A study of the thermospheric forces at a high latitude site on two days of differing geomagnetic activity

Data from the Fabry-Perot Interferometer and Dynasonde at Halley (75.5°S, 26.6°W, L ∼ 4.2), Antarctica, have been used to calculate the forces acting on the high latitude thermosphere. Two case studies of the forces have been undertaken to study why the thermospheric zonal wind speeds are typically so different on nights with different geomagnetic activity. One case study analyses the forces on a geomagnetically active night and the other analyses them on a geomagnetically quiet night. Even on the geomagnetically active night, it is found that the ion drag force is not necessarily the largest force at any one time. Simple comparison of the magnitudes of the forces does not make it very clear which ones dominate in controlling the motion of the thermosphere. This can be seen more clearly by rewriting the momentum equation so that the neutral velocity is expressed in terms of the ion velocity, and the other forces normalized by the ion density. It then becomes clear that, in the evening, the differences in the neutral velocity are due to increases in both ion density and ion velocity, while in the morning, only changes in ion density are important. Thus, although the ion drag force is often not the largest force, it appears that changes in it can account for the variations in neutral velocity between the two nights that were studied.It has also been shown as part of the analysis that whether or not the viscosity needs to be considered when calculating the ion drag force at an altitude of 240 km depends on the ion density profile. If the profile has a single peak then it is only necessary to consider the ion density at 240 km. It is, however, possible that just considering the ion density at this altitude may lead to an underestimate of the effective ion drag force if more than one peak is present.     

Spherical cap harmonic modelling of high latitude magnetic activity and equivalent sources with sparse observations

The new method of spherical cap harmonic analysis is used for modelling solely high latitude magnetic activity and equivalent sources. Data from 13 Canadian magnetic observatories, during the 6–8 February 1986 great storm, are used to model the perturbation fields and their equivalent internal and external currents. The mean hourly values are used to reduce the spatial aliasing of short period variations. Both the spatial and temporal variations of the 3 components of the field are modelled. Only those coefficients that are statistically significant are retained in the analysis. The harmonic degree is less for the internal sources than the external ones, and the analyses are anisotropic to optimize the modelling and computation. The standard deviation of fit is less than 10 nT during less active periods of the storm, and 20–50 nT (~ 15%) during peaks in the activity (500–1300 nT). Errors range from 5 to 25 nT when both the spatial and temporal variations are modelled during the first day of activity (<360 nT). During the peak of the storm the equivalent ionospheric currents are 0.9 A m⁻¹ and they extend into southern Canada. Large scale features of both regular and irregular magnetic activity and equivalent sources, can be well modelled with sparse magnetic observations using this technique.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Observations of high latitude ionospheric conductances

A brief historical review of the development of models of the ionospheric conductivities with special emphasis on high latitude regions and the auroral zone is presented. It is with great admiration that we must conclude that the physical understanding of the importance of the ionospheric conductances was well perceived by pioneers like Schuster and Birkeland a hundred years ago. Progress in the basic theoretical fundamentals was achieved in the late 1920s and 1930s. Realistic estimates were not derived until the first rocket probes measured the electron and ion content at different altitudes in the 1950s.Today we have a superior technique in resolving electron density profiles of high time and height resolution by incoherent scatter radars on the ground. The challenge that we are facing is to obtain global conductivity maps, especially at high latitudes, with a time and spatial resolution which match the details in auroral substorm phenomena. If that can be achieved, great progress in the understanding of detailed dynamical coupling in the ionosphere, magnetosphere, and thermosphere systems is expected. The imaging technique as demonstrated by the DE-satellite can be the tool which eventually materializes our desires for increased knowledge.     

Observations of the high latitude trough using EISCAT

Data taken by EISCAT are presented as contours of electron density, ion and electron temperature and plasma velocity versus invariant latitude and local magnetic time.Three nights near midsummer were studied and in each case a trough in electron density occurred north of invariant latitude 64° shortly after local midnight (MLT 0200) and remained a prominent feature for about 3 h before moving poleward. The minimum in electron density was associated with a marked increase in ion temperature, but the electron temperature showed litttle change. In this respect the high latitude trough is clearly different from the mid-latitude trough.Full velocity measurements were not available for all three nights, but it seems that the appearance of the trough followed the start of a strong eastward plasma velocity combined with a strong upward velocity along the magnetic field line. The sudden change in plasma velocity causes frictional heating, which explains the increase in ion temperature. Upward plasma velocity is a major factor in the formation of the trough, with enhanced recombination making a smaller contribution.     

Aircraft measurements of the geomagnetic latitude effect on air-earth current density

Recent investigations of the electrode effect and the phenomenon of bubble electrification processes at the air-sea interface throw doubt on the applicability of using surface atmospheric electric observations made at sea by the Carnegie for proving the latitude effect in the columnar resistance of the atmosphere. Conduction current measurements were taken on flights during a period of decades by two instrumented aircraft in oceanic areas remote from sources of pollution. A composite of these measurements is given and confirms the notion that there is a latitude variation in air-Earth current. With the reasonable assumptions of an equal ionospheric potential and either low concentrations of Aitken nuclei or at least suitably small variations in their density with respect to latitude, the observed variation is apparently the integrated result of the Earth's magnetic field acting on cosmic ray activity throughout the troposphere.     

