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Astron. Astrophys. 333, 1053-1068 (1998)

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5. Discussion of results

5.1. Weak fields at the tops of rising loops

The observations presented herein give the first quantitative glimpse of the full vector magnetic field and related flows as new flux begins to emerge into the photosphere. They demonstrate that, at the site of emergence where the field is horizontal to the surface, the intrinsic strength of the field is indeed rather weak: 200 to 600 G. In contrast, just adjacent to the weak, horizontal emerging fields are the intrinsically strong, nearly vertical fields of pores, azimuth centers, and plage. This suggests that as the magnetic loops rise through the convection zone, their tops may rise and expand rapidly, decreasing in strength, while the main trunk of flux (represented by Feature GP of Region 3) is oriented nearly vertically and is constrained in such a way as to preserve a large field strength. The decrease in field strength near the top of the loops may be a natural consequence of magnetic fields rising through the highly stratified outer solar atmosphere. The reason why strong fields might be maintained on the legs of this loop is not as obvious, but it could be due to the tendency noted in our observations, and in other references cited herein, for the mass draining out of the loops to fall towards the leading polarity, initiating an enhancement of the field strength via convective collapse (Parker 1978).

5.2. Up flows and down flows as a consequence of buoyantly rising, filamented fields

The inference of modest upward moving plasma in the flux emergence zone by Brants (1985a,b) and Strous (1994) is demonstrated in our observations to be associated with the weak emerging fields. The upward moving flux elements are mostly small, isolated, and transient, strongly suggesting that the emergence process takes place as the dynamical emergence of a filamented flux bundle, not a monolithic flux tube. We do not know if this property is deep-seated, or arises very close to the surface. The rising fields carry mass upward at least to the chromosphere, where the transient flows of arch filament systems may represent the same process seen in our photospheric observations. This indicates that the material comprising the chromospheric arch filaments is not maintained in a stable equilibrium above the emerging active region: a situation quite apart from non-erupting filaments in either the quiet Sun or in active regions.

Material drains out of the rising loops towards their photospheric foot points, giving rise to the large down flows near growing sunspots and pores, as observed both in the photosphere and chromosphere. Our observations of these down flows in the magnetic field near a pore are consistent with those of Brants (1985a). Note also that photospheric down flows of much larger magnitude have been observed near pores when viewed with higher angular resolution (Scharmer & Larsson 1992).

5.3. Magnetic canopies from arched fields

Our measurements show extensive areas of weak, horizontal fields between the two polarities of bipolar emerging flux, only parts of which are associated with the transient upflows suspected to represent the loci of actual flux emergence. We suspect that regions not participating in actual flux emergence may reveal a low-lying arched magnetic "canopy" resulting from previously emerged flux. As the emergence rate decreases, an increasing fraction of the nearly horizontal flux between the leading and following polarities may arise from canopy fields. Magnetic canopies around sunspots and other smaller structures are well-documented as having weak magnetic fields similar to that reported here for emerging flux.

The mottled appearance of the field zenith angle and field strength increases with age of the active region, i.e. as the rate of flux emergence decreases. This mottling arises from an increasing fraction of high field strength plage in the emergence zone. The observed distribution of the field strength vs. zenith angle (Fig. 7) as the active region approaches the limb is also suggestive of the presence of an arched canopy.

In emerging regions the arched canopies might also be precursors to the formation of sunspot penumbrae: we note that the penumbra of the main leader sunspot of Region 3 first forms on the side of the spot closest to the emergence zone.

5.4. Growth of active region magnetic fields as process dominated by buoyancy

A prominent unanswered issue regarding the emergence of flux at the solar surface is that of the process responsible for the formation of sunspots and pores: do they form from the action of convergent flows, or is sunspot formation largely the passage of a magnetic flux system through the photosphere as a result of buoyancy? The present observations provide some indicators that both processes may be at work in the formation of a single sunspot, but buoyancy appears to be the dominant process.

