Evidence for a siphon flow ending near the edge of a pore

Transcription

1 Evidence for a siphon flow ending near the edge of a pore H. Uitenbroek, K.S. Balasubramaniam and A. Tritschler National Solar Observatory/Sacramento Peak 1, P.O. Box 62, Sunspot, NM huitenbroek@nso.edu, bala@nso.edu ABSTRACT Observations of active region NOAA 9431, taken with the Vacuum Tower at Kitt Peak on 21 Apr. 18 in the Ca II nm line in both circular polarizations, show evidence for a strong supersonic down flow ending near the edge of a magnetic pore. The observed supersonic motion is interpreted as a siphon flow along a magnetic loop connecting a patch of weaker field to the pore of opposite polarity in the same active region. The nm line data reveals the up flow at one foot point of the loop, as well as the acceleration of the flow towards the foot point at the pore, where the flow reaches line-of-sight velocities of well over 2 km s 1, substantially larger than the critical speed. Numerical radiative transfer modeling of the nm line indicates the presence of a strong discontinuity in the flow velocity, which we interpret as evidence for a tube shock in the downwind leg of the siphon. Subject headings: line: formation polarization radiative transfer magnetic fields Sun: photosphere Sun: chromosphere 1. Introduction Siphon flows along loops that connect magnetic field elements of different polarity have been proposed as an alternative mechanism to convective collapse for strengthening photospheric magnetic fields above their equipartition value (Meyer & Schmidt 1968; Thomas 1988). The Bernoulli effect associated with the flow, which is driven by a difference in gas pressure between the loop foot points, reduces the internal gas pressure in the loop and concentrates the magnetic field. Theoretical arguments for the existence of such siphon flows 1 Operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for the National Science Foundation

2 2 have been advocated by Thomas (1988); Montesinos & Thomas (1989, 1993); Degenhardt (1989, 1991); Thomas & Montesinos (199, 1991). Little observational evidence in support of these theoretical models has, however, been presented outside the context of the Evershed flow along penumbral filaments. Degenhardt et al. (1993) presented simulations of Stokes V spectra of the two Fe I infrared lines at and nm, which reproduced the behavior of these lines observed by Rüedi et al. (1992), and indicated the existence of a siphon flow across the neutral line in the observed active region. They concluded that the observed infrared line profile shapes could only be explained by the siphon flow mechanism if the loop carried a critical flow with a standing tube shock in its downstream leg. In the thin tube limit of an isolated magnetic flux tube surrounded by field-free gas the critical speed is the tube speed c t given by c t = [ c 2 a 2 /(c 2 + a 2 ) ] 1 2, (1) where c is the internal sound speed and a is the Alfvén speed, rather than the sound speed (Thomas 1988). Steady solutions of siphon flows along arched isolated thin flux tubes include sub-critical flows with speeds less than c t everywhere, critical flows with speeds greater than c t in part of the solution, and supercritical flows with speeds greater than the tube speed everywhere Thomas (1988). In a critical flow, the flow velocity increases to the tube speed at the top of the arch and continues to accelerate to supercritical flow speeds in the descending part of the arch. At some point in the downstream half of the arch, depending on the internal gas pressure in the downstream footprint the supercritical flow abruptly decelerates to subcritical values through a standing tube shock (Thomas & Montesinos 1991). Just beyond the tube shock the Bernoulli effect associated with the flow is much reduced resulting in a sharp increase of the tube cross section and corresponding reduction of the magnetic field strength. The observational signature of a siphon flow on the solar surface is a pair of magnetic elements of opposite polarity with up flow in one element and a down flow and a somewhat stronger magnetic field (and hence lower internal gas pressure) in the other element. In a sub-critical solution the flow accelerates to its maximum speed (still below c t )atthetopof the arch, and decelerates in the downstream leg of the loop. In a critical siphon flow the up flow is sub-critical, reaches the tube speed at the top of the arch, and then accelerates to supercritical values towards the downstream foot point. At the standing tube shock in the downstream leg a sudden jump in flow speed from super- to sub-critical takes place. Although the siphon flow equations in thin flux tube arches allow for stationary flows, the flow will eventually vanish when it relieves the pressure difference between the two foot points. In this paper we present further evidence of the existence of siphon flows from an

