D. Pancheva

Two-day wave coupling of the low-latitude atmosphere-ionosphere system

[1]xa0Vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 2-day wave in the tropical MLT region has been investigated. The problem is studied from an observational point of view. Three different types of data were analyzed in order to detect and extract the 2-day wave signals. The 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations. The 2-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) and in the F-region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes. Two features for each kind of wave were investigated in detail: the variation with time of the wave amplitude and the zonal wave number. The results show that the westward propagating global 2-day wave with zonal wave number 2 seen in the ionospheric currents and in F-region plasma is forced by the simultaneous 2-day wave activity in the MLT region. The main forcing agent in this atmosphere-ionosphere coupling seems to be the modulated tides, particularly the semidiurnal tide. This tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere. The parameter that appears to be affected, and thus drives the observed 2-day wave response of the ionosphere, is the dynamo electric field.

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.

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.

Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

[1] The vertical coupling of the stratosphere-mesosphere system through quasi-stationary and traveling planetary waves during the major sudden stratospheric warming (SSW) in the Arctic winter of 2003/2004 has been studied using three types of data. The UK Met Office (UKMO) assimilated data set was used to examine the features of the global-scale planetary disturbances present in the winter stratosphere of the Northern Hemisphere. Sounding the Atmosphere using Broadband Emission Radiometry (SABER) satellite measurements were used as well for extracting the stationary planetary waves in the zonal and meridional winds of the stratosphere and mesosphere. Radar measurements at eight stations, four of them situated at high latitudes (63–69N) and the other four at midlatitudes (52–55N) were used to determine planetary waves in the mesosphere-lower thermosphere (MLT). The basic results show that prior to the SSW, the stratospheremesosphere system was dominated by an upward and westward propagating � 16-day wave detected simultaneously in the UKMO and MLT zonal and meridional wind data. After the onset of the SSW, longer-period (� 22–24 days) oscillations were observed in the zonal and meridional MLT winds. These likely include the upward propagation of stationary planetary waves from below and in situ generation of disturbances by the dissipation and breaking of gravity waves filtered by stratospheric winds. Citation: Pancheva, D., et al. (2008), Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004, J. Geophys. Res., 113, D12105, doi:10.1029/2007JD009011.

Planetary waves and variability of the semidiurnal tide in the mesosphere and lower thermosphere over Esrange (68°N, 21°E) during winter

[1] The main features of the planetary waves and the variability of the semidiurnal tide with planetary wave periods observed by meteor radar over Esrange (68°N, 21 °E) have been investigated. The interval of 39 months covering continuous measurements from October 1999 to December 2002 has been examined. The planetary waves most frequently observed by meteor radar measurements in the mesosphere and lower thermosphere (80-100 km) over Esrange are: 5-, 8- to 10-, 16-, and 23-day waves (the quasi-2-day wave is excluded in this study). They are strongly amplified in the winter. Some differences between high- and middle-latitude planetary waves notwithstanding, the 5-, 10-, and 16-day waves are most probably related to the well-known normal mode. There are some reasons to believe that the vertically upward propagating 23-day wave could be generated by solar forcing. The variability of the semidiurnal tide with periods of planetary waves has been thoroughly studied as well. It is found that in the winter when the planetary waves are significantly amplified, a very strong periodic variability of the semidiurnal tide is observed as well. This result indicates that the most probable mechanism responsible for the periodic tidal variability during winter is in situ nonlinear coupling between tides and planetary waves. Two winter periods have been examined (1999/2000 and 2001/2002) in order to find strong evidence supporting this suggestion. The validity of the frequency, phase, and vertical wavenumber (wavelength) relationship between the prime (the planetary wave and semidiurnal tide) and secondary waves has been established. The novel aspect of this work is that we show for the first time that the calculated vertical structures (vertical wavelengths) of the sum and difference secondary waves, which have very close periods, are actually very different.

