The Earth’s ionosphere is forced by Sun and the magnetosphere (i.e by space weather and climate) from above and by “meteorological” (neutral atmosphere) processes from below. The forcing from below means predominantly forcing by atmospheric waves, namely by planetary, tidal, gravity and infrasonic waves.
Method of the acoustic-gravity wave (AGW) detection has been developed and successfully applied on the high sampling ionospheric sounding campaigns performed during solar eclipses events (11 August 1999, 3 October 2005 and 29 March 2006). Our analysis reveals that ionosphere reacts differently during annular and total solar eclipses (Šauli et al., 2006a, Šauli et al., 2007, Jakowski et al., 2008). The type of the eclipse is more important than the solar disc coverage itself. AGW waves generated/excited by total solar eclipse were found to be present in the ionospheric plasma already during initial phase. Using wavelet based tool all the detected structure are further analysed to obtain all propagation characteristics.
A 2-D AGW detection toolbox is available on the webpage: http://www.ufa.cas.cz/html/climaero/sauli.html. It allows to detect gravity waves and analyse their characteristics in 2-D. A new generalized 3-D algorithm and toolbox has been developed, but it needs for proper work the electron density profiles of spatial-temporal coverage and quality expected to be provided by radio occultation measurements with the future Galileo system.
Analysis of the rapid sequence sounding (5 minute) for several consequent days over two distant observatories shows gravity wave activity enhancements for periods of morning and evening passages of the solar terminator with the time lag corresponding to the local time shift between observatories (Šauli et al., 2006b). The morning enhancement has been established as a very regular feature of the ionosphere over Central Europe in general.
A review of effects of infrasonic waves on the ionosphere with special emphasis paid to recent advances and problems was published (Krasnov et al., 2006). Infrasound investigations dealt predominantly with five topics, transient peculiar phenomena on the infrasound time scales (Chum et al., 2006), modelling of ionospheric infrasound (Krasnov et al., 2007), studies of the effect of infrasound of meteorological origin on the ionosphere (Šindelářová et al., 2008), effects of geomagnetic micro-pulsations in the infrasound period range, and infrasound associated with solar eclipses (Šauli et al., 2006).
Two types of transient peculiar phenomena were observed, S-shapes and oblique quasi-linear shape (QLS) traces. S-shapes occur predominantly near sunrise and sunset, therefore they are mostly related to solar terminator. They are thought to be caused by concave disturbances of the reflecting level in the ionosphere. All most distinct QLS events, reported for the first time by us, have been observed during late evening or early night-time hours. A typical QLS has a frequency span around 10 Hz, duration of about 20 s and a slope about 0.4-0.5 Hz/s. We excluded/discarded several potential sources of QLS events such as aircrafts, satellites, bolides, meteors, meteorites, thunderstorms, or geomagnetic storms.
Model calculations show that a sinusoidal signal launched at or near the surface is destroyed by nonlinear processes during its upward propagation; it transforms into two, initial and final, impulses. The location of the “transformation region” depends on period; shorter periods deposit energy in much narrower and much lower located height intervals. The acoustic waves can heat the upper atmosphere and, thus, thermally affect the ionosphere.
Strong active weather systems influence the ionosphere mainly through the upward propagating waves. In Central Europe, the ionospheric infrasound has so far been observed only during exceptionally severe tropospheric weather events; waves of periods ~2.5 to 5 minutes dominated. During convective storms of lower intensity, clear signatures in the infrasonic range were not revealed. Our results show that in general, the effect of severe convective storm on the ionospheric bottomside F region electron density is in the range of 20%, which equals to mean diurnal variability.
Ionospheric oscillations in the infrasound period range may be caused by geomagnetic micro-pulsations. Together with the Department of the Upper Atmosphere we run and scientifically utilize Doppler system for ionospheric measurements, described at http://www.ufa.cas.cz/html/upperatm/M_Doppler_system.pdf . As we found, this is the case when they are observed simultaneously on all five Doppler measuring paths of our new monitoring system, otherwise the oscillations are attributed to infrasound; this separation is supported by micro-pulsation observations at nearby observatory Budkov. New system of three ground-based microbarographs, installation of which finished in June 2008, should confirm this statement.
A campaign of one-minute separated ionosonde measurements during the solar eclipse of 11 August 1999 provided data which allowed detect distinguished long-period acoustic mode waves excited by the eclipse (even though in gravity wave mode the excited waves were substantially stronger). Our field Doppler measurements in Spain during the solar eclipse of October 2005 confirmed existence of infrasonic waves excited in the ionosphere by solar eclipses.
In recent years we have focused on investigations of persistency of planetary waves, which occur in the form of bursts of several wave cycles, not as a permanent or long-term oscillation. Our results show that the persistency of planetary wave type oscillations in foF2 (i.e. in maximum ionospheric electron density) in northern middle latitudes is very similar in Europe, northern U.S.A., and northern Japan, typically 4 wave cycles for the 5-day wave, for the 10-day wave, it is rather 3.5 wave cycles, and for the 16-day wave, the typical persistence is no more than 3 wave cycles. In terms of the number of wave cycles in the planetary wave type events, the persistence decreases towards longer periods. However, the persistence of wave events in terms of days increases towards longer periods. There is a large temporal and partly spectral variability of the planetary wave type activity. The longitudinal size of the planetary wave type events increases with increasing period, making the 5-day and 10-day period events in Europe, America and Japan essentially dissimilar, and the 16-day oscillations much more similar among the three regions. The spectrum of event duration is very broad. The character of the spectrum does not allow predict the duration of an event when we observe its beginning or, say, first 2-3 wave cycles. While the typical persistence of the planetary wave type oscillations in foF2 and the lower ionosphere over Europe is similar, the correspondence of occurrence of individual events is rather poor.
