T. A. Blix

First detection of charged dust particles in the Earth's mesosphere

Some theories for the observed anomalous radar backscatter during the summer (polar mesospheric summer echoes, or PMSE) and electron bite outs measured by rockets require the presence of charged dust. To investigate this, two dust probes have been launched in 1994 from Andoya Rocket Range and we here report the results from the dust and an electron probe on the two payloads. The dust probes were designed to block out the electron and ion components at the mesopause but to detect primary currents due to impacts of charged dust and also to detect secondary plasma production during dust impacts. The results indicate that both during PMSE and noctilucent cloud (NLC) conditions, large amounts of dust, with average sizes apparently of about 0.1 μm and less, were present. The number densities Nd can be up to many thousand per cubic centimeter, and the charge density NdZd likewise. Large local gradients in density and charge density of dust are detected. Dust carrying both positive and negative charges can apparently be present on different occasions. In some parts of the NLC/PMSE layers we find that the negative charge density locked in grains is so large that the number of free electrons is significantly reduced there because the dust acts like sinks for electrons, and an electron bite out results. We also find that in one case the presence of positive dust leads to an increase in the local electron density by photoionization. The main uncertainties in the data analysis are the structure of the dust and the secondary plasma production at the comparatively low dust impact velocities (1 km s−1) in the experiment.

Neutral air turbulence in the upper atmosphere observed during the Energy Budget Campaign

A number of different experimental techniques employed in the campaign provided measurements on the fine scale structure of the upper atmosphere, from which information about turbulent intensity, eddy transport and eddy dissipation rates may be extracted. The turbulent state of the mesosphere was shown to be highly variable and significant differences were found between observations obtained during the four salvoes launched during different degrees of geomagnetic disturbance.

In situ measurements of turbulent energy dissipation rates and eddy diffusion coefficients during MAP/WINE

Abstract In situ measurements of turbulent energy dissipation rates 6 and eddy diffusion coefficients in the 65–120 km altitude range using mass spectrometers, positive ion probes and foil clouds are presented. The mass spectrometer and the ion probe data were both analyzed with two independent theoretical approaches to derive e. In general, the derived quantities agree with the values discussed in the literature, although noticeable differences occur in certain rocket flights. An intercomparison of the spectral power densities derived from the observed neutral gas and positive ion density fluctuations supports the assumption that under certain circumstances positive ions can be used as passive tracers for neutral gas number density fluctuations. The observed e profiles exhibit a local minimum around 80 km altitude.

First in‐situ observations of neutral and plasma density fluctuations within a PMSE layer

The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, CUPRI and EISCAT observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.

First common volume observations of layered plasma structures and polar mesospheric summer echoes by rocket and radar

We report the results from simultaneous radar and rocket measurements of a PMSE event where for the first time the rocket measured dust and plasma within the radar beam. We find very clear correspondence between the measured dust charge density profile and the radar backscatter profile as a function of height. We find that even very small amounts of charged dust is associated with an appreciable PMSE radar backscatter. Although we find it likely that the dust layer corresponds fully with the PMSE layer there is a possibility that the upper part of the PMSE layer may be influenced by ion clusters which are too small to be detected by the rocket dust probe.

Microphysical and turbulent measurements of the Schmidt number in the vicinity of polar mesosphere summer echoes

During the ECHO campaign in 1994 neutral and electron density fluctuations were measured together with charged aerosols on the same sounding rocket launched close to a VHF radar detecting polar mesosphere summer echoes (PMSE). For the first time this combination of measurements allows for an independent test of the microphysical and the turbulence interpretations of the Schmidt number (Sc). The Schmidt number characterizes the reduction of the electron diffusivity by charged aerosols, which leads to an enhancement of the electron density fluctuations at small spatial scales. In one of the flights charged aerosols were observed at ∼83–89km together with correlated depletions in electron density (‘biteouts’). We have applied a model of aerosol charging to the measured plasma profiles and determined a mean aerosol radius of ∼8nm and a mean aerosol charge of 1e-. In the microphysical description of electron diffusion these parameters correspond to Sc∼420. Spectral analysis of the electron density fluctuations showed enhancements of spectral densities at small scales suggesting likewise a Schmidt number much larger than unity. Using an energy dissipation rate of 67mW/kg as derived from neutral air turbulence measurements on the same rocket we get from the electron spectra Sc=385 which is in excellent agreement with the microphysical result. Apart from this turbulent layer we observe no significant disturbances in neutral air number densities below ∼87km which confirms earlier indications that processes must exist to create PMSEs which are not directly coupled to neutral air turbulence.

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

Abstract The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.

First simultaneous measurements of neutral and ionized iron densities in the upper mesosphere

We report on the results of a multi-instrument study of a sporadic Fe layer and a sporadic E layer which took place in the night of September 20, 1991, at the Andoya Rocket Range (69°N, 16°E). A ground-based lidar was used to measure mesospheric Fe densities, while simultaneously rocket-borne probes and an ion mass spectrometer measured the profile of total ion density and ion composition, respectively. All instruments observed the sporadic layers near 97 km altitude. Radar tracking of chaff clouds established the wind field in the 90- to 100-km region with high spatial resolution, while the EISCAT incoherent scatter radar measured the E fields in the F region. The major ion in the sporadic E layer turned out to be Mg+. The narrow Fe+ layer was 1 km lower than the sporadic Fe+ layer. The ratio of densities [Fe+]/[Fe] in the peaks of the sporadic layers was found to be 1.75.

Mesospheric temperatures and the OH layer height as derived from ground-based lidar and OH∗ spectrometry

Abstract Night-time mesospheric temperatures were simultaneously determined from the Doppler broadening of the D2 resonance line of atmospheric sodium excited by a laser and from the rotational distribution of the P1(1), P1(3) and P1(4) lines of the OH(3,1) band by an i.r. spectrometer. Both instruments were located at the Andoya Rocket Range (69°N, 16°E). The mesospheric temperature gradient permits determination of the altitude of the OH∗ emitting layer from a comparison of the equivalent layer temperatures calculated from the height-resolved Na Doppler temperatures with the observed OH∗ rotational temperatures. The altitude of the OH∗ layer maximum is determined with an accuracy of ±4 km. For 3 nights in January 1986 the OH∗ emission layer is found near an altitude of 86 km.

Studies of Polar Mesosphere Summer Echoes by VHF radar and rocket probes

Abstract At radar frequencies in the range 50 MHz to 250 MHz, at times even to over 1 GHz, strong enhancements of scattering cross section occur between ≅ 80km and ≅ 95km altitude in summer at high latitudes. These echoes, termed “ Polar Mesosphere Summer Echoes” (PMSE), have attracted considerable experimental effort. Observations of this phenomenon are reviewed in the context of atmospheric dynamics and of scattering processes. Recent rocket and radar measurements indicate that a partial reflection from a multitude of ion layers and constructive interference causes at least some of the PMSE. It is discussed which further observations are necessary and some possible practical consequences of PMSE are pointed out.