C. Haldoupis

Observations of the relationship between sprite morphology and in-cloud lightning processes

[1] During a thunderstorm on 23 July 2003, 15 sprites were captured by a LLTV camera mounted at the observatory on Pic du Midi in the French Pyrenees. Simultaneous observations of cloud-to-ground (CG) and intracloud (IC) lightning activity from two independent lightning detection systems and a broadband ELF/VLF receiver allow a detailed study of the relationship between electrical activity in a thunderstorm and the sprites generated in the mesosphere above. Results suggest that positive CG and IC lightning differ for the two types of sprites most frequently observed, the carrot- and column-shaped sprites. Column sprites occur after a short delay (<30 ms) from the causative +CG and are associated with little VHF activity, suggesting no direct IC action on the charge transfer process. On the other hand, carrot sprites are delayed up to about 200 ms relative to their causative +CG stroke and are accompanied by a burst of VHF activity starting 25–75 ms before the CG stroke. While column sprites associate with short-lasting (less than 30 ms) ELF/VLF sferics, carrot sprites associate with bursts of sferics initiating at the time of the causative +CG discharge and persisting for 50 to 250 ms, indicating extensive in-cloud activity. One carrot event was found to be preceded by vigorous IC activity and a strong, long-lived cluster of ELF/VLF sferics but lacking a +CG. The observations of ELF/VLF sferic clusters associated with lightning and sprites form the basis for a discussion of the reliability of lightning detection systems based on VHF interferometry.

A study of tidal and planetary wave periodicities present in midlatitude sporadic E layers

The diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic E layers, acting through their vertical windshear forcing of the long-living metallic ions in the lower thermosphere. Also, recent studies suggested that planetary waves play a role on sporadic E generation as well, a fact that went unnoticed in the long-going research of sporadic layers. In this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic E critical frequencies foEs. In this approach, standard analysis techniques used in neutral atmospheric dynamics are applied on foEs time series obtained during summertime when sporadic E occurrence is nearly continuous. It is shown that besides the dominant and known 24-hour and 12-hour tidal periodicities in foEs, there is often a weaker terdiurnal (8-hour) oscillation present as well. In addition, there are planetary wave periodicites in foEs with periods near the normal Rossby modes, that is, 2, 5, 10, and 16 days. It is also found that the tidal oscillations in foEs undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data. Our results are in line with recent findings based on a single event study which suggested that sporadic E layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes. The close relationship between neutral wave dynamics and midlatitude sporadic E periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere.

Observation of the modified two‐stream plasma instability in the midlatitude E region ionosphere

We present the first clear evidence of the occurrence of the modified two stream (Farley-Buneman) instability and excitation of pure type 1 irregularities in the midlatitude ionospheric E region. The observations are made with a bistatic 50-MHz Doppler radio experiment set up recently in Crete, Greece. The system can perform high-frequency resolution coherent backscatter measurements along a fixed direction, from 3-m magnetic aspect sensitive irregularities inside a limited ionospheric volume in the E layer at the invariant geomagnetic latitude of 30.8° (L = 1.35). The observations presented here are from an event of backscatter characterized by large Doppler motions caused, presumably, by an impulsive electric field reaching a magnitude at least 14 mV/m. Apparently, the unusually high electron drifts along the radar viewing direction were sufficient in this case to excite pure Farley-Buneman waves. This had been manifested convincingly by the measured power Doppler spectra, which are reminiscent of the typical spectral signature of type 1 echoes observed regularly in the equatorial 50-MHz backscatter. Further, the spectral data confirmed the anticipation that in the midlatitude E region plasma exist two irregularity types corresponding to those of type 1 and type 2 echoes in the equatorial electrojet. The important difference with the equatorial results, however, is that the threshold conditions for the two-stream instability are seldom met at moderate latitudes, thus the medium is highly suitable for studying secondarily generated short-scale plasma turbulence.

An explanation for the seasonal dependence of midlatitude sporadic E layers

[1] The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory of E s formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long-term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause-and-effect explanation for the long-going mystery of sporadic E layer seasonal dependence.

Large polarization electric fields associated with midlatitude sporadic E

Recent 50 MHz E region coherent backscatter observations and in situ rocket measurements established the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley-Buneman instability. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field-aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridional polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated Es layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley-Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter.

Evidence for planetary wave effects on midlatitude backscatter and sporadic E layer occurrence

In this paper a large database of midlatitude E region coherent backscatter, obtained with a 50 MHz Doppler system, is used to investigate the long-term variability in echo occurrence. The backscatter is found to be dominated by pronounced quasi-periodic variations with periods in the range from about 2 to 9 days that persist for time intervals from about 10 to maybe more than 20 days and have no relation to geomagnetic activity. The most commonly observed periods appear in two preferential bands, that is, the 2 to 3 day and the 4 to 6 day band. Using concurrent ionosonde data we find the variations in backscatter to be exactly in-phase with similar periodicities in the occurrence of relatively strong sporadic E layers. The present findings support the possibility that planetary waves are responsible for the observed long-term periodicities which indicates also a close relation between planetary waves and the well known, but not well understood, seasonal Es dependence. We suggest, the planetary wave option constitutes a new component into the research of midlatitude sporadic E layer formation and occurrence that needs to be considered and closely investigated.

