and mid-latitudes ionospheric electric field satellite measurements

Ionospheric electric field measurements are provided by the Langmuir probe onboard the DEMETER satellite. Because there are four probes, various combinations of them form double probes that yield measurements. However, only the combination of probe antennas (E3) and (E4) enables possible measurements of the ambient (natural) ionospheric electric field. For other combinations, the measured field is almost identical to the induced field resulting from satellite motion. On the basis of this, we conducted comparative studies of the measured and induced electric fields during different seasons (summer, autumn, winter, and spring) to determine the best period for ionospheric electric field measurements. Multiple orbits were selected during different periods when the satellite was exposed to solar radiation. In all seasons except winter, the profiles of the induced and measured ionospheric electric field are similar. This masking effect leads to an underestimation of the ambient electric field. However, the highest accuracy for the ambient field can be obtained during winter, when solar flux radiation is weaker than that during other seasons. The probe position and orbital altitude also play important roles in the behavior of the induced electric field.

First radar measurements of ionospheric electric fields at sub‐second temporal resolution

A new multipulse sounding technique currently being used at the Wallops Island and Goose Bay SuperDARN radars has produced significant improvements in the temporal resolution of Doppler velocity measurements from which plasma velocities and electric fields are determined. The new technique allows Doppler velocities to be determined from every 200 ms multipulse sequence transmitted by the radar (equivalent to a 5 Hz measurement rate). To our knowledge, this is the highest Doppler measurement rate that has ever been attained with ionospheric radars. Tests of the new technique with the Wallops radar and Ottawa magnetometer revealed bursts of subauroral electric and magnetic field pulsations with periods of 13–20 s during a substorm expansion phase. These results indicate that SuperDARN measurements can be used to study highly dynamic processes in the coupled magnetosphere‐ionosphere system, including storm and substorm electrodynamics, short‐period pulsations and short‐term variability in Joule heating.

Auroral <i>E</i>-region electron density gradients measured

Abstract. In the theory of E-region plasma instabilities, the ambient electric field and electron density gradient are both included in the same dispersion relation as the key parameters that provide the energy for the generation and growth of electrostatic plasma waves. While there exist numerous measurements of ionospheric electric fields, there are very few measurements and limited knowledge about the ambient electron density gradients, ∇Ne, in the E-region plasma. In this work, we took advantage of the EISCAT CP1 data base and studied statistically the vertical electron density gradient length, Lz=Ne/(dNe/dz), at auroral E-region heights during both eastward and westward electrojet conditions and different ambient electric field levels. Overall, the prevailing electron density gradients, with Lz ranging from 4 to 7 km, are found to be located below 100 km, but to move steadily up in altitude as the electric field level increases. The steepest density gradients, with Lz possibly less than 3 km, occur near 110 km mostly in the eastward electrojet during times of strong electric fields. The results and their implications are examined and discussed in the frame of the linear gradient drift instability theory. Finally, it would be interesting to test the implications of the present results with a vertical radar interferometer.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)

Ionospheric electric field estimates from radar observations of the equatorial electrojet

Estimates of the zonal ionospheric electric field have been made on the basis of JULIA radar observations of the equatorial electrojet at Jicamarca. Two observing techniques were used and their merits compared. One technique (here referred to as the “oblique” technique) made use of a new, small antenna array with a broad beam in the equatorial plane. Electric fields were estimated from the Doppler shifts of type II radar echoes at low elevation angles as a function of range in the manner of Balsley [1969a]. For the other technique the main Jicamarca antenna array was used, and interferometry was employed to measure altitude profiles of the phase speeds of intermediate‐scale primary gradient drift waves in the electrojet directly overhead. Electric fields were inferred from these profiles following a modeling approach similar to the one described by Murthy and Ravindran [1994], Zonal ionospheric electric field measurements comparable to seasonal average incoherent scatter radar measurements were obtained using both methods. However, the interferometric observations reveal the presence of intense waves and jets in the zonal neutral wind similar to what has been seen in recent chemical release sounding rocket experiments. Zonal winds mainly control the zonal phase speeds of the primary gradient drift waves in the late afternoon and evening. Although the prereversal enhancement of the zonal electric field is evident in data from the oblique technique, there is no evidence of it in measurements derived from the interferometric technique.

