Magnetoacoustic Shocks and Spectropolarimetric Signals in He I 10830 Å

Abstract

Umbral flashes are manifestations of magnetoacoustic shocks in the solar chromosphere. These phenomena are thought to influence the evolution of chromospheric umbral magnetic fields. However, the impact of these shocks on inferred chromospheric magnetic field oscillations remains unclear. We examined five different sunspots located near the solar disk center, observed with the GRIS instrument installed at the GREGOR telescope. The HAZEL2 Spectropolarimetric inversion code is used to obtain the photospheric and chromospheric line-of-sight velocities and magnetic fields in Si 10827 Å and He 10830 Å spectral lines, respectively, using various inversion strategies. In the inversions with one chromospheric component, three of the sunspots exhibit remarkably stronger magnetic fields accompanying the shocks, while the other two sunspots show striking reductions in the magnetic field. Alternatively, the Stokes profiles can be reproduced by models with two chromospheric slabs, one on top of the other, through two-component inversions. These inversions provide excellent fits even when magnetic field fluctuations are discarded by imposing a constant magnetic field during the whole temporal series. In this scenario, the observed Stokes profiles are interpreted as the result of strong velocity gradients, where the He 10830 Å line is sensitive to both sides of the shock front. Both competing models explaining the spectral profiles during the shocks, either large magnetic field fluctuations or velocity gradients, are critically discussed.

Figures (7)

• Figure 1: Sample G-Band images of the photosphere obtained from the HiFI and HiFI+ instruments showing the approximate position of the GRIS slit (black and white line) for all active regions. The white regions of the lines represent the inverted areas, as illustrated in Figure \ref{['AR12345_Bz_Vlos_Map']}, in which the arrows are oriented such that their heads indicate the upward direction in Figure \ref{['AR12345_Bz_Vlos_Map']}. The diamonds denote the pixel used to illustrate the temporal evolution of the velocity and magnetic field in Figure \ref{['AR12345_Vlos_Bz_Temp']}.
• Figure 2: Temporal evolution of the chromospheric line-of-sight velocities (left panels) and line-of-sight magnetic fields (right panels) along the spectrograph slit for AR1, AR2, AR3, AR4 and AR5, respectively, inferred from the inversion of chromospheric He I triplet at 10830 Å line. Horizontal black solid lines indicate the locations illustrated in Figure \ref{['AR12345_Vlos_Bz_Temp']}.
• Figure 3: Temporal evolution of the chromospheric line-of-sight velocity (black) and line-of-sight magnetic field (red) at selected locations indicated by black solid lines in Figure \ref{['AR12345_Bz_Vlos_Map']} and diamonds in Figure \ref{['AR12345_HiFi']}. In case of AR1, AR2 and AR3, magnetic field fluctuations have been smoothed by averaging in three time step windows. Each panel correspond to a different active region, from AR1 (top panel) to AR5 (bottom panel).
• Figure 4: Top panel: Zoom of the temporal evolution of the chromospheric line-of-sight velocity (black) and line-of-sight magnetic field (red) from AR2 (second panel from Figure \ref{['AR12345_Vlos_Bz_Temp']}). Lower panels show the observed Stokes I and V spectral profiles (blue dots) and the fits from 1-component variable magnetic field (black lines) and 2-component with fixed chromospheric magnetic fields (red lines) inversions, at the times indicated by the red, black, blue, green and cyan dashed vertical lines in the top panel (a) as well as in the left panels. The black dot-dashed vertical line in panels (b), (d), (f ), (h), and (j) highlight the rest wavelength at 10830.25 Å.
• Figure 5: Same as Figure \ref{['AR2_Spectral_Profiles']} but for AR4. Middle panels illustrate the shock (vertical dashed line in panel a) and bottom panels the quiescent state (vertical solid line in panel a).