Proposer:
Name: Carsten Denker, Horst Balthasar, Rohan E. Louis, Klaus G.
Puschmann, and Meetu Verma (AIP)
Na Deng, Chang Liu, Haimin Wang (NJIT, SWRL)
Christian Beck (IAC)
E-mail: cdenker@aip.de
Contact person in HINODE team:
Name: Tom Berger
E-mail: berger@lmsal.com
Name: Takashi Sekii
E-mail: sekii@solar.mtk.nao.ac.jp
Abstract of observational proposal:
Our scientific interest for the present HOP is focused on complex active regions and regular sunspots. We therefore propose three different science cases:
The first science case is focused on photospheric shear flows and the evolution of magnetic shear along the magnetic neutral line in potential CME source regions. There are two important aspects concerning the evolution of active regions that are closely associated with solar flares and CMEs: new flux emergence and unusual flow motions. Emerging flux regions (EFRs) are closely
associated with solar flares and the onset of CMEs (e.g., Feynman and Martin 1995, JGR 100, 3355). The newly emerged flux perturbs the existing magnetic
configuration above the solar surface and thus triggers a flare or CME. The strong, long-lasting shear flows near the magnetic inversion line have also been identified as precursors of flares and CMEs (e.g., Meunier and Kosovichev 2003,
A&A 412, 541; Yang et al. 2004, ApJL 617, 151; Deng et al. 2006, ApJL 617, 151).
Flux emergence and complicated flow patterns on the solar surface may also be associated with each other (Tanaka 1991, SoPh 136, 133; Zirin and Wang 1993,
Nature 363, 426; Deng et al. 2006, ApJL 617, 151). Flare-associated shear flows are not a new phenomenon. During the major flares in August 1972, Zirin and
Tanaka (1973, ApJL 617, 151) measured spectroscopically strong photospheric velocity discontinuities of up to 6 km s1 in the vicinity of neutral lines. In more recent observations, converging Doppler velocities were observed in the
vicinity of delta-spots (e.g., Lites et al. 2002, ApJ 575, 1131). These flows were interpreted as interleaved systems of field lines, where the outward Evershed flow sharply bends downward and returns to the solar interior. The observed photospheric flows are related to the magnetic fields of the more horizontal Evershed flow channels. A connection to the more vertical fields of the uncombed penumbra (Solanki and Montavon 1993, A&A 275, 283; Martinez Pillet 2000, A&A 361, 734) still needs to be established, since these are the fields straddling the magnetic neutral line. However, this task requires observations with diffraction-limited spectroscopy and polarimetry with good spectral resolution. This would also help to establish a closer link between the build-up of magnetic shear and photospheric shear flows.
The second science case will focus on the Evershed Flow (EF; Evershed 1909, MNRAS 69, 454), which is a characteristic property of sunspot penumbrae that comprises a nearly horizontal, radially directed outflow of plasma. The EF starts as weak upflows (Rimmele & Marino 2006, ApJ 645, 593) in the deep layers of the inner penumbra and ends in a ring of downflow channels in the mid and outer penumbra (Bellot Rubio et al. 2004, A&A, 427, 319). The Evershed mass flux returning to the photosphere is associated with a polarity opposite to that of the sunspot and these downflows can sometimes be supersonic (del Toro Iniesta et al. 2001, ApJ 549, 139; Bellot Rubio et al., 2004 A&A, 427, 319). Apart from the EF, there exist at least two different mass motions that are unrelated to the Evershed effect. The first are small downflowing sites measuring ~1 km/s that are typically 0.5" in size. Katsukawa & Jurcak (2010, A&A 524, A20) find these flows to be distributed in the penumbra, predominantly on the centre side and have the same polarity as the sunspot. Some of these downflows are associatedwith penumbral microjets seen in the chromosphere (Katsukawa et al. 2007 Science 318, 1594). Another example of a non-Evershed type phenomenon is the existence of supersonic downflows (>7 km/s) that are seen in localized patches in light bridges (Louis et al. 2009, ApJ 704, L29) and the umbra-penumbra boundary (Louis et al. 2011, ApJ 727, 49) of sunspots. These locations exhibiting supersonic red-shifts are much bigger in size (1.5-6 arcsec^2) and also have the same polarity as the sunspot. The chromosphere above these downflowing patches is characterised by highly intense and long-lived brightness enhancements which are 1-2 arcsec^2 in area. These downflows could be produced by a slingshot effect from magnetic reconnection between adjacent penumbral filaments (Ryutova et al. 2008, ApJ 686, 1404) or by magneto-convection as suggested by Katsukawa & Jurcak (2010, A&A 524, A20) for the subsonic velocity patches. The possibility of an inverse Evershed effect which is known to be a chromospheric phenomenon also cannot be ruled out. Our study will address the following - (i) what are the typical lifetimes of these downflows and are they spatially recurrent, (ii) what are the corresponding signatures in the magnetic field, (iii) how does the mass motion change in the solar atmosphere, (iv) is there a one-to-one correspondence between the photospheric downflows and chromospheric brightness enhancements.
