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

3. The character of the vector field within emerging regions

We have identified a few properties of the vector character of newly emerging flux that are common to all regions observed. In this section we summarize these properties and their association with other observables, specifically the Doppler velocity, which appear to be universally associated with emerging flux.

3.1. Horizontal, weak emerging flux

Fig. 1a presents (clockwise from the upper left) constructed images of the continuum intensity and vector magnetic field for Region 1. All images in this paper are presented on a uniform grid of solar latitude and longitude with solar north upward and solar west to the right. All vector field quantities presented here have been transformed into the "local solar frame", where they appear as if viewed from radially above the region. For the present work we adopt the customary (but conservative) polarization threshold of 0.4% integrated over the 630.25 nm line (Lites et al. (1993)), where is the continuum intensity. Uniformly-shaded areas within images presented in this paper represent regions where no LSQ inversion was attempted.

Fig. 1. a (left): Gray-scale images of (clockwise from upper left): continuum intensity, field azimuth (measured counter-clockwise from solar west), zenith angle , and strength are presented in the local solar frame (see text) for Region 1. Horizontal dark lines at the top and bottom of the continuum image result from fiducial "crosshairs" on the slit, and the direction toward disk center is indicated. b (right): Shown from bottom to top are images viewed in perspective of (from bottom to top): the continuum intensity , the flux F (white = positive), the vector magnetic field of a subarea, and the measured Doppler velocity (blue shift = ) of the same subarea. Contours in the top two planes outline the pores as determined from the continuum spectroheliogram. Times listed in all figures represent the approximate midpoint of the duration of the corresponding map.

The azimuth angle image shows the field diverging (converging) radially from (to) the pore on the west (east). In the flux emergence zone between the pores, the field points in a nearly west-to-east direction. The strength of the field is fairly weak and nearly horizontal to the surface in this region, but the polarity reverses once along this direction, revealing a quadrupolar structure in the emergence zone.

The gray-scale images of in Fig. 1a reveal the fine structure present in these quantities, but they do not present immediately an intuitive picture of the vector magnetic field. A spatially coarser but more graphic illustration of the vector field is provided in Fig. 1b by the perspective representation of 3-dimensional arrows, where we present, from bottom to top, , the mean (pixel and LOS averaged) radial field (Semel & Skumanich 1998) (where is the strength of the field in Gauss and is the inferred fill factor: see Skumanich et al. (1994)), the vector field representation, and the measured absolute Doppler velocity of the magnetized plasma relative to the rest frame of the solar surface. Throughout the remainder of the paper we refer to the mean radial field F as simply the "flux". The vector field image shows the field strength via the intensity scale associated with that plane, with superimposed field vectors for every 7th pixel, having either their base emerging from the plane () or the arrow points touching the plane (). The lengths of the arrows are proportional to , and the 3-D orientation of the arrowheads indicate . Fig. 2 presents for Regions 2 and 3 the same vector field representation as Fig. 1b.

Fig. 2. a Same as Fig. 1b but for Region 2. b Same as Fig. 1b but for Region 3.

The images of Figs. 1 and 2 show that none of the sunspots of these young active regions have yet developed substantial penumbrae. In the case of Region 2, the spots themselves are each collections of smaller pores. These images reveal fundamental properties of the emerging vector field:

• Bipolar field structure: The flux images show these three regions to be fundamentally bipolar with leading (westward) polarity being positive as is the case for southern hemisphere regions during this cycle. Between the positive and negative sunspots for all three regions, the field points more or less directly from positive toward negative polarity. We define this region as the "emergence zone". In the emergence zone of Regions 1 and 2 the field vectors point roughly eastward. For Region 3, the fields initially point in a more northerly direction, but rotate gradually towards the eastward direction as the region evolves.
• Low field strength in horizontal emerging flux: The field magnitude images reveal that the field strength falls in the range  G where the field is nearly horizontal. This property is illustrated more clearly in the left panel of Fig. 3. In none of these three emerging active regions do we find strong, nearly horizontal fields resembling those reported by Brants (1985b).

Fig. 3. Scatter plots are presented for zenith angle vs. field strength (left) and vs. Doppler velocity v (right) for the subarea of Region 3 shown in Fig. 5 and in the images as 16:43 in Fig. 8. In the left image, squares represent the pore, triangles the flux emergence zone, and dots the plage outside either the emergence zone or the pores. In the right panel, blue shifts are positive values and all features are represented by dots. As the emerging region is basically bipolar and observed away from disk center, one may identify negative polarity fields () with the side of the bipolar region closest to disk center (see text).

Fig. 3 (left panel) presents a scatter plot of as a function of field strength for a subarea of the observed map of Region 3 which encompasses the emergence zone and adjacent pores. ¹ In the left panel of Fig. 3 we distinguish the pore by squares, the emergence zone by triangles, and the remaining plage  ² by dots. The pore was outlined manually using the continuum intensity as a guide. The emergence zone was also identified visually from a map of where we isolate the region of predominantly highly inclined fields between the spreading opposite polarity, and more vertical, flux. The plage region is defined as everywhere else in the subarea that is neither pore nor emergence zone. Note that the largest field strengths are clearly associated with the pore, where the fields are more vertical than horizontal. Likewise, the well-known clustering of plage fields near vertical orientation (Martinez Pillet et al. 1997a) is clearly visible, as is the dominance of plage field strengths in the range 1200-1500 G. In contrast, the emergence area is dominated by nearly horizontal fields ranging in strength from about 200 to 600 G. The lower limit of this range may be influenced by the polarization threshold of the analysis. Fig. 3 verifies the qualitative impression presented by Figs. 1 and 2. Similar scatter plots of vs for this region at other times, and for Regions 1 and 2, are found.

