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.     

Ionospheric electrodynamic model with an eccentric dipole geomagnetic field

A computer model of ionospheric electrodynamic processes using an eccentric dipole (ED) for the geomagnetic field has been developed. This is a development from existing models which are based on the centred dipole (CD) coaxial with the geographic axis. The ED dynamo model introduces or modifies the effects of hemispherical asymmetry and longitudinal variation in the dynamo processes through two explicit parameters—the geomagnetic field intensity and the length of the field lines. These parameters of the ED field have been quantified and displayed. An additional contribution to the above effects comes implicitly from the ionospheric parameters—plasma density and atmospheric tidal winds—which become asymmetric relative to the ED dip equator. The integrated effect of the geomagnetic and ionospheric parameters produces significant variation in the field line integrated ionospheric conductivity. The ED dynamo model shows that the peak height of the equatorial electrojet (EEJ) moves by over 2 km and height profiles of the EEJ display strong hemispherical asymmetry.     

Spread-F occurrence in mid- and low-latitude regions related to various levels of geomagnetic activity

The occurrence of spread-F from seven latitude regions for ionosonde stations (78 in all) located from L-shell = 3.3 to 1.05 has been investigated (using the superposed-epoch technique) relative to four different levels of geomagnetic activity. Data for 14.5 years were used. For moderate, high and very-high geomagnetic activity a significant peak in spread-F occurrence is found for the four latitude regions closest to the auroral zone. These peaks are delayed (after the geomagnetic activity) by a matter of days, the delays being greater for the lower levels of activity and also greater for regions further from the auroral zone. Similarly, delayed dips in spread-F occurrence are found for very-low geomagnetic activity. Analyses for the remaining three regions (those closest to the equator) failed to show corresponding delayed peaks or dips in the occurrence of spread-F relative to the appropriate levels of geomagnetic activity. It is suggested that (for the three highest levels of geomagnetic activity) the mechanism which is responsible for the suppression of spread-F in equatorial regions may operate at these low latitudes and thus counterbalance the other mechanism which is responsible for the positive correlation found for the higher-latitude regions.     

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 at the magnetic equator using the A1 technique

A₁ absorption data on four operating frequencies, 1.8, 2.0, 2.2 and 2.5 MHz over a period of two years (1972–74) at Trivandrum (dip 0.6°S, geograph. lat. 8.6°N, long. 76.9°E) has been analysed to study its diurnal, day to day, long term and frequency variations. An empirical relation has been established between noon absorption and solar X-ray and 10.7 cm fluxes. Using this relation, the seasonal cosχ index has been evaluated. The deviation of the estimated absorption using this empirical relation from the observed absorption is found to be less than 10%.     

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and Z) were included in the study.The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude.The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq(H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude.The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.     

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.     

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.     

Lunar and solar daily variations of the geomagnetic field at Italian stations

Mean hourly values of magnetic declination D, horizontal intensity H and vertical intensity Z observed at Italian stations have been analyzed to determine solar and luni-solar diurnal components, together with the corresponding terms O₁ and N₂ of the lunar tidal potential.The results, showing the variations of the first four harmonic components with season, degree of magnetic activity and annual sunspot number, are tabulated and discussed. Differences between the dependence of S and L on season and sunspot number are considered and tentative explanations offered. The oceanic tidal effect has been determined and it is apparent that this is more likely to show the influence of the Atlantic Ocean rather than the Mediterranean Sea.     

A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

Ionospheric F2-kegion storms at midlatitudes starting in the daytime and the night-time

Disturbances in the F2-region during geomagnetic storms have been studied statistically, using foF2 data for two midlatitude stations in Japan. It is found

• 1.(i) that during the initial stage of the storm foF2 increases on the average for storms starting in the daytime and decreases for storms starting in the night-time, in all seasons,
• 2.(ii) foF2 decreases in summer and at the equinoxes and increases in winter irrespective of the local time of the storm onset.
• 3.(iii) disturbances of foF2 in winter for storms starting in the night-time are larger than those for storms starting in the daytime. Greater positive disturbances occur on the average for strong geomagnetic storms.

