Structures and properties of fan-shaped jets: H$\alpha$ dark blobs and transparent regions

Abstract

We investigated three fan-shaped jets observed above sunspot light bridges or nearby sunspot regions. The study aimed to explore the dynamics and physical properties of jets' features that appear as blob-like structures at the tips of the jets, which we call `dark blobs'. A transparent region is observed beneath the dark blobs, creating a visible gap between the dark blobs and the trailing body of the jets. These phenomena were studied in chromospheric and transition region imaging and spectral high-resolution co-observations from the Visible Imaging Spectrometer of the Goode Solar Telescope at the Big Bear Solar Observatory and the Interface Region Imaging Spectrograph (IRIS), together with data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. We analyzed the jets' morphology and fine structure. We obtained the spatial scale and the dynamics of the dark blobs that are seen mostly in the wings of the H$\alpha$ line and have a cross-section of about $0.2^{\prime\prime}-0.3^{\prime\prime}$. The dark blobs and the transparent regions are seen bright (in emission) in the IRIS slit-jaw 1330 $\unicode{0x212B}$, 1400 $\unicode{0x212B}$, and AIA 304 $\unicode{0x212B}$ images. The IRIS Si IV 1394 $\unicode{0x212B}$ spectrum of the brightenings showed blue-shifted emission of about $16$ km s$^{-1}$ with non-thermal velocities of up to $40$ km s$^{-1}$. We also estimated the electron density of the blue-shifted brightenings to be $10^{12.1\pm0.2}$ cm$^{-3}$. Our findings likely suggest that we detect the observational signatures of shock waves that generate and/or contribute to the evolution of fan-shaped jets.