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Register a new userDynamics of Sunspot Shock Waves in the Chromosphere and Transition Region
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Added by
Pradeep Kumar Kayshap Dr.
Publication date
2020
fields
Physics
and research's language is
English
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We study the dynamics of shock waves observed in the umbra of a sunspot using the spectroscopic observations from the Interface Region Imaging Spectrometer (IRIS). The presence of the shock significantly deforms the shape of the spectral lines of Mg II , C II , and Si IV . We found that C II 1335.66 {AA} and Si IV 1393.75 {AA} show double-peaked profiles that change to a single peak later on. However, the Mg II h 2803.53 {AA} line first shows flat-top profiles that change into double-peaked followed by the single peak. To study the shock dynamics, we isolate the shock component from the spectra by fitting two Gaussians. We find that the lifetime of the shock is largest in Mg II h 2803.53 {AA} line. Moreover, the plasma motion shows both acceleration and deceleration phase of the shock. Yet, in C II 1335.66 {AA} and Si IV 1393.75 {AA}, only deceleration phase is observed. We observe a strong correlation between the largest blueshift of the shock and deceleration for all three spectral lines. We find a positive (negative) correlation between intensities contributed due to the shocks in Mg II and C II (Si IV ). This is suggestive that the shocks are first amplified in C II , followed by a decline in the height range corresponding to Si IV . These results may indicate the dissipation of shocks above the formation height of C II , and the shocks may have important roles in the dynamics of the upper chromosphere and transition region above sunspots.
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The instrumental advances made in this new era of 4-meter class solar telescopes with unmatched spectropolarimetric accuracy and sensitivity, will enable the study of chromospheric magnetic fields and their dynamics with unprecedented detail. In this regard, spectropolarimetric diagnostics can provide invaluable insight into magneto-hydrodynamic (MHD) wave processes. MHD waves and, in particular, Alfvenic fluctuations associated to particular wave modes, were recently recognized as important mechanisms not only for the heating of the outer layers of the Suns atmosphere and the acceleration of the solar wind, but also for the elemental abundance anomaly observed in the corona of the Sun and other Sun-like stars (also known as first ionisation potential; FIP) effect. Here, we take advantage of state-of-the-art and unique spectropolarimetric IBIS observations to investigate the relation between intensity and circular polarisation (CP) fluctuations in a sunspot chromosphere. Our results show a clear link between the intensity and CP fluctuations in a patch which corresponds to a narrow range of magnetic field inclinations. This suggests the presence of Alfvenic perturbations in the sunspot.
The solar chromosphere and transition region (TR) form an interface between the Suns surface and its hot outer atmosphere. Here most of the non-thermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33-arcsec) observations with NASAs Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub-arcsecond scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-m Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere.
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 3rd September 2015 measured the linear polarization produced by scattering processes in the hydrogen Ly-$alpha$ line of the solar disk radiation, whose line-center photons stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the $Q/I$ line-center signals. Using an analytical model, we first show that the geometrical complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the $Q/I$ and $U/I$ line-center signals. Secondly, we introduce a statistical description of the solar atmosphere based on a three-dimensional (3D) model derived from a state-of-the-art radiation magneto-hydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.
As the interface between the Suns photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80-250 km/s from the narrow bright network lanes of this interface region. These jets have lifetimes of 20-80 seconds and widths of 300 km or less. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with ~20 km/s amplitudes. Many jets reach temperatures of at least ~100000 K and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.
Aims. We analyse SUMER spectral scans of a large sunspot within active region NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and dynamics of the sunspot atmosphere at different heights/temperatures. Methods: The data analysed here consist of spectroheliograms in the continuum around 142.0 nm and in the Si iv 140.2 nm, O iii 70.3 nm, N iv 76.5 nm, and O iv 79.0 nm spectral lines. Gaussian-fitting of the observed profiles provides line-of-sight velocity and Doppler-width maps. Results: The data show an asymmetric downflow pattern compatible with the presence of the inverse Evershed flow in a region within roughly twice the penumbral radius at transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous on small scales, seem to occur in a collar of radially directed filamentary structures, with an average width less than the 1 Mm spatial resolution of SUMER and characterised by different plasma speeds. Assuming that the flows are directed along the field lines, we deduce that such field lines are inclined by 10 deg to 25 deg with respect to the solar surface.
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