An ionosphere is a “buffer” between the space environment of a planet and its atmosphere. It is created within its atmosphere mostly by solar extreme ultra violet radiation (EUV), although minor contributions from energetic particle impacts and cosmic rays are often also present. Studies of ionospheres provide us with understanding of an atmosphere’s losses, outflows and energy inputs, as well as explanations of atmospheric composition. Studies of ionospheric currents tell us about interactions with space environment, including how energy is transferred between the magnetosphere and the ionosphere, and where it is dissipated.
Since the main ionization source is solar EUV, every planet and moon with an atmosphere also possesses an ionosphere. At IRF-Uppsala we study ionospheres of Earth, Mars, Saturn’s moons Titan and Enceladus. The latter does not have a substantial atmosphere, but instead the continually active water plume (from a number of geysers) at its south pole provides a localised source of ionisable material. For the particular case of Enceladus, measuring the properties of charged particles of the plume gives us insight directly into the subsurface ocean of the moon.
Measurements by different instruments are always compared to each other to complete the picture and for cross-referencing. For this purpose we are collaborating (internationally) with scientific groups responsible for various instruments on-board a range of spacecraft.
The ionosphere of Earth is studied both from above and below. IRF-Uppsala has a long history of the study of Earth’s ionosphere using both spacecraft- and rocket-based instrumentation such as Langmuir probes and electric field sensors. Additionally, ground-based radars such as EISCAT, and simple ionosondes have featured heavily in our research. More recently, we have investigated the outflow of planetary plasma from the ionosphere using the Cluster spacecraft, conducting measurements of very low-temperature escaping plasma at the upper reaches of Earth’s ionosphere. ESA’s low-altitude Swarm mission regularly traverses the ionosphere making high-resolution measurements of the properties of the plasma and magnetic fields encountered, and is accumulating a new and exciting data set that we hope to analyse extensively over the coming years.
On Swarm, we have provided the Langmuir probe instrumentation, which measures the plasma density and temperature encountered by the spacecraft on its low-altitude orbits through the ionosphere. These data are combined with precise magnetic field measurements to investigate the coupling between the ionosphere and magnetosphere. Figure [Swarm Ionosphere] shows the presence of density irregularities associated with the equatorial anomaly. These irregularities can give significant disruption to radio signals, such as those used by the GPS system.
Saturn’s largest moon Titan has a dense atmosphere which may be similar to that of early Earth. Therefore, one of the main goals of studying Titan is investigating the role complex ionospheric chemistry may have played in the origin of life on Earth. The upper layers of Titan’s atmosphere are ionized into an ionosphere by various energy sources, supplying complex ion chemistry leading to formation of organic (pre-biotic) molecules and eventually nm-sized aerosols (tholins). These aerosols and their pre-cursors are charged negatively and can therefore be detected with our Langmuir Probe along with the other ionospheric ions present.
Enceladus is an icy moon of Saturn that is known for the plume coming from a subsurface sea at the south polar region. The plume consists of water group ions, gas and dust and is made up of single jets/geysers from the cracks in the surface called tiger stripes. The plume is often compared to a cometary coma as well as a ‘local’ ionosphere and is especially scrutinized after the arrival of Rosetta mission at comet 67P and prior to the JUICE mission to the Jupiter system, since Jupiter’s moon Europa is also covered with ice and plume sighting has been reported by Hubble telescope.
The ionosphere of Mars is formed by photo-ionisation of CO2 in its atmosphere. Mars’ lack of a dynamo magnetic field leads to a much more direct interaction of the ionosphere with the solar wind. Mars does have a weak, spatially varied crustal magnetic field, which in places is in fact strong enough to dominate the interaction with the solar wind, forming so-called “mini magnetospheres”.
We at IRF-Uppsala have been involved in studying the Martian ionosphere using data from the MARSIS radar on board ESA’s Mars Express, which provides profiles of the plasma density below the spacecraft. The MARSIS radar is somewhat analogous to ground-based ionosondes operated at Earth. We use these data to study the interaction of the ionosphere with the planet’s weak remnant crustal fields, and how it is influenced by variations in the solar wind. Recently, NASA’s MAVEN mission has also entered orbit, and IRF-Uppsala is involved in the analysis of data from the Langmuir Probe and Waves (LPW) instrument on board this spacecraft. MAVEN comprises a comprehensive suite of plasma instruments, and provides a rare opportunity to do “multi-spacecraft” plasma science at a planet other than Earth.
We are also involved in the Mars Upper Atmosphere Network (MUAN), which aims to foster collaboration between scientists on these missions, and has held several productive meetings on the topic over the last ~5 years.
Ganymede is the largest moon of Jupiter and, in fact, in our entire solar system, unique in its class because it has its own full-scale magnetosphere. Jupiter Icy Moon Explorer (JUICE) mission, to be launched in 2022, will be exploring the ionosphere of the moon. One of the main goals of the JUICE mission is to study habitability: apart from magnetosphere and atmosphere with ionosphere, Ganymede has a subsurface ocean. By studying the currents in the ionosphere we can infer currents in the subsurface ocean. This, together with the energy input from the Jovian magnetospheric environment, will point to the habitability of the moon.
- Mats André is the PI of the EFW instrument on Cluster, and studies low energy ions at Earth
- Jan-Erik Wahlund is the PI of the Cassini Langmuir Probe experiment, and the RPWI package on the up-coming JUICE mission
- Anders Eriksson is a Co-I on the LPW instrument on MAVEN
- Stephan Buchert is involved in operations and scientific analysis of data from Swarm, and EISCAT
- Niklas Edberg studies Titan’s ionosphere using Cassini data and is involved in JUICE preparations
- Erik Vigren develops models of Titan’s and Saturn’s ionospheres
- Dave Andrews studies the ionosphere of Mars using Mars Express and MAVEN data
- Michiko Morooka studies Saturn’s ionosphere and Enceladus plasma interactions using Cassini data and is involved in JUICE preparations
IRF-Uppsala has provided Langmuir probes and associated electronics for the ESA Swarm mission.