# Reconstruction of a helical prominence in 3D from IRIS spectra and images

## TL;DR

This paper reconstructs the 3D structure and dynamics of a helical solar prominence using IRIS spectra and images, revealing the trajectories, velocities, and projection effects of plasma knots.

## Contribution

It introduces a method to derive 3D velocity vectors of prominence knots from IRIS spectral and imaging data, providing detailed insights into prominence structure and motion.

## Key findings

- Knots travel along elliptical trajectories.
- Velocities reach up to 65 km/s.
- Projection effects cause spiral-like appearance.

## Abstract

Movies of prominences obtained by space instruments e.g. the Solar Optical Telescope (SOT) aboard the {\it Hinode} satellite and the Interface Region Imaging Spectrograph (IRIS) with high temporal and spatial resolution revealed the tremendous dynamical nature of prominences. { Knots of plasma belonging to prominences} appear to travel along both vertical and horizontal thread-like loops, with highly dynamical nature. The aim of the paper is to reconstruct the 3D shape of a helical prominence observed over two and a half hours by IRIS. From the IRIS \ion{Mg}{ii} k spectra we compute Doppler shifts of the plasma inside the prominence and from the slit-jaw images (SJI){ we derive the transverse field in the plane of the sky. Finally we obtain the velocity vector field of the knots in 3D. We reconstruct the real trajectories of nine knots travelling along ellipses. The spiral-like structure of the prominence observed in the plane of the sky is mainly due to the projection effect of long arches of threads (up to 8 $ \times 10^4$ km). Knots run along more or less horizontal threads with velocities reaching 65 km s$^{-1}$. The dominant driving force is the gas pressure.