Some peculiarities of altitudinal distribution of atomic oxygen at low latitudes during magnetic storms

Several storms were examined during the period of the ESRO-4 mission to study the altitudinal behaviour of atomic oxygen and molecular nitrogen at low latitudes. A special method of data reduction was applied to distinguish composition changes due to thermal expansion and those which resulted from some non-thermal process. It was shown that in the majority of cases studied there exists some latitudinal interval where changes of the N₂/O ratio from quiet to storm days are small compared with that observed in the purely thermal case. This latitudinal interval was found to be displaced to the winter hemisphere (not always across the equator) during solstice cases and the corresponding altitudinal interval was sometimes equal to the whole height range covered by the satellite. The effective temperature of atomic oxygen turned out to be greater than that deduced from the altitudinal distribution of N₂. This implies that there exists a non-thermal component of the [O] increase on a storm day at low latitudes and thus [O] is distributed in a non-barometric way. A possible mechanism responsible for this behaviour is downwelling due to the disturbed global thermospheric circulation.     

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

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.     

Positive ion composition in the lower ionosphere at high latitudes during MAP/WINE

Positive ion composition, total ion and electron density and ion production by energetic electrons were measured by rocket-borne experiments above Andøya (69.3°N, 16.0°E) in northern Norway. Observed altitudes of transition from molecular ions to proton hydrates and from electrons to negative ions are compared to results from an ion-chemical model. Nitric oxide and water vapour densities are inferred from the ion composition.     

Geomagnetic bays and Pc5 pulsation substorms at high latitudes

Morphological features of Pc5 pulsation substorms and of geomagnetic bays are examined in the light of current theories using data from Fort Churchill—a high latitude Canadian station in the auroral zone. The bays are classified using actual magnetograms into four categories:

• 1.(1) sleeping bays,
• 2.(2) pulsating bays,
• 3.(3) loaded bays and
• 4.(4) transition bays. Characteristics of all these bays are detailed and special circumstances are described in which they tend to occur on the magnetograms. A comprehensive model of the magnetosphere which largely fits the observations has been detailed.

     

GOES satellite timekeeping for field stations at high Arctic latitudes

The feasibility of using the GOES satellite time signal is discussed for field stations at high Arctic latitudes. Results are presented for three ground stations on islands in the Canadian Archipelago. The stations range in latitude from 74 to 81° 30' N. At all locations, time code reception was found to be satisfactory and capable of providing accurate time reference for remote experiments. A simple design for a high gain helical antenna, used successfully at these latitudes for time signal reception, is also presented. The antenna, primarily intended for a small research field station, is portable, inexpensive and readily constructed.     

Magnetic variations at other latitudes during reverse equatorial electrojet

Several reverse equatorial electrojet events are examined. Magnetic variations from a sample of worldwide stations on the same days are compared with the quiet daily average for the same month. In most cases departures can be found elsewhere at the same time as the reverse jet. Various wind systems in the dynamo region might produce the observed results but different systems are required on different occasions.     

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.     

An observational study of the D-region winter anomaly in lower latitudes during sudden stratopsheric warmings

An observational study of the D-region winter anomaly of HF radio wave absorption in lower latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. By means of large-scale isopleth analysis of the absorption index, ƒ_min, and of meridional winds near 70 km height along 60°N, it is found that there exists a winter anomaly in lower latitudes which is comparable in order to that in middle latitudes, resulting from a nitric oxide (NO) increase due to southward transport from higher latitudes by well-developed planetary wave winds. From the daily changes of absorption in the equatorial region, it is found that the enhanced absorption reveals an oscillation with a period of about 2 weeks and has its maximum in the region south of 20°N. The period is similar to that of planetary wave amplitudes in the winter stratosphere and mesosphere, suggesting that an effect of planetary waves could contribute to the equatorial anomaly of the absorption in the D-region.     

Magnetic field of the magnetospheric ring current and its dynamics during magnetic storms

This review examines models existing in the literature which describe the magnetic field produced by the ring current (DR) at the Earth's surface based on the energy balance equation. The parameters of this equation, the injection function F and decay parameter τ are considered to depend on parameters of the interplanetary medium and the DR intensity. The existing models are shown to be able to describe the DR variations with sufficient accuracy (r.m.s. deviation δ between the experimental and modelled values of DR for 170 magnetic storms is 5 < δ < 15 nT and the correlation coefficient between the two is 0.85 <r<1). The models describe that part of the geomagnetic field variation at low latitudes during a magnetic storm that is controlled by the geoeffective characteristics of the interplanetary medium and which thus responds immediately to its variations (the driven part).The values of τ are significantly less during the main phase of a magnetic storm than during the recovery phase. This reflects the difference in the main mechanisms of ion loss from the ring current during the two phases of the storm. These are the interaction of ions with hydromagnetic waves during the main phase of the storm with its intervals of intense plasma injection into the inner magnetosphere and charge exchange with the cold hydrogen geocorona during the recovery phase.     