Evidence of a role for modulated atmospheric tides in the dependence of sporadic E layers on planetary waves

[1]xa0A large amplitude, 7-day period westward propagating S = 1 planetary wave (PW) of global response has been reported from ground radar and satellite wind measurements in the mesosphere-lower thermosphere (MLT) during the second half of August and well into September 1993. Following recent suggestions that PW might play a role in the formation of midlatitude sporadic E layers (Es), Haldoupis and Pancheva [2002] found a strong 7-day periodicity present in all stations concurrently with the 7-day planetary wave reported elsewhere, by analyzing sporadic E critical frequency (foEs) time series from eight midlatitude ionosonde stations covering a longitudinal zone from about 58°E to 157°W. This study provided the first direct proof in favor of a PW role on Es formation. In the present paper we further investigate this role by considering the same PW event and correlating the 7-day periodicity in foEs directly with concurrent variations in the mesospheric neutral wind measured with atmospheric radars in Saskatoon, Canada, and in Sheffield, United Kingdom. Although our analysis cannot exclude a direct PW role on Es formation, it shows clearly that Es is affected indirectly by the PW through the action of the diurnal and semidiurnal tides which are strongly modulated by the same PW, apparently through a nonlinear interaction process at altitudes below 100 km. This 7-day PW modulation was found to be clearly present simultaneously in the amplitude of the zonal 12-hour tidal wind, the meridional 24-hour tidal wind, and in both, the 12-hour and 24-hour periodicities which existed in the foEs time series. The results here provide a new physical explanation for the observed relation between sporadic E layers and planetary waves.

Signatures of ultra fast Kelvin waves in the equatorial middle atmosphere and ionosphere

[1]xa0In the equatorial atmosphere, oscillations with periods of 3 to 4 days have been observed in the meteor radar zonal wind at Cariri (7.4°S, 36.5°W), in the ionospheric minimum virtual height hF and the maximum critical frequency foF2 at Fortaleza (3.9°S, 38.4°W), and in the TIMED/SABER satellite temperature data in the stratosphere-mesosphere. Wavelet analyses of these time series reveal that the 3–4-day oscillation was observed for all of these data during the period from March 1 to 11, 2005. From the characteristics of the downward phase propagation (wavelength of ∼40 km), longitudinal and latitudinal extension, we conclude that this oscillation must be a 3.5–day Ultra Fast Kelvin (UFK) wave. This is the first report of clear evidence of propagation of a UFK wave from the stratosphere to the ionosphere. The UFK wave could have an important role in the day-to-day variability of the equatorial ionosphere evening uplift.

The 8‐hour tide in the Arctic mesosphere and lower thermosphere

An all-sky VHF meteor radar at Esrange (68°N, 21°E) near Kiruna in Northern Sweden has been used to investigate the 8-hour tide in the Arctic mesosphere and lower thermosphere. We present a climatology of the 8-hour tide over the period October 1999 to April 2001. The tide appears to be a persistent feature of the Arctic atmosphere, although a large day-to-day variability of the tidal amplitude is observed. At times the 8-hour tide reaches amplitudes over 30 m s -1 . The amplitude of the tide increases with height across the observed height range of ∼80-100 km. Monthly mean tidal amplitudes range from <2 m s -1 to values as large as 10 m s -1 . A clear seasonal behavior is apparent with maximum amplitudes observed in the autumn. Vertical wavelengths are shortest in winter and spring (25-35 km) and longest in summer and autumn (50-90 km). At least on some occasions the vertical wave number relationships between the 8-, 12-, and 24-hour tides suggest that the 8-hour tide is being generated by nonlinear interaction between the 12- and 24-hour tides.

Long-term trends in planetary wave activity (2-15 days) at 80-100 km inferred from radio wave absorption

Abstract The daytime radio wave absorption in the lower ionosphere measured by the A3 method (oblique incidence on the ionosphere) in central and southeastern Europe is used to study long-term trends in the planetary wave activity in the period range of 2–15 days in the upper middle atmosphere. In central Europe we have found no trends in the 1960s and 1980s, but a positive trend in the 1970s (early 1970s- early 1980s); in southeastern Europe we have not established any trend in the 1970s, but a positive one in the 1980s (beginning in late 1970s). These trends are of non-solar origin. They are possibly an indication of changes of anthropogenic origin in the Earths atmosphere.

Response of the mesopause region dynamics to the February 2001 stratospheric warming

Abstract The response of the mesosphere/lower-thermosphere (MLT) region to a major stratospheric warming in Europe during winter 2000/2001 has been investigated using mesopause-region winds measured by meteor radar or the LF-D1 method over three stations (Castle Eaton, 52°N; Collm, 52°N; and Esrange 68°N). The vertical wind structure measured over the three sites, and its time evolution, are found to be quite similar despite the different techniques used in the measurements. The effects of stratospheric warming are very clear, and are similar over both the mid-latitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was apparently associated with a planetary-wave oscillation with a period of ∼10 days. The effect was most conspicuous in the vertical prevailing wind gradients. The mesopause-region effects thus seem to be the results of a superposition of an intensifying planetary wave and a slow overall decrease in the strength of the zonal prevailing winds.