Why is the sun’s corona hotter than its surface? 

A recent study led by Syed Ayaz, a graduate research assistant at the University of Alabama in Huntsville (UAH), has investigated kinetic Alfvén waves and their potential role in heating the solar corona. 

Ayaz’s research, under the guidance of the Center for Space Plasma and Aeronomic Research (CSPAR), addresses the long-standing mystery of why the corona – the sun’s outer atmosphere – is significantly hotter than the sun’s surface.

The mechanism behind coronal heating

“For decades, Alfvén waves have been proven to be the best candidates for transporting energy from one place to another,” said Ayaz. 

“This paper utilizes a novel approach to model energetic particles in space plasmas, as observed by satellites like Viking and Freja, to answer how the electromagnetic energy of the waves, interacting with particles, transforms into heat during the damping process as the waves move through space.”

This energy transport could be crucial in understanding the mechanism behind coronal heating, as these waves might transfer energy from the sun’s magnetic fields to the plasma in the corona, raising its temperature.

Perplexing heat of the sun’s corona

The study primarily focuses on the perturbed electromagnetic fields, Poynting flux vector, and the power delivery rate of kinetic Alfvén waves within the solar atmosphere. 

The corona extends about eight million kilometers above the sun’s surface and is perplexing because it reaches temperatures over a million degrees Kelvin, vastly exceeding the sun’s surface temperature of approximately 6,500 degrees.

Significance of the study

Dr. Gary Zank, director of CSPAR and the Aerojet Rocketdyne chair of the UAH Department of Space Science, emphasized the significance of Ayaz’s work. 

“Syed is one of our outstanding students who is just starting out on his research career. His abiding interest in Alfvén waves has now resulted in his investigation of these waves at very small scales, the so-called kinetic scale in a plasma.”

Dr. Zank noted that this research offers important insights into the critical problem of how energy in a magnetic field is transformed to heat a plasma comprising charged particles like protons and electrons.

The role of kinetic Alfvén waves

Kinetic Alfvén waves (KAWs) are oscillations of ions and magnetic fields in the solar plasma, originating from movements in the photosphere, the sun’s outer layer that emits visible light. These waves are crucial in the energy transfer process in plasma environments. 

“My primary 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,” said Ayaz.

The research team focused on how KAWs facilitate heating and energy exchange within the corona. Observations from various spacecraft and theoretical studies have consistently shown that KAWs dissipate and contribute to heating the solar corona as they travel through space. 

Transferring energy to plasma particles 

Kinetic Alfvén waves operate on small kinetic scales and are capable of supporting parallel electric and magnetic field fluctuations, which enable them to transfer energy to plasma particles.

“The present work utilizes and explores the Landau damping mechanism, which occurs when particles moving parallel to a wave have velocities comparable to the wave’s phase velocity,” noted Ayaz. 

The Landau damping mechanism allows for an exponential decrease in particular waveforms within the plasma, either transferring energy from the wave to the particles or vice versa, depending on their interaction.

The study’s findings suggest that KAWs dissipate rapidly, transferring their energy to plasma particles, which then accelerates them over large distances. This energy transfer has significant implications for the dynamics of the solar plasma, contributing to the overall heating of the corona.

Energy dynamics within the sun’s corona 

The insights gained from this research are essential for understanding the complex processes at work in the sun’s atmosphere. 

The study advances our knowledge of the energy dynamics within the corona and provides a framework for future investigations into solar and space weather phenomena. 

Ayaz’s work, supported by CSPAR and the Department of Space Science, represents a significant step forward in heliophysics research, with potential applications in understanding the behavior of plasmas in various astrophysical settings.

“The analytical insights gleaned from this study find practical application in understanding phenomena within the solar atmosphere, particularly shedding light on the significant role played by nonthermal particles in the observed heating processes,” concluded the researchers.

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