The equatorial ring-current energy during magnetic storms as related to Dst activity indices

On the basis of geomagnetic activity indices Dst and solar wind parameters (velocity and density), we calculate the kinetic energy of the equatorial ring-current during the development of a magnetic storm, following the hypothesis of Sckopke's theorem.Supposing that the relaxation mechanisms of the plasmaspheric drift current which lead to the pre-storm state are always present and depend solely on the kinetic energy, it is found that the major part of the energy gain is lost, except during the main phase, when the interplanetary magnetic field permits it to be stored.     

Observations of the reflection coefficient of the sporadic E-layer at high latitudes

A method to measure the reflection coefficient of the sporadic E-layer using a conventional ionosonde is described, and some results obtained at Sodankylä, Finland (Φ=63.8°, φ= 120.0°), are presented. The observations are often found to be in agreement with the theoretical frequency dependence of the reflection coefficient of a thin horizontal layer in the presence of mode coupling. Some of the results can be interpreted as indicative of scattering from small-scale irregularities or reflections from larger inhomogeneities in the E_s layer.     

The influence of sunspot number and magnetic activity on the diurnal variation of the geomagnetic field at mid to high latitudes

The variations of the diurnal range of the geomagnetic field with sunspot number and with magnetic activity was studied at mid and high latitude stations in the northern hemisphere at different seasons. The effect of increasing sunspot number is small at lower latitudes and increases with geomagnetic latitude, while the effect of increasing magnetic activity is to increase the range at all latitudes, very greatly at the higher geomagnetic latitudes.     

The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

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.     

Non-ducted whistler-mode signals at low latitudes

In an earlier paper (Andrews, 1974) it was shown that a component of VLF signal from station NLK, near Seattle, received on a trans-equatorial path at Wellington appeared to have left the Earth-ionosphere waveguide at low latitudes and crossed the equator in the whistler mode. Results from a VLF recorder at Rarotonga (lat. 21°S) show that similar signals are indeed common at low latitudes. Recent calculations by Singh (1976) have established that a non-ducted VLF mode could exist at a latitude of about 20° in the presence of horizontal gradients of ionisation. It is pointed out that a second non-ducted path, first implied by Scarabucci (1970), appears possible at still lower latitudes when the O⁺ to H⁺ transition height in the ionosphere is high. While firm conclusions cannot be drawn, the low Doppler shift on the observed signals and the terrdency for their occurrence to maximise before dawn and at solar maximum are features which tend to favour the very low latitude path.     

Downward mapping of quasi-static ionospheric electric fields at low latitudes during fair weather

The problem of downward mapping of equatorial ionospheric electric fields is studied in two dimensions employing the finite elements and finite differences numerical techniques. The solutions obtained for low latitudes are compared with known results for high latitudes. It is found that equatorial ionospheric electric fields of scale lengths of the order of 100 km or more reach balloon heights (30–40 km) without undergoing noticeable attenuation. However, in the case of equatorial ionospheric electric fields of scale lengths of a few tens of kilometers it is found that these fields reach balloon heights with severe attenuation. The corresponding attenuation factors are significantly larger than those known for high latitudes. It is also shown that the presence of mountains with a fairly large height as well as of a large-scale conductivity irregularity in the middle atmosphere, such as that expected in the South Atlantic Magnetic Anomaly (SAMA) region during energetic electron precipitation events, can considerably distort the mapped ionospheric electric fields at the middle atmosphere.     

Day-time Pi pulsations at equatorial latitudes

Low latitude Pi2 pulsations are considered to be the best indicators of the onset of magnetospheric substornis (Rostoker and Olson, 1978; Saito, 1979) and are hitherto believed to be mainly night-time phenomena. It is seen from this study utilising the pulsation records from Choutuppal (geomagnetic: 7°.5, 149°.3 E)and Etaiyapuram (geomagnetic: –0°.6.147°.5 E)and the “Common Scale Magnetograms” from the Auroral Electrojet (AE) stations during January–April 1976, that Pi2s do appear even during day-time on many occasions at equatorial latitudes in simultaneity with the onset of magnetospheric substorms at AE stations located in the night hemisphere. It is also found that the day-time Pis, unlike the night-time Pi2s, show enhancement in their amplitudes of Hx component at Etaiyapuram, situated at the dipequalor as compared to those at Choutuppal, well away from it. The results thus not only show the appearance of Pi pulsations during daytime in the equatorial zone, but also bring out the possible influence of the equatorial electrojet on their amplitudes at the dip equator.     

On the problem of a connection between ozone dynamics at high latitudes and magnetospheric processes

Two data sets of ozone density measurements over a wide latitudinal range in the Arctic during summer and winter seasons arc presented. It is shown that geophysical effects manifest themselves in the O₃ dynamics in the high latitude region under various circumstances. Therefore a type of total ozone content diurnal variation is a distinctive feature in the auroral oval as well as the polar cap and must be taken into account in any full model of ozone dynamics in the polar regions.     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.