Magnetically Driven Resonance Helps Heat Sun’s Atmosphere

Why is the Sun’s corona even hotter than its core? The combined data from two satellites hints at an answer.

AsianScientist (Sep. 2, 2015) – Solar physicists have captured the first direct observational signatures of resonant absorption, thought to play an important role in solving the ‘coronal heating problem’ which has defied explanation for over 70 years. Their results have been published in The Astrophysical Journal.

Coronal heating problem is the question why the sun’s atmosphere—also known as the solar corona—is about two-fold higher than its surface. The solar corona reaches temperatures of 1,000,000 degrees Celsius and is composed of extreme high temperature gas, known as plasma. As the outer layer of the Sun, the part farthest from the core where the nuclear reactions powering the Sun occur, it would logically be expected to be the coolest part of the Sun. But in fact, it is 200 times hotter than the photosphere, the layer beneath it.

Currently, two theories explaining the coronal heating problem prevail. Firstly, the wave heating theory involves sound and magnetic waves coming from the surface releasing energy to heat up the surface. Magnetic reconnection theory, on the other hand, relies on magnetic fields to induce electric current on the sun’s surface.

Resonant absorption is a process where two different types of magnetically driven waves resonate, strengthening one of them. In particular this research looked at a type of magnetic waves known as Alfvénic waves.

Thanks to space missions such as the Japanese Hinode mission (launched in 2006), we now know that the solar atmosphere is permeated with Alfvénic waves. These magnetically driven waves can carry significant amounts of energy along the magnetic field lines, enough energy in fact to heat and maintain the corona. But for this theory to work, there needs to be a mechanism through which this energy can be converted into heat.

To look for this conversion mechanism, the research team led by Dr. Joten Okamoto from Nagoya University combined data from two state-of-the-art missions: Hinode and the IRIS imaging and spectroscopic satellite (the newest NASA solar mission, launched in 2013).

Both instruments targeted the same solar prominence, a filamentary bundle of cool, dense gas floating in the corona. Although denser than the rest of the corona, a prominence doesn’t sink because magnetic field lines act like a net to hold it aloft. The individual filaments composing the prominence, called threads, follow the magnetic field lines.

Hinode’s very high spatial and temporal resolution allowed researchers to detect small motions in the two-dimensional plane of the image (up/down and left/right). To understand the complete three-dimensional phenomenon, researchers used IRIS to measure the Doppler velocity (i.e. velocity along the line of sight, in-to/out-of the picture). The IRIS spectral data also provided vital information about the temperature of the prominence.

These different instruments allow the satellites to detect different varieties of Alfvénic waves: Hinode can detect transverse waves while IRIS can detect torsional waves. Comparing the two data sets shows that these two types of waves are indeed synchronized, and that at the same time there is a temperature increase in the prominence from 10,000 degrees to more than 100,000 degrees. This is the first time that such a close relationship has been established between Alfvénic waves and prominence heating.

But the waves are not synchronized in the way scientists expected. In the case of the prominence threads, the torsional motion is half-a-beat out of sync with the transverse motion driving it. In other words, there is a delay between the maximum speed of the transverse motions and the maximum speed of the torsional motion, like the delay between the motion of the hips of a dancer in a long skirt and the motions of the skirt hem.

To understand this unexpected pattern the team used NAOJ’s ATERUI supercomputer to conduct 3D numerical simulations of an oscillating prominence thread. Of the theoretical models they tested, one involving resonant absorption provides the best match to the observed data. In this model, transverse waves resonate with torsional waves, strengthening the torsional waves; similar to how a child on a swing can add energy to the swing, causing it to swing higher and faster, by moving his body in time with the motion.

The simulations showed that this resonance occurs within a specific layer of the prominence thread close to its surface. When this happens, a half-circular torsional flow—known as the resonant flow—around the boundary is generated and amplified. Because of its location close to the boundary, the maximum speed of this flow is delayed by half-a-beat from the maximum speed of the transverse motion, just like the pattern actually observed.

The simulations further reveal that this resonant flow along the surface of a thread can become turbulent. The appearance of turbulence is of great importance since it is effective at converting wave energy into heat energy. Another important effect of this turbulence is to enlarge the resonant flow predicted in the models to the size actually observed.

This model can explain the main features of the observations as the results of a two-step process. First resonant absorption transfers energy to the torsional motions, producing a resonant flow along the surface of the prominence thread. Then turbulence in this strengthened resonant flow converts the energy into heat.

Here, for the first time, researchers were able to directly observe resonant absorption between transverse waves and torsional waves, leading to a turbulent flow which heats the prominence.

The articles can be found at:
Okamoto et al. (2015) Resonant Absorption of Transverse Oscillations and Associated Heating in a Solar Prominence. I. Observational Aspects
Antolin et al. (2015) Resonant Absorption of Transverse Oscillations and Associated Heating in a Solar Prominence. II. Numerical Aspects.

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Source: National Astronomical Observatory of Japan.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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