Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

Characteristics of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar and possible explanations of their origin

We describe experiments carried out with the EISCAT VHF radar during the MAC/SINE campaign. These experiments included observations of the polar mesosphere summer echoes (PMSE), which were studied with a high spectral resolution program. The fine structure of the spectra imply that very thin and non-random transient structures of reflectivity occur frequently in the mesopause region. We find no clear relation between the echo power and the coherence time which could support the hypothesis of scatter from turbulence or partial reflection. In addition, the estimates of radar reflectivity let us discard incoherent scatter and pure turbulence scatter as the cause of the PMSE. We also discuss the relation of the PMSE and cluster ions, electric fields, charge accumulation and atmospheric gravity waves.     

Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes

The characteristics of polar mesosphere summer echoes (PMSE) are studied at 53.5 and 224 MHz. Observations at 2.78 MHz, simultaneous with the ones at the other two frequencies, were carefully compared for indications of PMSE, but no obvious relation was found. Relationships between relative scattering cross-section, spectral width and vertical velocity are studied for the 224 MHz radar, and observations at 53.5 MHz are compared with those at 224 MHz. Results of aspect sensitivity measurements at 53.5 MHz are presented. The implications of these characteristics for several possible scattering mechanisms are discussed. We rule out incoherent scatter and chemically induced fluctuations from the evidence that we have. In view of the extremely low temperatures near the high-latitude mesopause in summer, we discuss several scenarios involving heavy cluster ions and charged aerosol particles.     

On the role of dust in the summer mesopause

We propose that dust formed at the cool summer mesopause may have optical properties very different from that measured for bulk material of ice. The smallness of the dust and possible surface impurities may lead to high photoelectric yields and low workfunctions. For such reasons the dust in the summer mesopause may, at least occasionally, be charged to substantial positive surface potentials while pure ice, with its high photoelectric workfunction, would be charged to low and negative potentials by collisions with plasma particles. The presence of ‘dressed’ dust particles, with surface potentials of some volts, can lead to enhanced radar backscatter. We also suggest that the apparent reductions in electron density (‘bite-out’), which have been observed in the radar backscatter region, can be caused by the inability of an electrostatic probe to deflect the massive dust particles.The dust density which is required by our model to explain radar backscatter and electron bite-outs is of the order of 10 cm⁻³ for dust of radius above 5 × 10⁻⁶ cm.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

Consistency of rocket and radar electron density observations : implication about the anisotropy of mesospheric turbulence

We report about a quantitative comparison of rocket observations of electron density fluctuations and simultaneous 53.5 MHz radar measurements that were obtained during the MAC/SINE campaign in northern Norway in summer 1987. Out of three rockets launched during the Tur-bulence/Gravity Wave salvo on 14 July 1987, two were flown during conditions that allowed a detailed investigation. For a large part of the data from these rocket flights it is found that the radar reflectivity is about 10 dB, enhanced over what would be expected from the rocket observations in the case of isotropic electron density fluctuations. The observations can be reconciled under the assumption of an anisotropic turbulence. Assuming a simple model spectrum for the electron density fluctuations, we derive a relation between the rocket and radar observations that covers the whole range from isotropic turbulent scatter to Fresnel scatter at horizontal density stratifications. For the observed dataset, an anisotropy which typically corresponds to a ratio of the horizontal to the vertical coherence length of about 10 is consistent with the comparison of rocket and radar observations. A similar anisotropy is found also from the observed aspect sensitivity of the radar echoes. The variation of the anisotropy with height and time shows an anticorrelation with the turbulence level of the mesosphere as deduced from the spectral width of the radar echoes. The anisotropy is found to maximize in heights where the electron density displays deep ‘bite-outs’. These depletions in the electron density were independently observed by a Langmuir and an admittance probe on board two of the rockets.     

