Abstract:
The Visible Emission Line Coronagraph on board Aditya-L1 provides continuous spectroscopic observations of the solar corona in the Fe XIV 5303 Å emission line, which are a valuable tool to understand the plasma properties and dynamics in the near-Sun solar corona. One of the widely debated topics in the spectral line observations are the observed line widths, which are always in excess of the thermal width due to the ion temperature. In this context, we report estimates of the intensity, width, and Doppler velocity of the 5303 Å emission line along a preflare coronal loop in the heliocentric distance (r) range 1.10─1.24R⊙. The Doppler velocity of the line shows a significant blueward shift (≍11 km s─1) along one of the legs of the loop, indicating upflow of plasma. There is a simultaneous increase in the line width (≍0.2 Å) also. The changes in the above two parameters are strongly correlated. The correlation coefficient is ≍−0.9. The results indicate preflare activities are likely the cause for the excess line widths in the present case. The large change in the Doppler velocity, toward the blue side, confirms the related plasma upflows. The respective line parameters along either leg are nearly equal at the apex of the loop. The changes in the width and Doppler velocity in the other leg are comparatively minimal. The estimated upper limit for the Alfvén wave energy flux density at r ≍ 1.1R⊙ in the loop is ≍39%.