High resolution bistatic HF radar observations of ULF waves in artificially generated backscatter

High resolution HF radar observations of ULF waves in the ionosphere are possible by artificially generating irregularities using the EISCAT heater at Tromsø, Norway. The line‐of‐sight velocities from the CUTLASS radars have been combined for the first time to form bistatic flow vectors during a ULF wave, generating ionospheric electric field measurements with a higher accuracy than ever previously recorded by any instrument. A high‐m wave observed in the ionosphere exhibits a frequency which is a harmonic of that of a low‐m field line resonance which is observed simultaneously in the ionosphere and at the ground. The high‐m pulsation resembles a class of particle driven wave previously recorded on VHF radars. These results are consistent with recent HF radar observations of small scale waves possessing similar characteristics to field line resonances and with the theory that the low‐m wave may be driving the high‐m wave through a non‐linear Kelvin‐Helmholtz instability.

Equatorial 150‐km irregularities observed at Pohnpei

First observations of meter‐scale equatorial 150‐km plasma irregularities outside the American longitude sector are reported. The observations conducted at Pohnpei (western Pacific) with a wind profiling ST radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150‐km VHF radar echoes.

Ionospheric F region currents at middle and low latitudes estimated from Magsat data

Two methods for determining the F region current density at satellite heights by means of a decomposition of the magnetic field into toroidal and poloidal parts are presented. The first method allows for determination of the radial current component only by neglecting the height variation of the satellite orbit. Application to Magsat data yields the well‐known field‐aligned currents of polar latitudes (average current density Jr ≲ 200 nA/m2) and the meridional current system of the equatorial electrojet (Jr ≈ 10 – 20 nA/m2) during dusk. In addition, a midlatitude interhemispheric current system could be observed for the first time. Current direction is from the winter to the summer hemisphere during dusk and in opposite direction during dawn, as predicted by three‐dimensional models of the ionospheric dynamo. Presumably, for the first time a lunar contribution to the meridional current system of the equatorial electrojet was found, too. Amplitude at the dip equator is Jr ≈ 1.8 nA/m2; time of maximum is in agreement with magnetic ground observations, ionospheric electric field measurements, and ionospheric dynamo theory. The second method allows for estimation of both the radial and the horizontal current density by expanding the radial dependence of the magnetic field in Taylor series. First results confirm the current direction of the low‐latitude meridional current system as suggested by dynamo calculations: upward currents at the dip equator and field‐aligned downward currents at low latitudes.

Ionospheric convection response to changing IMF direction

By combining ground‐based and satellite‐based measurements of ionospheric electric fields, conductivities and magnetic perturbations, we are able to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the Interplanetary Magnetic Field (IMF). In response to a rapid southward‐to‐northward turning of the IMF on 23 July 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two‐cell pattern to a contracted four‐cell pattern, with reversed convection cells in the high‐latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the −Y. toward the −Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two‐cell pattern was reestablished, largely in accord with the qualitative model of Crooker [1988]. Our results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.

Reconstruction of the ionospheric influence on ground-based observations of a short-duration ULF pulsation event

Recent theoretical advances concerning the influence of the ionosphere on ULF pulsations indicate clear differences depending on whether uniform or non-uniform ionospheric conductivity is regarded. To test these new theoretical concepts, simultaneous observations of a Pc5 event on 2 February 1977 made by the Scandinavian Magnetometer Array (SMA) and the STARE radar are analysed. Taking into account a horizontally uniform conductivity, ground magnetic observations below the STARE field of view could not be predicted in a satisfactory manner from the ionospheric electric field measurements. To allow for non-uniform conductivity in our model calculations we analysed the spatial characteristics of the pulsation wave field using both the SMA and STARE data. We thus derived a first-order approximation of the incident wave field from which the electric field of the reflected wave has been computed for various non-uniform conductivity distributions. From the resulting ionospheric and field-aligned currents the ground magnetic field is determined. In a trial-and-error procedure conductivity and partly also the incident electric field have been changed until satisfactory agreement between our model ionospheric electric field and model ground magnetic fields with those observed by the SMA and STARE, respectively, results. From the final model the spatial variation of the ionospheric rotation angle has been derived. It significantly deviates from the earlier predicted 90° value.

Low-frequency fluctuations of the electric field in the equatorial ionosphere

High time-resolution radar measurements of ionospheric electric fields have recently permitted studies of low-frequency hydromag-netic waves, but most of the research has depended on electron drift velocity from data obtained from the Scandinavian Twin Auroral Radar Experiment (STARE), taken at high latitudes (λm ∼ +70°) and used to study oscillations with periods of 200–300 s. At high latitudes, the low-frequency pulsations provide important information on the magnetospheric plasma process1–3, but, at low latitudes, coupling of the ionosphere and magnetosphere remain unexplored. The purpose of our study is to examine the fluctuations in the electric field on a time scale longer than ion cyclotron periods, Tci ∼ 0.6 s (ref. 4), or for frequencies ω ≪ ωci ∼ 102 rad s−1 (the ion cyclotron frequency in the equatorial ionosphere), which haye not been studied at low latitudes. We report here observations of these waves with periods larger than 45 s from the high time-resolution data taken at Jicamarca, Peru.

