Steps towards a high precision solar rotation profile: Results from SDO/AIA coronal bright point data

Abstract

Context. Coronal bright points (CBP) are ubiquitous small brightenings in the solar corona associated with small magnetic bipoles. Aims. We derive the solar differential rotation profile by tracing the motions of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). We also investigate problems related to the detection of CBPs resulting from instrument and detection algorithm limitations. Methods. To determine the positions and identification of CBPs we used a segmentation algorithm. A linear fit of their central meridian distance and latitude vs time was used to derive velocities. Results. We obtained 906 velocity measurements in a time interval of only 2 days. The differential rotation profile can be expressed as ω rot = (14.• 47 ± 0.• 10 + (0.• 6 ± 1.• 0) sin 2 (b) + (−4.• 7 ± 1.• 7) sin 4 (b)) d −1 . Our result is in agreement with other work and it comes with reasonable errors in spite of the very short time interval used. This was made possible by the higher sensitivity and resolution of the AIA instrument compared to similar equipment as well as high cadence. The segmentation algorithm also played a crucial role by detecting so many CBPs, which reduced the errors to a reasonable level. Conclusions. Data and methods presented in this paper show a great potential for obtaining very accurate velocity profiles, both for rotation and meridional motion and, consequently, Reynolds stresses. The amount of CBP data that could be obtained from this instrument should also provide a great opportunity to study changes of velocity patterns with a temporal resolution of only a few months. Other possibilities are studies of evolution of CBPs and proper motions of magnetic elements on the Sun.

Meridional motions and Reynolds stress from SDO/AIA coronal bright points data

“…Given that the spatial resolution of SDO/AIA is ≈0.6 /pixel, we can estimate that the error in position induced by CBP apparently changing position by 1 pixel is about 0.04 • in solar coordinates around the equator. This is much smaller than observed by Sudar et al (2015). The error in velocity between two subsequent images would be less than 1 m s −1 .…”

“…We used a similar procedure to that used in our previous paper (Sudar et al 2015) to obtain CBP positions. The segmentation algorithm is a modification of similar algorithms described in McIntosh & Gurman (2005) and Martens et al (2012).…”

“…It is quite common to filter out such outliers by selecting a fixed range of acceptable rotational velocity (Brajša et al 2002b;Vršnak et al 2003;Sudar et al 2014Sudar et al , 2015. This approach neglects the fact that the solar rotation varies with latitude and that such a fixed cut-off does not have a uniform effect on all latitudes.…”

“…For example, Brajša et al (2001Brajša et al ( , 2002bBrajša et al ( , 2004, Vršnak et al (2003), Wöhl et al (2010) used SOHO/EIT data, Hara (2009) analysed Yohkoh/SXT measurements, while Kariyappa (2008) used both Yohkoh and Hinode data. Recently, Sudar et al (2015) have used SDO/AIA measurements in the 19.3 nm channel.…”

An evolutionary computation based algorithm for calculating solar differential rotation by automatic tracking of coronal bright points

“…CBPs are also very good tracers since they extend to much higher latitudes than sunspots (Sudar et al 2015). They are also one of the solar corona's most ubiquitous features (McIntosh & Gurman 2005), quite numerous in all phases of the solar cycle while, for example, sunspots are often absent in the minimum of the cycle (Sudar et al 2015).…”