Images of magnetic flux F (e.g. Figs. 1b, 2, 5, 6, 8, 9 in this paper) present almost no information about the weak horizontal emerging fields visible in the image of the acute field angle (as defined in Figs. 5, 6). Our flux images resemble standard high resolution longitudinal magnetograms taken near disk center. Thus, Zwaan's (1992) conjecture that the strength of emerging flux is greater than a few hundred Gauss, based upon the identification of facular elements in longitudinal magnetograms, may be correct in fact, but the reasoning underlying that inference is probably not correct because the facular elements he observed likely represent kiloGauss fields. Facular elements visible in our very sensitive flux images are all of kiloGauss strengths, while the emerging, horizontal flux is clearly identifiable only in images of intrinsic field strength (where G) and field inclination (where ).

3.2. Observed Doppler velocities: up flows near the tops of loops

The Stokes profile data present evidence for upward-moving fluid near the apex of loops as they penetrate into the photosphere. This flow has been firmly established for a single, isolated emerging loop (Martinez Pillet et al. 1998b), has been inferred from sequences of high resolution magnetograms and Dopplergrams (Strous 1994 , Strous et al. 1996), and was suspected to be the cause of modest blue shifts seen by Brants (1985b). Here we show that it is a persistent feature of flux emergence zones as identified by horizontal magnetic fields. The right panel of Fig. 3 presents a scatter plot of absolute (within  100 m s^-1) Doppler velocity v - inferred from the shift of the Stokes polarization profiles and thus indicating LOS flows in the magnetized atmosphere - relative to the rest frame of the local solar surface. These scatter plots result from the fit to the Stokes profiles within the same subarea represented by the left panel of Fig. 3. We note a clear preference for the largest blue shifts, up to 1.5 km s^-1, in the emerging flux region for fields that cluster around the horizontal. This region was observed rather close to disk center (), but similar plots for Region 1 (and to a lesser extent for Region 2), shown in Fig. 4, give the same result somewhat farther from the center of the disk. The observed fact that the clustering occurs for fields that are nearly horizontal in the local solar frame, and not for fields that are transverse to the LOS, demonstrates that the inferred blue shifts are real, i.e. they are not an artifact of analysis of regions that are nearly transverse to the LOS showing anomalous Stokes V profiles.

Fig. 4. Scatter plots of vs. v for subareas of Regions 1 (left) and 2 (right) centered on the flux emergence zone are presented. The subareas used are indicated in Figs. 1 and 2. See caption of Fig. 3 for further details.

These observations indicate that emerging flux consists of nearly horizontal ropes oriented in a bipolar configuration bridging the interior region between the previously emerged opposite polarities. If we presume this rather simple geometry for the flux, then within the emergence zone we may associate one polarity of the field with location closest to disk center, as indicated on the scatter plots. (For all three regions studied here, the direction toward disk center is oriented very approximately along the line joining opposite polarities in the emergence zones: see the arrows in Figs. 3 and 4 indicating the polarity of fields associated with the disk-center side.) All three regions show a clustering of data points representing emerging flux with fields nearly horizontal, but slightly skewed in towards the opposite direction of the arrows. Mean blue shifts are also larger, especially for Regions 1 and 3, on the disk-center side. This latter property might be explained by the viewing angle of the region: for material being lifted bodily into the atmosphere by a ballooning magnetic flux rope (assuming that flows along the rope are negligible  ³), measured Doppler velocities would be largest at the point where flux rope expansion has the greatest LOS component, i.e. the point on a uniformly expanding loop where the expansion is directed along the LOS. We have no ready explanation for the former property. More statistics are needed to determine if there is indeed an association with the orientation to disk center, or if this might be due to some observational selection effect.

Figs. 1b and 2a,b show the spatial distribution of the Doppler motions (top planes) of the emergence zones for Regions 1-3. In the flux emergence zone the spatial distribution of blue shifted material differs from that of the field zenith angle (planes second from top). Blue shifted regions usually occur in smaller patches within larger areas of nearly horizontal field. The correlation of blue shift with horizontal field is least distinct for the more mature Region 2, as is also suggested by the right panel of Fig. 4.

We also note the common occurrence of significant motions a few arcseconds outside of the indicated boundaries of the pores (e.g., in Fig. 1b, the dark (indicating red shift) patches to the right of the pore to the east, and the light patches (indicating blue shift) to the left of the pore to the west). If we interpret these Doppler images as indicating flows along the vector field, then all three regions show the presence of flows toward the pores, rather than away from them as would be expected from the Evershed effect. This is especially evident in Fig. 1b, and it is consistent with the interpretation (Sect. 5.2) of material carried upward above the surface by the recently emerged fields, then draining back down towards the pores along arched magnetic loops.

In Fig. 5 we examine in detail Doppler velocities (lower right image) in the region of rapid emergence of flux for Region 3 on Sept. 23, when the pore near the center of the image was growing rapidly in size and flux. Hereafter, we designate this pore as "Feature GP" (Growing Pore). The velocity image reveals the up flow (white) associated with the horizontal fields, but also shows significant down flow at the edge of the pore just to the south of the up flow. Note that within Feature GP itself, any flows along the LOS that are present are of very small amplitude, as is the case at the photospheric level in most sunspot umbrae. Flow patterns similar to that of the photosphere, but larger in both scale and amplitude, are visible in chromospheric Doppler images of Region 3, as is typical for arch filament systems.

Fig. 5. An expanded view of Region 3 at UT on 1994 Sept. 23 shows (clockwise from upper left) continuum intensity , flux (  G = white), Doppler velocity (  km s-1 = white), and acute field angle (0 = dark). The contour outline of the pore is superimposed on each image.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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