Midlatitude E region plasma accumulation driven by planetary wave horizontal wind shears

Recent findings suggested the possibility that planetary waves play a role in the occurrence of midlatitude sporadic E layers. To account for this, we propose here a new mechanism for large-scale accumulation of metallic ions in the midlatitude E region ionosphere driven by planetary waves in the lower thermosphere. In this process, the plasma is forced to converge horizontally and accumulate inside areas of positive vorticity set up by cyclonic neutral wind shears within a planetary wave. In its simplest form, the proposed model is similar to the well-known vertical wind shear mechanism of Es formation, but with the geometry “turned on its side.” Because of the long times required for ambipolar diffusion, the new mechanism can lead to significant plasma accumulation, acting as complementary to the vertical wind shear process so that dense Es can form more efficiently and frequently. The present model provides a physical base for understanding the long-term periodicities in occurrence and also the seasonal dependence of strong sporadic E layers at midlatitude.

A Tutorial Review on Sporadic E Layers

The sporadic E layers (Es) form in the dynamo region of the ionosphere when metallic ions of meteoric origin are converged vertically in a wind shear. This paper provides a comprehensive update on sporadic E, a topic that has been studied for many years. The aim is to render useful information and physical understanding for both the general and specialized reader, and construct an integrated picture of sporadic E through a critical synthesis of recent findings. The basic aspects of the layer windshear theory are reviewed and then selected observations are presented which are tested against the theoretical predictions. The emphasis is placed on the tidal wind control of the diurnal and semidiurnal variability and altitude descent of sporadic E layers. There is now enough evidence to suggest that mid- and low-latitude sporadic E is not as “sporadic” as the name implies but a regularly occurring ionospheric phenomenon. This suggests that E layer physics could also be incorporated in existing atmosphere-ionosphere coupling models. Furthermore, the present review summarizes recent findings which provide physical insight into long-going problems and questions about the seasonal dependence and the global occurrence of Es. The experimental results, which are in favor of the windshear theory, imply that the key agents controlling sporadic E are: tidal wind atmospheric dynamics, the Earth’s horizontal magnetic field component, and the meteoric deposition of metallic material in the lower thermosphere.

Simultaneous 50-MHz coherent backscatter and digital ionosonde observations in the midlatitude E region

SESCAT, a coherent backscatter radar system located in Crete, Greece, was operated for one summer together with a (CADI) digital ionosonde observing nearly the same scattering volume. The purpose of the experiment was to further investigate the origin of midlatitude E region VHF echoes which occur almost exclusively during summer nighttime. It was found that 50-MHz midlatitude backscatter always occurs in association with sporadic E layers. A statistical analysis indicated significant correlations between SESCAT total echo power and E s characteristics such as the layers top frequency f t E s (a measure of maximum E s electron density) and the apparent E s trace spread which results from range spreading due to oblique reflections from a nonuniform and horizontally inhomogeneous layer. Similar correlations were obtained for SESCAT spectral width and the same sporadic E characteristics. The experiment confirmed that the presence of an E s layer in the scattering volume, which could provide destabilizing electron density gradients perpendicular to the magnetic field, is necessary but not sufficient for the occurrence of 50-MHz backscatter. We suggest that in addition there is need for an enhanced electric field to be present inside the layer as well, a notion that is in line with the observed correlation of backscatter with a dense but strongly inhomogeneous E s layer and a recently proposed mechanism for strong polarization fields at midlatitudes.

High‐frequency Doppler radar observations of magnetic aspect sensitive irregularities in the midlatitude E region ionosphere

In this paper, an experiment designed for dual-frequency azimuthal Doppler spectrum studies of decameter-scale field-aligned irregularities in the midlatitude E region ionosphere is introduced, and the first results are presented. The observations were made in July 1993 with a large HF radar facility near Valensole (43.8°N, 6.1°E), in the south of France. The radar system is an oblique ionospheric sounder that employes long antenna arrays of vertical monopole pairs and covers the HF frequency band from 4 to 30 MHz in 1-kHz steps. A scheme of broad beam width transmission and narrow-beam, phased-array, multireceiver coverage was used to scan with 2° step an azimuthal sector from about 24° east to 60° west of geomagnetic north. The 15-gate viewing region was confined in range between 100 and 370 km in order to cover an area of E region magnetic aspect sensitive backscatter near 37° invariant magnetic latitude (L ≃ 1.7, magnetic dip, ∼60°). In this configuration, each azimuthal scan was completed in 80 s over a 15×42 spatial grid with the full Doppler spectrum at each grid point recorded in real time. The experiment provided observations simultaneously at two frequencies, 9.0 MHz and 14.8 MHz, that correspond to backscatter from plasma waves with wavelengths of 16.7 and 10.1 m, respectively. Here, we present an overview of the observations that include azimuthal and range-time echo characteristics as well as mean Doppler shift and spectrum width properties. The first results show aspect sensitive decameter-wavelength irregularities having mean phase velocities at least as large as 120 m/s to act as tracers of wavelike dynamic structures that drift westward with speeds in the 40- to 80-m/s range and have characteristic times between 10 and 30 min and typical scale lenghts between 40 and 90 km. In our interpretation, we consider these structures to be sporadic Es ionization patches, possibly affected by the passage of atmospheric gravity waves, which are accompanied by intense horizontal electron density gradients and enhanced electric fields. Although secondary generation cannot be excluded entirely, the evidence supports the possibility that the gradient drift instability is the basic physical mechanism for direct generation of decameter aspect sensitive plasma waves in the midlatitude E region ionosphere.