Our investigations of effects of strong-to-great geomagnetic storms on the ionospheric F1 region over Europe Burešová (2005): (1) Independent of the sign of the geomagnetic storm effect on NmF2, the effect on electron density at the F1 region heights for European higher middle latitudes is negative, if any at all; at European lower middle latitudes (Arenosillo, Athens) the effect is weaker and less regular. (2) There is a substantial summer/winter asymmetry of storm effects in the F1 region electron density. (3) Geomagnetic super-storm (Dst < -300 nT) effects penetrate deeper into the ionosphere than the effects of strong storms (-200 < Dst < - 100 nT). (4) The maximum of the storm effect may occur sometimes well below the height of the F region maximum electron density.
Burešová et al. (2007) summarized manifestations of strong geomagnetic
storms in ionospheric botomside F region above Europe. Analysis of the
stormy ionosphere behaviour above middle latitudes shows that
storm-induced variations of the F2 region ionisation during storm main
phase often change from large enhancements (positive phase) to depletions
(negative phase). Such a change of sign of the storm effect makes a
systemic description and prediction of the disturbed ionosphere rather
complicated. The results show that the changeover from one type of the
effects to the other is more common for winter than for summer, and the
occurrence of such behaviour increases with decreasing latitude.
Two significant effects on the ionosphere during the superstorm of November 2003 have been observed over Europe (Blanch et al., 2005): (1) Strong auroral E layer was observed at latitudes as low as 37°N. (2) The presence of two thin belts; one of enhanced and other of depressed total electron content (TEC), both over the mid-latitude European evening sector.
Investigations of pre-storm enhancements of foF2 (Burešová and Laštovička, 2007) revealed their occurrence frequency to be 20-25% for strong storms. They occur both day and night. They tend to appear more often in summer half of the year. They seem to be absent under solar cycle maximum conditions. The pre-storm enhancements do not exhibit a systematic latitudinal dependence and are not accompanied by a corresponding change in hmF2. They are confined to F2-region altitudes; they are not observed in E and F1 regions. Their longitudinal extent at middle latitudes seems to be typically 120-240 degrees of longitude. Several potential sources of the pre-storm enhancements were excluded: solar flares (they can only occasionally strengthen the effect), soft particle precipitation in dayside cusp, magnetospheric electric field penetration, auroral region activity (AE index), and Mikhailov’s quiet-time F2-layer disturbances. However, the origin of pre-storm enhancements remains uncovered.
IRI-2001 model has still large discrepancies for ionospheric F region bottomside parameters B0, B1 and D1. Local Model (LM) has been developed for Ebro (40.8°N, 0.5°E) to improve predictions of the above parameters (Blanch et al., 2005). Model validation shows that the LM provides more reliable variation of the analysed bottomside parameters than IRI-2001. At mid-latitudes and under quiet ionospheric conditions LM improves the IRI-2001-predicted B0 and B1 by a factor of two and D1 by a factor of three (Altadill et al., 2008).
Quality of the empirical STORM model and the effectiveness of the IRI-2001-predicted electron density (N(h) profile updating with real-time measurements have been tested for severe geomagnetic storms over Europe. The IRI-2001 model with STORM option generally describes better the distribution of NmF2. Nevertheless, the model not always estimates correctly the phase and the magnitude of intense storm effects in the daytime NmF2. The IRI-2001 model does not enable storm correction of hmF2. Therefore, updating IRI model with the near-real time measured ionospheric parameters makes resulting N(h)-profile more realistic (Burešová, 2005; Burešová et al., 2006; Stanislawska et al., 2004).
Since January 2004 Digisonde DPS4 provides routine ionospheric F-region drift measurements in the Průhonice Observatory (typically with 15 minutes sampling rate). Ionogram autoscaling process “ARTIST” automatically finds the F-region critical frequency foF2, from which convenient sounding frequencies are calculated for F-region drift measurements – autodrift regime.
Since May 2005, the Pruhonice Digisonde also measures E-region drifts every 15 minutes, using four fixed frequencies between 2.0 and 2.6 MHz. On the contrary to the autodrift setting, E region sounding frequencies do not depend on critical frequency: they are set and fixed for all the measurements. During summer 2006 the first special campaign for monitoring drifts in Es layer was performed. Drift-measurement on a higher sounding-frequency window 3.2-4.7 MHz was run every 15 minutes in addition to the standard E-region drift measurement. E-region drift measurement with two frequency-window setting represent an important source of information about the dynamics of the E region ionosphere and bring new pieces of information about sporadic E layer formation and its behavior. Differences of the plasma motion confirm different dynamics of E and Es layers.
We included a newly developed quality control - skymap point selection method (Kouba et al., 2008) to plasma drift evaluation. The method consists of a three-step selection of skymap points and application of the standard DDA algorithm on the corrected skymaps: (i) robust height range selection, (ii) setting limits on the Doppler frequency shift, and (iii) setting limits on the echo arrival angle. This selection method guarantees a better quality of obtained drift velocities.