SOHO JOINT OPERATIONS PROGRAMME 082 STUDY OF THE PROCESS OF MAGNETIC FLUX DISAPPEARANCE IN CANCELING BIPOLES Collaborators: Karen L. Harvey (SPRC), M. Penn (NSO/NOAO), H. P. Jones (NASA), D. Hassler (SWRI), J. B. Gurman (EIT), A. Fludra (CDS), C. Schrijver (MDI, TRACE) Objectives: (1) study of the process of magnetic flux disappearance in small-scale canceling bipoles to determine whether or not the magnetic fields in these structures are retracting below the photosphere. (2) determine as a function of height and time the magnetic field, velocity field, and physical parameters (e.g. temperature and density) in canceling bipoles. Scientific Background: --------------------- The mutual disappearance (`cancellation') of opposite polarity magnetic flux in small bipoles is a common occurrence in surface photospheric magnetic field observations, removing a substantial fraction of the observed magnetic flux from the surface. There is little observational evidence, however, to establish if the magnetic flux in these canceling bipoles is submerging below the surface or continuing to emerge into the solar atmosphere from the photosphere. On time-scale of a solar cycle, essentially all of the magnetic flux that emerges in active regions (a total of 10^25 Mx) disappears from the surface. Studies of the evolution of vector magnetic fields in active regions, expanding active region loops, and coronal mass ejections suggest that the disappearance of magnetic flux from the surface occurs by the submergence or retraction of bipolar fields (Rabin et al., 1984; Parker, 1984; Howard, 1992) and by the motion of the fields from the photosphere into the corona as a closed system (Lites et al., 1995), by expansion (Uchida et al., 1992), and by ejection (Low and Hundhausen, 1994). Outside of active regions, a substantial amount of magnetic flux on the sun disappears within small-scale canceling bipoles as magnetic network element of opposite polarity converge (Harvey and Harvey, 1976; Livi et al., 1985). This observing proposal is proposed a collaborative research effort to acquire and compare a set of time-sequence observations that sample simultaneously and as a function of time the height structure of the magnetic field, velocity flows, intensity, temperature, and density in small-scale canceling bipoles. This proposal overlaps with JOP001. Observational Program: ---------------------- The observational requirements are high-spatial resolution (1-3 arc-seconds), moderate cadence (5-15 minutes), large area coverage (> 2 x 2 arc-minutes), and an observing period of at least 6 hours each day. We are requesting a five-day observing run during the second half of June 1998. The area targeted will be the quiet sun at disk center, which at this time of the cycle is devoid of active regions and is comprised primarily of mixed polarity network elements. Individuals have been contacted at each of the experiments and observatories for preliminary discussions of the types of observations to be taken as part of this collaborative effort. The requested observations are presented below in general terms based on those conversations. Additional discussions will follow in the next few weeks to specify in more detail the observational program at each participating facility. To follow the process of cancellation as long as possible, both ground- and spaced-based observations are requested to continue for approximately 8 hours each day. Ground-Based Observations: -------------------------- NSO/Kitt Peak Spectromagnetograph (K. Harvey, M. Penn, H. Jones): Central to this study are simultaneous observations of the chromospheric and photospheric line-of-sight magnetic and velocity fields. Using this instrument, the full line profile of the CaII 8542\AA will be recorded at each 1.15 arc-second pixel within a 8.5 x 8 arc-minutes field-of-view at a cadence of 7 minutes. An analysis of the right- and left-circular polarization states in the CaII 8542 chromospheric line and of the FeI 8538\AA photospheric line yields images of the magnetic fields, velocity fields, and intensity at these two levels of the solar atmosphere. These observations will be made under identical seeing and instrumental conditions, eliminating