3 3 observation of an active region containing pores of opposite polarity performed in the Ca II nm line. The structure of this paper is as follows. The observations are described in Section 2. In Section 3 the evidence for a supercritical is outlined, and conclusions are given in Section Observations Intensity Intensity y [arcsec] Fig. 1. Images in the continuum (left panel) and line core of the Ca II nm line constructed from the spectra-spectroheliogram of region NOAA 9431 on 21 Apr. 18 at 15:1 UT. The slit was oriented North South with the top of the image towards the North. The rectangle outlines the subfield with several pores that is studied in greater detail. The vertical lines in the middle of the continuum image are caused by passing high clouds. A pair of full spectra-spectroheliogram of active region NOAA 9431 (located at heliographic latitude S 11 and longitude E 4, corresponding to µ =.93) was obtained on April 18, 21 in opposite circular polarizations I +V and I V over a 1. nm window centered on the Ca II nm line using the now decommissioned NSO/KP Vacuum Tower Telescope

4 4 and Spectromagnetograph (Jones et al. 1992). Under spectra-spectroheliogram we understand a scan of a long-slit spectrograph over the solar surface. At each slit position a full pair of spectra (I + V and I V ) is obtained to build up a data cube with two spatial dimensions and the third dimension going over wavelength. Spatial sampling was 1. 14/pixel along the slit and the step size of the slit in the direction of the dispersion was the same. The spectral sampling was 4.5 pm/pixel. The spectral scan spanned 261 steps in the East West direction with a slit length of 485. The observed spectral images were corrected for dark-current and flat fielded, and Stokes I and V were determined by simple addition and subtraction of the two oppositely polarized signals. Figure 1 shows maps of scanned active region in relative intensity at the shortest wavelength in our spectral window (left panel, at about a relative intensity of.9 of the continuum), in the line core of the Ca II nm line (right panel). A comparison of the mean quiet-sun spectrum (obtained by averaging the line profile over the upper one third of the full field displayed in Figure 1 with the corresponding profile from the McMath Pierce/KP Fourier Transform Spectrometer (FTS Brault & Neckel 1987; Neckel 1999) was used to determine the amount of scattered light in the instrument at 9%, which was accounted for. B LOS [Gauss] y [arcsec] Fig. 2. Subarea of the MDI full-disk magnetogram taken at 15:1 UT, the beginning of the spectra-spectroheliogram. In the remainder of the paper we will concentrate on the region marked by the black rectangle in the left panel of Figure 1, which has magnetic field patches of mixed polarity. A

5 5 subarea of the MDI full-disk magnetogram taken at 15:1 UT, coinciding with the start of the spectral scan and covering the same region, is shown in Figure 2. Magnetic field values in this graph were scaled with the calibration factor of 1.56 as suggested by Berger & Lites (23) from comparison of MDI magnetograms with field strength values obtained with the HAO/NSO Advanced Stokes Polarimeter (ASP). The magnetogram clearly shows the two pores in the field as the two strongest field concentrations with opposite polarity. In between the two pores there there is a field with smaller magnetic elements of mixed polarity. 3. Supercritical flows When we tune in wavelength through the data cube of nm line profiles, some remarkably large redshifts appear in the mixed polarity region marked in Figure 1. In the next section we examine these large shifts and make plausible that the associated velocities are consistent with a siphon flow from a weak positive polarity patch of magnetic field towards the stronger negative polarity field of one of the pores in region Observed flows From our data cubes of left- and right-circularly polarized intensity we made maps of the (near) continuum intensity, the intensity just redward of line centre, the line-of-sight (LOS) velocity in the nm line, and the LOS magnetic field. These are displayed clockwise, starting from the upper left, in Figure 3. The LOS velocity was calculated from the shift of the line core with respect to the centre of the quiet-sun profile used to calibrate the observations to the atlas profile. We used the difference in the center-of-gravity (COG) of the two opposite polarizations in the Ca II line to compute the chromospheric LOS magnetic field B LOS (see Rees & Semel 1979; Uitenbroek 23). The derived magnetic field configuration in the nm line is very similar to the one obtained by MDI (Figure 2) in the photosphere, again showing the pores of opposite polarity and the field of mixed polarity elements in between them. As an exception the string of negative polarity near the bottom of Figure 2 does not have an equivalent in the chromospheric magnetogram. Tuning through the data cube of nm line profiles we see the absorption minimum move along the line segment indicated in the four panels in Figure 3, following as it were the path of the flow along the tube in the three-dimensional space of the data cube. This path is especially obvious when the tuning is done rapidly back and forth on a computer screen. To make the endpoints of this velocity pattern clear we display the intensity at λ nm, just

6 6 Intensity Intensity λ = nm λ = nm y [arcsec] 6 4 y [arcsec] B LOS [Gauss] v LOS [km s 1 ] y [arcsec] 6 4 y [arcsec] Fig. 3. Clockwise from the upper left: Maps of continuum intensity, intensity just redward of the core, LOS velocity, and LOS magnetic field strength determined from the circular polarization in the nm line. The line segment marks the path of the flow with the characteristics of a siphon. Wavelengths of the top panels are marked in Figure 4.

