Studies of middle atmosphere dynamics: the research projects Middle Atmosphere Co-operation/Summer in Northern Europe (MAC/SINE), and MAC/EPSILON

The Earth's middle atmosphere covers the height region 10–100 km and has been extensively studied in recent years. The international programmes Middle Atmosphere Program (MAP) and Middle Atmosphere Co-operation (MAC) have made significant contributions to our understanding of this part of the atmosphere. This special issue reports on the results from two campaigns, MAC/SINE and MAC/EPSILON, carried out within the framework of MAP and MAC. The two projects both studied the dynamical state of the middle atmosphere at high latitudes and had different, but complementary, aims:

• 1.1. Middle Atmosphere Co-operation/Summer in Northern Europe (MAC/SINE) studied the general circulation and temperature structure as well as waves, turbulence and winds in high latitudes during summer solstice conditions. The experiments comprised a series of regular meteorological rocket firings and ground-based observations as well as four launches of sounding rockets. The campaign was carried out in July-August 1987.
• 2.2. MAC/EPSILON was a case study of middle atmosphere turbulence by means of instrumented sounding rockets, meteorological rockets and ground-based observations. The campaign comprised four salvoes in which all rocket and ground-based techniques were concentrated in time and space to make detailed measurements during events with strongly developed turbulence. During the campaign period the ground-based techniques were exploited to map the general behaviour of the middle atmosphere during autumn/early winter conditions. The campaign was carried out in October–November 1987.

Both campaigns were very successful and yielded a number of significant and interesting results, as demonstrated in the 21 articles in this special issue.     

Non-homogeneous structure of neutral winds in the lower thermosphere during the MAC/EPSILON campaign

The paper presents the results of an investigation of the height variations of dynamic processes in the 80–110 km height region, carried out in Kazan, U.S.S.R. (56°N, 49°E) by the radiometeor method during the MAC/EPSILON campaign. Experimental results show that the largest values of vertical wind gradients, as well as zonal and meridional temperature gradients can be found at heights of ~ 83 km. At heights of 80 ⩽ h ⩽ 100 km, we can observe energy absorption of IGW and tides which are the major sources of turbulent energy in the above-mentioned height interval. Using the effects of IGW energy absorption, values of the turbulent eddy diffusion coefficient K_l ranging from 1600 to 4400 m²/s were calculated for October 1987. The energy dissipation rate ϵ was estimated to be from 0.1 to 0.4 W/kg.     

Middle atmosphere electrical structure during MAC/EPSILON

Extensive use of rocket-launched probes during the MAC/EPSILON campaign at the Andøya Rocket Range, Norway, has enabled the characterization of the region's electrical environment for all four flight series. The first rocket salvo was conducted during daylight (15 October 1987) and the subsequent three occurred at night (21 and 28 October and 12 November 1987), all of them during geomagnetically disturbed conditions. Measurements of polar electrical conductivity, ion mobility and number density are presented, and their associated structure is investigated for local auroral ionization effects. This is believed to be the first time that Gerdien condenser mobility measurements have indicated a heavy-ion presence (positively charged aerosols) in the auroral mesopause region.     

Electric field measurements during the MAC/EPSILON campaign

As part of the MAC/EPSILON campaign in northern Norway during October and November 1987, five rocket-borne payloads made three-axis electric field measurements of the middle atmosphere. Flights 31.066 and 31.067 consisted of large multi-experiment packages, while the other three flights (30.036, 30.037 and 30.038) were devoted primarily to electrical measurements. Simultaneous measurements of the horizontal electric field made by flights 31.066 and 30.036 were in general agreement in their limited altitude region of overlap. A simultaneous small temporal feature was observed in both datasets. The relatively more extensive horizontal E-field datasets from 31.066 and 31.067 both exhibited a decreasing mapping function with decreasing altitude, which is an indication of the observation of fields from a local auroral patch. Small-scale variations in the horizontal fields of the flights were similar to observed wave-like variations in the neutral wind field. No unusual features were observed at high altitudes in the measured E₁ field. Two of the payloads observed small vertical layer structures between 40 and 50 km. No electric field structure was observed in association with the presence of a sudden sodium layer.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

A study of the vertical motion field near the high-latitude summer mesopause during MAC/SINE

