linkages between the atmosphere on the Earth and sun Phần 5

206 L.R. Bellot Rubio measurements at very high spatial resolution. With 0.100 it should be possible to determine the flow field across penumbral filaments, resolving internal fluctuations smaller than the width of the filaments themselves. Hopefully, this kind of observa-tions will be provided soon by instruments like IMaX aboard SUNRISE or CRISP at the Swedish Solar Telescope. 5 Conclusions The Evershed flow exhibits conspicuous fine structure at high angular resolution. It occurs preferentially in the dark cores of penumbral filaments, at least in the inner penumbra. The flow is magnetized and often supersonic, as demonstrated by the observation of Stokes V profiles shifted by up to 9kms1. At each radial distance, the flow is associated with the more inclined fields of the penumbra; in the inner penumbrathis happens in the bright filaments, while in the outer penumbrathe dark filaments havethe largestinclinations.Theflow is also associatedwith weakerfields (except perhaps near the edge of the spot). High-resolution magnetograms by Hinode show the sources and sinks of the Evershed flow with unprecedented clarity, confirming earlier results from Stokes inversions at lower resolution: on average, the flow points upward in the inner penumbra,then becomes horizontalin the middle penumbra,and finally dives down below the solar surface in the outer penumbra. The Hinode observations reveal tiny patches of upflows concentratedpreferentiallyin the inner penumbraand patches of downflows in the mid and outer penumbra; presumably they correspond to the ends of individual flow channels. Recent numerical calculations by Ruiz Cobo and Bellot Rubio (2008) have demonstrated that Evershed flows with these properties are capable of heating the penumbra very efficiently, while reproducing many other observational features such as the existence of dark-cored penumbral filaments. This result strongly sug-gests that the radial Evershed flow is indeed responsible for the brightness of the penumbra. At the same time, there have been observationsof small-scale motionsin penum-bral filaments that could reflect the existence of overturning convection (Ichimoto et al. 2007b; Zakharov et al. 2008; Rimmele 2008). Convection is an essential in-gredient of the field-free gap model proposed by Spruit and Scharmer (2006) and seems to occuralso in MHD simulationsofsunspots(Rempel et al. 2009).However, other spectroscopic observations at 0.200 do not show clear evidence for downflows in filaments near the umbra/penumbraboundary (Bellot Rubio et al. 2005). It is important to clarify whether or not convection exists in the penumbra. To investigate this issue we need spectroscopic observations at 0.100. Narrow lanes of downflows should show up clearly in those measurements. Only then will it be pos-sible to assess the contribution of overturning convection to the brightness of the penumbra and compare it with that of the supersonic Evershed flow. Ultimately, these efforts should reveal the primary mode of energy transport in the penumbra. A Topology for the Penumbral Magnetic Fields J. Sanchez Almeida Abstract We describe a scenario for the topology of the magnetic field in penumbrae that accounts for recent observations showing upflows, downflows, and reverse magnetic polarities. According to our conjecture, short narrow mag-netic loops fill the penumbral photosphere. Flows along these arched field lines are responsible for both the Evershed effect and the convective transport. This scenario seems to be qualitatively consistent with most existing observations, including the dark cores in penumbral filaments reported by Scharmer et al. Each bright filament with dark core would be a system of two paired convective rolls with the dark core tracing the common lane where the plasma sinks down. The magnetic loops would have a hot footpoint in one of the bright filament and a cold footpoint in the dark core. The scenario fits in most of our theoretical prejudices(siphon flows alongfield lines, presence of overturning convection, drag of field lines by downdrafts, etc). If the conjecture turns out to be correct, the mild upward and downward velocities observedin penumbraemust increase uponimprovingthe resolution.This andother observational tests to support or disprove the scenario are put forward. 