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.     

On the occurrence of equatorial spread-F in the evening hours

By comparing electron drift velocities at Jicamarca with corresponding ionograms and VHF radio scintillation records at Huancayo it has been shown that the day-to-day variability in the occurrence of equatorial spread-F irregularities in the post-sunset period depends critically on the time of reversal of the Sq electric field. The field reversal before sunset does not produce any spread-F in the evening hours, while the continuation of the day-time electric field for a couple of hours after sunset at normal strength is a favourable condition for generating spread-F.     

The nature of ionospheric spread-F irregularities in mid-latitude regions

Initially, this paper considers earlier experimental results (some of them hitherto unpublished) obtained by making observations on signals returning from mid-latitude spread-F irregularities. These results suggest associations between spread-F irregularities and nighttime travelling ionospheric disturbances. Statistical analyses are then described which investigate the spread-F phenomenon at a number of mid-latitude stations with approximately the same latitudes but distributed over a range of longitudes. An east-west movement of spread-F irregularities is revealed when the occurrence at these stations is considered relative to days of enhanced occurrence at a particular station. All the experimental evidence presented in the paper supports the idea that the appearance of mid-latitude spread-F ionograms results primarily from specular reflections from relatively-large-scale structures which can be imagined as being in fact nighttime travelling ionospheric disturbances. These are, in turn, possibly related to internal gravity waves in the neutral atmosphere. It is suggested that the small-scale ionospheric structures (which are undoubtedly also present) are effective in inhibiting some of the specular reflections thus contributing to the diffuse nature of some records. This idea is quite contrary to the generally-accepted view that the spread-F traces are a direct consequence of scattering from these small-scale structures.     

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.     

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.     

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.     

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.     

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.     

Periodic amplitude variations as precursors of plumes of equatorial ionospheric irregularities

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called “amplitude waves” exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic eastwest direction. The velocity of these wavelike disturbances has been found to be approximately 140 ms⁻¹ eastward with a corresponding wavelength of 25 km. No wavelike behavior of Faraday rotation, a measure of the background changes in TEC, was observed during these times. Several mechanisms are examined as the cause of these amplitude waves; however, none was found to be completely satisfactory in explaining the observations.     

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earth's horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst < −150 nT have been found to be most effective in producing the above scintillation activity. From the present observations, a threshold value of the maximum negative excursion of Dst for producing scintillation may be obtained, Dst < −75 nT being significantly associated with the post-midnight scintillation occurrences. The results are interpreted in terms of the reversal of the equatorial horizontal electric field, under magnetically disturbed conditions, due to a coupling of the high latitude and magnetospheric current systems with the equatorial electric field.     

Structure in the seasonal variations of ionospheric Es occurrence in the South Pacific

A survey is presented of the occurrence of strong ionospheric sporadic-E for South Pacific ionosonde stations covering a period of many years. The seasonal characteristics indicate the presence of strong non-solar effects with, for example, subtropical data showing strong afternoon enhancements of E_s activity in the autumn.     

Day-to-day ionospheric variations in a period of high solar activity

In November and December 1979 the solar 10.7 cm radio flux density, sunspot number, X-ray flux and EUV flux were high and very variable. The day-to-day variations of noon F2-layer height and Elayer electron density at three ionosonde stations (Slough, Port Stanley and Huancayo) are found to be well correlated with the day-to-day variations of solar activity. The short-term E- and F-layer variations are consistent with those derived from longer-term studies.     

Satellite and rocket observations of equatorial spread-F irregularities: a two-dimensional model

Recent rocket and satellite measurements of equatorial F-region irregularities have been able to resolve wavelengths comparable to the meter-size sensitivities of the Jicamarca and Altair radar backscatter techniques. In a July 1979 rocket campaign at the Kwajalein Atoll, vertical profile measurements by ‘in situ’ plasma probes showed the F-region marked by a number of large scale plasma depletions, each having its own distribution of smaller scale irregularities and a trend toward a co-location of the more intense irregularities with positive gradients of larger scale features. Similar measurements on the S3-4 Ionospheric Irregularities Satellite have shown large scale depletions (1–3 orders of magnitude) with east-west asymmetries that point toward the western wall as the sight for the more intense plasma density fluctuations. The combined rocket and satellite measurements provide a two-dimensional model of macroscopic F-region depletions with small structures tending to develop more readily on the top and western boundaries. The model and associated power spectral analyses is in concert with a developing catalog of radar observations and the predictions of numerical simulations which employ the Rayleigh-Taylor instability as the primary mechanism for the generation of intermediate wavelength irregularities.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

The role of acoustic gravity waves in the generation of spread-F and ionospheric scintillation

In the aggregate, acoustic gravity waves in the F-region constitute a spectrum of geophysical noise extending from the frequencies involved in diurnal variations up to the Brunt-Väisälä buoyancy frequency. They drive a roughly uniform power spectrum of travelling ionospheric disturbances (TIDs) with vertical scales of the order of the atmospheric scale height H and with horizontal scales extending from the radius of the Earth down to H. It has been known since the 1950s that this permits multiple normals onto the F-region from an ionosonde, thereby creating the multiple-trace type of spread F on ionograms. At shorter scales the spectrum of TIDs decreases in strength and, below the mean free path of the neutral atmosphere, creates a spectrum of plasma turbulence aligned along the Earth's magnetic field. Progressively shorter scales are responsible for phase scintillation, for amplitude scintillation and for blur-type spread F on ionograms. A weak extension of the spectrum to scales less than the ion gyroradius is responsible for spread F and transequatorial propagation in the VHF band. Under evening conditions in equatorial regions a band of TIDs with wavelengths of the order of 600 km can, at times, have a phase velocity that matches the drift velocity of the plasma (Röttger 1978). This band of TIDs is then amplified until it breaks (Klostermeyer 1978). The associated explosive increase in plasma turbulence creates the plume phenomenon discovered by Woodmn and La Hoz (1976).     

VHF radio wave scattering due to range and frequency types of equatorial spread-F

Comparing vertical incidence ionograms during spread-F conditions at the equatorial station Huancayo and modified range time interrsity records of 50 MHz scatter echoes at Jicamarca, it has been shown that the range type of spread-F is very efficient for the back-scattering of VHF radio waves. On the otherhand, the frequency type of spread-F does not seem to produce strong echoes. It is suggested that the range type of spread-F ionogram is due to the reflection (ω = ω_p) of radio waves from large scale irregularities with structure as small as 3 m below or near the base of the F-region The frequency type spread-F ionogram is suggested due to scattering from large scale irregularities with no 3 m counterpart situated around the region of peak ionisation density.     

North-south asymmetry in the ionospheric equatorial anomaly in the African and the West Asian regions produced by asymmetrical thermospheric winds

The effect of asymmetrical thermospheric winds on NmF2 at the dip I = 30° and its magnetic conjugate point have been computed for equinox conditions to study asymmetry in the ionospheric equatorial anomaly in the African and West Asian regions. The wind models of I11 et al. and Chan and Walker have been used in our computations. During the daytime, due to the winds NmF2 in the northern crest becomes greater than NmF2 in the southern crest; at night the reverse is true in both regions. It is shown that the observed asymmetry in NmF2 at the equatorial crest in the African sector can be well explained by considering the effects of asymmetrical winds with respect to those in the West Asian sector.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.