Imaging coherent backscatter radar studies of equatorial spread F

A new interferometric imaging technique has been used at Jicamarca to study the morphology and dynamics of plasma irregularities occurring during spread F conditions. The technique produces two-dimensional, in-beam images of the coherent scatter from these irregularities, instantaneously mapping the structure of the underlying plasma instabilities in the equatorial plane. We present sequences of images depicting backscatter from bottom-type layers, bottomside layers, and topside plumes drifting and rising through the ionospheric regions illuminated by the Jicamarca radar. The high spatial and temporal resolution of the images permits us to observe dynamical phenomena missed by conventional fixed- and steered-beam techniques. The new images show evidence of intermediate- and large-scale plasma depletions bifurcating and pinching off from the bottomside as they ascend. Analytic models are presented to describe these dynamical effects. The images also differentiate between bottom-type and bottomside spread F layers, the former being shown to have much smaller scale primary waves than the latter. Ample evidence of secondary plasma instabilities appear throughout the images. The consequences of these secondary instabilities and of shear flow are discussed.     

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.     

E-region mid-latitude decametre irregularities observed at four radar frequencies. Experiment and first results

In this paper an experiment designed for multifrequency azimuthal Doppler-spectrum investigations of decametre-scale plasma irregularities in the midlatitude E region is introduced and some preliminary results are presented. The observations were made with the high-frequency Valensole radar in the south of France. The radar operated in a multifrequency mode that allowed simultaneous measurements of Doppler spectra at the four frequencies of 9.23, 11.03, 12.71 and 16.09 MHz, which correspond to scatter from fieldaligned irregularities with wavelengths of 16.2, 13.6, 11.8 and 9.3 m, respectively. In addition, a digital ionosonde was operating beneath a small part of the radar viewing region. The data show that lower-frequency echoes are stronger, more frequent and more spatially extended than higher-frequency ones, in general agreement with theory and rocket measurements. On the other hand, the preliminary analysis shows no pronounced differences of the Doppler spectrum with radar frequency. Some trends in the spectral moments do exist, however, which can be understood as the result of temporal and/or spatial mixing of backscatter from directly generated (primary) plasma waves by the gradient drift instability and secondary low-velocity waves. Finally, a close relation of mid-latitude coherent backscatter with patchy sporadic E-layers is present in the data, but the details of this relation remain unresolved.     

Scattered reflections and multiple traces in the range of 7–10 MHz on ionograms of the interkosmos-19 satellite

The scattered reflections and multiple traces regularly recorded on the topside sounding ionograms of the Interkosmos-19 satellite in the frequency range of 7–10 MHz are considered. The reflected radio signals in this frequency range appear both above and below the critical frequency of the regular layer F2. They are observed at all altitudes of the topside ionosphere from hmF2 to a satellite altitude of 1000 km. It is shown that these phenomena regularly appear at high latitudes (≥60° ILAT) and, less often, in the equatorial region. The scattered reflections indicate the presence of small-scale irregularities, and continuous traces are a consequence of total internal reflection from large-scale irregularities. Small-scale irregularities evidently form within a large-scale irregularity. Ray tracing shows that the size of large-scale irregularities is hundreds of kilometers in height and tens of kilometers in latitude. The appearance of scattered reflections and multiple traces at high latitudes is nearly independent of local time; in the equatorial region, they appear only in the interval of 20–08 LT. All of this agrees well with other observations of irregularities in the ionospheric plasma of different scales.     

Disturbances in the topside ionosphere during solar flares

The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O⁺ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma.     

Equatorial plasma bubbles at altitudes of the topside ionosphere

A consistent patter, indicating that subtroughs in the He⁺ density and plasma bubbles can be considered as phenomena of the same origin, has been obtained within the scope of the existent model of equatorial plasma bubbles. The study has been performed based on the measurements of the ISS-b satellite, which flew during the period of high solar activity. The conclusion has been made based on a comparative analysis of the characteristics of subtroughs with the parameters of the known equatorial phenomena. (1) The similarity of the LT variations in the latitude of the minimums of subtroughs in the He⁺ density has been revealed. (2) It has been displayed that the variations in the averaged depth of subtroughs change from season to season similarly to the LT variations in the average velocity of the equatorial vertical plasma drift. (3) Good correlation (R = 0.67) between the occurrence probability of subtroughs and equatorial spread F statistics, constructed as the functions of LT and month, has been obtained. (4) The obtained velocity of the possible rise of plasma irregularities (observed as regions depleted in He⁺) is in good agreement with the ionosonde, satellite, and radar measurements of the equatorial plasma bubble velocities of the same period. (5) It has been indicated that plasma irregularities, reaching the altitudes of the topside ionosphere in the low-latitude and midlatitude regions during high solar activity, are most observable as depleted regions (subtroughs) of He⁺ density.     

