Atmosphere Radar Observations Research Articles

Semidiurnal Tidal Influence on the Occurrence of Postmidnight F Region FAI Radar Echoes

AbstractThe Equatorial Atmosphere Radar observations at Kototabang (0.2°S, 100.3°E) are used to study the possible semidiurnal tidal influence on the occurrence of postmidnight echoes from the field‐aligned irregularities (FAIs) due to spread F. It is found that the postmidnight FAI echoes show high percentage of occurrence (PO) during June–July and low PO in December–January of low solar activity years. As solar activity approaches minimum, the PO increase is extended to May, August, and September. The space‐time Fourier analysis on the temperature information obtained from the Sounding of Atmosphere by Broadband Emission Radiometry instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite reveals that at the low‐latitude upper mesospheric heights, though the migrating diurnal tide propagating westward with zonal wavenumber (k) 1 (DW1) dominates the migrating semidiurnal tide propagating westward with k = 2 (SW2) particularly during the equinox months, the SW2 tidal amplitudes are larger than DW1 during June–July. The postmidnight FAI echoes appear to be more frequent during the sudden stratospheric warming of the Solar Minimum Years 2018–2019 and 2008–2009 but not in the Solar Maximum Year 2013. The dominance of SW2 over DW1 is also noted during these years. This indicates that the SW2 plays a major role in the occurrence of postmidnight spread F. It is suggested that the dominant semidiurnal variation of zonal electric field can become eastward and lift the F layer to higher heights during midnight hours, which can favor the growth of the Rayleigh‐Taylor instability to cause delayed spread F around midnight.

On the Solstice Maxima and Azimuth‐Dependent Characteristics of the 150‐km Echoes Observed Using the Equatorial Atmosphere Radar

AbstractWe present long overdue statistical analysis of the 150‐km echoes based on 10 years of observations made using the Equatorial Atmosphere Radar located at Kototabang, Indonesia. Analysis shows that while the basic features of the echoes are very similar to those observed from other equatorial/low‐latitude stations, occurrence and power/signal‐to‐noise ratio of the echoes are much higher in the solstices than in the equinoxes. Furthermore, both occurrence rate and power/signal‐to‐noise ratio of the echoes in the December solstice are marginally lower than those of the June solstice. The observed solstice‐equinox asymmetry has important implications with regard to the generation mechanism of the 150‐km echoes since this seasonal behavior cannot be accounted for by the recently proposed photoelectron‐induced enhanced plasma wave process alone. Equatorial Atmosphere Radar observations also display remarkable azimuthal variations of the echo characteristics, which indicate magnetic aspect angle dependence of the echoes. The implications of these results are discussed in terms of the origin of the 150‐km echoes.

Intercomparison of various SA techniques on wind estimation using multi-receiver phased array radar

In this paper we have applied various spaced antenna (SA) techniques to derive horizontal winds with Middle and Upper (MU) atmospheric radar observations. SA techniques comprise different analysis methods like Full Correlation Analysis (FCA), Cross Correlation Ratio (CCR), Slope at Zero lag (SZL) and Intersection (INT) methods etc. Among these methods, FCA is considered one of the classical approaches. In the present study, the MU radar antenna array is regrouped (in software) into three different antenna segments, each segment consists of 8 channels and forming an equilateral triangle. Grouping of antenna has been done with 4 different orientations by interchanging the channels. FCA wind has been estimated with an equilateral triangle grouping of antenna array in all orientations. With the same grouping in different orientations, winds are estimated with CCR, SZL and INT methods. Winds estimated with different orientation are averaged to obtain the mean winds component. In most of studies reported earlier using CCR, SZL and INT methods, winds have been estimated with pair of antenna having baseline parallel to the antenna array plane. We have retrieved the zonal wind with a pair of antennas having baseline parallel to the antenna array plane in east–west direction and meridional wind is estimated with a pair of antennas having baseline parallel to the antenna array plane in north–south direction. The mean winds obtained with various SA techniques are compared with that observed by the Doppler Beam swinging (DBS) technique and GPS-sonde. It is observed that mean winds obtained with the approach adopted using different spatially distributed array grouping has yielded higher height coverage up to ~20km and in good agreement with the results obtained using DBS observations even with a temporal resolution of ~1.3min in clear air condition. Various statistical analyses like correlation analysis, standard deviation, root mean square error (RMSE) and error analysis have been performed to understand the performance of other techniques in comparison with FCA. The analysis is carried out for an observation of 6h for two different observation periods.

