Meteor Radar Observations Research Articles

Nonlinear coupling between quasi 2 day wave and tides based on meteor radar observations at Maui

AbstractAn observational study of nonlinear interaction between the quasi 2 day wave (QTDW) and the diurnal and semidiurnal tides from meteor radar measurements at Maui is reported. The diurnal and semidiurnal tides show a short‐term variation with the QTDW activity. The variation of amplitude of the semidiurnal tide is opposite to that of the QTDW. The minimum amplitudes of the diurnal tide appear several days later than the maximum amplitudes of the QTDW, and the diurnal tide obviously strengthens when the QTDW drops to small amplitudes. The bispectrum analysis shows significant nonlinear interactions among the QDTW and the tidal components. The two quasi 16 h modes with periods of 16.2 h and 15.8 h generated in the interactions of the QTDW with the diurnal and semidiurnal tides can clearly be distinguished because of the slight deviation of the QTDW period from 48 h. The bicoherence spectrum demonstrates that the QTDW and the semidiurnal tide have quite strong levels of coherence, indicating that the nonlinear interaction is a mechanism responsible for the variability of the semidiurnal tide. Although there is also some interaction between the QTDW and the diurnal tide, their coherence level is low. When the QTDW drops to very weak amplitudes, the background wind decreases and reverses. During this time, the diurnal tide holds large amplitudes. These results support the notion that the variability of the diurnal tide is mainly attributable to the strong QTDW‐induced changes in the background atmosphere, which was shown in the modeling study by Chang et al. (2011). Hence, both the nonlinear interaction and the background flow changes are responsible for the observed variation of the diurnal tide.

Mean winds in the MLT, the SQBO and MSAO over Ascension Island (8° S, 14° W)

Abstract. Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8° S, 14° W) have been measured at heights of approximately 80–100 km by a meteor radar. The results presented in this study are from the interval October 2001 to December 2011. In all years, the monthly-mean meridional winds display a clear annual oscillation. Typically, these winds are found to be southward during April–October, when they reach velocities of up to about −23 m s−1, and northward throughout the rest of the year, when they reach velocities up to about 16 m s−1. The monthly-mean zonal winds are generally westward throughout most of the year and reach velocities of up to about −46 m s−1. However, eastward winds are observed in May–August and again in December at the lower heights observed. These eastward winds reach a maximum at heights of about 86 km with velocities of up to about 36 m s−1, but decay quickly at heights above and below that level. The mesospheric semi-annual oscillation (MSAO) is clearly apparent in the observed monthly-mean zonal winds. The winds in first westward phase of the MSAO are observed to be much stronger than in the second phase. The westward phase of the MSAO is found to maximise at heights of about 84 km with typical first-phase wind velocities reaching about −35 m s−1. These meteor-radar observations have been compared to the HWM-07 empirical model. The observed meridional winds are found to be generally more southward than those of the model during May–August, when at the lower heights observed the model suggests there will be only weakly southward, or even northward, winds. The zonal monthly-mean winds are in generally good agreement, although in the model they are somewhat less westward than those observed. Throughout the observations there were eight occasions in which the first westward phase of the MSAO was observed. Strikingly, in 2002 there was an event in which the westward winds during the first phase of the MSAO were much stronger than normal and reached velocities of about −75 m s−1. This event is explained in terms of a previously proposed mechanism in which the relative phasing of the stratospheric quasi-biennial oscillation (SQBO) and the MSAO allows an unusually large flux of gravity waves of large westward phase speed to reach the mesosphere. It is the dissipation of these gravity waves that then drives the MLT winds to the large westward velocities observed. It is demonstrated that the necessary SQBO–MSAO phase relationship did indeed exist during 2002, but not during the other years observed here. This demonstration provides strong support for the suggestion that extreme zonal-wind events during the MSAO result from the modulation of gravity-wave fluxes.