1 Introduction We arecelebratingthecentenaryofthe discoverybyJohnEvershed(1909)ofthe ef-fectnowbearinghisname.Photosphericspectrallinesinsunspotsaresystematically shifted toward the red in the limb-side penumbra, and toward the blue in the center-side penumbra. A 100 years have passed and, despite the remarkably large number of works on the Evershed effect,1 we still ignore how and why these line shifts are produced (see, e.g., the review paper by Thomas and Weiss 2004). Thus, the Evershed effect is among the oldest unsolved problems in astronomy. Although its study has never disappeared from the specialized literature, the Evershed effect has J. Sa´nchez Almeida () Instituto de Astrof´ısica de Canarias, La Laguna, Tenerife, Spain 1 The NASA Astrophysics Data System provides more than 1,400 papers under the keyword penumbra, 70 of them published during the last year. S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 210 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 16, Springer-Verlag Berlin Heidelberg 2010 A Topology for the Penumbral Magnetic Fields 211 undergonearecentrevivaltriggeredbytheadventofnewinstrumentation(Scharmer et al. 2002; Kosugi et al. 2007), original theoretical ideas (Weiss et al. 2004; Spruit and Scharmer 2006), as well as realistic numerical simulations (Heinemann et al. 2007; Rempel et al. 2009). Unfortunately, this renewed interest has not come together with a renewal of the diagnostic techniques, that is, the methods and pro-cedures that allow us to infer physical properties from observed images and spectra. Often implicitly, the observers assume the physical properties to be constant in the resolution element, a working hypothesis clearly at odds with the observations. Spectral line asymmetries show up even with our best spatial resolution (Ichimoto et al. 2007a;Sa´nchez Almeidaet al. 2007,Sect.2). This lack of enoughresolutionis not secondary. The nature of the Evershed flow has remained elusive so far because we have been unable to isolate and identify the physical processes responsible for the line shifts. Different measurements provide different ill-defined averages of the same unresolved underlaying structure, thus preventing simple interpretations and yielding the problems of consistency that plague the Evershed literature (e.g., non-parallelism between magnetic field lines and flows, Arena et al. 1990; violation of the conservationof magnetic flux, Sa´nchez Almeida 1998;non-parallelismbetween continuum filaments and magnetic field lines, Ka`lma`n 1991). Understanding the observed spectral line asymmetries complicates our analysis but, in reward, the asymmetries provide a unique diagnostic tool. They arise from sub-pixel variations of the magnetic fields and flows; therefore, by modeling and interpretation of asymmetries, one can get a handle on the unresolved structure. Although indirectly, such modeling allows us to surpass the limitations imposed by the finite resolution. The idea has tradition in penumbral research, starting from the discoveryof the asymmetries almost 50 years ago (e.g.,Bumba 1960;Grigorjevand Katz 1972). Sa´nchez Almeida (2005, hereinafter SA05) exploits the tool in a sys-tematic study that encompasses a full round sunspot. The unresolved components found by SA05 inspire the topology for the penumbral magnetic fields proposed here. According to SA05, the asymmetries of the Stokes profiles2 can be quantita-tively explained if magnetic fields having a polarity opposite to the sunspot main polarity are common throughout the penumbra. The reverse polarity holds intense magnetic field aligned flows which, consequently, are directed downward. Counter-intuitive as it may be, the presence of such ubiquitous strongly redshifted reverse polarity has been directly observed with the satellite HINODE (Ichimoto et al. 2007a). This new finding supports the original SA05 results, providing credibility to the constraints that they impose on the magnetic fields and mass flows. The exis-tence of such ubiquitous return of magnetic flux, together with a number of selected results from the literature, are assembled here to offer a plausible scenario for the penumbral magnetic field topology. Such exercise to piece together and synthesize information from different sources is confessedly speculative. It will not lead to 2 Weuse Stokes parameters tocharacterize thepolarization;I for theintensity, Q andU for thetwo independent types of linear polarization, and V for the circular polarization. The Stokes profiles are representations of I, Q, U, and V vs. wavelength for a particular spectral line. They follow well defined symmetries when the atmosphere has constant magnetic field and velocity (see, e.g., Sa´nchez Almeida et al. 1996). A Topology for the Penumbral Magnetic Fields 213 Voort et al. 2004), and the width of the narrower penumbral filaments is set by the resolution of the observation (Scharmer et al. 2002; see also Fig.1). This interpretation of the current observations should not be misunderstood. The penumbrae have structures of all sizes starting with the penumbra as a whole. However, the observations show that much of its observed structure is at the resolution set by the present technical limitations and, therefore, it is expected to be unresolved. This impression is corroborated by the presence of spectral line asymmetries as discussed in item 11. 