Jan Lastovicka

On the transformation of planetary waves of tropospheric origin into waves in radio wave absorption in the lower ionosphere

SummaryCalculations are carried out of upward propagation of a tropospherically forced 10-day planetary wave into the upper middle atmosphere with the use of the COMMA-R model of the University of Cologne, of its transformation into a wave in electron density by means of the model of the Comenius University, and of its final transformation into a wave in radio wave absorption in the lower ionosphere applying the computer code of the Geophysical Institute. The calculations show that the absorption may be used for investigating the planetary wave activity, particularly of its long-term trends. The possibility of propagation of planetary waves from the winter hemisphere to the summer hemisphere is illustrated, which could contribute to explanation of the occurrence of travelling planetary waves in the mesosphere in summer.

Long-term changes and trends in the lower ionosphere

A brief overview of long-term trends in the lower ionosphere (h<100 km, ionized component) is presented. The trends have been studied with the use of several different types of data sets: A3 radio wave absorption (LF–HF, oblique incidence on the ionosphere, continuous wave), A2 (cosmic radio noise) riometric absorption, IPHA (indirect measurements of phase reflection height), rocket measurements of electron density and ion composition. The A3 radio wave absorption in the lower ionosphere over Europe has been analyzed for trends in: (1) absolute values of absorption; (2) amplitudes of annual and semiannual variations; (3) inferred planetary wave activity. Trends in absolute values of absorption do not provide a consistent pattern, which may be due to their sensitivity to changes in the shape of electron density profile and to instrumental problems. Similarly, there is no consistent pattern in trends in relative amplitudes of annual variations. However, there appears to be a systematic tendency to the positive trend in amplitudes of semiannual variations. Some increase of the absorption-inferred planetary wave activity was found in 1970s and 1980s, whereas no change was found in 1960s and 1990s. Analyses of cosmic noise absorption measured by a chain of riometers in Finland do not reveal an evident trend in absorption. IPHA analyses in mid-latitudinal Europe point to a systematic decrease of the reflection height (level of constant electron density) near 80 km. This has been interpreted in terms of decreasing atmospheric pressure due to decreasing columnar temperature in 50–80 km. Rocket measurements of electron density display for non-winter electron density at 75 and 80 km an increase at middle latitudes (contrary to IPHA results). Rocket measurements of ion composition in the lower ionosphere are too rare, but models reveal dramatic changes with a large decrease of NO+ concentration. Since the lower ionosphere is substantially affected by the 11-year solar cycle, data sets length of more than 20 years is very desirable in long-term trend studies. Results based on various methods and various data sets do not provide sufficiently consistent observational pattern of trends in the lower ionosphere. Therefore, further investigations, tests of data quality and homogeneity, and careful application of various methods are necessary.

Long-term trends in fo F2: their estimating and origin

This paper deals with two problems, methods of fo F2 trend determination and origin of trends in fo F2, both being controversial in current literature. We found that various regression-based methods and artificial neural network-based method of Yue et al. (2006) provided comparable results within uncertainties caused mainly by various ways of removing/suppressing the dominant solar cycle effect. The role of geomagnetic activity in the observed trends in fo F2 was probably substantial and might be still even rather dominant in the last quarter of the 20th century.

Seasonal Variation of Gravity Wave Activity in the Lower Ionosphere in Central Europe

The seasonal variation of gravity wave activity at altitudes around 95 km is investigated using digital measurements of low-frequency nighttime radiowave absorption at Průhonice (50°N, 15°E) between 1989 and 1993. The analysis of 5 years of data allows two conclusions to be drawn: (1) under high solar activity conditions, there is no clearly detectable seasonal variation of gravity wave activity; (2) under medium solar activity conditions (1992, 1993), there is a tendency to a pronounced summer maximum.

Total ozone response to major geomagnetic storms and changes in meteorological situations

SummaryThe total ozone response to strong major geomagnetic storms (Ap≥60) in winter along the 50° N latitudinal circle is studied. The results add to the recent results of Laštovička et al. (1992) obtained for European middle latitudes (∼50°N) and to the results of Mlch (1994). A significant response of total ozone is only observed in winter under high solar activity/E-phase of QBO conditions (E-max) and seems to be caused by geomagnetic storm-induced changes of atmospheric dynamics. There are two sectors along latitude 50°N, which are sensitive to forcing by geomagnetic storms both in total ozone and the troposphere — north-eastern Atlantic-European and eastern Siberia-Aleutian sectors. The total ozone response under E-max conditions manifests itself mainly as a large decrease in the longitudinal variation of ozone after the storm, which means an increase of ozone in Europe. The observed effects in total ozone consist in redistribution, not production or loss of ozone.

The vertical propagation of disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake over Taiwan

In this paper, concurrent/colocated measurements of seismometers, infrasonic systems, magnetometers, HF-CW (high frequency-continuous wave) Doppler sounding systems, and GPS receivers are employed to detect disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake. No time delay between colocated infrasonic (i.e., super long acoustic) waves and seismic waves indicates that the triggered acoustic and/or gravity waves in the atmosphere (or seismo-traveling atmospheric disturbances, STADs) near the Earths surface can be immediately activated by vertical ground motions. The circle method is used to find the origin and compute the observed horizontal traveling speed of the triggered infrasonic waves. The speed of about 3.3 km/s computed from the arrival time versus the epicentral distance suggests that the infrasonic waves (i.e., STADs) are mainly induced by the Rayleigh waves. The agreements in the travel time at various heights between the observation and theoretical calculation suggest that the STADs triggered by the vertical motion of ground surface caused by the Tohoku earthquake traveled vertically from the ground to the ionosphere with speed of the sound in the atmosphere over Taiwan.

