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Ultraviolet and Extreme Ultraviolet Spectroscopy of the Solar Corona at the Naval Research Laboratory

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Ultraviolet and Extreme Ultraviolet Spectroscopy of the Solar Corona at the Naval Research Laboratory F222 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article Ultraviolet and extreme ultraviolet spectroscopy of the solar corona at the Naval Research Laboratory 1, 1 1 2 1 1 J. D. MOSES, * Y.-K. KO, J. M. LAMING, E. A. PROVORNIKOVA, L. STRACHAN, AND S. TUN BELTRAN 1Space Science Division, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington DC 20375, USA 2University Corporation for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307, USA *Corresponding author: [email protected] Received 8 June 2015; revised 3 August 2015; accepted 17 August 2015; posted 18 August 2015 (Doc. ID 242463); published 17 September 2015 We review the history of ultraviolet and extreme ultraviolet spectroscopy with a specific focus on such activities at the Naval Research Laboratory and on studies of the extended solar corona and solar-wind source regions. We describe the problem of forecasting solar energetic particle events and discuss an observational technique designed to solve this problem by detecting supra-thermal seed particles as extended wings on spectral lines. Such seed particles are believed to be a necessary prerequisite for particle acceleration by heliospheric shock waves driven by a coronal mass ejection. OCIS codes: (300.2140) Emission; (300.6540) Spectroscopy, ultraviolet; (350.1270) Astronomy and astrophysics; (350.6090) Space optics. http://dx.doi.org/10.1364/AO.54.00F222 1. INTRODUCTION with the launch of the Solar and Heliospheric Observatory [3] Many activities of the sun that affect the terrestrial environment (SoHO) in 1995, and the Solar Terrestrial Relations Observatory [4] (STEREO) in 2006. Instruments on SoHO and human society lie in the solar corona. Such activities in- – clude the solar wind, solar flares, and coronal mass ejections studied CME eruptions from about 1.5 30 solar radii (R⊙), (CMEs). Tousey and Friedman at the Naval Research while STEREO provided a 3D view of these events from Laboratory (NRL) made the initial discoveries of ultraviolet 1.5 R⊙ to the orbit of Earth. and x-ray emission from the solar corona shortly after This paper describes new instrumentation designs required to attack two specific problems arising out of the SoHO ob- World War 2 with the aid of captured German V2 rockets servations: the role of wave-particle interaction in the acceler- [1]. In 1958 Friedman demonstrated that solar x-ray emission ation of the solar wind and the initiation of solar energetic is extended beyond the visible solar disk and that the solar particle (SEP) events by shock waves driven by CMEs. corona is structured across the disk. This was achieved through Section 2 describes observations of the solar corona in a broader observation during a solar eclipse with a series of Nike-Asp context with the ultimate goal of extending the initial SoHO flights carrying nonimaging x-ray sensors. This use of the moon results on solar wind acceleration. Section 3 describes the as an optical element from a space platform was reprised in particular problem of particle injection into the CME shock 1964 with Freidman’s experiment to measure the angular size acceleration process while Section 4 describes the observables of the x-ray source in the Crab nebula. that should be associated with it. Finally, in Section 5 we dis- Koutchmy [2] gives an account of the early history of the cuss the instrumentation required both for a scientific valida- development of the technique of white-light coronagraphy by tion of our hypothesis and for an experiment designed to yield groups at NRL and other institutions where outside of solar real-time monitoring. eclipses, an artificial occulter is used to block out light from the solar disk allowing the outer corona to be studied. Coronal mass ejections were discovered from space in 1971. 2. SPECTROSCOPY OF THE CORONA AND NRL instruments on Skylab in 1973/74 revolutionized our SOLAR WIND SOURCES view of the solar upper atmosphere, and the SolWind The extended solar corona far off the solar limb is the site where Coronagraph observed the first Earth directed halo coronal the solar wind is accelerated and coronal plasma is nonther- mass ejection (CME) in 1979. Our view of not just the solar mally heated to millions of degrees. It is also the region where atmosphere but the extended corona was further transformed propagating CME shocks are formed (e.g., see [5]) and Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F223 subsequently produce SEPs [6,7]. Observations of the extended understanding how the fast solar wind is accelerated. corona are thus essential for understanding the formation and Cranmer et al. [10] present a summary of these and other physi- evolution of these activities. Especially valuable are observations cal