MF Radar Research Articles

A case study of gravity waves in noctilucent clouds

Abstract. We present a case study of a noctilucent cloud (NLC) display appearing on 10-11 August 2000 over Northern Sweden. Clear wave structures were visible in the clouds and time-lapse photography was used to derive the parameters characterising the gravity waves which could account for the observed NLC modulation. Using two nearby atmospheric radars, the Esrange MST Radar data and Andoya MF radar, we have identified gravity waves propagating upward from the upper stratosphere to NLC altitudes. The wave parameters derived from the radar measurements support the suggestion that gravity waves are responsible for the observed complex wave dynamics in the NLC.

Simultaneous mesosphere/lower thermosphere and thermospheric F region observations during geomagnetic storms

Simultaneous mesosphere/lower thermosphere (MLT) and thermospheric F region observations made during geomagnetic storms are presented using the MU radar (35°N, 136°E) in alternate meteor and incoherent scatter (IS) modes, MF radar (45°N, 136°E), optical mesosphere thermosphere imagers (OMTI) (35°N, 136°E) and over 1000 GPS receivers in Japan. The observations were made during one of the MTEC‐S (mesosphere‐thermosphere experiments for coupling studies) campaigns conducted during 23 March to 2 April 2001 when a major storm with Dst reaching −358 nT and a moderate storm with Dst reaching −112 nT occurred and solar activity varied from high to very high (F10.7 = 220 to 270). The campaign has provided zonal and meridional wind velocities at MLT altitudes (80–95 km) over the MU and MF radars; meridional wind velocity in the upper thermosphere (220–450 km) and ionospheric electron density (150–600 km), peak height and plasma drift velocity over the MU radar; all‐sky airglow intensities at 557.7 nm, 630.0 nm, and 777.4 nm wavelengths over the MU radar location; and vertical GPS‐TEC all over Japan. Study of the data indicates that following the major storm, the meridional wind in the upper thermosphere over the MU radar (35°N) becomes weak (less poleward) during daytime and its diurnal amplitude reduces. The winds and tides at MLT altitudes show changes during both nonstorm times and storm times, but this study is not able to confirm any statistically significant changes related to the storms. In the ionosphere, following the onset of the major morning storm, the electron density increases at altitudes near and above the ionospheric peak, which after about 5 hours is followed by large increase in density at all altitudes above about 200 km. On the following night, though the density is generally low, there is a large increase in density in the bottomside for about 3 hours centered at 0230 LT. During this time, a rare intense optical activity has occurred at all wavelengths (557.7 nm, 630 nm, and 777.4 nm) over the low midlatitude location (35°N), and an associated large‐scale traveling ionospheric disturbance (LSTID) has propagated from north to south at F region altitudes.

Longitudinal and latitudinal variations in dynamic characteristics of the MLT (70−95km): a study involving the CUJO network

Abstract. The newly-installed MFR (medium frequency radar) at Platteville (40°N, 105°W) has provided the opportunity and impetus to create an operational network of middle- latitude MFRs stretching from 81°W–142°E. CUJO (Canada U.S. Japan Opportunity) comprises systems at London (43°N, 81°W), Platteville (40°N, 105°W), Saskatoon (52°N, 107°W), Wakkanai (45°N, 142°E) and Yamagawa (31°N, 131°E). It offers a significant 7000km longitudinal sector in the North American-Pacific region, and a useful range of latitudes (12–14°) at two longitudes. Annual climatologies involving both height and frequency versus time contour plots for periods from 8h to 30 days, show that the changes with longitude are very significant and distinctive, often exceeding the local latitudinal variations. Comparisons with models and the recent UARS-HRDI global analysis of tides are discussed. The fits of the horizontal wave numbers of the longer period oscillations are provided in unique frequency versus time contour plots and shown to be consistent with the expected dominant modes. Annual climatologies of planetary waves (16 day, 2 day) and gravity waves reveal strong seasonal and longitudinal variations. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides; climatology)

Mesopause dynamics from the scandinavian triangle of radars within the PSMOS-DATAR Project

