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 β 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 Hα 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 recently, an observational picture of the chromospheric-coronal mass and energy transport has developed. De Pontieu et al. 2009, McIntosh & De Pontieu (2009a), and McIntosh & De Pontieu (2009b) have investigated the connection of these “type-II” spicules and the ubiquitous insertion of plasma, preheated to typical corona temperatures, in quiet, coronal hole, and active regions using a combination of spectral and imaging observations. These observations in some measure dispel the common belief that the coronal “tail” wags the chromospheric “dog” and ties directly into all of the issues raised above.

3/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System

Figure 2: Popularized depiction of space weather; grossly out of scale, but nicely illustrating the impact of wind, UV radiation, and mass ejections on Earth. The Modern Relevance of the Chromosphere The relentless emergence and convective forcing of magnetic flux through the solar surface determines the cycle of mass and energy inside the chromosphere and into the corona, producing the bulk of the Sunʼs UV and EUV emission. Understanding the mass-energy cycle and the formation of this emission provides essential physical insight into the radiative forcing of the terrestrial atmosphere, as embodied in state-of-the-art climate models (e.g., Marsh et al. 2007). Recent observational results discussed above have indicated that the release of mass and non-thermal energy at or near the base of the chromosphere is the seed of the solar wind, another key process whose physical origin remains elusive. Understanding the solar wind is relevant for society not only through direct space weather effects, but also because of more subtle indirect forcing of the terrestrial atmosphere, for example by modulation of Earth-impacting energetic particles. Our models of coronal magnetism and eruptive phenomena rely almost exclusively on photospheric vector field measurements to extrapolate the magnetic force lines into the corona. These efforts typically fail to meet desired metrics on the model field (DeRosa et al. 2009), with the Lorentz forces acting on the field in the upper photosphere being the likely culprit. These forces can be so large that the inferred electrical currents are not field-aligned, and the magnetic force lines either never make it into the corona, or are deflected to very different positions before they enter the corona. The obvious remedy to this issue is to measure the vector field directly in the low-β regime of the upper chromosphere, and extrapolate from there. In principle, measurements of the three-dimensional, time varying, magnetic field at the heliospheric base can be made using imaging spectro-polarimetry or radio measurements. At the very least the line-of-sight magnetic fields can be accurately determined, and, under favorable conditions, so can the transverse components. Very recent observations also have demonstrated the presence of ubiquitous high speed (magnetized) flows that are closely aligned with the background magnetic field (McIntosh & De Pontieu 2009a). Thus, the combination of kinematic velocities and vector field polarimetry offers the highest potential of success for making routine measurements of the vector magnetic field at the heliospheric base (see also the white paper by Philip Judge).

Obstacles to Progress in Traversing the Heliospheric Frontier Four effects mainly set the chromosphere apart from the much denser, near equilibrium photosphere. Each of these presents barriers to numerical simulations of chromospheric structure and physical processes; these also are potential obstacles to remote sensing.

4/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System

1. The chromosphere encompasses the domain in which magnetic fields begin to gain dominance over purely hydrodynamic forces. It contains intermixed regions where the plasma β is greater than and less than unity, with substantial spatial and temporal fluctuations. Strong gradients can develop in the chromospheric field owing to the small magnetic diffusivity. 2. The chromosphere is optically thin at most visible wavelengths, apart from a few strong spectral lines. Because these lines also dominate the transport of radiative energy, they must be handled with so-called non-LTE formation: challenging to formulate, dependent on often poorly known atomic physics, and demanding to compute. 3. Recombination time scales of hydrogen - the heart of the “ionization valve” discussed earlier - can be much longer than dynamical time scales. Consequently, the ionization state at any instant depends on the previous history of the plasma. Chemical equilibrium time scales also are long compared with the dynamics. This can affect key species like CO, instigator of “molecular cooling catastrophes” in places where mechanical heating has temporarily abated (Ayres 1981). As with the magnetic field, strong gradients in ionization and chemistry can develop, with consequences for the plasma energy balance (e.g., Leenaarts et al. 2007). 4. There also is mounting evidence that the partial ionization leads to multi-fluid effects, very different from single-fluid MHD (e.g., Arber et al. 2007). This has clear consequences for wave propagation and mode couplings, but also might affect the outcomes when Lorentz forces act during, say, flux emergence, fibril dynamics, type-II spicule excitation, or filament disruption. The severe departures from the equilibrium state make it much harder to model the chromosphere at a level of realism approaching what has been achieved for the photosphere.

