L Jalonen

The role of electric field and neutral wind direction in the formation of sporadic E-layers

Abstract The effect of an electric field and a homogeneous neutral wind on the vertical ion motion in the ionospheric E -region is investigated. An electric field pointing, in the northern hemisphere, in the quadrant between geomagnetic north and west is found to he capable of driving plasma towards a certain height from both above and below. A homogeneous neutral wind blowing in a direction between east and north has a similar effect. Unlike in the wind shear model, the resulting plasma sheet may be created within a quite limited height interval only. It seems possible that the midnight occurrence maximum of mid-latitude type E s -layers, observed at high latitudes, is caused by electric fields in the Harang discontinuity region. It is also suggested that the flat type E s -layers often observed before a substorm onset are caused by electric fields. The wind shear theory is investigated using a screw-like neutral wind profile. The effects of right- and left-handed wind screws are compared and rules are derived which define the conditions leading to convergent and divergent nulls in the vertical ion velocity. In the northern hemisphere, a right-handed screw is found to be more effective than a left-handed one with equal pitch in compressing plasma into thin sheets.

On the morphology of energetic (≥30keV) electron precipitation during the growth phase of magnetospheric substorms

Abstract The morphology of energetic (≥30 keV) electron precipitation during the growth phase of magnetospheric substorms has been investigated using measurements of auroral-zone bremsstrahlung X-rays obtained from multiple balloon flights and supplementing riometer recordings. Growth-phase precipitation typically starts about one hour before the onset of a negative magnetic bay and occurs in a limited region parallel to the auroral oval around local midnight. The precipitation is first observed in the northern part of the auroral zone and moves southwards with a speed of 5–10 km/min. To the north this precipitation therefore ceases well before bay onset whereas a continuous transition from ‘prebay’ precipitation to bay-associated precipitation takes place in the south. A decrease in the intensity or at least a levelling off may occur some minutes before bay onset. The southward movement of the precipitation region is associated with a similar movement of a weak ionospheric current system. The events studied were all associated with a southward-pointing interplanetary magnetic field and with growth-phase conditions in the magnetotail. It is suggested that growth-phase precipitation originates from the ‘horns’ of the plasma sheet. The equatorward motion of the precipitation is then a consequence of an expansion of the polar cap, a thinning of the plasma sheet, and an equatorward motion of its inner edge. It is also suggested that this precipitation provides a stabilization of the outer boundaries of the plasma sheet by restricting the ionospheric mobility of the bordering field lines through enhanced conductivity.

High resolution EISCAT observations of the ion-neutral collision frequency in the lower E-region

Abstract A multichannel Barker coded EISCAT experiment with basic spatial and temporal resolutions of 600 m and 10 s, respectively, is described. The experiment was run on 13–17 February 1984 and data obtained in a diffuse aurora as well as in an auroral arc are used for determining the ion-neutral collision frequency in the lower E -region. Post-integration over 20 and 3 min, respectively, is carried out in the two cases and the plasma parameters are fitted with the assumption of equal ion and electron temperatures. The effect of varying the temperature ratio is discussed. The obtained collision frequencies are generally found to be in accordance with profiles based on the CIRA (1972) model atmosphere. In addition to collision frequencies, vertical ion velocities and electron temperature profiles are presented. It is found that the velocities are consistent with the observed electric fields.

Interference of tidal and gravity waves in the ionosphere and an associated sporadic E-layer

Abstract Observations made on 10 July 1987 with the EISCAT UHF radar are presented. The F -region measurements of both electron density and field-aligned ion velocity show that an upward propagating gravity wave with a period of about 1 h is present. The origin of the gravity wave is probably auroral. The E -region ion velocities show a tidal wave and both upward and downward propagating gravity waves. The gravity waves have three dominant periods with a possible harmonic relationship and similar vertical wavelengths. These waves are either reflected at a single reflection level, ducted between two levels, or they are generated in a non-linear interaction between gravity and tidal waves. The E -region electron density is dominated by particle precipitation. After a short burst of more intense precipitation, a sporadic E - layer forms at 105km and then disappears 40min later. Within this time, the layer rises and falls by a few kilometres, following closely the motion of a convergent null in the velocity profile. We suggest that the formation and destruction of this layer is controlled by both the precipitation, which indirectly provides a source of metal ions through charge exchange, and the superposition of gravity waves and the tidal wave.

A new method of measuring the ion-neutral collision frequency using incoherent scatter radar

Abstract A new method of measuring the ion-neutral collision frequency using the Doppler shift instead of the shape of the incoherent scatter spectrum is introduced. The method is based on the fact that, in the absence of neutral wind, the relation between electric field and ion velocity is determined by collision frequency. If both the electric field and the ion velocity are measured, the collision frequency can be solved from the equation of ion motion. The method is tested using the EISCAT radar. The applied experiment gives profiles of vertical ion velocity in the E-region and electric fields in the lower F-region. Data from a period with a weak neutral wind and a sufficiently strong electric field are analysed and the results are compared with those obtained using the conventional method. It is found that, under favourable conditions, the new method allows the determination of the collision frequency up to 130 km altitude. This is a considerable improvement as compared to the conventional method, which usually gives the collision frequency profile no higher than to the 110 km level. At altitudes where both methods can be used, a good agreement is found. The main drawback in the new procedure is that it can be used only during periods of negligible neutral wind. It is suggested that this difficulty can be avoided by using an experiment with at least two transmitter beam directions.