insights that were gained from the UVCS observations. The by spectroscopic means. The very same technique has long most important finding from UVCS has been the discovery of been used to understand our universe. The power of spectros- broadened emission line profiles of O VI (the spectrum of five- copy is that it allows us to probe the local physical state of the times ionized oxygen). These broad line widths suggest that the corona and other remote locations in the universe where direct O5 ions are heated by transverse Alfvén waves, which are in in situ sampling is still not possible. resonance at the ion cyclotron frequency for the ion. While The solar corona presents dozens of ultraviolet and extreme there is no direct detection of the waves themselves, over ultraviolet (UV/EUV) emission lines that allow the plasma the years a number of authors (e.g., [11] and references therein) properties (e.g., densities, temperatures, outflow velocities, have developed models to show how these waves are generated and abundances) in the corona to be determined from remote by motions at the coronal base. Another key UVCS observation sensing diagnostics. In fact, NRL’s Space Science Division has a that supports the presence of Alfvén waves is the fact that the rich history of solar spectroscopy of the disk and limb (see oxygen ions in fast solar wind appear to flow out of coronal Doscheck et al., this issue). However, to make such measure- holes faster than the protons above ∼2.5 R⊙ [12]. This differ- ments at heights beyond about 1.5 R⊙ (from sun-center) ence in speed is preserved in the distant solar wind as measured, requires that the solar disk be occulted in order to keep the for example, by the Advanced Composition Explorer at 1 AU, intense brightness of the disk from overwhelming the faint which shows that, in general, ions have outflow speeds above UV and EUV emissions in the extended corona. the protons by an amount on the order of the local Alfvén The observational problem is shown in Fig. 1. It shows coro- speed [13]. nal-hole intensities, relative to the disk, for a few of the bright- In addition to the investigations of the fast solar wind, more est emission lines observable by the ultraviolet coronagraph recent work involves understanding the sources of the slow- spectrometer (UVCS) on SoHO [8] (plus He II 30.4 nm, speed wind. The energy requirements are comparable for the which is not observed with UVCS). The He II intensity for slow wind but the large variability in many parameters, including all heights and the intensities of the weaker lines (Mg X and wind speed [14], density, elemental abundances, and the Si XII) above 2.5 R⊙ are extrapolated based on the density first ionization potential (FIP) effect (see below), makes this variation with height. (Intensities for coronal streamer and problem difficult. Theoretical ideas involving the magnetic field CMEs can be 3–10 times higher.) As can be seen in the figure, foot-point interchange reconnection [15], expansion factor radiation from the disk is orders of magnitude greater than the (e.g., [16]), and unique field-line topologies (e.g., S-Web, [17]) emission lines we wish to observe. The solution is to use a com- have been proposed to explain various properties of the slow bined spectrograph and coronagraph [9] to make the required wind. A surprising outcome of this work is that it is now becom- observations of emission lines far off-limb in the extended ing clear that the wind speed alone is no longer sufficient to corona. As described in Section 5, the coronagraph reduces identify slow solar wind from fast wind. Elemental and charge the disk radiation to acceptable levels with occultation, while state abundances may be a better discriminant for determining the spectrograph is used to reject off-band radiation from both the type of solar wind and its coronal source regions [18]. Off- nearby lines and scattered disk radiation within the instrument. limb coronal abundance determinations (e.g., [19–22]) have The selection of optical coatings also plays a role in the rejection proved useful in identifying sources of slow wind in streamers of off-band radiation. and other localized regions of the corona. Elemental abundance The UVCS mission on the SoHO was the first to use measurements on coronal loops have provided constraints on these techniques to make groundbreaking advances in our ion-neutral fractionation processes occurring at the loop chromospheric foot points that point to the important role of magnetohydrodynamics (MHD) waves [23]. This leads to the FIP effect, where elements with low FIP that are ionized in the chromosphere (e.g., Fe, Si, and Mg) are enhanced in abun- dance in the corona with respect to high FIP elements (e.g., O, Ne, and Ar) that remain neutral. New observations with a dedicated coronal spectroscopy mission are needed to determine the properties of these waves and to understand their role in FIP fractionation and the solar-wind acceleration processes.
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