The focus of the department "Radar Remote Sensing" lies on continuous observations of dynamical and thermal parameters of the mesosphere and lower thermosphere, as well as the troposphere and lower stratosphere. Standard parameters to be determined are winds, wave parameters and turbulence in these regions, as well as mean temperatures and mean neutral densities at an altitude of around 90 km. These observations are performed using partly unique MF and VHF radar systems as well as meteor radars which are located at mid and polar latitudes. Such radar systems provide the only existing possibility suitable for the continuous observation of the above-mentioned height region.
Neutral dynamics in the mesosphere and lower thermosphere (MLT / 60–110 km altitude) span a wide range of spatial scales. At planetary scales, strong disruptions of the stratospheric polar vortex may extend upwards into the MLT, modulating the amplitude and phase of solar tides, and impacting the seasonal evolution of the background winds (Conte et al., 2019). These large-scale dynamics can, in turn, influence the propagation and dissipation of smaller-scale motions.
Mesoscale dynamics in the MLT refer to wind fluctuations at horizontal scales from tens to about 1000 km. Although they play a vital role in shaping the atmosphere both globally and regionally, they have been poorly investigated until recently.
SIMONe systems have provided new insight, showing that mesoscale motions at high latitudes account for at least 25% of the total neutral wind kinetic energy. However, this estimate only includes scales larger than 400 km, suggesting the true energy contribution is likely higher.
To resolve mesoscales of less than 400 km and better quantify the energy spectrum, advanced techniques like the Wind Field Correlation Function Inversion (WCFI) (Vierinen et al., 2019) are essential. WCFI uses pairs of Doppler shift measurements to derive spatial and temporal correlations of wind fluctuations. It can also be used to estimate divergence and vorticity correlations, which are key to distinguishing between gravity waves and strongly stratified turbulence.
WCFI also provides estimates of the energy dissipation rate (ε), quantifying how energy cascades toward smaller scales. This complements measurements made in situ by sounding rockets, or remotely by radars like MAARSY, deepening our understanding of upper-atmospheric energy dynamics.
The atmosphere is a thin gaseous envelope surrounding the Earth that protects life from harmful solar and extraterrestrial radiation. It is a dynamically coupled system in which different layers interact through wave processes, vertical transport from below, and solar insolation and particle precipitation from above.
Lower atmospheric forcing strongly influences the mesosphere and lower thermosphere (MLT) region, particularly at low and mid-latitudes. Vertically propagating gravity waves, tides, and planetary waves play key roles in transferring energy and momentum from their source regions to dissipation altitudes.
Several impulsive phenomena—lasting from a few hours to several weeks—such as earthquakes, volcanic eruptions, severe meteorological storms, sudden stratospheric warmings (SSWs), and polar vortex variability can significantly affect the MLT region and ionosphere (link). These impacts arise through the generation or modulation of atmospheric waves and the deposition of ash and complex chemical species into the stratosphere.
Longer-term oscillations in the troposphere and stratosphere, such as the Madden–Julian Oscillation (MJO, lasting 30–60 days), the Quasi-Biennial Oscillation (QBO, spanning 22–32 months), and the El Niño–Southern Oscillation (ENSO, occurring every 3–5 years), also impact the middle and lower thermosphere (MLT) region by influencing wave generation and propagation. However, our understanding of the coupling process involved remains limited.
At the IAP, vertical coupling processes are investigated using seven SIMONe radar systems deployed globally, along with two partial reflection radars, an MST radar, and an optical airglow imager. Combined with strong international collaborations, these facilities provide a comprehensive framework for studying global atmospheric coupling and variability.
Exploring long-term trends in the Earth's atmosphere is essential for advancing our scientific understanding and identifying potential human impacts. A significant focus of this research is the ability to differentiate between sustained changes and natural variations, such as those resulting from the 11-year solar cycle. Studies have shown that changes in greenhouse gases lead to warming in the lower atmosphere while causing cooling in the middle and upper atmosphere.
To investigate these effects and their impact on the mesosphere and ionosphere, the IAP has been operating one of the longest continuous radio measurement systems, called an ionosonde, since mid 1957, alongside standard phase height measurements since 1959. Results from these observations indicate that the middle atmosphere is cooling and shrinking, although the specific sources of these changes have not yet been quantified.
