Electron Scavenging in the Mesosphere

The free electrons in the ionosphere are formed by ionisation through various processes such as solar UV, solar X- rays, cosmic rays or energetic particles originating in the solar wind. The reverse reaction which limits the fraction of the atmosphere which is ionised, is either recombination by which the original neutral molecules are restored, or attachment to neutrals, including aerosols. These negatively charged particles thus formed in a further stage recombine with positive ions and lead to the original neutral atmosphere or indeed produce atmospheric trace constituents. The attachment process of electrons to the most abundant atmospheric constituents is reasonably well understood from laboratory measurements, but observations in the real atmosphere show that the attachment of electrons ("scavenging") varies much more than could reasonably be expected due to any uncertainty in the laboratory data or the assumptions made concerning the background atmosphere. The most likely explanation for this large scatter is that attachment of free electrons does not only occur with molecules, but - notably in the mesosphere - with ablated meteorites which are theoretically expected and experimentally known to abound at these altitudes, but whose number density varies erratically. Most of the hitherto available data concerning night-time electron scavenging were obtained from rocket flights without associated measurements of parameters expected to be relevant to test the concept of electron attachment to meteoric smoke particles. Four sounding rockets which will be launched in two campaigns carry instruments that are very relevant to test the hypotheses by which night-time electron loss in the mesosphere can be explained. The prime goals of these campaigns are to study the dynamics and energy balance of the mesosphere, and to establish the size and height distribution of meteoric dust particles. The inclusion of the Austrian plasma diagnostic instruments in these four payloads is mutually beneficial, i.e. on the one hand crucial for the prime objectives of the two campaigns, and on the other hand the data on atomic oxygen, solar Lyman-alpha flux and meteor dust distribution contribute importantly towards the understanding the highly variable loss of free electrons. Three of the four sounding rockets will be launched in the same campaign, and one on its own a year later, but in a different season.

The ionised part of the atmospheres mesosphere (50 to 90 km) is otherwise known as the ionospheric D-region. The main processes have been explained by gas phase chemistry for a long time, but some systematic discrepancies between model and measurements persist. Deviations leading to more free electrons in thin layers are understood by sporadically occurring metal ions with a much lower recombination rate and are consequently termed sporadic E (Es). Similarly, pronounced electron density reductions (bite-outs) have been observed for many years at the high latitude, summer mesopause and are today positively identified as being due to electron attachment to ice particles prevalent in thin layers. However, a more ubiquitous process leading to fewer electrons than predicted by gas phase chemistry, is attachment to prevalent meteoric dust particles. Early attempts to detect the surmised meteoric smoke was by Faraday cup instruments. The shortcoming of these instruments is that inside the detector the pressure due to stagnating air by the fast flying rocket is such that small (light) smoke particles will not reach the collecting electrode and will therefore pass undetected; hence these results are always biased towards heavy particles and only the upper altitude limit of the dust layer can reliably be established. Aboard two sounding rockets a novel mass spectrometer, dedicated to cover the expected mass range of smoke particles, was flown from the Norwegian Andøya Rocket Range. It was designed to avoid excessive stagnation pressure; moreover, the two payloads were actively controlled to assure that the rocket (mass spectrometer) axis was always aligned with the velocity vector. Hence for the first time the bottom altitude of the meteoric dust layer could reliably be established since on downleg, i.e. when the instrument was well evacuated, the lower boundary of these particles was found at exactly the same altitude as on upleg. Another pair of sounding rockets had the emphasis on understanding the distribution of minor species in the mesosphere. For this purpose high resolution temperature profiles were established from which one can draw conclusion pertaining to turbulence and in consequence to the distribution of important minor species, notably atomic oxygen. This atmospheric constituent was measured by several, novel instrument which will allow to assess their strengths and weaknesses. The inclusion of meteoric dust and properly established atomic oxygen profiles will greatly improve the outcome theoretical atmospheric models of the upper atmosphere.