Propagation of gravity waves and spread F in the low-latitude ionosphere over Tucumán, Argentina, by continuous Doppler sounding: First results

Results of systematic analysis of propagation directions and horizontal velocities of gravity waves (GWs) and spread F structures in low-latitude ionosphere (magnetic inclination ~27°) in Tucuman region, Argentina, are presented. Measurements were carried out by multipoint continuous Doppler system during 1 year from December 2012 to November 2013. It was found that meridian propagation of GWs dominated and that southward propagation prevailed in the local summer. Oblique spread structures observed in Doppler shift spectrograms and associated with spread F propagated roughly eastward at velocities from ~70 to ~180 m/s and were observed at night from ~ September to ~ March. The velocities were computed for 182 events and the azimuths for 64 events. Continuous Doppler sounding makes it possible to analyze more events compared to optical observations often used for propagation studies since the measurements do not depend on weather.

Manifestation of Strong Geomagnetic Storms in the Ionosphere above Europe

The solar wind effects on Earth environment are studied for their basic science value as well as for their crucial practical impact on human technological systems. Increased dissipation of solar wind energy in the near-Earth environment is a significant source of consequent perturbations in the upper atmosphere and ionosphere. This chapter addresses the ionospheric manifestation of geomagnetic storms induced by solar wind. Changes in the electron density distribution at the ionospheric F region heights above Europe during strong-to-severe geomagnetic storms, which occurred over present solar cycle, have been analysed. As for the seasonal preference, during storm main phase only negative phases dominate in summer, while during winter occurrence of both negative and positive phases is probable. Enhancements of electron density have been sometimes observed several hours before the onset of geomagnetic storm. Also the existence of few-hours-long periods during storm main phase, when the deviation of the electron density from median was insignificant, has been observed. Independent of the sign of the storm effect on F2 region ionisation, the effect on electron density at the F1 region heights at European higher middle latitudes has been found negative, if any at all. The F1 region response to magnetic disturbances also shows substantial summer/winter asymmetry. The stormy high latitude F region is most variable compared with middle and lower middle latitudes, being strongly influenced by magnetospheric processes, in particular, strong electric fields, which are usually present during geomagnetic storms. Several specific features of the storm-time high latitude ionosphere will briefly be mentioned including behaviour of ionospheric scintillations. The comparative analysis illustrates that the improved IRI-2001 model with the activated STORM option provides better description of the ionisation distribution above Europe under geomagnetic storm conditions. Nevertheless, our results show that model not always estimates correctly the storm phase and the magnitude of the effects on F region electron density

Response of the mesosphere-thermosphere-ionosphere system to global change - CAWSES-II contribution

Long-term trends in the mesosphere, thermosphere, and ionosphere are areas of research of increasing importance both because they are sensitive indicators of climatic change and because they affect satellite-based technologies which are increasingly important to modern life. Their study was an important part of CAWSES-II project, as they were a topic of Task Group 2 (TG-2) ‘How Will Geospace Respond to Changing Climate’. Three individual projects of TG-2 were focused on important problems in trend investigations. Significant progress was reached in several areas such as understanding and quantifying the role of stratospheric ozone changes in trends in the upper atmosphere, reaching reasonable agreement between observed and simulated trends in mesospheric temperatures and polar mesospheric clouds, or understanding of why the thermospheric density trends are much stronger under solar cycle minimum conditions. The TG-2 progress that is reviewed in this paper together with some results reached outside CAWSES-II so as to have the full context of progress in trends in the upper atmosphere and ionosphere.

Very strong negative trends in laminae in ozone profiles

Abstract A very strong negative trend in the overall ozone content in the positive laminae in ozone profiles, a decrease by 50% during about 20–25 years, was found in the 1970s, 1980s and early 1990s in data of eight Northern Hemisphere ozone sounding stations. They were located at high and middle latitudes in Europe, Canada and Japan. Only the station Wallops Island in US ( 38 ° N ) revealed much smaller reduction of ozone in laminae. This means among others that the negative trend is basically the same for Resolute Bay at 75 ° N and Tateno at 36 ° N in spite of twice as high ozone content in laminae at Resolute Bay. The trends have been broadly studied for large laminae with the peak ozone concentration higher by 40 nbar and more than the background ozone concentration. However, analyses of laminae with the peak ozone concentration higher than 30 or 20 nbar against background for a few stations revealed similar strong negative trends. The trends are almost height independent, but the contribution of laminae to overall ozone content clearly peaks in the lowermost stratosphere. The overall ozone content in laminae has a strong seasonal variation (factor 5 or even more) with a maximum in late winter/early spring, when the negative trend in laminae may be responsible for as much as one third of the observed ozone depletion in European middle latitudes. Laminae seem to contribute substantially to seasonal variation of trends in total ozone.