Planetary Wave Periods Research Articles

Origin and development of vertical propagating oscillations with periods of planetary waves in the ionospheric F region

Starting from previous results of vertical propagating 6.5-day oscillations observed in the F region of the ionosphere during 1998, we investigated their possible forcing from both the planetary wave activity of the neutral winds in the mesosphere/lower thermosphere and the geomagnetic activity. For this purpose we used the electron density profiles and geomagnetic field components obtained at Observatori de l'Ebre station (40.8°N, 0.5°E), the mesopause region neutral wind data obtained at Collm Observatory (52°N, 15°E) and the 3-hourly geomagnetic index Ap. From the four events of 6.5-day oscillations observed in the ionosphere, we found that two of them are contemporary with two events of 6.5-day period in the neutral winds and the coherency between the neutral winds and ionosphere variations at 6.5-day periods is very large. The other ionospheric events are contemporary with the existence of two geomagnetic storms and the coherency between the ionospheric and geomagnetic activity variations is very large at periods at 6.5 days. The potential mechanisms linking the geomagnetic activity variations and the planetary wave activity of the neutral winds in the mesosphere/lower thermosphere with the wave events observed in the ionosphere are discussed in order to explain the possible origin of the ionospheric waves.

Non-linear interaction of tides and planetary waves in the mesosphere and lower thermosphere: observations over Europe

Observations of tidal modulations measured by meteor radars situated in UK (53.5°N, 3.9°W), Bulgaria (42.5°N, 26.6°E) and Sweden (67.9°N, 21.1°E) are shown in the present study. The observations indicate that the nonlinear interaction between tides and planetary waves contributes significantly to the tidal variability at planetary wave periods in the MLT region. The most frequently observed tidal modulations have periods of the well-known normal modes with periods of 2, 5, 10, 16 and 25–27 days. The planetary wave zonal wind components seem to be most important for the tidal modulations. In many cases the non-linear interaction between tides and planetary waves takes place in the stratosphere.

Wavelet analysis on transient behaviour of tidal amplitude fluctuations observed by meteor radar in the lower thermosphere above Bulgaria

Abstract. On the basis of bispectral analysis applied to the hourly data set of neutral wind measured by meteor radar in the MLT region above Bulgaria it was demonstrated that nonlinear processes are frequently and regularly acting in the mesopause region. They contribute significantly to the short-term tidal variability and are apparently responsible for the observed complicated behavior of the tidal characteristics. A Morlet wavelet transform is proposed as a technique for studying nonstationary signals. By simulated data it was revealed that the Morlet wavelet transform is especially convenient for analyzing signals with: (1) a wide range of dominant frequencies which are localized in different time intervals; (2) amplitude and frequency modulated spectral components, and (3) singular, wave-like events, observed in the neutral wind of the MLT region and connected mainly with large-scale disturbances propagated from below. By applying a Morlet wavelet transform to the hourly values of the amplitudes of diurnal and semidiurnal tides the basic oscillations with periods of planetary waves (1.5-20 days), as well as their development in time, are obtained. A cross-wavelet analysis is used to clarify the relation between the tidal and mean neutral wind variability. The results of bispectral analysis indicate which planetary waves participated in the nonlinear coupling with the atmospheric tides, while the results of cross-wavelet analysis outline their time intervals if these interactions are local.Key words: Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) - Radio science (nonlinear phenomena)

Recent results with an MF radar at McMurdo, Antarctica: Characteristics and variability of motions near 12‐hour period in the mesosphere

We present an analysis of the first wind measurements obtained with an MF (medium frequency) radar installed at McMurdo Station, Antarctica in January 1996. The largest amplitude motions are the quasi‐12‐hr motions, with additional periodicities at 24 hr, ∼10 hr and 2 to 10 days. The quasi‐12‐hr motions occur at discrete periods distinct from 12 hr and exhibit pronounced amplitude and vertical wavenumber modulations at planetary wave periodicities. During the months analyzed in this report (mid‐January through April 1996) the 12‐hr wave is by far the dominant motion the mesosphere. The source of this wave is unknown, but our results provide additional support for the suggestion that such motions arise through nonlinear interactions involving the migrating semidiurnal tide and low‐frequency planetary wave motions.

