First Detection of Lyman-Alpha Scattering Polarization in Off-Limb Spicules and Its Constraint on Their Magnetic Field

The magnetic field of spicules is one of the critical physical quantity that needs to be observationally determined to quantitatively evaluate the energy transported to the corona. The CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) is a sounding rocket experiment launched in 2015 to obtain spectro-polarimetric data in the hydrogen Lyman-alpha line. CLASP performed the sit-and-stare observations in the quiet Sun near the limb for 5 minutes with a slit perpendicular to the limb and successfully captured off-limb spicules. The Lyman-alpha line is well suited to investigate how spicules affect corona because it is sensitive to higher temperatures than other chromospheric lines due to large optical thickness. Another advantage of this spectral line is the Hanle effect, namely the modification of the scattering polarization by the magnetic field, operates at the field strength approximately between 10 and 100 G, which is comparable to the typical field strength of spicules suggested by previous studies using other chromospheric lines in visible and infrared ranges. In this talk, we report the first detection of the scattering polarization of spicules in the Lyman-alpha line, which can be exploited to constrain the magnetic field information via the Hanle effect. The linear polarization Q/I signal (positive Q/I indicates polarization parallel to the limb) during the observing time is 0% to 2% in off-limb locations. The linear polarization U/I signal changes in time from +1% to -1%. The accuracy of the polarization signal is about 0.1–0.3%. The positive Q/I values during the observation period indicate that 90° scattering by the anisotropic incident radiation from the solar disk is dominant. The temporal variation observed in U/I can be considered to be due to the temporal variation of magnetic field vector (i.e., operation of the Hanle effect) and/or to the symmetry properties of the radiation field. We discuss possible constraints on the magnetic field in the spicules observed by CLASP.