Investigating Alfvénic Motions in Solar Loops

The study of waves in the solar atmosphere is important for understanding various solar phenomena. A particular focus is on Alfvénic Waves, which are seen frequently in the solar corona. These waves are closely connected to p-modes, which are oscillations based on sound waves from the photosphere, the visible surface of the sun. However, how these sound waves can lead to transverse motions in the sun's magnetic structures, known as Coronal Loops, remains a mystery.

This article looks into a specific scenario: how an inclined wave driver can create Alfvénic motions in coronal loops. We aim to create a better understanding of how these motions develop and their significance in the solar atmosphere.

#The Solar Atmosphere

The solar atmosphere consists of several layers, including the photosphere, transition region, chromosphere, and corona. The corona is the outer layer and is much hotter than the layers below it. Observations have shown that transverse oscillations are common in the corona. These motions are often classified as Magnetohydrodynamic (MHD) Waves, specifically kink waves, due to the movement of magnetic tubes in the region.

Researchers believe that Alfvénic waves might play a role in significant solar processes, such as coronal heating and the acceleration of solar wind. Understanding the relationship between these waves and photospheric p-modes is essential, as it potentially explains how energy moves from one layer to another.

#Wave Propagation

Solar p-modes are standing waves created by turbulence within the convection zone of the sun. They have specific periods and can leak energy into the lower atmosphere. The challenge arises when these acoustic waves try to move upward through layers of the solar atmosphere. Various factors, such as gravitational stratification and acoustic cut-off, can impede this upward motion.

In the lower atmosphere, the plasma is partially ionized, which introduces more complexity in the interactions between sound and magnetic waves. The Hall effect and ambipolar diffusion can affect how these waves convert from acoustic waves to Alfvénic waves. The process of generating Alfvénic motions from a predominantly acoustic driver is a key focus of this study.

#Numerical Model

To explore this phenomenon, we created a numerical model that simulates a coronal loop in a gravitationally stratified solar atmosphere. This model includes a transition region and chromosphere. The simulation applies a local wave driver at one end of the loop. This driver is similar to a sound wave but inclined at an angle to the magnetic structure.

The results show that transverse motions are generated in the magnetic loop. These motions displace the magnetic field's axis due to a loss of azimuthal symmetry. We analyzed the resulting oscillations using a theoretical framework based on a magnetic cylinder model.

#Simulation Details

The model for the simulations is based on a straight, expanding coronal loop extending from the photosphere to the corona. Unlike typical models, which may use density enhancements, we take an evacuated approach, where the structure is guided by the magnetic field strength alone.

Gravity is accounted for in the model, meaning that the gravitational force acts differently at various heights throughout the loop. The base of the numerical domain represents the photosphere, and we define the transition region as the point where the plasma temperature reaches a specific level.

#Boundary Conditions and Numerical Setup

In order to simplify calculations, we simulate only half of the loop. The numerical domain is defined in three dimensions, with various points set along the loop's length. The simulation uses specific boundary conditions for both the magnetic field and velocity.

The results are computed using a numerical code that solves the MHD equations in cylindrical coordinates, enabling us to analyze how the driver affects the loop's oscillations.

#Wave Excitation

We examine whether an inclined wave driver can excite different wave modes in the magnetic structure. The inclination of the wave driver is expected to create non-axisymmetric waves, particularly kink modes, within the loop.

The presence of these modes is important as they may carry significant energy to the corona. The expectation is that the modes generated can be identified through changes in the structure of the magnetic waveguide.

#Analysis of Motions

The study looks at non-axisymmetric motions that occur in different modes, focusing on how they affect the magnetic structure. We convert our data into Cartesian geometry to analyze the component of motion more clearly. By interpolating the signals from our simulations, we can observe how the axis of the waveguide moves over time.

Results indicate a periodic pattern in the amplitude of these motions, suggesting the excitation of higher order modes. Observations show clear distinctions between motions at different heights in the atmosphere, with variations in amplitude and the nature of oscillations.

#Vorticity and Energy Transfer

Vorticity, or the measure of rotation in a fluid, is also computed to observe how transverse motions manifest themselves throughout the atmosphere. The results show that vorticity is not concentrated in one place but is spread throughout the non-uniform space in the corona.

This behavior supports our theoretical expectations that Alfvénic waves propagate through inhomogeneous plasmas. The presence of non-zero vorticity indicates that the transverse waves are affecting the local plasma dynamics.

#Observational Characteristics

To relate our findings to real observations, we identify specific characteristics of the waves generated. The analysis indicates that the transverse motions we simulate exhibit velocity amplitudes similar to those found in observations of solar activity.

We note the significance of different oscillation periods present in the simulations. Two distinct periods stand out, which reflect the oscillations driven by the inclined wave driver and the eigenmode of the coronal loop. This aspect can help in comparing simulation data to observational results.

#Conclusion

This study has provided insights into how inclined acoustic-gravity wave drivers can create Alfvénic motions in the solar corona. We have shown through numerical simulation that transverse oscillations are induced, leading to significant displacement of the magnetic structure's axis.

The results indicate that both kink and sausage modes play a role in the dynamics of the loop. Further investigation could explore how these waves contribute to energy transport within the solar atmosphere and their implications for solar phenomena.

In conclusion, understanding the connection between p-modes and Alfvénic waves is crucial. This research paves the way for a more comprehensive analysis of the solar atmosphere's behavior, potentially revealing more about the underlying mechanisms driving solar activity. Future studies will delve deeper into the characteristics of these waves and their observational signatures, enhancing our comprehension of the sun's complex dynamics.