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Astronomy

Interferometer to observe solar roots of space weather

A new vacuum ultraviolet telescope will observe magnetic fields, plasma flows, and heating events in the Sun's atmosphere, providing key data for space weather models.
18 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200706.0709

The vacuum ultraviolet (VUV) region allows remote sensing of the upper levels of the solar atmosphere where the magnetic field dominates the physics: that is, where magnetism is stronger than gas pressures. A VUV interferometer will allow us to observe the magnetic field, flows, and heating events in the mid-transition region (between the chromosphere and corona). Observations of this region are needed to directly probe the magnetic structure and activity at the base of the corona where the magnetic field is approximately force-free, i.e., where gas pressures are very small. This is a key element in developing accurate models of the Sun's dynamics for space weather. The specific region of interest is the 100km-thick transition region, between the chromosphere and the much hotter corona, which strongly emits at 155nm from triply ionized carbon (CIV) at 100,000K.

This is best observed by an imaging interferometer that combines the best attributes of a spectrograph and an imager. Our goal at NASA Marshall Space Flight Center (MSFC) is to develop a high-spectral-resolution piezoelectric-tunable VUV Fabry-Perot interferometer (VUV FPI). This instrument, when flown above Earth's atmosphere on a suborbital rocket, will obtain narrow-passband images, magnetograms, and Dopplergrams of the transition region in the CIV 155nm line at a rapid cadence.

We recently measured the MSFC VUV FPI using the University of Toronto's F2 excimer laser as a proxy for CIV 155nm. The test demonstrated the first tunable interferometer with the passband required for a VUV filter magnetograph. The measured values have a full-width half-maximum (FWHM) passband of 10pm, a free-spectral range (FSR) of 61pm, and a transmittance of 58% at 157nm. The resulting VUV interferometer finesse is 5.9. With this success, we are developing an instrument suitable for a flight on a suborbital sounding rocket.


Figure 1. The tunable CIV interferometer is shown in this internal view of the assembly, as are the housing caps, cantilever spring assembly, the annular ring mounting plates which hold the 35mm MgF2 etalons, and the piezoelectric stacks.

By adding a polarimeter to the spectral filter, spectropolarimetry of the CIV 155nm line can be obtained. This will increase our understanding of the magnetic forces, evolution, and energy released within the solar atmosphere, which drives space weather. Although difficult, two-dimensional measurements of the full vector magnetic field at the height of maximum magnetic influence can be accomplished by measuring the Zeeman splitting of the CIV resonance pair. Peter (2001) and Solanki and Hammer (2001) have pointed out the importance of the mid-transition region for understanding the dynamics and heating below coronal features.1,2

Major optical elements

The major elements of the tunable CIV VUV FP interferometer are the 35mm MgF2 etalon plates with a plate finesse of F>25 at 155nm, the π-dielectric coatings, a Hansen mechanical mount in a pressurize canister, and the piezoelectric (PZT-8/lead zirconate titanate) control system (see Figure 1). The control system for the etalon is a capacitance-stabilized Hovemere Ltd. standard system. The special Cascade Optical Corp. reflectance coatings are 25 π-multilayers of high-low (LiF3–MgF2) refractive layers paired in phase.3 Decreasing the LiF3/MgF2 ratio in the paired configuration allows lower absorptance, and an ion deposition provides low-stress coatings. As an alternative, we could use aluminum with a MgF2 overcoat. Its advantages are: approximately zero-stress via two soft layers, broadband reflectance, and a shorter coating time, i.e., lower cost. Its major disadvantages are: non-reproducibility and an Al2O3 boundary, randomness of reflectivity and absorption, and extreme fragility of soft surfaces, i.e., sensitivity to the environment. The π-dielectric coatings have the advantages of high reproducibility, durable coating, easier handling, absorptance about 1/3 of Al, larger reflectivity, and use as a built-in prefilter. Their disadvantages are a low-stress ion deposition is required and a higher cost results due to the need for a minimum of 30 coating runs (see Figure 1).