Observation of a thin Es-layer by the eiscat radar

Abstract High resolution E-region measurements carried out on 16 November 1983 using the EISCAT incoherent scatter radar are presented. The experiment was monostatic with a vertical radar beam, and it was based on a Barker-coded four-pulse code on one frequency channel and Barker-coded single pulses on three channels. The basic integration time was 15 s and the spatial resolution 450 m. The results reveal a short-lived but intense thin sporadic E-layer at 18:00–18:06 U.T. at an altitude of about 106 km. Both before and during the event, downward ion velocities of the order of 100 m s−1 are observed above this height. A convergent null in the vertical ion speed is occasionally seen at the layer altitude. The layer occurrence is associated with auroral arcs drifting across the radar beam. Simultaneous observations of the STARE radar show an ionospheric electric field of 25–30 mV m−1. The field always has a westward component, which is in accordance with the observed downward plasma flow. Most of the time when the layer is intense, the field points into the NW-sector. Theoretically, this field direction should create convergent vertical plasma motion. Therefore it is suggested that the observed Es-layer is created by the action of the auroral electric field rather than by the wind shear mechanism.

Incoherent scatter studies of sporadic-E using 300 m resolution

Abstract A new EISCAT UHF experiment with 300m range resolution is introduced. It is based on Barker-coded five-pulse patterns and single pulses on 8 channels. The high resolution is obtained by using a bit length of 2 μs. A thin and intense sporadic- E -layer drifting vertically within the altitude region 102–105 km has been observed with this experiment. The drift speed of the layer is approximately equal to the measured vertical plasma velocity. Occasionally the layer remains stationary and is compressed by vertical convergent plasma flow. The decay of the layer can be followed and it is found that it is, at least partly, caused by wave motions in the neutral atmosphere. It is shown that the layer contains heavy ions, most probably Fe + . The composition fit for molecular and Fe + ions gives molecular ion concentrations which are much higher than those estimated on the basis of the electron density. Therefore, it is concluded that the layer must also contain light metal ions, perhaps Mg + . According to composition fits of Fe + and Mg + , the abundance of Fe + in the layer is 60–75%.

Wave activity, F1-layer disturbance and a sporadic E-layer over EISCAT

Abstract During July 1987 the EISCAT radars were used to study thin layers in the ionospheric E -region. This paper outlines the observing campaign, describes the GEN-type radar program used for the UHF experiments, and discusses the ‘descending’ or ‘sequential’ layer observed on the afternoon of 12 July during a period of strong wave activity, which could be traced throughout the whole E - F 1 transition region. Following the descent of one particularly marked wave, a thin layer developed around 120 km height and lasted about 100 min, with temporary disappearances and periods of upward motion which were related to variations of field aligned ion velocity, and in particular to ‘convergent nulls’ in the velocity profile. The layer was eventually dispersed by a rapid upward surge of ion velocity. Composition analysis shows that the layer contains both long-lived light ions and heavy ions, most probably Fe + .

Density profiles of sporadic E-layers containing two metal ion species

Abstract Ionospheric plasma containing two types of metal ions is investigated under the action of the wind shear mechanism or, alternatively, an electric field causing convergent vertical plasma flow. It is shown that the different ion species are separately collected into thin sheets with a height difference ranging from some hundreds of meters to several kilometers. Theoretical density profiles for Mg+ and Fe+ ions are calculated assuming a screw-like wind structure or a strong auroral electric field. It is found that the two ion layers usually partially merge forming a single Es-layer. If the height difference of the ion sheets is not too great as compared to their thicknesses, the Es-profile is single peaked and approximately symmetric. With increasing layer separation the two sheets will gradually be discerned, until finally a double peaked profile is created. It is suggested that some of the observed complexities in Es-profiles are caused by the presence of more than one monoatomic ion species.

The effect of electric field-induced vertical convection on the precipitation E-layer

Abstract The effects of perpendicular electric fields on the high-latitude nocturnal precipitation E -layer are studied in terms of model calculations. The deformations of the electron density profiles caused by vertical plasma movements are described assuming various incident electron energies and flux densities, as well as different field intensities and directions. Manifestations of the profile variations in vertical sounding observations and changes in the conductivity profiles are examined. The results show that field-induced vertical convection is important in plasma densities even higher than 10 11 m −3 . Erroneous results may be obtained if the effect of the electric field is neglected when determining the incident electron spectrum from a measured ionospheric density profile.