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Main research topics:
1) Physics of the Ionosphere
Forcing of the Ionosphere by atmospheric waves
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.
Ionosphere – gravity 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 of Petra Koucká Knížová. 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.
Ionosphere – infrasonic waves
Doppler type sounding of the ionosphere has been operated in collaboration with the Department of the Upper Atmosphere since 2004. This sounding technique enables observations of ionospheric phenomena down to periods of tens of seconds. Detailed description of the sounding system can be found here.
Effects of infrasonic waves on the ionosphere with special emphasis paid to recent advances and problems were reviewed by Krasnov et al. (2006). Krasnov et al. (2007) modelled propagation of quasi-sinusoidal infrasonic waves from near-surface sources to the upper atmosphere. 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.
Experimental studies dealt with acoustic-gravity waves occurring during the solar eclipse (Šauli et al., 2006), transient peculiar phenomena on the infrasound time scales (Chum et al., 2008), effects of geomagnetic micropulsations in the infrasound period range (Chum et al., 2009), observations of infrasound of meteorological origin in the ionosphere (Šindelářová et al., 2009), observations of ionospheric infrasound generated by extreme distant earthquake (Chum et al., 2012).
A campaign of ionosonde measurements during the solar eclipse of 11 August 1999 provided data in 1 minute resolution which allowed analysing wave activity in the acoustic-gravity wave domain. Long period acoustic waves excited by the solar eclipse were detected together with strong gravity waves. 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.
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 and therefore seem to be 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.
Strong active weather systems generate waves of a broad period range. Part of the emitted energy propagates upwards and may influence the upper atmosphere. Infrasonic waves generated by meteorological activity in the lower atmosphere have rarely been observed in the ionosphere above the Czech Republic. Waves of period 2.5-5 min occurred only during exceptionally severe tropospheric weather events - severe convective storms; windstorm of 18-19 January 2007 (“Kyrill”). During events of lower intensity, clear signatures in the infrasonic range were not revealed.
Ionospheric oscillations in the infrasound period range may be caused by geomagnetic micro-pulsations. 1-3 min ionospheric oscillations that occurred simultaneously on all Doppler sounding paths were highly correlated with Pi2 pulsation in the geomagnetic records. The Doppler signals were usually best correlated with the horizontal components of the geomagnetic field.
Ionospheric infrasound related with the Tohoku earthquake in March 2011 was observed. The analysis of ionospheric Doppler shift records together with seismic records from the Czech observatories revealed that the infrasound was most probably generated in situ from seismic waves that arrived from the epicentre.
The equipment for observations of atmospheric infrasound was completed in 2008-2010 when a network of five microbarographs was built. The microbarographs enable observations of infrasound in the troposphere and thus conveniently complete the Doppler sounder, particularly for the purpose of studies of vertical propagation of infrasound.
The microbarograph network in the Czech Republic consists of three sites at observatories Nový Kostel (50°13’N 12°26’E), Průhonice (49°59’N 14°32’E), and Panská Ves (50°31’N 14°34’E). Nový Kostel and Průhonice sites are equipped with one microbarograph each. An array of three sensors has been installed at Panská Ves. The sensors are arranged in a nearly equilateral triangle, the distance between the microbarographs is ~200 m. The microbarographs are of differential type – infrasound gage ISGM03. Optimal sensitivity is in the frequency band 0.02-4 Hz. Maximum accuracy of measurements is of the order of 10-3 Pa.
Case studies were conducted dealing with observations of tropospheric infrasound generated in the epicentral zone of weak earthquakes (Laštovička et al., 2010) and with infrasound emitted during nearby convective storms.
Ionosphere – planetary waves
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.
Forcing of the ionosphere by Space Weather
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.
Development of IRI (International Reference Ionosphere)
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).
Investigations of ionospheric drifts
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.
2) Global change in the upper atmosphere and ionosphere
In the upper atmosphere, greenhouse gases produce a cooling effect, instead of a warming effect. Increasing greenhouse gas concentrations induce changes in the mesosphere, thermosphere, and ionosphere. We constructed the first scenario of the long-term global change in the upper atmosphere, based on trend studies of various parameters, which shows general cooling and thermal contraction of the upper atmosphere and changes in the ionosphere due to chemical changes in minor atmospheric constituents as a consequence of cooling. The scenario is qualitative and contains still some gaps and a few discrepancies, the number of which has been reduced in recent years. The overall pattern of observed long-term changes throughout the upper atmosphere is qualitatively consistent with model predictions of the effect of greenhouse gas increases.
