Solar physics: analysing the magnetic field in the solar atmosphere.

Analyzing the Magnetic Field in the Solar Atmosphere

The magnetic field in the solar atmosphere exceeds the geomagnetic field strength by four orders of magnitude.
It greatly influences the processes of energy transport within the solar atmosphere, and dominates the morphology of the solar chromosphere and corona. Kinetic energy from convective motions in the Sun can be efficiently stored in magnetic fields and subsequently released - to heat the solar corona to several million degrees or to blast off coronal mass ejections.

The determination of the conditions in the lower layers of the solar atmosphere (photosphere - chromosphere), including the three-dimensional structure of the magnetic field therein, is the main task within this PhD project. Knowing these accurately is essential for many things, understanding turbulent magnetoconvection, finding out how energy is loaded onto the magnetic field lines that is then transported up to the corona and deposited there, uncovering the secrets of how heavy prominences can stay suspended in the thin corona, or gaining new insights into the solar cycle and solar irradiance variations.

MPS has access to world leading facilities to measure these conditions: the largest available ground-based solar telescopes and the balloon-borne Sunrise observatory, deliver maps of the emitted solar radiation, displaying details down to scales of about 70 km. Access to data from space borne observatories such as the Japanese Hinode spacecraft extends these rich data sets. For the ground-based observatories SLAM group members have recently built or are building our own, often highly novel instruments, which provide us with outstanding and unique data.

Unlocking the Sun's secrets with unique data from the balloon-borne observatory Sunrise

The Sunrise observatory on the launchpad in Kiruna, Sweden.

Sunrise is a balloon-borne solar observatory developed under the leadership of MPS. The 1-meter telescope is the largest solar telescope ever to leave the ground, allowing for highest spatial resolution measurements (~50 km on the Sun) in UV wavelength bands not accessible with ground-based observatories.

Two successful flights at an altitude of 37 km around the north pole have delivered a wealth of high-quality data sets, which have already resulted in immense new insights into the working of the solar photosphere (the layers of the solar atmosphere just at the solar surface), including a number of breakthroughs. These are documented in well over a hundred mostly high-impact scientific publications, making Sunrise by far the most successful balloon-borne mission to date. Currently, preparations are ongoing for the third flight of the Sunrise observatory, which will be even more powerful thanks to three new instruments that are being built for it. These include a unique and novel spectropolarimeter to make optimal use of the UV wavelength band. These new instruments will greatly expand the capabilities of Sunrise, enabling it to unlock not just hidden secrets of the photosphere, but also of the solar chromosphere, the most mysterious part of the solar atmosphere.

A PhD student will be directly involved in the first analysis of these data and will have a good chance of making/participating in the first discoveries from the Sunrise III data.

Also, a PhD with the correct skills and starting in 2020 has a chance of attending the launch from ESRANGE in north Sweden and participating in the operations of Sunrise during its flight in the stratosphere.

Flying over the poles of the Sun with Solar Orbiter

The flight model of the Polarimetric and Helioseismic Imager for Solar Orbiter during ground testing at MPS.

The Solar Orbiter space mission, scheduled for launch in February 2020, will be the first mission ever to leave the ecliptic plane with optical instrumentation on board. You could help in the in-flight calibration, the analysis of the first science data and in the preparation of the nominal mission phase of one of the major instruments on board Solar Orbiter. The Polarimetric and Helioseismic Imager (SO/PHI) is an imaging spectro-polarimeter which provides both full-disk and high resolution vector-magetograms, Dopplergrams and continuum images of the Sun. The SO/PHI instrument has been built by an international consortium led by MPS. After assembly, testing and integration into the Solar Orbiter spacecraft it is now ready to fly!

Together with world leading experts you will prepare observation programs and data analysis tools for SO/PHI. Your tasks could consist in participating in the development of the software tools simulating the SO/PHI instrument, the science observations and data analysis procedures. Alternatively, your task could be comparing ground calibration data to data obtained during early in-flight operations.

SO/PHI's first data are to be received at the beginning of your PhD project. The successful PhD student will, therefore, get a head start for Solar Orbiter's exciting science phase!

The Sun in 4D: "How to capture the properties of the solar atmosphere fast enough?"

Example observation with a MiHI prototype at the Swedish 1-meter Solar Telescope [http://www.isf.astro.su.se/NatureNov2002/telescope_eng.html]. The red square on the left panel shows the spatial field of view of MiHI within a larger context scene of the solar surface. The small panels on the right show different cuts through the multi-dimensional data cube with spatial, spectral and polarization information recorded simultaneously with MiHI. Panels S1 to S4 show example spectra for the region around 6302 Å containing 2 spectral lines of neutral iron (denoted by ‘Fe I’ in panel S1). S1 shows an intensity (Stokes I) spectrum. Panels S2 to S4 show spectra of the degree of linear polarization (Stokes Q/I and U/I) and circular polarization (Stokes V/I). The horizontal axes represent the relative wavelength. Panels I1 to I4 show two-dimensional spatial images of the MiHI field of view in the 4 polarization states and spectrally averaged across a small region of the spectrum (denoted by the vertical yellow lines in panels S1 to S4). Panels C1 to C8 represent different spectral cuts through those images. The cuts have one spatial dimension and one spectral dimension. In cuts C1 to C4 the horizontal axis represents the spectral dimension. The vertical axis represents the spatial dimension cutting through the images along the blue lines. In cuts C5 to C8 the spectral dimension is vertical and the spatial dimension horizontal, cutting through the images along the red lines. The small structures seen in the images as well as in the Fe I lines in the spectral cuts are signatures of the Zeeman effect, which results from the interaction of the iron atoms with light in the presence of a magnetic field in the solar atmosphere.