2. The best penumbral images show dark cores in penumbralfilaments (Scharmer et al. 2002). We prefer to describe them as dark filaments outlined by bright plasma. This description also provides a fair account of the actual observation (Fig.1), but it emphasizesthe role ofthe darkcore.Actually,darkcores without a bright side are common, and the cores seldom emanate from a bright point (Fig.1). a b c d Fig. 1 Time evolution of one of the dark cores in penumbral filaments discovered by Scharmer et al. (2002). (The UT of observation is marked on top of each snapshot.) Note that one of the bright sides is partly missing in (c) and (d). Note also that the bright points are not on the dark filament but in a side. These two properties are common. The arrow indicates the emergence of a new bright point in a side of the preexisting dark filament. Note the narrowness of the bright filaments, and their large aspect ratio (length over width). The spatial scales are in Mm, and the angular resolution of the image is of the order of 0.09Mm 214 J. Sa´nchez Almeida The widths of the dark core and its bright boundaries remain unresolved, although the set formed by a dark core sandwiched between two bright filaments spans some 150–180km across. 3. There is a local correlation between penumbral brightness and Doppler shift, so that bright features are blueshifted with respect to dark features (Beckers and Schro¨ter 1969; Sa´nchez Almeida et al. 1993, 2007; Johannesson 1993; Schmidt and Schlichenmaier 2000). The correlation maintains the same sign in the limb-side penumbraand the center-side penumbra,a propertyinvokedby Beckers andSchro¨ter(1969)toconcludethat it is producedby verticalmotions. A positive correlation between vertical velocity and intensity is characteristic of the nonmagnetic granulation. The fact that the same correlation also ex-ists in penumbrae suggests a common origin for the two phenomena, namely, convection. 4. The limb-side and center-side parts of a penumbra are slightly darker than the rest, an observational fact indicating that the bright penumbral filaments are elevated with respect to the dark ones (Schmidt and Fritz 2004). The behav-ior seems to continue down to the smallest structures. Dark cores are best seen where the low resolution penumbra is darkest according to Schmidt and Fritz (2004), that is, along the center-to-limb direction (e.g., Langhans et al. 2007; Ichimoto et al. 2007b). The two observations are probably connected, suggest-ing that dark cores are depressed with respect to their bright sides. 5. There is a local correlation between magnetic field inclination and horizontal velocity. The largest velocities are associated with the more horizontal fields (e.g., Title et al. 1993; Stanchfield et al. 1997). 6. The largehorizontalmotionsoccurin the darkpenumbralfilaments (e.g.,Ru¨edi et al. 1999;Pennet al. 2003;Sa´nchezAlmeidaet al. 2007).Thistrendcontinues down to the dark cores in penumbral filaments (Langhans et al. 2005, 2007). 7. The observationson the correlationbetween magneticfield strength and bright-nessarecontradictory.Someauthorsfindthestrongestfieldstrengthsassociated with the darkest regions, and vice versa (c.f. Beckers and Schro¨ter 1969; Hofmann et al. 1994). What seems to be clear is the reduced circular po-larization signal existing in dark cores, which is commonly interpreted as a reduced field strength (Langhans et al. 2005, 2007). We show in Sect.3 that such dimming of the circular polarization admits a totally different interpreta-tion, consistent with an increase of field strength in dark cores. 8. Theoretical arguments indicate that the convective roll pattern should be the mode of convection for nearly horizontal magnetic fields (Danielson 1961; Hurlburt et al. 2000). The rolls have their axes along the magnetic field lines. Unfortunately, this is not what results from recent numerical simulations of magneto-convectionin stronghighlyinclinedmagneticfields (Heinemannet al. 2007; Rempel et al. 2009). Here the convection takes place as field-free plasma intrusions in a strong field background,resembling the gappy penumbra model by Spruit and Scharmer (2006). However, these numerical simulations may not be realistic enough. They are the first to come in a series trying to re-duce theartificial diffusivitiesemployedby the numericalschemes. It is unclear ... - tailieumienphi.vn