moving magnetic features (mmfs) jun zhang national astronomical observatories chinese academy of...

Moving Magnetic Features (MMFs)

Jun ZhangNational Astronomical Observatories

Chinese Academy of Sciences

Collaborators: Sami Solanki and Jingxiu Wang

Outline

1. Definition of MMFs 2. Properties of MMFs3. A qualitative model4. Results

1. Definition of MMFs

• Moving magnetic features (MMFs) are small magnetic features that move away from a sunspot to the periphery of the surrounding moat (Vrabec 1971; Harvey and Harvey 1973; Lee 1992).

• The MMFs are generally thought to be transported by the outward moat flow. There are two kinds of MMFs: unipolar and mixed polarity (Harvey and Harvey 1973; Yurchyshyn et al. 2001).

Magnetograms and Dopplergrams of AR 8375 observed by SOHO/MDI

Magnetograms and Dopplergrams of AR 9236 observed by SOHO/MDI

Left: H images; middle: MDI magnetograms; right:MDI continuum images. The FOV is 172"172".

• Definition of angles and (see also Yurchyshyn et al. 2001).

2. Properties of MMFs

• It is still an open question about the nature of MMFs.

• We have identified 144 pairs of opposite magnetic polarity MMFs in two active region ARs 8375 and 9236.

Flux distributions of the positive and negative elements of 144 MMF pairs.

Distance of first appearance of MMF pairs from the sunspot boundary.

Histogram of the proper motion speeds of the MMFs.

Histogram of MMF lifetimes.

Histogram showing the separationbetween polarities of MMF pairs.

The difference between the separation of each MMF pair near the end and near the

beginning of its life.

Histogram of angle measurements of the two investigated active regions.

Histogram of the number of MMFs for a given angle in AR NOAA 8375.

Relationship between and the change of over MMF lifetime.

To find the evolution of MMFs’ velocity fields, we have traced 42 MMF pairs in active region AR 8375.

The relationship between horizontal velocity and distance to the sunspot penumbra ofAR 8375.

The horizontal velocity of MMF elements is plotted versus relative distance from MMF birth place.

Left: MDI magnetograms showing the northern part of the sunspot of AR 8375 . A box outlines the field-of-view of the following magnetograms. Three arrows denote a negative element of an MMF pair; Right: Dopplergrams. White patches show downflow, and black, upflows. Three arrows point to the Dopplershift of the negative element.

Upper panel: histogram showing the Doppler shift of the individual MMFs; Lower panel: histogram showing the difference of the Doppler shift between negative and positive MMF elements.

Sketch of a possible explanation of the creation(upper frame) and motion ( lower frame) of MMF pairs.

3. A qualitative model

Relationship among an umbra, a penumbra, an MMF pair and a canopy

base.

4. Results

(1) The majority of MMF pairs first appears at a distance of 1000 to 5000 km from the outer boundary of the sunspot, although MMFs 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. The orientations of MMF pairs are associated with the twist of the sunspot superpenumbra;

(3) The mean lifetime of the studied MMFs is around 4 hour;

(4) The separation between the two polarities of the MMFs falls in the range of 1100 ~ 1700 km. This separation decreases slightly as the MMF pairs move outwards;

(5) MMFs tend 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 0.45 km s-1. Their motion is deflected towards large concentrations of magnetic flux of opposite polarity to that of the parent sunspot.

(7) The velocity of an MMF pair decreases significantly (from 0.6 km/s to 0.35 km/s) with increasing distance from the MMF's birth place.

In contrast, the decrease of the average velocity far less obviously (from0.5 km/s to 0.4 km/s) with increasing distance from the sunspot.

The change in MMF flow speed does not reflect the radial structure of the moat flow, but rather is intrinsic to the evolution of the MMF pairs. The vertical velocity indicates that the elements of MMF pairs with polarity opposite to that of the sunspot support a downflow (Doppler redshift).

• 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. According to this model the downflow is caused by the Evershed flow returning below the solar surface.