A Minimum Threshold for Wind Profiler Signal-to-Noise Ratios

Abstract One limiting factor in atmospheric radar observations is the inability to distinguish the often weak atmospheric signals from fluctuations of the noise. This study presents a minimum threshold of usability, SNRmin, for signal-to-noise ratios obtained from wind profiling radars. The basic form arises from theoretical considerations of radar noise; the final form includes empirical modifications based on radar observations. While SNRmin was originally developed using data from the 50-MHz profiler at Poker Flat, Alaska, it works well with data collected from a wide range of locations, frequencies, and parameter settings. It provides an objective criterion to accept or reject individual spectra, can be quickly applied to a large quantity of data, and has a false-alarm rate of approximately 0.1%. While this threshold’s form depends on the methods used to calculate SNR and spectral moments, variations of the threshold could be developed for use with data processed by other methods.

Equatorial Atmosphere Radar observations of short vertical wavelength gravity waves in the upper troposphere and lower stratosphere region induced by localized convection

Equatorial Atmosphere Radar (EAR) observations made at Koto Tabang (0.2°S, 100.32°E) revealed that gravity waves with a short vertical wavelength (∼2–4 km) were evident in the upper troposphere and lower stratosphere (UTLS). Results are based on two convective events observed on 11 April and 26 April 2004. Above the convective region (∼12–13 km heights) the decrease in the vertical wavelength (λz) found in vertical wind profiles was noticeable. Large portions of the induced gravity waves with short λz dissipated in the lower stratosphere and hence their role is considered to be of importance for the dynamics of UTLS region. Gradual decrease in λz of gravity waves between 12 km and 20 km heights is partly made by a steady increase in Brunt Väisälla frequency from ∼12 km to 20 km heights. The decrease in amplitude in UTLS may be due to damping mechanism.

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.

A climatology of middle and upper atmosphere radar observations of thermospheric winds

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time‐of‐day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) and with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong one in the early daylight hours and a weak one in the afternoon‐evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady wind of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward in winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.

Modeling studies of the middle and upper atmosphere radar observations of the ionospheric F layer

Observations of electron density and plasma drift velocity made during noon, June 25, 1990, to noon, June 27, 1990, by the middle and upper atmosphere (MU) radar at Shigaraki, Japan, are investigated by comparing them with values modeled by the Sheffield University plasmasphere‐ionosphere model. The model calculations, with inputs for the E×B drift and neutral wind velocities derived from the observed plasma drift velocities, give diurnal variations for the electron density profile in reasonable accord with the observations and show that at the magnetic latitude of the MU radar (25°N) the effects of the west‐east electric field on the electron density profile are comparable to the effects arising from the neutral wind. The observed feature that the F‐region electron density is lower at around noon than during the morning and presunset periods results mainly from the effect of the daytime poleward neutral wind. At night the observations show an increase in the F2‐layer peak electron density accompanied by a lowering of the F layer. This feature is shown to be caused by the postsunset eastward electric field which gives rise to a plasma flux from the plasmasphere to the ionosphere. It is found that the effects arising from the electric field are different from those arising from its replacement by an equivalent neutral wind derived from the E×B drift velocity.

Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we investigate the dispersion relation for these waves. This relation varies widely from day to day, even on two successive magnetically extremely quiet days. On average, however, we find that the wave horizontal phase trace speed is about 200 m/s for periods from 60 to 130 min with little variation with solar or magnetic activity. There is some indication that the speed increases slightly from equinox to winter.

