Waves in the solar interior and atmosphere

Waves in the solar interior and atmosphere

The solar photosphere and the various magnetic structures embedded in it are subjected to a constant buffeting and impulsive motions due to global and local resonant waves of the solar interior and due to convective motions. Recent observational work has provided evidence on the propagation of oscillatory signals from the transition region into the solar corona in the 3-min period band, and it has been suggested that they are connected to the 5-min global p-modes.

Local Helioseismology of sunspots

The future of helioseismology is recognised to be largely in the realm of new methods of analyses provided by local helioseismology (Duvall and Kosovichev2001, Gizon and Birch 2005), with focus on unravelling the complexities of near surface convection, magnetic activity, as well as the poorly understood tachocline region dynamics and large scale flows that control the solar dynamo. Local helioseismology uses solar oscillations to probe localized perturbations in the solar structure and dynamics. The NLST instruments with fast narrow band imaging and spectropolarimetric capabilities will be able to provide data for local helioseismic studies.

Understanding the sub-surface thermal and magnetic structure of sunspots is a basic necessity, because it is linked to several other fundamental problems such as the solar dynamo ---why and how the surface magnetism is related to a deeper operating dynamo --- and energy transport mechanisms and structuring in the higher atmospheric layers. Physics of sunspots will also provide key answers to interpreting the stellar activity phenomena (Schrijver and Zwaan, 2000).

Figure 6 A high resolution picture of a sunspot (left panel) showing the complex umbral brightness structure, understanding of which depends on deciphering the sub-surface thermal and magnetic constitution of sunspots. A local helioseismic attempt at imaging the 3-dimensional sound speed and flow structure beneath a sunspot (Courtesy: A. Kosovichev and T.L. Duvall 2000). Refinements of such studies to increase accuracies and reliability depend a lot on high-resolution spectropolarimetry that provides simultaneous velocity and 3-D magnetic mapping over the sunspot region.

Accounting for the directly observable photospheric structures (magnetic as well as thermal) of a sunspot, while using the oscillations observed within it to infer the subsurface conditions, is a challenging problem in solar physics (Thomas and Weiss 1991). Time-distance helioseismology, with its 3-D tomography imaging capabilities (Duvall et al., 1993, Zhao et al., 2001, Rajaguru et al.,2005) has led to some interesting revelations on the sound speed and flow structures beneath sunspots. But close scrutiny of these results has led to uncertainties and complications arising from dominant near-surface interactions between sunspot magnetic field and acoustic waves (p modes), parts of which are directly observable in the photospheres (e.g. waves in the penumbra, Evershed flows etc.) (Schunker et al., 2005, Lindsey and Braun 2005a, Lindsey and Braun (2005b), Zhao et al., 2006, Rajaguru et al., 2006, Rajaguru et al., 2007).

NLST will facilitate a closer look at the above problem through mapping of wave phases in the observable layers using a set of narrow band imaging and spectropolarimetric observations in the photospheric and lower chromospheric spectral lines. These, in combination with the helioseismic measurements, would help address the following crucial questions: (1) how are the quiet-Sun originating acoustic waves transformed into magnetic field guided magneto-acoustic waves within the sunspots?; (2) how do these mode transformations affect the helioseismic signatures of sub-surface changes in structure and dynamics due to sunspots?; (3) how and why are the penumbrae of sunspots fine-structured with the associated Evershed flows and how are these flows driven?; and (4) what are the sources of umbral flashes and associated oscillatory phenomena in the higher atmospheric layers?.

Waves and chromospheric heating

The question of chromospheric heating has been posed ever since it was recognized that the solar chromosphere is hotter than the underlying photosphere. Dissipation of magneto-acoustic waves in magnetic flux tubes has been considered as a possible way of heating the chromosphere and extensive theoretical, numerical and observational research has explored this idea. Magnetohydrodynamic modeling (Figure 7) (Hasan and van Ballegooijen, 2008) of such waves have progressed so as to provide detailed characteristics that can be matched with observations.

Figure 7 Schematic diagram showing the structure of a magnetic network element on the quiet Sun. The thin half-circles at the bottom of the figure represent the granulation flow field, and the thick curves represent magnetic field lines of flux tubes that are rooted in the intergranular lanes.

The Ca II bright grains were thought to be located inside flux tubes at height of about 1 Mm above the base of the photosphere. Recent MHD modelling work (Figure 8) (Hasan and van Ballegooijen, 2008), however, suggest that the Ca II "straws" (Rutten, 2006) may be located at the boundaries between the flux tubes. Numerical simulations of wave propagation in a two-dimensional gravitationally stratified atmosphere consisting of individual magnetic flux concentrations representative of solar magnetic network predict the amount of acoustic and Poynting fluxes carried upwards by photospherically excited waves.

Using NLST to study the photospheric and chromospheric signatures of such waves will form an important objective. This will be carried out using high spectral resolution and time cadence observations in some specific spectral lines.

Figure 8 Temperature perturbation, ?T (about the initial state), at (a) 75 s, (b) 122 s, and (c) 153 s in a network element due to periodic horizontal motion at the lower boundary, with an amplitude of 750 m/s and a period of 24 s. The black curves denote the magnetic field lines, and the color scale shows the temperature perturbation. The white curves denote contours of constant β corresponding to gas to magnetic pressure ratio of 0.1 (top), 1.0 (middle), and 10 (bottom)(Courtesy: Hasan and van Ballegooijen,2008).

Coronal oscillations and seismology

The idea of exploiting observed oscillations as a diagnostic tool for determining the physical conditions of the coronal plasma (Roberts, Edwin & Benz 1984) had not been successfully applied until recently due to the lack of high-quality observations of coronal oscillations. However, this situation has changed dramatically, especially due to space-based observations by the Solar and Heliospheric Observatory (SOHO), the Transition Region and Coronal Explorer (TRACE) and, most recently, with the high-resolution spectra from the Hinode spacecraft.

It has been shown theoretically that Alfven waves from the Sun can accelerate the solar wind to high speeds. The presence of such Alfven waves in the polar coronal holes has been inferred through variations in extreme ultra-violet line widths using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode (Banerjee et al., 2009).

Observations in the H-alpha line, from ground based telescopes, have been successfully used in the past to study such coronal oscillations. NLST, with its much higher resolution, can make sensitive measurements in H-alpha so as to be able to identify various fine scale features in the oscillations of coronal structures.