The Solar Chromosphere: The Inner Frontier of the Heliospheric

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 fourdimensional 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.
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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.
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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 convectiontriggered 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 pmode 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.
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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.
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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 nonradiative 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.
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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.
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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 MartinezSykora 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.
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