T.L. Hansen

Sudden neutral sodium layers : a strong link to sporadic E layers

Abstract A sodium LIDAR instrument located at Andenes, Norway (69°N; 16°E) observed several sudden developments of narrow sodium layers in the 90–100 km altitude region. These layers grow with typical time constants of 5 min and have a width of 1 km in altitude. We present the temporal and spatial properties for a number of these events. In a first step towards identifying the processes which create these layers we study the correlation of the growth phase of sudden sodium layers and of sporadic E layers. The latter have been recorded by an ionosonde located 129 km east of the LIDAR site. Within the mutual altitude and time resolution available in our common records a strong correlation of simultaneous occurrence of sudden sodium layers and E s − l layers is observed, which establishes a strong link between the formation of the two types of layers. We further discuss processes which potentially could give rise to the formation of sudden sodium layers.

Ionospheric modification experiments with the Tromsø heating facility

Abstract The experiments performed up to mid 1984 with the heating facility at Ramfjordmoen near Tromso, Norway, are summarized. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF frequencies, excitation of E -region small-scale irregularities and of F -region small- and large-scale irregularities, anomalous absorption of HF wave on long and short time scales, excitation of incoherent backscatter plasma and ion lines, stimulated radio wave emission and F -region in situ measurements.

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

Abstract The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.

Cusp/cleft auroral forms and activities in relation to ionospheric convection : Responses to specific changes in solar wind and interplanetary magnetic field conditions

This work is intended to be a first step toward a categorization of dayside auroral responses to various solar wind and interplanetary magnetic field (IMF) conditions and the corresponding states of plasma convection in the dayside magnetosphere. In this paper we relate cusp latitude auroral emissions observed during a 4-hour interval on December 17, 1992, to solar wind data supplemented by ground magnetograms. We focus on different types of variability associated with an interplanetary shock and with an IMF directional discontinuity. We further investigate the quasi-steady conditions related to periods of low activity versus periods of high activity, as well as periods of IMF By-related zonal convection in the cusp region versus periods of IMF BZ-related convection in the north-south direction. To these various conditions the dayside aurora responds differently. These observations are discussed in relation to entry of magnetosheath plasma into the magnetosphere associated with different modes of solar wind-magnetosphere coupling. Two main categories of persistent auroral forms in the cusp/cleft region are observed, a latitudinally wide (≥200 km) zone of weak 630.0-nm emission with no sharp boundaries extending to magnetic latitudes of ∼78°–79° during quiet intervals and another latitudinally narrow (∼100 km) zone of strong red line emission, located equatorward of 75° MLAT during disturbed periods. These forms occur in association with sunward and antisunward convection in the dayside polar cap, respectively. It is suggested that these two signatures of electron precipitation in the noon/near-noon sector reflect plasma entry by lobe reconnection and reconnection at low (subcusp) latitudes, respectively. The observed sudden transition from the former to the latter auroral condition may thus reflect a change of reconnection site. The associated brightening of the cusp/cleft aurora then corresponds to the larger efficiency of magnetosheath plasma entry at low magnetopause latitudes when the subcusp reconnection process is switched on, compared to the more limited plasma entry occurring at high latitude during lobe reconnection. The most intense and latitudinally narrow auroral forms at 630.0 nm are observed during intervals of strong IMF By-related zonal convection and a strong DPY mode of ground magnetic deflection. Information on the ionospheric convection pattern in the vicinity of the magnetic field separatrix is obtained for the different IMF orientations by combining the auroral and magnetic observations in the Arctic sectors of Greenland and Svalbard.

Development of D-region electron and ion densities under various auroral conditions during the Energy Budget Campaign (EBC)

Abstract D -region electron density profiles and time variations were obtained during the Energy Budget Campaign 1980 by a partial reflection radar at Ramfjordmoen, Tromso, located between the rocket ranges at Andoya and Kiruna. The observations were made under various geophysical conditions which are illustrated by riometer observations. The partial reflection measurements indicate that the rockets were launched into a relatively stable D -region on two occasions, while it was somewhat more disturbed on the third. A comparison between the electron density profiles derived by the partial reflection technique and rocket borne probes and Faraday rotation experiments does indicate fair agreement during the quiet conditions, but relatively large discrepancies during disturbed conditions. Simultaneously derived electron density profiles, by use of the Faraday technique, and ion density profiles, by gridded electrostatic spheres mounted on the rocket payload, have made it possible to estimate the negative ion to electron density ratio A versus height. These values of λ are within the range of model calculations.

HF modification of the auroral D-region detected by a partial reflection experiment

Abstract During an auroral event with strong ionization in the lower D -region, the ionosphere was modified by a powerful (260 MW ERP) transmitter operating at 4 MHz in the extraordinary mode. The effects on the received partial reflection in an experiment using 2.75 HMz waves were observed. The results indicate a very abrupt increase in electron temperature, with a factor of ~ 3 at the level of maximum heating. This seems to be followed by a change in electron density, probably due to electron temperature variation of recombination reaction rates. The effect of the heating wave on the irregularities giving rise to the partial reflections is discussed.

Variability of d-region electron densities at tromsø

Abstract The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromso 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

Gyro line observations with the EISCAT VHF radar

Abstract This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.

Preliminary results from a study of the F-region heating during an intense aurora observed by EISCAT

Abstract Observations of large time variations in the ionospheric F -region temperature derived from EISCAT are compared with simultaneous observations of the E - and F -region plasma densities. The observations suggest that the F -region may be heated by current driven instabilities generated during intense precipitation of auroral electrons.

Synoptic models of high latitude magnetic activity and equivalent ionospheric and induced currents

Abstract Hourly models of the northern high latitude magnetic activity and the equivalent ionospheric and induced currents have been made for the entire 1980 period using data from 55 magnetic observatories. Vector observations and the method of spherical cap harmonic analysis are used to simultaneously model the external and internal scalar potentials from which the disturbance vector field and the corresponding equivalent currents can be determined anywhere and at anytime on, over and under the cap. This modelling technique, when used with observatory mean hourly values to reduce spatial aliasing of irregular disturbances and with the quiet nighttime (undisturbed) reference level, provides reasonable synoptic (basic) models (errors 5–80 nT). Only the statistically significant coefficients are retained in the hourly models so those for a full year (8784 models) can be readily stored and used on small computer systems. However, there are numerous regions that are in need of observatories and such additional data could significantly improve the delineation of large irregular disturbances. These synoptic models of the high latitude disturbance fields and the equivalent currents have a variety of applications. The models of the disturbance field might be used for correcting some magnetic surveys for the temporal variations, for the analyses of the large scale equivalent ionospheric and induced current systems and for determining parameters of the activity. Maps of the external currents provide an indication of their spatial extent and, with other observations, can be used to study the electrodynamics and the heating of the upper atmosphere and the magnetosphere-ionosphere coupling. Similarly, maps of the induced currents show their spatial extent, variability and the influences of the conducting oceans and other crustal features. Timely data and models of the activity could be used to help mitigate the effects of active conditions on surveys and also the hazardous effects of storms on some technological facilities.