Abstract
The measurement of the Sun's surface motions with a high spatial and temporal resolution is still a challenge. We wish to validate horizontal velocity measurements all over the visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic and Magnetic Imager (SDO/HMI) data. Horizontal velocity fields are measured by following the proper motions of solar granules using a newly developed version of the Coherent Structure Tracking (CST) code. The comparison of the surface flows measured at high spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI, 0.5 arcsec) allows us to determine corrections to be applied to the horizontal velocity measured from HMI white light data. We derive horizontal velocity maps with spatial and temporal resolutions of respectively 2.5 Mm and 30 min. From the two components of the horizontal velocity Vx and Vy measured in the sky plane and the simultaneous line of sight component from SDO/HMI dopplergrams v_D, we derive the spherical velocity components (Vr, Vtheta, Vphi). The azimuthal component Vphi gives the solar differential rotation with a high precision (+-0.037km/s) from a temporal sequence of only three hours. By following the proper motions of the solar granules, we can revisit the dynamics of the solar surface at high spatial and temporal resolutions from hours to months and years with the SDO data.
Highlights
The dynamics of the Sun’s surface is one of the major elements in understanding the time evolution of its magnetic activity
From the two components of the horizontal velocity vx and vy measured in the sky plane and the simultaneous line of sight component from Solar Dynamics Observatory/ Helioseismic and Magnetic Imager (SDO/Helioseismic and Magnetic Imager (HMI)) dopplergrams vD, we derive the spherical velocity components
By following the proper motions of the solar granules, we can revisit the dynamics of the solar surface at high spatial and temporal resolutions from hours to months and years with the Solar Dynamics Observatory (SDO) data
Summary
The dynamics of the Sun’s surface is one of the major elements in understanding the time evolution of its magnetic activity. By following the proper motions of the solar granules, representative of solar plasma evolution, it is possible to define the flow field on the solar surface (Roudier et al 2012) from a small spatial scale of 2.5 Mm up to nearly 85% of the solar radius (Fig. 6 of that paper). In order to identify whether improvements could be made in the determination of the horizontal velocities beyond that limit, we used simultaneous observations of the Sun in white light at low (SDO/HMI) and high (Hinode) spatial resolution. We describe a comparison between velocity fields projected on the sky plane that are obtained in the south pole region with Hinode data and SDO/HMI data using the coherent structure tracking (CST) code (Rieutord et al 2007; Roudier et al 2012).
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