Additionally, the IAP operates very high-frequency radar systems, known as MAARSY and OSWIN, which are capable of studying polar mesospheric summer echoes (PMSE) and polar mesospheric winter echoes (PMWE). The occurrence frequency of these echoes is influenced by factors such as ionization, temperature, water vapor content, and turbulence in the mesopause region, making them potential indicators for investigating the impact of climate change on the mesosphere.
Furthermore, IAP’s long-term observations continue in conjunction with other radar systems, such as meteor radars and partial reflection radars, which can estimate neutral winds in the mesosphere and lower thermosphere (MLT) region. These systems have been operational for at least the last two solar cycles, and their data can be utilized to examine long-term changes in mesospheric winds.
Many of the measurement techniques used by the Radar Remote Sensing Department at IAP rely on the interactions between the charged and neutral atmosphere in the mesosphere-lower-thermosphere (MLT) region. The charged atmosphere provides significant variations to the index of refraction, which allows the scattering of radio waves to probe the underlying atmospheric processes. Due to the high collisions between the neutral atmosphere and the charged component in the MLT region, we are able to consider that the charged atmosphere and neutral atmosphere are strongly coupled and move with the same motion. We can therefore obtain MLT winds and dynamics by measuring the charged atmospheric component with radars, whether that be ionized meteor trails with the SIMONe and other meteor radar systems, polar mesospheric echoes with MST radars, or the mesospheric winds with the partial reflection radars. The ionization of the atmosphere can occur from meteor ablation, solar photo-ionization, and/or particle precipitation at high latitudes (Aurora Borealis/Australis). Also, by determining the diffusion rate of ionized meteor trails, it is possible to obtain estimates of the MLT temperature.
At altitudes above approximately 100 km, the charged-neutral atmospheric coupling changes, where the electrons start to be dominated by electromagnetic forces as the collision rate to gyro-frequency ratio decreases, but the ions are still unmagnetized and follow the neutral atmosphere dynamics. Due to the separate motion of the charged species, this then allows currents to flow in the region, and it is at altitudes of approximately 100-150 km where magnetospheric currents can close their circuit, generated from precipitating charged particles. The electric fields in this region can be quite large and result in frictional heating of the atmosphere from the charged species colliding with the neutral atmosphere. This is one way by which the solar wind inputs energy to the Earth's atmosphere. There are also processes driven by solar heating and neutral-charged coupling at low latitudes that drive ionospheric dynamics (for example, the equatorial electrojet and the equatorial fountain effect).
Plasma turbulence caused by the ionospheric currents can be detected with the SIMONe meteor radar systems, both at equatorial and high latitudes. An example of auroral plasma turbulence measured by one of the SIMONe meteor radar links in Norway is shown below.
The D-region ionosphere plasma density can be measured with the partial reflection radars, which can be used to determine the neutral atmospheric winds at altitudes of 50-100 km. Further, IAP also operates an ionosonde that can make bottomside measurements of ionospheric plasma density up to approximately 250 km, though this varies based on solar activity and time of day.
Radio Science is a combination of methods and techniques that can be applied to our studies in the MLT region and aim to either enable or enhance the individual measurements.
Although many observations can be done with commercial radar/radio instruments, the quality, precision, as well as spatial and temporal resolutions can be considerably improved with radio science techniques. This is particularly true for existing unique IAP instruments like MAARSY and Saura radars, as well as for the MMARIA concept, including the specular meteor radar networks SIMONe. Radio science activities at IAP involve among others:
Antenna design and testing as well as passive and active calibration experiments,
Design and development of acquisition radio/radar systems (like SANDRA),
Implementation of pulse compression, multi-pulse, and multi-frequency schemes,
Application of magnetoionic theory to derived parameters from Faraday rotation and absorption and/or riometer observations (e.g., D region electron densities),
Spatial as well as frequency domain radar imaging techniques, etc.
Given the advances in technology that allow for continuous sampling in time and at different receiving antennas, as well as the ability to change the phase and amplitude of pulses arbitrarily, robust inverse theory is required to accompany radio science techniques. This can be achieved through either backward or forward scattering model approaches (e.g. maximum entropy in radar imaging).
The recent application of bi- and multi-static measurements enhances viewing angles to the investigated targets in the MLT, partly relaxing assumptions in the measurement schemes, but also allows new insights into the processes. Besides the accurate timing and synchronization, proper calibration is essential to correctly map the observed radar echoes.