On the frequency shift in modulated VLF emissions

The nature of the shift between the frequency of geomagnetic pulsations registered on the Earth and the modulation frequency of VLF emissions registered on board satellites is analyzed. The results obtained are compatible with the mechanism of VLF emissions modulations founded on the interaction between magnetosonic waves propagating perpendicular to the geomagnetic field and the energetic electrons exciting VLF emissions. It is shown that the frequency shift considered can be used for pulsations source localization in the magnetosphere.     

On the equatorial electrojet influence on geomagnetic pulsation amplitudes

It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.0–2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (<20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.     

A multistation study of long period geomagnetic pulsations at cusp and boundary layer latitudes

The characteristics of 1–20 mHz (Pc5) geomagnetic pulsations recorded during the daytime on the ground at cusp and boundary layer latitudes have been examined. On quiet and moderately disturbed days the major spectral contributions are due to three different mechanisms. Sustained oscillations whose properties are consistent with the Kelvin-Helmholtz instability at the low latitude boundary layer are the dominant mechanism at −70 to −75 geomagnetic latitude. Transient irregular pulsations are frequently seen at single stations at the foot of polar cap and boundary layer field lines. Occasionally similar transients occur essentially simultaneously at widely spaced stations accompanied by absorption spikes on riometer records. The latter signals are most likely due to solar wind pressure pulses on the magnetopause. At cusp latitudes the major spectral contribution arises from sustained irregular pulsations centred on magnetic noon. Although their occurrence is related to the proximity of the cusp's particle signature, it may be more appropriate to discuss these signals in terms of fluctuations in boundary layer or mantle currents.     

Ionospheric mechanism for the two maxima in the spatial distribution of Pc3 geomagnetic pulsations

A modelling of the spatial distribution of Pc3 geomagnetic pulsations on the Earth's surface is carried out. We propose that the main contribution to the PC3 amplitude is due to ionospheric currents fluctuating because of conductivity variations associated with the modulation of electron precipitations which occurs in the field of compressional waves coming, probably, from the solar wind. A coincidence of the two dayside maxima in Pc3 geomagnetic pulsation amplitude (at latitudes ~ 70° and 55–60°) with two maxima in electron precipitations is in favour of such a proposition.     

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.     

Ionospheric propagation of ULF-waves

Features of mid-latitude ionospheric propagation of geomagnetic pulsations are studied. It is shown that the orientation of the horizontal component of the incident wave vector with reference to the geomagnetic meridian has a strong influence on the amplitude transformation and on the angle of rotation of the polarization plane. The effect of different conductivities of the Earth's crust and the contribution of Hall currents to the total pulsation field in the case of an inclined geomagnetic field are also considered.     

Drift analysis of random and quasiperiodic scintillations in the ionosphere

Radio signals in the VHF/UHF range from the geostationary satellite ATS-6 were recorded using a system of three spaced antennas at Slough. Simultaneously, the integrated electron content (TEC) was monitored between the satellite and ground. Full correlation analysis and similar fade techniques were used to deduce the drift velocities of irregularities responsible for random and quasiperiodic (QP) ‘ringing’ scintillations. Similar drift velocities were found for the disturbances responsible for both types of scintillations at the times when QP and random scintillations occurred in a sequential pattern. A southward-drifting disturbance was responsible for rare, multiple QP scintillations which were followed by large scale fluctuations in electron density. In general, QP-scintillation-producing irregularities drifted southward, with velocities whose median magnitude and azimuth were 64 m s⁻¹ and 178°E of N respectively.The sequential occurrence pattern of QP-random scintillations as well as the time delay between occurrences of large fluctuations in TEC and QP scintillations, appear to be consistent with a reflection model of generation of the ringing fading of the signal.     

