Scientists in the Space Science Division (SSD) of the Naval Research Laboratory
have been studying the sensitivity of the Extreme-ultraviolet Imaging
Spectrometer (EIS) on the Japanese Hinode spacecraft since its launch in
late September 2006. This state-of-the-art spectrometer was built by an
international consortium that included SSD scientists funded by NASA.
The science objective of EIS is to measure spectroscopically physical
parameters of the solar atmosphere such as temperature, density, and
dynamical properties.
The instrument is 3 meters long and for reasons of weight, thermal design,
and mechanical stability, the optical elements were housed in a carbon
composite structure. One of the biggest challenges for EUV space instruments
is minimizing contamination that degrades the optical sensitivity. Chief
among these is carbon itself. The EIS composite structure was designed
for cleanliness at every step, and part of the process was a two month
vacuum bake at 80 ℃. Keeping the EIS spectrometer "clean" involved a
major effort throughout the build of EIS and Hinode.
Since launch, SSD scientists have been checking to see how successful the
team was in keeping EIS free of contaminants. Measuring sensitivity loss
is accomplished by monitoring the intensities of solar spectral lines in
the so-called quiet Sun using observational studies developed for this
purpose. Some of these EUV lines are known to have on average constant
intensities with time. An example of the data obtained by the monitoring
studies is shown below in
Figure 1.
Figure 1:
The intensity of a solar He II line as a function of time in the quiet Sun.
(Click figure to see full-size image)
The intensity of the spectral line of He II at 256.32 Å is shown as a
function of time between January 2007 and July 2009. A recent analysis
of these data, and the data from many other lines observed so far, has
shown that the time constant of the EIS sensitivity, or the time over
which the sensitivity will drop to 37% of its current value, is about
6.4 years. This e-folding time of 6.4 years is extremely long for an
EUV instrument and shows that the international EIS team achieved the
goal of producing an exceptionally clean instrument. The team looks
forward to many years of fruitful research for EIS, uncomplicated by
severe degradation due to contamination.
