K. Schlegel

Direct penetration of the polar electric field to the equator during a DP 2 event as detected by the auroral and equatorial magnetometer chains and the EISCAT radar

The quasi-periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.

Penetration of auroral electric fields to the equator during a substorm

We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low-latitude negative bay. This latitudinal profile implies that in addition to the three-dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field-aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66° corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijarvi (56° CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijarvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.

Schumann resonance parameter changes during high‐energy particle precipitation

A systematic study of Schumann resonance parameters during high-energy particle precipitation events is presented. Protons and electrons with energies above 1 MeV ionize the upper boundary of the Earth-ionosphere cavity, leading to an increase of the resonance frequency and a decrease of the damping of the first Schumann resonance, as derived from measurements at Arrival Heights, Antarctica. The study uses the nine strongest solar proton events of the past solar cycle 22 and high-energy electrons emitted periodically from corotating interaction regions in the solar wind during 1994–1995. The variation of the Schumann resonance parameters is in qualitative agreement with current theories of Schumann resonances. The study also shows that high-energy particle precipitation is not the only relevant source affecting Schumann resonance parameters. The reported findings constitute a so far little-explored aspect of solar terrestrial relations.

An explanation for type 1 radar echoes from the midlatitude E‐region ionosphere

Type 1 radar echoes from the midlatitude E-region are comparatively rare, but we now have many nighttime examples. These observations imply that large electric fields, of the order of 15 mV/m or more, must exist at times in the plasma, even during quiet magnetic conditions. We suggest here that such fields can be generated by the same polarization process as at the magnetic equator, but with the geometry “turned on its side.” That is, we assume that there are sharp horizontal conductivity gradients associated with patchy nighttime metallic ion layers that play the same role that vertical gradients play at the equator. Sharp gradients in the zonal direction, particularly, could lead to polarization fields more than an order of magnitude greater than the ambient dynamo fields and sufficient to excite the two-stream instability implied by the radar data. Only a small percentage of the metallic ion patches are like to have the right geometry and be at the right altitude, however, which perhaps explains why the type 1 echoes are seen only occasionally.

Observation of the modified two‐stream plasma instability in the midlatitude E region ionosphere

We present the first clear evidence of the occurrence of the modified two stream (Farley-Buneman) instability and excitation of pure type 1 irregularities in the midlatitude ionospheric E region. The observations are made with a bistatic 50-MHz Doppler radio experiment set up recently in Crete, Greece. The system can perform high-frequency resolution coherent backscatter measurements along a fixed direction, from 3-m magnetic aspect sensitive irregularities inside a limited ionospheric volume in the E layer at the invariant geomagnetic latitude of 30.8° (L = 1.35). The observations presented here are from an event of backscatter characterized by large Doppler motions caused, presumably, by an impulsive electric field reaching a magnitude at least 14 mV/m. Apparently, the unusually high electron drifts along the radar viewing direction were sufficient in this case to excite pure Farley-Buneman waves. This had been manifested convincingly by the measured power Doppler spectra, which are reminiscent of the typical spectral signature of type 1 echoes observed regularly in the equatorial 50-MHz backscatter. Further, the spectral data confirmed the anticipation that in the midlatitude E region plasma exist two irregularity types corresponding to those of type 1 and type 2 echoes in the equatorial electrojet. The important difference with the equatorial results, however, is that the threshold conditions for the two-stream instability are seldom met at moderate latitudes, thus the medium is highly suitable for studying secondarily generated short-scale plasma turbulence.

The generation and propagation of atmospheric gravity waves observed during the Worldwide Atmospheric Gravity-wave Study (WAGS)

Abstract During the Worldwide Atmospheric Gravity-wave Study (WAGS) in October 1985, the EISCAT incoherent scatter radar was used to observe the generation of atmospheric gravity waves in the auroral zone in conjunction with a network of magnetometers and riometers. At the same time a chain of five ionosondes, an HF-Doppler system, a meteor radar and a radio telescope array were used to monitor any waves propagating southwards over the U.K. The EISCAT measurements indicated that in the evening sector both Joule heating and Lorentz forcing were sufficiently strong to generate waves, and both frequently showed an intrinsic periodicity caused by periodic variation in the magnetospheric electric field. Two occasions have been examined in detail where the onset of a source with intrinsic periodicity was followed by a propagating wave of the same period which was detected about an hour later, travelling southwards at speeds of over 300 m s−1, by the ionosondes and the HF-Doppler radar. In both cases the delay in arrival was consistent with the observed velocity, which suggests a direct relationship between a source in the auroral zone and a wave observed at mid-latitude.

