| Abstract : | Recent high-resolution observations revealed the presence of low-lying loops in the chromosphere/transition region that rapidly change within a time scale of a few minutes. These loops are often heated to a transition region and higher temperature. These loops were previously termed Unresolved Fine Structures (UFS). Their formation mechanism remains unclear until now. We comprehensively analysed the Interface Region Imaging Spectrograph (IRIS) and the Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO), USA, observations to understand the formation mechanism and dynamics of the UFS loops. GST observation shows that these UFS loops are distinctively observed as a bright emission structure in the Hα wing images, particularly in the red wing, causing asymmetries in the Hα line. High-resolution Hα observations reveal that the length is in the range of 3.3-3.6 Mm, and their height is 1-1.2 Mm. IRIS observation shows these loops are clearly visible in the 1330 ̊A slit-jaw images. The detailed analysis of the IRIS spectra shows that the transition region line intensities are highly enhanced and are enormously broadened compared to the average line profile, particularly in C II 1330 ̊A and Si IV 1403 ̊A lines.
Furthermore, we notice that these lines show enhancement in the blue wing, likely due to the eruptions of a jet above the loops. We also found the presence of the chromospheric absorption lines in the transition region spectra. The loops and the jets might have occurred due to the magnetic reconnection and the presence of the absorption lines, indicating that reconnection-driven heated loops formed in the lower chromosphere. These observations suggest that the solar atmosphere is more complex than conventionally thought and shed light on the intricate energy conversion in magnetic reconnection. |
