Properties of the solar velocity field indicated by motions of coronal bright points

Abstract

Abstract. Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse properties of the solar velocity field by tracing coronal bright points from June 4, 1998 to May 22, 1999. Rotation velocity residuals, meridional motions and their relationship are investigated. Zones of slow and fast rotation found in motions of coronal bright points are consistent with the pattern of torsional oscillations, indicating that the statistical velocity pattern of bright point motions reflects the large-scale plasma flows. A complex pattern of meridional motion is deduced: The equatorward flows are found to dominate at low (B < 10• ) and high (B > 40 • ) latitudes, whereas at mid-latitudes (B ≈ 10• −40 • ) a poleward flow is inferred. The complete data set shows no significant correlation between rotation residuals and meridional motions. However, when a subsample of coronal bright points including only the "point-like structures" (predominantly young bright points) is considered, a statistically significant correlation is found. On average, faster tracers show equatorward motion and the slower ones show poleward motion. Such a segregation is reflected in a statistically significant covariance of the rotation residuals and meridional velocities in the order of −1000 m 2 s −2 , revealing an equatorward transport of angular momentum. The negative value of the covariance is provided by the high velocity tail in the velocity distribution of point-like structures, representing less than 15% of the population. The latitude dependence of the covariance can be expressed as Q = −62 B + 200 m 2 s −2 covering the range B = 0 • -60• .

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

“…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.…”

“…Coronal bright points (CBPs) have also been used very frequently by using the data obtained by different satellites. 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.…”

Height correction in the measurement of solar differential rotation determined by coronal bright points

“…In Brajša et al (2002a) the results on differential rotation for the two methods, for the northern and southern solar hemispheres, and for the three subtypes of tracers (pointlike structures, small loops, and small active regions) were presented and compared. Properties of the solar velocity field indicated by motions of coronal bright points expressed through the rotation velocity residual, meridional motions and Reynolds stresses were discussed in Vršnak et al (2003).…”

Multiscale optical flow probing of dynamics in solar EUV images

“…We applied a displacement field to an EIT image I 1 and obtained a second image I 1,dr using the parametric models of rigid and differential rotation estimated by Vršnak et al (2003). The interpolation procedure is based on the IDL function of cubic interpolation (Park & Schowengerdt 1983).…”

“…Recent works (e.g. Brajša et al 2001;Vršnak et al 2003) have studied differential rotation of the atmosphere, and the present paper uses their results for comparison. In a future publication, the method will be used for studying the solar rotation over long periods.…”