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

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1. Introduction

The magnetic fields of solar active regions are believed to be generated at or just below the base of the convection zone. Then, presumably as a result of some instability (Moreno-Insertis 1994), the fields escape the convectively-stable layers and begin to buoyantly rise through the convection zone, carrying the more tenuous plasma constrained within it, and ultimately emerge into view at the solar surface. The phenomenon of emergence of bipolar magnetic regions at the solar surface has long been studied (see Zwaan 1985, Zwaan 1992 , Harvey-Angle 1993 , Strous et al. 1996 for reviews) using magnetograph observations which present some measure of the longitudinal (i.e., line-of-sight, LOS) component of the magnetic field. The "foot points" of loops of emerging flux, i.e. the locations of opposite magnetic polarity where the magnetic lines of force intersect the photosphere, separate in time, forming the very common bipolar active region configuration. Although more complex field configurations (e.g. delta sunspots, Lites et al. 1994) sometimes appear, by far the most common pattern for emerging fields is this simple bipolar configuration, especially after the first few days of the the life of an active region (Zwaan 1992).

Presuming that the majority of the emerging active region fields are generated by a dynamo process deep within the solar interior - i.e., that they are not the result of a near-surface photospheric dynamo - then magnetic flux rising from the deep interior must first appear as the horizontal field of the tops of magnetic loops (or ropes, in the case that the field is significantly twisted). Thus, the time history of the field evolution observed at the surface presents us with an indication of topology of the field that existed below the surface prior to emergence. The rapid evolution early in the life of an active region provides us with perhaps the best avenue towards an understanding of the 3-dimensional structure of the subsurface field. This method has been employed in the past to interpret the structural evolution of active regions (Piddington 1974; Tanaka 1991; Leka et al. 1996).

The field topology measured at the solar photosphere may be radically modified by near-surface effects such as: granular and other convective processes, the dynamical influence of the small surface scale height resulting in a "ballooning" distortion, and the rapid increase of radiative losses near the surface. In addition to providing clues about the structure of the flux in the convection zone, studies of emerging magnetic flux then also provide information about such near-surface processes and their relative importance to the photospheric evolution of the field. Even if these near-surface effects obliterate most of the information regarding the dynamo, the observed history of the field at the surface is of significant interest from the standpoint of understanding these near-surface processes.

Another important goal for the study of emerging flux is to gain an understanding of the physical processes which form and which ultimately disrupt magnetic surface features such as pores, sunspots, and faculae (plage). In particular, are sunspots assembled from a field of previously emerged small scale flux tubes through the action of a shallow, convergent flow? Or are they simply the "trunks" of a tree of magnetic flux that rises buoyantly into the atmosphere? At what point in a spot's evolution does decay set in? Is decay ongoing from the time of first emergence? What are the mechanisms causing the decay? Detailed observational study of the emerging vector field patterns at the photosphere, and its evolution, may be the only means to adequately address these questions.

Emerging flux has been studied for many decades without the benefit of vector field measurements. As prominent indicators of newly emerging flux, "arch filament" systems visible in the [FORMULA] line have long been used to identify emerging flux regions and to study the dynamical response of the chromosphere to these events (Bruzek 1967; Zwaan et al. 1985; Chou & Zirin 1988; Zwaan 1992). Most spectroscopic evidence indicates that the tops of these arch filaments are rising in the chromosphere with velocities of about 10 km s-1, while even larger descending velocities, 30-50 km s-1, exist on at least one side of the arch filament. A widely held interpretation is that buoyantly-rising magnetic fields lift mass out of the photosphere to the chromosphere, which then drains out of the arched chromospheric loops, accelerating towards the photosphere.

