A Midsummer Solar Physics Update

From spacecraft that dive into the Sun’s atmosphere to insights from complex models, we’re learning more about the Sun than ever before. Today’s highlight introduces five recent research articles that tackle hot topics in solar physics.

Tracking Down Dark Regions in the Corona

The first two articles touch on phenomena seen by the Parker Solar Probe, a NASA spacecraft launched in 2018 that aims to study the Sun’s hot and tenuous upper atmosphere, or corona, up close. In the first article, Guillermo Stenborg (Johns Hopkins University) and coauthors examined white-light images of the solar corona taken by Parker Solar Probe. These images show light that has been scattered off of electrons and dust within the corona. Many of the images show areas where the corona is very faint in white light, which could indicate a depletion of electrons, dust, or both.

To investigate further, Stenborg and coauthors compared the locations of the darkened regions to the locations of coronal holes — places in the corona where the solar magnetic field extends out into the solar system rather than looping back to the solar surface, allowing particles to stream out from the Sun. The team found that many of the darkened regions can be explained as the spacecraft’s line of sight passing through the “zone of influence” of a coronal hole. Other darkened regions had a different cause: as coronal mass ejections exit the corona, they leave a plasma-depleted region in their wake. The largest coronal mass ejection witnessed by the spacecraft excavated both plasma and dust.

Seeking the Source of Switchbacks

When the Parker Solar Probe made its first journeys toward the Sun, it discovered rapid back-and-forth changes in the direction of the solar wind magnetic field that researchers dubbed switchbacks. There are many theories as to the origins of solar switchbacks, including magnetic field lines rearranging into a new configuration (i.e., magnetic reconnection), turbulence in the solar wind, or a combination of both.

To delve into the origins of switchbacks, Pankaj Kumar (American University and NASA Goddard Space Flight Center) and collaborators investigated a closely associated phenomenon called microstreams. Microstreams are changes in the speed and direction of the solar wind that last about 10 hours and seem to be linked to switchbacks. Using data from the Parker Solar Probe as it surfed the solar wind, and combining it with photographs taken from afar by the Solar Dynamics Observatory, Kumar’s team found that microstreams in the solar wind and jets produced deeper in the solar atmosphere vary over time in the same way. The jets are also associated with close-set opposing magnetic field lines and bursts of protons and ions — clear signs that magnetic reconnection is underway. This suggests that magnetic reconnection creates the jets, and the jets in turn create switchbacks and microstreams.

Improving Forecasts with Machine Learning

Correctly forecasting the arrival times of coronal mass ejections — immense explosions of tangled solar plasma and magnetic fields — at Earth is important for testing our understanding of how these eruptions travel through space as well as for developing an early warning system. Even as simulations of coronal mass ejections unfurling across space have grown more complex, however, the typical error in the arrival times they predict has remained the same, around 12 hours.

A team led by Talwinder Singh (The University of Alabama in Huntsville) developed a new method to predict when a coronal mass ejection will arrive at Earth. Singh and collaborators used observations to shape their modeling of coronal mass ejections when they are poised to erupt, as well as their modeling of the background solar wind. This new model cut the arrival-time error by a third, which the authors attributed to their model’s ability to capture realistic interactions between the coronal mass ejection and the solar wind. Using machine-learning methods improved the model further, yielding a typical arrival-time error as low as 4 hours.

Solving the Cool Chromosphere Problem

Samuel Evans (Boston University) and collaborators focused on the chromosphere, the region between the sunspot-dotted solar surface and the tenuous, superheated corona. The chromosphere represents a unique challenge for modelers because one of our best tools — fluid dynamics models — can’t quite capture what’s going on there; the corona, where fluid dynamics models excel, is so rarefied that different components of the plasma can be treated individually, as they rarely interact. The chromosphere, however, is dense enough that the interactions between different components of the plasma can no longer be ignored. So far, fluid dynamics models have struggled to reproduce the temperature in the solar chromosphere — likely somewhere in the 3000–4000K range, based on observations — predicting a relatively chilly 2000K.

Evans and collaborators suggested that the mismatch between theory and observations is due to a missing heating process in the chromosphere. Specifically, the team proposed that a plasma instability is at work, creating fleeting meter-sized waves too small to be captured by existing fluid models — but perhaps impactful enough to warm our chromospheric models to the right temperature. The team used simulations to understand how this instability would affect chromospheric plasma composed of multiple interacting components. The instability appears to ramp up fastest in regions where the chromospheric temperature is low, heating those regions and potentially solving the cool chromosphere problem.

Gaining X-ray Insights into Charged-Particle Acceleration

Solar flares are likely the most well-known solar phenomenon, but there’s still plenty we don’t know about them. We know that solar flares accelerate charged particles to high velocities by releasing pent-up magnetic energy, but the details of this process are fuzzy.

Morgan Stores (Northumbria University) and collaborators explored this issue by modeling the acceleration of charged particles as they encounter a region of turbulent plasma in the Sun’s outer atmosphere. The team’s goal was to determine how factors like the size of the turbulent region, the distribution of the turbulent plasma, and the timescale of particle acceleration affect observable properties like X-ray brightness. Based on the results of this modeling, Stores and coauthors found that images and X-ray spectra of the Sun can be analyzed together to determine not just where electrons are being accelerated, but also when and how quickly. Next, the team plans to use their simulations to analyze solar flare observations made by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar Orbiter.

Citation

“Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images,” Guillermo Stenborg et al 2023 ApJ 949 61. doi:10.3847/1538-4357/acd2cf

“New Evidence on the Origin of Solar Wind Microstreams/Switchbacks,” Pankaj Kumar et al 2023 ApJL 951 L15. doi:10.3847/2041-8213/acd54e

“Improving the Arrival Time Estimates of Coronal Mass Ejections by Using Magnetohydrodynamic Ensemble Modeling, Heliospheric Imager Data, and Machine Learning,” Talwinder Singh et al 2023 ApJ 948 78. doi:10.3847/1538-4357/acc10a

“Multifluid Simulation of Solar Chromospheric Turbulence and Heating Due to Thermal Farley–Buneman Instability,” Samuel Evans et al 2023 ApJ 949 59. doi:10.3847/1538-4357/acc5e5

“Spectral and Imaging Diagnostics of Spatially-Extended Turbulent Electron Acceleration and Transport in Solar Flares,” Morgan Stores et al 2023 ApJ 946 53. doi:10.3847/1538-4357/acb7dc