February 27, 2019 – Most people have heard about tsunamis in the Earth’s oceans. Those occur when large masses of water get violently moved at the ocean floor, as a result of earthquakes or volcanic eruptions. The Indian Ocean Tsunami of 2004, often called the “Boxing Day Tsunami”, is just such an event – it caused catastrophic destruction in many coastal regions around the Indian ocean.
But tsunamis are not limited to the Earth’s oceans — they can occur inside the Sun too, as new research highlights. An international team of scientists led by Dr. Mausumi Dikpati of the NCAR’s High Altitude Observatory have discovered that tsunami can occur in the Sun’s interior. Specifically, in the shear-fluid layer known as the “tachocline,” where a “magnetic dam” forms at the Sun’s equator.
That dam is constructed from the large magnetic bands that give rise to sunspots and migrate towards the suns migrator over nearly 20 years. As the bands cancel one another across the Sun’s equator, the dam eventually breaks, rapidly releasing trapped fluid – like the oceanic equivalent. The tsunami then propagates poleward at the speed of about 1000 kilometers per hour, carrying the surplus fluid to mid-latitudes. As it roars through the Sun’s interior it changes the pressure of the environment. This change makes the magnetic field bands of the next solar cycle, that have been slowly migrating to mid-latitudes over the past decade, unstable. The unstable pressure rapidly starts the emergence of magnetism from those bands through the Sun’s surface – in other words, it triggers the sunspots of the next cycle.
The next solar tsunami is anticipated in 2020, an event that will kick-off sunspot cycle 25. The tsunami will cause a rapid growth in space weather events that will appear as massive eruptions of matter, magnetism and energy that blast out into space, sometimes directed toward Earth. These space weather events are extremely hazardous to our technology-based society, so understanding the start of the Sun’s next decade-long stormy season can help prevent damage to critical pieces of infrastructure on the ground, and in space.
In a paper recently published in the Nature Journal’s Scientific Reports, a team of scientists, including Mausumi Dikpati (HAO/NCAR, Boulder), Scott McIntosh (HAO/NCAR, Boulder), Subhamoy Chatterjee (IIA, Bangalore), Dipankar Banerjee (IIA, Bangalore), Ron Yellin-Bergovoy (Tel-Aviv University, Israel) and Abhishek Srivastava (IIT-BHU, Varanasi) demonstrated with a novel computer model of the Sun’s interior shear-fluid, employing a nonlinear magnetohydrodynamic shallow-water formulation and India’s Kodaikanal Observatory data, that the sudden cessation of emergence of old magnetic flux near the equator can be caused by mutual annihilation near the equator of sunspot-producing magnetic fields in North and South hemispheres deep inside the Sun, which sets off a ‘solar tsunami’ that propagates poleward to perturb the previously unseen new cycle magnetic fields there. In turn, this disruption leads to new magnetic flux coming to the surface at these higher latitudes in the form of new cycle sunspots.
Oceanic tsunamis are generally modeled using a hydrodynamic shallow-water framework; the solar tsunami is essentially a magnetohydrodynamic tsunami. The speed of this tsunami, a few hundred meters per second, is what allows the equatorial event to communicate so fast (within a few weeks) with the higher latitudes, a distance of several hundred thousand kilometers.
This research was performed using 10 million core hours of Cheyenne supercomputer time at NCAR, and is supported by NCAR and NSF (National Science Foundation) and an Indo-US grant.
The discovery of these events in the Sun’s interior opens a new frontier into the workings of the Sun and the processes that make its magnetic field. Further, the team’s ability to observe and model them can significantly enhance the ability to forecast solar weather events.