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.     

Ionospheric absorption and Pc1-type micropulsations following enhanced geomagnetic activity

Data of Pc 1-type micropulsations and ionospheric absorption derived from measurements at mid-latitudes were analysed during and after geomagnetically disturbed periods. In comparison to the normal Pc 1 activity, a rather reduced one has been found during disturbed conditions. A distinct increase of Pc 1 activity occurred, however, after selected key days in the investigated interval between days ‘+2’ and ‘+7’. A clear after-effect could also be shown for ionospheric absorption, measured at three frequencies in Kühlungsborn (φ∼55°N) and a quite moderate one at somewhat lower geographic latitudes (φ ∼ 48°–49°N). In Kühlungsborn the ionospheric after-effect is particularly prominent at all investigated frequencies in case of a simultaneous after-effect in Pc 1 pulsations. It is restrained in the LF-range and missing in the MF-range when the after-effect is lacking in Pc 1 pulsations.     

A study of wave packets and phase jumps in low latitude Pc3 geomagnetic pulsation

An array of four low latitude induction coil magnetometer stations has been used to study the spatial and temporal characteristics of Pc3 pulsations over a longitudinal range of 17° at L = 1.8 to 2.7 in southeast Australia. A preliminary study of individual Pc3 wave packet structure at the azimuthal stations has established the existence of phase jumps between wave packets at low latitudes, similar to those observed at synchronous orbit and at higher latitude ground stations. However, there did not appear to be any obvious pattern in phase jump occurrences between stations or signal components.     

Theory of generation of ULF pulsations by ionospheric modification experiments

A model characterized by three regions is considered. A thin dynamo region is separated by a region of free space from a perfectly conducting earth below. Above, the dynamo region is bounded by a magnetosphere in which hydromagnetic waves in the fast and in the Alfvén mode can propagate. Two limited extensions of a previous theory that is valid for frequencies well below 1 Hz and that neglects the effect of currents flowing into the magnetosphere, are considered. First, the frequency dependence of the ionospheric current system is determined, neglecting currents flowing into the magnetosphere. Secondly, the effects of currents flowing into the magnetosphere are evaluated in the low frequency limit and the Alfvén wave fields excited in the magnetosphere in the equatorial plane are calculated.     

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.     

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.     

Some effects of geomagnetic activity and ionospheric height rises on mid-latitude spread-F occurrence

The analysis investigates the base heights of the ionosphere (h′F) when spread-F is recorded at Brisbane, Australia, for 2 separate periods, namely July and August 1966 when spread-F occurs frequently and September and October 1966 when the activity is much lower. It is found that for July and August there is little tendency for spread-F to occur preferentially when the base height is above the average for the period. However, for the September–October period, spread-F occurs more often (by a factor of 2 or 3 depending on the geomagnetic activity) when the base height is above rather than below the average height. Also, this analysis shows that the overall spread-F occurrence (for both periods investigated) decreased to some extent following increased geomagnetic activity. This suppressed activity in the hours following geomagnetic activity is confirmed by superposed-epoch analyses using K indices (for Macquarie Island) as controls. It is suggested that the results of all the analyses might be explained by invoking a transition height in the ionosphere (controlled by the neutral-particle density of the upper atmosphere). Ionospheric off-vertical reflections from above this height would be recorded as spread-F traces in this model. This transition height would be low in July and August when the neutral-particle density is low and higher in September and October. It is further proposed that changes in the neutral-particle density could also be associated with reduced spread-F activity following increased geomagnetic activity, as well as influencing the diurnal, annual and sunspot-cycle variations of spread-F occurrence in mid-latitudes.     

Dependence of ionospheric response on the local time of sudden commencement and the intensity of geomagnetic storms

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.     

