Dynamic spectra of pulsation events at L ∼ 1.9 and L ∼ 3.3

High resolution dynamic spectra of 12 pulsation events are compared at NCK (L ∼ 1.9) and KVB (L ∼ 3.3), lying on the same meridian. Common features of the pulsation activity at the two stations include periods, period shifts and switches on and off. These parameters are investigated for the sample of pulsation activity and demonstrate that the two stations record activity from both a common and from different sources. Shell resonances are discussed in relation to the common source of activity.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

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.     

VLF transmissions observed at anomalously high latitude above the ionosphere in the conjugate hemisphere

Ariel 3 and 4 satellite observations of the GBR 16 kHz and NAA 17.8 kHz transmissions above the ionosphere in the conjugate hemisphere show that their wave-fields generally show a rapid reduction in signal strength for geomagnetic latitudes greater than 55°–60°. Sometimes, however, the signal strength has been observed to be high in the invariant latitude range > 60°. At certain times during these observations, the signal showed clear evidence of amplification, whilst at other times the pattern of signal strength was displaced to higher latitude with the signal strength integrated over latitude being unchanged from that normally observed.It is shown that the plasmapause can guide both the NAA and GBR signals but that the efficiency of this guiding depends on the plasmapause position. The important condition is found that the plasmapause must be situated sufficiently equatorwards that half the equatorial electron gyrofrequency at the plasmapause position is greater than (or approximately equal to) the transmitter signal frequency. Ray-tracing calculations in a realistic magnetosphere model indicate that for the 16 kHz GBR signal, the efficiency of guiding falls off for L_pp, (the L-value of the plasmapause) > 3.0 and guiding effectively ceases for L_pp > 3.5.Guidance by the plasmapause results in a wave-field at higher latitude than for non-guided propagation. This will only occur when, following geomagnetic storms, the plasmapause position is at a sufficiently low L-value. This is in agreement with the experimental observations of anomalously high latitude signal reception following strong magnetic storms (K_p ≥ 4+).     

The maintenance of the night-time ionosphere at mid-latitudes. II. The ionosphere above St. Santin

The total rate of recombination in the night-time ionosphere above St. Santin (at L = 1.8) was estimated using a model atmosphere and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was measured but at the latitude of St. Santin this was always small and averaged over the night as a whole the contribution was negligible. Downward diffusion provided the main source of plasma and the flux predicted was compared with the flux measured at 450 km. The comparison was good provided the model atmosphere was modified to use exospheric temperatures based on actual measurements by the incoherent scatter radar. A comparison with the results obtained at Malvern (Paper I) confirmed that the saturation time for the protonosphere at L = 1.8 is far less than at L = 2.6 and that the downward flux from the saturated protonosphere was also less.     

Average electron density profiles in the plasmasphere between L = 1.4 and 3.2 deduced from whistlers

985 whistlers recorded at Tihany, Hungary (L = 1.9) between December 1970 and May 1975 have been processed for equatorial crossing radius L, equatorial electron density n_eq and tube electron content N_T. The obtained L values lie in the range L = 1.4-3.2. The median values of n_eq and N_T as a function of L fit well to the density profiles published by Park et al. The results also indicate that at lower L values the whistlers are ducted by stronger density enhancements.     

The nocturnal intermediate layer over South Georgia : solar and magnetic influence on occurrence

Ionograms from South Georgia (54°S, 37°W; L = 1.9) are used to investigate the influence of magnetic and solar activity upon the occurrence of the nocturnal intermediate layer. The diurnal variation in occurrence of this layer exhibits a peak at about 2000 LT and a subsidiary peak after midnight. It is shown that magnetic activity has no significant influence on the behaviour before midnight, but is positively correlated with the size of the post-midnight peak. The effect of varying solar activity is to introduce a local time shift (42 min for a change in mean solar activity from 80 to 130× 10⁻²² Wm⁻²Hz⁻¹)inthediurnal variation without otherwise changing the overall morphology of the layer significantly. Most of the features of the intermediate layer before midnight can be explained by a wind shear mechanism associated with the solar semi-diurnal tide. Some possible causes for the post-midnight observations are considered, but no firm conclusions can be made.     

