Abstract
This investigation is based on Hα observations of high spatial resolution. They stem from an enhanced network region near disk centre of the sun and consist of broad-band and narrow-band images taken with the two-dimensional “Göttingen” Fabry-Perot spectrometer mounted in the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The “lambdameter method” was applied to derive intensity and velocity maps over the two-dimensional field of view reflecting the behaviour of these parameters in the solar chromosphere. The determination of the source function, the line-of-sight (LOS) velocity, the Doppler width and the optical depth was based on Beckers' cloud model (Beckers [CITE]). From the LOS velocity distribution along the Hα structures we conclude that ballistic motions are unlikely to prevail. Especially the bright Hα features cannot be explained by the cloud model. It is shown how, instead, two-dimensional non-LTE radiative transfer calculations of embedded chromospheric structures lead to reasonable agreement with the observed line profiles from these bright features. It is found that many of the bright fibril-like structures near dark fibrils can be explained by radiation which is blocked by absorbing material at large heights and escapes through less opaque regions. We estimate the number densities and the temperature. With these and with the measured velocities, the kinetic energy flux and the enthalpy flux related to the motions of the fine structures can be calculated. Both fall short by at least an order of magnitude of covering energy losses by radiation of active chromospheric regions.
Highlights
Mottles form the most prominent structures of the solar chromosphere seen against the disk in the wings of the Hα line
They stem from an enhanced network region near disk centre of the sun and consist of broad-band and narrow-band images taken with the two-dimensional “Gottingen” Fabry-Perot spectrometer mounted in the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife
Instead, two-dimensional non-LTE radiative transfer calculations of embedded chromospheric structures lead to reasonable agreement with the observed line profiles from these bright features
Summary
To determine the line-of-sight velocity as well as other physical quantities of chromospheric structures like the source function, the optical depth and the Doppler width, often Beckers’ cloud model (1964) is employed (Grossmann-Doerth & von Uexkull 1971, 1973; Alissandrakis et al 1990; Tsiropoula et al 1993, 1994; Lee et al 2000). For diagnostics of filaments and of highly elevated filamentary structures, the original cloud model by Beckers (1964) has been refined using non-LTE radiative transfer calculations by Mein et al (1996) who allow for a variation of the source function within the cloud. We shall comment on this criticism below when we describe our two-dimensional (2D) non-LTE calculations This investigation deals with the application of the cloud model to highly resolved Hα data and the resulting variation of the quantities mentioned above within the structures under investigation, i.e. dark mottles. We shall show by means of 2D non-LTE radiative transfer calculations how bright Hα structures can be reproduced by parameter variations within the atmosphere forming the undisturbed Hα profile.
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