The dynamics of ion layer generation in the 80–150 km altitude region at low and mid-latitudes

The association of sporadic ion and sporadic sodium layers in the low-latitude, 90–100 km altitude region suggests that we must look beyond the windshear theory for details of the formation mechanism of sporadic layers in the 80–150 km altitude region. We present evidence, including specific 85–105 km results from the AIDA-89 and the ALOHA-90 campaigns, that 80–150 km altitude sporadic layers—including sporadic sodium layers—are generated in a complex interplay of tidal and acoustic-gravity wave (AGW) dynamics with temperature-dependent chemistry where wave-produced temperature variations are both adiabatic and dissipative or turbulent (non-reversible) in origin. We suggest that layering processes are best studied with an instrument cluster that includes sodium and iron lidars, MST radar (turbulence), incoherent scatter radar (electron concentration and winds), meteor radar techniques (winds), passive optical/IR imaging techniques, and appropriate rocket payloads to study a significant volume of the 80–150 km altitude region. We introduce the concept of volumtric radar and lidar techniques.     

The amplitude of auroral backscatter—I. Model estimates of the dependence on electron density

A model of the auroral backscatter amplitude, in the form discussed by Uspensky and Oksman et al., has been derived for the radar geometry appropriate to joint observations by the PGI auroral radars at Karmaselga and Essoyla and the EISCAT incoherent scatter radar. The model shows how refraction effects cause a strongly non-linear dependence of backscatter amplitude on electron density in the E-region. It also explains why the macro aspect sensitivity for auroral radar operating at a frequency of about 45 MHz is only 1–2 dB per degree for aspect angles greater than 5°.     

Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8, and Søndrestrøm incoherent scatter radar observations

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft and the Søndrestrøm incoherent scatter radar coincide in space. Such coincidence offers a unique opportunity to study temporal vs spatial variations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP satellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distribution and line-of-sight plasma velocities. We combine these data sets to construct a two-dimensional map of a possible auroral pattern above Søndrestrøm. It is characterized by the following properties. No difference is seen between the gross precipitation patterns measured along the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic east-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells. However, F6 and F8 observed minor differences in the small-scale structures. More significant differences are found between small-scale features in the DMSP precipitation measurements and in radar observations of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4°, in magnetic longitude, and 0–40 s in time along the spacecraft trajectories (varying with magnetic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide. The combined data suggest that the plasma flow reversal delineates the polar cap boundary, that is, the boundary between precipitation characteristic for the plasma mantle and for the plasma sheet boundary layer.     

A narrow auroral arc observed with EISCAT

On the evening of 13 January 1983 we made simultaneous observations of optical and radar aurora using low light television cameras together with the EISCAT radar system. At 19 h 16 m 06 s UT an extremely bright auroral arc moved rapidly (about 2 km s⁻¹) through the EISCAT radar beam. The associated rapid rise and fall in the E-region electron density indicates that there was an intense narrow electron beam associated with the optical arc. We estimate that the ionisation rate in the E-region increased at least 20-fold (from 1 × 10¹⁰ m⁻³ s⁻¹ to >2 x 10¹¹ m⁻³ s⁻¹) for 1 or 2 s as the arc passed by. In addition, there was a brief (<4 s) increase of 130% in the signal returned from 250 km altitude which coincided with the arc crossing the radar beam at that height. In view of this coincidence, we find that a possible explanation is that the increase arose from short-lived molecular ions, for example vibrationally excited N⁺₂ ions, produced in the F-region by soft precipitation associated with the arc.     

Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

EISCAT observations of a sequential Es layer

An experiment was carried out during the summer of 1982, using the EISCAT incoherent scatter radar, to investigate sequential Es: layers of enhanced electron density in the ionospheric E-region. Results are presented for an event occurring on 30 July, and the observations compared in a simple way with those expected from the wind-shear theory. Comparisons are made with data available from ionograms from Tromsø, Kiruna and Sodankylä, at which sites Es layers are also seen, though with differences that may be due to differences of wind direction.     

The E-region electron density diurnal asymmetry at Saint-Santin: observations and role of nitric oxide

Measurements of the E-region electron density were made with the Saint-Santin incoherent scatter radar during consecutive days in June 1978, March 1979 and December 1980. On the basis of a statistical study, the observations show the presence of a diurnal asymmetry of the electron density, with morning values usually exceeding the afternoon densities by 3–20%. Two possible causes of the dissymmetry are examined: the asymmetry in the diurnal variation of the neutral composition and the effect of nitric oxide. The presence of NO partly converts O₂⁺ into NO⁺ ions and increases the effective recombination rate of the electrons in the afternoon. Numerical simulations assessing the relative importance of the two factors are, in general, in good agreement with the measurements.     

