Given the grandeur of a total solar eclipse, it is likely that prehistoric humans marveled at the Sun’s corona. Artificially rejecting light from the solar disk to see the million times fainter corona is a significant challenge, so it is not surprising that efforts to reproduce an eclipse were not successful until relatively recently.
Now observations of the Sun’s extended atmosphere with coronagraphs (see Figure 1 for an example of a coronagraph image) have become indispensable to both basic and applied research—similar to solar magnetograms and total irradiance measurements.
In the early 1930s, Bernard Lyot at Pic du Midi was the first person to successfully produce a coronagraph by reducing instrumental stray light caused by diffraction and scattering, but he could only detect the emission line corona using narrowband filters [Lyot, 1933]. His original design relied on internal occultation (i.e., allowing all the Sun’s photospheric light to enter the telescope before removing it) and other innovative optical improvements; Evans  was able to improve stray light rejection by adding external occultation (i.e., removing light before it reaches the telescope).
In the 1960s, Audouin Dollfus at the Meudon Observatory, Gordon Newkirk and Robert MacQueen at the High Altitude Observatory, and Richard Tousey and Martin Koomen at the Naval Research Laboratory began experimenting with externally occulted instruments, which were adapted for balloons, airplanes, and, ultimately, spaceflight. But imaging of the broadband white light corona was not achieved until the space age when Tousey  demonstrated with the 7th Orbiting Solar Observatory OSO-7 coronagraph that the corona could be seen in white light, and he reported several transients that would now be called coronal mass ejections (CMEs). The basic characteristics of white light CMEs have been measured by subsequent spaceborne coronagraphs on Skylab, P78-1, the Solar Maximum Mission, the Solar and Heliospheric Observatory (SOHO), the Solar Terrestrial Relations Observatory (STEREO), and the ground-based MK3/MK4 instruments on Mauna Loa.
The Sun’s Atmosphere
White light coronagraph images are extremely complementary to emission line observations in the low corona. A white light coronagraph records Thomson-scattered light by coronal electrons, a process that is independent of the plasma temperature and falls off with distance from the Sun’s visible surface as inverse density of the plasma.
These qualities enable white light coronagraphs to observe out to heights that cannot be viewed in coronal emission lines and do not suffer from loss of signal from Doppler shifts and plasma temperature changes that impact single–emission line observations. These properties make white light coronal observations indispensable for studying the expansion of the Sun’s atmosphere into the solar system as it becomes the solar wind. It is this medium that is the background that is punctuated by the violent expulsion of plasma and magnetic fields—the CMEs.
The Importance of CMEs
Since Jack Gosling’s provocative “solar flare myth” paper [Gosling, 1993], which emphasized the central importance of CMEs rather than solar flares in producing major geospace events, CMEs have occupied a central role in studies of geomagnetic storms, solar flares, radio bursts, interplanetary shocks, and energetic particle storms in space. Space weather forecasting was transformed by the recognition that coronagraphs on SOHO could detect disturbances at the Sun and provide warning 1 to 3 days in advance of potential geomagnetic storms.
Some researchers reported that an extreme CME during 2012 would have had significant impact on technological systems if it had struck Earth [Baker et al., 2013]. For this reason, CMEs, as the driver of the most severe geomagnetic events, are frequently the focus whenever coronagraphs are discussed, but these instruments also contribute significantly to many other scientific topics.
The Utility of Coronagraphs as Measured by Publications
The number of refereed papers that used observations provided by the SOHO Large Angle and Spectrometric Coronagraph (LASCO) instrument [Brueckner et al., 1995] can provide a measure of the utility of these instruments. As with many observatories and space science missions, the SOHO project scientists have maintained a database of publications utilizing measurements from the payload http://soho.nascom.nasa.gov/publications/publications.html). As of April 2014, the SOHO publication database contained almost 5000 refereed papers by more than 3000 authors where data from one or more SOHO instruments were used (this total was gleaned by searching relevant journals). The international solar physics community is estimated to have about 1000 active researchers, so it is clear that SOHO has been a tremendously productive observatory, reaching far beyond the audience of solar physicists.
This database shows that LASCO ranks second of the 12 instruments in the payload, with 1424 entries (averaging more than 90 refereed publications per year since 1999), trailing only the magnetograph/Doppler imager (MDI), which has 1776 records. The extreme ultraviolet imaging telescope (EIT) has 1027 entries, and the remaining nine instruments in the SOHO payload have fewer than 700 records each. Both MDI and EIT have counterparts on the Solar Dynamics Observatory (SDO) [see Pesnell et al., 2012], so it was not surprising to see that the annual usage of data from each of those instruments has significantly dropped since the February 2010 launch of SDO.
Who are the authors of these papers using coronagraph data? Going by the institutional affiliation of the first author, 43% are from the United States, and 30% are European. This result is not surprising given that SOHO was a collaborative mission between the European Space Agency (ESA) and NASA, and the LASCO coronagraph suite was composed of hardware from the United States, France, Germany, and the United Kingdom. Recent emphasis in heliophysics topics has attracted scientists from China, India, Russia, and Japan, who account for 23% of all papers using LASCO observations.
It may initially seem surprising that only 60% of the LASCO records in the database could be categorized as CME related, whereas the remaining 40% were considered non-CME related. The ratio of CME versus non-CME papers using LASCO data has changed only slightly over time—from about equal during the first 6 years to about 2:1 more recently (for data current through July 2014, see Figure S1 in the additional supporting information in the online version of this article).
A closer examination of the topics covered in the non-CME papers shows that about a third of them concerned the corona and its continuation into interplanetary space as the solar wind. The Sun itself, theory and modeling, and energetic particles each account for slightly more than 10% of the topics. Space weather, geospace effects, comets, and solar wind conditions at other planets account for about 15%; the remainder cover radio emissions, image processing, instrumental papers, and reviews. These results show clearly that coronagraphs enable scientific study and understanding far beyond the dynamic eruptions of CMEs at the Sun.
SOHO, designed for a 2.5-year nominal mission, is nearing 2 decades in orbit. It suffered a near-fatal accident in 1998, and its recovery is still heralded as a remarkable feat in the aerospace community [e.g., Vandenbussche, 1999]. Because a coronagraph was not included as part of the SDO payload, one of the primary arguments for the continued operation of SOHO has been to provide continuity of those observations.
The international science community and the forecasting community should work together to secure these measurements in the future to support this broad range of research across the field of heliophysics. It is the unique ability of coronagraphs to remotely sense this region of space that connects our star to our planet and to the solar system.