To study the dynamics of the continuously evolving solar atmosphere, it is necessary to obtain morphological information by imaging the solar surface on a time scale that is sufficiently small to effectively "freeze" the solar scene. With the European Solar Telescope - EST we expect to resolve solar structures of order 30 km which evolve at a time scale of seconds. While an image can be successfully recorded within this short time scale, only a small fraction of the information required for understanding the solar atmosphere is encoded in the image properties alone.

By analyzing in detail the strength, shape and the polarization state of the many spectral lines that are present in the solar spectrum, detailed information about the atmospheric stratification of temperature, vertical bulk motion, magnetic field strength and direction, encoded by atomic line transitions of the many elements that constitute the solar atmosphere, can be obtained. However, for a detailed analysis of the spectral properties of the Sun, a spectrograph with a high spectral resolution must be used.

There is currently no solar instrument in operation that can record spectral, polarization and spatial information within the limits of the short timescale of solar evolution that will be seen by the European Solar Telescope - EST, and with sufficient resolution and sensitivity. Present-day instruments are only able to capture all this information by scanning either spectrally or spatially which takes more time.

A new type of instrumentation is therefore needed for the EST. The Microlens-fed Hyperspectral Imager (MiHI) under development at the Max Planck Institute for Solar System Research is one example of such a new instrument that overcomes this problem by recording the spectral information for each image pixel simultaneously. In order to accomplish this, the pixel must first be reduced in size to create enough space on the two-dimensional image sensor for the spectral information to be dispersed without overlap with the neighboring pixels. The microlens spectrograph uses an assembly of microlens arrays to reduce the pixel size and create space for the spectral information, while minimizing the loss of light. Then the light is dispersed in a spectrograph, forming an array of spectra on the detector.

Digging deep: peering into the deepest observable layers of the Sun

Continuum intensity, line-of-sight velocity and magnetic field strength of a sunspot with granular light bridges, derived from observations by the Hinode spacecraft.

The rapid increase of the particle density with depth impedes the escape of photons from deep layers of the solar atmosphere, making the measurement of the physical conditions in these layers extremely difficult. Observations in a spectral region around the opacity minimum, located in the near-infrared region of the solar spectrum, are essential to improve our understanding about the convective processes in the near-surface layers.

The challenge of this PhD project is to first convert high-resolution observations of photospheric spectral lines in the visible and the infrared into reliable, 3D maps of the solar atmospheric conditions, and second to deepen the understanding of the physical mechanisms working in the giant natural plasma physics laboratory, the Sun. Very often comparisons to state-of-the-art magneto-hydrodynamic simulations, performed within the solar group at the MPS, are used to gain insight into these mechanisms.

Unique data from the world-largest ground-based solar telescopes will deliver the information about the conditions in the solar atmosphere in the deepest accessible layers. Complemented with observations from space-based observatories (Hinode, SDO) the 3-dimensional stratification of the solar atmospehre can be reconstructed.

The applicant should have basic knowledge in the fields of spectral line formation (atomic physics, Zeeman effect, radiative transfer), very good programming skills (Python or IDL, C/C++ or Fortran), and should have a good understanding of observational data (e.g. measurement and statistical errors).

Mapping the Sun's mysterious chromosphere and its driver, the magnetic field

Intensity and circular polarization maps in the photosphere and the chromosphere obtained with the GRIS instrument at the GREGOR telescope in 2014.

Between the photosphere, where most of the solar energy escapes, and the hot Corona, the outer atmosphere of the Sun where the temperature reaches millions of degrees, sits the chromosphere, a layer above the solar surface in which the temperature reaches a minimum before rising again. The chromosphere is characterized by magnetically dominated, highly dynamic structures, but is visible only in a small number of the strongest atomic lines in the solar spectrum. Due to their strength, these lines form in conditions far away from thermal equilibrium, making the interpretation of their line profiles in terms of physical quantities very challenging. Several codes for the interpretation of such spectral lines are currently available for general use, and we are currently developing a new code to extend our capabilities.

Data from  instruments like GRIS+ will allow the key chromospheric magnetic field to be observed as never before. Obtaining your own data set during one or two observing campaigns on Tenerife will be part of this project.

In this PhD project you observe and record data in a chromospheric line, with the focus on the He 10830 triplet. You will reduce and analyze the data using available interpretation tools or by writing new analysis software. The focus of the project is on the physics and dynamics of solar magnetic structures on and above the solar surface.

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