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Astron. Astrophys. 363, 279-288 (2000)

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1. Introduction

The solar chromosphere is highly structured at various spatial scales and observations and theory point to the fact that the magnetic field is the agent responsible for this structuring. The H and K lines of singly ionized calcium are the proven best spectral diagnostics from the ground to probe the physical conditions of these structures in the chromosphere of the sun (Rutten & Uitenbroek 1991) as well as the chromospheres of sun - like stars (Wilson & Bappu 1957; Bappu & Sivaraman 1977; Noyes 1981 and the references therein).

On a two-dimensional image of the sun in the H or K line, the structures seen in emission in the quiet chromosphere are the network and the bright points that populate the interior of the network. Plages are an active region component with striking emission that appear during the years of solar maximum (Hale & Ellerman, 1903; Bappu & Sivaraman, 1971; Zirin, 1974). Of these, the plages and the active network as well as the network on the quiet sun are cospatial with the photospheric magnetic fields (Leighton, 1959). Furthermore, the brightness of the network bears a linear relation with the underlying photospheric magnetic field with which it is cospatial (Skumanich et al. 1975; Stenflo & Harvey, 1985). An excellent review of the properties of K2V bright points in the interior of the network (also called the K2V grains) exists in the literature (Rutten & Uitenbroek, 1991 and the references therein). Magnetic elements in the interior of the supergranular cells, although detected as early as 1975 by Livingston & Harvey (1975), have attracted renewed interest in the last decade because of the possibility of measuring these small-scale fields, with improved precision (Keller et al., 1994; Wang et al., 1995; Lin, 1995). These magnetic elements in the interior of the supergranular cells are referred to as intranetwork fields by Keller et al. (1994), Zhang et al. (1998b, 1998c) and as internetwork fields by Lites et al. (1999). Now, the important question is whether these K2V bright points (referred to as INBPs in this paper) are cospatial with the magnetic elements (referred to here as IN magnetic elements) or not.

Direct evidence for the spatial correspondence between the INBPs and IN magnetic elements was provided by Sivaraman & Livingston (1982). They compared Fe I 8688 Å magnetic scans with sequential 1.1 Å band pass K-line scans both obtained at the vacuum solar telescope on Kitt Peak.

They found that:

  1. the INBPs spatially correspond to the IN magnetic elements,

  2. the fields of the associated IN magnetic elements are in the range 10-20 Mx cm-2, the maximum fields reaching 70-80 Mx cm-2,

  3. the INBPs have no preference for either polarity, and

  4. there are many IN magnetic elements which do not have corresponding INBPs.

However, some authors report that the INBPs are of hydrodynamic origin and bear no correlation with the IN magnetic elements. Steffens et al. (1996) found that some of the INBPs travelled within the network cells with horizontal speeds of 30-80 km s-1, comparable to the phase velocity of the solar p-modes and the spatial distribution of the INBPs showed no prominent clustering within the network. Based on these results, they concluded that the INBPs are just the prominent peaks of the p-mode wave field and have therefore no correlation with the IN magnetic elements. However, Wellstein et al. (1998) recent measurements show that the INBPs move with horizontal velocities 6-8 times smaller than the values of Steffens et al. (1996). Remling et al. (1996) reached the conclusion that K2V grains are non-magnetic phenomena from their analysis of simultaneous spectral scans in the CN band (3883 Å) and the K-line. They used the CN scans as proxy for magnetic flux and did not find any statistically significant increase in CN intensity at those locations where the K-line INBPs appear.

All these conclusions are from observations that do not contain measurements of magnetic field and must be considered indirect evidence. This controversy according to Rutten has now been resolved (Rutten, 1996; Hoekzema, 1997) by the hydrodynamic simulations by Carlsson & Stein (1997, 1998). The propagation of acoustic waves through a non-magnetic chromosphere reproduces the dynamical evolution of the H-line profiles of the INBPs very well. According to this scenario, the INBPs are due to the interference between the shocks in the upward propagating acoustic waves with the back-falling material of the previous shocks and these are of non - magnetic origin and have a stochastic distribution within the network. On the other hand, Kalkofen (1996 and references therein) favours an impulsive excitation at the site of the IN magnetic elements as a plausible mechanism for the 3-minute oscillations in K2V emission in the bright points since other mechanisms like chromospheric cavity or the Lighthill mechanism cannot explain either the temporal or spatial intermittancy in the emission exhibited by the INBPs. He argues in favour of a possible connection between the INBPs and the IN magnetic elements on the basis of the positional stability of the K2V bright points, on the equality of the numbers of the INBPs and the IN magnetic elements within a cell and the agreement between the magnetic field values estimated by him and those measured for the IN magnetic elements (Keller et al., 1994; Lin, 1995).

The way to resolve this controversy is to obtain K-line and magnetic field observations and look for correlations or their absence. The only attempt in the recent past in this direction is by Nindos & Zirin (1998) where they analysed two sets of K-line filtergram and deep magnetogram pairs and concluded that there are two classes of intranetwork bright points: one associated with intranetwork magnetic fields and the other not associated. While we were in the final stages of completing this paper we came across another attempt by Lites et al. (1999) using the Stokes V data from the Advanced Stokes Polarimeter and H[FORMULA] spectra obtained synchronously at the NSO / Sacramento Peak Dunn Solar Telescope with the aim of "utilising spectrographic techniques to enhance the measurement accuracy at the expense of field of view". Specifically they compare the H[FORMULA] index (the H-line intensity index over a 0.008 nm wide band displaced from line center by [FORMULA] = 0.016 nm) and the Stokes V profile space-time charts for a "small internetwork area" and find that "there is no obvious association of H[FORMULA] brightenings with either localized field excesses or with field deficits".

In this paper we report new observations of K2V INBPs and IN magnetic elements and demonstrate that the two structures do show spatial correspondence. Our study points out in particular the possibility of misinterpretation without some key insights. In the cases of excess K2V emission associated with bipoles, the magnetic field measurements do not show up because of their intrinsic low fields. However, if such cases are not included for this reason, then the percentage of coincidences of K2V or H[FORMULA] bright points with magnetic fields will be lower and this could be interpreted as a no obvious correlation. On the contrary, when the cases of bipoles are included, then the percentage of coincidences rises as high as 85%.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000