Planetary Wave Type Oscillations Research Articles

On the use of Very Low Frequency transmitter data for remote sensing of atmospheric gravity and planetary waves

Continuous ground-based monitoring of Very Low Frequency (VLF) transmitter signals is an efficient remote sensing tool for studying of the lower ionosphere (60–90km). Here, we present the use of VLF radio data to study short-period (∼min–hrs) atmospheric gravity waves and long-period (∼days) planetary waves. We analyse VLF data from several receiving stations obtained by ICSP-VLF network during the total solar eclipse of July, 2009 to show the existence of short-period atmospheric gravity waves. We find dominant wave periods range from 10min to 1h around the time of maximum eclipse phase which could be associated with atmospheric gravity waves excited due to the eclipse. We also analyse VLF amplitude data of 2007 received at ICSP, Kolkata from VTX (18.2kHz) transmitter for planetary wave-type oscillations in the mesosphere–lower ionosphere system. Fourier and wavelet analysis show presence of periodic structures with periodicity in the range of 5–27days. We compare VLF planetary spectrum with spectrum obtained from total column density of Ozone and mesospheric average temperature data which may indicate vertical coupling between the stratosphere and ionosphere in winter to early spring time.

Thermospheric planetary wave‐type oscillations observed by FPIs over Xinglong and Millstone Hill

AbstractThree‐year (2010–2013) observations of thermospheric winds (at ~250 km) by Fabry‐Perot interferometers at Xinglong (XL, 40.2°N, 117.4°E) and Millstone Hill (MH, 42.6°N, 71.5°W) are used to study the climatology of atmospheric planetary wave‐type oscillations (PWTOs) with periods of 4–19 days. We find that (1) these PWTOs occur more frequently in the months from May to October. They are consistent with the summertime preference of middle‐latitude ionospheric electron density oscillations noted in other studies. (2) The month‐to‐month variations in PWTOs show phase changes between MH and XL, switching from antiphase to in phase when PWTO periods vary from short to long. (3) Typical PWTOs show annual and semiannual variations. The relative intensity of annual over semiannual components for PWTOs is different between XL and MH. (4) Magnetic storms and substorms have little influences on the annual and semiannual variations of the typical PWTO amplitudes. (5) Meridional wind PWTOs with typical periodicity bands around 5, 8, and 16 days appear to be correlated to both solar wind speed and Kp oscillations, suggesting a possible influence of the solar wind corotating interaction regions on neutral wind dynamics.

Manifestation of planetary wave-type oscillations in variations in the critical frequencies of the ionospheric F2 layer in the Asian region

Results of studies of the wave structure of the critical frequencies of the ionospheric F2 layer with periods of planetary waves for two Asian stations—Irkutsk and Wuhan (China)—are presented. Estimates of the appearance frequency, amplitudes, and the lifetime of oscillations with periods typical of planetary waves (2–25 days) are obtained. It is shown that these characteristics depend on the season and place of observation. The appearance of joint periodicities in the critical frequencies at both stations, as well as in the planetary index of geomagnetic activity Ap, is noted.

Characteristics of F2‐layer planetary wave–type oscillations in northern middle and high latitudes during 2002 to 2008

A first description of the characteristics of the F2‐layer ionosphere oscillations with periods between 2 and 30 days (so‐called planetary wave–type oscillations, PWTO) is derived using regional maps of total electron content (TEC), covering the northern hemisphere from 50°N to the North Pole. Oscillations forced by quasiperiodic variations of solar signals (e.g., extreme ultraviolet irradiation and solar wind) are identified and separated using wavelet transformation. It is found that up to 50% of the PWTO intensity in the F2‐layer ionosphere occur due to solar variability. The signals that are not obviously related to solar variability are spectrally decomposed to characterize oscillations by their period and wave number. Climatological analyses of the occurring oscillations in TEC reveal similarities to stratospheric planetary waves (PW). These are typical periods of PW, the strongest wave activity during winter and the enhancement of the wave activity at similar latitudes. But there are also major differences to stratospheric PW. In the ionosphere zonal mean oscillations are dominant, stationary waves are not observed, and the dominant periods are shorter than the typical periods of PW in the stratosphere. However, the obtained results are in good agreement with recent numerical modeling results that showed no stationary wave propagation into the F2‐region ionosphere and only fast, short‐period waves that propagate up to the lower thermosphere.