     

Faraday polarization fluctuations associated with amplitude scintillations at Lunping

The diurnal, seasonal and solar cycle variations of Faraday polarization fluctuations (FPF) associated with amplitude scintillations observed at Lunping, Taiwan (25.0°N, 121.2°E : geographic) during the period 1978–1981 are presented. The occurrence of polarization fluctuations is maximum in the premidnight hours. FPFs occur either simultaneously or with a time lag after the onset of amplitude scintillations. There is an increase in FPF activity with an increase in sunspot activity. Occurrence of FPF peaks in the equinoxes. There had been a moderate activity in summer while the winter occurrence is a minimum. The seasonal occurrence pattern compared with reports from other locations indicates a longitudinal control on FPF activity. The maximum probable duration of FPF ranges from 15 to 30 min. It was found that the association of FPF with range spread-F is much better than that with frequency spread-F. Large ambient ionization densities corresponding to plasma frequencies greater than 15 MHz appear to create a favourable environment for the occurrence of FPF.     

The use of sunrise and sunset terminators to calibrate ionospheric movement measurements

The velocity of the terminators may be used to calibrate the measurements of ionospheric movements and this has been successfully carried out for an H.F. radar system.     

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.     

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.     

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.     

Range indices of geomagnetic activity

The simplest index of geomagnetic activity is the range in nT from maximum to minimum value of the field in a given time interval. The hourly range R was recommended by IAGA for use at observatories at latitudes greater than 65°, but was superceded by AE. The most used geomagnetic index K is based on the range of activity in a 3 h interval corrected for the regular daily variation. In order to take advantage of real time data processing, now available at many observatories, it is proposed to introduce a 1 h range index and also a 3 h range index. Both will be computed hourly, i.e. each will have a series of 24 per day, the 3 h values overlapping. The new data will be available as the range (R) of activity in nT and also as a logarithmic index (I) of the range. The exponent relating index to range in nT is based closely on the scale used for computing K values.The new ranges and range indices are available, from June 1987, to users in real time and can be accessed by telephone connection or computer network. Their first year of production is regarded as a trial period during which their value to the scientific and commercial communities will be assessed, together with their potential as indicators of regional and global disturbances' and in which trials will be conducted into ways of eliminating excessive bias at quiet times due to the rate of change of the daily variation field.     

Radar measurement of the seasonal variation in the velocity of the sunrise terminator

The HF phased-array pencil beam radar at Bribie Island, Australia, used to measure horizontal movements of the ionosphere, has been calibrated using the known velocity of the sunrise terminator. The seasonal variation in the velocity of the terminator has been resolved, both in magnitude and direction.The technique uses single-station ionospheric sounding, and requires the angle of arrival and Doppler shift of ionospheric echoes to be measured as the terminator passes overhead. Pfister's theorem [(1971), J. atmos. terr. Phys. 33, 999] then allows calculation of the velocity of the reflecting surface. The difference between theory and experiment is less than 3% in speed and 2° in direction on average.     

Modifications in the nighttime low-latitude ionosphere after southward turning of the IMF

Sharp decreases in Φ_oF2 are found to occur frequently in the nighttime low-latitude ionosphere after southward turning of the IMF B_z component, especially under isolated B_z turnings, i.e. when the IMF has been northward for at least 6 h before its turning. These decreases occur simultaneously (within a 1-h time interval) with the B_z turning. The effect is observed both when a substorm or a magnetic storm begins after B_z has turned southward, and when a noticeable substorm does not occur. The effect is more pronounced after midnight and a maximum at 03 LT. Short-term (with scale times of about 1 h) variations of Φ_oF2 and h_mF2 for B_z southward turning are analysed using a large amount of ground-based and topside sounding data. The decreases in Φ_oF2 are shown to occur at first over the magnetic equator and then, during the second hour after the turning, at the crests of the equatorial anomaly. The ionosphere returns to its undisturbed state, on average, in 4–5 h (if other disturbing agents do not arise). These decreases are suggested to be caused by modifications in the electric field in the low-latitude ionosphere associated with B_z southward turning.