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The Solar Chromosphere: the Inner Frontier of the Heliospheric System

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The Solar Chromosphere: the Inner Frontier of the Heliospheric System Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System The Solar Chromosphere: The Inner Frontier of the Heliospheric System A White Paper Submitted to the Heliophysics 2012 NRC Decadal Survey S. W. McIntosh (High Altitude Observatory; corresponding author) T. Ayres (University of Colorado Boulder) T. Bastian (National Radio Astronomy Observatory) T. Berger, B. De Pontieu, C. Schrijver (Lockheed Martin Solar and Astrophysics Laboratory) G. Cauzzi, K. Reardon (INAF - Arcetri Astrophysical Observatory) M. Carlsson, V. Hansteen (University of Oslo) C. DeForest (Southwest Research Institute) D. Gary (New Jersey Institute of Technology) S. Keil, A. Tritschler, H. Uitenbroek (National Solar Observatory) R. J. Rutten, J. Leenaarts (University of Utrecht) S. Solanki (Max Planck Institute for Solar System Research) S. White (Space Vehicles Directorate, AFRL) Executive Summary: The highly intricate, dynamic, phenomena-rich solar chromosphere results from the profound interaction of plasma motion and magnetic fields. The fruitful combination of high resolution multi-wavelength imaging and polarimetry with detailed numerical simulations offer the promise to unlock the mysteries of this crucial interface between the photosphere and corona - as it did so successfully in the study of photospheric granular convection. Understanding the solar chromosphere will open a new window onto mass and energy transport at the crucial innermost boundary of the heliospheric system, from which the corona and solar wind are powered, and the bulk of the Earth-impacting UV radiation is released, all key players in “space weather.” Recent momentum in the development of new instruments, theory, and facilities must be recognized, encouraged, and sustained to fully exploit ongoing four- dimensional observational dissection of the “magnetic complexity zone.” We are at a time when comprehensive modeling of chromospheric physics is tractable and reaching maturity. As a result, we may be able to slide this crucial, but currently missing, piece of the picture into the grand heliospheric puzzle. 1/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System Figure 1: Cartoon evolution of the chromosphere (Schrijver 2001). It has grown from a simple stationary “layer” of the 1950's; to topologically more diverse view of the 1980's, mixing cool (”CO”) and hot gas at similar altitudes, pervaded by magnetic “flux tubes”, albeit still largely static; to contemporary highly dynamic and spatially chaotic, “magnetic complexity zone.” Recent observational evidence now points to the major influence of this highly complex region as the lower boundary of the heliosphere. Introduction: Observational Context From ancient times, the Sun's chromosphere has been a subject of fascination. With the naked eye, it is seen only at eclipse, as a string of brilliant rosy beads above the solar limb, a bright and colorful contrast to the rather pale visage of the white light corona. In the modern era, the chromosphere retains its fascination, but now as a key interface region of the solar outer atmosphere, the de facto inner boundary of the heliosphere. While the photosphere above the convective overshoot layer is relatively quiescent (with the notable exception of the abundant acoustic and internal gravity wave flux) in the chromosphere this placidity gives way to the highly non-equilibrium, dynamic, mechanically heated, forced temperature inversion of the higher altitudes; a segue into the equally enigmatic super-hot (few MK) corona. Like many boundaries, the transition from apparent order to disorder is intriguing and is determined by magnetism that permeates the outer solar atmosphere. The chromosphere at once hosts some of the coldest material in the whole solar atmosphere (T~3500K) as seen in off-limb emissions of 4660nm CO lines (Solanki et al. 1994) and very warm gas (T>104K) associated with ionizing shocks. The magnetic fields that pervade this region are highly twisted and tangled because of the shift from gas dominance in the deep photosphere (field lines frozen to the plasma, dragged about by surface flows) to field dominance in the corona (ionized plasma forced to flow along the field lines, bottled up in coherent large scale structures). At either extreme, the field takes on relatively simple forms: small scale intense flux concentrations in the high-β photosphere 1 and delicate near-potential loop-like structures in the low-β corona. Paradoxically, the field in the intervening β≈1 zone takes on a chaotic topology, reconnecting willy-nilly, and blossoming out into an overarching “canopy,” the conspicuous source of the meandering mottling in Hα filtergrams (see cover page). In fact, the view presented in the rightmost panel of Fig. 1 would support a renaming of the chromosphere as the “magnetic complexity zone.” The signature property of the chromosphere is its enormous variability in vertical extent (Mms “thick” in internetwork regions to only a few 100km in hot, plage-rooted “mossy regions”). The prime suspect 1$ The plasma ! is the ratio of gas pressure to magnetic pressure. 