5:15 PM - 6:45 PM
[PEM14-P06] Investigation of the wave propagation in the solar atmosphere with relation to the magnetic field and the view angle
Keywords:chromosphere, atmospheric heating, oscillations
Understanding how the energy balance in the chromosphere and corona is achieved is a major problem in modern solar physics. The theory that waves excited in the photosphere dissipate energy in the upper atmosphere is one of the most promising.
In the photosphere, acoustic waves of various frequencies are excited by convective motion. In general, acoustic waves can only propagate in the upper layers if they are higher in frequency than a threshold value called the cutoff frequency, which is a quantity that depends on the local magnetic field. However, it is believed that where there is a large inclined magnetic field (chromosphere network), the effective magnetic field becomes smaller and the cutoff frequency decreases somewhat with the local field inclinations, so that lower frequency waves leak into the chromosphere. In fact, many observational studies have detected waves in the upper atmosphere with frequencies lower than cutoff, and recent simulation studies suggest that acoustic waves in particular contribute significantly to chromospheric heating.
However, the detailed mechanisms and rigorous estimates of the dissipated energy have not been successfully elucidated. The following three reasons are given for this. (1) Observational studies have only detected the presence of low frequency waves in the upper atmosphere and have rarely examined the relationship with the inclination of magnetic field lines. (2) The field of view is limited to the quiet regions or plages at the disk center; while the planar structure can be accurately investigated near the disk center, it is difficult to detect oscillations propagating along inclined magnetic field lines as line-of-site velocity components. (3) The accuracy of the instruments, length of observation time, lines used and analysis methods differ from paper to paper. Therefore, what is required in this research field is to observe various magnetic regions at various distances from the disk center using the same instrument.
Based on this, we are conducting two studies: first, we investigate the propagation of waves between the photosphere and the chromosphere over the entire disk; second, we follow the waves from the photosphere to the corona and study their behavior in various magnetic regions at various distances from the disk center. In this poster talk, the first study will be presented.
We have investigated how acoustic waves excited by a photosphere propagate through a magnetic field to the corona on the whole disk. Specifically, we obtained the full surface data of the Doppler velocity of the Hα line center from the SMART/SDDI archive at the Hida Observatory, Kyoto University, and the full surface data of the Doppler velocity of the Fe I line center from the SDO/HMI archive. Both data were obtained on May 4, 2022, and are continuous time series over 12 hours. The seeing was extremely good throughout the day and various magnetic regions were present at various distances from the disk center. Next, the FFT was used to calculate the respective oscillation powers and their spatial distribution was examined.
As the results, we obtained some characteristic power maps. In the photosphere, we obtained typical 5-minute oscillations power map. In the chromosphere, in addition to the 3- and 5-minute oscillations that have often been observed, long-period oscillations of 10-40 minutes were found to be present over the entire disk. Considering the characteristics of the global power distribution, these long-period oscillations can be divided into the following two categories; (a) oscillations with a period longer than 20 minutes, and (b) oscillations with a period shorter than 20 minutes. In the full-disk power map of (a), the power is significantly higher in the filament. In the full-disk power map of (b), the power is significantly higher in magnetic regions such as the chromospheric network, enhanced network, plages, and active regions. Such period oscillations have not been reported in previous studies due to the short observation time, but they suggest the possibility that very low frequency oscillations also propagate to the upper layers through interactions with the magnetic field. Finally, we examined how these four types of chromospheric power vary from disk center to limb in the quiet region in order to investigate the correlation between wave propagation and the inclination of magnetic field lines. We found that all four types of chromospheric oscillations propagate along magnetic field lines with inclination of about 50 degrees, and that long-period oscillations propagate well in the chromospheric network.
In the photosphere, acoustic waves of various frequencies are excited by convective motion. In general, acoustic waves can only propagate in the upper layers if they are higher in frequency than a threshold value called the cutoff frequency, which is a quantity that depends on the local magnetic field. However, it is believed that where there is a large inclined magnetic field (chromosphere network), the effective magnetic field becomes smaller and the cutoff frequency decreases somewhat with the local field inclinations, so that lower frequency waves leak into the chromosphere. In fact, many observational studies have detected waves in the upper atmosphere with frequencies lower than cutoff, and recent simulation studies suggest that acoustic waves in particular contribute significantly to chromospheric heating.
However, the detailed mechanisms and rigorous estimates of the dissipated energy have not been successfully elucidated. The following three reasons are given for this. (1) Observational studies have only detected the presence of low frequency waves in the upper atmosphere and have rarely examined the relationship with the inclination of magnetic field lines. (2) The field of view is limited to the quiet regions or plages at the disk center; while the planar structure can be accurately investigated near the disk center, it is difficult to detect oscillations propagating along inclined magnetic field lines as line-of-site velocity components. (3) The accuracy of the instruments, length of observation time, lines used and analysis methods differ from paper to paper. Therefore, what is required in this research field is to observe various magnetic regions at various distances from the disk center using the same instrument.
Based on this, we are conducting two studies: first, we investigate the propagation of waves between the photosphere and the chromosphere over the entire disk; second, we follow the waves from the photosphere to the corona and study their behavior in various magnetic regions at various distances from the disk center. In this poster talk, the first study will be presented.
We have investigated how acoustic waves excited by a photosphere propagate through a magnetic field to the corona on the whole disk. Specifically, we obtained the full surface data of the Doppler velocity of the Hα line center from the SMART/SDDI archive at the Hida Observatory, Kyoto University, and the full surface data of the Doppler velocity of the Fe I line center from the SDO/HMI archive. Both data were obtained on May 4, 2022, and are continuous time series over 12 hours. The seeing was extremely good throughout the day and various magnetic regions were present at various distances from the disk center. Next, the FFT was used to calculate the respective oscillation powers and their spatial distribution was examined.
As the results, we obtained some characteristic power maps. In the photosphere, we obtained typical 5-minute oscillations power map. In the chromosphere, in addition to the 3- and 5-minute oscillations that have often been observed, long-period oscillations of 10-40 minutes were found to be present over the entire disk. Considering the characteristics of the global power distribution, these long-period oscillations can be divided into the following two categories; (a) oscillations with a period longer than 20 minutes, and (b) oscillations with a period shorter than 20 minutes. In the full-disk power map of (a), the power is significantly higher in the filament. In the full-disk power map of (b), the power is significantly higher in magnetic regions such as the chromospheric network, enhanced network, plages, and active regions. Such period oscillations have not been reported in previous studies due to the short observation time, but they suggest the possibility that very low frequency oscillations also propagate to the upper layers through interactions with the magnetic field. Finally, we examined how these four types of chromospheric power vary from disk center to limb in the quiet region in order to investigate the correlation between wave propagation and the inclination of magnetic field lines. We found that all four types of chromospheric oscillations propagate along magnetic field lines with inclination of about 50 degrees, and that long-period oscillations propagate well in the chromospheric network.