Abstract. The "Scandinavian Triangle" is a unique trio of radars within the DATAR Project (Dynamics and Temperatures from the Arctic MLT (60–97km) region): Andenes MF radar (69°N, 16°E); Tromsø MF radar (70°N, 19°E) and Esrange "Meteor" radar (68°N, 21°E). The radar-spacings range from 125-270km, making it unique for studies of wind variability associated with small-scale waves, comparisons of large-scale waves measured over small spacings, and for comparisons of winds from different radar systems. As such it complements results from arrays having spacings of 25km and 500km that have been located near Saskatoon. Correlation analysis is used to demonstrate a speed bias (MF smaller than the Meteor) between the radar types, which varies with season and altitude. Annual climatologies for the year 2000 of mean winds, solar tides, planetary and gravity waves are presented, and show indications of significant spatial variability across the Triangle and of differences in wave characteristics from middle latitudes. Key words: Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides: instrument and techniques)

Mesosphere/lower thermosphere prevailing wind model

The mesosphere/lower thermosphere (MLT) wind data from the 46 ground-based (GB) MF and meteor radar (MR) stations, located at the different latitudes over the globe, and the space-based (SB) HRDI data were used for constructing of the empirical global climatic 2-D prevailing wind model at 80–100 km heights for all months of the year. The main data set is obtained during 1990–2001 period. It is shown that the three datasets (MF, MR, HRDI) are mainly well correlated. However, a certain systematic bias between the GB and SB data at 96 km exists, as well as that between the MF and MR data higher 88 km. Simple correction factors are proposed to minimize these biases. The 2-D distant-weighted least-square interpolation procedure for some arbitrary collection of points was used for drawing model contour plots. The model is available in the computer readable form and may be used for construction of the new CIRA model.

MF radar observation of mean wind and tides of winter mesopause (80– [formula omitted]) region over Wuhan (30°N, 114°E)

Data obtained from the medium-frequency radar at Wuhan (30°N, 114°E) from 11 to 25 February and from 28 February to 13 March have been used to study the mean wind and tidal oscillations at mesopause (80– 98 km ) region over Wuhan in winter. The observed zonal and meridional mean winds show obvious temporal variation and usually decrease with the increasing heights. In most observational time, the zonal mean wind is eastward. A dynamic spectral analysis of disturbance amplitude illustrates a prominent peak at a period of 24 h at all heights and almost uninterrupted, and there are occasionally slightly strong semidiurnal and weak terdiurnal tidal disturbances, indicating that in the mid-latitude regions, the diurnal tide is the dominant tidal disturbance at mesopause in winter. The temporal variation of tidal amplitudes and their wave kinematical energies illustrates that there may be significant resonant interaction among the diurnal, semidiurnal and terdiurnal tides. A quantitative comparison with the global-scale wave model (GSWM) demonstrates that the observed diurnal and semidiurnal tidal amplitudes are less and larger than the results of GSWM-02, respectively. The essentially linear GSWM cannot predict the decrease of diurnal tidal amplitude at higher heights, which may be due to several possible nonlinear processes, such as wave–wave interaction and wave break. Although the height variations of tidal phases are consistently well with the predictions of GSWM, there are evident phase differences between our observation and the model. The profiles of the diurnal tidal phases show obvious decrease trend with the increasing height, indicating the observed diurnal tide is propagating upward. The vertical wavelength of the observed diurnal tide is estimated to be about 37 km , and the corresponding downward phase velocity is about 1.5 km −1 h .

Responses of the mesospheric wind at high latitudes to the April 2002 space storm

An intense space storm occurred on 17–20 April, 2002. The effects of this storm on the mesospheric wind system were observed by two meteor wind radars at Andenes (69N, 16E) and Esrange (68N, 21E). At those locations, neutral winds showed a pattern similar to the ion convection pattern on 17 April for several hours at altitudes of 94–97 km. It is the first time that this kind of wind pattern is detected at such low altitudes. In contrast, winds by a medium frequency radar at Poker Flat (65N, 213E) showed no obvious storm effect, reflecting different responses of the dayside atmosphere and the nightside atmosphere. The ion convection wind pattern at Andenes and Esrange only appeared on the first day of the storm. We hypothesize that this behaviour may be related to a short‐lived but prominent electron density enhancement in the local thermosphere on the same day observed by an incoherent scatter radar at Svalbard (78N, 16E).

