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Emerging Ux As the Source of Down Ows in the Chromosphere

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Emerging Ux As the Source of Down Ows in the Chromosphere Mem. S.A.It. Vol. 81, 792 c SAIt 2010 Memorie della Emerging ux as the source of down ows in the chromosphere B. W. Lites High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA, e-mail: [email protected] Abstract. Downward flowing plasma is a common signature of Doppler diagnostics in spectral lines forming in the chromosphere. Examples of such are the foot points of arch fil- ament systems associated with emerging flux and the inverse Evershed flow in and around the penumbrae of sunspots. This contribution puts forth the hypothesis that these down- ward flows, at least those occurring in and around active regions, trace their origin to the ubiquitous emergence of magnetic flux. In this scenario, the buoyantly rising, emerging flux carries mass upward from lower levels, then drains downward along magnetic lines of force to produce the observed downflow signatures in the chromosphere and below. The hypoth- esis is discussed and illustrated by chromospheric observations of sunspots and filaments. Key words. Sun: chromosphere – Sun: filaments – Sun: prominences 1. Introduction witness to a global-scale emergence of active region flux. The advent of the Yohkoh space mission This presentation explores the possible re- brought a new perspective to studies of the so- lation between the ever-rising fields and the lar corona, in that it provided the first con- frequent occurrence of downflows in the ac- tinuous movies of the entire solar disk in x- tive region chromosphere. Buoyant rise of rays over long periods. One of the first and fields from the interior into the atmosphere and most notable results from that mission is that corona will result in the upward advection of the corona, mainly seen as the plasma as- the mass frozen to those fields. The high con- sociated with fields arising from active re- ductivity of the solar atmosphere will constrain gions, appeared to expand outward from the the plasma contained in the rising fields to solar surface in a quasi-continuous manner move along the magnetic lines of force. Thus, (Uchida et al. 1992). The expansion is cer- the rising, dense plasma will tend to fall back tainly related to the evolution of flux at the so- along inclined field lines. lar surface: as new flux emerges from the inte- Perhaps the arch filament system (Bruzek rior, the field lines rise rapidly upward into the 1967, 1969) is the solar phenomenon for highly-stratified atmosphere where they then which this mechanism of buoyant rise of mass- fill the volume above that is occupied by much laden plasma followed by draining toward weaker fields. Evidently, the corona is bearing the magnetic foot points is most widely ac- cepted. Through spectroscopy and imaging of Send offprint requests to: B. W. Lites Lites: Downflows in the chromosphere 793 Fig. 1. Images are shown from spectro-polarimetric observations taken with the Advanced Stokes Polarimeter of NOAA AR 10397 near the east solar limb (9N, 54E) on 29 June 2003. The images at the top display the continuum intensity at 630 nm (left) and the intensity at the center of the Hα line (right). Lower images show the Doppler velocity taken as the wavelength shift of the intensity minima in the Fe i 630.15 nm line (left) and Hα (right). Solar north and west are up and to the right, respectively, and the east limb is close to the left edge of the images. Blue (red) shifted profiles are dark (light). These observations clearly show the chromospheric inverse Evershed flow surrounding the large sunspot at the right of the image. the Hα line, that work identified moderate that come and go on time scales of a few min- (∼ 10 km s−1) chromospheric upflows near the utes or less. The photospheric Evershed ef- mid-points of the apparent loops, and high ve- fect is confined to the penumbra of sunspots, locity downflows (∼ 50 km s−1) at their appar- but the CIEE mainly exists in the regions ent foot points. A similar structure, but with just outside of the sunspot. Figure 1 illus- correspondingly much slower velocities, has trates both the photospheric Evershed effect been identified with flux emerging into the and the CIEE as observed with the Advanced photosphere (Lites et al. 1998). Here I raise the Stokes Polarimeter. The inverse Evershed flow possibility that this process may be much more is clearly seen in the Hα Doppler image at widespread in the solar chromosphere. lower right of Fig. 1. It is apparent that the flow exists mainly in the moat region outside of the sunspot penumbra. The CIEE shows much 2. The Chromospheric Inverse coarser structure than its photospheric counter- Evershed Effect part, and it follows the superpenumbral chro- The chromosphere above and surrounding mospheric fibril structures