The mesosphere and lower thermosphere (MLT) is the transition region between the terrestrial atmosphere and near-Earth space. Neutral and plasma parameters interact creating the ionospheric dynamo. The electric fields generated in the E-region (collocated with the MLT) influence the plasma distribution of the ionosphere at low and middle latitudes. The interaction and modulation of atmospheric waves and tides play a crucial role for the variability of this region. It is thus of utmost scientific relevance to investigate the neutral-plasma interactions and their relation to MLT dynamics at different spatial and temporal scales. At equatorial latitudes, the equatorial electrojet is a pronounced daytime electric current that is mainly directed eastward, however, its direction sometimes turns westward. This turning is thought to be due to changes of the neutral wind, but the relationship between the equatorial electrojet and neutral wind has not been established. Using new observations of the high precision magnetic field at the Swarm satellites and neutral winds from MIGHTI on board ICON mission, it is now possible to determine their relationship empirically. The analysis benefits from global coverage of Swarm and ICON/MIGHTI data. Satellite data can be complemented by those from ground-based observations, that provide continuous time series at selected scientific sites at the magnetic equator, e.g. SIMONe radar systems, and magnetometers in Peru surrounding the Jicamarca Radio Observatory. This presentation will give examples of the variability of the neutral wind in the MLT, as well as addresses open scientific questions and instrumental needs to advance the understanding of the MLT and therefore, their impact on the ionosphere.

In the recent years, regular burst-mode measurements campaigns of the Absolute Scalar Magnetometers (ASM) onboard two of the Swarm satellites have been conducted. During one week every month for each satellite, the total intensity of the Earth’s magnetic field has been measured using a sampling frequency of 250 Hz, enabling the detection of electromagnetic signals in the Extremely Low Frequencies (ELF).
It has been possible to observe a large number of whistlers excited by the most powerful lightning strikes occurring in the lower atmosphere. These detections can occur several thousand km away from the lightning strike location, because these signals propagate in the wave-guide between the Earth surface and the ionosphere before entering the ionised layers and reaching the satellites.
At the end of 2021 the orbital planes of the Swarm satellites overlapped while they were orbiting in counter-rotation. Several simultaneous burst-mode campaigns were acquired on both satellites, providing unique opportunities of detections of multiple whistlers generated by a single lightning. They revealed the complexity of the permeability of the ionosphere in this frequency range: some events have been detected by both satellites at several thousand km distance, while other events have been detected only by one satellite even if they were closer to each other.
The dispersion of the whistler signals as detected at satellite altitude depends on the electrons and ions present along the propagation path of these signals. It can be modeled by computing the propagation time of each frequency component using ray-tracing techniques. By also taking advantage of the simultaneous in-situ electron density measurements of the Electric Field Instrument (EFI) of Swarm, it is possible to constrain the ionosphere in the region below the satellite. This has been validated using data from ionosondes and the Ionosphere Real-Time Assimilative Model (IRTAM), used to specify real-time foF2 and hmF2 global maps, (improve the climatological International Reference Ionosphere (IRI) model - suggest to be deleted). Even if the whistler signals occur randomly, this opens new observational capabilities in areas where no ground-based observations of the ionosphere are possible. Whistler analysis will be particularly important in the coming years, when we will reach the solar maximum and the increase of ionisation will produce larger dispersions. No other LEO mission explored the ELF under this condition.
Extended opportunities to detect and characterise whistlers in the ELF will also be obtained with the NanoMagSat mission, for which continuous 2 kHz measurements will be acquired. The larger spectrum will enable the observation of proton whistlers and the multiple observation points provided by all available satellites of Swarm and NanoMagSat constellations could provide unique opportunities to study the permeability of the ionosphere to ELF signals and monitor the lower ionosphere from space.