Ionospheric disturbances during the severe magnetic storm of November 7–10, 2004

Results of the study of the behavior of the F ₂ region and topside ionosphere during the magnetic storm on November 7–10, 2004, which was a superposition of two sequent Severe magnetic disturbances (Kp = 9–) are presented. The observations were conducted by the incoherent scatter radar at Kharkov. Considerable effects of a negative ionospheric disturbance are registered, including a decrease in the electron density in the F ₂-layer maximum by a factor of 6–7 and of the total electron content up to a height of 1000 km by a factor of 2, a lifting up of the ionospheric F ₂ layer by 300 km at night and by 150–180 km in the daytime, unusual nighttime heating of the plasma with an increase of the ion and electron temperatures up to 2000 and 3000 K, respectively, and a decrease in the relative density of hydrogen ions N(H⁺)/N _e by a factor of up to 3.5 because of the emptying of the magnetic flux tube passing over Kharkov. The effects usually observed in the high-latitude ionosphere, including the coherent echoes, are detected during the main phase of the storm. The results obtained manifest a shift of the large-scale structures of the high-latitude ionosphere (the auroral oval, main ionospheric trough, hot zone, etc.) down to latitudes close to the latitude of the Kharkov radar.     

Long term trends in the frequency of occurrence of the F3 layer over Fortaleza,Brazil

Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F₃ layer. The observations using bottomside ionograms from locations close to the magnetic equator in Brazilian region have shown that the occurrence of the layer is very high from December to February (local summer) and from June to August (local winter). In fact, for the year 1995 the occurrence of the F₃ layer is >75% during the months of January, February and December, and it is >65% for the period of June, July and August (Geofisica Int. 39 (2000) 57). In this work, we use 25 years of data for the months of January and August to investigate how the layer occurrence varies with the magnetic dip angle and solar activity.     

Asymmetric response of the topside ionosphere to large-scale IGW generated during the November 30, 1979, substorm

We used bottomside ground observations and topside sounding data from the Intercosmos-19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to Large-Scale Internal Gravity Wave (LSIGW) propagation during a substorm on November 30, 1979. We built a global scheme for the wavelike ionospheric variations during this medium substorm (AE_max ～800 nT). The area where the TID was observed looks like a wedge since it covers the nighttime hours at subauroral latitudes but contracts to a ～02 h local sector at low latitudes. The ionospheric response is strongly asymmetric because the wedge area and the TID amplitude are larger in the winter hemisphere than in the summer hemisphere. Clear evidence was obtained indicating that the more powerful TID from the Northern (winter) hemisphere propagated across the equator into the low latitude Southern (summer) hemisphere. Intercosmos-19 observations show that the disturbance covers the entire thickness of the topside ionosphere, from h_mF2 up to at least the 1000 km satellite altitude at post-midnight local times. F-layer lifting reached ～200 km, N_e increases in the topside ionosphere by up to a factor of ～1.9 and variations in N_mF2 of both signs were observed. Assumptions are made concerning the reason for the IGW effect at high altitudes in the topside ionosphere. The relationship between TID parameters and source characteristics determined from a global network of magnetometers are studied. The role of the dayside cusp in the generation of the TID in the daytime ionosphere is discussed. The magnetospheric electric field effects are distinguished from IGW effects.     

Spectral analysis of plasma drift measurements from the AE-E satellite: evidence of an inertial subrange in equatorial spread F

Plasma drift data from the AE-E satellite are spectrally analyzed to investigate the characteristics of the flow in the topside equatorial F region ionosphere during strong spread F conditions. Plasma flow around rapidly rising depletions is thought to exhibit behavior similar to two-dimensional Kolmogorov turbulence, but only on flux tubes with sufficiently small integrated ion–neutral collision frequencies. We find that one-dimensional spectra computed from vertical plasma drift measurements made in such depletions on such flux tubes tend to display a −5/3 spectral index over scale sizes from about 1 to $\text{100}\text{km}$, suggesting the operation of an inverse energy cascade. This universal spectral form is evidence of an inertial regime of the underlying ionospheric interchange instability.     

