Douglas Braun

HELIOSEISMIC MEASUREMENTS OF THE SUBSURFACE MERIDIONAL FLOW

We measure the mean frequencies of acoustic (p-mode) waves propagating toward and away from the poles of the Sun from observations made with the Solar Oscillations Investigation‐Michelson Doppler Imager on board the Solar and Heliospheric Observatoryand the ground-based Global Oscillations Network Group. We demonstrate that there is a significant frequency shift between poleward- and equatorward-traveling waves measured over solar latitudes 207‐607, which is consistent with the Doppler effect of a poleward meridional flow on the order of 10 m s 21 . From the variation of the frequency shifts of p-modes with degree between 72 and 882 as a function of the lower turning point depth, we infer the speed of the meridional flow, averaged over these latitudes, over a range in depth extending over the top half of the solar convection zone. We find no evidence for a significant equatorward return flow within this depth range.

Local acoustic diagnostics of the solar interior

The observed absorption of p-modes by sunspots and solar magnetic fields has raised the possibility of developing holographic techniques to probe local magnetic features within the solar interior. Two simple diagnostic utilities are developed and tested to search for evidence of subsurface p-mode absorption. These consist of maps of acoustic power and maps of what we term the surface acoustic flux vector. Maps of acoustic power for a 50 hr sequence of solar Ca II K-line images show power deficits at 3 mHz corresponding to surface magnetic flux and power enhancements surounding active regions (halos) at 6 mHz

Helioseismic imaging of sunspots at their antipodes

Recent work by Braun, Duvall, and LaBonte has shown that sunspots absorb helioseismic waves. We propose that sunspot absorption causes a seismic deficit that should be imaged at the antipode of the sunspot. If these images are observable, it should be possible to produce seismic maps of magnetic regions on the far side of the Sun. This possibility opens a broad range of synoptic and diagnostic applications. Diagnostic applications would include lifetimes of higher-frequency modes, and possibly rotation of the solar interior and detection of subsurface magnetic structure. We outline elements of the theory of seismic imaging and consider some applications. We propose the extention of acoustic holography to solar interior diagnostics in the context of antipodal imaging.

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.

Seismic Imaging of the Far Hemisphere of the Sun

We apply phase-sensitive helioseismic holography to Solar and Heliospheric Observatory/Michelson Dopper Imager data to demonstrate how acoustic travel-time perturbations may be mapped over the entire portion of the Sun facing away from the Earth, including the polar regions. In addition to offering significant improvements to ongoing space weather forecasting efforts, the procedure offers the possibility of local seismic monitoring of both the temporal and spatial variations in the acoustic properties of the Sun over all of the far surface.

Principles of Seismic Holography for Diagnostics of the Shallow Subphotosphere

We develop the wave-mechanical formalism for phase-correlation computational seismic holography of the shallow subphotosphere under the plane-parallel approximation and apply it to helioseismic Doppler observations from the Michelson Doppler Imager on the SOHO spacecraft of both the quiet Sun and active regions. We compare holographic signatures computed wave-mechanically with similar signatures computed under the widely used eikonal approximation. The major difference between the hydromechanical and eikonal computations can be expressed in terms of acoustic dispersion effects within a few Mm of the solar surface. With an appropriate account for dispersion, the eikonal computations are remarkably accurate over a broad range of practical applications. A major imposition that confronts local diagnostics of the shallow subphotosphere is a phenomenon we call ‘‘ghost signatures,’’ artifacts introduced by a local ambiguity in the origin of the waves that give rise to the helioseismic signatures observed in the photosphere. Phase-correlation holographic signatures of the shallow subphotospheres of active regions are predominated by strong, stochastic phase shifts associated with magnetic fields at the solar surface. These introduce effects similar to those of an optical showerglass, significantly impairing the coherence of waves impinging into the magnetic photosphere from beneath, smearing the holographic signatures of possible subphotospheric anomalies. Subject heading

HELIOSEISMOLOGY OF PRE-EMERGING ACTIVE REGIONS. II. AVERAGE EMERGENCE PROPERTIES

We report on average subsurface properties of pre-emerging active regions as compared to areas where no active region emergence was detected. Helioseismic holography is applied to samples of the two populations (preemergence and without emergence), each sample having over 100 members, which were selected to minimize systematic bias, as described in Leka et al. We find that there are statistically significant signatures (i.e., difference in the means of more than a few standard errors) in the average subsurface flows and the apparent wave speed that precede the formation of an active region. The measurements here rule out spatially extended flows of more than about 15 m s −1 in the top 20 Mm below the photosphere over the course of the day preceding the start of visible emergence. These measurements place strong constraints on models of active region formation.

Stochastic Seismic Emission From Acoustic Glories and the Quiet Sun

Helioseismic images of multipolar active regions show enhanced seismic emission in 5mHz oscillations in a halo surrounding the active region called the ‘acoustic glory’. The acoustic glories contain elements that sustain an average seismic emission 50% greater than similar elements in the quiet Sun. The most intense seismic emitters tend to form strings in non-magnetic regions, sometimes marking the borders of weak magnetic regions and the separation between weak magnetic regions of opposite polarity. This study compares the temporal character of seismic emission from acoustic glories with that from the quiet Sun. The power distribution of quiet-Sun seismic emission far from solar activity is exponential, as for random Gaussian noise, and therefore not perceivably episodic. The distribution of seismic power emanating from the most intense elements that comprise the acoustic glories is exponential out to approximately 4 times the average power emitted by the quiet Sun. Above this threshold the latter distribution shows significant saturation, suggesting the operation of a hydromechanical non-linearity that sets limits on the acoustic power generated by the convection zone. This could give us considerable insight into the physical mechanism of seismic emission from the near subphotosphere.

Helioseismology of a Realistic Magnetoconvective Sunspot Simulation

We compare helioseismic travel-time shifts measured from a realistic magnetoconvective sunspot simulation using both helioseismic holography and time-distance helioseismology, and measured from real sunspots observed with the Helioseismic and Magnetic Imager instrument on board the Solar Dynamics Observatory and the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory. We find remarkable similarities in the travel-time shifts measured between the methodologies applied and between the simulated and real sunspots. Forward modeling of the travel-time shifts using either Born or ray approximation kernels and the sound-speed perturbations present in the simulation indicates major disagreements with the measured travel-time shifts. These findings do not substantially change with the application of a correction for the reduction of wave amplitudes in the simulated and real sunspots. Overall, our findings demonstrate the need for new methods for inferring the subsurface structure of sunspots through helioseismic inversions.

Physical Properties of Wave Motion in Inclined Magnetic Fields within Sunspot Penumbrae

At the surface of the Sun, acoustic waves appear to be affected by the presence of strong magnetic fields in active regions. We explore the possibility that the inclined magnetic field in sunspot penumbrae may convert primarily vertically-propagating acoustic waves into elliptical motion. We use helioseismic holography to measure the modulus and phase of the correlation between incoming acoustic waves and the local surface motion within two sunspots. These correlations are modeled by assuming the surface motion to be elliptical, and we explore the properties of the elliptical motion on the magnetic-field inclination. We also demonstrate that the phase shift of the outward-propagating waves is opposite to the phase shift of the inward-propagating waves in stronger, more vertical fields, but similar to the inward phase shifts in weaker, more-inclined fields.