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Solar observations at a resolution of 0.2 arc seconds show the reconnection and relaxation of magnetic braids in a coronal active region, leading to the dissipation of sufficient energy to heat the structures to about 4,000,000 K. A glimpse of solar corona heating The Sun's outer atmosphere, or corona, is millions of degrees hotter than its surface. The search for mechanisms capable of transferring energy from the Sun's interior to the periphery on such a scale has identified two strong candidates: wave heating is thought to heat the corona to 1.5 million K, and the reconnection and unravelling of magnetic braids has been proposed as the means of boosting the temperature towards 4 million K. New evidence to support that latter mechanism is provided by a five-minute series of images obtained by a high-resolution camera on-board a sounding rocket. The images reveal fine-scale braiding to a resolution of about 150 km in a coronal active region, and the images are consistent with energy production sufficient for the observed heating. It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona 1 , 2 , 3 , 4 , 5 . Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1 , 2 , 3 ). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed 6 , 7 , 8 , 9 , 10 , 11 , 12 to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred 13 , 14 , but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought 1 , 2 , 3 , 15 , 16 , 17 to have an angular width of about 0.2 arc seconds have not been resolved 10 , 18 , 19 , 20 . Fine-scale braiding has been seen 21 , 22 in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.

The solar wind consists of continuous streams of charged particles that escape into the heliosphere from the Sun, and is split into fast and slow components, with the fast wind emerging from the interiors of coronal holes. Near the ecliptic plane, the fast wind from low-latitude coronal holes is interspersed with a highly structured slow solar wind, the source regions and drivers of which are poorly understood. Here we report extreme-ultraviolet observations that reveal a spatially complex web of magnetized plasma structures that persistently interact and reconnect in the middle corona. Coronagraphic white-light images show concurrent emergence of slow wind streams over these coronal web structures. With advanced global magnetohydrodynamics coronal models, we demonstrate that the observed coronal web is a direct imprint of the magnetic separatrix web (S-web). By revealing a highly dynamic portion of the S-web, our observations open a window into important middle-coronal processes that appear to play a key role in driving the structured slow solar wind.Using detailed solar coronal observations and advanced magnetohydrodynamics simulations, the authors find that a coronal web above a region of coronal holes and active regions dynamically evolves and persistently drives the highly structured slow solar wind.

Coronal loops, seen in solar coronal images, are believed to represent emission from magnetic flux tubes with compact cross sections. We examine the 3D structure of plasma above an active region in a radiative magnetohydrodynamic simulation to locate volume counterparts for coronal loops. In many cases, a loop cannot be linked to an individual thin strand in the volume. While many thin loops are present in the synthetic images, the bright structures in the volume are fewer and of complex shape. We demonstrate that this complexity can form impressions of thin bright loops, even in the absence of thin bright plasma strands. We demonstrate the difficulty of discerning from observations whether a particular loop corresponds to a strand in the volume, or a projection artifact. We demonstrate how apparently isolated loops could deceive observers, even when observations from multiple viewing angles are available. While we base our analysis on a simulation, the main findings are independent from a particular simulation setup and illustrate the intrinsic complexity involved in interpreting observations resulting from line-of-sight integration in an optically thin plasma. We propose alternative interpretation for strands seen in Extreme Ultraviolet images of the corona. The “coronal veil” hypothesis is mathematically more generic, and naturally explains properties of loops that are difficult to address otherwise—such as their constant cross section and anomalously high density scale height. We challenge the paradigm of coronal loops as thin magnetic flux tubes, offering new understanding of solar corona, and by extension, of other magnetically confined bright hot plasmas.

Remote observations of the solar photospheric light scattered by electrons (the K-corona) and dust (the F-corona or zodiacal light) have been made from the ground during eclipses 1 and from space at distances as small as 0.3 astronomical units 2 – 5 to the Sun. Previous observations 6 – 8 of dust scattering have not confirmed the existence of the theoretically predicted dust-free zone near the Sun 9 – 11 . The transient nature of the corona has been well characterized for large events, but questions still remain (for example, about the initiation of the corona 12 and the production of solar energetic particles 13 ) and for small events even its structure is uncertain 14 . Here we report imaging of the solar corona 15 during the first two perihelion passes (0.16–0.25 astronomical units) of the Parker Solar Probe spacecraft 13 , each lasting ten days. The view from these distances is qualitatively similar to the historical views from ground and space, but there are some notable differences. At short elongations, we observe a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone 9 – 11 . We also resolve the fine-scale plasma structure of very small eruptions, which are frequently ejected from the Sun. These take two forms: the frequently observed magnetic flux ropes 12 , 16 and the predicted, but not yet observed, magnetic islands 17 , 18 arising from the tearing-mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but also reveal that, as recently predicted 19 , streamers are composed of yet smaller substreamers channelling continual density fluctuations at all visible scales. Observations of the solar corona by the Parker Solar Probe reveal evidence for the predicted dust-free zone and confirm that streamers comprise smaller substreamers that channel continuous multiscale density fluctuations.

