On the nature of moving magnetic feature pairs around sunspots

Abstract

Abstract. Employing data recorded by the Michelson Doppler Imager (MDI) instrument on the Solar and HeliosphericObservatory (SOHO), we have identified 144 pairs of opposite magnetic polarity moving magnetic features (MMFs) in two active regions (NOAA ARs 8375 and 9236). The following results are obtained: (1) The majority of MMF pairs first appears at a distance of 1000 to 5000 km from the outer boundary of the sunspot, although MMF pairs appearing closer to the sunspot may be missed. (2) MMF bipoles are not randomly oriented. The member of an MMF pair further from the sunspot has the polarity of the parent sunspot in 85% of the cases. Furthermore, the orientations of MMF pairs are associated with the twist of the sunspot superpenumbra deduced from Hα images. (3) The mean lifetime of the studied MMFs is around 4 hours. (4) The separation between the two polarities of the MMFs falls in the range of 1100-1700 km. This separation remains almost unchanged, even decreases slightly as the MMF pairs move outwards. (5) MMFs are observed to cluster at particular azimuths around the parent sunspot, in particular in AR 8375. (6) MMF pairs move approximately radially outward from sunspots at an average speed of around 0.5 km s −1 . Their motion is deflected towards large concentrations of magnetic flux of opposite polarity to that of the parent sunspot. A qualitative model based on these and other observations is presented. MMF pairs are proposed to be part of a U-loop emanating from the sunspot's magnetic canopy. Possible mechanisms leading to the formation of such a loop are discussed.

Observation of bipolar moving magnetic features streaming out from a naked spot

“…Based on high-resolution observations, analyses of MMF characteristics have demonstrated that some MMFs are manifestations of the penumbral Evershed flow in the sunspot moat, supporting earlier results about the magnetic connection between MMFs and the sunspot penumbra (see, e.g., Yurchyshyn et al 2001;Zhang et al 2003;Sainz Dalda & Martínez Pillet 2005;Cabrera Solana et al 2006;Kubo et al 2007). On the other hand, statistical analyses of sunspot groups also showed type II MMFs flowing out from naked spots, i.e., sunspots without a penumbra (Harvey & Harvey 1973).…”

Evolution of Active Regions

“…Small magnetic flux concentrations called moving magnetic features (MMFs) stream radially out from the sunspots. Though MMFs have a mixed-polarity nature , and their ability to carry away flux from the sunspot has been questioned (Zhang et al, 2003), at least the mono-polar MMFs do seem to be effective in that respect. The flux shed by spots will be subjected to a random walk along the network buffeted by supergranular motions (Lawrence and Schrijver, 1993).…”

“…The bipolar MMFs are not randomly orientated, but instead are aligned so that a line connecting between the centres of each polarity is roughly parallel to the radial of the sunspot, with some MMFs' orientation reflecting the twist in the sunspot penumbral magnetic field . In most cases, it is the bipolar MMF fragment furthest from the sunspot that has the same magnetic polarity as the sunspot Zhang et al, 2003). Such an orientation suggests that MMFs are formed either by U-loops in a field that originates in the canopy emanating from the sunspot which has formed dips that reach down to to the photosphere (Zhang et al, 2003;Bernasconi et al, 2002), or by Ω-loop sections of flux that have detached themselves from the sub-photospheric portion of the sunspot that have subsequently risen to the photosphere to form a sea-serpent (Wilson, 1973;Spruit et al, 1987).…”

“…In most cases, it is the bipolar MMF fragment furthest from the sunspot that has the same magnetic polarity as the sunspot Zhang et al, 2003). Such an orientation suggests that MMFs are formed either by U-loops in a field that originates in the canopy emanating from the sunspot which has formed dips that reach down to to the photosphere (Zhang et al, 2003;Bernasconi et al, 2002), or by Ω-loop sections of flux that have detached themselves from the sub-photospheric portion of the sunspot that have subsequently risen to the photosphere to form a sea-serpent (Wilson, 1973;Spruit et al, 1987). In this case the field remains attached to the spot at the photospheric level but the flux concentration below the photosphere is being eroded.…”

“…For the bipolar MMFs where the leading polarity is opposite to that of the sunspot, flux originating in the sunspot's canopy could be dipping down into the photosphere so that the MMF dipole is formed by Ω-loops and their outward flow could be directly peeling away from the spot at the photospheric level and fully detaching the field from the spot. The first appearance of MMFs can be outside of the sunspot penumbral boundary (Zhang et al, 2003) or inside the penumbra (Sainz Dalda and Martínez Pillet, 2005) and an MMF mechanism that invokes the propagation of a kink along a highly-inclined field, rather than sunspot decay, has also been proposed (Ryutova and Hagenaar, 2007).…”