Equivalent electron densities at reflection heights of tweek atmospherics in the low-middle latitude D-region ionosphere

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20—28 el./cm3 at ionospheric reflection heights of 80—85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

Planetary wave activity and rotational effects in the mid-latitudes of the lower and middle atmosphere (0–100KM)

Investigation of the wind regime during solstices (1998–1999) allowed the presentation of the vertical structure of the intensity of planetary waves in the zonal and meridional wind. The seasonal dependence of the vertical structure of these planetary wave amplitudes is determined at middle atmosphere heights. Daily mesosphere/lower thermosphere (MLT) experimental data, measured at Collm (52N 15E, LF D1 radar), Kazan (56N 49E, meteor radar) and Saskatoon (52N 103W, MF radar) in the height range 80–100 km, and UKMO assimilated wind data from the British Atmospheric Data Center in the height range 0–55 km are used. In winter, maxima of the planetary wave intensity are observed near the tropopause and in the stratosphere. A decrease of the planetary wave intensity in the mesosphere coincides with the weakening of the stratospheric jet, and with the decay of the zonal flow in the mesosphere. Processes such as radiative suppression of planetary waves in the mesosphere and their additional reductions as a result of wave interactions and interactions with the zonal flow may explain these significant decreases of the intensity. Rotational spectra and estimates of rotational amplitudes in the height range of the middle atmosphere allow the investigation of the change of rotation direction of the horizontal wind vector dependent on height and period of the wave perturbations.

Ionospheric and dynamical characteristics of the mesosphere‐lower thermosphere region over Platteville (40°N, 105°W) and comparisons with the region over Saskatoon (52°N, 107°W)

The newly installed medium frequency radar (MFR) at Platteville provides unique opportunities to assess latitudinal effects in both the ionosphere and mesosphere‐lower thermosphere (MLT) by comparisons with the long‐established MFR at Saskatoon. The influence of the D region “winter anomaly” is evident in both ionospheres, and descending “sporadic layers” (110–90 km) are identified, especially at 40°N, for the first time for MF radar systems. Preliminary comparisons with the wind measurements are made, and the processes are identified as complex. Contour plots of mean winds, tides (12‐ and 24‐hour), and planetary waves (PW) (2‐ and 16‐day) demonstrate significant trends over 12° of latitude (1100 km). The 24‐hour tide dominates at 40°N, the 12‐hour tide dominates at 52°N, and PW structures demonstrate spatial and temporal intermittency. The two radars are now part of a new network, Canada U.S. Japan Opportunity (CUJO), stretching from 81°W to 141°E.

Validation of imaging Doppler interferometer winds using meteor radar

There has been some debate over the years concerning the accuracy of mesospheric wind observations made using the imaging Doppler interferometer (IDI) technique. The high potential and increasing use of IDI wind data in joint studies with spaced‐antenna MF and meteor radar systems make it important to quantify the IDI results. This paper presents a novel comparison of wind measurements between a dynasonde implementation of IDI and winds derived from an all‐sky meteor radar system, a widely‐accepted standard for such measurements. Both radars were located at the USU Bear Lake Observatory and operated almost continuously for a four‐month period. The winds and tides derived from IDI were found to closely match those measured by meteor radar, not only during the day but also at night, and at all overlapping heights from 80–95 km.

Rapid, large‐scale temperature changes in the polar mesosphere and their relationship to meridional flows

Mesospheric temperatures derived from spectroscopic measurements of the hydroxyl (OH) nightglow have been observed from Rothera (67.6°S, 68.1°W) and Halley (75.6°S, 26.6°W) stations in Antarctica during the 2002 austral winter season. In addition to the normal seasonal changes, the temperatures at both sites, separated by 1658 km, showed several simultaneous shifts in temperature of between 10 and 20 K. These changes abruptly occurred in the space of 2–3 days and lasted for several days. These rapid variations in temperature were associated with large swings observed in the meridional component of the mesospheric wind measured by the Rothera MF radar. As there appeared to be no phase shift between the temperature variations at the two longitudinally separated stations indicating that planetary‐waves caused the changes, these large‐scale changes have been interpreted as variations in the inter‐hemispheric meridional jet, and a corresponding modulation of the mesospheric descent and adiabatic heating rates over the polar region.

