The existence of the Sun’s hot atmosphere and the solar wind acceleration continues to be an outstanding problem in solar-astrophysics. Although magnetohydrodynamic (MHD) modes and dissipation of magnetic energy contribute to heating and the mass cycle of the solar atmosphere, yet direct evidence of such processes often generates debate. Ground-based 1-m Swedish Solar Telescope (SST)/CRISP, Hα 6562.8 Å observations reveal, for the first time, the ubiquitous presence of high frequency (~12–42 mHz) torsional motions in thin spicular-type structures in the chromosphere. We detect numerous oscillating flux tubes on 10 June 2014 between 07:17 UT to 08:08 UT in a quiet-Sun field-of-view of 60” × 60” (1” = 725 km). Stringent numerical model shows that these observations resemble torsional Alfvén waves associated with high frequency drivers which contain a huge amount of energy (~105 W m−2) in the chromosphere. Even after partial reflection from the transition region, a significant amount of energy (~103 W m−2) is transferred onto the overlying corona. We find that oscillating tubes serve as substantial sources of Alfvén wave generation that provide sufficient Poynting flux not only to heat the corona but also to originate the supersonic solar wind.
Small scale magnetic fields (magnetic elements) are ubiquitous in the solar photosphere. Their interaction can provide energy to the upper atmospheric layers, and contribute to heat the solar corona. In this work, the dynamic properties of magnetic elements in the quiet Sun are investigated. The high number of magnetic elements detected in a supegranular cell allowed us to compute their displacement spectrum (∆r) 2 ∝ τ γ (being γ > 0, and τ the time since the first detection), separating the contribution of the network (NW) and the internetwork (IN) regions. In particular, we found γ = 1.27 ± 0.05 and γ = 1.08 ± 0.11 in NW (at smaller and larger scales, respectively), and γ = 1.44 ± 0.08 in IN. These results are discussed in light of the literature on the topic, as well as the implications for the build up of the magnetic network.
Magnetohydrodynamics (MHD) wave propagation inside the Sun's atmosphere is closely related to the magnetic field topology. For example, magnetic fields are able to lower the cutoff frequency for acoustic waves, thus allowing the propagation of waves that would otherwise be trapped below the photosphere into the upper atmosphere. In addition, MHD waves can be either transmitted or converted into other forms of waves at altitudes where the sound speed equals the Alfvén speed. We take advantage of the large field-of-view provided by the IBIS experiment to study the wave propagation at two heights in the solar atmosphere, which is probed using the photospheric Fe 617.3 nm spectral line and the chromospheric Ca 854.2 nm spectral line, and its relationship to the local magnetic field. Among other things, we find substantial leakage of waves with five-minute periods in the chromosphere at the edges of a pore and in the diffuse magnetic field surrounding it. By using spectropolarimetric inversions of Hinode SOT/SP data, we also find a relationship between the photospheric power spectrum and the magnetic field inclination angle. In particular, we identify well-defined transmission peaks around 25• for five-minute waves and around 15• for three-minute waves. We propose a very simple model based on wave transmission theory to explain this behavior. Finally, our analysis of both the power spectra and chromospheric amplification spectra suggests the presence of longitudinal acoustic waves along the magnetic field lines.
The formation of shocks within the solar atmosphere remains one of the few observable signatures of energy dissipation arising from the plethora of magnetohydrodynamic waves generated close to the solar surface. Active region observations offer exceptional views of wave behavior and its impact on the surrounding atmosphere. The stratified plasma gradients present in the lower solar atmosphere allow for the potential formation of many theorized shock phenomena. In this study, using chromospheric Ca II 8542Å line spectropolarimetric data of a large sunspot, we examine fluctuations in the plasma parameters in the aftermath of powerful shock events that demonstrate polarimetric reversals during their evolution. Modern inversion techniques are employed to uncover perturbations in the temperatures, line-of-sight velocities, and vector magnetic fields occurring across a range of optical depths synonymous with the shock formation. Classification of these non-linear signatures is carried out by comparing the observationally-derived slow, fast, and Alfvén shock solutions to the theoretical Rankine-Hugoniot relations. Employing over 200 000 independent measurements, we reveal that the Alfvén (intermediate) shock solution provides the closest match between theory and observations at optical depths of log 10 τ = −4, consistent with a geometric height at the boundary between the upper photosphere and lower chromosphere. This work uncovers first-time evidence of the manifestation of chromospheric intermediate shocks in sunspot umbrae, providing a new method for the potential thermalization of wave energy in a range of magnetic structures, including pores, magnetic flux ropes, and magnetic bright points.
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