Persistence of planetary wave type oscillations in the mid-latitude ionosphere

Planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the iono- sphere, including the ionospheric F2 layer. Here we deal with the oscillations in foF2 analysed for two Japan- ese and two US stations over a solar cycle (1979-1989) with the use of the Morlet and Paul wavelet transforms. Waves with periods near 5, 10 and 16 days are studied. Only events of duration of three wave-cycles and more are considered. The results are compared with the results of a similar analysis made for foF2 and the lower ion- osphere over Europe (Lastovicka et al., 2003a,b). The 5-day period wave events display a typical duration of 4 cycles, while the 10- and 16-day wave events are less persistent with typical duration of about 3.5 cycles and rather 3 cycles, respectively, in all three geographic regions. The persistence pattern in terms of number of cy- cles and in terms of number of days is different. In terms of number of cycles, the typical persistence of oscilla- tions decreases with increasing period. On the other hand, in terms of number of days the typical persistence ev- idently increases with increasing period. The spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration. Thus the predictability of the planetary wave type oscillations in foF2 seems to be very questionable. The longitudinal size of the planetary wave type events increases with increasing wave period. The persistence of the planetary wave type events in foF2 and the lower ionosphere is similar in Europe, but the similarity in occurrence of individual events in foF2 and the lower ionosphere is rather poor.

Climatology of planetary wave type oscillations with periods of 2–20 days derived from O<sub>2</sub> atmospheric and OH(6-2) airglow observations at mid-latitude with SATI

Abstract. The presence of planetary wave type oscillations at mid-latitudes in the mesosphere/lower thermosphere region has been investigated using airglow observations. The observations were taken with a Spectral Airglow Temperature Imager (SATI) installed at Sierra Nevada Observatory (37.06° N, 3.38° W) at 2900 m height. Airglow data of the column emission rate of the O2 Atmospheric (0-1) band and of the OH Meinel (6-2) band and deduced rotational temperatures from 1998 to 2007 have been used in this study. From these observations a climatology of planetary wave type oscillations at this location is inferred. It has been found that the planetary wave type oscillations of 5-day period is predominant in our data throughout the year, with activity greater than 50% during March/April and October/November months. The planetary wave type oscillations of 2-day period is predominant during both solstices, being predominant during winter solstice in O2 while a 10-day oscillation appears throughout the year with activity around 20% and with maximum activity during spring and autumn equinoxes. The 16-day oscillation has maximum occurrence during autumn-winter while its activity is almost disappeared during spring-summer. No clear seasonal dependence of the amplitude of the planetary wave type oscillations was observed in the cases considered in this study. The waves simultaneously detected in the rotational temperatures deduced from both OH and O2 emissions usually show an upward energy propagation and are affected by dissipation processes.

Case study on total electron content enhancements at low latitudes during low geomagnetic activities before the storms

Abstract. Sometimes the ionospheric total electron content (TEC) is significantly enhanced during low geomagnetic activities before storms. In this article, we investigate the characteristics of those interesting TEC enhancements using regional and global TEC data. We analyzed the low-latitude TEC enhancement events that occurred around longitude 120° E on 10 February 2004, 21 January 2004, and 4 March 2001, respectively. The TEC data are derived from regional Global Positioning System (GPS) observations in the Asia/Australia sector as well as global ionospheric maps (GIMs) produced by Jet Propulsion Laboratory (JPL). Strong enhancements under low geomagnetic activity before the storms are simultaneously presented at low latitudes in the Asia/Australia sector in regional TEC and JPL GIMs. These TEC enhancements are shown to be regional events with longitudinal and latitudinal extent. The regions of TEC enhancements during these events are confined at narrow longitude ranges around longitude 120° E. The latitudinal belts of maxima of enhancements locate around the northern and southern equatorial ionization anomaly (EIA) crests, which are consistent with those low-latitude events presented by Liu et al. (2008). During the 4 March 2001 event, the total plasma density Ni observed by the Defense Meteorological Satellite Program (DMSP) spacecraft F13 at 840 km altitude are of considerably higher values on 4 March than on the previous day in the TEC enhanced regions. Some TEC enhancement events are possibly due to contributions from auroral/magnetospheric origins; while there are also quasi-periodic enhancement events not related to geomagnetic activity and associated probably with planetary wave type oscillations (e.g. the 6 January 1998 event). Further investigation is warrented to identify/separate contributions from possible sources.

