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Register a new userThe Magnetic Field of Solar Spicules
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R. Centeno
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Determining the magnetic field of 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 magnetic spicule properties in a very quiet region of the off-limb solar atmosphere, as inferred from new spectropolarimetric observations in the HeI 10830 A triplet. 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 (HAZEL) to interpret the observations. Magnetic fields as strong as 40G were unambiguously detected in a very localized area of the slit, which may represent a possible lower value of the field strength of organized network spicules.
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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 A 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 (HAZEL) to interpret the observations. Magnetic fields as strong as ~40G 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.
We aim to study the formation and evolution of solar spicules by means of numerical simulations of the solar atmosphere. With the use of newly developed JOANNA code, we numerically solve two-fluid (for ions + electrons and neutrals) equations in 2D Cartesian geometry. We follow the evolution of a spicule triggered by the time-dependent signal in ion and neutral components of gas pressure launched in the upper chromosphere. We use the potential magnetic field, which evolves self-consistently, but mainly plays a passive role in the dynamics. Our numerical results reveal that the signal is steepened into a shock that propagates upward into the corona. The chromospheric cold and dense plasma lags behind this shock and rises into the corona with a mean speed of 20-25 km s$^{-1}$. The formed spicule exhibits the upflow/downfall of plasma during its total lifetime of around 3-4 minutes, and it follows the typical characteristics of a classical spicule, which is modeled by magnetohydrodynamics. The simulated spicule consists of a dense and cold core that is dominated by neutrals. The general dynamics of ion and neutral spicules are very similar to each other. Minor differences in those dynamics result in different widths of both spicules with increasing rarefaction of the ion spicule in time.
Hydrodynamic jets are unstable to the kink instability (m=1 mode in cylindrical geometry) owing to the centripetal force, which increases the transverse displacement of the jet. When the jet moves along a magnetic field, then the Lorentz force tries to decrease the displacement and stabilises the instability of sub-Alfvenic flows. The threshold of the instability depends on the Alfven Mach number (the ratio of Alfven and jet speeds). We suggest that the dynamic kink instability may be of importance to explain observed transverse motions of type II spicules in the solar atmosphere. We show that the instability may start for spicules which rise up at the peripheries of vertically expanding magnetic flux tubes owing to the decrease of the Alfven speed in both, the vertical and the radial directions. Therefore, inclined spicules may be more unstable and have more higher transverse speeds. Periods and growth times of unstable modes in the conditions of type II spicules have the values of 30 s and 25-100 s, respectively, which are comparable to the life time of the structures. This may indicate to the interconnection between high speed flow and rapid disappearance of type II spicules in chromospheric spectral lines.
Spicules are intermittently rising above the surface of the Sun eruptions; EUV jets are now also reported in immediately above layers. The variation of spicule orientation with respect to the solar latitude, presumably reflecting the confinement and the focusing of ejecta by the surrounding global coronal magnetic field, is an important parameter to understand their dynamical properties. A wealth of high resolution images of limb spicules are made available in H CaII emission from the SOT Hinode mission. Furthermore, the Hough transform is applied to the resulting images for making a statistical analysis of spicule orientations in different regions around the solar limb, from the pole to the equator. Spicules are visible in a radial direction in the polar regions with a tilt angle (less than 200). The tilt angle is even reduced to 10 degrees inside the coronal hole with open magnetic field lines and at the lower latitude the tilt angle reaches values in excess of 50 degree.
The weak-field approximation implying linear relationship between Stokes $V/I$ and longitudinal magnetic field, $B_{Vert}$, often suffers from saturation observed in strong magnetic field regions such as sunspot umbra. In this work, we intend to improve the magnetic field observations carried out by the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station, China. We propose using non-linear relationship between Stokes $V/I$ and $B_{Vert}$ to derive magnetic field. To determine the form of the relationship, we perform a cross-calibration of SMFT data and magnetograms provided by the textit{Helioseismic and Magnetic Imager} on board the textit{Solar Dynamics Observatory}. The algorithm of the magnetic field derivation is described in details. We show that using non-linear relationship between Stokes $V/I$ and $B_{Vert}$ allows us to eliminate magnetic field saturation inside sunspot umbra. The proposed technique enables one to enhance the reliability of the SMFT magnetic field data obtained even long before the space-based instrumentation era, since 1987.
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