Investigations of ionospheric radio wave absorption processes using imaging riometer techniques

The recent development of imaging riometer techniques has enabled a range of new, interesting observations of the complex dynamics of auroral and polar radio wave absorption events. These events mostly relate to the precipitation of energetic particles, creating enhanced ionization in the D-region. However, E-region heating by large electric fields and F-region electron density enhancements may also—at times—be responsible for observable absorption effects. Observations of ionospheric radio wave absorption processes using imaging riometer techniques may provide detailed characteristics of the spatial and temporal structures of small-scale disturbance events, velocity vectors for drifting features and frequency spectra for modulated events. This presentation will give a brief summary of imaging riometer techniques and a survey of existing and planned imaging riometer installations. Furthermore, the characteristics of frequently occurring absorption event types are summarized. In a companion paper imaging riometer observations are presented for some selected absorption events.     

D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10⁶ cm³ at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.     

On the prediction of auroral radio absorption on the equatorial side of the absorption zone

Observations of auroral radio absorption made in Finland over the years 1972–1983 have been compiled for use in absorption predictions. The data are presented as values of Q(1), the probability of at least 1 dB absorption being recorded by a 30 MHz riometer, and the results are compared with the predictions of an existing model regarding the latitudinal variation, the influence of solar and magnetic activity indices, and the pattern of daily variation. New formulae for basing absorption predictions on the A_p index are proposed.     

Morphology of the D-region in northern Scandinavia during the campaigns MAC/SINE and MAC/EPSILON in 1987

The morphology of the lower ionosphere was examined by the riometer and ionosonde networks in Scandinavia during the campaigns MAC/SINE and MAC/EPSILON. The campaigns were carried out during 1987 when the solar activity was low. The minimum sunspol number 14 was observed in 1986. The sunspot number was already increasing during the year 1987, while the Ap-value and riometer absorption were still decreasing. During the campaign MAC/SINE the rockets were launched during quiet periods, and during the campaign MAC/EPSILON during more disturbed periods. The variations in ionospheric parameters, ionospheric absorption and foF2 are presented for the campaign periods. Some interesting events are pointed out.     

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.     

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.     

Quasi-periodic scintillation events at southern auroral latitudes

Quasi-periodic (QP) radio scintillations were observed during (1987) on 244 MHz and 1.5 GHz geostationary satellite transmissions in the southern auroral zone from Davis station (68.6°S, 78.0°E geographic, 74.6°S Aλ) in Antarctica. Three distinct types of OP events were identified, with occurrence times mainly restricted to the period 18-00 MLT. The substantial loss of signal associated with these events appears to be an important factor in determining the reliability of satellite links on 1.5 GHz in auroral regions. Previous observations at mid-latitudes of QP scintillations have noted a preference for large zenith angles and equatorward azimuths. It is demonstrated that a height transition in a densely ionized layer can produce QP scintillations in a manner analogous to a dense column of ionization but at lower ionization densities, as well as demonstrating a zenith angle and azimuthal dependence that is more consistent with observations than a column of ionization. At the occurrence times noted, the raypath may be intersecting the poleward edge of the trough where sporadic-E is a regular feature. QP scintillation events may result when the E_s-layer is height modulated by the passage of acoustic-gravity waves originating in the auroral zone.     

Storm after-effects observed at Ushuaia and Kerguelen

D-region disturbances have been detected at mid-latitudes after intense magnetic storms by means of a wide range of radiowave signals. Two different mechanisms have been suggested to account for these storm ‘after-effects’: one implies precipitation of energetic electrons from the radiation belt; the other, transport of neutral constituents from the auroral zones.This paper presents observations of abnormal enhancements of ionospheric absorption arising after two major magnetic storms occurring in March and April 1976. The measurements were made at Ushuaia (54.8°S, L = 1.7) and at Kerguelen (49.4°S, L = 3.7). The former were obtained by means of the pulse reflection method (A1) at MF and the latter by the riometer technique. It is shown that electron precipitation can explain the effects observed at Kerguelen but not those at Ushuaia which also depart significantly from the ‘winter anomaly’ trend observed at that site. The abnormal ionization at Ushuaia is attributed to transport from the southern auroral zone.     

High-latitude plasma densities and their relation to riometer absorption

A large number of D- and E-region electron density profiles from high latitudes have been analysed. These were derived from rocket-borne wave propagation experiments and—after careful screening—arranged according to riometer absorption. Statistical profiles for various degrees of absorption, including 0 dB, were established both for day and night. Furthermore, the height region predominantly contributing to the absorption has been identified. Finally a mean variation of the density of negative ions has been derived.     

Interpretation of optical substorm onset observations

The ionospheric location of substorm onset is generally found to be at the most equatorward arc in the poleward portion of the diffuse aurora. The observation that most activity occurs in this region provides a reference from which the source region in the magnetotail may be assessed. This reference can be examined in two ways. First, magnetic field mappings of these onset locations to the equatorial plane suggest that the onset is associated with processes quite near the Earth. For example, for 14 cases the average GSM X value was found to be ≈ −7.8 R_E. However, this identification is based on a static magnetic field model and while these results are consistent with some earlier findings there is not sufficient confidence in this technique to discriminate between topological regions in the magnetotail. A second way to examine the ionospheric onset location is in relation to the open/closed field line boundary. It is evident from Viking satellite images that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and active periods. There is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore unrelated to the open/closed field line boundary, a result consistent with some (but not all) near-Earth mechanisms but only under some conditions with the distant tail boundary layer theory.     