Observations of F layer electron density profiles modulated by planetary wave type oscillations in the equatorial ionospheric anomaly region

A digital ionosonde of the type known as the Canadian Advanced Digital Ionosonde (CADI) is operational at São José dos Campos (23.2°S, 45.9°W), Brazil. This ionospheric sounding station is located under the southern crest of the ionospheric equatorial anomaly. The F layer electron density profile presents considerable day‐to‐day variability, even during undisturbed conditions, and this variability is still one of the less understood aspects of the physics of the ionosphere. The propagation of waves into the ionosphere may play an important role in this day‐to‐day ionospheric variability. In this paper we present a new technique that uses multifrequency virtual height variations, from ionospheric sounding observations at this low‐latitude station, to investigate how the F layer is modulated by planetary wave type oscillations. We have also considered the possible influence of oscillations due to solar origin (solar rotation variation). In the present study, observations during the months of June to September 2003 (season of low‐range type equatorial spread F occurrence at this location) have been used. The present study indicates the presence of 2‐day (the observed 3‐day periods are possibly associated with the quasi 2‐day oscillations), 5‐day, 10‐day, and 16‐day periods related to planetary wave type oscillations due to tropospheric sources during all the day. Also, it appears that during the day and dusk times, the longer periods 24–30 days are related to solar rotation (27 days), whereas the shorter periods 9–17 days have possibly some influence from half solar rotation (13.5 days). During the nighttime, possibly the short period oscillations 3–9 days may have some influence from the geomagnetic disturbances.

Planetary wave-type oscillations in the ionosphere and their relationship to mesospheric/lower thermospheric and geomagnetic disturbances at Wuhan (30.6°N, 114.5°E)

Using wind, ionospheric and geomagnetic intensity data observed simultaneously at Wuhan (30.6°N, 114.5°E) in 2002 and 2003, planetary wave type oscillations (PWTO) in the ionosphere and their relationship with oscillations in the mesosphere/lower thermosphere (MLT) and geomagnetic intensity have been investigated. Two cases of 2- and 5-day oscillations in the ionosphere are analyzed to distinguish MLT drivers and geomagnetic drivers. The geomagnetic oscillations are the most important drivers for the PWTO with dominant periods of 5-, 10-, 13.5-day. But the planetary waves in MLT winds are the main drivers for the quasi-2-day oscillations in the ionosphere. The correlation coefficient of the occurrence rates of the 2-day oscillation in winds and in ionospheric f 0F 2 is 0.82 for the data from February to November. The oscillations in the geomagnetic intensity can drive 30–50% of 5-, 10- and 13.5-day PWTO events in the ionosphere. If we evaluate all of the possible events driven by geomagnetic oscillations, more than 70% of the events have a relationship with geomagnetic oscillations with 5- and 10-day periods. The 39.7 h PWTO in ionospheric plasma frequencies observed from day 161 to 166 in 2002, are generated by an upward propagating wave with period 43.8 h. Two events of 6-day oscillations in the ionosphere observed during May to July in 2003, have different drivers. The first one is probably generated by 5.4-day upward propagating waves in MLT winds, but enhanced during a moderate geomagnetic storm. The second one is generated by the geomagnetic oscillations during a strong geomagnetic storm.

Persistence of the planetary wave type oscillations in <i>fo</i>F2 over Europe

Abstract. Planetary waves are oscillations of very predominantly tropospheric origin with typical periods of about 2–30 days. Their dominant zonal wave numbers are 1, 2 and 3, i.e. the waves are of large-scale (global) character. The planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the ionosphere, including the ionospheric F2-layer. Here, we deal only with the oscillations analyzed for four European stations over a solar cycle with the use of the Meyer and Morlet wavelet transforms. Waves with periods near 5, 10 and 16 days are studied. Only events with a duration of three wave-cycles and more are considered. The 5-day period wave events display a typical duration of 4 cycles, while 10- and 16-day wave events are less persistent, with a typical duration of about 3.5 cycles and 3 cycles, respectively. The persistence pattern in terms of number of cycles and in terms of number of days is different. In terms of number of cycles, the typical persistence of oscillations decreases with increasing period. On the other hand, in terms of number of days the typical persistence evidently increases with increasing period. The spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration. Thus, the predictability of the planetary wave type oscillations in foF2 seems to be very questionable.Key words. Ionosphere (ionosphere-atmosphere interaction, mid-latitude ionosphere, ionospheric disturbances) – Meteorology and atmospheric dynamics (waves and tides)

