[Paper Review] Phase diversity restoration of sunspot images I. Relations between penumbral and photospheric features

José Bonet, I. Márquez+3 more|ArXiv.org|May 27, 2004

Adaptive optics and wavefront sensing35 references12 citations

TL;DR

This study uses phase diversity restoration on 2-hour high-resolution G-band and blue continuum observations from the Swedish Vacuum Solar Telescope to analyze the dynamic relationship between penumbral and photospheric features near a decaying sunspot. It finds that one-third of outward-moving penumbral grains escape the penumbra and evolve into granules or bright features, while G-band bright points form independently near strong magnetic fibrils, indicating distinct origins for these features.

ABSTRACT

We investigate the dynamics of and the relations between small-scale penumbral and photospheric features near the outer penumbral boundary: penumbral grains (PGs), dark penumbral fibrils, granules, and photospheric G-band bright points. The analysis is based on a 2 h time sequence of a sunspot close to disc center, taken simultaneously in the G-band and in the blue continuum at 450.7 nm. Observations were performed at the Swedish Vacuum Solar Telescope (La Palma) in July 1999. A total of 2564 images (46 arcsec x 75 arcsec) were corrected for telescope aberrations and turbulence perturbations by applying the inversion method of phase diversity. Our findings can by summarized as follows: (a) One third of the outward-moving PGs pass through the outer penumbral boundary and then either continue moving as small bright features or expand and develop into granules. (b) Former PGs and G-band bright points next to the spot reveal a different nature. The latter have not been identified as a continuation of PGs escaping from the penumbra. The G-band bright points are mostly born close to dark penumbral fibrils where the magnetic field is strong, whereas PGs stem from the less-magnetized penumbral component and evolve presumably to non-magnetic granules or small bright features.

Motivation & Objective

• To investigate the dynamical relationship between small-scale penumbral and photospheric features near the outer penumbral boundary in a decaying sunspot.
• To determine whether penumbral grains (PGs) that escape the penumbra evolve into G-band bright points or granules.
• To clarify the origin and formation mechanism of G-band bright points near the penumbral-granulation boundary.
• To assess the role of magnetic field structure and plasma motions in feature formation and evolution.

Proposed method

• Simultaneous high-cadence observations in G-band and blue continuum (450.7 nm) were acquired over 2 hours at the Swedish Vacuum Solar Telescope (La Palma).
• A total of 2,564 images (46″ × 75″) were processed using the phase diversity inversion technique to correct for atmospheric and instrumental aberrations.
• Image restoration achieved high temporal and spatial stability, enabling precise tracking of small-scale features over time.
• Features such as penumbral grains, dark fibrils, granules, and G-band bright points were manually and systematically tracked across the time sequence.
• Spatial and temporal coincidence analysis was performed to test whether escaping PGs correspond to newly formed G-band bright points.
• Magnetic field strength and flow patterns near the penumbral-granulation boundary were analyzed to infer formation mechanisms of G-band bright points.

Experimental results

Research questions

• RQ1 Do outward-moving penumbral grains that cross the outer penumbral boundary continue as granules or bright features?
• RQ2 Are G-band bright points near the penumbral boundary formed from escaping penumbral grains?
• RQ3 What is the role of magnetic field strength and horizontal plasma motions in the formation of G-band bright points?
• RQ4 How do convergent or expanding motions of granules and penumbral features influence the concentration of magnetic flux at the penumbral boundary?
• RQ5 What is the nature of the magnetic and dynamic structure at the penumbral-granulation interface?

Key findings

• One-third of outward-moving penumbral grains (PGs) located near the outer penumbral boundary escape into the surrounding photosphere and continue moving outward as small bright features or expand into granules.
• Escaping PGs exhibit velocities slightly higher than average speeds in the outer penumbra and the sunspot moat, indicating enhanced outflow dynamics.
• G-band bright points near the penumbral boundary do not originate from escaping PGs; they are spatially and temporally distinct features.
• G-band bright points predominantly form near dark penumbral fibrils or their tips, where the magnetic field is strong and nearly horizontal, suggesting a direct link to magnetic flux concentration.
• The formation of G-band bright points is associated with convergent horizontal motions caused by expanding granules and outward-moving penumbral features, which may compress and concentrate magnetic flux.
• No existing sunspot penumbral model in the literature can fully explain the observed dynamics and independent formation pathways of PGs and G-band bright points.