An In-Flight Solar Eclipse

The 9.5-hour C130 flight to Panama from Boulder on February 16, ten days before the eclipse was noisy and tiring, but uneventful. Haosheng and I had cooled dewars and prepared the IR spectrograph and imaging system for operation before the flight -- the dewars even went to Panama cooled. By the next day, Tuesday, we were testing and aligning optics. From Howard Air Force Base, where we were based, we used a simple mirror on top of the plane and "sea level flights" (pushing the airplane around the tarmac) to set up for a series of three test flights over the Pacific later that week and the day immediately before the eclipse. The air base provided superb logistical support including LN₂, air conditioning, and weather data for us while we worked in the plane in the tropical sun on the airport tarmac. Most of our prep time before the eclipse was devoted to devising photometric and spectroscopic calibration schemes, although we had some interesting experiences learning, for example, how to store and transfer LN₂ in the tropical heat and humidity. Our experiment crew of nine (two from Max Planck/Lindau, three from Rhodes, three from NSO and MSU, and one from HAO) was notably busy, handling tasks ranging from turning mirrors on the roof of the C130 to dealing with the press crews that toured the experiment. There were also eight people in the aircraft crew and all of us were lodged across the only bridge in the Republic of Panama that crosses The Canal. During the height of Carnaval, crossing this bridge was a daily adventure. Luckily, our rather pessimistic fears for how many of us or our cars would be lost or damaged by the traffic were never realized.

Several scientific goals were to be met with these instruments. The spectrograph was designed to confirm and search for new IR emission lines between 1-2.5 µm, and to measure spectroscopically the dynamics of F-coronal dust around the Sun. We were also interested in looking for a peculiar HeI (cool) outer coronal component, which we observed in the 1994 experiment. The new 1-5 µm, high dynamic range IR imager (newly designed and built at MSU in collaboration with the NSO) would be used with a tunable IR Lyot filter and fixed narrow and broad-band IR filters and polarizers for several experiments aimed at diagnosing the spatial temperature and density structure of the corona, and the thermal properties of the circumsolar dust. A high priority of the new photometer was to look for evidence of a predicted, and highly magnetically sensitive emission line at a wavelength of 3.9 µm.

Since the small opening in the roof of the airplane restricted the range of unvignetted viewing angles of the instrument pointing spar, it was always necessary to use the airplane as part of the "telescope." This complicates the airplane flight path since the instantaneous airplane heading had to match the proper azimuth range for the current solar position. Since the lunar shadow was only 150 km in diameter and moving at 1800 km/hr while the airplane was moving at 450km/hr, our flight path required careful planning. Our three test flights were used to good advantage, calibrating and learning the operational principles needed to steer airplane and telescope.

On Thursday, 26 February, the day of the eclipse, we arrived at Howard Air Force base before 7:00 am (Carnaval had just ended), in time to study the satellite imagery the Howard AFB weather group was gathering for us. Growing cirrus clouds to the west of Panama left few options but to choose our primary intercept point with the moon's shadow at a far rendezvous point, 800 km into the Pacific. We decided to approach this intercept by flying southwest along the computed eclipse path trajectory. We arrived at our second contact point early enough to complete additional calibration, and we "waited" for the lunar shadow by executing a standard aircraft holding pattern at about 18,000 ft. The shadow arrived on time, and with the C130 properly aligned on the eclipse centerline and heading parallel to the shadow. We were rewarded with the longest period of totality (almost 5 minutes) that was possible. All instruments and experimenters performed as designed and rehearsed, and we acquired data with few technical disappointments. The instruments accumulated nearly 100 Mbytes of data, which, as revealed in the accompanying Figures 1 through 4, are exceptional and unique.

Infrared imaging spectroscopy, using fixed and tunable (/= 200-400) filter elements, reveals evidence of a new Si IX emission line. This line is far into the infrared and may allow direct measurements of coronal magnetic fields (heretofore impossible). This experiment was conducted from an open C130 aircraft (operated by NCAR). Scientists from NSO/SP, Rhodes College, MSU, Max Planck (Lindau), and HAO participated in a broad range of IR experiments. A new Rockwell HgCdTe high dynamic range infrared array camera (sensitive between 1-5 µm and developed at MSU in collaboration with NSO/SP) was used to obtain these results.

Figure 1. Inner corona at = 1.100 µm.                   Figure 2. Inner corona at = 1.450 µm.

The upper figures show the K corona (continuum only) brightness at 1.1 µm and 1.458 µm in a 7 nm bandpass. A tunable liquid crystal Lyot filter was used to obtain these images.

Figure 3. Continuum + FeXIII = 1.075 µm.                 Figure 4. Continuum + Si IX = 3.932 µm.

These figures show the continuum + line emission near Fe XIII and the predicted Si IX wavelengths. The bright region on the lower (west) limb of Figure 4 is likely evidence of Si IX emission. Tentative intensity calibration suggests that this may be one of the brightest coronal emission lines visible. The long wave observations were obtained with a 10 nm bandwidth interference filter centered at 3.932 µm.

Jeff Kuhn, for the C130 Eclipse Experiment Group
(Bob MacQueen, Ingrid Mann, Haosheng Lin,
Jack Streete, Dan Edmunds, Phil Judge,
Peter Hillebran, Gerry Tansey).