The State of Modeling Efforts In high-resolution modeling efforts, the near-equilibrium photosphere has served as an important laboratory to vet our understanding of kinetic transport, culminating in the sterling success of modern 3D convection models (e.g., Stein & Nordlund 2000; Vögler et al. 2005). The important lesson is the pivotal role played by small scale phenomena in controlling transport processes. Solar granular convection is a highly non-local phenomenon, driven from a thin surface thermal boundary layer. Here, radiative cooling produces low entropy gas that, in turn, forms strong downdrafts concentrated in the narrow intergranular lanes, smaller than 100km in size (less than a tenth the diameter of the parent granule). Most of the buoyancy work occurs in such tiny structures. Similar comparisons currently underway for the more complex physical situation of magneto-convection show that dynamical control by small scale elements holds generally: in magnetic fibrils of the quiet Sun (e.g., Stein & Nordlund 2006), sunspot umbrae (Schüssler & Vögler 2006; Rimmele 2008), and the surrounding penumbae (Scharmer et al. 2008; Rempel et al. 2009). Now, it is the chromosphere that stands as the frontier for the palpably more challenging (Non-LTE) radiation-magnetohydrodynamic (R-MHD) simulations required when the classical transport processes begin to break down, and the magnetic field and non-linear acoustic disturbances take over (e.g., Wedemeyer-Böhm et al. 2004; Abbett 2007; Martinez-Sykora et al. 2008). Most of the mass and non- radiative energy that sustain the outer atmosphere resides in the chromosphere, and from here are channeled upwards, thanks mainly to the magnetic field (e.g., Gudiksen & Nordlund 2005). However, it still is far from clear exactly how the transport and heating are accomplished, and in particular the roles of the copious small and rapid phenomena, routinely observed in different guises as spicules, mottles, fibrils, jets, Ellerman bombs, and other events with similarly evocative names. Some of these phenomena have physically consistent explanations, for example the creation of dynamic fibrils and mottles by means of magneto-acoustic shocks - driven themselves by wave leakage from the solar interior (De Pontieu et al. 2007; Rouppe van der Voort et al. 2007). However, for the most part, a complete description of the link between the photospheric drivers and the resulting hot outer atmosphere remains elusive. Fully 3D, R-MHD numerical simulations spanning from the upper convection zone through the corona - treated as a coherent system - are starting to appear, but cannot yet address (all) the salient physical ingredients on scales larger than a few granules or one super-granule. Thus, a close collaboration between observers and theorists, similar to the successful model for photospheric processes, is essential to make progress in the chromospheric arena.

5/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System

Figure 3: Snapshot of a 3-D R-MHD quiet Sun simulation from De Pontieu et al. (2007). The coloring illustrates the plasma temperature from the low chromosphere (red) to higher temperatures near the transition region (green). New Opportunities To achieve a successful close comparison between chromospheric observations and theory, much work is needed on both sides. For example, from the observational perspective, the necessary polarimetric accuracy required to capture chromospheric vector fields is close to achievable at the desired spatial and temporal scales. With the present instrumentation and telescopes (dividing up the photons into space, time, frequency, and polarization states quickly exhausts the supply) although progress is very rapidly evolving in this area.

Innovations in both areas are actively being pursued, especially telescope aperture. Notably, the 4m Advanced Technology Solar Telescope (ATST) will be the flagship international facility in the coming decade to carry out pioneering high (spatial/temporal/spectral) resolution studies of the solar atmosphere. On the instrumental front, designers are attempting to address several fundamental, sometimes conflicting, requirements. First, recording multiple spectral lines is necessary to probe the many scale heights spanned by the chromosphere. Second, to investigate the discrete components of the system and adequately trace the 3D interconnectivity forced by the magnetic field, these detailed measurements must be obtained over a field of view capturing several cells of the supergranulation “network”, a substantial portion of a prominence or filament. Third, it is essential to collect the multi-line diagnostics simultaneously, to avoid ambiguities introduced by fast structure evolution and solar surface kinematics. Prototype instruments like IBIS at NSOʼs Dunn Solar Telescope and CRISP at the Swedish Solar Telescope on La Palma already are driving significant breakthroughs in understanding (e.g., de la Cruz Rodriguez et al. 2010). We should stress, however, that the detailed ATST/CRISP/IBIS observations cannot exist in a vacuum, if we are to fully understand the transfer of mass and energy across the disparate scales of the chromosphere, assess its impact on the heliosphere, and support missions like SDO and IRIS that are blind to chromospheric magnetism. Here is where innovative synoptic instrumentation will play a key role, exploiting the same observational diagnostics, while pushing the limits of filter, detector, and image post-processing technology.