Middle and upper atmosphere radar observations of turbulence and movement of midlatitude sporadic E irregularities

Midlatitude field‐aligned irregularities (FAI) associated with sporadic E layers were continuously observed on June 17–19, 1989, with the middle and upper atmosphere radar to investigate spatial and temporal behaviors of the 3.2‐m‐scale FAI. Using Doppler spectrum, mean Doppler velocity, and echo intensity data, we present some features of the plasma turbulence and irregularity movement pertinent to two types of radar echoes: “continuous echoes” appearing at 90‐ to 100‐km altitudes at night and in the morning, and “quasi‐periodic (QP) echoes” appearing at 95‐ to 125‐km altitudes at night. For both types of echoes, echo intensities and Doppler velocities were modulated with periods of 5–20 min due to gravity wave activities. Most of the enhanced echoes were located within the region where Doppler velocities tend to be close to zero. On many occasions there were velocity shears across the enhanced echo region. The Doppler velocities and echoing‐patch movements for the continuous echoes are compared with the existing F region plasma drift and neutral wind models, respectively, to find a partial consistency. General characteristics of the Doppler velocity and spectral width associated with the QP echoes are consistent with previous observational results. Unusual movement of the FAI sheet related to the QP echoes is discussed in the light of recent theory.

Comments on “Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets” by W. L. Oliver et al.

Jacobson [this issue] has written a comment on our article Oliver et al. [1994], subsequently referred to as OEA, describing various geometric effects neglected in our analysis of middle and upper atmosphere (MU) radar multiplebeam observations of ionospheric gravity waves. Jacobson [this issue] shows that these effects can be important in the assessment of wave structure and, in the worst case, possibly void the findings altogether. We wish here to acknowledge the validity of the effects discussed by Jacobson and to comment on their applicability to our results. The MU radar in Japan can observe in multiple beams simultaneously. OEA used this capability to look at electron density differences between beams and considered these differences and their time sequences to be evidence of horizontal gradients caused by gravity waves. The first F region gravity wave seen with the MU radar occurred during the large magnetic storm of February 6-8, 1986. It had a horizontal wavelength of 2500 km, large compared with our 200-km beam spacing, and we have tacitly assumed since that time that the MU radar beams enclose only a small portion of a wave. Jacobson points out that this was not true for all waves reported in OEA. The waves reported there had horizontal wavelengths between 260 and 1400 km. The longer of these waves are appropriately seen with beams 200 km apart, but the shorter are not. Jacobson also points to the problem arising from beams that intersect the wave fronts at different aspect angles (see his Figure 1); one beam may cut across wave trough and crest while another may lie entirely within a trough or crest. Again, this problem is exacerbated when the wavelength is small.

Reply [to “Comments on ‘Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets’ by W. L. Oliver et al.”

Reply [to “Comments on ‘Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets’ by W. L. Oliver et al.”

Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.

Middle and upper atmosphere radar observations of ionospheric electric fields

Middle and upper atmosphere (MU) radar observations of electric fields over the period September 1986 to January 1991 are reported in the form of time‐of‐day variations for several different seasonal and solar activity cases. The perpendicular north velocity has on average a smooth and distinct northward excursion from 0600‐1500 LT followed by a period of constant southward velocity until midnight and then a slow linear change to merge with the daytime northward excursion period. This excursion feature is small in summer and enhanced in winter but, seasonally averaged, has only small solar activity variation. The individual seasonal averages show afternoon/evening excursion behavior apparently tied to the time of sunset. The perpendicular east velocity is characterized by a major eastward excursion in the evening and a minor eastward excursion in the morning, with a tendency to level off or form a slight additional local eastward excursion after midnight. The evening excursion is enhanced during winter and at high solar activity, and the postmidnight excursion is enhanced during summer. The morning excursion has no comparable counterpart in existing electric field models or data averages from Arecibo or Jicamarca. We suggest that this may be a semidiurnal component that evidences enhanced propagation of the semidiurnal tide at the location of the MU radar. The maximum daily amplitudes and the time‐of‐day and solar activity trends at the Arecibo and MU radar locations have strong resemblances, but the seasonal trends in the MU data, the Arecibo data, and existing radar models disagree strongly.

