Characterizing the Motion of Solar Magnetic Bright Points at High Resolution

Abstract

Magnetic bright points in the solar photosphere, visible in both continuum and G-band images, indicate footpoints of kilogauss magnetic flux tubes extending to the corona. The power spectrum of bright-point motion is thus also the power spectrum of Alfvén wave excitation, transporting energy up flux tubes into the corona. This spectrum is a key input in coronal and heliospheric models. We produce a power spectrum of bright-point motion using radiative magnetohydrodynamic simulations, exploiting spatial resolution higher than can be obtained in present-day observations, while using automated tracking to produce large data quantities. We find slightly higher amounts of power at all frequencies compared to observation-based spectra, while confirming the spectrum shape of recent observations. This also provides a prediction for observations of bright points with DKIST, which will achieve similar resolution and high sensitivity. We also find a granule size distribution in support of an observed twopopulation distribution, and we present results from tracking passive tracers which show a similar power spectrum to that of bright points. Finally, we introduce a simplified, laminar model of granulation, with which we explore the roles of turbulence and of the properties of the granulation pattern in determining bright-point motion.From these MURaM simulations, we extracted granule size distributions, the motion paths of passive tracers, and the motion paths of automatically-identified bright points. This

Magnetic Activity of F-, G-, and K-type Stars in the LAMOST–Kepler Field

“…These two populations are divided in size: the large granules have diameters in a Gaussian distribution of approximately 1.2 ± 0.5 Mm and are believed to be traditional convective cells, whereas the smaller granules (or granule-like visible features) follow a decreasing power-law distribution in diameter, with typical sizes under 0.5 Mm. This population of smaller features may have its origin in turbulent processes at the solar surface rather than convection (Van Kooten & Cranmer 2017), meaning they may follow different distributions in timescale τ c and temperature contrast Θ RMS than those assumed in the present work's model. Abramenko et al (2012) quantify the importance of a given size of granule to the overall appearance of the photosphere with the area contribution function (ACF), defined as the ratio of the total area of granules of a given size to the total area available, and this should serve as a good proxy of the influence of granules (or granule-like features) of a given size on F 8 .…”

Energy Transport and Heating by Torsional Alfvén Waves Propagating from the Photosphere to the Corona in the Quiet Sun

“…The choice of a Kolmogorov spectrum at high frequencies is motivated by the observational indications that photospheric motions are turbulent. Regarding the form of the spectrum at low frequencies, recent computations by Van Kooten & Cranmer (2017) suggest a rather flat spectrum for low frequencies, i.e., L ≈ 0, while Tu & Song (2013) and Arber et al (2016) propose a low-frequency exponent of L = 5/6. These two possible choices for L are considered to determine if this dependence has any impact on the results.…”

Study of the Dynamics of Convective Turbulence in the Solar Granulation by Spectral Line Broadening and Asymmetry

“…They suggested that the mini-granules behave in a turbulent manner compared with their regular-size counterparts. van Kooten & Cranmer (2017) analyzed the motions of bright points in a numerically simulated photosphere and concluded that mini-granules cause high-frequency motions and regular-size granules cause low-frequency motions. They further suggested that there might be more power observed in small-scale and high-frequency phenomena.…”