Figure 2. A solar payload configuration using a dual CIV interferometer filter system is shown within a sounding rocket envelope. Shown are the Solar Ultraviolet Magnetograph Investigation (SUMI) cold-mirror telescope elements (A, primary; B, secondary), the waveplate assembly (C), the folding mirrors (C and D), dual Fabry-Perot interferometers (F), order selection filters (G), and VUV detector (H).

Initial results

Our initial program has accomplished several things. Two pairs of MgF2 etalon plates have been polished to better than λ/150 at 633nm or λ/24 at 155nm (FD>12). A primary set of test plates was coated with the designed 77% reflectance at 155nm. These are stress-free VUV dielectric coatings with low-absorption π coatings. These coatings were then applied to the second pair of etalon plates. The tunable interferometer was assembled with MgF2 plates mounted to annular rings of silica glass with a matching coefficient of thermal expansion in a novel design. It was then mounted as a piezoelectric-tunable, capacitance-stabilized etalon. The etalon was placed in a Hovemere's Hansen optical mount for low induced mechanical stress. The result is a robust, operational CIV VUV interferometer.4 To complete the filter design, a static etalon will be added to bridge the spectral isolation requirement between the tunable etalon and VUV interference filters.

Sounding rocket program

Our sounding rocket program instrument will incorporate both tunable and fixed etalons to provide a fully-functional filter magnetograph for the CIV resonance lines. The tunable etalon tested is an integral part of the CIV narrow passband filter (∼10 pm FWHM) consisting of a dual etalon system with three sub-elements: a scanning high-spectral resolution interferometer (HRI) providing a high-resolution passband, a static low-resolution interferometer (LRI) allowing a large effective free spectral range, and multiple reflective interference filters acting as prefilters. The HRI, a VUV piezoelectric-controlled etalon, has been tested using the surrogate laser line at 157nm. The LRI of the filter can be built using existing MgF2 test plates and coated with π-stack dielectric coatings that employs a fixed gap. The prefilter is conceived as multiple folding mirrors with dielectric multilayer stacks, following the design concept of the ultraviolet imager of NASA's Polar spacecraft. The two etalons allow the effective free spectral range to be compatible with the prefilter profile, and provide a complete CIV narrow passband filter. (Furthermore, an additional etalon could provide a 2pm spectral resolution for full CIV spectro-polarimetry.) In Figure 2, we show our overall sounding rocket concept, with dual etalons, in a package which contains the MSFC Solar Ultraviolet Magnetograph Investigation (SUMI) cold-mirror telescope. The SUMI telescope and its spectrograph are scheduled for flight in 2008.5

The development phase of this research was supported by a NASA/Marshall Space Flight Center Independent Research and Development grant.


G. Allen Gary, Edward West
Solar Physics, Marshall Space Flight Center
Huntsville, AL

G. Allen Gary has been a member of the Solar Physics Team at the NASA Marshall Space Flight Center for 25 years. During this time he has done extensive research into the nature of coronal structures and solar magnetic fields. His research includes both theoretical and instrumental programs to understand the solar magnetic field configuration, evolution, and morphology.

Edward A. West has been a member of the Solar Physics Team at the NASA Marshall Space Flight Center since the Skylab mission in 1973-74. During this time he has worked with the Skylab mission data, the MSFC Vector Magnetograph program, and the ultraviolet spectropolarimeter on the Solar Maximum Mission, and has developed vector magnetograph concepts for Solar B (Hinode), Solar Probe, and Solar Orbiter. He is currently the lead engineer for SUMI.

John Davis
Space Science Office, Marshall Space Flight Center
Huntsville, USA

John M. Davis started his career studying the solar wind at MIT. He transitioned to the study of coronal structures as a co-investigator of the AS&E (American Science and Engineering) x-ray telescope on Skylab. He has been a member of the Solar Physics Team at the NASA Marshall Space Flight Center for 21 years where his interests have migrated from the x-ray corona to the magnetic fields that control the x-ray coronal structures and their dynamics.

David Rees
Hovemere Ltd.
Sevenoaks, UK

David Rees has worked for 40 years on the development of stable, high-resolution Fabry-Perot interferometers and their application to a wide range of scientific applications. His research areas include studies of Earth's upper atmosphere, development of Doppler lidar systems for wind measurements and high-performance filters permitting efficient daytime atmospheric measurement by lidar.