More work remains ahead. While IR observations have already changed our physical understanding of the Sun in important ways, at the current time the behavior of our Sun presents us with many problems. The strange trends seen in the current sunspot Cycle 24 and the difficulty with making accurate predictions of the solar magnetic dynamo loom large as of this writing. The exact heating mechanism for the solar corona remains elusive. The evolution of magnetic fields in the chromosphere and the corona, especially the conditions which generate solar flares and coronal mass ejections, is a key problem with a direct connection to spaceweather here at Earth. Having enjoyed the advancements from the past 20 years of work in this field, we must recognize that for continued advancement in the next two decades, we have to continue in the tradition of the original experimentalists. While some infrared observations have become a new comfort zone for solar physics, the hard work of observing new wavelengths must be done. New technology must be used to develop new instrumentation to allow those windows to be opened, and these instruments must be used by careful observers to explore the questions posed by our Sun.
We are at an exciting time in the history of infrared solar physics research with the development of new instruments and new telescopes. At the recently built NST a cooled-grating spectrograph spectropolarimeter is planned to come on-line soon, the Cyra instrument (Cao et al., 2010). Cyra will have dual-beam polarimetric capability from 1000 to 5000 nm, and the cryogenic spectrograph will give much lower background signals for studies from 3000 to 5000 nm. The 1.5-m GREGOR solar telescope at the Observatorio del Teide on Tenerife is home to the GRIS instrument (Schmidt et al., 2012; Collados et al., 2012), which is designed to perform spectroscopy from 1000 – 2300 nm and spectropolarimetry from 1000 – 1800 nm. Initial intensity spectra from GRIS show excellent image quality can be obtained with the system, and this promises that future polarimetric data from the instrument will be scientifically very interesting. Update
A new 4-m all-reflecting IR optimized telescope at the NSO was proposed by Livingston (1994); currently under construction, the 4-m DKIST telescope will deliver 3 times the spatial resolution of the McM-P or the NST facilities. The telescope and the AO system is designed to achieve 0.08 arcsecond resolution at 1565 nm. The infrared instruments planned for the DKIST, the Cryo-NIRSP, and the DL-NIRSP (Lin, 2003; Rimmele et al., 2005) will provide unprecedented observations from 1000 to 5000 nm, on the solar disk, at the limb, and into the solar corona. With these instruments, the DKIST will deliver all of the scientific and technical advantages of observing in the IR solar spectrum at the highest spatial resolution we can now achieve from the ground.
Building on work done by the COMP instrument, Gallagher et al. (2012) have proposed a large coronagraph for the COSMO project which will examine the coronal magnetic fields of the Sun using the polarization of the 1075 nm [Fe xiii] spectral lines. This instrument would operate in a more synoptic manner than would the DKIST Cryo-NIRSP, and it would have a larger field-of-view.
Spectropolarimetric observations analyze the spatial distribution of solar radiation across wavelength and polarization state, and the result is that the measured intensity is a function of four variables . The solar radiation varies inherently as structures change on the Sun, and also as the Earth’s atmosphere alters the incoming wavefronts of sunlight, and so ideally all four of these parameters would be sampled simultaneously. Unfortunately detectors are only 2d, and so modern solar instrumentation divides these four variables in different ways and samples them across short intervals of time. As detector sizes become larger though, it becomes possible to limit the area of the Sun which is observed and to measure more variables simultaneously. Using a slit spectrograph fed with a 64 × 32 array of fiber optics, the SPIES instrument Lin and Jaeggli (2012) simultaneously measures using a 2048 × 2048 array detector. Preliminary results have been shown for spectropolarimetry of 1083 and 1565 nm. Nearly simultaneous measurements are possible with quantum well infrared photodetector (QWIP) cameras as changing the bias voltage applied to the array can alter the wavelength response (Li et al., 2002). Dual beam systems measuring orthogonal polarization are common in solar physics now, and coupled with a slit spectrograph and they minimize the problems caused when atmospheric distortion changes between polarimetric measurements. However, a quad-beam polarimetric system to measure linear I, Q, U Stokes vectors simultaneously has been used for comet observations (Geyer et al., 1996) and, more recently, it has been coupled with a large format array (Kawabata et al., 2008). With the introduction of circular polarization analysis as is currently done with optimal chopping techniques, one may truly measure simultaneously. Stokes measurements at the pixel level using QWIP detectors has been discussed by Serna Jr (2002); one may envision sampling simultaneously with a device like this. While the problem of sampling the solar intensity in all of the desired ways is not a unique problem to infrared observations, the unique capabilities of some IR detectors may facilitate a solution in the IR spectrum sooner than at shorter wavelengths.
