Science Cases for DKIST

Spectral vs. Spatial Diagnostics The Sun sustains life on Earth and impacts human endeavors in space through variations in its radiative, magnetic and particle output as caused by magnetic activity. The solar atmosphere is controlled by magnetic fields. The Daniel K. Inouye Solar Telescope (DKIST) is the first major instrument designed by the astronomical community in all of its aspects as a tool for magnetic remote sensing. Its collecting area, spatial resolution, wavelength performance and integral focal plane instrumentation are all targeted for understanding how magnetic fields affect the physical properties of the Sun. DKIST will be the largest leap in ground-based solar capabilities since Galileo's telescope.

DKIST will be a four-meter off-axis reflecting telescope, which will have the spatial, temporal, spectral resolution and dynamic range that is needed to see and measure the basic magnetic structures (magnetic fibrils) at the solar surface and into the outer atmosphere. Currently much of the magnetic field is invisible. We therefore will depend on the DKIST for quantifying, understanding, and predicting the consequences of such magnetism on solar-terrestrial and astrophysical plasmas.

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The DKIST addresses the basic questions: What is the nature of solar magnetism; how does that magnetism control our star; and how can we model and predict its changing outputs that affect the Earth? DKIST will observe solar plasma processes and magnetic fields with unprecedented resolution in space and time. It will provide critical information needed to solve the mysteries associated with the generation, structure, and dynamics of the surface magnetic fields, which govern the solar wind, solar flares, and short-term solar variability. Used in combination with space-based and other ground-based solar observing assets, DKIST will underpin a revolution in our understanding of the sun.

The following pages provide short descriptions of just some of the critical science topics scientists will use the DKIST to address and solve.

Flux Tubes: The Building Blocks of Solar and Stellar Magnetic Fields

Solar Granulation Observations have established that the photospheric magnetic field is organized in small fibrils or flux tubes (associated with the bright g-band points seen in the photo). These structures are mostly unresolved by current telescopes. Flux tubes are the most likely channels for transporting energy into the upper atmosphere, which is the source of UV and X-ray radiation from the Sun, which in turn affects the Earth's atmosphere. Detailed observations of these fundamental building blocks of stellar magnetic fields are crucial for our understanding not only of the activity and heating of the outer atmospheres of late-type stars, but also of other astrophysical situations such as the accretion disks of compact objects, or proto-planetary environments. The diffraction limited resolution of DKIST will be 0."03 at 500nm and 0."08 at 1.6 microns. At this resolution DKIST will provide the required spectroscopy and polarimetry at an angular resolution to explore the enigmatic flux tube structures.

Magnetic Field Generation, Local Dynamos and the Solar Cycle

Small Scale Dynamo To understand solar activity and solar variability, we need to understand how magnetic fields are generated and how they are destroyed. The 11-year sunspot cycle and the corresponding 22-year magnetic cycle are still shrouded in mystery. Global dynamo models that attempt to explain large-scale solar magnetic fields are based on mean field theories. Dynamo action of a more turbulent nature in the convection zone may be an essential ingredient to a complete solar dynamo model. Local dynamos may produce the small-scale magnetic flux tubes recently observed to cover the entire Sun. This "magnetic carpet" continually renews itself on a time scale of a few days at most and its flux may be comparable to that in active regions. The DKIST will make it possible to directly observe such local dynamo action at the surface of the Sun. The DKIST will measure the turbulent vorticity and the diffusion of small-scale magnetic fields and determine how they evolve with the solar cycle. The DKIST will address the following fundamental questions: How do strong fields and weak fields interact? How are both generated? How do they disappear? Does the weak-field component have global importance and what is its significance for the solar cycle? The DKIST will address these fundamental questions by resolving individual magnetic flux tubes and observing their emergence and dynamics. It will measure distribution functions of field strength, field direction and flux tube sizes and compare these with theoretical models. The DKIST will observe plasma motions and relate them to the flux tube dynamics.

Magnetic and Current Helicity and its Relevance to the Dynamo Problem

Twisted Magnetic Fields Helicity plays a fundamental role in evolution and topology of solar magnetic fields on different spatial and temporal scales. Helicity is essential for the effective operation of a dynamo. On the other hand, excessive helicity may suppress the dynamo action. To ensure efficient operation of the dynamo, helicity has to be removed from the dynamo region and transported to the coronal. On their way to the surface magnetic fields can accumulate addition helicity by interacting with turbulence in the convection zone.

