1 Introduction

“Why does anyone still observe the Sun using visible wavelengths of light?” a colleague recently asked of me. Certainly night-time astronomers have been exploiting the infrared spectrum for many years to address critical science questions in that field. The spatial resolution, flux, and background problems are even more extreme at night than they are for observations of the Sun. Infrared detector and instrument technology is clearly robust, and the scientific advantages are important. So then why is not the majority of solar data, particularly ground-based solar data, taken in the infrared spectrum? The best answer I could conceive was simply that of tradition. Solar observations have been made in visible wavelengths for hundreds if not thousands of years, and many of those observations have produced breakthrough science. But one thing is certain: the Sun is still stubbornly keeping secrets from us. The goal of this review is to clearly show how we can attack those outstanding questions in solar physics by looking through the window provided by the infrared spectrum.

From the discovery of infrared radiation by William Hershel (Herschel, 1800) infrared referred to light with a wavelength too long to be detected by the human eye. As the human eye is replaced in astronomy by a plethora of different types of detectors, this definition falters, and we must look for other ways to define the infrared spectrum. One technique which is often used (Jefferies, 1994*) simply defines the infrared spectrum as three decades of wavelength, from a wavelength of 1000 nm to a wavelength of 1 mm. On the short wavelength side, this definition roughly agrees with a detector-based definition. Silicon detectors are sensitive to visible wavelengths, and drop to very little sensitivity at a wavelength around 1000 nm. On the long wavelength end, this definition includes some of the sub-mm or terahertz radation (usually 0.1 to 1 mm). For the purposes of this review paper, these three decades of wavelength will be used to define infrared radiation.

Since the visible spectrum covers a wavelength range of only a factor of two, and the infrared spectrum covers a factor of 1000, it is useful to sub-divide the infrared spectrum into smaller wavelength ranges. Table 1 shows a list of commonly used names for the different parts of the infrared spectrum. While this nomenclature is often used, it is not strictly defined for the three regions of Near-IR, Mid-IR, and Far-IR, and one should expect to see the terms used only loosely especially in fields outside of astronomy. Table 1 also includes lists of the corresponding wavenumber and frequency intervals for these bands, again only approximately computed due to the informal nature of the definitions. The last column shows the temperatures corresponding to black body radiation curves with peaks at the wavelength boundaries, computed using Wien’s displacement law, where T = 2897∕λmax in degrees Kelvin when the wavelength is written in microns. Recalling the fact that solar telescopes and their associated optics operate near room temperature of 300 K, the meaning for the term thermal IR becomes clear: at wavelengths from about 4000 nm and longer, photon flux emitted from background sources can dominate even solar photon flux. Measurements at these wavelengths become background dominated, and clever observational techniques must often be used.

Table 1: Informal Infrared Nomenclature.
Name Wavelength Wavenumber Frequency Temperature
[nm] [cm–1] [GHz] [K]
Near IR 700 – 5000 14 300 – 2000 428 000 – 60 000 4100 – 580
Mid-IR 5000 – 25 000 2000 – 400 60 000 – 12 000 580 – 120
Far-IR 25 000 – 106 400 – 10 12 000 – 300 120 – 3

Instead of using these labels, a simple division of the infrared spectrum by factors of ten is appealing. In the first decade of wavelengths (1000 to 10 000 nm) the spectrum is explored using array detectors which are similar to those used at visible wavelengths. In the longest wavelength decade (from 0.1 to 1 mm) measurements are often made using heterodyne receivers or radiation waveguides which resemble radio detectors. Here I will focus on results in the first decade of the infrared wavelength range, from about 1000 to 10 000 nm including just a few notable exceptions outside of this range. Studies of the Sun at longer wavelengths have been reviewed by Deming et al. (1991b) and new exciting results continue to be made, especially by Kaufmann et al. (2013*); but these wavelengths will be left for future discussion.

Within this smaller wavelength range, it is useful to consider both atmospheric transmission and detectors again. Figure 1* from Hinkle et al. (2003*) shows a diagram of the Earth’s atmospheric transmission. There are several clear wavelength regions where a high percentage of light is transmitted to the surface of the Earth, and there are several blocked wavelength regions where light is very effectively absorbed by the Earth’s atmosphere. In astronomy, these atmospheric transmission windows were exploited for stellar photometry, and the early work of stellar astronomers, (especially Johnson, 1962) established the nomenclature for the infrared wavelength bands in the 1000 to 5000 nm range known as J, H, K, L, and M. The central wavelengths of these bands are roughly 1300, 1600, 2200, 3600, and 5000 nm. and the names are often used to describe infrared solar spectral observations.

Finally, it is useful to briefly discuss the detectors used for study in the Near-IR region. In order to detect a photon with a reasonable level of efficiency, a detector pixel needs to be roughly the size of the photon wavelength or larger. At these wavelengths, array detectors with pixel sizes of order 10 or 100 microns (just as for visible wavelengths) can be used as efficient detectors. Silicon array detectors, including both CMOS and CCD cameras, have a quantum efficiency which drops to near zero at roughly 1100 nm, as the silicon substrate becomes transparent to longer wavelength photons.

Infrared detectors are highly desirable items for many industrial purposes, and so the technology is changing constantly. Several reviews of the applications of these new arrays to astronomy have been published and make useful resources (for two examples, see Wynn-Williams and Becklin, 1987; Rieke, 2007). In general, arrays which use new technology strive for high efficiency within the wavelength range of interest, as well as high uniformity of response with a low dark current. Ease-of-use and expense are also factors which enter into the development process. At the current date, infrared observations at different wavelengths are regularly made with a handful of different types of detectors. These include arrays with detectors of HgCdTe (roughly 1000 – 2200 nm), InGaAs (roughly 1000 – 1800 nm), InSb (1000 – 5000 nm), Si:X doped silicon arrays (2000 – 30 000 nm), Ge:X doped germanium arrays (28 000 – 20 0000 nm), and PtSi diode arrays Ge bolometer arrays. There are many new detector technologies under development, and solar physics research is often the first place that these new technologies can achieve scientific results. The quantum well infrared photodetector (QWIP) cameras offer a new and inexpensive route for measuring infrared photons, and are currently being tested at the NSO McMath–Pierce Solar Facility (McM-P).

View Image
Figure 1: Atmospheric transmission from 560 to 21 000 nm as measured from the McMath–Pierce facility at Kitt Peak. The molecules which are responsible for the various absorption bands are marked. Image reproduced with permission from Hinkle et al. (2003), copyright by AAS.

  Go to previous page Scroll to top Go to next page