Organisation and dynamics of surface magnetic field
Magneto-convective processes in the photospheric and sub-photospheric layers of the Sun play crucial
roles in the magnetic structuring and dynamics of upper atmospheric layers. The near-surface layers
are the regions where the plasma β (= the ratio of gas to magnetic pressure) transitions from
β>>1 in the interior to β<<1 in the outer atmospheric layers. The β~1 photospheric boundary
layer is also the region where the continuum optical depth τc~1, which separates the optically thick
interior from the thin outer atmosphere. Hence, this boundary layer is subject to a strong coupling of
very different scales of dynamical interests, and is difficult to study both observationally and
Both very fast imaging and fast spectroscopic measurements are necessary to study the highly
complex processes in this region. Spectropolarimetry, the study of Stokes vectors (Polarization) of a
spectral line to infer physical conditions prevailing in a dynamic magnetized medium is the most
sensitive tool that is available.
Flux emergence and active region dynamics
Emerging magnetic flux through the β~1 layers expand and merge above a certain height called the
"magnetic canopy height" and fill the available space in the β<<1 region. Below the photosphere,
due to the dominant gas pressure forces, the magnetic fluxes are passive and are confined to
individual and separated flux tubes. The emergence from below is, thus, in the form of individual
flux tubes, which are expected to carryoppositely directed currents on either side of the flux tube
boundaries as well as twists.
Measurements of currents and twists in emerging flux tubes is important both to infer the dynamical evolution
of the magnetic flux tubes while they rise through the solar interior to the surface as well as to understand
the role of the twist leading to instabilities and eventual dissipation of magnetic energy in the
solar atmosphere. Studies of helicity and energy fluxes in active regions (e.g. Ravindra et al., 2008)
give important insights into the coronal dynamics and activity.
Present vector magnetograms show a persistent pattern of electric currents (see Figure 2) and
helicity associated with strong magnetic fields of active regions. NLST will be able to address the
following questions: (1) how are the electric currents in the emerging flux tubes structured and how
do they interact with the current systems already developed in the
overlying canopy fields?; (2) what are the basic mechanisms of flux submergence?; (3) how does the
magnetic helicity interact with mass flows down the legs of raising flux tubes and how does it affect
the overall helicity flux into the upper atmosphere?; and (4) what are the dynamical consequences of
helicity driven vortical flows in the photosphere?.
Dynamical evolution of small-scale magnetic field
Extensive theoretical and numerical studies (Parker 1978, Spruit and Zweibel 1979, Spruit 1979, Hasan 1984,
Venkatakrishnan 1986, Rajaguru and Hasan 2000) as well as 2- and 3-D MHD simulations (Nordlund 1985,
Steiner 1999) have shown that there are two important processes in action in the formation, organisation
and stabilization of small-scale strong magnetic flux tubes that dot the super-granular boundaries. They
are flux expulsion and convective collapse. While the existence of the former process, viz. flux expulsion,
finds easy confirmation through the established pattern of magnetic flux residing in the convective down
flow regions, observational evidence for the latter process is very challenging to establish. This is due
to the fact this process happens very fast and over a small spatial scale. Though recent high resolution
observations from HINODE have provided evidences, a routine mapping of this process so as to be able to
study it statistically and test theoretical predictions requires high sensitivity and high cadence
Observations of different layers of Sun using different instruments leading to inference of vector magnetic field maps is shown in Figure 3.5.
This figure shows that multi wavelength observations are needed to map vector magnetic fields at different heights in solar atmosphere. NLST
will provide an opportunity to undertake such observations. High resolution numerical simulations leading to
MHD waves and corresponding Stokes vector calculations are routinely made these days (Figure 4).
They will need high resolution observations for validating purposes. Small scale features can be easily
identified with high resolution. NLST instruments, with adaptive optics, will test the presence and efficiency of the
convective collapse mechanism by both spatially and temporally resolving it in action.
The kilo Gauss strength network elements provide mechanical and energetic coupling between the photospheric,
chromospheric and higher layers through waves as well as through associated shock dynamics related transients such
as jets, coronal bright points, blinkers etc. This makes solar atmosphere highly dynamic. These strong flux concentrations
constantly undergo buffeting by the granular
flows reaching speeds of 1-5 km/s. Observations of these structures require high time cadence with fast cameras.
Spectropolarimetry and 3-D structure of sunspots and magneto-convection
Sunspots are classic examples that portray the complexity and rich physics that magnetohydrodynamic phenomena exhibit
on stellar surfaces. Despite several centuries of observations and intense study, we still lack a consistent
scientific explanation of the observed nature of a sunspot (Solanki, 2003). The way they remain stable and survive
over a month in a turbulent environment continues to be a puzzle. Understanding what drives the radial Evershed flow
in the penumbra of a sunspot is still a challenging problem.
We are currently in a period of revived attention mostly fuelled by our increased numerical computational
capabilities to simulate MHD processes. These observed features are indicative of a modified pattern of convection.
However, identifying and understanding the exact physical causes and mechanisms behind these complex magneto-convective processes
require solving the full 3-D MHD equations.
With prescribed magnetic field geometries representative of sunspot like situations, modelers have performed 3D nonlinear simulations of
interactions between turbulent convection and magnetic fields, thereby obtaining a new magneto-convective origin for
the Evershed flows (Scharmer et al., 2008, Rempel and Schussler, 2008). Here, the convective interactions between an
upward hot plume and magnetic field produce all the necessary ingredients to drive a horizontal Evershed flows
depicted in Figure 5.