Persistence of Planetary Waves in the Lower Ionosphere

The transient planetary waves in the atmosphere and ionosphere seem to occur in the form of bursts of a couple of waves with limited persistence. To study persistence of planetary wave events in the lower ionosphere, data from two radio paths from Central Europe are used, Luxembourg – Panska Ves (f = 6.09 MHz, f eq = 2.1-2.2 MHz) and Deutschlandfunk – Panska Ves (f = 1539 kHz, f eq = 650-700 kHz). The absorption along the former radio paths is formed very predominantly at altitudes of about 90-100 km, whereas the latter absorption is formed mostly at altitudes of about 85-90 km. The persistence of planetary wave type oscillations is studied in three period bands centred at 5, 10 and 16 days. Waves with period T near 5 days reveal a typical persistence of wave events around 5 cycles. Waves with T = 10 days are less persistent with a typical persistence of 3-4 cycles. The typical persistence of waves T = 16 days is no more than 3 cycles. In terms of number of cycles, the persistence of oscillations evidently decreases with increasing period. On the other hand, in terms of number of days, the persistence seems rather to increase with increasing period.

Effects of gravity and planetary waves on the lower ionosphere as obtained from radio wave absorption measurements

This paper is an overview of effects of gravity and planetary waves on the lower ionosphere as obtained from radio wave absorption measurements. To transform gravity waves from the neutral atmosphere into the ionized component is not easy. The uncertainty caused by problems with transfer of oscillations from neutral to ionized component makes the use of daytime absorption measurements for gravity wave investigations largely impossible and allows us to use only nighttime radio wave absorption. Some results of gravity wave activity studies in the lower ionosphere, based on 5.5 year long data set, are presented: (i) The gravity wave activity is almost insensitive to QBO. (ii) It decreases from the solar cycle maximum to the solar cycle minimum by ∼30%. (iii) The Mt. Pinatubo volcanic effect is well expressed for longer periods ( > 2 hours) but not detectable at short periods (< 1 hour). (iv) There is no detectable seasonal variation at high solar activity but there is a tendency to summer maximum at medium solar activity. The planetary waves are transformed from the neutral to the ionized component adequately as for the period. It was proved that planetary wave type oscillations in the lower ionosphere are caused by the neutral atmosphere oscillations, not by solar or geomagnetic activity. Various features of planetary wave activity, inferred from ionospheric measurements, have been studied. Perhaps the most interesting results are those on long-term trends. A general increase of planetary wave activity has been observed in the 1970s and 1980s, while the level of planetary wave activity has been stable in the 1960s and 1990s.

Planetary wave type oscillations in the ionospheric F region

The existence of quasi-periodic oscillations (from 2- to 6.6-day) in the ionospheric F region, which may be connected with the planetary wave activity in the mesosphere/lower thermosphere, and the main results based on studies of the critical frequency forF2 of the F region in both hemispheres are presented. The existence of quasi-periodic oscillations with periods of about 2, 3, 5 and 6.5 days is statistically evident in the foF2 and they the following characteristics: the probability of existence and the occurrence frequency are maximum in local summer, the oscillation amplitude is maximum near equinoxes, and the dominant zonal structure of the oscillations at middle latitudes is westward travelling with zonal wave number s=1 and stationary waves with zonal wave numbers s=1 and 2. The zonal structure of the foF2 quasi-2-day oscillation with s=1 differs from the expected s=2 or s=3 of the Rossby normal modes (2,0) and (3,0) in the atmosphere. These quasi-periodic oscillations contribute significantly to the day to day variability of foF2 and their contributions have been evaluated in relation to the solar cycle, season and latitude. The altitude progression of the 2-day oscillation in the northern mid-latitude F region is also presented. The possible mechanisms for generating the wave type oscillations, from 2 to 6.5 days, in the F region are discussed.

