How Coronal Rain Functions

When observed through the high-resolution telescopes aboard NASA's Solar Dynamics Observatory (SDO), the Sun appears as a chaotic sphere of plasma, interwoven with magnetic field lines and fiery loops. While it may seem vastly different from Earth, we can draw parallels, especially with the concept of rain.

On our planet, rain is a component of the water cycle—a constant interaction between heat and gravity. It initiates when the Sun heats liquid water in oceans, lakes, or rivers, causing evaporation. The vapor rises, cools, and condenses into clouds, which eventually release the water back to the Earth in the form of rain.

Mason explains that coronal rain operates similarly on the Sun, "but instead of 60-degree water, you're dealing with a million-degree plasma." This electrically charged gas, instead of pooling, moves along the magnetic loops that extend from the Sun's surface. At the base of these loops, where they connect to the Sun, plasma is heated to temperatures exceeding 1.8 million degrees Fahrenheit. As it rises along the loop, it cools and condenses, ultimately falling back down as coronal rain.

Mason initially focused on helmet streamers due to their known association with the slow solar wind, a denser gas stream that escapes the Sun. Previous measurements indicated that this slow solar wind was once subjected to extreme heating before cooling and departing the Sun. The cyclical heating and cooling process behind coronal rain could potentially clarify this phenomenon.

Additionally, the coronal heating problem remains unresolved—specifically, why the Sun's outer atmosphere is about 300 times hotter than its surface. Simulations indicate that coronal rain forms only when heat is applied to the base of the loop. “If a loop has coronal rain on it, that means that the bottom 10% of it, or less, is where coronal heating is happening,” Mason noted.

After nearly six months of searching, Mason had not observed any coronal rain in helmet streamers but noticed a series of smaller, bright magnetic structures that were unfamiliar to her.

“When I finally took a look at them, sure enough, they had tens of hours of rain at a time,” Mason stated.

A Shift in Focus

The smaller structures she discovered differed significantly from helmet streamers, particularly in size. According to Spiro Antiochos, another solar physicist at Goddard, these loops were much smaller than those initially targeted, indicating that coronal heating is more localized than previously assumed.

Mason's findings revealed loops approximately 30,000 miles in height—only about 2% of the size of the helmet streamers she had originally been studying. This indicates that the regions where coronal rain forms are critical to understanding coronal heating.

Interestingly, some observations contradicted established theories. Traditionally, coronal rain is thought to develop only on closed loops, where plasma can accumulate and cool. However, Mason found instances of rain forming on open magnetic field lines, which are anchored at one end to the Sun but extend out into space.

To explain this anomaly, Mason's team proposed a new theory linking the rain observed on these smaller magnetic structures to the origins of the slow solar wind. According to their hypothesis, the plasma begins its journey on a closed loop and, through magnetic reconnection, shifts to an open one. This reconfiguration occurs often on the Sun and is frequently associated with solar storms and sunspots.

When a closed loop intersects with an open field line, the system can rewire itself, causing the superheated plasma to transition to the open line. Some of this plasma may cool and return to the Sun as coronal rain, while other portions escape into the solar wind.

Mason is currently developing a computer simulation to validate this new explanation. Additionally, with the Parker Solar Probe, which launched in 2018, now venturing closer to the Sun than any previous spacecraft, researchers anticipate gathering observational evidence that could confirm these theories.

NASA | Fiery Looping Rain on the Sun - YouTube

This video showcases the stunning phenomena of coronal rain on the Sun, highlighting the intense dynamics within solar magnetic fields.

The future is promising for Mason and her team as they continue the quest for coronal rain in helmet streamers.

As Antiochos suggests, “Maybe it's so small you can't see it? We really don't know.”

Despite the initial challenges, Mason views her research as rewarding. “It sounds like a slog, but honestly it's my favorite thing. I mean that's why we built something that takes that many images of the Sun: So we can look at them and figure it out.”