2/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System driving chromospheric weight fluctuation is the local and temporal variation in the ionization of its dominant constituent, hydrogen. Given its large abundance and First Ionization Potential (FIP), hydrogen ionization acts as a sponge to soak up energy, buffering the gas somewhat from local heating spikes (like acoustic shocks), moderating the temperatures. Further, ionization frees electrons to fuel steady radiative cooling by collisional excitation of abundant species such as Fe, Mg, and Ca. This “ionization valve” works effectively because of the large dynamic range of the electron fraction, ne/nH - only 10-4 at the base of the “classical” inter-network chromosphere (where the electrons come from singly ionized metals); which increases dynamically (sometimes by a factor of 100; Leenaarts et al. 2007) as convection- triggered shocks (Rouppe van der Voort et al. 2007) propagate through the system. It approaches unity at the top where hydrogen is mostly ionized. These ionization effects permit the gas to balance heating even while the overall density falls outward (Ayres 1979). In the “non-classical” magnetic network and plage regions of the chromosphere a host of other phenomena are excited, as we will see below. So, in an overall sense, the chromosphere is a creature of hydrogen ionization. Although this simple picture is appealing, it obscures the deep complexity of the region. In active regions the sunspot umbrae, penumbrae, and moats show a thick fibril web outlining the field topology as well as a host of dynamic phenomena such as umbral flashes, penumbral waves, penumbral microjets, Ellerman bombs and more. Filaments and prominences (on- and off-disk, respectively) represent enormous clouds of chromospheric gas embedded in the corona - that have the propensity to produce CMEs when erupting. A goal of contemporary solar physics, then, is to dig deeper into this intricate interface layer. Fundamental unresolved issues linking the chromosphere to the “Big Picture” are: •How is the atmosphere heated and the solar wind accelerated? •How is energy transported into, and across, the chromospheric interface? •What is the source of the conspicuous chromospheric structure, both long-lived and transient? •What is the role of the chromosphere as a reservoir of mass and energy to feed the overlying corona (whose heating and mass loading requirements are but a small fraction of those of the chromosphere itself)? •What role does the ubiquitous magnetic field play in all of the above? The chromosphere was fertile ground in the 1950's and 1960's for the early development of specialized treatments of radiation transport and spectral line formation to confront the extreme departures from classical Local Thermodynamic Equilibrium (LTE) at the very low densities and high temperatures of the region. “Scattering” in strong resonance transitions on the one hand becomes a dominant radiation transport mechanism, but on the other hand, the induced non-locality of the radiation fields creates serious challenges for remote sensing and numerical simulation. The Solar Optical Telescope of the Hinode satellite has motivated something of a renaissance in chromospheric physics and relevance. Underscoring the complexity of the region, Ca II and H# imaging sequences at the solar limb recorded by the Hinode satellite reveal an incredibly rich, relentlessly dynamic, and highly structured environment. Indeed, the Hinode observations were of such quality that two distinct populations of “spicule” (Roberts 1945) could be discerned (De Pontieu et al. 2007). The first, evolving on timescales of a few minutes, typical of the underlying photospheric convection and internal p- mode envelope, showed material rising and falling along the same magnetically determined path at the speeds between 20 and 40km/s in a process described by Hansteen et al. (2006). The second, previously unresolved, class is taller (>1Mm taller), faster (50-150km/s), and evolves faster (10-100s) before quickly fading, a hint of heating to higher temperatures. Hinode also revealed that both of these spicule flavors undergo significant transverse (Alfvénic) motions. In coronal holes the estimated wave flux is enough to accelerate the fast solar wind (De Pontieu et al. 2007). More
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