Ioanna Tsagouri

Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms

The morphology of middle latitude ionospheric disturbances in response to geomagnetic storms has been investigated to determine the phenomenological differences between positive and negative ionospheric storm effects, using foF2 observations from azimuthal chain of stations. To better organize the disturbance signatures, two ionospheric indices were introduced to describe the maximum positive (Dfu- index) and negative (Dfl-index) deviation observed during an ionospheric storm. A systematic appearance of nighttime positive effects was determined with a 24-hour recurrence. The thermospheric-ionospheric view associated with positive and negative storm effects proposed by Prolss (1993) was extended to encompass our observations. This test clearly demonstrates that such a model can capture most of the basic aspects of ionospheric storms, nevertheless the prominent feature of large nighttime enhancements in the ionization density have yet to be explained.

Time evolution of cosmic-ray intensity and solar flare index at the maximum phase of cycles 21 and 22

Abstract Analysis of the time series into trigonometric series allows the investigation of cosmic-ray (CR) intensity variations in a range of periodicities from a few days to 1 year. By this technique the amplitude and the phase of all observed fluctuations can be given. For this purpose, daily CR intensity values recorded at Climax Neutron Monitor station for the time intervals 1979–1982 and 1989–1991, which correspond to the epochs of maximum activity for solar cycles 21 and 22, respectively, have been studied. The data analysis revealed the occurrence of new periodicities, common or not, in the two solar maxima. A search of our results was done by a power spectral analysis determining independently possible systematic periodic or quasi-periodic variations. Based on the fact that during these maxima the CR intensity tracks the solar flare index better than the sunspot number, the same analysis was performed on these data, which are equivalent to the total energy emitted by the solar flares. Both analyses result in periodicities with different probability of occurrence in different epochs. Occurrence at peaks of 70, 56, 35, 27, 21 and 14- days were observed in all time series, while the periods of 140–154 and 105 days are reported only in the 21st solar maximum and are of particular importance. All of the short-term periods except of those at 27 and 154-days are recorded for first time in CR data, but they had already been observed in the solar activity parameters. Moreover, each parameter studied here has a very different power spectrum distribution in periods larger than 154 days. The possible origin of the observed variations in terms of the CR interaction in the upper atmosphere and the solar cavity dynamics is also discussed here.

The normal state of the F2 ionospheric layer at high latitudes

Abstract The use of monthly median values to determine the normal level of the undisturbed F2 layer, leads often to inconsistent results regarding the type of ionospheric disturbance and quietness as well. Recently Belehaki (2000) proposed an hourly local ionospheric index f n F2 to define the normal level of the F2 layer at middle latitudes. In this paper we apply the f n F2 index to observations from a high latitude station, where geomagnetic activity strongly influence the temporal and spatial behaviour of the ionospheric layers, for investigating and testing the usefulness of the f n F2 index on a planetary scale. From the analysis that follows, it was revealed that during intervals of low magnetospheric activity, f n F2 index shows a very good fit to the observed f o F2 parameter, comparing to the monthly median f o F2 behaviour. On the other hand, enhanced magnetospheric activity does not affect the f n F2 index, which daily variation is a function of the sunspot number itself. In summary, the f n F2 index can be used as a measure of the undisturbed F2 layer at middle and high latitudes as well, caution though is needed in the definition of the quiet magnetospheric intervals, which for the needs of this work it was approached as a function of the aa -index for middle latitudes, whereas the AE -index was used for auroral latitudes.

Pilot Ionosonde Network for Identification of Traveling Ionospheric Disturbances

Travelling Ionospheric Disturbances (TIDs) are the ionospheric signatures of atmospheric gravity waves (AGWs). Their identification and tracking is important because the TIDs affect all services that rely on predictable ionospheric radio wave propagation. Although various techniques have been proposed to measure TID characteristics, their real-time implementation still has several difficulties. In this contribution, we present a new technique, based on the analysis of oblique Digisonde-to-Digisonde (D2D) “skymap” observations, to directly identify TIDs and specify the TID wave parameters based on the measurement of angle-of-arrival, Doppler frequency, and time-of-flight of ionospherically reflected high-frequency (HF) radio pulses. The technique has been implemented for the first time for the Net-TIDE project with data streaming from the network of European Digisonde DPS4D observatories. The performance is demonstrated during a period of moderate auroral activity, assessing its consistency with independent measurements such as data from auroral magnetometers and electron density perturbations from Digisondes and GNSS stations. Given that the different types of measurements used for this assessment were not made at exactly the same time and location, and that there was insufficient coverage in the area between the AGW sources and the measurement locations, we can only consider our interpretation as plausible and indicative for the reliability of the extracted TID characteristics. In the framework of the new TechTIDE project (European Commission H2020), a retrospective analysis of the Net-TIDE results in comparison with those extracted from GNSS TEC-based methodologies is currently being attempted, and the results will be the objective of a follow up paper.

Magnetosphere energetics during substorm events: IMP8 and GEOTAIL observations

Abstract Magnetospheric energetics during substorm events is studied in this paper. Three events were selected, a weak substorm, a large isolated one and finally a prolonged period of substorm activity with multiple intensifications. It is assumed that the energy, that entered the magnetosphere due to electromagnetic coupling with the solar wind, is described by the epsilon parameter, proposed by Perreault and Akasofu (1978) . High resolution, magnetic field and plasma data from the MGF and LEP experiments on board GEOTAIL were analyzed to determine the timing of plasmoid release, its dimensions, its convection velocity and finally the energy carried by each plasmoid. Plasmoids were defined as structures with rotating magnetic fields and enhanced total pressure. Tailward plasmoid bulk speed in the distant tail varied from 350 to 750 km / s . Their dimensions in XGSM direction was found to be from 4.5 to 28Re, and their duration did not exceed 5 min . The average energy carried by each plasmoid in the distant tail was estimated to be equal to (2.4±1.0)×10 14 J . The thermal energy flux dominates when the plasmoid structure is a closed loop. In cases of flux rope plasmoids the kinetic energy is comparable to or even greater than the thermal energy flux. The study of the overall magnetospheric energetics yielded the result that the input solar wind energy, estimated by the integral of the epsilon parameter over the lifetime of a substorm, represents the energy stored in the magnetotail and the convective dissipation in the ionosphere as well. The energy required for the plasmoid release is thought to be provided by the night side reconnection in the near-Earth neutral line location. Finally, it was found that 20% of the total input solar wind energy is dissipated in the auroral ionosphere.