problems in the co-alignment of the set of chromospheric and photospheric images. M. Penn finds a height difference of about 2000 km between the CaII 8542 and FeI 8538 lines, providing an adequate separation to determine the respective magnetic field changes between these two levels of the solar atmosphere. HAO CHIP Imager: We will request access to the HAO/CHIP daily full-disk He I 10830 images. This instrument takes full-disk images in He I $\lambda$ 10830 at a cadence of 3 minutes and with 2 arc-second pixels. BBSO (Phil Goode, Bill Marquette): We will request high-spatial and temporal resolution images in H$\alpha$ (line center, blue wing), CaII K, and of the longitudinal magnetic fields within the target area. SOONSPOT (Alan Kiplinger): Activity permitting, we will request high-resolution H$\alpha$ observations of the quiet sun target area. Acquisition of these data will likely depend on the level of activity. Space-Based Observations: ------------------------- We request that the SOHO CDS, EIT, and MDI observations be taken for 8 hours each observing day and during the daylight hours in the US. The Yohkoh/SXT observations would be restricted only to those direct contact orbits, activity permitting. SOHO CDS (Andrzej Fludra): we are requesting high-resolution observations in several lines as temperature, density diagnostics and the MgIV/NeIV abundance. For the latter parameter, the ratio of these two lines should give us some idea of whether the abundances are typical of the photosphere or corona (Sheeley, 1995) within the structures associated with canceling bipoles. We are requesting high-spatial resolution images within a 2 x 2 arc-minute field-of-view. The selected sequence of images should have a cadence of no longer than 30 minutes. An observing sequence for this program is currently being developed by A. Fludra. Within the target area, CDS will target the brightest features to optimize the measurements with this instrument. We will request that ground-based observations with field-of-views smaller than that of the NSO/KP images be centered on the CDS target. SOHO EIT (Joe Gurman): we request images in the HeII 304 and FeXII 195 lines within a subfield (~8 x 8 arc-min) of the target region. A cadence of 15 minutes should be sufficient for the purposes of this observing program. SOHO MDI (Ted Tarbell?, Karel Schrijver?): we request high-resolution magnetograms. Our target area (at disk center) will fall within the fixed MDI area. Requested cadence of the observations should be sufficient to allow summing successive images to help in reducing the noise level. Yohkoh/SXT (Hugh Hudson?): we will request high resolution (2.5 arc-pixels) images in the AlMg filter within a 5 x 5 arc-minute area. Acquisition of these images (and their exposure times) depends on the level of activity during the collaborative observing effort. TRACE (Karel Schrijver): we request images in Ly$\alpha$ and several other lines for temperature and density diagnostics. A program for these observations is being developed by Karel Schrijver. Observing Run: -------------- We are requesting one five-day observing run sometime during an interval extending from June 12 to June 30, 1998. The latter part of this period coincides with the overlap in the direct contact orbits between ISAS and Yohkoh and the observing times at NSO/KP. However, the monsoon weather also can begin in the southwest in late June. References: ----------- Harvey, K. L. and Harvey, J. W.: 1976, Air Force Report AFGL-TR-76-0255, p. 35; also in Doctoral Thesis, K.L. Harvey, Chapter 6. Howard, R. F.: 1992, in K. L. Harvey, `The Solar Cycle', ASP Conference Series, 27, 297. Livi, S. H. B., Wang, J., and Martin, S. F.: 1985, Austral. J. Phys., 38, 855. Lites, B. W., Low, B. C., Mart\'enez Pillet, V., Seagraves, P., Skumanich, A., Frank, Z. A., Shine, R. A., and Tsuneta, S.: 1995, ApJ, 446, 877. Low, B. C. and Hundhausen, A. J.: 1994, ApJ, 443, 818. Parker, E. N.: 1984, ApJ, 280, 423. Rabin, D., Moore, R., and Hagyard, M. G.: 1984, ApJ, 287, 404. Sheeley, N.R., Jr.: 1995, ApJ 440, 884. Uchida, Y., McAllister, A., Strong, K. T., Ogawara, Y., Shimizu, T., Matsumoto, R. and Hudson, H. S.: 1992, PASJ, 44, L155.