Observations of 3-m auroral irregularities during the ERRRIS campaigns

In the late winter of 1988 and 1989, three NASA sounding rockets were flown through the auroral electrojet from ESRANGE (Sweden) as part of the E-region Rocket-Radar Instability Study (ERRIS). Many ground-based instruments supported these flights, including the EISCAT, STARE, and CUPRI radars, as well as all-sky cameras, riometers, and magnetometers. In this paper we summarize the observations of the Cornell University Portable Radar Interferometer (CUPRI), which detected coherent backscatter from 3-m irregularities in the auroral E-region. Twenty hours of power spectra and interferometry data are available, and, during the 1989 campaign, three weeks of nearly continuous Range-Time-Intensity (RTI) and first moment data were recorded.     

F-layer irregularities' formation at auroral latitudes: radio wave scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using orbital satellite beacons and CP-3-F program measurements by means of the EISCAT ionospheric radar facility is described. The results reveal the location of patches, containing kilometre-scale irregularities, in the vicinity of a region of an electron density minimum and an electron temperature increase. In the daytime under quiet geomagnetic conditions, the region of scintillations coincided closely with the southwards gradient in electron density, while a plasma drift velocity was mainly westwards V_E-W ≲ 0.3 km/s. In the evening, the region of the most intense irregularities was transformed to the northwards sense of the electron density gradient simultaneously with the plasma drift velocity reverse and the arrival of a significant southwards component V_N-S ≲ (1.5−1.0) km/s. EISCAT data demonstrated the patches' location in regions of an electron temperature increase. Processes operating to create kilometer-scale irregularities were analysed and estimated according to the data obtained. The assessments suggest that irregularities with a cross-field scale, equal to or greater than 1 km, and a field-aligned scale, equal to or greater than 30 km, were the result of growth of the thermomagnetic instability.     

EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Small scale sub-auroral F-region irregularities were observed on 6–7 February 1984 by the two HF radars of the EDIA experiment while the EISCAT UHF system was scanning the ionosphere between 57° and 66° invariant latitude at a slightly different longitude. The bistatic EDIA system was mainly designed to detect the F-region irregularities at sub-auroral latitudes and to measure their perpendicular velocities. This paper is devoted to an examination of the morphology of the irregularity regions detected by the HF radars and of their production mechanisms, by comparison with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360–430 km height are not associated with any large scale horizontal density gradients in the F-region (350km). However, a strong north-south gradient observed at lower altitudes (150–200km), which is likely to indicate the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars. The EISCAT electron temperature measurements at 350km height also show horizontal gradients which are well correlated with the small scale F-region irregularities. We discuss implications of these observations on the mechanisms of production of irregularities in the sub-auroral F-region.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

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

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 K_p 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.     

The amplitude of auroral backscatter—I. Model estimates of the dependence on electron density

A model of the auroral backscatter amplitude, in the form discussed by Uspensky and Oksman et al., has been derived for the radar geometry appropriate to joint observations by the PGI auroral radars at Karmaselga and Essoyla and the EISCAT incoherent scatter radar. The model shows how refraction effects cause a strongly non-linear dependence of backscatter amplitude on electron density in the E-region. It also explains why the macro aspect sensitivity for auroral radar operating at a frequency of about 45 MHz is only 1–2 dB per degree for aspect angles greater than 5°.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

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.

     

Substorm activity precursors in the dayside magnetic perturbations

Magnetic data from a meridional chain of stations in Greenland and AL-indices of magnetic activity have been used to study the relationship between magnetic perturbations in the dayside cleft region and substorm activity in the night-time auroral zone. The analysis of 14 substorms, isolated and prolonged, has shown that intensification of westward currents in the postnoon sector of the cleft precedes or accompanies substorm development in the night-time auroral zone. Westward currents appear in the northern cleft as substorm precursors even under the adverse influence of the IMF positive B_y component. These currents trend to extend in the prenoon sector. To explain the relationship between the cleft currents and auroral electrojet the connection between neutral layer currents and noon Birkeland currents is proposed. This connection can be realized by means of the source region acting just inside the daytime magnetopause owing to stationary reconnection of geomagnetic field and IMF, the source region flowing downstream to the tail magnetopause.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

Spectral characteristics of high frequency waves backscattered by small scale F-region irregularities: evidence of strong sub-auroral ion flow

The spectra of high frequency waves backscattered at night by small scale (10–20 m) sub-auroral F-region irregularities often exhibit large Doppler shifts and widths in the local time sector 2000–2400. After local midnight the Doppler shifts and the widths of the spectra decrease rapidly. We present examples of experimental data, obtained with the two coherent backscatter radars of the EDIA¹ experiment, showing the spectral characteristics just mentioned. From the Doppler shift measured at the two sites we deduced the perpendicular velocity of the irregularities, which can reach values as high as 2000 ms ⁻¹. These observations are interpreted using results of theoretical models which predict strong sub-auroral ion flow in the trough region.     

Post-sunset F-region vertical velocity variations at magnetic equator

The vertical drift velocity of the F-region in the post-sunset period at the magnetic equatorial station Trivandrum has been studied using a HF phase path sounder. The study revealed the presence of quasi-periodic fluctuations with periods in the range 4 30 min superposed on a steady vertical motion as a regular feature of the equatorial F-region in the post-sunset period. The fluctuations in the vertical velocity arc attributed to the east west electric field fluctuations generated by internal atmospheric gravity waves. The vertical velocity fluctuations can provide the necessary seed perturbations for the growth of equatorial spread-F irregularities.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (V_p) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (E_y) are deduced by matching experimentally observed V_p profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental E_y values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ A_p ⩽ 30); (iv) a significant decrease of the average E_y on disturbed days compared to that on quiet days during 0900–1200 h L.T.     

Electrojet irregularity parameters from daytime equatorial scintillations

The second moment of the complex amplitude or the mutual coherence function (MCF) for transionospheric VHF radio waves transmitted from the geostationary satellite ATS-6 is computed from daytime amplitude and phase scintillations recorded at an equatorial station, in order to study the structure of electrojet irregularities. The shape of the correlation function for fluctuations in the integrated electron content along the signal path is deduced by using a theoretical relationship between this correlation function and the MCF which is based on the assumption that the irregularities are “frozen”. Further, using a power-law spectrum to describe the electrojet irregularities, the outerscale l_o associated with the spectrum as well as the r.m.s. density fluctuation are estimated from theoretical fits to the computed values. The irregularity drift speeds V_o transverse to the signal path, for the scintillation events studied here, are derived from power spectra of weak scintillations. On the basis of a relationship between l_o and V_o suggested by a linear theory of the gradient-drift instability, the effective Hall conductivity is estimated to be about five times the effective Pedersen conductivity in the electrojet region.     

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.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.