The enigma surrounding the solar corona continues to be the focus of observations from space and the ground. Recent space-based imaging experiments have added significantly to the complexity and dynamics of coronal structures. However, these are limited by the coverage of the inner corona over a distance range limited to half a solar radius at best. They thus fall short in placing the observed structures within the context of the expanding corona from the solar surface out into interplanetary space.
“Like eclipses of the sun and moon, by which all are attracted, but whose effects no one takes the trouble to calculate.”
—Henry David Thoreau (1817-1862)
The unique scientific wealth of eclipse observations is the motivation behind our team’s many expeditions. Solar eclipses allow us to view part of the solar atmosphere (the corona) that is not observed by satellites or would otherwise be blocked by the brightness of the solar surface. The image on the left shows the importance of the field of view of eclipse observations. The area between the black and yellow circles represent the section of the atmosphere that is “filled in” by eclipse observations.
Eclipse images allow us to complete a map of the coronal structure. Coronal dynamics (events) are observed starting from the solar surface and include jets, coronal mass ejections (CMEs) and plasma instabilities.
The goal of our total solar eclipse observations is to capitalize on the diagnostic properties of coronal forbidden emission lines, to infer electron temperatures, ion densities, abundances and charge states, and the properties of the coronal magnetic field. The scientific advantage of observing these lines stems from the strength of their radiatively excited component, which enables the emission to extend out to large heliocentric distances, matching that of white light.
The observational plans of the eclipse of 17 August 2017 in the USA will be carried out the same as 2015 in Svalbard and 2016 in Indonesia, namely imaging and imaging spectroscopy. The solar corona will be imaged in white light, Fe XI and Fe XIV. The latter lines detect plasma at 1 million and 2 million degrees respectively. Application of state-of-the-art image processing techniques will maximize the scientific return for exploring the details of coronal structures and their thermodynamic and dynamic properties. Furthermore, this eclipse corresponds to the declining phase of the solar activity in cycle 24, hence will extend our eclipse data base to span almost a full solar activity cycle.
Observing the Corona
“A picture painted by the Sun without instruction in art”
—Ambrose Bierce (1842-1914)
Anyone witnessing a total solar eclipse is struck by the filamentary structures defining the corona. These trace the magnetic fields that originate from the solar surface entraining charged particles as they expand outwards. The charged particles are responsible for the emission that the eye and cameras capture. What the eye sees is the Thompson-scattered emission from electrons that reflect the full spectrum of light emitted by the photosphere towards the observer. This emission is commonly known as continuum or white light radiation. The other component, indistinguishable to the eye, because it occupies a minute fraction of the coronal radiation, is produced by highly ionized heavier elements in the corona, with the dominant one being iron (Fe). It can only be captured by cameras or detectors retrofitted with filters that isolate the select ‘colors’ of the different stages of ionization of Fe, the dominant ones being Fe X 5303 A, Fe XI 7892 A, Fe XIII 10747 A and Fe XIV 5303 A, stripped of 9, 10, 12 and 13 electrons respectively. This can be achieved with narrow-band pass filters, tuned to the wavelength of the radiation emitted by these different charged states, or with spectrographs.
While most of these field lines, made ‘visible’ by the emission from the charged particles attached to them, escape into interplanetary space, some field lines form arches of all sizes that remain anchored to the Sun at both ends. The different ionization states of Fe and other elements also emit in the extreme ultraviolet part of the spectrum, as well as in X-rays. However, what is unique to the visible wavelength range, is that the emission can be captured to almost ten times further away from the Sun. This is a critical property as it enables one to study the expansion of the field lines to much larger distances from the Sun, and to distinguish between those field lines that escape into interplanetary space, and those that remain anchored to the Sun.
The ionization state is a direct reflection of the temperature of these ions. Our earlier eclipse observations have shown that the field lines escaping from the Sun, are characterized by Fe X/Fe XI emission, while the arch-like field lines are dominated by the twice as hot Fe XIII and Fe XIV emission. These observations thus indicate that there are two distinct temperatures characterizing the coronal plasma: a million degree expanding plasma, and a two million degree confined one. Hence, we decided to concentrate on imaging in Fe XI and Fe XIV for this eclipse.
