Richard S. Bogart

Evolving Submerged Meridional Circulation Cells within the Upper Convection Zone Revealed by Ring-Diagram Analysis

Using the local helioseismic technique of ring-diagram analysis applied to Michelson Doppler Imager (MDI) Dynamics Program data from the Solar and Heliospheric Observatory, we have discovered that the meridional flow within the upper convection zone can develop additional circulation cells whose boundaries wander in latitude and depth as the solar cycle progresses. We report on the large-scale meridional and zonal flows that we observe from 1996 to 2001. In particular, we discuss the appearance and evolution of a submerged meridional cell during the years 1998-2001, which arose in the northern hemisphere and disrupted the orderly poleward flow and symmetry about the equator that is typically observed. The meridional flows in the southern and northern hemispheres exhibit striking asymmetry during the past four years of the advancing solar cycle. Such asymmetry and additional circulation cells should have profound impact on the transport of angular momentum and magnetic field within the surface layers. These flows may have a significant role in the establishment and maintenance of the near-surface rotational shear layer.

Ring-Diagram Analysis of the Structure of Solar Active Regions

We measure differences in structure between active and quiet regions of the Sun using the frequencies of high-degree modes determined from ring-diagram analyses. We find that both the speed of sound and the adiabatic index Γ1 differ in active regions as compared with quiet regions. In the immediate subsurface layers, the sound speed is lower in active regions, but below a depth of about 7 Mm the opposite is true. A comparison of sound-speed inversion results with those for Γ1 indicates that at least a part of the differences between active and quiet regions is likely to be due to the structural and thermal perturbations caused by magnetic fields in the active region.

Confirmation of a 152 day periodicity in the occurrence of solar flares inferred from microwave data

Evidence for a periodicity about 155 +- 5 days in the production of energetic solar flares was reported in 1984 by Rieger et al. and Kiplinger et al. The data on which these analyses were based are restricted to the years 1980 through early 1984. To see whether this periodicity is a persistent phenomenon, we have examined the occurrences of flares inferred from microwave data, which are available for most of the present and previous solar cycles. We find strong confirmation of a 152 day periodicity in the time interval previously studied, demonstrating that these flares are a useful indicator for the observed periodicity. We find evidence for persistence of the periodicity in the previous cycle (cycle 20). In cycle 20 the periodic modulation of the flare occurrence rate was weaker than in cycle 21, but the phase has apparently remained coherent through both cycles.

Large-scale motions on the sun - An overview

The history and present status of observations of large-scale velocity fields in the solar atmosphere are reviewed. Observations of the torsional oscillation and of mean meridional circulation suggest a connection of large-scale dynamics with the solar cycle. Significant problems must be solved before Doppler observations can match the precision of tracer measurements, particularly allowing for the effects of changes in line-profile asymmetries and for scattered light. Coordinated observations would establish the reliability of Doppler techniques, but Doppler measurements with precision of order 1 m s−1 made in a proper spatial-temporal window appear necessary for the identification of sub-global velocity fields varying with time-scales less than that of the solar cycle. This survey is presented in the context of the Solar Cycle Workshop held at Big Bear Lake, Calif, August 17–20, 1986.

The Virtual Solar Observatory: status and initial operational experience

The Virtual Solar Observatory (VSO) is a bottom-up grassroots approach to the development of a distributed data system for use by the solar physics community. The beta testing version of the VSO was released in December 2003. Since then it has been tested by approximately 50 solar physicists. In this paper we will present the status of the project, a summary of the communitys experience with the tool, and an overview of the lessons learned.

A system for line profile studies at the 150-foot tower on Mount Wilson

We describe enhancements to the hardware and software for the 150-foot tower system on Mt. Wilson which make possible the acquisition of high precision line profile measurements. This system utilizes the 75-foot pit spectrograph with a photomultiplier detector system to scan line profiles repeatedly in order to study variations induced by the passage of waves vertically through the solar atmosphere. Oscillations of line profile parameters with an amplitude as low as 1.7 m s−1 have been detected with this system using integrated sunlight. Phase relations between oscillations of different parts of the line profile are appropriate to upward energy transport. Consistent with the previous conclusion by Mein and Schmieder (1981), we find that the magnitude of the energy transport is compatible with the 5-min oscillations making an important contribution to the heating of the low chromosphere.

EVOLUTION OF NEAR-SURFACE FLOWS INFERRED FROM HIGH-RESOLUTION RING-DIAGRAM ANALYSIS

Ring-diagram analysis of acoustic waves observed at the photosphere can provide a relatively robust determination of the sub-surface flows at a particular time under a particular region. The depth of penetration of the waves is related to the size of the region, hence the depth extent of the measured flows is inversely proportional to the spatial resolution. Most ring-diagram analysis has focused on regions of extent ~15{\deg} (180 Mm) or more in order to provide reasonable mode sets for inversions. HMI data analysis also provides a set of ring fit parameters on a scale three times smaller. These provide flow estimates for the outer 1% (7 Mm) of the Sun only, with very limited depth resolution, but with spatial resolution adequate to map structures potentially associated with the belts and regions of magnetic activity. There are a number of systematic effects affecting the determination of flows from local helioseismic analysis of regions over different parts of the observable disk, not all well understood. In this study we characterize those systematic effects with higher spatial resolution, so that they may more effectively be accounted for in mapping temporal and spatial evolution of the flows. Leaving open the question of the mean structure of the global meridional circulation and the differential rotation, we describe the near-surface flow anomalies in time and latitude corresponding to the torsional oscillation pattern in differential rotation and analogous patterns in the meridional cell structure as observed by SDO/HMI.

Evidence for Solar Frequency Dependence on Sunspot Type

High-degree solar mode frequencies as measured by ring diagrams are known to change in the presence of the strong magnetic fields found in active regions. We examine these changes in frequency for a large sample of active regions analyzed with data from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory spacecraft, spanning most of solar cycle 23. We confirm that the frequencies increase with increasing magnetic field strength, and that this dependence is generally linear. We find that the dependence is slightly but significantly different for active regions with different sunspot types.

The thermal structure of sunspots from ring diagram analysis

We present a large sample of 264 active regions from solar cycle 23, analysed using ring diagrams. The frequencies of these rings are inverted to determine the thermal structure (sound speed and adiabatic index) of these regions as a function of radius. The large sample allows us to describe in a statistically significant way how the thermal properties of the outer layers of the Sun change in the presence of magnetic fields.

Far-side helioseismic maps: the next generation

For more than a decade, far-side seismic maps of medium-to-large active regions have proven their capability as a space weather forecasting tool. In the last few years, these maps have started to serve another purpose: complementing the front side observations that are input to different solar models. Photospheric flux transport as well as solar spectral irradiance models have been shown to produce improved results when incorporating the far-side seismic maps as well as providing better forecasting. The challenge for the future is twofold: Far-side seismic monitoring needs to be more sensitive, and it needs to offer more information. We present here initial steps towards fulfilling these goals using higher resolution input images, adding extra skips to the analysis and changing the presentation of the maps.