Unintentional man-made modification effects in the magnetosphere

Evidence continues to accumulate that even very small input signals, such as power-line harmonic radiation (PLHR), can give rise to wave-particle interactions in the magnetosphere by stimulation of emission at VLF and contribute to the formation of the electron slot (2<L<3). Over North America and its geomagnetic conjugate, the satellite data indicate a permanent zone of VLF emission which appears to originate in the industrial areas of northeastern U.S.A. and southern Canada. The more intense stimulated emission in the summer (northern hemisphere) occurs poleward of the electron slot at 3<L<5. The resultant increased electron precipitation may explain the increase in thunderstorms at L>4 in the period 1935–1970 relative to 1900–1935. There is probably an intensity threshold at which PLHR becomes important. This is consistent with ground-based data from longitudinally separated stations near L = 4 (Siple, Halley, Yakutsk). The discovery that certain industrial plants (e.g. cement works) give rise to relatively much more intense PLHR requires further investigation. Also, geomagnetically induced currents in long power lines at high geomagnetic latitudes in Canada and Alaska can result in transformer saturation and possible damage, as well as greatly increased PLHR.Powerful VLF transmitters in the range 15–25 kHz can produce peaks in the spectra of energetic electrons observed on low altitude satellites. Morphological studies of the NAA wavefield intensity above the ionosphere show strong amplification immediately to the east of the South Atlantic Anomaly.     

Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, σ_v, less than ∼5 × 10⁻¹³ m⁻¹, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (∼50 m/s per 10dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (∼200 m/s per 10dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.     

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.     

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.     

Identification of the magnetospheric cusp and cleft using Pc1–2 ULF pulsations

Geomagnetic pulsations in the 0.1–2.5 Hz (Pc1–2) range recorded over 12 quiet summer days at six Antarctic stations between −62.3 and −80.6° invariant latitude were examined in order to map the spatial and temporal distribution of spectral characteristics. Ionospheric particle signatures associated with the magnetospheric cusp and boundary layer were deduced for three of these days using ground riometer, magnetometer and ionosonde measurements, and in-situ ionospheric particle data. Comparison with the magnetic pulsation data shows that specific Pc1–2 emissions are associated with these regions. Within the cusp, intense unstructured ULF noise in the 0.15−0.4 Hz range is observed. Less intense waves of this type are seen near the cusp location on mantle and plasma sheet boundary layer flux tubes. These emissions are quite distinct from the discrete, structured and narrowband emissions seen equatorward of the cusp. Whereas past discussions of cusp and cleft identification have usually focused on optical or satellite data, we conclude that ground-based observations of Pc1–2 pulsations can provide a more convenient, although less precise, monitor of high latitude features.     

A study of gravity waves in ionospheric electron content at L = 4

Using satellite radio beacon transmissions, travelling ionospheric disturbances have been observed in the electron content at L = 4. Waves are a common feature at this latitude, present for at least 98% of all daylight hours. The amplitude is usually 1–4% of the mean electron content and periods range between 15 and 90 minutes. Simultaneous observation of two satellite beacons, giving an effective east-west separation of 350 km, indicated apparent east-to-west velocities of 200–700 m/s.A search was made for a likely source of the waves, using data from magnetometers and riometers, from incoherent scatter radar measurements of Joule heating, and from orbiting satellite measurements of electron influx, but no definite source could be established.It is also shown that travelling disturbances are closely related to occurrences of spread-F on ionograms at high latitudes.     