The quasi 2‐day wave observed in the polar mesosphere

Using MF radar data obtained at Tromsø (69.6°N, 19.2°E) for ∼37 months between 1 November 1998 and 8 December 2001, we have examined characteristics of the quasi 2‐day wave (Q2DW) in the polar mesosphere in the height range from 70 to 91 km. The activity of the Q2DW is higher in winter months (November–February) than in summer months (May–August) over this height region. The maximum values of the amplitude are ∼25 m s−1 in winter and ∼15 m s−1 in summer. Between 70 and 82 km, the amplitude appears to maximize near winter solstice over the 3 years. The average ratio of meridional to zonal amplitudes is ∼1.1–1.2 over the height region. Thus, there is no significant preference towards either components. The variations of the periods of maximum amplitude are also examined, and periods with 45.2 and 54.9 hours occur more frequently than those with 48.0 and 51.2 hours. However, no clear seasonal trend of the variation of the period is found. In addition, the amplitude is modulated at 4–10 day rate. With simultaneous observational data from the MF radar and the EISCAT UHF radar colocated in Tromsø, we have examined whether or not the Q2DW was able to penetrate to the lower thermosphere for the period of 1–9 July 1999. It is found that the amplitude of the Q2DW with period of 51.2 hours maximized at 95 km height with values of ∼25 m s−1 in both meridional and zonal components. The Q2DW was attenuated above 95 km, but it appears to survive up to 108 km height in the meridional component. Between 85 and 95 km for the meridional component during this 8‐day interval, the Q2DW and semidiurnal tide were the strongest among the periodic components.

Modulation of gravity waves by planetary waves (2 and 16 d): observations with the North American-Pacific MLT-MFR radar network

In an earlier study based upon medium frequency radar (MFR) data from Saskatoon (52°N) the variability of time-sequences of gravity wave (GW) variances was linked to tidal (12,24 h) and planetary wave (2 d) oscillations of the wind. Fifty days of data were chosen from each of winter, spring and autumn seasons for this most comprehensive assessment of wave interactions. While modulations of the GW variances were observed, the results indicated considerable intermittency in the strength and direction of waves from the GW sources. Here we extend the study to other sites in the MLT-MFR (mesosphere, lower-thermosphere) network, and focus upon the planetary waves (PWs) and their modulating influences upon the GW variances. The PW events include the 2 d waves as seen at Saskatoon, London, Hawaii and Christmas Island during the summer of 1994; and 16 d wave activity evidenced at Saskatoon and London throughout 1994, and also during 12 and 7 years of observations, respectively. The modulations of the sequences of GW variances (10– 100/150 min , 2– 6 h periods) are significant, and the phase-differences between the PW (2 d,16 d) oscillations in the time-sequences of the winds and of the GW variances allow the propagation directions for the GW fluxes to be inferred. These are eastward in summer months and westward in winter at MLT (60– 90 km) altitudes.

Differential absorption measurements of mesospheric and lower thermospheric electron densities using the Buckland Park MF radar

The differential absorption experiment (DAE) was first proposed in the 1950s for the estimation of mesospheric and lower thermospheric electron density using MF/HF radars. The technique was used extensively until the late 1970s, when interest in the technique declined, due to experimental limitations and questions regarding the assumptions of the technique. This paper describes the application of the DAE within the online observations of the Buckland Park MF (BPMF) radar. The experimental limitations of the technique for the BPMF radar are discussed, with particular attention paid to effects of complex gain differences between receiving channels used to decompose the linearly received signals into circular components. Hourly and monthly averaged midday DAE electron densities are presented, revealing good agreement with IRI model estimates. Monthly averaged midnight DAE electron densities are also presented, revealing good qualitative agreement with the IRI model estimates.

Collision frequencies in the D-region

Partial reflection differential absorption and differential phase observations have been used to systematically study collision frequencies ( ν) in D-region of the lower ionosphere. The observations made with the large MF radar located at Buckland Park (35° S, 138° E) near Adelaide in the period September 1996–December 2000 show ν values larger than predicted by previously used models. The new estimates are compared with values calculated using new collision frequency momentum cross-sections for N 2( σ) measured in the laboratory. The two types of measurements are found to be in good agreement, with a moderate seasonal variation of ν at a constant height. No change due to solar cycle variations is found.

Climatological studies of the quasi 16-day oscillations in the mesosphere and lower thermosphere at Yamagawa (31.2° N, 130.6° E), Japan

Abstract. The 16-day wave climatology at Yamagawa (31.2° N, 130.6° E) is investigated by analyzing the mesosphere and lower thermosphere (MLT) wind data collected with the recently installed MF radar. We have utilized the continuous data gathered during the five-year period 1996–2000. The wave climatology clearly shows some seasonal variations. The period of late autumn-spring is marked with larger wave activity, with the strongest waves being observed in the winter months. The maximum amplitude observed at Yamagawa is about 20 m/s, which is comparatively larger than the amplitudes observed at mid-latitude stations. The height dependence of the 16-day wave suggests that the maximum amplitude is observed at altitudes below 80 km. The summer months are characterized with much weaker wave activity. The vertical wavelength appears to be larger in the winter months and shorter in the summer months. The present analysis again confirms that the 16-day wave is highly sensitive to the background mean winds. Eastward motion of the background winds is a more favourable condition for the 16-day wave penetration to the MLT heights. The wave features show some signs of interannual variability. Overall, the observed features of the 16-day wave at Yamagawa, which is located at the edge of the subtropical latitudes, show some correspondence with the results reported for mid-latitude stations.Key words. Meteorology and atmospheric dynamics (climatology; thermospheric dynamics)