We present the results of high-resolution observations of the vertical velocity field obtained with the EISCAT and SOUSY VHF radars near the high-latitude summer mesopause during the MAC/SINE campaign in northern Norway in 1987. The data reveal an energetic motion field with maximum amplitudes of ~ 10 m/s and characteristic periods of ~5–30 min. Motions exhibit a high degree of vertical coherence and a quasi-periodic structure, with typical durations of 5–10 cycles. Estimates of the mean vertical velocity are downward at lower levels and are near zero or positive at greater heights. The mean vertical velocity variance is found to be ~5 m²/s², consistent with other high-latitude measurements. Frequency spectra computed for each radar are found to exhibit considerable variability, while vertical wavenumber spectra are seen to be somewhat variable in amplitude and to have slopes approaching −3 at lower wavenumbers. These results are suggestive of an energetic spectrum of gravity wave motions near the mesopause that has a large vertical flux of wave energy, that may have observed wave frequencies differing significantly from intrinsic frequencies due to Doppler shifting by large horizontal winds, and that is consistent with the separability of the frequency and wavenumber dependence of the motion spectrum and with gravity wave saturation at sufficiently small vertical scales.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Investigations of ionospheric radio wave absorption processes using imaging riometer techniques

The recent development of imaging riometer techniques has enabled a range of new, interesting observations of the complex dynamics of auroral and polar radio wave absorption events. These events mostly relate to the precipitation of energetic particles, creating enhanced ionization in the D-region. However, E-region heating by large electric fields and F-region electron density enhancements may also—at times—be responsible for observable absorption effects. Observations of ionospheric radio wave absorption processes using imaging riometer techniques may provide detailed characteristics of the spatial and temporal structures of small-scale disturbance events, velocity vectors for drifting features and frequency spectra for modulated events. This presentation will give a brief summary of imaging riometer techniques and a survey of existing and planned imaging riometer installations. Furthermore, the characteristics of frequently occurring absorption event types are summarized. In a companion paper imaging riometer observations are presented for some selected absorption events.     

Some regularities of the lower thermosphre wind regime in summer and autumn from meteor radar measurements in obninsk (55°N, 38°E)

Meteor wind measurements made at Obninsk during MAC/SINE and MAC/EPSILON, and in the summer and autumn of other years since 1973, are reported. The zonal wind, which is presumed to be in the 93–95 km height region, is generally westerly, and the meridional wind is mainly northerly. Quasi-two-day oscillations are studied, as are semidiurnal tides. There is some evidence for a 22-yr periodicity in the amplitude of the semidiurnal tide     

High-latitude D- and E-region investigations using imaging riometer observations

The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B_Y component of the interplanetary magnetic field. A typical example is examined.     

Case study of modelled and measured D-rmregion plasma densities

Two of the many sounding rockets launched as part of the MAC/EPSILON campaign are particularly suited to test the validity or quality of D-region models under night-time conditions. The measurements covered atomic oxygen, neutral temperatures and fluxes of charged particles. With these parameters as inputs, one is in the position to assess their relevance for the plasma parameters ion composition, transition heights of clusters and negative ions, and total plasma density. The two IOMAS payloads are believed to be the only night-time flights with associated measurements of atomic oxygen extending below the ledge at about 80 km. The relevance of atomic oxygen for the ion chemistry of the lower D-region could be experimentally demonstrated.     

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A., Fritts D. C. and Van Zandt T.E., (1987, J. atmos. Sci. 44, 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.     

Ionospheric ELF radio signal generation due to LF and/or MF radio transmissions—I. Experimental results

A new non-linear ionospheric effect has been discovered from the analysis of ground based FLF/VLF radio data obtained in Scandinavia during 1979. Apart from signals of natural origin, timing signals (six pips which occurred on the hour) were received. The pips of frequency 1000±0.5 Hz, duration 105 ± 8 ms, and field strength ~ 0.1 pT at Sodankyla, Finland, exhibit a favoured source location ~ 150 km south-south-east of Sodankyla. A close association between the reception of these ELF pips and the auroral electrojet is demonstrated by the positive temporal correlation between ELF pip generation and periods of enhanced local magnetic activity, and also by the spatial correlation between source location and the latitude over F'inland at which riometer absorption is a maximum. In the evening and midnight sectors the latter is interpreted as indicating the electrojet position. The originating signals are shown to emanate from one or more Soviet LF/MF broadcast transmitters, all of which are several hundred kilometres or more from the favoured generation region.     