Simulation of ionospheric currents and geomagnetic field variations of Sq for different solar activity

Variations of ionospheric Sq electric currents and fields caused by changes in electric conductivity due to changes in solar activity are studied using the International Reference Ionosphere (IRI) model. Calculations are made for R (sunspot number) = 35 and 200 on the assumption of constant (1, −2)mode tidal winds. It is shown that electric fields grow when solar activity is high, because the ratio of the conductivity in the F-region to that in the E-region increases. Currents in the F-region become stronger than those in the E-region, and nocturnal currents are not negligible when solar activity becomes high. F-region currents also play an important role in the westward currents on the high latitude side of the current vortex. The calculated geomagneticH component at the equator has a depression around 1600 LT for R = 35, while it decreases smoothly from 1100 LT to 1900 LT for R = 200. This difference is consistent with the observed geomagnetic field variation. The ratio of total Sq currents obtained by our simulation is about 3.5, which is a little larger than is found in the observed results.     

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.     

Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.     

Comparison of one year of data on geomagnetic pulsations at geostationary orbit and at a low latitude ground station

In spite of several satellite-ground comparisons of pulsation data, many questions remain open for future investigation. This paper reports on a comparison of the satellite ATS 6 and Nagycenk data (L ∼ 1.9) on pulsation occurrence, activity, period and switches. This low latitude ground station sees a lot of activity which is less evident at L ∼ 6.6, i.e. these pulsations are due to amplification in the inner magnetosphere. The ATS harmonic structure is shown to have little influence on the ground activity. The inner magnetospheric amplification is changing and is influenced by solar wind velocity. The switches confirm that a large part of the two pulsation activities are of different origin, supposedly at least partly from a Kelvin-Helmholtz source at L ∼ 6.6 and from the upstream source at L ∼ 1.9.     

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.     

Deconvolution of ionospheric parameters in the F1-region

In order to study the influence of altitude smearing on the determination of ionospheric parameters by incoherent scatter, we take advantage of the fact that EISCAT single pulse measurements in the F1-region have been made simultaneously with two different altitude resolutions. It is shown that the measured parameters can be very far from the real parameters, due to this altitude smearing. An attempt to deconvolve the profiles is made, which works well in the case of smooth power variations within the diffusive volume, but diverges in the case of strong structured power variations, such as those produced by energetic auroral particle precipitation into the upper atmosphere. In this last case it was, however, possible to deduce the real profile with quite good accuracy by a trial and error method. The geophysical consequences of the large discrepancy between measured and actual parameters (density and temperatures) are finally discussed.     

The theory of ionospheric focused heating

Ionospheric modification by high power radio waves and by chemical releases are combined in a theoretical study of ionospheric focused heating. The release of materials which promote electron-ion recombination creates a hole in the bottomside ionosphere. The ionospheric hole focuses high power radio waves from a ground-based transmitter to give a 20 dB or greater enhancement in power density. The intense radio beam excites atomic oxygen by collisions with accelerated electrons. Airglow from the excited oxygen provides a visible trace of the focused beam. The large increase in the intensity of the radio beam stimulates new wave-plasma interactions. Numerical simulations show that the threshold for the two-plasmon decay instability is exceeded. The interaction of the pump electromagnetic wave with the backward plasmon produces a scattered electromagnetic wave at 3/2 the pump frequency. The scattered wave provides a unique signature of the two-plasmon decay process for ground-based detection.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

The variability of ionospheric dynamo currents

The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents.     

The components of the annual line in geomagnetic field elements

The annual cycle of the strength of the geomagnetic horizontal field shows a local time dependence. Recent explanations for this invoke a ring current or magnetospheric source for the night-time, and an ionospheric source for the daytime annual waves, but the relative importance of each source has been uncertain. Harmonic dials of the annual waves in H from harmonic analysis at several local times and observatories are presented. Comparison with corresponding analyses of observed ionospheric winds supports the two source model. It is shown that both the magnetospheric and ionospheric components in H are 180° out of phase across the equator. But because of differing amplitudes of the two components across the equator, the net annual waves are about 160° out of phase across the equator.     

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.