On the variability of the terdiurnal tide over a Brazilian equatorial station using meteor radar observations

Features of the terdiurnal tide in the mesosphere and lower thermosphere (MLT) are investigated using four years (2005–2008) of meteor radar wind data from a southern hemisphere equatorial station, São João do Cariri (7.4°S, 36.5°W). Prominent existence of the terdiurnal tide is found to be a permanent feature of the MLT over the present site. It exhibits considerable inter-annual variability with smaller amplitude during 2007. Maximum monthly mean amplitudes of the terdiurnal tide are found to be ~11 and 14m/s in the zonal and meridional winds, respectively. The meridional wind shows sharp amplitude enhancement during fall equinox and a broader peak during late winter and spring equinox. Phase profiles indicate large vertical wavelength and possible wave evanescence. The generation mechanism of the terdiurnal tide has been investigated and it is inferred that nonlinear interaction between the diurnal tide and semidiurnal tide/gravity waves can partially excite the terdiurnal tide during fall equinox and solar heating is believed to be the primary source at the present latitude throughout the year.

Diurnal tides at low latitudes: Radar, satellite, and model results

Mean winds and tidal signatures in the mesosphere and lower thermosphere (MLT) region are derived from meteor radar observations at three sites around 22°S acquired in 2005. The observed differences of mean winds and tides are discussed in relation to the meteorological situation in the lower atmosphere and the possible generation of non-migrating tides. The longitudinally well separated radar sites allowed the evaluation of the migrating tidal component. The seasonal variation of signatures of the diurnal tide derived from ground-based radar observations, TIDI measurements aboard TIMED satellite, and model results obtained with HAMMONIA (Hamburg Model of the Neutral and Ionized Atmosphere) are compared. The ground-based, satellite, and model results of the total diurnal tide are in good agreement. The same is true for the migrating diurnal tide obtained from the radar observations, TIDI observations and from the model studies of HAMMONIA and GSWM00 (Global Scale Wave Model).

Terdiurnal signatures in sporadic <i>E</i> layers at midlatitudes

Abstract. Global Positioning System radio occultation measurements by the FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate satellites were used to analyse the behaviour of the signature of the terdiurnal tide in sporadic E (ES) layers at midlatitudes (43–63° N). According to theory, the occurrence of ES is expected when the vertical zonal wind shear, which is mainly owing to solar tides, is negative. 4 yr means, based on 3-monthly running mean zonal means from December 2006–November 2010, were constructed for the terdiurnal oscillation in the occurrence frequency of ES. Comparison of the results with VHF meteor radar observations of the terdiurnal tide and the 8 h oscillation in the vertical zonal wind shear at Collm, Germany (51.3° N, 13° E) shows a clear correspondence between the 8 h in ES and in wind shear signature.

Study of the ultra-fast Kelvin wave with meteor radar observations over a Brazilian extra-tropical station

Characteristics of the 3.5-day ultra-fast Kelvin (UKF) wave are investigated with 4 years of meteor radar wind observations over Cachoeira Paulista (22.7°S, 45°W). The UFK signature manifests itself as discrete bursts spanning over various times of the year with generally less amplitude during fall equinox and early winter. Daily as well as seasonal mean amplitude of the UFK component is found to be considerably smaller than that observed previously from equatorial stations. Estimated vertical wavelengths of the UFK wave come out to be larger than 40km. Comparison of the present results with another Brazilian equatorial station implies latitudinal invariance of the wave as well as transient local dynamical influence to its variability. High zonal acceleration caused by momentum flux convergence due to the UFK wave just after equinoxes is believed to be responsible for driving the westerly phase of the mean zonal wind immediately after the equinoxes and hence the mesospheric semiannual oscillation to some extent. There is a clear signature of modulation of the UFK wave amplitude by various harmonics of the annual oscillation.

