New Players in the Mesosphere: Dust and Atomic Oxygen

The mesosphere is a complex region of the upper atmosphere, notably its ion chemistry is more intricate than at higher altitudes, such as in the thermosphere (or the E- or F-layers). Modern theoretical models describing the mesosphere include not only gas phase chemistry, but also consider transport of atmospheric constituents, including meteoric dust. Recent in-situ measurements have revealed the importance of this ubiquitous dust for the plasma equilibrium, but impact on the neutral chemistry can also be expected according to model results. Atmospheric models are better than their inputs, i.e. the most serious weakness of the models is that they suffer from often poorly known, but important input parameters. Many of them can only be estimated, or are based on only scarce data of reliable instruments. Notably the important measurements of turbulence, atomic oxygen, and dust are largely incidental and uncertain in their absolute values due to the intricacy of measuring in situ in a relatively dense background atmosphere. Ground-based or satellite observations generally do not provide the necessary height resolution or are averages of over larger areas. In order to remedy this situation four sounding rocket flights are foreseen which will yield data on meteoric dust with unprecedented accuracy and resolution pertaining to mass and charge and number density through the application of several instruments developed and provided by different institutions. All flights will take place from Northern Scandinavia with international cooperating partners from Germany, Norway, Sweden and the United States. Another important parameter is atomic oxygen which at mesospheric heights is very difficult to measure. An ensemble of largely novel instruments will measure its concentration in two flights under different geophysical conditions. The results will provide realistic input values for theoretical atmospheric models and help to identify weaknesses in the assumptions underlying the model calculations. A better understanding of the processes responsible for polar mesospheric echoes and noctilucent clouds will allow interpretation of these ground based observations in terms of atmospheric parameters, notably temperature. Noctilucent clouds have - at least sporadically - been observed for well over a century and can serve as a proxy for temperatures near the mesopause. In addition, the proposed plasma measurements will be used to augment the data base on which several empirical models of ionospheric parameters are based on.

The original application was for participation in a total of four rocket soundings aimed at assessing the role of the minor species atomic oxygen and meteoric dust for the state of the mesosphere (<85 km). Due to budged restraints of the respective Project Scientists the schedule of the campaigns were delayed, but fortunately in the end the number rockets flights was doubled (!). The importance of atomic oxygen, notably for the ionised atmosphere has long been recognised, but the measurement of it below, say 80 km is problematic and very few measurements are considered reliable in their absolute values. More recently, dust from ablating meteors which accumulates near the mesopause (ca. 85 km) was identified as an important factor impacting on the charge balance in that height region; but again, to measure these particles constitutes a challenge and different instruments tend to yield different results. Three differently instrumented rocket payloads were used, some of them to be recovered and to be reflown. All flights took place from Northern Scandinavia, but most of the obtained results are also relevant for the understanding of the upper atmosphere outside the polar region. The first two flights (O-states in 2015, from ESRANGE, Sweden) were aimed at oxygen in various states. Nine different photometers were used, as well as a novel electrolytic sensor for atomic oxygen. The Graz contribution was with instruments to determine ion and electron densities; their number densities are closely tied to both dust and oxygen. The payload which was recovered over land was reflown after 18 days within the same campaign, but under somewhat different geophysical conditions. The next two flights (MaxiDusty in 2016 from Andøya, Norway) were specifically aimed to quantitatively determine meteoric dust with a number of different dedicated instruments, including a mass spectrometer. Finally, the PMWE flights (in 2018 from Andøya) were aimed at investigating radar echoes from the 70 to 80 km region which occasionally occur in polar winter. The PMWE payloads were recovered from the sea and will be flown again in 2019, i.e. beyond the current project. The activities described above have hitherto only led to a few final publications, but led to invitations by scientists not directly involved in these rocket campaigns, but interested in the topics, to contribute to more global investigations/publications of the lower ionosphere with emphasis on dust and oxygen.