Variability of d-region electron densities at tromsø

The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromsø has been used for a study of D-region electron densities by the differential-absorption method on a number of days during 1978–79. Received signals are generally stratified in several layers, typically over 60–80 km. Strong stable echoes are seen down to 55 km during periods of enhanced riometer absorption. Inferred electron densities vary between ~ 100–1000 cm⁻³ at ~ 60–80 km and show well-defined features which persist for ~ 10–20 min. During periods of high absorption, enhanced electron densities (~ 600 cm⁻³) are observed below 65 km. During a Polar Cap Absorption event, the inferred electron densities at 60–70 km show a very stable profile. Possible sources of D-region ionization at high latitudes are briefly discussed     

Wave activity,F1-layer disturbance and a sporadic E-layer over EISCAT

During July 1987 the EISCAT radars were used to study thin layers in the ionospheric E-region. This paper outlines the observing campaign, describes the GEN-type radar program used for the UHF experiments, and discusses the ‘descending’ or ‘sequential’ layer observed on the afternoon of 12 July during a period of strong wave activity, which could be traced throughout the whole E-F1 transition region. Following the descent of one particularly marked wave, a thin layer developed around 120 km height and lasted about 100 min, with temporary disappearances and periods of upward motion which were related to variations of field aligned ion velocity, and in particular to ‘convergent nulls’ in the velocity profile. The layer was eventually dispersed by a rapid upward surge of ion velocity. Composition analysis shows that the layer contains both long-lived light ions and heavy ions, most probably Fe⁺.     

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.     

Dynamics of the upper middle atmosphere (80–110 km) at Tromsø (70°N) and Saskatoon (52°N), June–December 1987, using the Tromsø and Saskatoon MF radars

A real-time-winds (RTW) system from Saskatoon operated with the Tromsø MF (partial reflection) radar system on a continuous basis in the period June–December 1987. This interval includes MAC/SINE and EPSILON. Profiles with 3-km resolution were obtained every 5 min—weak ionization and few geomagnetic disturbances limited the observations normally to 80–110 km. However, daily mean winds, diurnal and semidiurnal tidal characteristics (amplitudes, phases and wavelengths) and gravity wave characteristics (intensities, mean directions) are available throughout this interval. This is particularly valuable in defining the background state for some experiments, e.g. rockets, and for comparison with related parameters from the lidar and other radars (EISCAT, SOUSY-VHF). Comparisons with these dynamical parameters from Saskatoon (52 N) are made : the zonal circulation was weaker at Troms0, tidal amplitudes smaller, and summer 12-h tidal wavelengths shorter ( ~ 80 km vs ~ 100 km). The fall transition for this tide occurred in September at Troms0, earlier than observed elsewhere.     

Observational evidence of local turbulence production due to gravity wave saturation in the summer mesosphere

Five foil chaff and two falling sphere rockets flown during the MAC/SINE Campaign on 15 July 1987 at Andenes, Northern Norway (69°17′N). From these rocket measurements, turbulent energy dissipation rates, vertical wind shears and Richardson numbers as functions of height were derived in the range from 82 to 92km. Turbulent energy dissipation rates generally range from 1.4 × 10⁻⁵ to 2.0 × 10⁻²W/kg and are consistent with other experiments performed at the same latitude. Strong wind shears of the order of 50–90 m/s/km are observed at various heights. Good correspondence between turbulence intensity peaks, regions of strong wind shear and low Richardson number is found. Vertical wavenumber spectra of the five scalar winds measured by the foil chaff rockets indicate that there is an excellent agreement with the saturation hypothesis, suggesting that the turbulence intensity peaks measured in this salvo are linked directly to the saturation of gravity wave motions via dynamical instabilities.     

High-latitude plasma densities and their relation to riometer absorption

A large number of D- and E-region electron density profiles from high latitudes have been analysed. These were derived from rocket-borne wave propagation experiments and—after careful screening—arranged according to riometer absorption. Statistical profiles for various degrees of absorption, including 0 dB, were established both for day and night. Furthermore, the height region predominantly contributing to the absorption has been identified. Finally a mean variation of the density of negative ions has been derived.