The formation of the leading-polarity pore, Feature GP shown in Fig. 8, provides a strong indication that the "trunk" of the "tree" representing the positive polarity leg of the emerging magnetic flux rope existed below the surface well before emergence. This conclusion is drawn from the proximity of the emergence zone to Feature GP. However, unlike prior observations, we see that Feature GP also gains flux from the side away from the emergence zone (Fig. 9), both from MMFs ejected by the decaying sunspot and from other directions not clearly associated with the moat flow from the decaying spot. In contrast, observations on the next day reveal that some MMFs from the decaying sunspot pass close to Feature GP, but then they continue on past it to form plage rather than being assimilated into Feature GP itself. Thus, some of the flux assimilated into Feature GP from the side away from the emergence zone suggests a subsurface connection of the decaying spot and Feature GP, and some does not. The animation of the time sequence of Region 3 strongly indicates that the MMFs assimilated by Feature GP represent only a small fraction of the rate of growth of this flux element, so we conclude that, at this early stage of emergence, the buoyant rise of flux appears to dominate the growth Feature GP. Any systematic flows at and just below the photosphere therefore must play a subsidiary role in the growth of this region. This viewpoint will be strengthened further in Paper 2 of this series through the study of the twist observed in the magnetic field.

5.5. Convective collapse during emergence?

The development of Feature GP in Region 3 supports the notion of pre-existing strong fields below the surface rather than of the process of convective collapse (Parker 1978) of weak vertical fields into intense, small flux tubes. In contrast, we have seen the continual creation of kiloGauss plage fields on the following polarity side of the emergence zone. These strong fields also arise from the same emerging weak horizontal fields that create the compact pore of Feature GP. One might argue either that this is the same process that creates the pore - reflecting only the prior existence of very small, strong flux elements below the surface - or that convective collapse of weak flux is indeed taking place at sites of flux emergence. The limitations of angular resolution and cadence of the present and prior observations do not allow one to distinguish between these alternatives. What is clear from these observations is that the emerging flux attains kiloGauss strength only after it becomes oriented nearly vertically. Future observations seeking to explore the proposed phenomenon of convective collapse should examine emerging flux regions with both better time resolution and extremely high angular resolution in order to capture unambiguous signatures of this process. Such conditions are not likely to be met by ground-based precision polarimeters such as the ASP without the aid of real-time correction of seeing.

5.6. Implications for global behavior of magnetic fields

In view of the widespread expansion of coronal loops over active regions as documented clearly by Yohkoh SXT time sequences (Uchida et al. 1992), the entire life history of most solar magnetic activity might be dominated by the buoyant rise of the field and its entrained plasma, and by the subsequent draining of this material, sometimes at high velocity, back down to the photosphere along arched magnetic lines of force. This might even take place well after the initial rapid emergence phase of an active region. If fields are continually rising, the associated transfer of material upward from the interior probably does not cease after this initial emergence phase. A process similar to that described above (as an explanation for observed Doppler shifts in newly emerging regions) also may be operative in older regions. If so, one may have a simple, consistent explanation for the "inverse chromospheric Evershed effect" (i.e., chromospheric inflow) observed near sunspots, and for the ubiquitous down flows in the chromosphere and transition region above photospheric plage and network magnetic elements.

The inverse Evershed flow occurs above the outer penumbra and the region immediately outside of sunspots. Here one observes on average an inflow of material toward the sunspot when one observes in lines forming above the temperature minimum. When observed at high resolution, the inverse Evershed flow is even more time-variable than its photospheric counterpart, but it persists for much of the lifetime of a sunspot. These inflows might be confined (Martinez Pillet 1998; Lites 1997) to the more vertically oriented component of the penumbral magnetic field (the "spines" of Lites et al. 1993), which in turn may be also associated with the large-scale coronal structures that appear to be rooted in the penumbrae of sunspots (Sams et al. 1992). A number of explanations of this phenomenon have been set forth, including siphon flows. We speculate that it might be the continuance of the same flows seen in arch filaments.

The observations presented herein strongly point to the need for more observations of this type having continuous coverage of developing active regions with quantitative vector field measurements, with accompanying measurements of the chromosphere. With the present observational capability much of the development of active regions is missed, even when the full daily observing window of a single observing site is used. Furthermore, much of the development is masked by less than excellent seeing conditions, especially for the examination of processes related to the conversion of weak, horizontal flux into kiloGauss, vertical flux elements.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998