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In a new study published in The Astrophysical Journalresearcher at the University of Alabama at Huntsville (UAH), part of the University of Alabama System, is exploring critical aspects of a phenomenon called kinetic Alfvén waves (KAW) to provide fresh insights into a centuries-old heliophysical mystery.

Syed Ayaz, a graduate scientist at the UAH Center for Space Plasma and Aeronomic Research (CSPAR), explored the potentially key role of KAW in heating the solar corona, bringing science one step closer to solving the puzzle of why the corona is many times hotter from the surface of the sun itself.

“Alfvén waves have been shown for decades to be the best candidates for transporting energy from one place to another,” says Ayaz, noting the potential role of KAWs in driving coronal heat.

“This paper uses a new approach to modeling energetic particles in cosmic plasmas 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 decay process as the waves move through space.

“Our investigation examines the perturbed electromagnetic fields, the Poynting flux vector, and the KAW power delivery rate in the solar atmosphere.”

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

“Sayed is one of our outstanding students who is just beginning his research career,” said Dr. Gary Zank, director of CSPAR and Chair of Aerojet Rocketdyne at UAH’s Department of Space Sciences. “His continuing interest in Alfvén waves, which began while he was a student in Pakistan working with his mentor, Dr. Imran A. Khan, has now led to his investigation of these waves at very small scales, the so-called kinetic scale in plasma.

“His work offers important insight into the critical problem of how energy in a magnetic field is transformed to heat a plasma consisting of charged particles such as protons and electrons.” One of the reasons Syed’s work is important is that we still don’t understand why the sun’s atmosphere is more than 1 million degrees compared to the sun’s surface, which is a relatively cool 6,500 degrees.”

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

“My main interest in these waves was sparked by the launch of the Parker Solar Probe and Solar Orbiter missions, which raised the crucial question of how the solar corona is heated,” says Ayaz. “Until now, no spacecraft mission has provided predictions regarding these near-sun phenomena, specifically in the range 0-10 solar radii. Our main focus is to investigate the heating by KAW in these ranges in the solar corona.”

“We focused on the heat and energy exchange facilitated by the KAW,” notes the researcher. “The reason for the great interest in these waves lies in their ability to transport energy. Observational data from multiple spacecraft and theoretical studies consistently show that KAWs dissipate and contribute to solar coronal heating during their propagation through space.”

Because of these unique properties, waves provide a critical energy transfer mechanism important for understanding the energy exchange between electromagnetic fields and plasma particles.

“KAWs operate on small kinetic scales and are able to sustain parallel electric and magnetic field oscillations, allowing energy transfer between the wave field and plasma particles through a phenomenon called Landau interactions,” says Ayaz.

“The present work exploits and investigates 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 the exponential decay as a function of time of certain waves in a plasma. “When the particles interact with the wave, they gain/lose energy – a term called a ‘resonance state,'” says Ayaz.

“This can result in the wave either delivering its energy to the particles or receiving energy from them, causing the particles to moisten or grow. Our study finds that KAWs quickly dissipate, completely transferring their energy to the plasma particles in the form of heating. This energy transfer accelerates the particles over longer spatial distances, which significantly affects the dynamics of the plasma.”

The analytical insights gathered from this study will find practical application in understanding the phenomena in the solar atmosphere, especially by shedding light on the significant role that non-thermal particles play in the heating processes.