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We study point-like explosive events (EE), characterized by emission in the far wings of spectral lines, in a quiet region near the South Pole, using Interface Region Imaging Spectrograph (IRIS) spectra at two slit positions, slit-jaw (SJ) observations, and Atmospheric Imaging Assembly (AIA) images. The events were best visible in Si iv spectra; they were weak in SJs, occasionally visible in 1600 Å and 304 Å AIA images, and invisible in higher temperature AIA images. We identified EEs from position–time images in the far wings of the Si iv lines and measured their distance from the limb. A Gaussian model of the height distribution showed that EEs occur in a narrow (0.9′′) height range, centered at 3.2′′ above the continuum limb at 2832.0 Å. On the disk, we found that they occur in network boundaries. Further, we studied the line profiles of two bright EEs above the limb and one on the disk. We found that what appears as broad-band emission is actually a superposition of 2 – 3 narrow-band Gaussian components with well-separated line profiles, indicating that material is expelled towards and/or away from the observer in discrete episodes in time and in space. The expelled plasma accelerates quickly, reaching line-of-sight (LOS) velocities up to 90 km s−1. Overall, the motion was practically along the LOS, as the velocity on the plane of the sky was small. In some cases, tilted spectra were observed that could be interpreted in terms of rotating motions of up to 30 km s−1. We did not find any strong absorption features in the wing of the Si iv lines, although in one case, a very weak absorption feature was detected. No motions indicative of jets were detected in SJ or AIA images. Reconnection in an asymmetric magnetic-field geometry, in the middle or near the top of small loops, is a plausible explanation of their observational characteristics. [ABSTRACT FROM AUTHOR]

The objective of this review article is to critically analyze turbulence and its role in the solar atmosphere and solar wind, as well as to provide a tutorial overview of topics worth clarification. Although turbulence is a ubiquitous phenomenon in the sun and its heliosphere, many open questions exist concerning the physical mechanisms of turbulence generation in solar environment. Also, the spatial and temporal evolution of the turbulence in the solar atmosphere and solar wind are still poorly understood. We limit the scope of this paper (leaving out the solar interior and convection zone) to the magnetized plasma that reaches from the photosphere and chromosphere upwards to the corona and inner heliosphere, and place particular emphasis on the magnetic field structures and fluctuations and their role in the dynamics and radiation of the coronal plasma. To attract the attention of scientists from both the fluid-dynamics and space-science communities we give in the first two sections a phenomenological overview of turbulence-related processes, in the context of solar and heliospheric physics and with emphasis on the photosphere-corona connection and the coupling between the solar corona and solar wind. We also discuss the basic tools and standard concepts for the empirical analysis and theoretical description of turbulence. The last two sections of this paper give a concise review of selected aspects of oscillations and waves in the solar atmosphere and related fluctuations in the solar wind. We conclude with some recommendations and suggest topics for future research. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we summarize the work done to build a UV-EUV spectral atlas of a prominence and we introduce some recent scientific results obtained from these data. For this work we used SOHO/SUMER data covering the full wavelength range accessible by the instrument (from about 800 to 1600Å), when a prominence and the quiet Sun (used as spectral reference) were observed in October 1998. We present here only a fraction of the data, which results in the analysis of more than 600 spectral lines, including some new identifications. Here we also present the differential emission measure obtained from the data, both for the quiet Sun and the prominence. The main differences in characteristics are discussed. [Copyright &y& Elsevier]

Abstract: With knowledge gained from the recent ULYSSES, SOHO, TRACE and other missions, a great deal of progress has been made in our understanding of the structure and dynamics of the various regions of the solar atmosphere: e.g., transition region and “quiet” corona, coronal holes and active region loops. However, no clear conclusions can yet be drawn about the fundamental processes that determine the existence and the underlying physics of these regions, especially their observed high temperature and flows. The lack of adequate spatial resolution has limited the study of the continuum between cool/hot, source/sink, and magnetic/non-magnetic regions. We consider here, magnetic reconnection as an example of the basic processes that are most likely involved at many different scales in heating and particle acceleration. Starting with a possible mechanism for reconnection (flux emergence), we consider different remote-sensing signatures of reconnection: flows, field rearrangement, impulsive heating, particle acceleration, wave generation, propagation and dissipation. We discuss their small-scale properties derived from current measurements and ways to improve their observation. We also consider how remote sensing and in situ measurements could be coordinated. To demonstrate how these advances might be achieved, we discuss the potential for discoveries offered by the Solar Orbiter, resulting notably from its relatively high spatial resolution and its capacity to observe for a substantial time in co-rotation with the Sun. [Copyright &y& Elsevier]

Abstract: The results from missions such as Yohkoh, Trace or SOHO have advanced our understanding of the solar corona and heliosphere, but also have raised many new questions. Measurements at higher spatial resolution and cadence are needed, some regions such as the solar poles or the inner heliosphere remain unexplored. The Solar Orbiter mission addresses those questions. UV imaging spectrometers may be included in the remote sensing instruments package for this mission. Fourier transform spectrometry (FTS) is an ideal solution for imaging systems as there is no need to scan the region of interest with a slit to get a 2D map, and the spectral range covered is only limited by the bandpass of the filter and the maximum optical path difference that can be obtained. FTS systems also allow to make observations at high cadence (100Hz). Working in the far UV (below 200nm) requires the use of reflective only optical systems. A solution for such an instrument has been designed at IAS and a mock-up has been built in order to test and validate the working principle. The first results of the alignment phase are shown. [Copyright &y& Elsevier]

We present SUMER/SOHO UV measurements of chromospheric oscillations of intensity, velocity, and linewidth observed in C i, S i, O i, and C ii lines, which are formed in the altitude range from 1000 km to 2000 km above τ 500=1. Oscillations in lines originating at similar altitudes exhibit different behaviors which we discuss in terms of the formation of the lines. [ABSTRACT FROM AUTHOR]