Investigation of Solar Torsional Oscillations and Their Relation to Activity Cycles

Overview

All manifestations of solar activity, from spectral irradiance variations to solar storms and geomagnetic disturbances, are caused by magnetic fields generated through a dynamo mechanism operating in the convection zone of the Sun. Despite substantial modeling and simulation efforts, we still have a poor understanding of how the magnetic fields are generated, transported to the surface, and form the solar activity cycles.

The most prominent feature of the solar cycle is the sunspot “butterfly” diagram, which develops at the beginning of an 11-year sunspot cycle when magnetic sunspot regions emerge at about 30 degrees latitude and the sunspot formation zone migrates toward the equator. During this period of the sunspot maxima, the polarity of the global magnetic field in the Sun's polar regions is reversed. This cyclical pattern is known as a torsional oscillation, in which the solar rotation is periodically sped up or slowed down in certain latitude zones while the rotation remains essentially steady elsewhere. Torsional oscillations represent bands of fast and slow zonal flows beneath the visible surface of the Sun, and are similar to stream jets in the Earth's atmosphere.

Project Details

Our computational analysis of solar data obtained from two NASA missions in 1996 and 2022 for two sunspot cycles reveals zones of slowdown of the torsional oscillations inside the Sun. The flow deceleration is caused by magnetic fields generated by the solar dynamo. It originates about 120,000 miles beneath the solar surface in a high latitude region. The zonal flows migrate through the convection zone, revealing patterns of magnetic dynamo waves. The analysis explains the phenomenon of the “extended solar cycle” observed in the evolving shape of the solar corona, and why the polar magnetic field strength predicts the solar maxima.

In the absence of observational evidence of dynamo waves, alternative scenarios, called flux-transport models, were developed. These models suggest that the cyclic evolution of the magnetic field is controlled by meridional circulation. Similar to a conveyor belt, it transports magnetic flux to the bottom of the convection zone from high to low latitudes, where it emerges to form sunspots.

Results and Impact

Our new results provide strong evidence of the dynamo waves and reveal their migration pattern in the form of two branches migrating toward the poles and the equator. The polar branch reaches the surface in 1–2 years, while it takes about 8–9 years for the equatorial branch to reach the solar surface and form the sunspot butterfly diagram. Because the polar branch comes to the surface more quickly, it explains why the strength of the polar magnetic field predicts the subsequent sunspot maximum. Recent measurements show a significant decrease in the zonal deceleration within the tachocline, indicating that the current sunspot cycle may be weaker than the previous one. Thus, the long-term trend of a decline in the Sun's magnetic field is likely to continue—which may impact the state of Earth's atmosphere, space weather, and climate.

Why HPC Matters

The measurements are performed for 22 years by two NASA space missions: the Solar and Heliospheric Observatory (1996-2010) and Solar Dynamics Observatory (2010-present). The solar acoustic oscillations with a characteristic period of 5 minutes are observed every 45 seconds, and it takes 72 days of uninterrupted observations to obtain one measurement of the subsurface flows in the Sun. The total amount of data processed at the Joint Science Operations Center at Stanford University is about five terabytes. The flow analysis, numerical modeling and visualization were performed at the NASA Advanced Supercomputing (NAS) facility at NASA’s Ames Research Center.

What's Next

The next steps are to improve the spatial and temporal resolution of the data analysis techniques and numerical simulation models. It is particularly important to merge models of emerging magnetic flux and sunspot formation with the global models of solar activity. Such synergy will substantially advance our understanding of the dynamo process in the deep solar interior and develop innovative tools for space weather prediction.

More Information

• Advances and Challenges in Observations and Modeling of the Global-Sun Dynamics and Dynamo
• NASA Solar Dynamics Observatory

Researchers

• Alexander Kosovichev, New Jersey Institute of Technology
  
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