Interaction between plasma structures in an unstable ionospheric plasma

A vortex structure renders additional stability to plasma irregularities stretched along magnetic field lines. Plasma irregularities extended over several tens of kilometers are registered with rocket and satellite equipment in the topside ionosphere. The registered scale of irregularities depends on the spatial and time resolution of the equipment used. Irregular structures were registered in the ionosphere during experiments with barium clouds and jets, when a plasma irregularity separated into strata extended over several meters and several kilometers across the geomagnetic field. It has been indicated that plasma vortices can be generated in an unstable plasma in a situation when its quasi-neutrality is disturbed. Local geomagnetic field disturbances will be caused by the appearance of a proper vortex magnetic field. Plasma vortices can interact in an inhomogeneous plasma with an unstable electron component. Such interactions are related to the transformation of the phase volume of free electrostatic oscillations in the frequency-wave vector space.     

Radar observations of ionospheric irregularities at Syowa Station, Antarctica: a brief overview

We briefly overview the radar observations that have been made for 30 years at Syowa Station, Antarctica for studying small-scale electron-density irregularities in the southern high-latitude E- and F-region ionosphere. Some observational results (i.e., long-term variations of radio aurora, Doppler spectra with narrow spectral widths and low Doppler velocities, and simultaneous observations of radar and optical auroras) from VHP radars capable of detecting 1.3- to 3-m scale irregularities are presented. A new 50-MHz radar system equipped with phased-antenna arrays began operation in February 1995 to observe two-dimensional behaviors of E-region irregularities. An HF radar experiment also began in February 1995 to explore decameter-scale E- and F-region irregularities in the auroral zone and polar cap. These two radars will contribute to a better understanding of the ionospheric irregularities and ionospheric physics at southern high latitudes.     

Longitudinal statistics of plasma bubbles observed as He+ density depletions at altitudes of the topside ionosphere

This work presents a new examination of the hypothesis regarding the equatorial origin of low He⁺ density plasma depletions (or subtroughs). For this purpose, we have conducted a detailed comparative analysis of longitudinal variations in the occurrence probabilities of subtroughs in both hemispheres and variations in the occurrence probabilities of equatorial F-region irregularities (EFIs), equatorial spread F (RFS and ESF), and equatorial plasma bubbles (EPBs). Taking into consideration the seasonal dependence and some peculiarities of magnetic field variations in different hemispheres, a conclusion has been reached regarding the similarity between longitudinal statistical occurrences of subtroughs and equatorial ionospheric F-region irregularities. In addition, another piece of evidence in favor of the similarity of the nature of the above-mentioned phenomena has been obtained. We have got a confirmation once again that low He⁺ density depletions (or subtroughs) can be rightfully considered as equatorial plasma “bubbles,” which can be observed at altitudes of the topside ionosphere as depletions in the He⁺ density.     

A comparison of veloCity measurements from the CUTLASS Finland radar and the EISCAT UHF system

The CUTLASS Finland radar, which comprises an integral part of the SuperDARN system of HF coherent radars, provides near continuous observations of high-latitude plasma irregularities within a field-of-view which extends over some four million square kilometres. Within the Finland radar field-of-view lie both the EISCAT mainland and EISCAT Svalbard incoherent scatter radar facilities. Since the CUTLASS Finland radar commenced operation, in February 1995, the mainland EISCAT UHF radar has been run in common programme 1 and 2 modes for a total duration exceeding 1000 h. Simultaneous and spatially coincident returns from these two radars over this period provide the basis for a comparison of irregularity drift veloCity and F-region ion veloCity. Initial comparison is limited to velocities from four intervals of simultaneous radar returns; intervals are selected such that they exhibit a variety of veloCity signatures including that characteristic of the convection reversal and a rapidly fluctuating veloCity feature. Subsequent comparison is on a statistical basis. The velocities measured by the two systems demonstrate reasonable correspondence over the veloCity regime encountered during the simultaneous occurrence of coherent and incoherent scatter; differences between the EISCAT UHF measurements of F-region ion drift and the irregularity drift velocities from the Finland radar are explained in terms of a number of contributing factors including contamination of the latter by E-region echoes, a factor which is investigated further, and the potentially deleterious effect of discrepant volume and time sampling intervals.     