The third science case will attack the open question concerning the presence of overturning convection in sunpsots. Sunspots, the locations of the strongest magnetic fields in the solar photosphere, appear darker than the surrounding field-free quiet Sun. The reason is the suppression of the convective energy transport by magnetic fields. Without magnetic fields, overturning convective cells exist that transport hot material upwards in their centers. The hot material cools radiatively at the solar surface, flows to the side, and descends to be re-heated again in deeper atmosphere layers, and the cycle starts anew. The necessary lateral motion is impossible for an ionized plasma in the presence of vertical or only slightly inclined magnetic fields. Sunspots, however, show two different intensity levels, the extremely dark umbra and a halo with a brightness intermediate between the umbra and the surrounding quiet Sun, the filamentary penumbra. The energy flux through the penumbra is about 75% of the quiet Sun (e.g., Schlichenmaier et al. 1999, A&A 349, 961), but without convective processes such a high intensity level is difficult to explain (Spruit & Scharmer 2006, A&A 477, 343; Scharmer & Spruit 2006 A&A 460, 605; SS06 from now on). SS06 thus suggested the existence of field-free gaps inside the penumbra in which convective energy transport is possible. The evidence on the existence of such field-free gaps is, however, not conclusive (Borrero & Solanki 2008, ApJ 687, 668; Zakharov et al. 2008, A&A 488, L17; Bellot Rubio et al. 2010, A&A 725, 11; Bharti et al. 2010, ApJ 722, L194; Puschmann et al. 2010, ApJ 720, 1417; Rempel 2011, ApJ 750, 14R), with both confirmations and rejections of the existence of field-free gaps or overturning convection in the penumbra. The main signature of overturning motions will be in the line-of-sight (LOS) velocity that would have to show close-by upflows and downflows. The necessary field-free volumes would show up in the polarization signal ofZeeman-sensitive spectral lines as a reduction or complete absence of polarization signal. Observations with SOT/SP will provide important information about the magnetic field while coobservations with GREGOR will provide information about the flow field at the smallest spatial scales. Such data will be ideally suited to confirm or reject the presence of overturning convection inside sunspot penumbrae. Our study will address the following: (i) Derivation of LOS velocities from the line-core position in Stokes I (GREGOR, HINODE, VTT) and the zero-crossing in Stokes V (HINODE, VTT). (ii) Determination of the field strength and magnetic flux from an inversion of the SP-, TESOS- and TIP-spectra.
(iii) Search for up/downflows, investigate their locations relative to (a) filaments seen in Hinode G-band data, (b) flow filaments in Stokes I and V, (c) the magnetic field strength/flux. (iv) Investigate the flow pattern at difference heliocentric angles. Overturning convection should not show strong differences between the limb and the center side of the sunspot. v) Reduced field strength/flux or polarization signal can indicate field-free areas. vi) Determine the flow direction from azimuthal curves and magnetic field orientation. Agreement/mismatch indicates field-aligned flows or overturning convection unrelated to the field direction. (vii) The Calcium imaging data can be used to confirm/reject a continuation of the flows into the lower chromosphere.
