Imaging three-dimensional heliosphere in EUV

Abstract

The glow of interstellar plasma and solar wind pickup ions and solar wind emissions at 30.4 nm provide a way of exploring important physical processes in the heliosphere. Imaging the heliosphere at this wavelength with high spectral resolution will map the heliopause, probe pickup ions in the solar wind, and reveal the three-dimensional flow pattern of the solar wind, including in the regions over the sun's poles. The required high-throughput, high-resolution spectrometer for diffuse radiation should be able to measure 1 milli-Rayleigh irradiance in 10000 seconds with a 0.005-nm spectral resolution across pixels subtending a few degrees of celestial arc. The desired performance characteristics can be achieved by combining multiple entrance slits with an optimized spectrometer design. We present a concept of a space experiment to image the heliosphere at 30.4 nm and discuss the scientific rationale and required instrumentation.Keywords: heliosphere, heliopause, extreme ultraviolet, EUV, solar wind, diffuse radiation, spectrometry 1.SOLAR SYSYTEM GALACTIC FRONTIER Global interactionThe interaction of the expanding solar wind plasma with the surrounding galactic medium -Local Interstellar Medium (LISM) -creates the heliosphere.1-4 The heliosphere is a complex phenomenon where the solar wind and interstellar plasmas, interstellar gas, magnetic field, and energetic particles all play important roles. [5][6][7][8] The region where the solar wind meets the galactic medium remains poorly explored, with the present concepts of the heliospheric interface based on scarce and mostly indirect experimental data. The most distant operating spacecraft, Voyager 1, had reached the termination shock region and the spacecraft entered the slowed down solar wind at 94 AU (ecliptic latitude β = 34.8° and ecliptic longitude λ = 253.3°) in December 2004. (The first inconclusive indications of the possible arrival at this region were reported two years ago.9,10 ) This is a major milestone in exploration of the outer heliosphere, although the spacecraft is instrumented for planetary flybys and not optimized for interstellar studies. Voyager 1 will be turned off in 2020-2025 at the distance 150 AU because of the decreasing efficiency of its radioisotope thermoelectric generators.A possible two-shock Sun-LISM interaction scenario 5 illustrates the main features of the heliosphere (Fig. 1). The interstellar wind approaches the heliosphere with a supersonic velocity and forms a bow shock. The dynamic pressure of the expanding, highly supersonic solar wind decreases with the heliocentric distance. At a certain distance from the Sun, this pressure would equal the external LISM pressure of the interstellar wind and magnetic field. The solar wind expansion transitions to a subsonic flow at the termination shock. There the kinetic energy of the supersonic flow is largely converted into thermal energy in the subsonic plasma beyond the shock. The subsonic postshock solar wind plasma (Fig. 1, left) flows in the heliospheric sheath around the ...

Imaging the global solar wind flow in EUV

“…Consequently, a space experiment should realistically focus on measuring the angle x 0 and require obtaining a few all-sky images per year. This latter requirement is consistent with another goal of heliosphere EUV imaging, mapping of the heliopause [Gruntman, 2001a[Gruntman, , 2001bGruntman et al, 2005].…”

“…[5] Remote probing of the solar wind flow constitutes integral part of global heliosphere imaging in EUV, a concept that also aims at exploring remotely the properties of the boundary separating the solar wind and galactic plasmas, the heliopause, and magnetic field and the ionization state and flow field of interstellar plasma beyond [Gruntman, 2001a[Gruntman, , 2001b[Gruntman, , 2005Gruntman et al, 2005].…”

“…In fact, the envisioned observations (from a spacecraft outside the geocorona and radiation belts) would provide a dozen or so all-sky images (maps) in spectral bins 0.005 nm wide near 30.4 nm [Gruntman, 2001a[Gruntman, , 2001bGruntman et al, 2005]. Imaging the heliosphere at this spectral range will map the heliopause; probe pickup ions in the solar wind; and reveal the three-dimensional flow pattern of the solar wind, including in the regions over the Sun's poles [Gruntman and Fahr, 1998;Gruntman, 2001aGruntman, , 2001bGruntman, , 2005Gruntman et al, 2005].…”

“…Radiance of one milli-Rayleigh equals one photon per square centimeter in a 6.4°Â 6.4°solid angle in one second. Gruntman et al [2005] and Lampton et al [2005] have developed an instrumental concept to achieve the required sensitivity and spectral resolution. The enabling instrumentation is based on a Rowland-type spectrometer with multiple tall off-plane-blur-free slits of a special shape, with a large number of slits forming an aperture-coding pattern.…”

“…Therefore these two distinct cases would translate into the requirements of one image per week and a few images per year, respectively. (We note that mapping of the heliopause, achieved simultaneously with imaging of the solar wind, would require one or two all-sky images each year [Gruntman, 2001a[Gruntman, , 2001bGruntman et al, 2005].) The full sky (4p sr) contains about 1600 5°Â 5°angular (directional) pixels.…”