🔭 astrophysics

Multi-thermal dynamics and transverse oscillations of solar spicules revealed by coordinated SST, IRIS, and SDO observations

By coordinating high-resolution observations from SST, IRIS, and SDO, this study reveals that solar spicules are multi-thermal, dynamic structures connected to the transition region and corona, carrying substantial energy through transverse oscillations that may contribute to coronal heating.

Original authors: Ravi Chaurasiya, Tiago M. D. Pereira, A. Raja Bayanna

Published 2026-06-25

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ravi Chaurasiya, Tiago M. D. Pereira, A. Raja Bayanna

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun's atmosphere not as a smooth, glowing ball, but as a chaotic, bustling city street filled with millions of tiny, high-speed fountains shooting water straight up into the sky. These "fountains" are called spicules. They are short-lived jets of hot gas that shoot up from the Sun's surface (the chromosphere) and disappear just a few minutes later.

For a long time, scientists were like detectives trying to figure out what happens to these fountains once they reach the top. Do they just fizzle out? Or do they somehow heat up the Sun's outer atmosphere (the corona), which is mysteriously much hotter than the surface below it?

This paper acts like a high-definition, multi-camera investigation into these solar fountains. Here is what the researchers found, explained simply:

1. The "Three-Camera" Setup

To solve the mystery, the team didn't just look at the Sun with one telescope. They coordinated three different "cameras" to watch the same event from different angles and at different "heights":

The Ground Camera (SST): A powerful telescope on Earth that took super-sharp pictures of the lower atmosphere (the chromosphere) in visible light.
The Transition Camera (IRIS): A space telescope that looked at the "middle ground" where the gas gets very hot (the transition region).
The Corona Camera (SDO): A space observatory that watched the super-hot outer atmosphere (the corona).

By syncing these three, they could see if a fountain shooting up from the bottom actually caused a spark or brightening at the very top.

2. The Big Discovery: A Direct Connection

The researchers found a clear, direct link. When a spicule shot up from the bottom, they saw a corresponding brightening in the corona right at the tip of that jet.

The Analogy: Imagine a firework rocket launching from the ground. As it shoots up, you see the trail of fire. In this study, they saw that the "trail" (the spicule) didn't just stop; it actually ignited a small, bright explosion in the sky (the corona) right where the jet ended. This suggests these fountains are physically connected to the heating of the Sun's outer layers.

3. The "Two-Flow" Mystery

When they looked closely at the gas inside these fountains, they found something strange. The gas at the bottom and the gas at the top seemed to be moving in opposite directions at the same time.

The Analogy: Think of a crowded escalator where people are moving up, but on the side, a group of people is sliding down. The researchers saw that the cooler gas (chromosphere) and the super-hot gas (transition region) inside the same spicule were flowing in opposite directions. This proves that a single spicule is a complex, multi-layered structure, not just a simple stream of one type of gas.

4. The "Shaking" Energy

Perhaps the most exciting finding was that these fountains aren't just shooting straight up; they are also wiggling side-to-side like a whip cracking in the wind.

The Analogy: Imagine holding a jump rope. If you shake it, a wave travels up the rope. The researchers saw these solar fountains shaking back and forth very quickly (in less than 5 minutes).
The Energy: They calculated the energy carried by these wiggles. The amount of energy is massive—enough to potentially heat the Sun's outer atmosphere. It's as if the "wiggling" of the rope is powerful enough to warm up the entire room.

5. The Rhythm of the Sun

They also noticed a rhythm to the shaking. While some wiggles were very fast (high-pitched), there was a very common, slower rhythm of about 3 minutes. This is like a heartbeat that the whole solar atmosphere seems to share, suggesting that waves are traveling up from the surface to the top.

The Bottom Line

This paper confirms that solar spicules are not just random bursts of gas. They are:

Multi-layered: Containing different types of gas moving in complex ways.
Connected: Directly linking the lower atmosphere to the super-hot corona.
Energetic: Their side-to-side shaking carries enough energy to potentially help solve the mystery of why the Sun's outer atmosphere is so hot.

In short, these solar fountains are the "elevators" and "power lines" of the Sun, transporting both matter and energy from the surface to the sky.