Evidence for nonlinear coupling of planetary waves and tides in the Antarctic mesopause

Analyses of hourly‐mean meridional and zonal winds near 95 km, observed with the meteor radar system at Molodezhnaya station (68°S, 45°E) during 1990, reveal the presence of long‐period (2–18 day) waves with dominant periods corresponding to those of Rossby normal modes modified by mean zonal winds. Time–varying analyses indicate that although significant contributions at planetary wave periods persist with nearly constant periods for intervals of days to weeks, temporal variability of these periods is observed over longer intervals. Tidal motions, which mainly consist of the semidiurnal and the diurnal periods, demonstrate considerable modulation of their amplitudes that vary with time. The observed tidal amplitude modulations are investigated through bispectral analyses. The results of these analyses suggest nonlinear interactions of quadratic form between oscillations with periods in the range of 2 to 19 days and the diurnal and semidiurnal tides at various intervals during 1990.

Planetary waves in the lower thermosphere over East Siberia

The database of regular lower thermosphere horizontal wind measurements at Badary (52°N; 102°E) allows examination of variations with planetary wave periods. A summary of characteristics of these variations is presented. In addition to quasiperiodical variations of the prevailing wind, solar activity (F10.7 index) and geomagnetic activity (Ap-index) variations were also investigated.

Periods of planetary waves in geomagnetic variations

Abstract. Periods of planetary waves, especially the 10- and 16-day waves, were found in Fourier analyses of 10-year geomagnetic time series from two mid-latitude stations in the northern hemisphere. This suggests that planetary waves influence geomagnetic variations. Cross-spectral analysis of magnetic time series from seven stations located at around 50°N at the beginning of 1979, when a 16-day wave occurred in the stratosphere, also shows a 16-day oscillation. However, study of the phases does not reveal the horizontal direction of wave propagation. Furthermore, the temporal variations of the 16-day oscillation in magnetic time series are presented as dynamic spectra and the results are compared with global investigations of geopotential height data at 1 hPa (around 48 km) with respect to the 16-day wave for the same time interval. In some cases this comparison suggests a clear correlation between geomagnetic variations and planetary waves as well as a propagation of the 16-day wave up to the dynamo region (100-170 km).

Periods of planetary waves in geomagnetic variations

Periods of planetary waves, especially the 10- and 16-day waves, were found in Fourier analyses of 10-year geomagnetic time series from two mid-latitude stations in the northern hemisphere. This suggests that planetary waves influence geomagnetic variations. Cross-spectral analysis of magnetic time series from seven stations located at around 50°N at the beginning of 1979, when a 16-day wave occurred in the stratosphere, also shows a 16-day oscillation. However, study of the phases does not reveal the horizontal direction of wave propagation. Furthermore, the temporal variations of the 16-day oscillation in magnetic time series are presented as dynamic spectra and the results are compared with global investigations of geopotential height data at 1 hPa (around 48 km) with respect to the 16-day wave for the same time interval. In some cases this comparison suggests a clear correlation between geomagnetic variations and planetary waves as well as a propagation of the 16-day wave up to the dynamo region (100-170 km).

Relationship between the Radio Wave Absorption and Horizontal Winds in the Lower Ionosphere over East Siberia.

The horizontal wind field in the ionospheric D-region belongs to important parameter related to ionospheric radio wave absorption. The analysis of continuous ionospheric vertical radio sounding and wind measurements by the spaced receiver method in low-frequency range (Irkutsk, East Siberia 1975-1991) is applicated to define the possible coupling of absorption and dynamics in their seasonal and year-to-year dependence. For the analysis we used parameter dfmin =fmin -fmin median, prevailing winds, amplitudes and phases of semidiumal tides for zonal and meridional winds. For some seasons there are significant but changeable correlations between dfmin and prevailing wind. In most cases especially in winter the changes of wind outstrip the changes of absorption. Quasi-periodic fluctuations with planetary wave periods (18-30 days) exist in winter for both. The periodicity close to 27 days (26-30 days) exists for all seasons and all parameters. Application of correloperiodogram analysis reveals the response of ionospheric absorption to the dynamical processes, quantitatively depending on the solar and geomagnetic activity.

On tidal variability induced by nonlinear interaction with planetary waves

Short‐time variability of the atmospheric tides is frequently observed in the meteor region but is not yet fully explained in terms of production mechanisms. This is probably due to the existence of several such mechanisms acting together or separately. In this paper we show that many observations can be explained by nonlinear interactions between tides and planetary waves having periods corresponding to those of the observed tidal amplitude modulations. These nonlinear interactions generate two secondary waves whose frequencies are the sum and difference of frequencies of the primary waves. These two waves beat with the tide, modulating its amplitude with the planetary wave period. A numerical model is used to demonstrate that with primary waves of reasonable amplitudes the nonlinear interactions can be quite large. This is because the importance of nonlinearity depends essentially on the amplitude of the induced fluid velocity in the direction of wave propagation compared to the wave propagation velocity. When two waves propagate simultaneously, the fluid velocity can have a large component in the direction of propagation of one of the waves, and advective (nonlinear) terms can be large. This point is further illustrated in the case of two gravity waves interacting together. Finally, some observational campaigns carried out above Garchy (45°N) are analyzed using a nonparametric method. The results indicate that nonlinear interactions between tides and planetary waves really take place in the upper mesosphere and lower thermosphere.

