The Sun is a giant ball of plasma that emits heat and light through nuclear fusion.
However, the Sun's atmosphere, or corona, is much hotter than its surface, reaching temperatures of over a million degrees Celsius.
This is puzzling, because the heat source of the Sun is located at its core, and the temperature should decrease as the distance from the core increases.
This is known as the coronal heating problem, and it has been baffling solar physicists for decades.
A new study by researchers from the New Jersey Institute of Technology (NJIT) and their collaborators has shed some light on this mystery, by discovering a source of intense wave energy in the Sun's coldest region, the sunspot umbra.
The study, published in Nature Astronomy, suggested that these waves can travel across the solar atmosphere and heat up the corona.
How sunspots store wave energy
ANDREW CABALLERO-REYNOLDS/AFP via Getty Images
Sunspots are dark regions on the Sun's surface that appear when strong magnetic fields emerge from the Sun's interior and suppress the flow of heat, as per Phys.org.
Sunspots have a dark center called the umbra, where the magnetic field is strongest and the temperature is lowest, around 3,000 degrees Celsius.
Surrounding the umbra is a lighter region called the penumbra, where the magnetic field is weaker and the temperature is higher, around 5,000 degrees Celsius.
The researchers used the Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO), operated by NJIT's Center for Solar-Terrestrial Research (CSTR), to observe an active sunspot on July 14, 2015.
They detected numerous dark features in the umbra, called fibrils, which are cone-shaped structures of plasma that follow the magnetic field lines.
They also noticed that these fibrils were oscillating transversely, or sideways, with high speeds and amplitudes.
The researchers measured the wave energy carried by these fibrils and found that it was much higher than expected.
They estimated that each fibril could carry up to 10 megawatts of power, which is equivalent to the power output of a large wind turbine.
Furthermore, they calculated that the total wave energy stored in the umbra was enough to heat up a large volume of plasma to over a million degrees Celsius.
The researchers proposed that these waves can escape from the umbra and propagate through the solar atmosphere, transferring their energy to other plasma structures along the way, as per NJIT.
For example, they could interact with spicules, which are jet-like features that extend from the Sun's surface to the lower corona.
Spicules are known to carry plasma and magnetic fields upward, and they could also carry wave energy from the umbra to higher altitudes.
The researchers also suggested that these waves could reach the corona directly, by passing through gaps or windows in the overlying magnetic field.
These windows could be created by magnetic reconnection, which is a process that rearranges the magnetic field lines and releases energy.
Magnetic reconnection is often associated with solar flares and coronal mass ejections, which are explosive events that can affect space weather.
The researchers compared their results with data from other telescopes and satellites, such as NASA's Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS).
They found that their findings were consistent with a global pattern of wave energy generation and dissipation in sunspots and their surroundings.
The implications: a new piece of the puzzle
The study has important implications for our understanding of the Sun and its influence on Earth.
It reveals a new mechanism for coronal heating that involves waves generated in sunspots, which are common features on the Sun's surface.
It also shows that sunspots are not only sources of magnetic fields and plasma but also sources of wave energy that can affect the whole solar atmosphere.
The study also demonstrates the power of high-resolution observations from ground-based telescopes such as GST, which can capture details that are invisible to other instruments.
GST is one of the largest solar telescopes in the world, and it can resolve features as small as 50 kilometers on the Sun.
The researchers hope that their study will inspire more investigations into the role of waves in coronal heating and solar activity.
They also plan to continue observing sunspots with GST and other telescopes to further explore their dynamics and evolution.
