Predicting the Sun’s Coronal Magnetic Field: Insights from the 2019 Total Solar Eclipse

By [Soumyaranjan Dash], [01/05/2020]

The Sun’s corona—the outermost layer of its atmosphere—presents a complex and dynamic environment shaped by magnetic fields. Understanding the structure and behavior of these coronal magnetic fields is essential for comprehending solar phenomena such as coronal heating and solar eruptions. However, direct measurements of these fields are challenging due to the corona’s high temperatures and low densities. In this context, computational models become invaluable tools for predicting and interpreting coronal structures.

A significant contribution to this field is the study titled “Prediction of the Sun’s Coronal Magnetic Field and Forward-modeled Polarization Characteristics for the 2019 July 2 Total Solar Eclipse” by Soumyaranjan Dash, Prantika Bhowmik, Athira B S, Nirmalya Ghosh, and Dibyendu Nandy, published in The Astrophysical Journal (DOI: 10.3847/1538-4357/ab6a91). This research focuses on predicting the coronal magnetic field structure during the total solar eclipse on July 2, 2019, and modeling the polarization characteristics of the coronal emission.

The Significance of Total Solar Eclipses

Total solar eclipses offer unique opportunities to observe the Sun’s corona, which is otherwise obscured by the bright solar disk. During an eclipse, the Moon blocks the Sun’s photosphere, allowing the faint corona to become visible. Observations during such events provide critical data for validating models of the coronal magnetic field and understanding solar-terrestrial interactions.

Methodology: Combining Surface Flux Transport and Potential Field Models

The authors employed a two-step modeling approach to predict the coronal magnetic field structure:

1. Surface Flux Transport Model: This model simulates the evolution of the Sun’s surface magnetic fields over time. By incorporating data on the emergence, movement, and cancellation of magnetic flux, the model provides a time-dependent map of the Sun’s surface magnetic field distribution.

2. Potential Field Source Surface (PFSS) Model: Using the surface magnetic field map as input, the PFSS model extrapolates the magnetic field into the corona, assuming it to be current-free (potential field). This extrapolation predicts the large-scale structure of the coronal magnetic field, including features like streamers and coronal holes.

By running the surface flux transport model forward for 51 days leading up to the eclipse, the researchers generated a predicted surface magnetic field map for July 2, 2019. This map served as the boundary condition for the PFSS model, resulting in a three-dimensional representation of the coronal magnetic field at the time of the eclipse.

Predicting Coronal Structures: Streamers and Pseudo-streamers

The model predicted two prominent streamer structures located on the east and west limbs of the Sun. Streamers are bright, dense regions in the corona associated with closed magnetic field lines and are often observed during solar eclipses. The predicted locations and shapes of these streamers were found to be in reasonable agreement with observations made during the eclipse, validating the modeling approach.

Additionally, the study discussed the potential development of a pseudo-streamer—a coronal structure resembling a streamer but with a different magnetic topology. By analyzing the field line connectivity, the researchers identified regions where pseudo-streamers might form, providing insights into the complex magnetic configurations present in the corona.

Forward Modeling of Polarization Characteristics

Understanding the polarization of light emitted by the corona offers insights into the coronal magnetic field and electron density distributions. The authors forward-modeled the polarization characteristics of the coronal emission based on their predicted magnetic field structures. These models can be compared with polarization measurements obtained during the eclipse to further validate the predictions and refine our understanding of coronal magnetism.

Implications for Coronal Magnetometry and Future Observations

This study has significant implications for coronal magnetometry—the measurement of magnetic fields in the corona. Accurate predictions of coronal structures and their polarization characteristics are essential for interpreting observations from ground-based facilities like the Daniel K. Inouye Solar Telescope and the Coronal Multichannel Polarimeter, as well as upcoming space-based instruments such as the Solar Ultraviolet Imaging Telescope and the Variable Emission Line Coronagraph onboard India’s Aditya-L1 mission.

The research underscores the importance of continuous monitoring and modeling of the Sun’s surface magnetic fields. Notably, the study highlights that the Sun’s polar fields have a substantial influence on the modeled corona, suggesting that accurate polar field observations are critical for reliable predictions.

Conclusion

The study by Dash et al. demonstrates the efficacy of combining surface flux transport models with potential field extrapolations to predict the Sun’s coronal magnetic field structure. The reasonable agreement between their predictions and actual eclipse observations validates this approach and provides a framework for future predictive modeling efforts. As observational capabilities advance, such predictive models will play an increasingly vital role in planning observations, interpreting data, and enhancing our understanding of the Sun’s dynamic corona.

For a more detailed exploration of this study, refer to the full article in The Astrophysical Journal: DOI: 10.3847/1538-4357/ab6a91.