First observation of bald patches in a filament channel and at a barb endpoint

Abstract

Context. The 3D magnetic field topology of solar filaments/prominences is strongly debated, because it is not directly measureable in the corona. Among various prominence models, several are consistent with many observations, but their related topologies are very different. Aims. We conduct observations to address this paradigm. We measure the photospheric vector magnetic field in several small flux concentrations surrounding a filament observed far from disc center. Our objective is to test for the presence/absence of magnetic dips around/below the filament body/barb, which is a strong constraint on prominence models, and that is still untested by observations. Methods. Our observations are performed with the THEMIS/MTR instrument. The four Stokes parameters are extracted, from which the vector magnetic fields are calculated using a PCA inversion. The resulting vector fields are then deprojected onto the photospheric plane. The 180• ambiguity is then solved by selecting the only solution that matches filament chirality rules. Considering the weakness of the resulting magnetic fields, a careful analysis of the inversion procedure and its error bars was performed, to avoid over-interpretation of noisy or ambiguous Stokes profiles. Thanks to the simultaneous multi-wavelength THEMIS observations, the vector field maps are coaligned with the Hα image of the filament. Results. By definition, photospheric dips are identifiable where the horizontal component of the magnetic field points from a negative toward a positive polarity. Among six bipolar regions analyzed in the filament channel, four at least display photospheric magnetic dips, i.e. bald patches. For barbs, the topology of the endpoint is that of a bald patch located next to a parasitic polarity, not of an arcade pointing within the polarity. Conclusions. The observed magnetic field topology in the photosphere tends to support models of prominence based on magnetic dips located within weakly twisted flux tubes. Their underlying and lateral extensions form photospheric dips both within the channel and below barbs.

Measuring an Eruptive Prominence at Large Distances From the Sun. Ii. Approaching 1 Au

“…Some models (e.g., Karpen et al 2001;López Ariste et al 2006, and references therein) describe the prominence as settling into the base of the CME flux rope structure, while others (e.g., Sturrock 1989;Lynch et al 2008;Panasenco et al 2011) treat the prominence as a separate magnetic structure to the CME, either lying beneath the base of the flux rope or otherwise becoming separated from the CME postlaunch. Given that prominences are entirely ionized beyond around 10 R  (Paper I), we can safely assume that their behavior will be entirely governed by its embedded field.…”

An active region filament studied simultaneously in the chromosphere and photosphere

“…According to observational studies, the magnetic field topology under active region (AR) filaments has sheared or twisted field lines along the PIL that can support dense plasma in magnetic dips (Lites 2005;López Ariste et al 2006). Previous findings about prominence magnetic structure have shown models with dipped field lines in a normal (Kippenhahn & Schlüter 1957) or inverse 1 (Kuperus & Raadu 1974;Pneuman 1983) polarity configuration, the latter having a helical structure.…”

“…Recently, several studies have presented analyses of the vector magnetic field in filaments or prominences from observations either in the photosphere (e.g., Lites 2005;López Ariste et al 2006;Okamoto et al 2008;Guo et al 2010;Lites et al 2010) or the chromosphere (e.g., Lin et al 1998;Casini et al 2003;Merenda et al 2006;Kuckein et al 2009), but none of them have inferred the field at both heights at the same time. The 10 830 Å spectral region, which includes a chromospheric He i triplet and a photospheric Si i line, offers a unique spectral window to understanding the physical processes that take place in AR filaments as already shown by Sasso et al (2011).…”

Prospects of Solar Magnetometry—From Ground and in Space

“…Due to its low instrumental polarization, THEMIS is well-suited for high-precision polarimetric studies. Studies have focused on magnetic fields in prominences (e.g., López Ariste et al 2006;Schmieder et al 2014) and the second solar spectrum (Faurobert & Arnaud 2003;Faurobert et al 2009). López Ariste et al (2012) investigated the scattering polarization of Na in the exosphere of Mercury, which could be used in the future to study its magnetic field.…”