We currently know that the Mg i 12 318 nm line is the most sensitive probe of the solar magnetic field. However, with a value for the wavelength of the line is the key in determining its magnetic sensitivity. A line at a shorter wavelength which was found to have a significantly larger value would supercede the Mg i line.
A set of weak lines at 4135 nm have been observed recently and are a key target for the Cyra instrument. The lines are weak and have a strong dependence on temperature; they almost completely disappear at the low temperatures of a sunspot umbra. Some of the lines are currently not well-understood, and one shows some very large Zeeman splitting according to Clark (2005) and as seen in more recent data taken at the McM-P (see Figure 26*). While the line may be absent from sunspots and active regions, it has the potential to provide critical diagnostics for the weak magnetic fields of the quiet Sun.
Until the DKIST comes on-line, the NST and McM-P are the two telescopes that can explore this region of the spectrum, with the McM-P having the advantage of being at a drier site. The Celeste instrument (McCabe et al., 2003) currently represents the only cooled-grating instrument dedicated to solar use at these wavelengths, though other instruments may have abilities here. For use on warm spectrographs, a camera system which functions at these wavelengths is the Si:Ga array detector used by Gezari et al. (1992) and employed for solar observations by Gezari et al. (1999). This array is nominally sensitive from 5000 to 18 000 nm. Newer camera systems which can be used in this wavelength range include QWIP cameras, which were used for astronomical imaging by Ressler et al. (2001) and have been fabricated in large formats of 1024 × 1024 (Jhabvala et al., 2004). This particular array is sensitive from 8400 to 9000 nm, although during fabrication the wavelength response of the array can be modified.
Because the IR spectrum is dominated by spectral lines from molecules, a full understanding of the magnetic sensitivity of molecules is imperative if this part of the solar spectrum is to be fully exploited. While the development of some spectropolarimetric diagnostics has been mentioned already, a more thorough review is presented in Berdyugina (2011). An entire session of the Solar Polarization Workshop 4 (Casini and Lites, 2006) was devoted to a description of the state of the art in molecular line spectropolarimetry, and especially interesting work was presented by Trujillo Bueno et al. (2006) and Asensio Ramos (2006). Observations must be done in order to push the development of these theories, and so recent efforts for spectropolarimeters to explore the 1000 to 5000 nm region will provide important results, and efforts beyond 5000 nm are needed as well.
With the exception of only one option for the upcoming Japanese Aerospace Exploration Agency mission to be an infrared spectropolarimeter, prospects for infrared space-based solar observations are surprisingly lacking, although past proposals for space-based solar IR missions have been made, including the SIRE mission (Deming et al., 1991c). This oversight needs to be corrected. While the spatial resolution of a space-based solar telescope operating in the IR is less than if it operated at shorter wavelengths, the magnetic sensitivity such an instrument gains opens new realms of possible science. Night-time astronomy has long exploited the advantages offered in the infrared spectrum through a variety of space missions from IRAS to WISE, from Spitzer to Herschel, and now to the upcoming JWST. Clearly the technology is available to develop cutting-edge solar physics missions to observe the Sun from space in the infrared spectrum. The recent excellent results from the Hinode spacecraft (i.e., Lites et al., 2008) clearly show that a new solar mission with three times the magnetic sensitivity of Hinode would be revolutionary.