Comparison of height‐integrated current densities derived from ground‐based magnetometer and rocket‐borne observations during the Porcupine F3 and F4 flights

On March 19, 1979, the Porcupine F3 rocket was launched from Esrange (Sweden) during a period of low magnetospheric activity while the Porcupine F4 rocket launched on March 31, 1979, flew during a rather disturbed interval. Besides other experiments the rockets took measurements of ionospheric electric fields and energetic particle spectra. The electron spectra measured by the rocket were used to calculate height‐integrated ionospheric conductivities by using Rees' method. Together with the electric field measurements, height‐integrated current densities could be calculated from the rocket measurements. During both flights the magnetic perturbations recorded on the ground by the Scandinavian Magnetometer Array indicated a westward electrojet. Since during both flights the electric field was nearly southward directed and parallel to the ground magnetic disturbance vectors the westward electrojet could be attributed to a nearly pure Hall current in both cases. From the magnetic data height‐integrated ionospheric current densities have been computed at the magnetic footprints of the rockets in the ionosphere by means of field separation and upward continuation. A comparison of the current densities derived from ground‐based magnetometer and rocket‐borne observations shows a very good agreement in the quiet period during the Porcupine F3 flight, while there are differences during stronger magnetospheric activity (Porcupine F4 flight) due to the different spatial and temporal resolution of rocket and ground observations.

Evidence for the E×B drift of pulsating auroras

Simultaneous measurements of ionospheric electric fields by STARE and TV imaging of pulsating auroral forms strongly suggest that the latter undergo E×B drift. A display of pulsating auroras is considered as the projection on the ionosphere of an assembly of ducts of enhanced plasma density convecting with the background plasma. Energetic electrons, convecting and with additional gradient and curvature drift, sweep through these ducts which are ‘illuminated’ by pulsating precipitation of these energetic electrons. Both the self‐sustained pulsation model of Davidson (1979) and the driven pulsation model of Coroniti and Kennel (1970) are compatible with our observations.

Hydromagnetic waves in the magnetosphere

The study of hydromagnetic waves in the magnetosphere has seen some notable advances over the last four years. We owe a large debt to the IMS observational programs both in space and on the ground. These programs have produced extremely valuable data sets which should be worked on for many years to come.The improvement in instrumentation has been a major factor in observational work. The new generation of spacecraft are exemplified by the ISEE and GEOS missions, both of which have novel aspects. Both missions have played a large part in helping us understand how important heavy ions are in the propagation of hydromagnetic waves of all frequencies. The dual spacecraft, ISEE 1 and ISEE 2, are helping to unravel the spatial structure of these waves in space, while both missions are providing the first measurements of electric field oscillations associated with hydromagnetic waves. The ionospheric electric field measurements made by the STARE radar system have introduced a new dimension to the field, and help provide a link between ground based and spacecraft measurements. Several ground based magnetometer networks were either introduced or expanded, largely as a result of the IMS.

Chatanika Radar observations of the electrostatic potential distribution of an auroral arc

Observations of a premidnight auroral arc were made with high temporal and spatial resolution, using the Chatanika incoherent scatter radar. The arc was a quiet east‐west aligned arc, about 40 km wide, that drifted slowly through the radar beam without appreciably changing in shape or luminosity. The distribution of ionospheric electric fields and currents associated with the arc were computed from the ion density and line of sight velocity. The electric field was 30 mv/m outside the arc and decreased to a few mv/m within the arc. The horizontal potential drop was only ∼ 0.6 kV across the arc. By deconvolving the electron density profiles, differential energy spectra and velocity distributions were obtained within the arc. From these the accelerating potential parallel to the magnetic field lines was estimated by several techniques. One technique makes use of an empirical relationship between the parallel potential drop and the total electron current. The accelerating potential reached 7 kV at the arc center. The parallel potential drop within the arc is combined with ionospheric electric field measurements to reconstruct the equipotential contours above the arc.

Characteristics of Pc 5 micropulsations as determined with the STARE Experiment

Recently, Walker et al. (1979) have shown the suitability of the Scandinavian twin auroral radar experiment facility for Pc 5 pulsation studies. They consider that in the ionosphere the hydromagnetic wave causing the geomagnetic pulsation produces a periodic electric field superimposed on the ambient background field. In this paper STARE measurements of ionospheric electric fields are used to obtain Pc 5 polarization maps covering all the STARE field of view. These fields are deduced from electron Hall drift velocities measured with high spatial (20 × 20 km²) and temporal (20 s or 60 s) resolution over a large area (160,000 km²). The fast Fourier transform algorithm is applied to time series of both the north‐south and east‐west electric field components for each of the ∼400 geographic points within the STARE field of view. By retaining both the amplitudes of and relative phase between the north‐south and east‐west Fourier transformed components, the complete polarization state of the Pc 5 electric field can be determined at each geographic point and for each of the discrete frequencies obtained from the fast Fourier transform algorithm. The analysis of a number of selected events that support the hydromagnetic field line resonance theory of micropulsations is presented.

