Monochromatic X-ray Observations of Solar Cusps

Abstract

In this work, we investigate the possibility of observing cusps—hot, pointed structures above solar flare loops—using the Mg XII spectroheliograph onboard the ‘‘CORONAS-F’’ satellite. This instrument produced monochromatic images of the solar corona in the Mg XII \(\lambda\) 8.42 \(\text{\AA}\) line, which is formed at temperatures exceeding 4 MK. Observations of cusp tips, which are otherwise challenging due to the dominance of the cool coronal background, can aid in the analysis of reconnection dynamics and magnetic field topology. We examined three cusps observed using the Mg XII spectroheliograph and compared them with observations from the Extreme Ultraviolet Imaging Telescope (EIT) and the Soft X-ray Telescope (SXT). We concluded that EIT can observe cusps only when they are located on the limb, while the visibility of cusps in SXT images depends on the relative positioning of the cusps and adjacent coronal structures. In contrast, Mg XII images are free from these issues and are the most suitable for studying cusps. The characteristic sizes and lifetimes of the analyzed cusps are 300–500 Mm and 4–16 hours, respectively. For two cusps, motion of the cusp tip (i.e., the X-point) was observed over 2–3 hours, with characteristic speeds of 10–40 km s\({}^{-1}\). This suggests that reconnection can occur on timescales of several hours after the impulsive phase of the flare. Using the measured cusp tip velocities, we estimated the magnetic reconnection rate: \(M_{A}\approx 0.13\).