Lidar sounding of the mesospheric sodium layer at high latitude

Long series of laser sounding of the sodium layer have been performed at Heyss Island (80.4°N) during the polar winters of 1977–1978 and 1978–1979. The measurements show large and rapid variations of the sodium total content (a factor of 2, about 1000s). Those variations and the correlated modification of the sodium layer could be interpreted as the response of the layer to internal gravity waves.     

Incoherent scatter radar contributions to high latitude D-region aeronomy

Recent aeronomical work on the high latitude D-region is reviewed, restricting the discussion to observations of the D-region by the incoherent scatter technique. Emphasis is given to chemical aeronomy, which governs part of the coupling between the neutral atmosphere and the ionosphere, and forms the basis for the global role of the high latitude D-region. Details of the dynamics of the high latitude D-region, and thus the actual coupling with regions below and above, are, however, not discussed in this context. The aeronomical consequences of special high-latitude phenomena are discussed. These include the effects of the polar summer, precipitation of high energy electrons during auroral substorms and high ionization of the D-region during solar proton events. A detailed discussion is given on selected studies concerning the series of solar proton events that occurred in 1989. Problems of ion and neutral chemistry are readily accessible with incoherent scatter measurements through chemical modelling of the D-region. In this way the continuous nature of incoherent scatter measurements can be utilized to expand our knowledge of the D-region, which earlier was mainly based on momentarily sounding rocket experiments. However, it is pointed out how the interpretation of incoherent scatter data from the D-region strongly benefits from multi-instrument configurations. An outline is given of a possible new development based on the combined use of the Tromsø heating facility and the EISCAT incoherent scatter radars.     

Comparisons between EISCAT observations and model calculations of the high latitude ionosphere

Calculations using a numerical model of the convection dominated high latitude ionosphere are compared with observations made by EISCAT as part of the UK-POLAR Special Programme. The data used were for 24–25 October 1984, which was characterized by an unusually steady IMF, with Bz < 0 and By > 0; in the calculations it was assumed that a steady IMF implies steady convection conditions. Using the electric field models of Heppner and Maynard (1983) appropriate to By > 0 and precipitation data taken from Spiroet al. (1982), we calculated the velocities and electron densities appropriate to the EISCAT observations. Many of the general features of the velocity data were reproduced by the model. In particular, the phasing of the change from eastward to westward flow in the vicinity of the Harang discontinuity, flows near the dayside throat and a region of slow flow at higher latitudes near dusk were well reproduced. In the afternoon sector modelled velocity values were significantly less than those observed. Electron density calculations showed good agreement with EISCAT observations near the F-peak, but compared poorly with observations near 211 km. In both cases, the greatest disagreement occurred in the early part of the observations, where the convection pattern was poorly known and showed some evidence of long term temporal change. Possible causes for the disagreement between observations and calculations are discussed and shown to raise interesting and, as yet, unresolved questions concerning the interpretation of the data. For the data set used, the late afternoon dip in electron density observed near the F-peak and interpreted as the signature of the mid-latitude trough is well reproduced by the calculations. Calculations indicate that it does not arise from long residence times of plasma on the nightside, but is the signature of a gap between two major ionization sources, viz. photoionization and particle precipitation.     

Production and control of ion-cyclotron instabilities in the high latitude ionosphere by high power radio waves

The possible generation and suppression of ion-cyclotron waves in a collisional plasma by external high power electromagnetic (EM) waves with frequency close to the local upper-hybrid frequency is considered. It is shown that the ion cyclotron instability can be destabilized (stabilized) for ω₀ <ω_UH (ω₀ > ω_UH), where ω₀ is the pump frequency of the EM wave. The results are applied to naturally occurring ion-cyclotron instabilities in the high latitude ionosphere.     

A case study of a high latitude ionospheric electron density depletion

Incoherent scatter measurements inside and outside an ionospheric electron density depletion are described. The density depletion is probably caused by an enhancement of NO⁺-ions and subsequent dissociative recombination. The NO⁺-ions are increased because high electric fields present at the geographical location of the density depletion speed up the reaction O⁺ + N₂ → NO⁺ + N. The electron as well as the ion temperature within the density depletion are strongly enhanced, the latter due to Joule heating, also caused by the electric field.     

Penetration of hydromagnetic waves of cylindrical symmetry through the high latitude ionosphere

The problem is formulated, and boundary conditions are developed, in order to solve numerically the equations for the penetration of hydromagnetic waves of horizontal cylindrical symmetry through a stratified high latitude ionosphere and atmosphere due to sources above the ionosphere. There are two orthogonal polarization modes. The numerical results are shown for various cases. It is found that near the ground the horizontal components of both the electric and magnetic fields along the circumference direction are much smaller than the radial components.     

The trouble with thermospheric vertical winds: geomagnetic,seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms⁻¹ during periods of quiet geomagnetic activity (K_p < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms⁻¹ may occur.     

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of

• 1.(i) a southward neutral wind
• 2.(ii) a vertical wind driven by Joule heating
• 3.(iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

     

Dayside high latitude magnetic impulsive events: their characteristics and relationship to sudden impulses

On 17 December 1990 a series magnetic impulsive events (MIEs) were observed at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive electron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The correlated EISCAT and riometer observations indicate that there was an elongated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electron precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitation and VLF emissions as signatures of a global compression of the Earth's magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magnetometers. We conclude that the small scale MIEs with their drifting ionospheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs have a common origin: the interaction of solar wind inhomogeneities with the Earth's magnetosphere. They do, however, represent different effects of the same primary agent.     

Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).