A comparison study of the auroral lower thermospheric neutral winds derived by the EISCAT UHF radar and the Tromsø medium frequency radar

A comparative study of the neutral wind in the polar upper mesosphere/lower thermosphere is conducted using two radars, the European Incoherent Scatter (EISCAT) UHF radar (ν = 931 MHz) and the Tromsø MF radar (ν = 2.8 MHz) colocated in northern Scandinavia. Comparisons of winds are made in case studies as well as on a statistical basis. On the basis of simultaneous observations obtained by the two radars, comparisons over a height range from ∼90 to ∼100 km are made for about 20 days between February and October 1999. In these comparisons we directly compare temporal wind variations. On the other hand, mean winds as well as diurnal and semidiurnal components are derived using 1‐month‐averaged wind data obtained by the medium frequency (MF) radar for 1999. They are compared with those derived from 56 days of EISCAT data studied by Nozawa and Brekke [1999a, 1999b]. In the case studies, we have also utilized altitude profiles of electron density obtained by the EISCAT radar at and above 62 km height to determine the total reflection height as well as to estimate the effect of group retardation for 2.8 MHz radio wave. It can be seen that the effect of particle precipitation sometimes penetrates into the D region (down to ∼84 km). Generally, wind velocities derived by EISCAT with the monostatic method exhibit significant scatter with time below 100 km, while the number of the MF radar wind data sharply decreases above 90 km except for summer. These facts make the comparison difficult, but no significant departures between different radar winds are identified on an instantaneous basis. In the statistical studies, generally both altitude profiles are well connected, or continuous, but significant disagreements are observed for the mean wind and the tidal components for summer. On the basis of these comparisons, we raise strong concerns regarding the use of summer wind data above 91 km obtained by MF radar located at high latitude under high solar activity conditions. The wind values during the winter and equinox observed by the two radars are complementary. Thus here is a good opportunity to utilize wind data from the two different radars through a year, which then enables us to cover a wide height range (approximately 70–120 km). This wide range gives us good opportunities to examine the mesosphere/lower thermosphere coupling processes based on observational data.

On the response of fading times of upper homosphere radar echoes to solar and geomagnetic disturbances

Four years of auroral latitude radar echo signal fading time data from the Tromso MF radar compared with geomagnetic and solar indices yield strong evidence to suggest, although not prove, that upper mesospheric turbulence may be enhanced during periods of geomagnetic activity. We present a critique of hypotheses to explain such effects.

Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2–70° N, modelled tides (GSWM, CMAM)

Abstract. In an earlier paper (Manson et al., 1999a) tidal data (1990–1997) from six Medium Frequency Radars (MFR) were compared with the Global Scale Wave Model (GSWM, original 1995 version). The radars are located between the equator and high northern latitudes: Christmas Island (2° N), Hawaii (22° N), Urbana (40° N), London (43° N), Saskatoon (52° N) and Tromsø (70° N). Common harmonic analysis was applied, to ensure consistency of amplitudes and phases in the 75–95 km height range. For the diurnal tide, seasonal agreements between observations and model were excellent while for the semi-diurnal tide the seasonal transitions between clear solstitial states were less well captured by the model. Here the data set is increased by the addition of two locations in the Pacific-North American sector: Yamagawa 31° N, and Wakkanai 45° N. The GSWM model has undergone two additional developments (1998, 2000) to include an improved gravity wave (GW) stress parameterization, background winds from UARS systems and monthly tidal forcing for better characterization of seasonal change. The other model, the Canadian Middle Atmosphere Model (CMAM) which is a General Circulation Model, provides internally generated forcing (due to ozone and water vapour) for the tides. The two GSWM versions show distinct differences, with the 2000 version being either closer to, or further away from, the observations than the original 1995 version. CMAM provides results dependent upon the GW parameterization scheme inserted, but one of the schemes provides very useful tides, especially for the semi-diurnal component.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)