Why the solar corona is much hotter than the surface of the sun

In a new study published in The Journal of AstrophysicsA researcher at the University of Alabama in Huntsville (UAH), part of the University of Alabama system, is exploring critical aspects of a phenomenon called kinetic Alfvén waves (KAWs) to provide new insights into an age-old heliophysical mystery.

Syed Ayaz, a graduate research assistant at UAH’s Center for Space Plasma and Aeronomy Research (CSPAR), has examined the potentially central role of KAWs in heating the solar corona, bringing science closer to solving the puzzle of why the corona is many times hotter than the sun’s surface itself.

“For decades, Alfvén waves have proven to be the best candidates for transporting energy from one place to another,” Ayaz says, highlighting the potential role of KAWs in transmitting coronal heat.

“This paper uses a new approach to model energetic particles in space plasmas, as observed by satellites such as Viking and Freja, to answer how the electromagnetic energy of the waves, interacting with the particles, is transformed into heat during the damping process as the waves move through space.

“Our study explores the perturbed electromagnetic fields, Poynting flux vector and energy distribution rate of KAWs in the solar atmosphere.”

The corona, or solar atmosphere, is an enigmatic region surrounding our star that extends far beyond the visible disk of the Sun, about 8 million kilometers above the Sun’s surface. Yet the corona is also characterized by extraordinarily high temperatures, a mystery that has captivated astrophysicists for nearly seventy years.

“Syed is one of our most remarkable students and is just beginning his research career,” said Dr. Gary Zank, CSPAR director and chair of UAH’s Department of Space Sciences for Aerojet Rocketdyne. “His lifelong interest in Alfvén waves, which began while he was a student in Pakistan working with his mentor, Dr. Imran A. Kahn, has now led him to study these waves at very small scales, called kinetic scales in a plasma.”

“His work provides important insights into the crucial problem of transforming the energy of a magnetic field to heat a plasma composed of charged particles such as protons and electrons. One reason Syed’s work is important is that we still don’t understand why the sun’s atmosphere is over a million degrees, compared to the sun’s surface, which is a comparatively cool 6,500 degrees.”

Kinetic Alfvén waves, abundant throughout the plasma universe, are oscillations of ions and the magnetic field as they move through the solar plasma. These waves are formed by the movements of the photosphere, the outer layer of the sun that emits visible light.

“My interest in these waves was sparked by the launches of the Parker Solar Probe and Solar Orbiter missions, which raised the crucial question of how the solar corona is heated,” Ayaz explains. “So far, no space mission has provided predictions of these phenomena near the Sun, especially in the range of 0 to 10 solar radii. Our main goal is to study heating by KAWs in these frequency ranges in the solar corona.”

“We focused on the heating and energy exchange facilitated by KAW waves,” the researcher notes. “The reason for the great interest in these waves lies in their ability to transport energy. Observational data from many spacecraft and theoretical research have consistently demonstrated that KAW waves dissipate and contribute to the heating of the solar corona as they propagate through space.”

Because of these unique properties, waves are an essential energy transfer mechanism, important for understanding the energy exchange between electromagnetic fields and plasma particles.

“KAWs operate at small kinetic scales and are able to support parallel fluctuations of electric and magnetic fields, enabling energy transfer between the wave field and plasma particles through a phenomenon called Landau interactions,” Ayaz explains.

“The present work uses and explores the Landau damping mechanism, which occurs when particles moving parallel to a wave have velocities comparable to the phase velocity of the wave.”

Landau damping is an exponential decay over time of certain waves in plasma. “When particles interact with the wave, they gain/lose energy, a term called the ‘resonance state,’” Ayaz explains.

“This can result in the wave either transmitting its energy to the particles or capturing energy from them, causing the particles to dampen or grow. Our research shows that KAWs dissipate rapidly, transferring all of their energy to the plasma particles as heat. This energy transfer accelerates the particles over longer spatial distances, which has a significant impact on plasma dynamics.”

The analytical results from this study will find practical application in understanding phenomena within the solar atmosphere, in particular by highlighting the important role played by non-thermal particles in heating processes.