Observations of neutral winds in the polar cap during northward IMF

Long term remote observations of neutral winds at F-region altitudes have been performed at Thule Air Base (lat. 76.5°N, long. 69.0°W), Greenland, and Søndre Strømfjord (lat. 67.0°N, long. 50.9°W), Greenland. The former site is very close to the geomagnetic pole, while the latter site is within the polar cap for several hours each night on either side of geomagnetic midnight. Wind data corresponding to clear sky conditions and Kp ⩽ 4 were sorted according to the sign of the IMF B_z component. The averaged maximum poleward flow near midnight LST was reduced by approximately one third during B_z northward conditions. If the magnitude of B_y was less than the magnitude of the northward B_z component, then the averaged poleward flow was further reduced by one half. In addition, if B_z > 5 nT, then sunward directed horizontal neutral winds were observed at the very highest latitudes near noon LST.     

Optical measurements of a nightside poleward expanding aurora

Coordinated optical observations were performed from the poleward side of the midnight auroral oval. Height measurements of the auroral emissions at 4278, 5577 and 6300 Å, as well as their intensity ratios in the poleward expanded auroral substorm, have been carried out. The findings indicate a significantly softened electron spectrum compared with similar data from the equatorward part of this substorm. Typical values for the poleward expanded aurora are 300 eV and lower, while keV particles dominate the auroras at 10° lower latitudes. Emission altitudes and spectral characteristics are comparable to the transient burst emissions frequently observed from the same site in the post-noon sector, i.e. within the cusp.The 6300 Å atomic oxygen emission is used as a tracer of F-region wind and temperature. Interferometer observations show that there exists a prevailing crosspolar antisunward wind, increasing with geomagnetic activity to several hundred m s⁻¹. The temperature shows an increase of 150 K associated with high geomagnetic activity.     

The effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Sudden sodium layers in polar latitudes

We discuss in this paper sudden sodium layers (SSLs), which we observe with a sodium lidar instrument at Andenes, Norway (69°N). We speak of a SSL if, in a narrow altitude range (typically less than 2km), the Na density increases over the normal Na density by a factor of at least 2 within 5 min. Between December 1985 and November 1987, we have observed 42 such layers in 378 h of lidar measurements. This number increases to 75 if we only require an increase of a factor of 1.5 within 8 min. At our observation site, SSLs have the following properties: (a) they develop between 90 and 110 km altitude, (b) they develop between 20 and 02 LT, (c) their appearance shows a strong, positive correlation with that of ƒ-type E_s layers, and (d) their appearance does not show a strong correlation with either riometer absorption or meteor showers. We discuss a number of potential processes for SSL formation. SSLs above 100km can be formed in ƒ-type E_s layers by the conversion of Na⁺ ions into neutral Na. The development of SSLs below 95 km requires the presence of an additional reservoir of Na, such as Na-bearing molecules, ions, and/or ‘smoke’ particles. We also evaluate the proposal that SSLs are the outcome of single meteoroids entering the upper atmosphere, a proposition for which we find little observational support.     

Variability of d-region electron densities at tromsø

The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromsø has been used for a study of D-region electron densities by the differential-absorption method on a number of days during 1978–79. Received signals are generally stratified in several layers, typically over 60–80 km. Strong stable echoes are seen down to 55 km during periods of enhanced riometer absorption. Inferred electron densities vary between ~ 100–1000 cm⁻³ at ~ 60–80 km and show well-defined features which persist for ~ 10–20 min. During periods of high absorption, enhanced electron densities (~ 600 cm⁻³) are observed below 65 km. During a Polar Cap Absorption event, the inferred electron densities at 60–70 km show a very stable profile. Possible sources of D-region ionization at high latitudes are briefly discussed     

Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations.Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.     

Auroral torch structures: results of optical observations

Auroral torch structures are frequently observed as large scale wave-like undulations on the poleward edge of the diffuse precipitation zone. Their eastward drift velocity has the same order of magnitude and direction as the electric drift velocity usually observed in the morning sector. From a study of all-sky camera and TV data, the main peculiarities of the torch structures are shown. Three cases were studied. Height distributions of luminosity along the boundaries of the torch were made using the triangulation method in the first case. It was shown that the poleward boundary of the torch had the lowest altitudes around the crest. The second case was the appearance of a train consisting of five torches that existed for a relatively long time (~20 min), drifting eastward. On the contrary the last case was the successive development of two torches recorded by the TV camera. For the two latter cases the field-aligned mapping of the torches into the magnetotail is made using the latest version of the Tsyganenko magnetic field model (1989). The results of mapping showed that the generation of torch structures takes place quite close to the Earth, at a distance of 5–6 R_E.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

D-region observations of polar cap absorption events during the EISCAT operation in 1981–1989

During the years 1981–1989, 71 solar proton events altogether were observed. Dividing the events into strong, p.f.u. > 1000 (p.f.u.—proton flux measured at geosynchronous satellite orbit in units of (cm² s sr)⁻¹), medium, 100 < p.f.u. < 1000 and weak events, p.f.u. < 100, only the strong and medium events have a considerable effect on the lower ionosphere. The mean daily absorption at 30 MHz (A), measured in the auroral zone, is >2 dB during strong events, <2 dB during medium events and < l dB during weak events. The most active year during the EISCAT operation was 1989 when 23 solar proton events were observed including six strong events. Diurnal variation of the electron density in the D-region during PCA is a function of the solar zenith angle. However, south of L = 5 a minimum in absorption is observed during the noon hours. During sunrise the absorption increases simultaneously with solar elevation angle, but during sunset there is about 2 h delay between the decrease of absorption and the solar elevation angle.