No abstract

During the 2007 -2008 minimum of solar activity, the internally occulted coronagraphs SECCHI-COR1 onboard the STEREO space mission recorded numerous jet-like ejections over a great range of latitudes. We have found more than 10000 white-light jets in the above-mentioned period. Sometimes they can be identified on the disk with bright points observed in ultraviolet images by EUVI. In this study we present a catalog consisting of jets observed by the SECCHI-COR1 instrument and their association with lower coronal activity (bright points, UV jets). Furthermore, their association with bright points in the context of previously proposed models is discussed. From the complete catalog we have selected 106 jets observed in both STEREO-A and STEREO-B images for which it is possible to derive their kinematics and point of origin.

We study broad red-shifted emission in chromospheric and transition region lines that appears to correspond to a form of post-flare coronal rain. Profiles of Mg II, C II and Si IV lines were obtained using the IRIS instrument before, during and after the X2.1 flare of 11 March 2015. We analyze the profiles of the five transitions of Mg II (the 3p − 3s h and k transitions, and three lines belonging to the 3d − 3p transitions). We use analytical methods to understand the unusual profiles, together with higher resolution observational data of similar phenomena observed by Jing et al. (2016). The peculiar line ratios indicate anisotropic emission from the strands which have cross-strand line center optical depths (k-line) of between 1 and 10. The lines are broadened by unresolved Alfvénic motions whose energy exceeds the radiation losses in the Mg II lines by an order of magnitude. The decay of the line widths is accompanied by a decay in the brightness, suggesting a causal connection. If the plasma is ∼ < 99% ionized, ion-neutral collisions can account for the dissipation, otherwise a of dynamical process seems necessary. Our work implies that the motions are initiated during the impulsive phase, to be dissipated as radiation over a period of an hour, predominantly by strong chromospheric lines. The coronal "rain" we observe is far more turbulent that most earlier reports have indicated, with implications for plasma heating mechanisms.

We take a broad look at the problem of identifying the magnetic solar causes of space weather. With the lackluster performance of extrapolations based upon magnetic field measurements in the photosphere, we identify a region in the near-UV (NUV) part of the spectrum as optimal for studying the development of magnetic free energy over active regions. Using data from SORCE, the Hubble Space Telescope, and SKYLAB, along with 1D computations of the NUV spectrum and numerical experiments based on the MURaM radiation–magnetohydrodynamic and HanleRT radiative transfer codes, we address multiple challenges. These challenges are best met through a combination of NUV lines of bright Mg ii, and lines of Fe ii and Fe i (mostly within the 4s–4p transition array) which form in the chromosphere up to 2 × 104 K. Both Hanle and Zeeman effects can in principle be used to derive vector magnetic fields. However, for any given spectral line the τ = 1 surfaces are generally geometrically corrugated owing to fine structure such as fibrils and spicules. By using multiple spectral lines spanning different optical depths, magnetic fields across nearly horizontal surfaces can be inferred in regions of low plasma β, from which free energies, magnetic topology, and other quantities can be derived. Based upon the recently reported successful sub-orbital space measurements of magnetic fields with the CLASP2 instrument, we argue that a modest space-borne telescope will be able to make significant advances in the attempts to predict solar eruptions. Difficulties associated with blended lines are shown to be minor in an Appendix.