Identification of the magnetospheric cusp and cleft using Pc1–2 ULF pulsations

Geomagnetic pulsations in the 0.1–2.5 Hz (Pc1–2) range recorded over 12 quiet summer days at six Antarctic stations between −62.3 and −80.6° invariant latitude were examined in order to map the spatial and temporal distribution of spectral characteristics. Ionospheric particle signatures associated with the magnetospheric cusp and boundary layer were deduced for three of these days using ground riometer, magnetometer and ionosonde measurements, and in-situ ionospheric particle data. Comparison with the magnetic pulsation data shows that specific Pc1–2 emissions are associated with these regions. Within the cusp, intense unstructured ULF noise in the 0.15−0.4 Hz range is observed. Less intense waves of this type are seen near the cusp location on mantle and plasma sheet boundary layer flux tubes. These emissions are quite distinct from the discrete, structured and narrowband emissions seen equatorward of the cusp. Whereas past discussions of cusp and cleft identification have usually focused on optical or satellite data, we conclude that ground-based observations of Pc1–2 pulsations can provide a more convenient, although less precise, monitor of high latitude features.     

Pulsing hiss,pulsating aurora and micropulsations

Simultaneous conjugate observations between the GEOS 2 geostationary satellite, an all-sky TV camera operated at Andenes, Norway and a magnetometer at Tromsø, are presented. A close correlation was found between the occurrence of pulsing ELF hiss on GEOS 2, pulsating aurora observed with the TV camera and the ground detected micropulsations. However, a one-to-one correlation between the individual pulses of all three phenomena was generally not found. Ten events were chosen for a case study where the optical pulsations could be classed into two types (either classical pulsating patches or propagating forms). Classical pulsations were seen to be associated with Pi(b) micropulsations while propagating forms showed a one-to-one correlation with Pi(c) micropulsations. Type matching between the optical pulsations and two different types of ELF hiss, agreed well on a statistical basis, but the exact relationship for individual events was not so clear. On occasions, the micropulsations possessed major frequency components whose periods were longer than the periods of the other two phenomena. This is shown to imply that the observed micropulsations were of ionospheric origin in these cases.     

The phase relationships of the ionospheric signatures of Pc1 geomagnetic pulsations

In this study we consider the phase relationships between the oscillations of various ionospheric signatures associated with Pcl geomagnetic pulsations. Investigations using a simple analytical method and a numerical model, which has proved successful when applied to longer period pulsations, both suggest that Doppler velocity oscillations should be predominantly in anti-phase with oscillations of the rates of change of group range and echo amplitude. However, observations indicate that the Doppler velocity oscillations are in quadrature with the other two types of oscillations. Possible causes for this discrepancy are suggested.     

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.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.     

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.     

Spatial dependence of day-time vertical polarisation of Pc 3–5 magnetic pulsations in Sri Lanka

The rate of change of the horizontal and vertical components of magnetic pulsations in the period range 20–600 s have been recorded at four stations in Sri Lanka, namely, Vavuniya, Maradankadawela, Maho and Colombo. An analysis of the records shows that the horizontal polarisation of the pulsations is predominantly along the magnetic meridian at all four stations. The vertical polarisation as measured by the ratio ΔZ/ΔH increases with increase in period and for signals in a given periodic band, except for those recorded at Maho ; there is also an increase of vertical polarisation with distance of the recording station from the magnetic equator. At Maho, there is a local decrease of the vertical polarisation at all periods probably due to anomalous electrical conductivity in this region. Although the spatial and period dependence of the vertical polarisation of the pulsations recorded at the other three stations can be explained in terms of an oscillating ionospheric current band of half-width about 105 km flowing east-west in the neighbourhood of the magnetic equator, and its image current in a uniform conducting earth, such a model may not be realistic in that it neglects possible effects due to induced currents in the ocean deflected round the coasts of Sri Lanka. It is suggested that the observed day-time polarisation may represent the effect of a wider equatorial ionospheric current system, such as the equatorial electrojet and additional effects due to induced currents in the ocean.     