Middle and upper atmosphere radar observations of ionospheric horizontal gradients produced by gravity waves

By observing simultaneously in multiple directions with the middle and upper atmosphere phased‐array radar, we have been able to measure the horizontal structure produced by waves in the ionospheric density. We find that our data are most clearly ordered on a contour plot of the horizontal gradient of the electron density versus time and altitude. These plots show a periodic oscillation of the gradient with a downward phase progression below 400 km altitude but very nearly vertical wave fronts above that altitude. A simulation of this effect with a simple wave formulation provides a strikingly good comparison with the measurements.

On the role of parametric instability of internal gravity waves in atmospheric radar observations

In the presence of a monochromatic finite‐amplitude internal gravity wave, many properties of tropospheric and mesospheric radar observations can be explained consistently in terms of parametric instability. At small primary wave amplitudes, the fastest growing instability modes can be divided into two classes. In one class, the instability modes have scales comparable to that of the primary wave leading to a broadening of the wave number spectrum. The second class consists of small‐scale instability modes with a continuous wave number spectrum so that the primary wave can be “seen” by radars even in the absence of conventional instability mechanisms such as shear or static instability. The small‐scale instability modes generated by long‐period primary waves propagate almost vertically and form extended layers moving with the phase velocity of the primary wave. Thus, in accordance with observational results, the signal power of backscattered radar signals is strongest at near vertical antenna beam directions and shows a layered structure. The small‐scale instability modes further satisfy Taylor's frozen turbulence field hypothesis so that the Doppler shift of scattered radar signals yields the space and time dependent fluid velocity of the primary wave. At sufficiently large primary wave amplitudes, there is in particular an isolated fast growing instability mode which has a frequency close to the mean Väisälä‐Brunt frequency and is probably due to shear instability. It may give rise to a cascade of successive instability modes that can explain organized substructures in layers of enhanced tropospheric UHF radar returns and the frequent occurrence of short‐period evanescent waves in mesospheric VHF radar observations which cannot be attributed to Kelvin‐Helmholtz instability of the mean shear flow.

A saturated inertia gravity wave in the mesosphere observed by the middle and upper atmosphere radar

We have found the dominance of a monochromatic inertia gravity wave with a vertical wavelength of approximately 5.6 km from middle and upper atmosphere radar observations of the mesosphere on February 8, 1985. By assuming linear dispersion and polarization equations for gravity waves, we have estimated the intrinsic period and propagation direction of the wave as 8 hours and northward, respectively. The minimum Richardson number for the atmosphere as modified by the inertia gravity wave was slightly negative near 72 km, which implies that the wave was saturated and generated turbulence through shear and convective instabilities. We observed an echo power enhancement near the altitude of the minimum Richardson number. At 68–78 km altitude we found intermittent fluctuations in radial wind velocities with a period of 9 min. Their phases rapidly reversed near 73 km, where their amplitudes reached a minimum and the Richardson number was less than 1. These fluctuations seem to be attributable to the shear or convective instabilities generated by the saturating gravity wave.

Indication of gravity wave‐meanflow interaction in upper atmospheric radar observations

Acoustic‐gravity waves detected by the Jicamarca VHF radar at tropospheric and stratospheric heights have properties that cannot be explained by simple theoretical models. The sign of the vertical wavenumber, in particular, reverses in a region with considerable background wind shear indicating wave‐wind interaction in a critical layer. A model is analyzed in which a gravity wave propagates into a critical layer with small Richardson number so that near total reflection and strong transmission take place due to the extraction of momentum and energy from the mean flow. Solutions of the linearized hydrodynamic equations describing gravity wave propagation in a viscid compressible atmosphere with shear flow show that this model is consistent with the observed data.