We use the most powerful telescopes and instruments in the ground and space to infer properties of the structures in the solar surface, specially focusing on the magnetic properties of such structures.
On the magnetic field of off-limb spicules
Determining the magnetic field related to solar spicules is vital for developing adequate models of these plasma jets, which are thought to play a key role in the thermal, dynamic, and magnetic structure of the chromosphere. Here we report on the magnetic properties of off-limb spicules in a very quiet region of the solar atmosphere, as inferred from new spectropolarimetric observations in the He I 10830 Å triplet obtained with the Tenerife Infrared Polarimeter. We have used a novel inversion code for Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects to interpret the observations (HAZEL, from HAnle and ZEeman Light). Magnetic fields as strong as ~50 G were detected in a very localized area of the slit, which could represent a possible lower value of the field strength of organized network spicules.
Magnetic ﬁeld inferred by the inversion from the Stokes proﬁles. The upper panel corresponds to data obtained at a distance of 2" outside the limb while the lower panel corresponds to data obtained at 3". The solid line represents the ﬁeld strength as a function of the position along the slit, and the arrows show the projection of the magnetic ﬁeld vector on the plane of the sky.
Small magnetic loops connecting the quiet surface and the hot outer atmosphere of the Sun
Sunspots are the most spectacular manifestation of solar magnetism, yet 99% of the solar surface remains “quiet” at any time of the solar cycle. The quiet sun is not void of magnetic ﬁelds, though; they are organized at smaller spatial scales and evolve relatively fast, which makes them difﬁcult to detect. Thus, although extensive quiet Sun magnetism would be a natural driver to a uniform, steady heating of the outer solar atmosphere, it is not clear what the physical processes involved would be, due to lack of observational evidence. We report on the topology and dynamics of the magnetic ﬁeld in very quiet regions of the Sun from spectropolarimetric observations of the Hinode satellite, showing a continuous injection of magnetic ﬂux with a well-organized topology of Ω-loop from below the solar surface into the upper layers. At ﬁrst stages, when the loop travels across the photosphere, it has a ﬂattened (staple-like) geometry and a mean velocity ascent of ∼3 km s−1 . When the loop crosses the minimum temperature region, the magnetic ﬁelds at the footpoints become almost vertical and the loop topology resembles a potential ﬁeld. The mean ascent velocity at chromospheric height is ∼12 km s−1 . The energy input rate of these small-scale loops in the lower boundary of the chromosphere is (at least) of 1.4 × 106 –2.2 × 107 erg cm−2 s−1 . Our ﬁndings provide empirical evidence for solar magnetism as a multi-scale system, in which small-scale low-ﬂux magnetism plays a crucial role, at least as important as active regions, coupling different layers of the solar atmosphere and being an important ingredient for chromospheric and coronal heating models.
Three-dimensional topology of the magnetic ﬁeld over a granule. Continuum image at the bottom shows granular (bright) and intergranular (dark) regions. Short lines indicate magnetic ﬁeld orientation (blue for the footpoint with positive, emergent polarity; red for the negative footpoint), derived from inversion at the points with high enough spectropolarimetric signal. Representative ﬁeld lines (tangent to these director vectors) are calculated starting at the height of formation of the Fe I 630 nm lines at one footpoint and followed until they reach the same height at the other end. Both footpoints happen to be connected. The projection of ﬁeld lines on the solar surface appears as colored lines on the bottom plane, showing azimuth spreading over nearly 90◦ . From low-lying bluish lines to high-lying ones the magnetic ﬁeld ﬁlls most of the volume from the photosphere to the low chromosphere. The colors of ﬁeld lines have been used for ease of eye.
A high-resolution three-dimensional model of the solar photosphere derived from Hinode observations
A new three-dimensional model of the solar photosphere is presented in this paper and made publicly available to the community. This model has the peculiarity that it has been obtained by inverting spectro-polarimetric observations, rather than from numerical radiation hydrodynamical simulations. The data used here are from the spectro-polarimeter onboard the Hinode satellite, which routinely delivers Stokes I, Q, U and V proﬁles in the 6302 Å spectral region with excellent quality, stability and spatial resolution (approximately 0.3”). With such spatial resolution, the major granular components are well resolved, which implies that the derived model needs no micro- or macro-turbulence to properly ﬁt the widths of the observed spectral lines. Not only this model ﬁts the observed data used for its construction, but it can also ﬁt previous solar atlas observations satisfactorily.
Horizontal (in the τ5000 depth scale) cuts of several interesting physical parameters in the multi-cube resulting from the inversion, including NLTE correction. The left column shows the temperature in kK at three diﬀerent heights. The top row also shows the line-of-sight magnetic ﬂux density (middle panel) in G and the transverse (on the plane of the sky) magnetic ﬂux density (right panel) at the base of the photosphere. The middle row shows the variation of the ﬂux density from log(τ5000)=0 to log(τ5000)=−1 (middle panel: line-of-sight component; right panel: transverse component). The bottom row shows the variation of the temperature in kK (middle panel) and the line-of-sight velocity in km s−1 (right panel) from log(τ5000)=0 to log(τ5000)=−1 inside the magnetic atmosphere.