Lidar observation of sudden increase of aerosols in the stratosphere caused by volcanic injections—II. Sierra Negra event

A striking disturbance in stratospheric aerosols over Fukuoka was observed by Nd-YAG laser radar in December 1979. It began with the appearance of a thin layer of enhanced scattering at an altitude of about 17 km and revealed remarkable variations of the layer in time and height. Measurements at two wavelengths suggest that the aerosols changed in size distribution, and the disturbance is inferred to be due to the Sierra Negra eruption. The integrated aerosol backscattering above the tropopause reached about 8 × 10⁻⁵sr⁻¹; i.e. some six times that of the Soufrière event when converted to the ruby wavelength. The mean meridional transport speeds of the dust clouds were much larger than ever observed previously and this may be due to the activation of meridional transport associated with the Canadian sudden stratospheric warming in November-December 1979     

The maintenance of the night-time ionosphere at mid-latitudes. II. The ionosphere above St. Santin

The total rate of recombination in the night-time ionosphere above St. Santin (at L = 1.8) was estimated using a model atmosphere and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was measured but at the latitude of St. Santin this was always small and averaged over the night as a whole the contribution was negligible. Downward diffusion provided the main source of plasma and the flux predicted was compared with the flux measured at 450 km. The comparison was good provided the model atmosphere was modified to use exospheric temperatures based on actual measurements by the incoherent scatter radar. A comparison with the results obtained at Malvern (Paper I) confirmed that the saturation time for the protonosphere at L = 1.8 is far less than at L = 2.6 and that the downward flux from the saturated protonosphere was also less.     

Heights of MF radar scatter (1986/87) and the wind field (55–95 km): Saskatoon,Canada

The Saskatoon MF radar (2.2 MHz) at 52°N, 107°W, has been used to measure the heights of occurrence of radar scatter during four seasons, and twelve months of 1986/87. Mean winds, and gravity waves are also available, by the spaced antenna method and from the same radar echoes. Certain heights, called elsewhere ‘preferred heights’, are identified near 60km, 70km, 75km in summer, and 80–86 km. Several layers have seasonal and diurnal variations. Associations with electron density gradients (rocket data), mean wind shear in summer, and gravity wave amplitude-minima in the equinoxes are effectively demonstrated. Case studies, involving 3 h data sets of radar scatter and wind elaborate the comparison: gravity waves of long period (τ > 6 h) are shown to modulate the scattering process.     

Comparison of ionospheric electrical conductances inferred from coincident radar and spacecraft measurements and photoionization models

Height-integrated electrical conductivities (conductances) inferred from coincident Sondrestrom incoherent scatter radar and DMSP-F7 observations in the high-latitude ionosphere during solar minimum are compared with results from photoionization models. We use radar and spacecraft measurements in combination with atmospheric and ionospheric models to distinguish between the contributions of the two main sources of ionization of the thermosphere, namely, solar UV/EUV radiation and auroral electron precipitation. The model of Robinsonet al. (1987, J. geophys. Res.89, 3951) of Pedersen and Hall conductances resulting from electron precipitation appears to be in accordance with radar measurements. Published models of the conductances resulting from photoionization that use the solar zenith angle and the solar 10.7-cm radio flux as scaling parameters are, however, in discrepancy with radar observations. At solar zenith angles of less than 90°, the solar radiation components of the Pedersen and Hall conductances are systematically overestimated by most of these models. Geophysical conditions that have some bearing on the state of the high-latitude thermosphere (e.g. geomagnetic and substorm activity and a seasonal variation of the neutral gas distribution) seem to influence the conductivity distribution but are to our knowledge not yet sufficiently well modelled.     

A servo-model of the night-time F2-layer above St. Santin

A modified form of the ionospheric servo-model is used to describe the night-time F2-layer above St. Santin. Data taken by the incoherent scatter radar on nine nights in 1974–1977 were used to determine the height profiles of electron density, electron and ion temperature and electric field. The servo-model was then used to compute the theoretical variation through the night of the height of the F2 peak and the field-aligned plasma velocity, using gas concentrations and horizontal pressure gradients derived from the MSIS79 atmospheric model. On magnetically quiet nights these calculated values agreed closely with the observations. On disturbed nights, however, the calculations and observations began to diverge an hour or so after the onset of a substorm. The divergence could be explained by an enhanced southward wind.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.