Heavy ions in the solar corona, often referred to as ‘minor’ ions, are important markers for the physical processes defining the properties of the coronal plasma and solar wind. Known as coronal forbidden lines, their emission in the visible wavelength range yields unique diagnostic capabilities: while they retain the same diagnostic potential of any EUV line from the same ion, they can be observed at far larger distances than their EUV counterparts (Habbal et al. 2007, 2012a,b, 2013; Judge et al. 2013; Landi et al. 2016). Hence, they offer precious tools for exploring the most critical region of the corona, namely the acceleration region of the solar wind. The considerable dimming of the sky brightness during total solar eclipses offers ideal observing conditions to fully exploit their unique properties.
The path of totality of the 21 August 2017 total solar eclipse covers an uninterrupted land mass of about 3000 miles. It thus offers a rare opportunity to maximize the chances for coronal emission line observations at multiple observing sites. At present, we are the only team worldwide that has been acquiring multi-wavelength imaging and spectroscopic observations of select coronal forbidden lines, coupled with high resolution broadband white light images. Our goal is to capitalize on the unique observing opportunities of the 2017 eclipse to enhance and expand our existing and unique data base, acquired since 2006, to a full solar cycle. The main scientific goals are to explore the underlying processes responsible for changes in the quiescent and dynamic corona that impact the acceleration of the solar wind. The instrumentation will enable us to characterize (1) the electron temperature distribution in the corona, (2) the evolution of ion charge states, and their freeze-in distance, (3) the thermodynamic properties, i.e. temperature and speed, of CME fronts, and (4) plasma instabilities.
To maximize our chances of data acquisition, we have identified three primary sites, 600 miles apart, and two secondary sites 1000 miles apart, thus covering approximately 2000 miles. Dynamic evolutions in the corona over time scales of 10 to 60 minutes can thus be captured as well. Each of the primary sites will have the following identical systems: (1) Imaging in Fe XI 7892 ̊A, Fe XIV 5303 ̊A, Ar X 5536 ̊A and Ar XI 6918 ̊A, (2) imaging spectroscopy with triple channel spectrographs, and (3) broad-band white light imaging. The secondary sites will be limited to imaging in Fe XI, Fe XIV and broad-band white light and one secondary site will also include a triple channel spectrograph. Since the measured coronal emission line intensity overlays the broad solar disk continuum emission, measurement of this continuum is needed to extract the coronal emission line intensity. This can be achieved with a pair of identical optical systems, with one filter center on the emission line, and the other 10 ̊A away in the neighboring continuum. Operating simultaneously under identical observing conditions, these image pairs are then subtracted to isolate the spectral line intensity.
Given the novel eclipse results from previous observations, we are in a unique position to fully exploit the observing opportunities of the 2017 eclipse, to expand our existing data base to a full solar cycle, and to exploit new diagnostics capabilities with coronal emission lines. With the short lead time until August 2017, RAPID funds are urgently requested to acquire components and assemble the following instrumentation: (A) Adding Ar X 5535 ̊A and Ar XI 6918 ̊A imaging to our existing Fe suite, will yield the first Ar X and XI images of the corona. These measurements will also connect the coronal measurements to in-situ ones. Since Ar X is formed at similar temperatures as Fe XI, and Ar XI at those characteristic of Fe XIV, their emission will appear in the same regions as Fe XI and Fe XIV, respectively. Through these measurements we will thus be able to determine the Fe/Ar ratio in both temperature regions mentioned earlier. We can then study: (1) how the abundance anomalies, known as the FIP effect (see Laming 2015), are distributed in these different regimes; (2) how they relate to the abundance anomalies observed in situ for the fast and slow solar wind, to identify candidate regions for slow wind origin throughout the entire corona; (3) whether there is FIP effect variation in different structures; and (4) whether gravitational settling affects the Fe/Ar ratio within the same structure.
In addition to imaging, we also use a spectrograph, specifically designed for the observations, to study the intensity and shape of the Fe XI and Fe XIV spectral lines as a function of distance from the Sun, and in different filamentary structures. The observations will give us information about the evolution of any plasma wave motion as the plasma propagates outwards.
Spectroscopy observations provide information on the composition of the gas being observed. The observed light that is being emitted is of the different components, or atoms, of the gas in the corona that have been highly ionized. Each ion emits light at a specific color, or wavelength. From that, the chemical composition of the gas can be determined.