Although there have been few published observations with spatial resolution adequate for study of emerging flux at the photospheric level, those studies hint of a behavior similar to that of arch filaments in the chromosphere. In a series of papers (Zwaan et al. 1985; Brants 1985a; Brants 1985b) the authors detail properties of emerging flux from spectroscopic and circular polarimetric data. They find 40 km s-1 down flows in the chromosphere on one side of an arch filament with corresponding down flows in photospheric lines of 1.5-2 km s-1. The photospheric down flow "occurs just outside of a rapidly growing pore" on the side closest to the site of flux emergence (which we define as the polarity inversion line separating opposite poles of a rapidly emerging region.) Furthermore, Brants (1985a,b) finds significant up flow ([FORMULA] 0.5 km s-1) in the vicinity of the inversion line, where he infers a transverse field strength of less than [FORMULA]  300 G, based upon observed widths of the Stokes I profiles. In a more recent high-resolution study, Strous (1994, 1996) analyzed filter magnetographic data of an emerging region. He has found small, transient, elongated darkenings in the continuum and at the center of the Zeeman-insensitive line Fe I 557.6 nm which he infers are the tops of emerging flux loops. Unlike the dark lanes occurring in non-magnetic solar granulation, these darkenings exhibit modest 0.5 km s-1 blue shifted line profiles. Strous (1994) also finds nearby brightenings of the circularly polarized photosphere which are associated with down flows of similar magnitude.

These observational results from the photosphere suggest a picture of flux emergence that is consistent with the observed behavior in the chromosphere, but they lack definitive information about the vector magnetic field. In the series of studies by Brants and co-workers, spectral information with circular polarimetry was obtained photographically, but at that time there was no way to accurately correct these data both for polarizing properties of the telescope and for the response of the polarimeter itself. More importantly, inference of anything about the transverse component of the magnetic field from only Stokes I and V profiles is highly questionable. We are able to demonstrate that the Brants (1985a,b) data cannot contain the information needed to determine either the strength, the orientation, or the fill factor of the magnetic field for the weak, emerging flux elements found in our present study. Similar issues surround the interpretation of the Strous (1994) circular polarimetry, which is also subject to further concerns regarding the use of filter polarimetry to infer quantitative measures of the vector field (Lites et al. 1994.)

Zwaan (1992) speculated that the fields arrive at the photosphere with strengths nearly in equipartition with convective motions (a few hundred Gauss), based both upon theoretical arguments and upon the visibility of facular magnetic elements in magnetograms. Such magnetograms, however, reveal the flux only after it has become vertically oriented (and as we show in this paper, with greatly enhanced field strength). It is the transverse field component which is most relevant to studies of emerging flux.

To date there has been little observational work describing the vector character of the initial stages of emerging flux. Especially important are measurements which characterize the vector magnetic field and associated thermodynamic parameters in quantitative detail free of a priori thermodynamic constraints: actual field strengths and orientation angles, estimates of area filling factors, and precise measures of Doppler shifts are all crucial to proper interpretation of the emergence process. This demands time series of precise, high resolution spectro-polarimetry, mapped over the entire emerging region, rather than the qualitative polarimetry rendered by most magnetographs. The few analyses to date of emerging flux which use data from vector magnetographs (Leka et al. 1996; Cauzzi et al. 1996) have not involved data of the precision needed to examine quantitatively the fields at the emergence site. The high spectral and spatial resolution Stokes polarimetry now available from the Advanced Stokes Polarimeter (ASP) permits one, for the first time, to address unambiguously the observational aspects of flux emerging into the photosphere. Not only do the ASP field measurements reveal the strength of the horizontal field at the site of emergence, they can also show if the field carries significant twist.

The present series of papers report on ASP observations that provide the most detailed picture to date of the site of newly emerging magnetic flux. In this first paper in the series, we focus on characterizing the properties of the field and associated atmospheric motions at the point of emergence, i.e. field strengths, Doppler shifts, emergence rates, etc. With such information we address the following questions: Do magnetic fields of plage regions emerge as kiloGauss flux tubes? Do we find any evidence for amplification of field strengths as a result of a convective collapse process? Is the emerging flux bodily carrying mass into the solar atmosphere, as suspected from arch filament behavior in the chromosphere and some prior measurements of flux emergence at the photosphere? The second paper in the series reports on detailed analyses of time series of one particular bipolar emerging region for which a number of indicators suggest a systematic twist, or helicity, exists over a range of sizes. In anticipation of the results of that paper, in this paper we often refer to emerging flux as "ropes" rather than "loops".

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998