Mechanisms for observed total electron content pulsations at mid latitudes

Total electron content variations in the Pc3–Pc4 range of frequencies of the order of 4 parts in 10⁴ have been reported in apparent correlation with simultaneous ground based magnetic pulsation observations. By means of a term-by-term analysis of the continuity equation for electrons, the plausibility of various mechanisms is investigated. The most likely explanation is in terms of localized increases in the electron density at F-region heights caused by the field-aligned (compressional) component of the pulsation magnetic field. The analysis predicts a tendency for the amplitude of the TEC pulsations to vary in antiphase with ground-based measurements of the north-south component of the pulsation field.     

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.     

Solar quiet geomagnetic variations and E-region neutral winds at equatorial latitudes

A model of the ionospheric current system valid at zones close to the geomagnetic equator, taking into account the contribution of neutral winds, is proposed. From this, the external magnetic field at ground is calculated. Also, ground records of the geomagnetic field variations at the Peruvian equatorial zone were separated into their external and internal contributions. Using an iterative process a local particular fitting was found by comparing the separated external field to the one calculated with the proposed model.     

Polarization characteristics and phase differences of Pi2 pulsations at conjugate stations

Spectral analysis was carried out on Pi2 pulsations occurring simultaneously at the conjugate stations St Anthony, Newfoundland and Halley Bay, Antarctica during the second half of 1976. For the strongest lines in each spectrum, the ellipticities and azimuths of the horizontal polarization ellipses at the two stations were approximately equal and opposite in sign generally consistent with the stations being conjugate, but there was noticeable scatter. When the differences in phase of the components between the stations were examined, they were found to be about 10° for the H-component and about 170° for the D (consistent with concept of an odd-mode oscillation of the field line) until about middle-late October. After this time, larger variations in the phase differences and decrease in coherence between the stations occurred and it is suggested that is a result of the presence of an ionosphere E-layer at Halley Bay throughout the night during Southern summer.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

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 sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

A study of pearl pulsations in the equatorial region of India

Pearl pulsations in the equatorial region, despite their relative paucity, are known to exhibit definite features in their diurnal, seasonal and annual occurrence patterns. In the present study, attempts are made to examine and bring out in detail some more aspects of these pulsations recorded at Choutuppal (Geomagnetic latitude: 7°28′N) over a period of nine years during 1967–1975. The mid-period (t) of pearls is found to bear a linear relation to the pearl repetition period (T) and is of the form T = 70t + 11.2. The periods of pearls are seen to be influenced by the average magnetic activity level prevailing during the preceding few days rather than that existing on the day of pearl occurrence. There is an increase in the mid-frequency of pearls that occurred succeeding a magnetic disturbance particularly when the disturbance persisted over some days. The amplitude ratio (H_x/H_y) averaged for each two-hourly interval is seen to attain a value of unity around local midnight and decrease towards the dawn and morning hours. From these results, namely the dependence of mid-frequency of pearls on the average magnetic activity (represented by (Σ Kp >) during the preceding few days and the occurrence of shorter period pearls even after a few days following a magnetic storm, it is suggested that the plasmapause takes much longer time to re-establish to the quiet time position. In the alternative, it may be visualized that possible presence of localized regions of high plasma density gradients inside the plasmasphere after the cessation of a storm might provide favourable conditions for the generation of shorter period pearls.     

Pi(c) magnetic pulsations: their relation to ionospheric currents

The association of the phase of the H and D components of the Pi(c) pulsations with the phase of the broadscale H component magnetic bays confirms that Pi(c) pulsations result from auroral electron precipitation induced conductivity enhancements of existing current systems. Statistically determined relationships between the time delays and phases of the H and D components of magnetic Pi(c) pulsations with respect to the optical pulsations are used to infer a delay between the E-region Hall and Pedersen current fluctuations associated with the pulsating electron fluxes. Theoretical modelling of an auroral pulsation patch, as per Oguti and Hayashi, is used to show that the polarization of the Pi(c) pulsations is controlled principally by the delay between the Hall and Pedersen currents and the direction of the background E-region electric field.     

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.     