The model presented here is publicly available and may be downloaded both as an IDL saveﬁle or in raw binary format. The ﬁles are licensed under the GPLv3 general public license which explicitly grants permission to copy, modify (with proper credit to the original source and explanation of the modiﬁcations) and redistribute the software. As a courtesy, potential users are kindly requested to contact the author explaining the nature of their investigations and the intended use of the model.
Socas-Navarro, H. 2010, A&A, in press
Multi-layer study of wave propagation in sunspots
We analyze the propagation of waves in sunspots from the photosphere to the chromosphere using time series of co-spatial Ca ii H intensity spectra (including its line blends) and polarimetric spectra of Si i λ10,827 and the He i λ10,830 multiplet. From the Doppler shifts of these lines we retrieve the variation of the velocity along the line of sight at several heights. Phase spectra are used to obtain the relation between the oscillatory signals. Our analysis reveals standing waves at frequencies lower than 4 mHz and a continuous propagation of waves at higher frequencies, which steepen into shocks in the chromosphere when approaching the formation height of the Ca ii H core. The observed nonlinearities are weaker in Ca ii H than in He i lines. Our analysis suggests that the Ca ii H core forms at a lower height than the He i λ10,830 line: a time delay of about 20 s is measured between the Doppler signal detected at both wavelengths. We ﬁt a model of linear slow magnetoacoustic wave propagation in a stratiﬁed atmosphere with radiative losses according to Newton’s cooling law to the phase spectra and derive the difference in the formation height of the spectral lines. We show that the linear model describes well the wave propagation up to the formation height of Ca ii H, where nonlinearities start to become very important.
Temporal evolution at one position in the umbral region of series 2. Left: Si i and He i intensity; center: Si i and He i Stokes V; right: Ca ii H intensity. The horizontal axis represents wavelength, with the origin at the position of the Si i λ10,827 Å rest wavelength in (a) and (b) and at the position of the Ca ii H λ3968 Å rest wavelength in (c). The vertical axis represents time, increasing from bottom to top.
An uncombed inversion of multiwavelength observations reproducing the net circular polarization in a sunspot's penumbra
The penumbra of sunspots has a complex magnetic ﬁeld topology whose three-dimensional organization remains unclear
after more than a century of investigation. I derive a geometrical model of the penumbral magnetic ﬁeld topology from an uncombed inversion setup designed to reproduce the net circular polarization (NCP) of simultaneous spectra in near-infrared (IR; 1.56 μm) and visible (VIS; 630 nm) spectral lines. I inverted the co-spatial spectra of ﬁve photospheric lines with a model that mimicked vertically interlaced magnetic ﬁelds with two distinct components, labeled background ﬁeld and ﬂow channels because of their characteristic properties (ﬂow velocity, ﬁeld inclination). The ﬂow channels were modeled as a perturbation of the constant background ﬁeld with a Gaussian shape using the SIRGAUS code. The location and extension of the Gaussian perturbation in the optical depth scale retrieved by the inversion code were then converted to a geometrical height scale. By estimating the geometrical size of the ﬂow channels, I investigated the relative amount of magnetic ﬂux in the ﬂow channels and the background ﬁeld atmosphere. The uncombed model is able to reproduce the NCP well on the limb side of the spot and less successfully on the center side; the VIS lines are better reproduced than the near-IR lines. I ﬁnd that the Evershed ﬂow happens along nearly horizontal ﬁeld lines close to the solar surface given by optical depth unity. The magnetic ﬂux that is related to the ﬂow channels constitutes about 20−50% of the total magnetic ﬂux in the penumbra. The gradients that can be produced by a Gaussian perturbation are too small for a perfect reproduction of the NCP in the IR lines with their small formation height range, where a step function seems to be required. Two peculiarities of the observed NCP, a sign change in the NCP of the VIS lines on the center side and a ring structure around the umbra with opposite signs of the NCP in the Ti i line at 630.37 nm and the Fe i line at 1565.2 nm, deserve closer attention in future modeling attempts. The large fraction of magnetic ﬂux related to the ﬂow channel component could suﬃce to replenish the penumbral radiative losses in the ﬂux tube picture.
Comparison between the NCP in the observed (left column) and best-ﬁt proﬁles (right). Top to bottom: 630.37 nm, 630.25 nm, 630.15 nm, 1565.2 nm, 1564.8 nm. For the ﬁt result of 630.37 nm and of 1565.2 nm, the display range is half that of the observed NCP.