The observed effects are predominantly caused by the increasing concentration of greenhouse gases, but in the mesosphere, lower thermosphere and lower ionosphere an important role is played by the ozone depletion and recovering, and a less important role by trends in water vapour concentration. In the F2 region ionosphere still the geomagnetic activity long-term changes may played a dominant role in the past, and some role is played also by secular changes of the Earth’s magnetic field. Important role might be played also by changes of atmospheric wave activity and circulation but these changes are to a large extent rather unknown. Our results indicate that anthropogenic emissions of greenhouse gases are affecting the atmosphere at nearly all altitudes between ground and space.
J. Laštovička is co-chairing both international bodies active in this area, the IAGA/ICMA WG II.F “Ling-Term Trends in the Mesosphere, Thermosphere and Ionosphere” (chairman 1999-2011) and SCOSTEP CAWSES-II TG-2 “How Will Geospace Respond to Climatic Change”. Recent review of the discipline may be found in:
Laštovička, J., S.C. Solomon, L. Qian (2012), Trends in the neutral and ionized upper atmosphere, Space Sci. Revs., 168, 113-145, doi: 10.1007/s11-214-011-9799-3.
Solar and geomagnetic effects
Results of various authors on trends in foF2 and hmF2 were controversial. We contributed to solving this problem. Even though still there is not full consensus, it seems that while in the past the geomagnetic activity was the dominant driver of trends, now greenhouse gases dominates, in hmF2 since early 1970s, in foF2 since about 2000.
We also showed that the impact of long-term changes of solar and geomagnetic activity on trends in the atmosphere-ionosphere system weakens from above (F-region ionosphere) down to the troposphere, and weakens from the beginning to the end of the 20th century.
3) Ozone and stratospheric dynamics
Long-term trends in laminae in ozone profiles
A very strong negative trend (decrease typically by about 50% over 20-25 years for the period 1970-1995) of the overall ozone content in laminae in ozone profiles was confirmed for all examined stations between 36° and 74°N. Such a strong trend was found in no other parameter in the mid-latitudinal middle atmosphere. However, this negative trend changed to a positive one in the mid-1990s. Cooling of Arctic winters in 1970-1995 and unusually warm Arctic winters in 1997/98-2003/04, which cause lower and higher release of laminae near polar vortex edge, seems to be important (if not dominant) contributor to the change of lamina trend. The Southern Hemisphere displays much less laminae and no evident trend in laminae and its change as the Northern Hemisphere does, very probably due to much more regular high latitude stratospheric circulation with much more stable vortex.
Ozone – other investigations
Data over several solar cycles reveal the existence of geomagnetic storm effects in total ozone only for strong storms, in winter, under high solar activity and E-QBO phase conditions, at 50oN, as a re-distribution leading to large suppression of longitudinal variation of total ozone (neither production, nor loss of ozone). Forbush decreases appear to play an important, if not decisive role in effects of strong geomagnetic storms in total ozone. Such effects of strong storms in total ozone have not been observed in 40-60oS, probably due to different dynamics and resulting much less longitudinal variation of total ozone.
We have long-term cooperation with the Institute of Atmospheric Physics in Beijing, China, focused on the comparison of the ozone vertical profiles at the European ozonosonde stations and at the Asian stations. A systematically higher ozone concentration is observed in Beijing in the lower troposphere compared to the European stations and Japanese station Sapporo. These differences are the highest in summer, when the enhanced ozone concentration in Beijing is due to photochemical production of ozone in polluted troposphere in Beijing. There is no systematic trend in the tropospheric ozone concentration at the European stations and in Sapporo, while in Beijing a significant positive trend is observed in the period 2002-2011 at all seasons and heights.
This area of research was re-opened in late 2007 due to a Czech-German project on this topic. Analysis of stratospheric winds at 100 and 10 hPa at northern higher middle latitudes revealed clearly poor quality of ERA-40 winds in the last four winters of ERA-40 data series; some disagreement between ERA-Interim and NCAP/NCAR reanalyses was found before but not after 2003. Long-term changes of stratospheric and mesopause region yearly average winds seem to be similar. Winter 100 hPa winds in the Atlantic sector display long-term changes very similar to those of NAO (North Atlantic Oscillation in the troposphere). Some influence of solar activity on long-term changes of stratospheric winds was detected as well.