Observations and modelling of ionospheric and thermospheric disturbances during major geomagnetic storms: A review

The thermosphere is primarily energised by the combination of three sources of energy and momentum. Solar UV and EUV energy is absorbed globally on the dayside within the middle and upper thermosphere. There is a persistent, but highly variable, inflow of energy and momentum from the magnetosphere. These magnetospheric inputs are usually confined to high latitudes, except at times of very large geomagnetic disturbances. Tides and gravity waves upwell from their sources in the troposphere and stratosphere to deposit energy and momentum at levels from the middle mesosphere to the upper thermosphere. Solar EUV radiation between 120 ran and 250 nm photo-dissociates the molecules which dominate the composition of the lower thermosphere, in particular producing atomic oxygen which dominates the composition of the upper thermosphere. The combination of solar EUV radiation at wavelengths shorter than 120 nm, plus energetic (mainly) charged particles from the magnetosphere, also ionise the neutral constituents of the thermosphere, creating the ionosphere. Particularly at high latitudes, within the geomagnetic polar caps and auroral ovals, the energetic, dynamical and chemical coupling and interactions between the thermosphere and ionosphere dominate the structural and dynamical response of both the thermosphere and ionosphere to solar and geomagnetic inputs of energy and momentum.Comparisons between predictions using global thermosphere-ionosphere coupled models and comparable observational sets have shown encouraging agreement during periods of relatively quiet geomagnetic activity. This indicates that the major energetic, ionisation, chemical and dynamical processes and interactions can be described in models with reasonable accuracy. During periods of high geomagnetic activity, and particularly during major geomagnetic storms, large rapid disturbances of the thermosphere occur with extremely rapid variations. These disturbances are observed as large increases of temperature, density, major changes of neutral composition, and with the development of high speed wind flows and large amplitude waves which may propagate to affect the entire globe. Since the ionosphere is formed from thermospheric constituents and affected by thermospherc dynamics, the gross disturbances of the ionosphere during highly disturbed periods are related to contemporary changes of density, composition and flows of the thermosphere, as well as changes of ionisation sources and electric fields. Observations which describe the nature and scale of disturbances of the thermosphere during geomagnetic storms will be used, in combination with appropriate global numerical simulations, to aid interpretation of storm-time ionospheric phenomena. The role of energetic, dynamical and chemical coupling between the thermosphere and ionosphere is emphasised.     

The vernal-autumnal asymmetry in the seasonal variation of geomagnetic activity

In intervals in which the polarity of the main solar dipole field is stabilized, a 12 month wave occurs in geomagnetic activity (indices aa, Ap, Dst) with its maximum in one of the equinoctial periods. Whether the vernal or the autumnal maximum is greater depends on the polarity of the main solar dipole; the existence of the wave may be explained by the north-south asymmetry in the main solar dipole field. The results favour the southward component of the interplanetary magnetic field as the decisive factor for geomagnetic activity.     

Quasi-periodic particle precipitation and Trimpi activity at Halley,Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interaction.     

Role of the plasmapause and ionosphere in the generation and propagation of pearl pulsations

The source of Pc 1 (pearl) pulsations observed in the course of the local morning hours on 7 December 1977 has been determined by the amplitude and group delay methods. The frequency of pulsations exhibit the typical diurnal variation with the maximum frequency during dawn hours. The source location of pearls during every 1-h interval is compared with the position of the plasmapause inferred from the GEOS I measurements and from previous statistical analysis. It is shown that the source of high-frequency pulsations (f > 1 Hz) is well inside the plasmapause whereas low-frequency pulsations (f < 1 Hz) occur near the plasmapause. The source of pulsations is displaced to higher L-values in the course of the local morning hours and this displacement is associated with the decrease of the frequency of pulsations. The source displacement is much more pronounced than the simultaneous movement of the plasmapause position. These observations imply that the model of the Pc1 generation which locates the source only at the plasmapause has serious shortcomings. A model is discussed which takes into account the generation of Pc1 pulsations also well inside the plasmapause and the properties of the waveguide propagation of waves in the ionspheric duct.     

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.     

Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, σ_v, less than ∼5 × 10⁻¹³ m⁻¹, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (∼50 m/s per 10dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (∼200 m/s per 10dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.