We present progress on the translational tomography technique for measuring the three-dimensional structure of the corona from near-perihelion Wide-field Imager for Parker Solar Probe (WISPR) image sequences. Translational tomography makes use of noncircular motion of a camera to extract three-dimensional information from an optically thin subject. Parker Solar Probe (PSP) presents a special case both because of the particular structure of the corona and because of the nonlinear motion of the vantage point. We show improvements to a previous direct analytic method (described in Paper I of this series) and an alternative inversion pathway using a synthetic sequence of WISPR images. The newer method successfully reconstructs the correct locations of modeled coronal rays in a synthetic WISPR image sequence, with curvilinear camera motion modeled on the PSP orbit. We present the refined methodology and validation study, and show that the technique is ready for application to actual WISPR data.

In the first of a planned sequence of articles, we present a simple method for reconstructing radial density structures of the solar corona in the vicinity of the Parker Solar Probe (PSP) near and during perihelion passes. We describe how we model the apparent kinematics of stationary K-corona striae from the PSP Wide-field Imager for Parker Solar Probe (WISPR) viewpoint using a simple two-parameter model, form a partial basis of the data space that is a WISPR image sequence, and change the basis from image coordinates to “tomographic coordinates” in order to determine the parameters of such features. We apply the method to a simple three-dimensional model of a WISPR coronal flythrough, demonstrate the ways that it succeeds and fails, and discuss possible improvements to the sensitivity and applicability of the method for real WISPR data.

In Fall 2008 NASA selected a large international consortium to produce a comprehensive automated feature-recognition system for the Solar Dynamics Observatory (SDO). The SDO data that we consider are all of the Atmospheric Imaging Assembly (AIA) images plus surface magnetic-field images from the Helioseismic and Magnetic Imager (HMI). We produce robust, very efficient, professionally coded software modules that can keep up with the SDO data stream and detect, trace, and analyze numerous phenomena, including flares, sigmoids, filaments, coronal dimmings, polarity inversion lines, sunspots, X-ray bright points, active regions, coronal holes, EIT waves, coronal mass ejections (CMEs), coronal oscillations, and jets. We also track the emergence and evolution of magnetic elements down to the smallest detectable features and will provide at least four full-disk, nonlinear, force-free magnetic field extrapolations per day. The detection of CMEs and filaments is accomplished with Solar and Heliospheric Observatory (SOHO)/ Large Angle and Spectrometric Coronagraph (LASCO) and ground-based Hα data, respectively. A completely new software element is a trainable feature-detection module based on a generalized image-classification algorithm. Such a trainable module can be used to find features that have not yet been discovered (as, for example, sigmoids were in the pre- Yohkoh era). Our codes will produce entries in the Heliophysics Events Knowledgebase (HEK) as well as produce complete catalogs for results that are too numerous for inclusion in the HEK, such as the X-ray bright-point metadata. This will permit users to locate data on individual events as well as carry out statistical studies on large numbers of events, using the interface provided by the Virtual Solar Observatory. The operations concept for our computer vision system is that the data will be analyzed in near real time as soon as they arrive at the SDO Joint Science Operations Center and have undergone basic processing. This will allow the system to produce timely space-weather alerts and to guide the selection and production of quicklook images and movies, in addition to its prime mission of enabling solar science. We briefly describe the complex and unique data-processing pipeline, consisting of the hardware and control software required to handle the SDO data stream and accommodate the computer-vision modules, which has been set up at the Lockheed-Martin Space Astrophysics Laboratory (LMSAL), with an identical copy at the Smithsonian Astrophysical Observatory (SAO).

Digital image data are now commonly used throughout the field of solar physics. Many steps of image data analysis, including image co-alignment, perspective reprojection of the solar surface, and compensation for solar rotation, require re-sampling original telescope image data under a distorting coordinate transformation. The most common image re-sampling methods introduce significant, unnecessary flaws into the data. More correct techniques have been known in the computer graphics community for some time but remain little known within the solar community and hence deserve further presentation. Furthermore, image distortion under specialized coordinate transformations is a powerful analysis technique with applications well beyond image resizing and perspective compensation. Here I give a brief overview of the mathematics of data re-sampling under arbitrary distortions, present a simple algorithm for optimized re-sampling, give some examples of distortion as an analysis tool, and introduce scientific image distortion software that is freely available over the Internet.``First get your facts straight. Then you can distort them as you please.'' - Mark Twain

On 7 and 8 March 1996, the SOHO spacecraft and several other space- and ground-based observatories cooperated in the most comprehensive observation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the morphology of the plumes observed over the south pole of the Sun during the SOHO observing campaign. Individual plumes have been characterized from the photosphere to approximately 15 R yielding a coherent portrait of the features for more quantitative future studies. The observed plumes arise from small ( 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrations, on chromospheric network cell boundaries. They are denser and cooler than the surrounding coronal hole through which they extend, and are seen clearly in both Feix and Fexii emission lines, indicating an ionization temperature between 1.0-1.5 x 10^sup 6^ K. The plumes initially expand rapidly with altitude, to a diameter of 20-30 Mm about 30 Mm off the surface. Above 1.2 R plumes are observed in white light (as 'coronal rays') and extend to above 12 R. They grow superradially throughout their observed height, increasing their subtended solid angle (relative to disk center) by a factor of 10 between 1.05 R and 4-5 R and by a total factor of 20-40 between 1.05 R and 12 R. On spatial scales larger than 10 arc sec, plume structure in the lower corona (R < 1.3 R) is observed to be steady-state for periods of at least 24 hours; however, on spatial scales smaller than 10 arc sec, plume XUV intensities vary by 10-20% (after background subtraction) on a time scale of a few minutes.[PUBLICATION ABSTRACT]