Equatorial F-layer irregularity patches at anomaly latitudes

Recent studies of the physics of F-layer irregularities in the equatorial ionosphere have been concerned with the development of plumes or patches. A series of observations in the equatorial anomaly region in a year of high solar flux has been analyzed for the radio propagation effect of scintillations. The observations were made on patches in the developing, mature and decay phases. Although irregularities develop on the west wall of the patches, the intensity of scintillation does not appear to diminish within the patch; the patches contain bursts of high level activity.Patch characteristics at microwave wavelengths match airglow depletion images when two considerations are introduced, i.e. the westward tilt of the patch as shown by optical and radar observations and the effective path length of the irregularities affecting the radio propagation path. Using optical images of depletions the effective thickness of the layer of irregularities above the peak of the F2-layer can be estimated; it is relatively short, i.e. of the order of 70 km for the gigaHertz frequencies and 150 km for the 257 MHz transmissions. The total path length is 110 km for the microwave frequencies and 220 km for the lower levels of scintillation at 257 MHz. The decrease in microwave scintillations compared to meter wavelength observations in the midnight and post-midnight time period in these anomaly observations is due to the combination of decay of electron density as well as the relatively rapid decay of smaller scale irregularities, as has previously been noted in observations at the magnetic equator.     

Quasi-periodic scintillation events at southern auroral latitudes

Quasi-periodic (QP) radio scintillations were observed during (1987) on 244 MHz and 1.5 GHz geostationary satellite transmissions in the southern auroral zone from Davis station (68.6°S, 78.0°E geographic, 74.6°S Aλ) in Antarctica. Three distinct types of OP events were identified, with occurrence times mainly restricted to the period 18-00 MLT. The substantial loss of signal associated with these events appears to be an important factor in determining the reliability of satellite links on 1.5 GHz in auroral regions. Previous observations at mid-latitudes of QP scintillations have noted a preference for large zenith angles and equatorward azimuths. It is demonstrated that a height transition in a densely ionized layer can produce QP scintillations in a manner analogous to a dense column of ionization but at lower ionization densities, as well as demonstrating a zenith angle and azimuthal dependence that is more consistent with observations than a column of ionization. At the occurrence times noted, the raypath may be intersecting the poleward edge of the trough where sporadic-E is a regular feature. QP scintillation events may result when the E_s-layer is height modulated by the passage of acoustic-gravity waves originating in the auroral zone.     

Winds,wind-shears and plasma densities during the initial phase of a magnetic storm from equatorial latitudes

Results of a sodium vapour release experiment carried out from SHAR (India), an equatorial rocket launching station, immediately after (⩽ 2 h) a storm sudden commencement (SSC) during the initial phase of a magnetic storm, followed by electron density measurements are presented. Many of the relevant atmospheric parameters, namely, neutral winds and their altitude variation, the magnitude of the shears in them, the neutral temperature with altitude by spectroscopic methods, diffusion measurements on the released trail, clues on the turbopause level and the electron densities including the structures (irregularities) in them were obtained. The results of the temperature measurements carried out independently on the sodium trail by means of a ground-based Fabry-Perot spectrometer, operating on the sodium D 1 line, resonantly scattered by the trail have already been reported by us (Ranjan Guptaet al., 1986). In this paper the effects of the excess temperature reported earlier and the rest of the measured parameters on the electron density profiles are evaluated using models and compared with the measurements.The formation of sharp layers of ionization have been explained on the basis of the electro-dynamical processes associated with the wind shears at a location, close to the edge of the equatorial electrojet belt. The significance of the changes in the neutral composition due to the enhanced neutral temperature and the low turbopause level, in increasing the base-level plasma densities by a factor of 3–5 are demonstrated and the possible role of plasma dynamics discussed.     

Scintillation at high latitudes during winter magnetic storms

Scintillation observations are described which were made at Kiruna in northern Sweden during three magnetic storm periods in the winter of 1984–1985. The results were obtained using transmissions from the multisatellite NNSS system, so that it has been possible to chart the development of scintillation activity over some 20° of geomagnetic latitude as a function of time for several days throughout each storm. A region of strong scintillation at the highest latitudes near magnetic noon is a common feature on all but the quietest days. This feature, probably associated with soft particle precipitation into the cusp, shows an abrupt boundary which moves equatorwards as the disturbance develops. In the magnetic midnight sector two latitudinally separate zones of scintillation are found, patchy at high latitudes although more sustained in the auroral zone. An absence of auroral scintillations around midnight UT can be followed by prolonged intense scintillation activity at auroral latitudes during the early morning hours on some disturbed days.     

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.

     

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Structure in the seasonal variations of Es at southern latitudes

The diurnal variations of the seasonal characteristics of sporadic-E occurrence have been studied by analyzing a large data set of ionosonde parameters for two southern hemisphere stations. The seasonal patterns are found to display anomalous short-term variations apparently not associated with solar control or the effects of dynamic meteorology.     

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.