The Geminid meteor shower during the ECOMA sounding rocket campaign: specular and head echo radar observations

Abstract. The ECOMA (Existence of Charge state Of meteoric smoke particles in the Middle Atmosphere) sounding rocket campaign was conducted during the Geminid meteor shower in December 2010 in order to explore whether there is a change of the properties of meteoric smoke particles due to the stream. In parallel to the rocket flights, three radars monitored the Geminid activity located at the launch site in Northern Norway and in Northern Germany to gain information about the meteor flux into the atmosphere. The results presented here are based on specular meteor radar observations measuring the radiant position, the velocity and the meteor flux into the atmosphere during the Geminids. Further, the MAARSY (Middle Atmosphere Alomar Radar System) radar was operated to conduct meteor head echo experiments. The interferometric capabilities of MAARSY permit measuring the meteor trajectories within the radar beam and to determine the source radiant and geocentric meteor velocity, as well as to compute the meteor orbit.

Study of the quasi-two-day wave during summer over Santa Maria, Brazil using meteor radar observations

Characteristics of the quasi-two-day wave (QTDW) have been investigated during late summer of 2005, 2006 and 2007 using meteor wind observations over Santa Maria (29.7°S, 53.7°W), Brazil. Significant inter-annual variability is evident in the derived wave parameters. Maximum wave amplitude is found to be 18 and 42m/s for zonal and meridional wind, respectively. Mean vertical wavelength shows the values of ∼62 and 74km for zonal and meridional components, respectively. In general high QTDW amplitude is observed when diurnal tide amplitude is lower suggesting strong wave-tide interaction and enhancement of the QTDW at the expense of the diurnal tide energy. Furthermore, the amplitude of the QTDW reveals a clear modulation of 7–8 days periodicity suggesting nonlinear interaction among planetary waves and a probable reason for the observed QTDW variability. Comparison of the present site with the other two Brazilian low latitude sites reveals overall latitudinal invariance except during a few local and/or transient events. There is an indication of wave ducting in the mesosphere and lower thermosphere through which the QTDW can extend latitudinally. Although the response of the QTDW to the mean background wind sometimes exhibits meridional (northward) acceleration, there is no evident signature in the zonal background wind. Both mechanisms suggested so far for exciting the QTDW, i.e. (i) baroclinic/barotropic instability and (ii) Rossby-gravity normal modes are found to be consistent with the present findings.

Temperature trends at 90 km over Svalbard, Norway (78°N 16°E), seen in one decade of meteor radar observations

Temperatures at 90 km altitude above Svalbard (78°N, 16°E) have been determined using a meteor wind radar and subsequently calibrated by satellite measurements for the period autumn 2001 to present. The dependence of the temperatures on solar driving has been investigated using the Ottawa 10.7 cm flux as a proxy. Removing the response of the temperatures to the seasonal and solar cycle variations yields a residual time series which exhibits the negative trend of −4 ± 2 K decade−1. We indicate that, given the month‐to‐month variability and memory in the time series, for a 90% confidence in this trend, we require only 55 months of data – considerably less than the amount available. Cooling of the middle atmosphere, which would be strongly supported by these results, would result in contraction and subsequent lowering of pressure surfaces; we explain that including a negative trend in the pressure model used to obtain temperatures from meteor train echo fading times would also merely serve to augment the observed 90 km cooling.

An enhancement of the lunar tide in the MLT region observed in the Brazilian sector during 2006 SSW

Meteor radar observations at São João do Cariri (7.4°S, 36.5°W), Cachoeira Paulista (23°S, 45°W) and Santa Maria (29.7°S, 53.7°W) have permitted estimates to be made of winds and temperature in the mesosphere and lower thermosphere (MLT). Using horizontal winds the lunar semidiurnal tide was determined from January 2005 to December 2008 for these three sites. In January 2006 an unusual enhancement was observed in the lunar tide amplitude at these stations, for meridional and zonal components. During this period, sudden stratospheric warming (SSW) events were observed in the northern hemisphere. Meridional and zonal winds observed in Brazil showed evidence of SSW effects in the MLT region. Moreover, the mesospheric temperatures at Cachoeira Paulista and Santa Maria presented cooling and a semimonthly oscillation, which followed the new–full–new moon phases. These results suggest that the enhancement of the lunar tide could be a response for the SSW in the MLT.

