The 17-24 
region is key for resolving the stratospheric
questions because the emission seen forms in the 50-500 mbar region of the
atmosphere, just above and below the tropopause. This part of the
spectrum is dominated by pressure-induced absorption (PIA) from H
.
As a homonuclear molecule H
does not have a permanent dipole
moment, however a dipole can momentarily be induced by collision with
another molecule. With the number mixing ratio of H
being 85% in
the atmosphere of Uranus, this can become a significant opacity
source. H
has two forms for which the characteristic behaviors
are slightly different. The molecule is in the para state when the
spins of the two H molecules are antiparallel or even. When they are
parallel or odd the molecule is said to be in the ortho state.
The transitions that exist in the spectrum of interest are
the S(0) 0-2 transition of para hydrogen centered at 28.2 
and
the S(1) 1-3 transition of ortho hydrogen centered at 17.0 
.
Because of the short timespan involved in the creation of the dipole,
the wings from these lines are broad enough to cover the entire
window. Since optical depth for molecular hydrogen reaches unity near
the tropopause, this is where we are able to probe.
The other terrestrial
window of interest allows probing of this region as well as
the upper stratosphere, because although PIA is important here,
hydrocarbons provide a significant portion of the overall opacity as
well. Both ethane with an emission band centered at 12.2 
and acetyle with a band at 13.7 
exist in the stratosphere as
optically thin hazes as well as in gaseous form. These are created by
the breaking apart of methane by UV photons. We have
laboratory measurements for the band strengths of both molecules and
will use them to compute mean opacities across the filter bandwidth.
Preliminary calculations show that these features are formed from 100
bar to 1 mbar.
Figure 3 shows the contribution function at the wavelengths we will be using computed with the model of Marley et al. (1995). The contribution function is the integrand of the emergent flux integral, determining how much each atmospheric level contributes to the thermal emission to space. Plotted over it is the temperature profile derived from the RSS experiment (Lindal et al. 1987). The contribution function is defined to be (Houghton et al. 1984),
where 
is the effective optical depth from the top of
the atmosphere to the atmospheric layer being examined and d
the
opacity contribution from each layer. Both this and figure 4 were
calculated using an emission angle of 0
.
Uranus will come into opposition on July 21, 1995, reaching its peak
angular diameter of 3.7
, however by September and early
October it will still project a 3.6 
disk across the sky.
The best FWHM point sources previously achieved using MIRAC2 at 11.5
was 0.8
so assuming a factor of 25% larger we will
have 3.6 resolution elements across the diameter of Uranus and 10 on
the disk in the short window. Using a 25% greater factor than the
best 1.4
achieved in the longer window gives 2.3 across the
diameter and 4.0 on the disk. The fact that we will have
spatially-resolved observation will allow us to probe the
mid-stratosphere due to limb-darkening.