visible in the Hα sunspots exhibits a persistant Doppler mo- image at the upper right of Fig. 1. tion indicating mass flow toward the cen- ter of the sunspot. This motion is known as 3. Active region filaments that the Chromospheric Inverse Evershed Effect terminate in sunspots (CIEE). As its name implies, the sense of the flow is opposite to that of the outward flowing The commonly accepted paradigm for fila- photospheric Evershed effect. Though persis- ments/prominences (hereafter denoted as fil- tent, the CIEE is by no means steady. Doppler aments) is that they are a quasi-static mag- movies in Hα reveal that the flow is com- netic structure containing material at chro- prised of many individual small-scale threads mospheric temperatures suspended within and 794 Lites: Downflows in the chromosphere Fig. 2. Left: Images of continuum intensity (top) Hα line center (middle), and Hα Doppler signal (bottom) are shown for NOAA AR 10390 observed near the center of the disk (N16, W10) on 29 June 2003. Red (blue) shifts appear as bright (dark) in the Doppler images. The long filament terminates in the sunspot penumbra, and is accompanied by rapid downward motions that persist over the duration of these measure- ments. Right: Stokes spectra of the Hα line show the dramatic downward velocity associated with the end of the filament at the sunspot. The vertical white line in the upper right image indicates the position of the spectrograph slit corresponding to the Stokes spectra below. isolated from the corona by the magnetic field. a vertical extent many times greater than the Although filaments do show a wide array of chromospheric scale height, as is commonly dynamic phenomena, notably revealed in great observed. detail recently by high resolution Hinode ob- Now consider the case of an active re- servations (for example, Berger et al. 2008), gion filament, one end of which terminates in much of the quiescent filament material must a sunspot (see Fig. 2). The filament is quasi- be subject to approximate hydrostatic equi- static, as demonstrated by its unchanging struc- librium and will therefore obey the stratifica- ture over some 22 minutes spanned by the ob- tion of material at chromospheric temperatures servations. Indeed, the filament had a simi- with a scale height of a few hundred km. In this lar appearance on the previous day, and has picture of filaments, each field line has a local a concave-upward (dipped) field geometry in minimum in height within the corona, and the the photosphere under the upper portions of cool filament material collects at this minimum the filament (Lites 2005). The right end of the with its stratification along field lines obey- filament appears to terminate in the sunspot ing the hydrostatic condition. Individual field penumbra. It is extremely unlikely that the field lines, each having a local dip, may be stacked structure above this simple, isolated sunspot vertically. Quasi-static cool material could re- could have a local minimum in height over, or side in the lowest few hundred km of each field just adjacent to, the outer penumbral boundary. line thereby allowing a prominence to have The field lines traced by the filament therefore Lites: Downflows in the chromosphere 795 must follow the converging sunspot field struc- lished if those field lines have a connection ture and terminate in the sunspot itself. with the superpenumbral filaments that harbor The Hα images presented in Fig. 2 come the CIEE. from a slit-jaw imaging sequence of this re- The persistant nature of the flows at the gion using the Universal Birefringent Filter at sunspot end of the filament shown in Fig. 2, NSO/Sac Peak, in which the filter was tuned al- coupled with the fact that the sunspot magnetic ternately to the continuum, red, blue, and line field almost certainly does not posess dips at center wavelengths. The Hα Doppler images this location, strongly suggests that the sunspot in the bottom two panels of Fig. 2 result from end of the filament is maintained by a dy- the simple Dopplergram processing: Doppler = namic rather than static equilibrium. The mass (blue-red) / (blue+red). Thus, for Hα in absorp- needed to supply this flow continuously may tion, redshifted material appears bright. These arise from a siphon flow, but it could also be Dopplergrams show highly redshifted material supplied by the continuous emergence of the in the segment of the filament leading down filament system through the photosphere into into the sunspot. the chromosphere and corona. The latter hy- Accompanying these slit-jaw image se- pothesis is strengthened by the fact that this quences is a spectro-polarimetric map in the filament had erupted and then re-formed on the Hα line. The right side of Fig. 2 shows the previous day (Lites 2005). Stokes profiles for one slit scan position (high- lighted in the image at upper right).
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