Atmospheric electricity coupling between earthquake regions and the ionosphere

We propose a mechanism to explain suggested links between seismic activity and ionospheric changes detected overhead. Specifically, we explain changes in the natural extremely low-frequency (ELF) radio noise recently observed in the topside ionosphere aboard the DEMETER satellite at night, before major earthquakes. Our mechanism utilises increased electrical conductivity of surface layer air before a major earthquake, which reduces the surface-ionosphere electrical resistance. This increases the vertical fair weather current, and (to maintain continuity of electron flow) lowers the ionosphere. Magnitudes of crucial parameters are estimated and found to be consistent with observations. Natural variability in ionospheric and atmospheric electrical properties is evaluated, and may be overcome using a hybrid detection approach. Suggested experiments to investigate the mechanism involve measuring the cut-off frequency of ELF “tweeks”, the amplitude and phase of very low frequency radio waves in the Earth–ionosphere waveguide, or medium frequency radar, incoherent scatter or rocket studies of the lower ionospheric electron density.     

Statistical observations of the MLT, latitude and size of pulsed ionospheric flows with the CUTLASS Finland radar

A study has been performed on the occurrence of pulsed ionospheric flows as detected by the CUTLASS Finland HF radar. These flows have been suggested as being created at the ionospheric footprint of newly-reconnected field lines, during episodes of magnetic flux transfer into the terrestrial magnetosphere (flux transfer events or FTEs). Two years of both high-time resolution and normal scan data from the CUTLASS Finland radar have been analysed in order to perform a statistical study of the extent and location of the pulsed ionospheric flows. We note a great similarity between the statistical pattern of the coherent radar observations of pulsed ionospheric flows and the traditional low-altitude satellite identification of the particle signature associated with the cusp/cleft region. However, the coherent scatter radar observations suggest that the merging gap is far wider than that proposed by the Newell and Meng model. The new model for cusp low-altitude particle signatures, proposed by Lockwood and Onsager and Lockwood provides a unified framework to explain the dayside precipitation regimes observed both by the low-altitude satellites and by coherent scatter radar detection.     

Equatorial electrojet plasma irregularities observed during late afternoon by the 30 MHz coherent scatter radar in São Luís,Brazil

Coherent scatter radar observations conducted right before twilight by the 30 MHz radar in São Luís (Brazil) during both quiet (December 10, 2001) and disturbed (December 12, 2001) conditions detected type I echoes on the shoulders of broad type II spectra (near zero Doppler velocity). Zonal electric fields inferred from the São Luís ionosonde have a larger magnitude during the period where these spectral observations were made than on December 18, 2001 when type I echoes were not observed during late afternoon. The IMF B_z and AE index indicate that the extended geomagnetic disturbance in the afternoon of December 12, 2001 has allowed the interaction between disturbance dynamo and prompt penetration electric fields. The latter electric field component could have exceeded the former to give rise to the observed electric fields, and make possible observation of type I echoes.     

电场产生的赤道扩展F的时空演变

利用计算机模拟研究了非均匀电场产生的赤道扩展Ｆ的时空演变。非均匀电场能在赤道电离层Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，导致等离子体泡形态。计算中非均匀电场的幅度取为０．２５－１．００ｍＶ／ｍ，所产生的等离子体泡在２０００ｓ以内就能穿过Ｆ峰达到５４０ｋｍ的高度。所得结果阐明了非均匀电场在赤道扩展Ｆ中的作用，指出了产生等离子体泡一种可能的扰动源。由于Ｅ区和Ｆ区电场是相互影响的，从而揭示了Ｅ区和Ｆ区扰动的相互联系。     

Effects of the solar eclipse of March 29, 2006, in the ionosphere and atmosphere

The observations of the effects of the partial (about 77%) solar eclipse (SE) of March 29, 2006, in the ionospheric plasma are presented. The experimental data were obtained using the Kharkov incoherent scatter radar. At the moment of the maximum phase of SE, a decrease in the critical frequency of the ionospheric F ₂ layer by 18%, a depletion of the density in the F ₂ layer maximum by 33%, and an increase in the maximum height z _m by 30 km were observed. The solar eclipse caused a decrease in the electron and ion temperatures by 150–300 and 100–200 K, respectively, within the height range 210–490 km. An increase in the relative density of the hydrogen ions during the maximum phase of SE by 20–25% within the height range 900–1200 km is detected. Calculations of the parameters of dynamical processes and thermal regime of the ionospheric plasma during SE are performed.