The primary objective of the 2-m National Large Solar Telescope (NLST) is to study the solar atmosphere with high spatial and spectral resolution. With an innovative optical design, NLST is an on-axis Gregorian telescope with a low number of optical elements and a high throughput. In addition, it is equipped with a high order adaptive optics system to produce close to diffraction limited performance.

NLST will address a large number of scientific questions with a focus on high resolution observations. With NLST, high spatial resolution observations of prominences will be possible in multiple spectral lines. Studies of magnetic fields, filament eruptions as a whole, and the dynamics of filaments on fine scales using high resolution observations will be some of the major areas of focus.

We study the correlation between the speed of two-ribbon separation and the magnetic flux density during the 2001 April 10 solar flare. A weak negative correlation is found between the ribbon separation speed (Vr) and the longitudinal magnetic flux density (Bz). In addition, we estimate the magnetic reconnection rate (Erec). Along the flare ribbons, Erec fluctuates in the small range except near the HXR source. The localized enhancement of the reconnection rate corresponds to the position of the HXR source.

The existence of the solar tachocline, a thin differentially rotating layer at the base of the convection zone which is inferred from helioseismology, leads to the concept of an interface dynamo. The tachocline is magnetically coupled to the radiative interior and the overlying convection zone. A multilayered interface dynamo is required to describe the dynamo process involved. We first discuss a two-dimensional multilayered interface dynamo model in cartesian geometry consisting of four horizontal layers with different magnetic diffusivities magnetically coupled by the three sets of interface matching conditions for the generated magnetic field. Exact solutions of the coupled dynamo system are obtained in this model. We then discuss a fully three-dimensional and multi-layered spherical dynamic interface dynamo using a finite element method based on the three-dimensional tetrahedralization of the whole spherical system. The spherical dynamic interface dynamo also consists of four magnetically coupled zones. In the convection zone, the fully three-dimensional dynamic feedback of Lorentz forces is taken into account. It is shown that the dynamo is characterized by a strong toroidal magnetic field, selects dipolar symmetry and propagates equatorward.

Kinematic solar dynamo models (KSDM) that use a solar like differential rotation profile usually fail to reproduce the latitudinal distribution of the toroidal magnetic fields. Usually, it is assumed that it is the larger radial shear present in the solar tachocline at higher latitudes that is the responsible for such results. We here consider variations in the shape and thickness of the tachocline and find that a better distribution of the toroidal fields is obtained when the thickness of the tachocline is $d_1=0.02R_{\odot}$, a smaller value than that conventionally used.

The relationship between the magnetic vector and the temperature of a large symmetric sunspot is studied on the basis of 1.56 µm spectra. From this relation we estimate the shape of the τ = 1 surface. i.e. the Wilson depression, as a function of radial distance in the sunspot. We find that the Wilson depression is relatively small throughout the penumbra and changes by 200–500 km at the umbral boundary. We also estimate the magnitude of magnetic gradients and curvature forces.

We present accurate measurements, based on the g = 3 line at 1.5648/µm, of the field strength and longitudinal velocity in the vicinity of the magnetic neutral-line of a solar active region. We find that very close to the neutral line the field strength and the flow velocity are correlated in the sense that the magnetic polarity with the larger field strength is associated with a downflow, while the polarity with the weaker field shows an upflow. This is exactly the signal predicted by theoretical models of siphon flows, and constitutes the first reliable detection of a siphon flow on the sun.

An analysis of Stokes I and V profiles of 1.56 µm lines of sunspots near the solar limb shows that the magnetic field continues outside the visible contours of sunspots in the form of a low-lying superpenumbral canopy. We also find that the V profiles formed in the canopy exhibit the Evershed effect (with line shifts of 1–2 km s−1), while the matter below it shows no sign of a flow. Therefore, the Evershed effect definitely is present beyond the visible sunspot boundary. However, if we interpret the line shifts in terms of stationary flows, then only a small fraction of the matter seen to be flowing outwards in the penumbra can be accounted for by the outward flow in the superpenumbral canopy. Therefore, although the Evershed “flow” does not stop at the boundary of the spot, most of the flowing matter stops or disappears there.