Charles A. Lindsey

The Local Helioseismology of Inclined Magnetic Fields and the Showerglass Effect

We present evidence for the dependence of helioseismic Doppler signatures in active regions on the line-of-sight angle in inclined magnetic fields. Using data from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory, we performed phase-sensitive holography in the penumbrae of sunspots over the course of several days as the spots traversed the solar disk. Control correlations, which comprise a correlation of the surface wave amplitude with the incoming acoustic wave amplitude from a surrounding region, were mapped. There is a direct dependence of control-correlation phase signatures on the line-of-sight angle in the plane defined by the vertical and magnetic field vectors. The phase shift of waves observed along directions close to the orientation of the magnetic field is smaller than the phase shift observed when the line of sight is at a significant angle with respect to the field orientation. These findings have important implications for local helioseismology. The variation in phase shift (or the equivalent acoustic travel-time perturbations) with line-of-sight direction suggests that a substantial portion of the phase shift occurs in the photospheric magnetic field. Observations of the vector components of the field may be used to develop a proxy to correct these phase perturbations (known as the acoustic showerglass) that introduce uncertainties in the signatures of acoustic perturbations below the surface.

Calibration of seismic signatures of active regions on the far side of the sun

Synoptic maps of the far hemisphere of the Sun calculated from seismic holography have proven to be very reliable in localizing large active regions before they rotate onto the visible hemisphere. We show here the first results toward a calibration of the far-side signatures of active regions in terms of active region size and magnetic field strength. We compare helioseismic maps of large active regions on the far side of the Sun, calculated from Global Oscillation Network Group (GONG) Doppler observations, with magnetic and visible-continuum images of the same active regions on the visible hemisphere before and after their far-side passage. The far-side seismic signature is expressed as a phase shift that a far-side active region introduces to waves from the near hemisphere as they are reflected into the solar interior on their way back to the near hemisphere. There is a significant correlation between this far-side signature and both the total area of the active region, as viewed on the near hemisphere, and the area of the sunspots contained in the active region. We have studied the relationship between the magnetic field strength and the phase signature for six of the larger, more stable active regions. We find an approximately logarithmic increase in the seismic phase signature with increasing magnetic field strengths above a critical field of ~10 G. This is roughly consistent with similar helioseismic signatures measured on the near solar hemisphere concurrent with associated magnetic fields.

ACTIVE REGION MORPHOLOGIES SELECTED FROM NEAR-SIDE HELIOSEISMIC DATA

We estimate the morphology of near-side active regions using near-side helioseismology. Active regions from two data sets, Air Force Data Assimilative Photospheric flux Transport synchronic maps and Global Oscillation Network Group near-side helioseismic maps, were matched and their morphologies compared. Our algorithm recognizes 382 helioseismic active regions between 2002 April 25 and 2005 December 31 and matches them to their corresponding magnetic active regions with 100% success. A magnetic active region occupies 30% of the area of its helioseismic signature. Recovered helioseismic tilt angles are in good agreement with magnetic tilt angles. Approximately 20% of helioseismic active regions can be decomposed into leading and trailing polarity. Leading polarity components show no discernible scaling relationship, but trailing magnetic polarity components occupy approximately 25% of the area of the trailing helioseismic component. A nearside phase-magnetic calibration is in close agreement with a previous far-side helioseismic calibration and provides confidence that these morphological relationships can be used with far-side helioseismic data. Including far-side active region morphology in synchronic maps will have implications for coronal magnetic topology predictions and solar wind forecasts.

Chromospheric Line Emission Analysis of the July 16, 2004 Sun Quake

Observations in chromospheric lines and the visible continuum together with photospheric helioseismic measurements make possible to image a 3‐dimensional profile of a sun quake in a flaring region. Chromospheric line observations show us the part of the solar atmosphere where high‐energy electrons are thought to cause thick target heating that then could also cause intense white‐light emission and could drive seismic waves into the active region subphotosphere, we present here the preliminary results of the sun quake of July 16, 2004.