DYNAMICAL MODEL FOR THE ZODIACAL CLOUD AND SPORADIC METEORS

The solar system is dusty, and would become dustier over time as asteroids collide and comets disintegrate, except that small debris particles in interplanetary space do not last long. They can be ejected from the solar system by Jupiter, thermally destroyed near the Sun, or physically disrupted by collisions. Also, some are swept by the Earth (and other planets), producing meteors. Here we develop a dynamical model for the solar system meteoroids and use it to explain meteor radar observations. We find that the Jupiter Family Comets (JFCs) are the main source of the prominent concentrations of meteors arriving to the Earth from the helion and antihelion directions. To match the radiant and orbit distributions, as measured by the Canadian Meteor Orbit Radar (CMOR) and Advanced Meteor Orbit Radar (AMOR), our model implies that comets, and JFCs in particular, must frequently disintegrate when reaching orbits with low perihelion distance. Also, the collisional lifetimes of millimeter particles may be longer (>10^5 yr at 1 AU) than postulated in the standard collisional models (10^4 yr at 1 AU), perhaps because these chondrule-sized meteoroids are stronger than thought before. Using observations of the Infrared Astronomical Satellite (IRAS) to calibrate the model, we find that the total cross section and mass of small meteoroids in the inner solar system are (1.7-3.5)x10^11 km^2 and 4x10^19 g, respectively, in a good agreement with previous studies. The mass input required to keep the Zodiacal Cloud (ZC) in a steady state is estimated to be 10^4-10^5 kg/s. The input is up to 10 times larger than found previously, mainly because particles released closer to the Sun have shorter collisional lifetimes, and need to be supplied at a faster rate.

Neutral air density variations during strong planetary wave activity in the mesopause region derived from meteor radar observations

Meteor radar observations of upper mesospheric/lower thermospheric winds and temperatures are suitable to investigate planetary waves in the mesosphere. Here we present a case study of the winter season 2009/2010 for two mid-latitude stations at the edge of the polar vortex and one station within the polar vortex on the island of Andøya. The selected interval begins with the build up of a long period oscillation at the end of November 2009 and ends in March 2010 after the January 2010 stratospheric warming. Planetary wave signature is visible in the temperature as well as the wind field derived from standard meteor radar and EOS MLS observations. Further, we infer neutral air density variations caused by this long period oscillation from meteor peak altitude variations of the sporadic meteor component. The results are compared with density variation computed from MLS temperature and pressure data.

Seasonal variability of the diurnal tide in the mesosphere and lower thermosphere over Maui, Hawaii (20.7°N, 156.3°W)

[1] The seasonal variability of the diurnal tide in the mesosphere and lower thermosphere over Maui, Hawaii (20.7°N, 156.3°W), is investigated using meteor radar horizontal wind measurements from the years 2002 to 2007. The semiannual oscillation (SAO) of tidal amplitudes is dominant above ∼88 km, with amplitudes at equinoxes 2–3 times larger than at solstices. Below 88 km, the annual oscillation (AO) dominates, and its magnitude is smaller than the SAO. The AO dominates in the phase variation of the diurnal tide, which advances in winter and lags in summer as compared with equinoxes. The vertical wavelength also has a noticeable seasonal variation with shorter vertical wavelengths found at equinoxes. The reconstructed diurnal tide from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Doppler interferometer (TIDI) and Sounding the Atmosphere using Broadband Emission Radiometry (SABER) measurements is compared with the meteor radar observation, and a consistency is found in the seasonal variation of the tidal amplitude. On the basis of the TIDI and SABER measurements, the migrating diurnal tide (DW1) is the dominant tidal component, while three other nonmigrating tides, DW2, DS0, and DE3, are nonnegligible. The seasonal variation of the diurnal tide is well captured by the global scale wave model and the Whole Atmosphere Community Climate Model, although important discrepancies still exist.