Technical Summary: Multi-thermal dynamics and transverse oscillations of solar spicules revealed by coordinated SST, IRIS, and SDO observations

Problem and Motivation
Solar spicules are ubiquitous, dynamic jets in the chromosphere that are hypothesized to play a critical role in the mass and energy balance of the solar atmosphere. However, their physical connection to the transition region and corona remains a subject of debate. While previous studies have identified spicules as potential contributors to coronal heating and mass supply, the exact nature of their multi-thermal evolution, their relationship to coronal emission, and the specific role of their wave motions in energy transport are not fully understood. Furthermore, while vertical velocity oscillations have been extensively studied, horizontal (transverse) oscillations and their associated energy flux require further investigation with higher resolution.

Methodology
The authors analyzed a coordinated dataset of a quiet-Sun region near the solar north pole, observed on June 17, 2014. The study utilized high-resolution observations from three instruments:

Swedish 1-m Solar Telescope (SST): Imaging spectroscopy of the H $\alpha$ line (chromosphere) using the CRISP instrument, providing a temporal cadence of 5.5 s and spatial sampling of 0.058 arcsec/pixel.
Interface Region Imaging Spectrograph (IRIS): Slit-jaw images (SJI) in Si iv 1400 Å (transition region) and Mg ii k 2796 Å, along with spectroscopic data in Si iv 1394 Å.
Solar Dynamics Observatory (SDO): EUV images from the Atmospheric Imaging Assembly (AIA) in the 171 Å and 131 Å channels (corona).

The analysis involved:

Co-alignment: Precise registration of datasets across different wavelengths and instruments.
Space–Time Analysis: Construction of space–time diagrams along virtual slits to track the temporal evolution of spicules from the chromosphere to the corona.
Spectral Analysis: Derivation of Doppler velocities from H $\alpha$ and Si iv line profiles to investigate multi-thermal plasma flows.
Wavelet Analysis: Examination of velocity oscillations to identify dominant periods and power distribution.
Transverse Wave Tracking: Manual tracking of spicule axis displacements in space–time diagrams to calculate oscillation periods, amplitudes, velocities, and energy flux ( $F = \frac{1}{2}\rho v_{rms}^2 V_A$ ).

Key Results

Multi-thermal Coronal Connection: Space–time analysis reveals a clear temporal and spatial correspondence between chromospheric spicules (observed in H $\alpha$ ) and enhanced coronal emission in AIA 171 Å. The coronal brightening evolves coherently with the spicule extension, appearing concentrated near the spicule tips. This suggests a direct physical link between spicular activity and coronal plasma heating.
Opposite Doppler Signatures: Spectral analysis of H $\alpha$ and Si iv 1394 Å at the same projected locations shows Doppler velocities with opposite signs. This indicates the presence of distinct, multi-thermal plasma components within individual spicules that do not always evolve coherently. The authors suggest this may be due to unresolved multi-stranded structures or torsional motions, though the precise mechanism remains uncertain.
Oscillatory Behavior:
- Chromospheric Oscillations: Wavelet analysis of H $\alpha$ velocities reveals frequently dominant $\sim$ 3-minute oscillations, consistent with upward-propagating magneto-acoustic waves.
- Transverse Oscillations: The study detected high-frequency transverse oscillations with periods ranging from 65 to 270 s (mean $\sim$ 159 s).
Energy Flux: The transverse oscillations exhibited velocity amplitudes between 3.3 and 9.9 km s $^{-1}$ (mean $6.23 \pm 1.14$ km s $^{-1}$ ) and displacement amplitudes of 121–394 km. The calculated mean energy flux carried by these waves is $(2.14 \pm 0.78) \times 10^3$ W m $^{-2}$ .

Significance and Claims
The paper claims that these results demonstrate spicules are multi-thermal, dynamic structures directly connected to the transition region and corona. The detection of coherent coronal brightening at the tips of limb spicules provides strong observational evidence for this connection.

Regarding coronal heating, the authors state that the estimated mean energy flux of the transverse waves substantially exceeds the typical energy requirement for heating the quiet solar corona ( $\sim$ 100–200 W m $^{-2}$ ). However, the paper maintains a modest stance on the ultimate role of spicules in coronal heating. It acknowledges that while the wave energy flux is sufficient in magnitude, the study does not quantify dissipation heights, damping efficiency, or mode conversion losses. Consequently, the authors conclude that while transverse oscillations in spicules represent a potentially important channel for energy transport into the upper atmosphere, their exact contribution to sustaining the hot corona remains uncertain and debated. The primary significance lies in establishing spicules as efficient channels for both mass and wave energy transport, highlighting their role in coupling the chromosphere to higher atmospheric layers.