Within the lower thermosphere and ionosphere (LTI), at altitudes roughly between 100 and 200 km, the atmosphere transitions from being well-mixed and electrically neutral to heterogeneous and partly ionised. Complex processes related to the interactions between its neutral and charged constituents uniquely characterise and shape this region. Even though a wealth of information comes from remote sensing measurements by instruments either onboard satellites or on the ground, key processes related to ion-neutral interactions within this region require co-temporal and co-spatial measurements with high spatial resolution, such that can only be provided by in-situ sampling. Required measurements include plasma density and temperature, ion drifts, neutral density and wind, ion and neutral composition, electric and magnetic fields, and energetic precipitating particles. In this talk we will provide an overview of key processes within this region related to energetics, dynamics and chemistry that require such comprehensive measurements for their unambiguous characterization; we will present an overview of the current status of understanding as indicated by discrepancies in the representation of these processes in current global circulation models and methodologies based on remote sensing measurements; and we will make the science case for Daedalus, a proposed in-situ mission targeting to provide the first ever systematic and comprehensive measurements to resolve key open questions within this critically unexplored region. Specific objectives for the Daedalus mission include: the provision of full and detailed estimates of the frictional heating (Joule heating) in the LTI resulting from the electro-magnetic connection to space as well as from energetic particle precipitation (EPP); obtaining detailed characterisation of the fluxes of Energetic Particle Precipitation that pass through the LTI and affect the middle atmosphere; retrieving altitude-resolved estimates of the electro-magnetic forcing as well as of the forcing due to changes and gradients in densities and temperatures; establishing the range of parameters that trigger radio wave-disturbing irregularities; and determining the response in the LTI to upward propagating atmospheric gravity waves, traveling atmospheric disturbances, and other dynamic features.

Since October 2008 hydroxyl (OH) airglow observations are performed at the environmental research station ‘Schneefernerhaus’ (UFS, 47.42°N, 10.98°E). Further observation sites at Catania (CAT, 37.51°N, 15.04°E), the Observatoire de Haute-Provence (OHP, 43.93°N, 5.71°E) and the Georgian National Observatory (ABA, 41.75°N, 42.82°E) were equipped with identical instrumentation in 2011 and 2012. These airglow emissions originate in the upper mesosphere lower thermosphere (UMLT) and provide an efficient approach to derive atmospheric temperatures of the emission layer between approximately 80 to 100km height.
At UFS on timescales from weeks to several years, temperatures at this height are strongly influenced by both, the general circulation of the middle atmosphere and the variability of the solar forcing. The strongest component is given by the annual cycle (caused by the residual meridional circulation of the mesosphere) with a yearly amplitude varying between 16.5 K to 18.5 K. The 11-year solar cycle provides the second strongest forcing mechanism with approximately 5.9 ±0.6 K/100 sfu during solar cycle 24. The uncertainty of the solar forcing term is governed by the question whether or not a lag term is to be applied in the cross-correlation of the two parameters. Concerning annual means of both parameters, a correlation of up to R²=0.91 is achieved, if solar flux is assumed to lead OH temperatures by 110 days. The phases of the quasi-biennial oscillation (QBO) originating in the tropical stratosphere play an important role in the explanation of this (potential) lag: the QBO-related signal of ca. 1 K, which is best observed from 2011 until 2015 during solar maximum conditions, strongly decreases after 2016 complicating the interpretation of the solar forcing term.
Recently, the semi-annual oscillation and its variability (between 2 K and 4.5 K), which appears to be decoupled from the variation of the annual oscillation comes more and more into focus. So far, UMLT airglow emissions have been considered to be only part of neutral atmospheric physics and chemistry. However, the emissions originate in the ionospheric D-region and potential feedback mechanisms from the strong semi-annual oscillations observed in the ionosphere should be considered.
In summary, the individual contributions to the temperature development at this height make it still difficult to decide whether this region is subject to the expected long-term cooling of mesospheric temperatures or the heating of the thermosphere. The best estimate for the 10-year change between 2010 and 2019 amounts to -0.14 K/dec ±0.59 K/dec. Observations at further sites of the Network for the Detection of Mesospheric Change (NDMC) help discriminating between local and large-scale effects in the data.