Long-period fluctuations of meteorological origin observed in the lower ionosphere

Application of a correloperiodogram analysis to day and night-time radio wave absorption values for five LF radio paths (164, 155, 185, 218 and 272 kHz) in central and southern Europe during the interval 1 June 1979–30 June 1980, fluctuations with the following quasi-periods were found: 2.3–3.2, 4–6 and 10.5–12 days. They exist in all the time series investigated. Other fluctuations with periods of 7–9 days are observed mainly in central Europe. Shorter period fluctuations are most active during the solstices and especially during winter, while the longer period fluctuations (10.5–12 days) have significant amplitudes also in spring. The analysis of satellite data on the intensity of ionizing solar H Lyman-alpha and X-ray (0.1–0.8 nm) radiation, of the variations of the geomagnetic activity ( aa( N)-index) and of lower thermospheric wind ( $ ̋ 95 km) in central Europe, shows that the fluctuations observed in absorption are caused not by fluctuations in the solar ionizing flux, but probably by fluctuations in the neutral atmosphere. This could offer a possibility of monitoring fluctuations of the planetary wave period range in the upper mesosphere and lower thermosphere by means of radio-wave absorption measurements.

Winds and waves in the middle atmosphere at Saskatoon (52°N, 107°W), Collm (52°N, 15°E) and Badary (52°N, 105°E)

Winds data from three radar systems in the U.S.S.R. G.D.R. and Canada, which are well-spaced along the 52°N latitude circle, are used to illustrate longitudinal/regional variations in the dynamics of the upper middle atmosphere 80–97 km. Responses to the stratospheric warming of 1982/3 are noted at all three locations, but the zonal wind does not reverse at Badary, consistent with the flow there being eastward during all months of the year. Planetary wave period oscillations (2–30 days) are observed at all locations, and highly significant cross-spectral coherences exist between the three stations.

Dynamics of the middle atmosphere at Saskatoon (52°N, 107°W): a spectral study during 1981, 1982

Planetary waves (2 day–30 day) and tidal wave (8 h, 12 h, 24 h) oscillations are studied by forming height (60–110 km) and time (January–December) contour cross-sections and comparing with mean winds for 1981, but also on occasion 1982. Strong seasonal variations with evident wave-mean flow couplings are demonstrated. The zonal mean wind momentum budget (80–90 km) is assessed using cross-sections of northward-eastward covariances for wind fluctuations in the planetary and tidal wave period ranges and also gravity wave cross-sections. Dominant contributions are due to coriolis torque in the meridional mean flow and vertical divergence of the gravity wave momentum flux, although planetary wave contributions may be large also in winter months.

Evidence for 50-day period planetary waves in the South Equatorial Current of the Indian Ocean

Records from moored current meters located off the northern tip of Madagascar in the South Equatorial Current during March–July 1975 indicate a 50-day period oscillation with an amplitude of 12 cm s −1. Meanders of similar amplitude and an approximate wavelength of 400 km could be seen in surface current vectors observed in August 1975 along a section at 11° S. These oscillations are interpreted as evidence of a barotropic Rossby wave. Further evidence can be seen in oscillations in drifter trajectories east of Madagascar. A likely cause for these waves appears to be a globally observed atmospheric oscillation with the same period.

The kinetic energy density in the troposphere, stratosphere and mesosphere: A preliminary study using the Poker Flat MST radar in Alaska

Continuous observations of the wind field obtained using the MST radar at Poker Flat, Alaska, over the past 5 years enable a determination of kinetic energy density (Joules per m−3) height profiles in the troposphere, stratosphere and mesosphere. Three specific frequency intervals of the horizontal wind velocity spectrum are processed to yield height profiles of kinetic energy density corresponding roughly to periods of gravity waves, tides and planetary waves (although the wind fluctuations do not necessarily correspond to wavelike processes). Preliminary results of this analysis are presented here for both summer and winter periods.