Institute for Geophysics and Extraterrestrial Physics, Technical University Braunschweig
The Suborbital Particle Aggregation and Collision Experiment (SPACE) is a novel approach to study the collisional properties of submillimeter-sized, highly porous dust aggregates. Studying these properties helps increasing our knowledge about the processes dominating the first phase of planet formation. During this phase the growth of planetary precursors occurs by agglomeration of micrometer-sized dust grains to aggregates of about a millimeter. However, the formation of larger bodies from the so-formed building blocks is not yet fully understood. Recent numerical models especially lack support by experimental studies in the size range of submillimeter-sized bodies colliding at very gentle relative velocities of below 1 cm/s. First observations of particle collisions and aggregate clustering with the SPACE experiment were recorded during a drop tower campaign at the ZARM (Zentrum für angewandte Raumfahrttechnologie und Mikrogravitation) in Bremen in August 2011. The high-speed video data produced during the REXUS 12 suborbital rocket flight in March 2012 still has to be analyzed and will deliver data on single particle collisions velocities and sticking probabilities, cluster formation and growth, sticking forces and low velocity particle fragmentation.
Institute for Astrophysics, University of Göttingen
Max Planck
Institute for Solar System Research
Cool main-sequence stars have convective envelopes, from which convective flows extend into the optically thin surface layers and thus measurably modify the stellar spectra. Many cool main-sequence stars also show magnetic activity and are known to have a substantial global magnetic field. While methods to probe the global geometry and strength of the magnetic field are available, the local structure of the field, which is governed by its interaction with the convective plasma flows, is unknown. Knowledge of the local structure of the magnetic field is, however, essential to interpret stellar spectra and measure the field. We have run and analysed 3D radiative magnetohydrodynamical simulations of the surface layers of main-sequence stars of spectral types F3 to M2. Our simulations show substantial differences in the magnetoconvection between stars of different spectral types. First steps have been undertaken towards a quantitative comparison of our results to observational data. Moreover, our results provide an independent test and calibration for techniques with which magnetic field strength and geometry are commonly assessed.
Max Planck Institute for Solar System Research
Absorption signatures in energetic electron distributions (microsignatures) caused by the interaction between electrons bouncing along the magnetic field lines and drifting around the planet with Saturn's icy moons orbiting in its inner magnetosphere provide a nice tool to investigate parameters of the global magnetospheric parameters.
In this talk we will present how radial displacements from the expected locations of the energetic electron microsignatures can be used as tracers of unidentified electric fields. On the basis of a statistical study from a large number of microsignatures, caused by the Saturnian moons Tethys and Dione recorded in the period July 2004 - January 2011, we report some local time asymmetries in their radial displacements that follow a systematic trend: inward displacements in the nightside and outward displacements on the dayside part. For this study we have used electron data in the energy range 20-300 keV from the MIMI/LEMMS detector as well as magnetometer data from the MAG instrument both aboard the Cassini mission.
We will show that these asymmetries cannot be explained by asymmetries of the magnetic field in the inner magnetosphere. We developed several methods to associate the properties of the displacements to magnetospheric electric fields. Our results are consistent with an electric field that has a noon-to-midnight orientation and amplitudes of around 0.11-0.18 mV/m. Such an electric field is not predicted by any theoretical model and its origin is still unknown.
Max Planck Institute for Solar System Research
The 11-year solar sunspot cycle has been known for centuries, however the investigation of smaller magnetic features and their relationship with the solar cycle could only studied in the past two decades. The space-borne Hinode mission has revealed copious amounts of magnetic flux covering the quiet Sun. With its 0.3 arcsecond resolution it has become possible to study the smallest magnetic features present on the solar surface over many years. The properties of this horizontal and vertical flux have nurtured the notion of local dynamo action operating close to the solar surface, independent from the global dynamo. We sought to investigate any solar cycle related variations in the polarisation signals detectable on the disc centre of the quiet Sun starting from November 2006 until May 2012. We studied in particular the weak signals found in the internetwork. The investigation used line-integrated circular and linear polarisation profiles obtained from the photospheric Fe I 630.25 nm absorption line measured by Hinode SOT/SP. After a careful consideration of the instrumental degradations affecting SOT/SP, the quiet Sun internetwork magnetic flux shows no cycle related variation and is constant throughout the entire period of investigation. This supports the idea that independent local dynamos are responsible for the majority of the quiet Sun internetwork flux detected by Hinode.
Institute for Astrophysics, University of Göttingen
Max Planck
Institute for Solar System Research
Observations of seismic waves on the solar surface can be used to probe the Sun's interior, particularly the convection zone. In time-distance helioseismology, the travel times of waves between pairs of points on the solar surface are measured using a cross-correlation technique. Wave travel times are sensitive to the properties of the medium through which the waves propagate. My project consists of developing inverse methods of time-distance helioseismology in order to characterize 3D convective flows in the upper convective zone. At first, the effort will focus on the formulation of the inverse problem: What travel times should be measured and used as input to the inversion procedure? Can we design an objective regularization procedure (typical inverse problems in time-distance helioseismology are ill-posed in the absence of regularization)? Using toy problems and synthetic observations, I intend to design more reliable inversion procedures in order to place constraints on solar convective velocities. These new procedures will then be applied to SDO/HMI observations.