Global ionospheric electric field measurements in April 1978

This paper presents an introduction to a global campaign of simultaneous quasistatic electric field measurements from radars, balloons, and satellites at various places within the earth's environment for April 8–14, 1978. The 7‐day time period encompassed both extended magnetically quiet times as well as two magnetic storms. These storms were related to SSC's that followed solar flares, one of which included a day‐long solar proton event. The wide variety of instrumentation and associated operating modes involved in this campaign is described. Sample conjunctions between satellites and ionospheric measurements are shown that demonstrate that field line mapping is valid under certrain circumstances. Some of the largest ionospheric electric fields ever reported with >100‐km scale size occurred on April 11, and these events are discussed in detail.

Measurements of high-latitude ionospheric electric fields by means of balloon-borne sensors

Measurements of ionospheric electric fields in the auroral and polar cap regions by means of balloon-borne sensors are described. Launches took place from the Norwegian station of Andenes (Andoya) (69degreeN, 16degreeE) during summer 1974. The Balloons drifted over the Atlantic to the west coast of Greenland nearly along 69.5degreeN geographic latitude (within +- 1.5degree) and were followed by telemetry stations at Andenes, Raufarhofn (Iceland), and Sondre Stromfjord (Greenland). The electric field data are discussed from the point of view of the two-cell magnetospheric convection pattern which has been established in recent years. Of particular interest are the well-recognized features of the Harang discontinuity as well as electric field data in the vicinity of the polar cusp region. (AIP)

Electric fields, Joule and particle heating in the high latitude thermosphere

A short review of the recent high latitude measurements of ionospheric electric fields is given. The importance of investigating large-scale and slowly-varying electric fields in order to study magnetospheric convection is stressed. The motion of such high energetic phenomena as auroral forms and spread E-region echoes must be treated by extreme caution when interpreted as a manifestation of convection motion. The relationship between the ionospheric source and polarization field is still an unanswered problem. It is indicated that progress can be made in this respect when electric fields and conductivities are measured simultaneously in the ionosphere. Evidence is shown at one occasion that the meridional component during an auroral substorm might be mainly a polarization field. The height-integrated Joule heating rate is occasionally found to be far larger than the solar radiation input at auroral altitudes. The presence of this additional heat source at any time of day is expected to have a strong impact on the global-scale atmospheric dynamics. From comparisons made between Joule and particle heating it appears that the two components are comparable. It is expected that high latitude incoherent radars will contribute substantially to our understanding of these phenomena in the near future.

Polar cap ionospheric electric field modulation by the solar wind sector structure

Measurements of ionospheric electric fields in the northern polar cap are reported for a nearly uninterrupted period of ∼8 days. The fields were measured by balloon-borne sensors. A clear modulation of the electric field strength as a function of the earth position within interplanetary sectors is observed. The field strength rises sharply after passage of a sector boundary and declines steadily in the trailing portion of the sector.

High-latitude electric fields and the three-dimensional interaction between the interplanetary and terrestrial magnetic fields

More than 200 hours of ionospheric electric field measurements taken on balloons flown from three polar cap sites have been analyzed to determine average properties of the large-scale polar cap electric field and its dependencies on the interplanetary magnetic field. The major component of this electric field was directed from dawn to dusk and produced an average polar cap potential drop of about 55 kV. The magnitude of this potential provides an upper limit of about 700 RE for the length of the magnetospheric tail and implies an energy input from the solar wind to the magnetosphere of about 5 × 1019 ergs/sec. The dawn to dusk component of the high-latitude polar cap electric field responds to Bz, the northward component of the interplanetary magnetic field, on a time scale ≲1 hour and with an average increase of about 3 mV/m for each 1 γ decrease of Bz. The hourly averages of the electric field data at each of the three sites are well described by a two-cell convection pattern whose location depends on the y component of the interplanetary magnetic field. When By is positive (negative), the two-cell convection pattern shifts toward dawn (dusk) in the northern hemisphere with the following consequences: the maximum intensity of the northern polar cap dawn to dusk electric field component occurs at local morning (evening), and the auroral zone return flow reaches higher latitudes in the evening (morning). Evidence of the vector nature of the interaction between interplanetary and terrestrial magnetic fields is provided by the observation that the above By dependent signatures are most evident when Bz is most negative.