Request to SOT
~1860 Mbits/day
Combination of SP and FG observation with SP as first priority. The observing procedure is the same for every day during the coordinated observing run. Only the target might change.
[SOT-SP]
~340 Mbits/hr, 1360 Mbits/day
Fe I 630.15 nm and 630.25 nm context SP IQUV scan (fast map, for 123" x 123", 0.32" slit, 23 min) every half an hour.
[SOT-FG] ~235 Mbits/hr, 940 Mbits/day
Ca II H 396.8 nm and G-band 430.5 nm image sequences (90 sec cadence, 111" x 111" FOV, 2 x 2 pixel binning).
Other participating instruments:
[Vacuum Tower Telescope (VTT)]
TESOS (Kentischer et al. 1998, A&A 340, 569) with the Vector Imaging Polarimeter (VIP): With TESOS we will take time series in H-alpha 656.3 nm, Fe I 630.2 nm, 612.2 nm, 617.3 nm, and Cr 578.2 nm with dispersion of 2-3 pm and a FOV of 70" x 40".
Tenerife Infrared Polarimeter (TIP, Collados et al. 2007, ASP Conf Ser. 368, 611) at the Echelle spectrograph: TIP will be used to obtain large area maps (100" x 80"), which will be repeated every hour. We will use the He 1083.0 nm, Fe 1078.3 nm and Si 1078.6 nm lines with a step width of 0.35" and a dispersion of 2.2 pm/pixel.
All spectroscopic data will be accompanied by Ca II H and G-band images to facilitate co-alignment with Hinode/SOT and GREGOR.
[GREGOR solar telescope]
GREGOR Fabry-Perot Interferometer (GFPI, e.g., Denker et al. 2010, Proc. SPIE 7735, 6M; Puschmann et al. 2012, 2011arXiv1111.5509P): With the GFPI two spectral lines can be sequentially scanned (FOV: 52" x 40", no polarimeter, cadence 10-60 s (depending on number of wavelength points and repititions), spectral lines: Fe I 543.4 nm, Fe I 557.6 nm, Fe I 569.1 nm, Na I D2 589.0 nm, Na I D1 589.6 nm, Fe I 617.3nm, Halpha). Observing without a polarimeter doubles the GFPI FOV. Therefore, we rely on VTT/TESOS/TIP and Hinode/SOT/SP data to provide the magnetic field information.
All spectroscopic data will be accompanied by Ca II H and G-band images to facilitate co-alignment with Hinode/SOT and VTT.
[ChroTel] (Kentischer et al. 2008, Proc. SPIE 7014, 13): Chromospheric full-disk images (Ca II K, H-alpha, He I 1083 nm, 2k x 2k pixels).
Remarks
[Time period] 2012/06/19-2012/06/30. This time period is flexible by +/- 3 days. The GREGOR solar telescope will undergo science verfication in 2012. The requested time period covers the first science verification campaign of the GFPI. Sunspots and pores are the targets of several observing runs with different setups (spectral lines, cadences, etc.) and scientific goals.
Observations on consecutive days are desired, but not necessary. Like other ground-based HOPs, it requires fairly high priority until some good data is obtained at Tenerife, then priority can drop down. We will inform the Hinode team well in advance about the local weather conditions.
[Time window] The best seeing conditions at Tenerife are from 8-12 UT in June.
Continuous observations within that time window are requested to cover the temporal evolution of sunspots and pores.
[Target] Any complex active region or regular sunspot, where a small (less than 10" in diameter) pore or umbral core can be used to lock the VTT and GREGOR AO systems. During science verification, several observing proposals by other scientists are available for queue observation, which involve sunspots and pores. Thus, also more quieter settings of active regions are potential targets.
[Contact person] Carsten Denker, e-mail: cdenker@aip.de, phone: +49-331-7499-297
(office Potsdam) and +34-992-329-142 (VTT/GREGOR) |
|