7 7 redward of the central wavelength at rest, in the upper right of Figure 3. The foot points are easily recognized as the bright and dark elements at the ends of the line segment marking the flow path. These patches are the result of the Doppler shifts at the foot points: with blue-shifted material appearing bright, and redshifted material appearing dark relative intensity KPNO atlas Upstream footpoint Downstream footpoint wavelength [nm] Fig. 4. Ca II line profiles at the up- (dot-dashed curve) and downstream foot point (dashed curve), compared with the spatially averaged disk center profile from the KPNO atlas. The dotted vertical lines mark the wavelengths of the images in the top panels of Figure 3. The bottom right panel in Figure 3 shows a Doppler map of the region determined by measuring the position of the central absorption of the nm line with respect to the average quiet-sun profile. An extremely high redshifts is seen near the bottom right of the pore with negative polarity on the left side in Figure 2 and 3 (lower left panel), at the upper left foot point of the flow channel. Translating this redshift to a velocity we find a down flow of well over 24 km s 1. The blueshift at the other foot point corresponds to an up flow of about 5-6 km s 1. Characteristic line profiles of the two foot points are compared with the average quiet-sun profile from the intensity atlas in Figure 4 The line profiles in the area of high red-shift at the ending foot point have an absorption satellite to blue of the core (dashed curve). In the very core of these strong down flows the shape of the line profiles is even more complicated than the profiles in figure 4 without a clear absorption minimum, making velocity determination ambiguous. Hence the seeming reversal of velocity in the centers of the strong down flows in the Doppler panel in figure 3.

8 8 At the upstream foot point the line profile is opposite in shape of that at the downstream foot, with a satellite absorption on the red side of the blueshifted absorption core (dot-dashed line in figure 4, indicating an up flow in the higher layers. While the down flow in the downstream foot point is clearly visible in the LOS velocity map in the lower right panel of Figure 3, the up flow at the opposite end is part of a more diffuse up flow region, perhaps containing foot points of other loops as well (a clear example is seen just right of the indicated flow channel). Smaller concentrated down flows are seen in the upper middle of the subarea, again indicating the presence of other flow channels in the field of view. 1 velocity [km s 1 ] path length [arcsec] Fig. 5. Line-of-sight velocity of the flow along the path from the upwind to the downwind footpoint indicated in Figure 3. The LOS velocity along the flow channel indicated in Figure 3 is plotted in Figure 5 as a function of the position along the line segment starting at the upstream foot point. At the upstream foot point material flows up with 5-6 km s 1 (projected to the LOS), then its projected velocity decreases, and the material accelerates downward till it reaches 27 km s 1 in the direction away from the observer. This is much larger than the sound speed of about 8kms 1 in the low chromosphere and the critical tube speed, which is comparable to the sound speed. Inspection of the two magnetograms shows that the flow originates in a small patch of positive polarity and end at the higher strength negative polarity at the edge of the upper left pore (lower left panel Figure 3). The flow therefore, has all the characteristics of a siphon flow

9 9 going between two opposite polarities while originating from the one with lower magnetic pressure, starting with a subcritical up flow, and then accelerating downwards toward the opposite foot point. In the next section we make clear with simple ad-hoc radiative transfer modeling that the shape of the line profile in the downstream foot point is characteristic of a shock front, thus exposing one more characteristic of a siphon flow Simple transfer modeling computation In this section we make plausible that the shape of the nm line in the downstream foot point is consistent with the presence of a velocity discontinuity between about 1 to 13 km, indicative of a shock front at this height. This shock front could be the termination shock of the siphon flow. We use the Non-LTE radiative transfer code developed by Uitenbroek (21) to calculate profiles of the Ca II nm line a model of the quiet Sun (Fontenla et al. 1993) with an ad-hoc macroscopic velocity. This code extends the Multi-Level Accelerated Lambda Iteration (MALI) scheme of Rybicki & Hummer (1991, 1992) to include the effects of Partial Frequency Redistribution (commonly abbreviated to PRD), which is needed in the case of the solar Ca II H and K lines. A standard five-level plus continuum model (Uitenbroek 1989) was used. The chosen velocity stratification, shown in the top panel of figure 6, features a flow that accelerates downward from a zero velocity to 17 km s 1 at a height of about 13 km, the flow then decelerates sharply to 7 km s 1 over a few hundred km, and finally decelerates more slowly to zero at the bottom of the atmosphere. The resulting line profile, shown with the dashed curve in the bottom panel of 6, is qualitatively very similar to the dashed profile in 4. The latter profile has a considerably higher intensity in its core and inner wings, indicating that the temperature in the region of the steep velocity gradient is hotter than the corresponding region in the quiet-sun model. This characteristic shape of the ad-hoc line profile with the absorption satellite on the blue side results from a static or relatively slow moving atmosphere below the discontinuity and the redshifted core results from a strong down flow above it. Varying the height at which the discontinuity in velocity occurs we find that the characteristic line shape only appears when it occurs slightly above 1 km. In addition, velocities of 17 km s 1 or higher are needed to create the second absorption satellite on the blue side (this is actually the almost unshifted component). The profile of the downstream foot point, therefore, is consistent with the presence of a sharp velocity discontinuity between the formation heights of the core and wing of the line, which corresponds to about 1 kilometers above the photosphere.