Are planetary wave type oscillations in the F2 region caused by planetary wave modulation of upward propagating tides?

Planetary wave type (T = 2–30 days) oscillations in F2 region parameters have been observed by several authors. Here we present such oscillations in foF2 from central European stations Juliusruh and Průhonice. They are compared with tides inferred from wind measurements at a nearby station Collm to check the hypothesis of planetary wave modulation of upward propagating tides at lower thermospheric heights as a mechanism responsible for planetary wave type oscillations at F2 layer heights. The results show that this mechanism does not play a dominant role but it can contribute to such oscillations.

Planetary wave type oscillations in the ionospheric F region

The existence of quasi-periodic oscillations (from 2- to 6.5-day) in the ionospheric F region, which may be connected with the planetary wave activity in the mesosphere/lower thermosphere, and the main results based on studies of the critical frequency foF2 of the F region in both hemispheres are presented. The existence of quasi-periodic oscillations with periods of about 2, 3, 5, and 6.5 days is statistically evident in the foF2 and they have the following characteristics: the probability of existence and the occurrence frequency are maximum in local summer, the oscillation amplitude is maximum near equinoxes, and the dominant zonal structure of the oscillations at middle latitudes is westward travelling with zonal wave number s=1 and stationary waves with zonal wave numbers s=1 and 2. The zonal structure of the foF2 quasi-2-day oscillation with s=1 differs from the expected s=2 or s=3 of the Rossby normal modes (2,0) and (3,0) in the atmosphere. These quasi-periodic oscillations contribute significantly to the day to day variability of foF2 and their contributions have been evaluated in relation to the solar cycle, season and latitude. The altitude progression of the 2-day oscillation in the northern mid-latitude F region is also presented. The possible mechanisms for generating the wave type oscillations, from 2 to 6.5 days, in the F region are discussed.

Observations of tides and planetary waves in the atmosphere-ionosphere system

Gravity, tidal, and planetary waves propagating from below represent sources of energy and momentum responsible for upward vertical coupling from “below” in the atmosphere-ionosphere system. A brief review of the observational evidence for tides and planetary waves in the neutral atmosphere (mesosphere and thermosphere) and, particularly, in the ionosphere, is given with a special emphasis on planetary wave type oscillations. Planetary waves of tropospheric origin detected in the middle atmosphere and lower ionosphere are unable to propagate efficiently deep into the thermosphere. However, planetary wave type oscillations are observed at the F2-region maximum heights. We must look for indirect ways for extending their influence upwards. Amplitudes of planetary wave oscillations in foF2 may be high (>1 MHz), with impact on predictions of radio wave propagation.

Solar cycle effect on oscillations in the period range of 2-20 days in the F region of the ionosphere

The results of Lastovicka and Mlch (1994) and Altadill and Lastovicka (1996) are extended by the use of f0F2 and h'F data from Juliusruh, Lannion and Rome. There is a strong solar cycle effect in amplitudes of oscillations in f0F2 but none or a weak solar cycle effect in relative amplitudes of oscillations in f0F2 and in amplitudes of oscillations in h'F. The conditions when the planetary wave type oscillations should be taken into account in short-term predictions and when, on the other hand, they need not be taken into account, are partly specified.

Long period meteor wind oscillations

Significant oscillations in the meteor wind with periods between 2 and 5 days have been found at Sheffield during several summer seasons and these appear to be linked with periodic variations of tropospheric parameters near the main meteor collecting areas. It was originally assumed that these oscillations were the result of planetary waves propagating upwards from the lowest atmospheric levels but the theory of such waves precludes direct propagation, particularly during the summer. It would appear, though, that theoretically sufficient planetary wave energy may leak upward during the autumn and winter to cause significant changes in the meteor wind patterns. Recent observations at Sheffield, however, do not show an enhancement of the amplitudes of planetary wave type oscillations at meteor heights at the beginning of the winter season although strong coupling is found between the lower and upper levels of the atmosphere. The absence of a marked seasonal effect would therefore suggest that the observed phenomena cannot be explained on the basis of the present theory of planetary waves and we would therefore propose that coupling is effected by a modulation-demodulation process involving short-period internal atmospheric gravity waves propagating obliquely to meteor heights.