Main research areas: Solar System Physics relates to planetary and interplanetary magnetic fields. Electrical discharges in the Earth's upper atmophere.
Electrical Discharges in the Upper Atmosphere
Electromagnetic Induction Studies
Magnetic fields at Mars
Electrical Discharges in the Upper Atmosphere: The study of Red Sprites, Blue Jets, and Elves
The upper atmosphere – reaching from the top of the troposphere where the weather and climate rule to the bottom of the ionosphere at 100 km altitude – is one of the least explored regions around the Earth. It constitutes a layer in which processes originating in outer space interplay with processes from the troposphere below.
Here, a surprising variety of optical emissions have been discovered above thunderstorms, including the 'Red Sprite', a gigantic electrical discharge in the mesosphere, the 'Blue Jet', a discharge propagating upwards into the stratosphere from cloud tops, and the 'Elf', a concentric ring of emissions propagating horizontally outwards at the bottom edge of the ionosphere. Observations have further documented super discharges with Blue Jets triggering Sprites, creating an electrical breakdown of the atmosphere from the top of thunderstorms to the bottom ionosphere.
The Upper Atmosphere Research Group at the National Space Institute is studying the physics of high altitude electrical discharges, through experimental and modeling activities. We conduct annual summer observational campaigns to Southern Europe under the EU Research Training Network CAL, develop instrumentation for the International Space Station ASIM, simulate sprite ignition using particle codes, and much more.....
Electromagnetic Induction Studies
What is the structure of a planet's interior? A better understanding of the mantle structure will help us learn more about the mechanisms of evolution of our planet. Geomagnetism is a technique that allows us to probe the interior of the Earth.
Knowledge of the structure of the Earth's mantle is important for the understanding of the mechanisms of evolution of our planet. Our present understanding of the processes acting in the mantle results from studies which combine the outcomes from various experimental fields like petrology, geochemistry, and laboratory experiments on the physics and chemistry of the Earth materials.
One of the major limitations is the lack of direct observations of the actual structure of the Earth's mantle. Very few techniques provide such data. Geomagnetism is one of them.
The Earth’s magnetic field is due to several sources of internal and external origins. Among these sources, the magnetization acquired by some minerals present in most rocks has been used for the investigation of the structure of the Earth’s crust and upper mantle. Another source of magnetic signal is the interaction between the magnetic field of the Earth and the solar wind which contains charged particles emitted by the sun; mainly protons, electrons and helium. This interaction gives rise to time-varying magnetic fields in the Earth’s ionosphere at 110 km altitude and in the magnetosphere which is situated at a distance of several Earth radii.
Figure 1: Time variation of the horizontal (d X) and vertical (d Z) components of the magnetic field measured at Fürstenfeldbruck near Munich/Germany. The vertical component Z is more affected by induced currents than horizontal component X. Induced currents lag the inducing currents, resulting in a time lag of d Z.
Figure 2. A magnetic signal penetrates deeper into the Earth if its period is longer or electrical conductivity is smaller.
The Earth’s interior is electrically conducting. Consequently the time-varying magnetic fields in the ionosphere and in the magnetosphere induce (Faraday’s law) time-varying electric currents in the interior of the Earth which in turn generate time-varying magnetic fields measured at the surface of the Earth.
The induced time-varying magnetic fields are called 'electromagnetically induced fields' and are a function of the electrical conductivity structure of the interior of the Earth.
Electrical conductivity is a physical parameter which is particularly sensitive to temperature, melt fraction and phase transformations inside the Earth. Most dynamic processes inside the Earth result from a thermal activation and are consequently linked to the temperature distribution within the medium. The knowledge of the electrical conductivity distribution inside the mantle will therefore provide a major clue for the understanding of the thermodynamic processes taking place inside the Earth: mantle convection, the origin of hot spots and the recycling of slabs.
Electrical conductivity studies at the National Space Institute
The external time-varying magnetic fields are characterized by a wide spectrum of time variations ranging from minutes to years.
Because of an effect known as the 'skin effect', magnetic fields of different periods of variation will induce electrical currents at different depths within the Earth. The longer the period of variation of the magnetic signal, the deeper it penetrates inside the Earth (Figure 2). Long period signals probe the electrical conductivity of deep regions of the Earth’s mantle.
The National Space Institute performs electromagnetic induction studies using magnetic field observations either taken by ground observatories distributed on the surface of the Earth or measured by satellites like Ørsted, CHAMP, SAC-C and Swarm.
Time-varying external, primary, currents in the ionosphere and magnetosphere induce currents in the interior of the planet, which are responsible for the secondary, induced, magnetic field. The 'transfer function' between external (inducing) and internal (induced) fields allows the estimation of the electrical conductivity of the planet. Transfer functions for different periods of variation of the magnetic signal provide information on the electrical conductivity structure of the Earth at different depths.
Magnetic field observations at Mars are used in a wide range of scientific investigations, including:
- study of the interaction between the solar wind and the upper atmosphere of Mars, and processes leading to atmospheric scape
- determination of the structure and thermal evolution of the deep interior of the planet
- recovery of deep sub-surface water reservoirs, by the determination of the planetary electrical conductivity
- study of the effect on the Martian environment of explosive events on the Sun, in particular the propagation and arrival of solar energetic particles at Mars
Hardly any other single physical quantity can be used in such a variety of studies related to planetary research as the magnetic field.
At the NSI we study these processes by analyzing magnetic observations from various satellite missions, in particular the Mars Global Surveyor (MGS) in cooperation with NASA's Goddard Space Flight Center. We cooperate with international groups to interpret the observations in terms of theoretical models and computer simulations of solar wind Mars interactions, and with other groups to provide improved models of crustal magnetization.
The Magnetometer Group at DTU has developed a miniaturized version of the Ørsted satellite magnetometer, well suited for operation at the surface of Mars. The NSI is currently leading an international effort to include this magnetometer as part of Geophysics and Environment Package on the ESA mission ExoMars, scheduled to land at the surface of Mars in 2011/2013.