Meteor radar observations of short-term variability of quasi 2 day waves and their interaction with tides and planetary waves in the mesosphere–lower thermosphere region over Thumba (8.5°N, 77°E)

[1] Meteor radar measurements of zonal and meridional winds over a low-latitude station, Thumba (8.5°N, 77°E), during March 2006 to March 2009 are used to study the quasi 2 day wave structure in the mesosphere–lower thermosphere (MLT) region. Emphasis is placed on the study of the day-to-day variations of quasi 2 day waves and their interaction with diurnal tides and planetary waves. The seasonal variation of quasi 2 day waves showed two maxima, one during summer and the other during winter, coinciding with the strong eastward winds in the MLT region along with a distinct peak during the month of October. Subsequent analysis of TIMED/SABER observations during October revealed the presence of a westward propagating quasi 2 day wave with zonal wave number 3 over the present latitude. The seasonal variation of quasi 2 day wave is also discussed with respect to the Stratospheric Semiannual Oscillation (SSAO), and we found that the quasi 2 day wave activity peaks in the MLT region during the phase transition of SSAO as well as during the eastward phase. Wavelet and subsequent bispectral analysis confirmed the wave-wave interactions of quasi 2 day wave and planetary waves with periods of 5–10 and 15–20 days. The 16 h oscillation, which is believed to be generated by interaction of quasi 2 day wave and diurnal tide, showed an excellent correlation with quasi 2 day wave activity. Apart from reporting day-to-day variation of quasi 2 day wave for the first time over this latitude, there are two new results from the present study: (1) a consistent peak in quasi 2 day activity during October, which has never been reported and has been observed for the first time over low latitudes, and (2) the relation between SSAO and quasi 2 day wave activity.

Meteor radar measurements of MLT winds near the equatorial electro jet region over Thumba (8.5° N, 77° E): comparison with TIDI observations

Abstract. The All-Sky interferometric meteor (SKYiMET) radar (MR) derived winds in the vicinity of the equatorial electrojet (EEJ) are discussed. As Thumba (8.5° N, 77° E; dip lat. 0.5° N) is under the EEJ belt, there has been some debate on the reliability of the meteor radar derived winds near the EEJ height region. In this regard, the composite diurnal variations of zonal wind profiles in the mesosphere-lower thermosphere (MLT) region derived from TIMED Doppler Interferometer (TIDI) and ground based meteor radar at Thumba are compared. In this study, emphasis is given to verify the meteor radar observations at 98 km height region, especially during the EEJ peaking time (11:00 to 14:00 LT). The composite diurnal cycles of zonal winds over Thumba are constructed during four seasons of the year 2006 using TIDI and meteor radar observations, which showed good agreement especially during the peak EEJ hours, thus assuring the reliability of meteor radar measurements of neutral winds close to the EEJ height region. It is evident from the present study that on seasonal scales, the radar measurements are not biased by the EEJ. The day-time variations of HF radar measured E-region drifts at the EEJ region are also compared with MR measurements to show there are large differences between ionospheric drifts and MR measurements. The significance of the present study lies in validating the meteor radar technique over Thumba located at magnetic equator by comparing with other than the radio technique for the first time.

Dusty Plasma Effects in Near Earth Space and Interplanetary Medium

We review dust and meteoroid fluxes and their dusty plasma effects in the interplanetary medium near Earth orbit and in the Earth’s ionosphere. Aside from in-situ measurements from sounding rockets and spacecraft, experimental data cover radar and optical observations of meteors. Dust plasma interactions in the interplanetary medium are observed by the detection of charged dust particles, by the detection of dust that is accelerated in the solar wind and by the detection of ions and neutrals that are released from the dust. These interactions are not well understood and lack quantitative description. There is still a huge discrepancy in the estimates of meteoroid mass deposition into the atmosphere. The radar meteor observations are of particular interest for determining this number. Dust measurements from spacecraft require a better understanding of the dust impact ionization process, as well as of the dust charging processes. The latter are also important for further studying nanodust trajectories in the solar wind. Moreover understanding of the complex dependencies that cause the variation of nanodust fluxes is still a challenge.

