Alexander Kosovichev1 and Junwei Zhao2
1 New Jersey Institute of Technology, Newark, NJ 07103, USA
2 Stanford University, Stanford, CA 94305, USA
Previous investigations by the ring-diagram technique and time-distance helioseismology revealed large-scale converging flows around active regions, which alter the mean meridional circulation1,2 and, thus, the magnetic flux transport affecting the strength and duration of the solar activity cycles. Detailed flow maps from the HMI Time-Distance Helioseismology Pipeline3 allow us to investigate the process of formation of these flows during the emergence of active regions.
As a case study we present a local helioseismology analysis of the subsurface dynamics of the emerging active region NOAA 11726, which is the largest emerging region observed by the SDO/HMI instrument during its first six years of operation.
During the initial emergence of the bipolar structure no specific large-scale flow pattern in the depth range 0-20 Mm is identified. Nevertheless, the region of the flux emergence is characterized by an enhanced horizontal flow divergence that corresponds to the spatial separation of the magnetic polarities. The speed of emergence determined from tracking the initial divergence signal with depth is about 1.4 km s-1, very close to the emergence speed in the deep layers4.
Figure 1 | Subsurface flow maps and the photospheric magnetograms at the initial moment of appearance of emerging bipolar magnetic flux on the solar surface at 20:00 UT, April 21, 2013, at three depths: (a) 1-3 Mm, (b) 3-5 Mm, and (c) 5-7 Mm. The point x = 0, y = 0 is located at the heliographic coordinates: latitude 13.0°, longitude 15.5°, and the Carrington longitude 322.0°. The travel times were calculated by using the Gabor-wavelet fitting technique, and the inversion for flows was performed by using the Born-approximation kernels.
As the emerging magnetic flux becomes concentrated in sunspots local converging flows are observed beneath the forming sunspots (Fig. 1). The converging flows are most prominent in the depth range 1-3 Mm, and remain converging after the formation process is completed. The structure of the converging flows is complicated and apparently reflects the sunspot structural evolution and interaction with the surrounding convection flows. The characteristic speed of these flows is about 0.3 km s-1. In the deeper layers the flows beneath the sunspots are predominantly diverging and occupy larger areas (Fig. 1c).
Figure 2 | Formation of the large-scale converging flow pattern during the active emergence illustrated for three moments of time: (a) 2013-04-16 21:00 UT; (b) 2013-04-19 16:00 UT; and (c) 2013-04-22 05:00 UT. The flow maps are obtained by applying a Gaussian smoothing filter with a standard deviation of 30 Mm.
To investigate the formation of large-scale converging flows around the active region we apply a Gaussian filter with the standard deviation of 30 Mm. This filter smooths the supergranulation-sized flows and reveals larger-scale patterns. The result of this filtering applied to the area of emergence of AR 11726 is shown in Fig. 2. Prior to the emergence the subsurface flow pattern represents a shearing zonal flow (Fig. 2a). At the start of the emergence the zonal flows are diverted towards the active region on the both sides of the emerging magnetic flux, forming a vortex-like structure in the northern part of the area (Fig. 2b). The formation of the converging flows appears as a diversion of the zonal shearing flows towards the active region, accompanied by the formation of a large-scale vortex structure. The scale of these flows is much larger than the size of the active region, and the typical flow speed is about 30 m s-1. This process occurs when a substantial amount of the magnetic flux emerges on the surface, and the converging flow pattern remains stable during the following evolution of the active region (Fig. 2c).
To investigate the relationship between the flux emergence and the formation of the converging inflow in Fig. 3a we compare the mean flow divergence with the mean unsigned magnetic field strength, calculated for the area shown in Fig. 2. It shows that the mean divergence becomes sharply negative after most of the magnetic flux emerged on the surface. The process of formation of the converging flow takes about 24 hours. Fig. 3b shows a similar comparison for the kinetic helicity proxy, <(∇×v)z·(∇·v)z>. While the divergence value quickly saturates the helicity value keeps increasing. The helicity increase means that a large-scale vortex structure is formed beneath the active region.
Our previous study showed that the large-scale inflows affect the mean meridional flow only in a relatively shallow 10 Mm deep layer at the top of the convection zone, by effectively reducing the flow speed at 20-40 degrees latitude5. As the solar cycle progresses the zone of the reduced speed migrates towards the equator.
These results show that the subsurface flows that develop in and around emerging active regions have a complex multi-scale structure, which is important for the understanding of how the active regions are formed and how they affect the global Sun’s dynamics and magnetic activity.
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