Height-dependent Velocity Structure of Photospheric Convection in Granules and Intergranular Lanes with Hinode/SOT

Abstract

The solar photosphere is the visible surface of the Sun, where many bright granules, surrounded by narrow dark intergranular lanes, are observed everywhere. The granular pattern is a manifestation of convective motion at the photospheric level, but its velocity structure in the height direction is poorly understood observationally. Applying bisector analysis to a photospheric spectral line recorded by the Hinode Solar Optical Telescope, we derived the velocity structure of the convective motion in granular regions and intergranular lanes separately. The amplitude of motion of the convective material decreases from 0.65 to 0.40 km/s as the material rises in granules, whereas the amplitude of motion increases from 0.30 to 0.50 km/s as it descends in intergranular lanes. These values are significantly larger than those obtained in previous studies using bisector analysis. The acceleration of descending materials with depth is not predicted from the convectively stable condition in a stratified atmosphere. Such convective instability can be developed more efficiently by radiative cooling and/or a gas pressure gradient, which can control the dynamical behavior of convective material in intergranular lanes. Our analysis demonstrated that bisector analysis is a useful method for investigating the long-term dynamic behavior of convective material when a large number of pixels is available. In addition, one example is the temporal evolution of granular fragmentation, in which downflowing material develops gradually from a higher layer downward.

Magnetoacoustic wave energy dissipation in the atmosphere of solar pores

“…Oscillations with similar velocity amplitudes, approximately 0.6 km s −1 , have been shown to be related to the presence of slow magnetoacoustic waves that propagate through multiple layers of the solar atmosphere [35]. Importantly, as can be seen in figure 4, the oscillations demonstrate reduced velocity amplitudes within the confines of the pore structures, where the root mean square (RMS) velocities are less than 100 m s 300-500 m s −1 in the surrounding quiet Sun, which are consistent with previous spectroscopic studies [36][37][38][39][40][41]. To investigate the plasma parameters associated with the detected wave activity, and subsequently evaluate the energy flux, it was imperative to undertake spectropolarimetric inversions of the Si I 10827 Å line.…”

Average Radial Structures of Gas Convection in the Solar Granulation

“…The SP data were calibrated using the standard routine SP PREP in the Solar SoftWare package (Lites & Ichimoto 2013); this procedure performs i) dark field correction, ii) flat-field correction, iv) the correction of curved spectral line, iv) the removal of periodic wavelength and spatial shifts in the period of the spacecraft, and vi) the calibration of intensity variation along the SP slit caused by the thermal deformation of the instrumental optics. Finally, a systematic error of the absolute velocity (i.e., the offset of the absolute wavelength position) was estimated in the same manner as Oba et al (2017a) such that the line core of our average spectral profile, integrated broadly over space and time, matches that in a well-calibrated spectral catalogue (Allende Prieto & Garcia Lopez 1998), falling within a systematic error of 0.18 km/s. This study focuses on normal convection in hydrodynamics, without disturbance by magnetic elements.…”

The Small-scale Structure of Photospheric Convection Retrieved by a Deconvolution Technique Applied to Hinode/SP Data

“…First, bisector analysis provided the velocity field at multiple heights (Stebbins & Goode 1987;Dravins et al 1981), and then a subsonic filter (Title et al 1989) removed the 5-min oscillations to obtain the convective velocities. Through these steps, we obtained convective velocity fields at six bisector levels (intensity level of I/I c =0.70, 0.65, 0.60, 0.55, 0.50, 0.45) using the Fe i 630.15 nm spectral line (the details are described in Oba et al 2017). We avoided sampling other intensity levels (more than 0.70 or less than 0.45), to make sure that our results are not adversely affected by pixels lost in intergranular lanes owing to their lower continuum and higher line core intensity, e.g., if a value of I/I c of 0.90 is chosen, then the bisector at this I/I c value cannot be determined in all the pixels at which the continuum intensity drops below this value.…”

“…The SP data were calibrated with the standard routine SP PREP in the Solar SoftWare package (Lites & Ichimoto 2013). Finally, the velocity was calibrated as in Oba et al (2017).…”

“…Using a Milne-Eddington inversion code, Jin et al (2009) found a velocity field dominated even more strongly by blueshifts with a range from −3.3 km/s to +2.0 km/s and a peak at -1.0 km/s. Finally, using a bisector analysis, Oba et al (2017) investigated the average convective velocity field of upflows and downflows separately. They found 1.5 times stronger upward than downward motions at photospheric heights from 40 to 160 km.…”