Storm-time variations of plasmaspheric ELF hiss

ELF and VLF observational data obtained from Intercosmos-3 (August 1970–November 1970) and Intercosmos-5 (December 1971–March 1972) satellites have been analyzed. During geomagnetic storms, the generation region of plasmaspheric hiss, which is usually located near the equatorial plane just within the plasmasphere, tends to move inwards, similar to the inward motion of the plasmapause. At any phase of the storm, the source region of the plasmaspheric hiss is found in the vicinity of the slot between the two radiation belts and the inner edge of the outer radiation belt. So the storm displacement of the source region is related to the inward diffusion of the radiation belt particles (Hess, 1968). In the main phase of the storm the relative change in the L-coordinate of the generation region of plasmaspheric hiss with respect to Dst variation is greater by one order of magnitude than that observed during the storm recovery phase. The storm variation of amplitude of the plasmaspheric hiss (in dB) is proportional, on the average, to Dst. These conclusions are in good agreement with the self-consistent theory of ELF hiss generation (Etchetoet al., 1973).     

Day-time Pi pulsations at equatorial latitudes

Low latitude Pi2 pulsations are considered to be the best indicators of the onset of magnetospheric substornis (Rostoker and Olson, 1978; Saito, 1979) and are hitherto believed to be mainly night-time phenomena. It is seen from this study utilising the pulsation records from Choutuppal (geomagnetic: 7°.5, 149°.3 E)and Etaiyapuram (geomagnetic: –0°.6.147°.5 E)and the “Common Scale Magnetograms” from the Auroral Electrojet (AE) stations during January–April 1976, that Pi2s do appear even during day-time on many occasions at equatorial latitudes in simultaneity with the onset of magnetospheric substorms at AE stations located in the night hemisphere. It is also found that the day-time Pis, unlike the night-time Pi2s, show enhancement in their amplitudes of Hx component at Etaiyapuram, situated at the dipequalor as compared to those at Choutuppal, well away from it. The results thus not only show the appearance of Pi pulsations during daytime in the equatorial zone, but also bring out the possible influence of the equatorial electrojet on their amplitudes at the dip equator.     

Morphology of low-frequency waves in the solar wind and their relation to ground pulsations

Three classes of low frequency waves (period range 20–80 s) were identified using data from the UCLA fluxgate magnetometer experiment on board the ISEE 2 spacecraft. These are continuous pulsations similar in type to Pc 3, band-limited oscillations distinguished by mixed period fluctuations, and relatively isolated wave bundles. The waves were preferentially observed when the interplanetary magnetic field (IMF) direction was sunward and were most common when the cone angle, i.e. the angle between IMF and the Sun-Earth line (θ_xb) was often between 15° and 45°. Their frequency is proportional to the IMF magnitude.Comparison between the waves observed on board the ISEE 2 spacecraft and the Pc 3–4 recorded simultaneously at a mid-latitude ground station, Oulu (L = 4.5), showed that similarity of spectra of the waves in the spacecraft and on the ground was very rare and that correspondence between the events in space and on the ground was extremely low.     

Electric field sources in the quiet plasmasphere from whistler observations

Existing evidence for the ionospheric dynamo being the source of quiet time electric fields in the plasmasphere is reviewed. Part of a 24 h set of whistler data recorded continuously at Sanae, Antarctica (L = 4), during quiet magnetic (average K_p = 1) is analysed to obtain westward electric fields in the equatorial plane. These electric fields are examined as a function of L-value in order to infer their source. It is found that for periods of outward flow of plasma during the noon-midnight local time period, the electric fields are consistent with the dominant source being the ionospheric dynamo. There is some evidence that during the evening period of inward flow the electric fields are magnetospheric in origin, although this could also be consistent with a refined dynamo model. The observed whistler duct convection patterns do not fit either of two theoretical models, which invoke a magnetospheric field but not a dynamo field.     

About the origin of high energy electrons in the inner radiation belt

A large flux of > 100 MeV electrons were registered in the inner radiation belt on low-altitude satellites. The origin of that flux is discussed. It appears that slow radial diffusion (D_o = 10⁻¹³ 1/s) gives a low probability for penetration of these electrons to small L from the boundary of magnetosphere because of synchrotron radiation energy losses. It is found that they can enter to the inner belt region without such losses after great magnetic storms when high speed radial diffusion sometimes takes place. Two great storms on 8–9 Feb.] 986 and 24 March 1991 are examples when one can directly observe a penetration of energetic electron fluxes into magnetosphere. The assumption about their Jovian origin is discussed.     

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.