Identification of the asymmetric spectral profiles in the solar transition region

Document Type : Research Article

Authors

Department of Physics, Faculty of Science, University of Zanjan, Zanjan, Iran.

Abstract

The temperature of the solar atmosphere steeply increases in the transition region from the chromosphere to the corona. In the coronal temperatures, the coronal hole, quiet sun, and active regions are visible in the solar corona. The magnetic field controls the solar corona. Different physical processes (e.g. magnetic reconnection, waves) play a role in the coronal dynamics, which cause plasma heating to millions of degrees. Then, it is essential to understand the role of these processes. A more exact analysis of the emission line profiles to investigate the dynamics and thermal behavior of the coronal and transition region plasma is spectroscopy. The spectral line profiles are proof of the structural evolution of the magnetic field and plasma temperature in the coronal holes and quiet sun. So far, the study of coronal spectral lines indicates that most line profiles are well-fitted based on a single Gaussian profile. However, some spectral lines avoid from a single Gaussian model because they have least one excess component. Observations display that 5 % to 10 % of line profiles have the blueward asymmetry in the quiet sun and coronal holes. Considering the mentioned advantages of spectroscopy, we use the formed Si IV 1394 Å spectral line in the transition region from the Interface Region Imaging Spectrograph (IRIS) raster in a central-equatorial region of Sun on 14 October 2015. Also, we make co-spatiotemporal raster images from Atmospheric Imaging Assembly (AIA) 193 Å, IRIS/SJI 1330 Å and Helioseismic and Magnetic Imager (HMI) magnetograms. Our data includes quiet sun, coronal hole and bright points features. The Si IV 1394 Å spectral line profiles are fitted with the single and then with double Gaussian function. We apply three essential conditions to certify any spectral profiles as asymmetric profile by double Gaussian model, that is, (1) the asymmetric profile must have a goodness-of-fit greater ( ) than one for the single Gaussian fits, (2) the minimum intensity of the first and second component to be 20 DN and (3) the distance between the centers of the two components to be more significant than 20 km/s. 1598 asymmetric profiles that are found out of a total of 103,000 profiles. The four types of profiles are dominated as only blue wing, only red wing, two clear peaks and none of the three types (it has two wings). The most significant number of asymmetric profiles corresponds to a profile with a component on its blue side. The lowest number is possessed by profiles inconsistent with single and double Gaussian fitting models (two wings). The asymmetries are concentrated on positions with high magnetic flux density. Also, asymmetric profiles arise in the large-scale bright lane-like areas in the SJI 1330 Å raster map. Corresponding to these areas, the magnetic flux concentration in the HMI raster map shows most probably, the network lanes. This correspondence can indicate the magnetic source. The asymmetric profiles may be owing to the reconnection of the open magnetic field of coronal hole with the bright points’ loops for bright points inside and the boundary of coronal hole. Also, we may contemplate a similar plan for asymmetric profiles at the coronal hole boundary, where the open magnetic fields of coronal hole may be reconnected with the quiet sun's close loops. However, the asymmetry of profiles at quiet sun may be owing to the reconnection of closed loops at this region. It is clear that the blueward and redward asymmetry are signatures of downflow, and upflow that may be caused by magnetic reconnection. However, the bidirectional jets derived from magnetic reconnection at the forming height of Si IV 1394 Å may be a reason for the asymmetric profiles with two clear peaks and two wings. Magnetic reconnections below the formation height of Si IV 1394 Å in the transition region may be a reason for upflows. Also, the profiles with blue wing may be relevant to the upflow spread of jets. These profiles are mainly sited away from the jet footpoints and on the network jets. The reconnection events above the formation height of Si IV 1394 Å or coronal return flows may be a reason for downflows. The red wing of the spectral line profiles is probably relevant to the downflow arising from reconnections that mainly placed around the footpoints of grid jets.

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Journal of the Earth and Space Physics
Volume 50, Issue 3
online publication date: 5 Oct 2024
October 2024
Pages 731-741

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