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.     

Small-scale ionospheric structures associated with mid-latitude spread-F

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

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.     

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.     

Some aspects of mid-latitude daytime ionospheric disturbances

This paper is concerned with the ionospheric irregularities produced during daylight hours in mid-latitude regions by medium-scale travelling ionospheric disturbances (MS-TIDs) as these disturbances pass overhead at a recording station. These irregularities are detected as ionogram trace distortions as well as by spread-F conditions particularly related to the second-hop reflections of HF radio waves. The existence of a significant amount of spread on second-hop ionogram traces during the passage of two particular MS-TIDs is illustrated and discussed. Experimental evidence is presented to suggest that, similar to the results for night-time spread-F, the occurrence of these daytime events is inversely related to variations in the neutral-particle density of the upper atmosphere, for the annual and sunspot-cycle variations. However, the results do not show a similar inverse relationship for the diurnal variation.     

A comparison of mid-latitude and equatorial-latitude spread-F characteristics

Experimental evidence using a fast-swept-gain technique on an ionosonde is presented to support the idea that mid-latitude spread-F irregularities are large-scale wave-like structures. Also,diurnal and annual distributions of spread-F occurrence at an equatorial station at times of low sunspot activity are shown to be similar to those found for mid-latitude stations. The sunspot-cycle variation of post-midnight spread-F occurrence is also found to be similar in the two latitude regions. The similarity of certain spread-F characteristics at both mid- and equatorial-latitude regions is discussed. An attempt is made to reconcile current spread-F models for these two latitude regions by proposing that the primary spread-F structures for equatorial regions are large-scale wave-like structures. It is further proposed that the small-scale plasma instabilities have a role of modifying the traces resulting from specular reflections from the large-scale structures.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.     

Observations of a day-time mid-latitude ionospheric trough

A summer, dayside, mid-latitude trough detected by a digital ionosonde located at Halley (76°S, 27°W, L = 4.2) is described. The trough is found to be present in the F2-region only and its movements are found to conform to known trough dynamics. The F1-layer shows a greater degree of development within the trough; slant type sporadic E reflections are present underneath the trough minimum. Satellite data from the northern hemisphere show a conjugate trough, with rapid ion flow occurring within it. Possible formation processes for the trough are examined. It is unlikely that depleted nightside plasma could have contributed to the trough. The trough is formed by the effect of enhanced F2 recombination rates combined with a differing solar production term for the plasma associated with the trough minimum and equatorial edge.     

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.     

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 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).     

Multiple beam observations of mid-latitude ionospheric disturbances by the MU radar

The pulse-to-pulse beam steerability of the M U radar of Kyoto University enables us to observe multiple beam positions simultaneously. Based on 560 h of this type of data, we present two typical patterns of mid-latitude ionospheric disturbances and their horizontal traveling characteristics. Wavy structures have not been found in large-scale disturbances. Isolated disturbances travel primarily southward (equatorward) in disturbed conditions, while no preferred direction is observed in quiet conditions.     

spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite.At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.     

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.     

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.     

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.     

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.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.