Eyeing the Sun. Probing the solar interface region. Introduction.

Abstract

![Figure][1] NASA/GSFC/CI LAB The Sun has been the subject of human curiosity since the dawn of time. It provides the energy that makes Earth habitable and is also the closest star to Earth. The Sun thus acts as a laboratory that provides detailed views of physical processes that occur in other, much more distant, astrophysical objects. Much progress has been made in understanding how nuclear fusion powers the Sun's 15-million-degree core and the mechanisms that transport this energy to the visible surface, where most of the light that reaches Earth is released. However, major unresolved questions linger about how the heliosphere, the Sun's outer atmosphere in which we live, is shaped and powered. We do not understand the counterintuitive rise of temperature from the 6000-K surface to millions of degrees in the Sun's outer atmosphere or corona. Equally puzzling is the solar wind, a high-speed continuous stream of particles that permeates space around Earth. These are not academic problems: Violent explosions such as flares and coronal mass ejections cause bouts of bad space weather that threaten power grids, satellites, and astronauts. These eruptions originate in and travel through this poorly understood solar atmosphere and wind. An important step in our quest to better understand such violent events is then to explore what drives the quiescent state of the solar atmosphere. In June 2013, NASA launched the Interface Region Imaging Spectrograph (IRIS), an Earth-orbiting small explorer satellite with a 20-cm telescope onboard. IRIS uses gratings to split the Sun's near- and far-ultraviolet light into its constituent wavelengths, in order to remotely probe the physical conditions in the interface region that consists of the chromosphere and transition region. Recent research suggested that it is here, at the interface between surface and corona, that answers to some of the more vexing unresolved questions in solar physics might be found. In this special section of Science , five Reports exploit the high-resolution images and spectra obtained with IRIS to present major advances toward a comprehensive understanding of how the solar atmosphere is energized ([sciencemag.org/special/iris][2]). Testa et al. find compelling evidence for the presence of high-energy particles generated during coronal nanoflares, small-scale heating events long hypothesized to drive coronal heating through the release of energy when magnetic field lines reconnect. These results provide constraints for models of the poorly understood mechanism that accelerates these electrons to such high energies and that probably acts under many other astrophysical conditions. Hansteen et al. reveal the presence of small-scale magnetic loops in high-resolution images of IRIS and advanced three-dimensional numerical models, resolving a long-standing debate about the nature of the transition region emission. These results vindicate the view that much of this emission does not originate in the “classical” transition region between the surface and the hot loops. Rather, the emission occurs in “unresolved fine structure” that has now been spatially resolved, thereby removing a major impediment to the modeling of coronal loops. Peter et al. exploit the power of high-resolution spectroscopy to reveal a solar atmosphere turned upside down: Hot plasma at 100,000 K is found closer to the solar surface than previously imagined, sandwiched by cool plasma both below and above. The hot plasma is heated by “bombs” in which the reconnection of magnetic fields leads to rapid heating. These unexpected results will likely lead to a reassessment of other phenomena in the low solar atmosphere, such as the mysterious Ellerman bombs discovered almost a century ago. De Pontieu et al. describe a chromosphere that is replete with twisting motions on very small scales that are associated with the heating of plasma to transition region temperatures. They are the signature of propagating Alfven wave pulses and provide support for recently developed models of atmospheric heating and dynamics and insight into the transport of helicity in the solar atmosphere. Tian et al. find evidence of high-speed jets at the root of the solar wind, fountains of plasma that appear to undergo rapid heating from chromospheric to transition region temperatures. These observations provide support for recent suggestions that the solar wind does not necessarily originate only from gentle evaporation in funnels rooted in strong field regions. Together, these results provide critical pieces in the still-unsolved puzzle of fully understanding of how the Sun shapes and affects the heliosphere. With solar activity at high levels, more advances from the imaging spectrograph onboard IRIS can be expected, especially with respect to flares and coronal mass ejections. [1]: pending:yes [2]: http://sciencemag.org/special/iris