The magnetic topology of the inverse Evershed flow

Abstract

The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights. We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field measurements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF. We derived chromospheric line-of-sight (LOS) velocities from spectra of H$\alpha$ and Ca II IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them. The magnetic field lines related to the IEF reach on average a height of 3 Mm over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference $\Delta B$ between inner and outer foot points is +400 G. The temperature difference $\Delta T$ is anti-correlated with $\Delta B$ with an average value of -100 K. The pressure difference $\Delta p$ is dominated by $\Delta B$ and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from $\Delta p$ reproduce the LOS velocities of 2-10 km s${}^{-1}$ with a square-root dependence. We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario.

23 pages, 27 figures, accepted for publication in Astronomy & Astrophysics

References in corpus (23)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
• Nonlinear force-free modeling of the solar coronal magnetic field
• Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768
• Magnetic properties of G-band bright points in a sunspot moat
• On fibrils and field lines: The nature of H$\alpha$ fibrils in the solar chromosphere
• A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating
• Convective blueshifts in the solar atmosphere: III. High-accuracy observations of spectral lines in the visible
• A Practical Approach to Coronal Magnetic Field Extrapolation Based on the Principle of Minimum Dissipation Rate
• Magnetohydrodynamic Simulation of Magnetic Null-point Reconnections and Coronal dimmings during the X2.1 flare in NOAA AR 11283
• Combining magneto-hydrostatic constraints with Stokes profiles inversions
• An Eruptive Circular-ribbon Flare with Extended Remote Brightenings
• Fast inversion of solar Ca II spectra
• The energy of waves in the photosphere and lower chromosphere: III. Inversion setup for Ca II H spectra in local thermal equilibrium
• Umbral chromospheric fine structure and umbral flashes modelled as one: the corrugated umbra
• A three-dimensional view of the thermal structure in a super-penumbral canopy
• Heating of the solar chromosphere in a sunspot light bridge by electric currents
• The energy of waves in the photosphere and lower chromosphere: II. Intensity statistics
• Three-dimensional magnetic field structure of a flux emerging region in the solar atmosphere
• Effects of Cowling Resistivity in the Weakly-Ionized Chromosphere
• Chromospheric magnetic field: A comparison of He I 10830 A observations with nonlinear force-free field extrapolation
• Magnetic Properties and Flow Angle of the Inverse Evershed Flow at Its Downflow Points
• The polarization signature of photospheric magnetic fields in 3D MHD simulations and observations at disk center
• Center-to-Limb Variation of the Inverse Evershed Flow

Cited by in corpus (3)

• Unveiling the True Nature of Plasma Dynamics from the Reference Frame of a Super-penumbral Fibril
• Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 Å lines of an active region
• Sustained heating of the chromosphere and transition region over a sunspot light bridge