Deployment of high performance instruments with advanced capabilities, and operating at solar telescopes of large aperture, supported by next-generation “synoptic”, or patrol, instruments is a clear priority for the coming decade.

6/7 Heliophysics 2012 NRC Decadal Survey - The Solar Chromosphere: The Inner Frontier of the Heliospheric System

A related issue is the need to achieve high spatial resolution, of order 0.1” (~70km) or better, over the extended fields of view, to resolve the fine scale structures in which much of the magnetic “action” likely takes place. Deployment of a large aperture solar telescope in space (e.g., Solar-C “Plan B”) employing advanced filter imaging technology will ensure a seeing-free, consistently high signal-to-noise observations. A complementary option for ground-based observation are multi-conjugate and ground layer adaptive optics systems.

Development and deployment of wide-field Adaptive Optics systems should be strongly supported.

In terms of theory, efforts to simulate the structure and dynamics of the photosphere, chromosphere, and corona as one coherent system are underway. Prototype codes, such as Oslo's BIFROST, approach realism by incorporating time-dependent hydrogen ionization, non-equilibrium radiation transport, multi-fluid behavior, and diverse magnetic field configurations, to mention a few key components; all challenging to implement and very demanding to compute (e.g., Fig. 3 and Martinez- Sykora et al. 2009). Forward modeling not only is a key to sort out the most relevant physical effects, “joining the observational dots,” but also a tool for vetting measurement techniques: model intensities can be subjected to a simulated observation to judge how much of the original physical information is recovered by that procedure.

State-of-the-art forward modeling should be strongly supported, not only to achieve physical insight, but also to validate and enhance observational strategies. This will require highly parallel high-end computing services with tens of thousands of cores and tens of millions of CPU hours. A necessary adjunct is access to diverse and accurate atomic physics data.

As a final remark, coordinating space-based UV, visible, and X-ray imaging (say, from Hinode) with ground-based visible and infrared spectroscopy continues to be a mainstay of contemporary solar astronomy. The chromosphere, however, presents a difficult remote-sensing target, because it is mostly transparent in the visible, aside from the cores of a few strong resonance lines. At the same time, the chromosphere is very opaque in the Lyman continuum below 91nm. This is a pity because many solar space imagers and spectrometers operate in the EUV around 20-30nm, and consequently cannot directly probe this region. Finding new windows into the state of the chromospheric gas thus is both a major challenge and an opportunity. For example, UV imaging spectroscopy longward of the LyC edge, as realized in the IRIS instrument currently under development, will allow us to “see” deep into the chromosphere, investigating the crucial temporal evolution of hydrogen ionization in the “clapotisphere” (Rutten 1995), and the upward extension of the explosive small-scale energy releases sprinkled across the magnetic network. Important additional views to the chromosphere lie in other regions of the electromagnetic spectrum. At millimeter wavelengths, for example, the Atacama Large Millimeter Array (ALMA; under construction in northern Chile) will probe the low-to-mid chromosphere with directly measured temperature contrast and high polarimetric accuracy at resolutions of 0.1” (or better). In addition, the Expanded Very Large Array (EVLA) will investigate explosive activity and particle acceleration at the top of the chromosphere in centimetric light, at the crucial level where magnetic complexity evolves into coronal heating. Unfortunately, these facilities mainly are dedicated to nighttime astronomy, and have relatively low solar duty cycles. In contrast, the Frequency Agile Solar Radiotelescope (FASR) will be a dedicated wide-band (1cm-6m) array that can probe the full-sun from the mid-chromosphere all the way to the corona, monitoring the thermal and magnetic conditions of the outer atmosphere in a “systems” context, providing complimentarity to missions like SDO.

Chromospheric physics is at the cutting edge of our understanding of the solar atmosphere. The chromosphere is a physical domain where current observations point to a convergence of complex effects - magnetic, kinematic, and radiative - whose impact on flows of mass and energy into the heliosphere is high and at the same time our potential to achieve physical insight is very promising. Consequently, there is clear need and drive to develop the observational diagnostics and interpretative tools vital to our understanding of this, the heliospheric systemʼs crucial innermost boundary.

7/7