10 1 5 Velocity [km s 1 ] Height [km] 1..8 Relative Intensity KPNO atlas Ad hoc model Wavelength [nm] Fig. 6. Ad hoc velocity stratification (upper panel) and resulting nm line profile (dashed curve) compared with the solar atlas profile. 4. Conclusions We have analyzed observations of the Ca II nm infrared triplet line. Since we registered both circular polarizations simultaneously these observations allow us to make maps of the chromospheric LOS velocity and magnetic fields. One remarkable feature in the observations is a flow that apparently originates with a subsonic up flow in a patch of weak magnetic field of positive polarity. We can follow this flow to an opposite foot point at a pore of opposite polarity with higher absolute field strength. At this downstream foot point the flow accelerates downward until it reaches speeds of 27 km s 1 in the LOS. With simple radiative transfer modeling with an ad-hoc velocity distribution we show that the shape of the nm line profile at this foot point is consistent with the presence of a sharp discontinuity in the downward velocity at about 1 13km. The flow remained visible for at least half an hour (it appears in another scan taken after the present one not

11 11 analyzed here). All these properties lead us to interpret the observed flow as a siphon driven flow from a patch of weaker positive polarity magnetic field to the stronger field of opposite polarity at the edge of one of the pores in the mixed polarity subregion indicated in Figure 3. The sharp velocity discontinuity at the downstream foot point is interpreted as the termination shock of the siphon flow. This shock must occur in the chromosphere as derived from the characteristic shape of the nm profile (Figure 4), but also from the fact that there is no discernible brightening in the continuum image (upper right panel in Figure 3) at the foot point. The help of Jack Harvey, Harry Jones and Elena Malanushenko with the observations at the KPVT is gratefully acknowledged. This research has made use of NASA s Astrophysics Data System (ADS). REFERENCES Berger, T. E., & Lites, B. W. 23, Sol. Phys., 213, 213 Brault, J. W., & Neckel, H. 1987, Spectral Atlas of Solar Absolute Disk-averaged and Diskcenter Intensity from 329 to 1251 Å, available with anonymous FTP at ftp.hs.unihamburg.de/pub/outgoing/fts-atlas Degenhardt, D. 1989, A&A, 222, 297 Degenhardt, D. 1991, A&A, 248, 637 Degenhardt, D., Solanki, S. K., Montesinos, B., & Thomas, J. H. 1993, A&A, 279, L29 Fontenla, J. M., Avrett, E. H., & Loeser, R. 1993, ApJ, 46, 319 Jones, H. P., Duvall, T. L., Harvey, J. W., Mahaffey, C. T., Schwitters, J. D., & Simmons, J. E. 1992, Sol. Phys., 139, 211 Meyer, F., & Schmidt, H.-U. 1968, Z. Angew. Math. Mech., 48, T218 Montesinos, B., & Thomas, J. H. 1989, ApJ, 337, 977 Montesinos, B., & Thomas, J. H. 1993, ApJ, 42, 314 Neckel, H. 1999, Sol. Phys., 184, 421

12 12 Rees, D. E., & Semel, M. D. 1979, A&A, 74, 1 Rüedi, I., Solanki, S. K., & Rabin, D. 1992, A&A, 261, L21 Rybicki, G. B., & Hummer, D. G. 1991, A&A, 245, 171 Rybicki, G. B., & Hummer, D. G. 1992, A&A, 262, 29 Thomas, J. H. 1988, ApJ, 333, 47 Thomas, J. H., & Montesinos, B. 199, ApJ, 359, 55 Thomas, J. H., & Montesinos, B. 1991, ApJ, 375, 44 Uitenbroek, H. 1989, A&A, 213, 36 Uitenbroek, H. 21, ApJ, 557, 389 Uitenbroek, H. 23, ApJ, 592, 1225 This preprint was prepared with the AAS L A TEX macros v5.2.