Interaction of Magnetic Fields and Mass Flows

Sunspot Fine StructuresIn sunspots, the total magnetic field is large enough to completely dominate the hydrodynamic behavior of the local gas, a regime very different from that of the rest of the solar photosphere. Numerical simulations and theoretical models predict dynamical phenomena, such as oscillatory convection in the strong-field regions of sunspot umbrae, flows at the speed of sound along penumbral filaments and oscillations and wave phenomena. To verify the predictions of numerical simulations of sunspots and ultimately answer such fundamental questions as "Why do sunspots exist?" require an extremely capable instrument. High-resolution (<0.1 arcsec) vector polarimetry combined with high sensitivity (requires high photon flux) and low-scattering optics are required. Understanding the interaction of magnetic flux and mass flows is crucial for our understanding of the behavior of magnetic fields from the scales of planetary magnetospheres, to star-forming regions, to supernova remnants, to clusters of galaxies. Sunspots allow us to test those theories in a regime where magnetic fields dominate mass flows.

Flares and Mass Ejections

H-alpha FlareIt is commonly believed that solar flares represent a process of rapid transformation of the magnetic energy of active regions into the kinetic energy of energetic particles and plasma flows and heat. Detection of variations of magnetic field associated with solar flares has been one of the most important problems of solar physics for many years. Such detection would provide direct evidence of magnetic energy release in the flares. An important goal for the DKIST will be to study the small-scale processes in solar flares. The DKIST will also provide a new set of tools, in particular in the infrared, to measure magnetic fields at higher layers of the atmosphere. There is limited observational evidence that the distribution of electric currents and current helicity inside an active region varies with flares. Highly uniform sequences of high-resolution vector magnetograms of an active region before and after a flare are required to address this important issue.

Coronal mass ejections (CMEs) originate in large-scale magnetic arcades known as helmet streamers. These structures are known to contain twisted magnetic fields. According to the prevailing view, the arcade becomes dynamically unstable when its fields are twisted beyond some critical point. Field line footpoint motions in the photosphere have long been considered efficient ways to supply (or drain) magnetic shear and energy into (from) the coronal field. MHD simulations have identified critical magnetic shear conditions above which the arcade field will form current sheets, and magnetic reconnection processes will occur to cause active phenomena such as flares, CMEs, and prominence formation and eruption. It is thus important to measure field line footpoint motions and, if possible, the magnetic shear in active regions as well. Although measurements of the footpoint motion (in particular the horizontal plasma flow velocity) have improved considerably, the DKIST will offer unprecedented high spatial and temporal resolution in measuring the field line footpoint motion.

More recently this view has been challenged by models in which the arcade emerges as a twisted flux rope or models in which small-scale photospheric reconnection events inject helicity into the corona. These hypothesized reconnection events occur when small (0."1) photospheric flux elements cancel along the active region's magnetic neutral line. This model seems to contradict those models in which CMEs result from excessive twist in the arcade. Detailed observations of the flux emergence can reveal whether the emerging flux is introducing magnetic twist into the arcade, or changing the arcade's topology through footpoint cancellation. Only with higher spatial resolution vector measurements and good temporal resolution as will be provided by the DKIST, however, can it be established, for example, that the rate and orientation of these cancellations is consistent with an observed change in the twist of the overlying arcade. Such observations are critical to distinguishing between competing models.

Inhomogeneous Stellar Atmospheres

Limb Spectrum of Solar CO LinesMeasurements of CO absorption spectra near 4.7µm show surprisingly cool clouds that appear to occupy much of the low chromosphere. Only a small fraction of the volume apparently is filled with hot gas, contrary to classical static models that exhibit a sharp temperature rise in those layers. The observed spectra can be explained by a new class of dynamic models of the solar atmosphere. However, the numerical simulations indicate that the temperature structures occur on spatial scales that cannot be resolved with current solar infrared telescopes. A test of the recent models requires a large-aperture solar telescope that provides access to the thermal infrared. Such observations would further explore the dynamical basis of the thermal bifurcation process, a fundamental source of atmospheric inhomogeneities in late-type stars. Spicules, the forest of hot jets that penetrate from the photosphere into the chromosphere, are clearly a MHD phenomenon that is not understood nor adequately modeled. Their role in the mass balance of the atmosphere is uncertain. Combined with UV observations (like those of TRACE), the DKIST will allow us to resolve their nature.

Magnetic Fields and Stellar Coronae

Coronal LoopsThe origin and heating of the solar corona, and the coronae of late-type stars, are still mysteries. Most of the proposed scenarios are based on dynamic magnetic fields rooted at the 0.1-arcsec scale in the photosphere. However, none of the processes has been clearly identified by observations or theory. EUV and X-ray observations have gained in importance, but ground-based observations are still critical, not only to determine the forcing of the coronal fields by photospheric motions, but also for the measurement of the coronal magnetic field strength itself. This is important for developing and testing models of flares and coronal mass ejections, which propel magnetic field and plasma into inter-planetary space and induce geomagnetic disturbances. In particular, precise measurements of the coronal magnetic field strength and topology are needed in order to distinguish between different theoretical models. The DKIST with its large aperture, low scattered light characteristics, and the capability to exploit the solar infrared spectrum will provide these critical measurements.

Quelle: DKIST

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