Characteristics of the semidiurnal tide in the MLT over Maui (20.75°N, 156.43°W) with meteor radar observations

Semidiurnal tidal features have been examined in the Mesosphere and Lower Thermosphere (MLT) from the long-term (2002–2007) meteor wind data over Maui (20.75°N, 156.43°W). Amplitude and phase obtained from the harmonic analysis exhibit large day to day variability. Mean amplitude obtained from the monthly mean data over the observation period is found to vary within ∼8–28 m/s and 10–32 m/s for the zonal and meridional winds, respectively. The amplitude has revealed clear semiannual oscillation (SAO) pattern with maxima during solstices and altitudinal growth in both wind components. Significant resemblance in its variability with other observations carried out from the low latitude sites all over the globe is obtained. Vertical wavelength estimated from the phase gradients exposes large values (>90 km) in all seasons. Contribution of the semidiurnal tide to the total tidal variability in the MLT is found to vary over wide range throughout the year with generally higher influence during winter season over diurnal and terdiurnal components.

Simultaneous observation of quasi 16 day wave in the mesospheric winds and temperature over low latitudes with the SKiYMET radar

[1] The seasonal characteristics of a 16 day planetary wave simultaneously in mesospheric temperature and winds over a low-latitude station Thumba (8.5°N, 76.5°E) using meteor radar observations are discussed for the first time. Four years (2005–2008) of meteor radar winds and temperature observations are used for the present study. It is observed that the amplitude of a 16 day wave in zonal component is more than that of meridional. Further analysis shows that the westerly phase of zonal wind is more favorable for the 16 day waves. The maximum amplitude of a 16 day wave in mesospheric temperature is observed during January–February and August–September. Climatology of a 16 day wave shows the signature of semiannual oscillation (SAO) in mesospheric temperature but not in winds. The vertical amplitude structure of zonal component shows the maximum amplitude at ∼88–92 km with constant phase. It is also noticed that zonal and meridional winds are in phase, whereas the temperature leads zonal wind by 5 ± 1 days. The significance of the present study lies in showing the 16 day wave characteristics, effect of background winds, and manifestation of SAO on their variability.

Extensive meteoroid fragmentation in V/UHF radar meteor observations at Arecibo Observatory

New 46.8/430 MHz (V/UHF) radar meteor observations at Arecibo Observatory reveal many previously unreported features in the radar meteor return that are consistent with meteoroid fragmentation. These signature features—observed at both V/UHF—include strong intra‐pulse and pulse‐to‐pulse fading as the result of interference between or among multiple meteor head‐echo returns and between head‐echo and impulsive “flare” event “trail‐echoes”. A few events are suggestive of differential ablation. These V/UHF radar observations are particularly useful as coaxial beams enable common volume observations that remove much ambiguity in the interpretation of the observations in terms of meteoroid fragmentation. These results, combined with simple modeling and two 500 sample statistical studies, lead us to conclude that a majority of the ∼17,000 AO radar meteor events included in this study exhibit fragmentation—manifested as non‐uniform lightcurves—implying that the form of meteoroid mass flux into the upper atmosphere goes well beyond simple ablation.

Genetic-Algorithm-Based Parameter Estimation Technique for Fragmenting Radar Meteor Head Echoes

Meteoroid fragmentation presents a serious problem for Doppler estimation using Fourier transform techniques. Radar returns from multiple closely spaced bodies traveling at nearly identical speeds result in an interference pattern which makes it difficult to estimate properties of individual bodies by traditional techniques. Here, we present a genetic-algorithm-based procedure to determine the properties of the individual fragments, such as relative scattering cross section, speed, and deceleration. The radar meteor observations presented here were made using the Poker Flat (Alaska) Incoherent Scatter Radar operating at 449.3 MHz.