Probing photospheric magnetic fields with new spectral line pairs

The magnetic line ratio (MLR) method has been extensively used in the measurement of photospheric magnetic field strength. It was devised for the neutral iron line pair at 5247.1 A and 5250.2 A (5250 A pair). Other line pairs as well-suited as this pair been have not been reported in the literature. The aim of the present work is to identify new line pairs useful for the MLR technique and to test their reliability. We use a three dimensional magnetohydrodynamic (MHD) simulation representing the quiet Sun atmosphere to synthesize the Stokes profiles. Then, we apply the MLR technique to the Stokes V profiles to recover the fields in the MHD cube both, at original resolution and after degrading with a point spread function. In both these cases, we aim to empirically represent the field strengths returned by the MLR method in terms of the field strengths in the MHD cube. We have identified two new line pairs that are very well adapted to be used for MLR measurements. The first pair is in the visible, Fe I 6820 A - 6842 A (whose intensity profiles have earlier been used to measure stellar magnetic fields), and the other is in the infrared (IR), Fe I 15534 A - 15542 A. The lines in these pairs reproduce the magnetic fields in the MHD cube rather well, partially better than the original 5250 A pair. The newly identified line pairs complement the old pairs. The lines in the new IR pair, due to their higher Zeeman sensitivity, are ideal for the measurement of weak fields. The new visible pair works best above 300 G. The new IR pair, due to its large Stokes V signal samples more fields in the MHD cube than the old IR pair at $1.56\,\mu$m, even in the presence of noise, and hence likely also on the real Sun. Owing to their low formation heights (100-200 km above tau_5000=1), both the new line pairs are well suited for probing magnetic fields in the lower photosphere.Comment: Accepted for publication in Astronomy & Astrophysic

Center to limb observations and modeling of the Ca I 4227 A line

The observed center-to-limb variation (CLV) of the scattering polarization in different lines of the Second Solar Spectrum can be used to constrain the height variation of various atmospheric parameters, in particular the magnetic fields via the Hanle effect. Here we attempt to model non-magnetic CLV observations of the $Q/I$ profiles of the Ca I 4227 A line recorded with the ZIMPOL-3 at IRSOL. For modeling, we use the polarized radiative transfer with partial frequency redistribution with a number of realistic 1-D model atmospheres. We find that all the standard FAL model atmospheres, used by us, fail to simultaneously fit the observed ($I$, $Q/I$) at all the limb distances ($\mu$). However, an attempt is made to find a single model which can provide a fit at least to the CLV of the observed $Q/I$ instead of a simultaneous fit to the ($I$, $Q/I$) at all $\mu$. To this end we construct a new 1-D model by combining two of the standard models after modifying their temperature structures in the appropriate height ranges. This new combined model closely reproduces the observed $Q/I$ at all the $\mu$, but fails to reproduce the observed rest intensity at different $\mu$. Hence we find that no single 1-D model atmosphere succeeds in providing a good representation of the real Sun. This failure of 1-D models does not however cause an impediment to the magnetic field diagnostic potential of the Ca I 4227 A line. To demonstrate this we deduce the field strength at various $\mu$ positions without invoking the use of radiative transfer.Comment: 20 pages, 10 figures, Accepted for publication in Ap

Observations of solar chromospheric heating at sub-arcsec spatial resolution

A wide variety of phenomena such as gentle but persistent brightening, dynamic slender features (~100 km), and compact (~1'') ultraviolet (UV) bursts are associated with the heating of the solar chromosphere. High spatio-temporal resolution is required to capture the finer details of the likely magnetic reconnection-driven, rapidly evolving bursts. Such observations are also needed to reveal their similarities to large-scale flares, which are also thought to be reconnection driven, and more generally their role in chromospheric heating. Here we report observations of chromospheric heating in the form of a UV burst obtained with the balloon-borne observatory, SUNRISE. The observed burst displayed a spatial morphology similar to that of a large-scale solar flare with circular ribbon. While the co-temporal UV observations at 1.5'' spatial resolution and 24s cadence from the Solar Dynamics Observatory showed a compact brightening, the SUNRISE observations at diffraction-limited spatial resolution of 0.1'' at 7s cadence revealed a dynamic sub-structure of the burst that it is composed of extended ribbon-like features and a rapidly evolving arcade of thin (~0.1'' wide) magnetic loop-like features, similar to post-flare loops. Such a dynamic sub-structure reveals the small-scale nature of chromospheric heating in these bursts. Furthermore, based on magnetic field extrapolations, this heating event is associated with a complex fan-spine magnetic topology. Our observations strongly hint at a unified picture of magnetic heating in the solar atmosphere from some large-scale flares to small-scale bursts, all being associated with such a magnetic topology.Comment: Accepted for publication in A&A, online movie of the observations is available at https://goo.gl/RRDuX