The gravity wave-TID relationship: insight via theoretical model-EISCAT data comparison

Abstract Atmospheric Gravity Waves (AGWs) in the thermosphere are of particular interest because of their role in the equatorward redistribution of auroral momentum and energy input. However, their direct measurement is difficult and so they are normally traced by their ionospheric signatures, the Traveling Ionospheric Disturbances (TIDs). These can be routinely observed, especially with incoherent scatter radars like the EISCAT-facility, which measure all the fundamental ionospheric parameters. In order to reliably infer AGW parameters from TID data, however, one needs to know the physics of the AGW-TID relationship as comprehensively as possible. We investigated this relationship by means of one-to-one comparison of theoretical model results with EISCAT data for several TID events. The relevant physics, the modeling procedure and the results of the comparisons are discussed. As a representative example, one typical event is presented in some detail. We found that the AGW-TID relationship can be quantitatively understood by means of careful physical modeling. A particular simulated TID shows quantitative consistency with a particular TID in EISCAT data only for a quite specific model-AGW ; thus, comprehensive AGW infonnation can be deduced by our method. We conclude that our use of TID ‘polarization information’ along a single incoherent scatter beam is basically as valuable for the unique determination of a causative AGW as is traditional TID ‘propagation/dispersion information’. The latter, however, requires several distributed stations. Finally, we address the possibility that radars like EISCAT could be used in future WAGS (Worldwide AGW Study) campaigns to provide almost real-time information on AGW activity for the benefit of mid-latitude monitoring stations.

A 50‐MHz radio Doppler experiment for midlatitude E region coherent backscatter studies: System description and first results

Contrary to equatorial and auroral electrojet plasmas, studies on scattering of VHF waves from midlatitude E region field-aligned irregularities are limited, and the physics of the phenomena is not well understood. In this paper we provide first an update of the subject and then present a new experiment, denoted as Sporadic E Scatter Experiment (SESCAT), designed for high-time resolution coherent backscatter measurements of 3-m E region irregularities from a middle latitude location. The technique, which is used successfully in auroral latitudes for years, is capable of measuring the Doppler spectrum of the echoing signal at a fixed range with great accuracy. The experiment is a state of the art bistatic CW Doppler radar operating at 50.52 MHz with the transmitting and receiving arrays beaming northward to a region perpendicular to the Earths magnetic field at the E region peak. The viewing area, determined by the narrow beam intersecting patterns and the field-aligned character of the irregularities, is fixed at about 15×40 km2 and located over the southern Aegean at 30.8° invariant magnetic latitude (L = 1.35). First results show that 50-MHz backscatter events, with snrs as large as 25 dB and lifetimes of several minutes to more than an hour, do exist at midlatitudes in accordance with previous experiments which related these echoes to sporadic E layers. The observed events, except for an early afternoon case, occurred during dark hours in the period between a few hours before and after local midnight. The backscatter can be continuous but in some events a quasi-periodic behavior was found with periods between 2 and 10 min. Associated to the backscatter are symmetric narrow Doppler spectra with mean velocities in the ±80 m s−1 range and mean widths about 30 to 60 m sminus;1. In terms of spectrum width, the midlatitude E region echoes do not compare to the low-velocity type 2 echoes observed in the equatorial and auroral electrojets. Finally, very strong meteor-induced backscatter with abrupt onsets and short lifetimes up to 3 min were also detected. These echoes have relatively large velocities at onset and in one case an ion acoustic velocity component was seen briefly at 320 m s−1, suggesting that the Farley-Buneman instability may also be functioning at times in the midlatitude E region.

Evidence for non-Maxwellian ion velocity distributions in the F-region

Abstract A study has been made of data taken with EISCAT using the Common Program CP-3-C (F-region meridian scan) which shows that regions of enhanced ion temperature (in excess of 3000K at all three EISCAT stations) are found on most days when Kp exceeds 2 or 3, usually accompanied by ion drift velocities of more than 1 km s−1. These periods are often accompanied by anisotropy of the ion temperature and abnormally low apparent electron temperature, consistent with the presence of a non-Maxwellian ion velocity distribution such as would result from large but not exceptional ion drifts. Data for a selected period have been fitted using theoretical ion velocity distributions based on the relaxation collision model and assuming that the ion composition is 100% O+. The results confirm the presence of non-Maxwellian distributions, but a detailed comparison with theory reveals some discrepancies, indicating that the analysis may need to be extended to include effects due to, for example, molecular ions and instabilities.

Large polarization electric fields associated with midlatitude sporadic E

Recent 50 MHz E region coherent backscatter observations and in situ rocket measurements established the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley-Buneman instability. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field-aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridional polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated Es layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley-Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter.