TIMED UPDATE

The Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED)
Mission has been facing increasingly stringent NASA budgets since the time
of its inception during the Space Physics Implementation Study of 1991.
These pressures led to a call for a cost-capped ($300 M) design-to-cost
mission in the TIMED Announcement of Opportunity, a selection of only a
subset of the strawman instrument payload, and immediate pressure on the
selected Principal Investigators to consider ways of significantly
decreasing the cost of the mission.  Pressure was unrelenting, however, and
early this year the Space Physics Division (SPD) convened an ad hoc review
panel of independent scientists to review options for downsizing the
mission  - to a program with development costs on the order of $100 M.
Although fitting a program into this budget requires a significant
downsizing in mission capability,  it was the conclusion of the ad hoc
panel that a viable first step towards understanding the mesosphere and
lower thermosphere (M/LT) could still be made within this cost.  The SPD
concurred with the conclusions of the panel and the TIMED program is being
restructured in a manner consistent with the panel's recommended approach.
An outline of the reconfigured program follows.  This core TIMED program is
the lead element in the new Solar Connections Program, which is a high
priority within the Office of Space Science for a Fiscal Year 1996 New
Start.


TIMED: Core Science Mission

The overall science goal for the TIMED mission, as defined by the AO, was
to measure the baseline state variables for the mesosphere/ lower
thermosphere/ionosphere (MLTI) region.  The down-sized mission has been
defined so as to provide the core subset of measurements that could define,
at least, to zeroth order  the state of the MLTI region. In this approach,
only the major parameters can be measured.  Reduced to the most fundamental
level, these parameters are: (1) energy input, (2) energy output, and (3)
the major consequences of this energy exchange, namely temperature, winds,
and density/composition. Moreover, only the most important physical
component of each of these parameters forms the core measurement subset.
Finally, the core measurement subset must provide those observations
throughout the entire altitude range (60 to 180 km) to insure that the
evolution in physical processes between low and high altitude is addressed.

The following set of measurements and instruments are necessary to carry
out this core TIMED mission.

Energy Input 

The most important and variable energy source into the region, one that is
most responsible for driving the dynamics, thermal, and chemical
properties, is the solar UV and FUV input (measured by SEE).

Energy Output 

The most important energy sink for the thermal balance of the region is
from radiatively active species present within the MLTI region.  Infra-red
radiation is the most effective cooling agent for the lower altitude MLTI
range (measured by SABER).

Basic State Parameters 

It is also imperative that the basic state parameters of wind, temperature,
and density/composition be obtained on a global scale as these parameters
are a direct result of the energy influences involved.

The neutral motions contribute to the transport of long-lived species, the
redistribution of thermal energy, and the understanding of the competing
forces involved in forming the wind.  As a result, measurement of the
neutral wind is considered a basic state parameter of the MLTI region
(measured by TIDI).

Direct measurements of temperatures in the lower altitude portion of the
MLTI regime are provided by SABER.
 
Above the mesopause, the atmosphere is no longer mixed -- the composition
is changing and the concept of temperatures becomes less meaningful.  The
major species are O, O2, and N2 but their relative concentrations are
rapidly changing.  The absorption of solar energy over altitudes above 120
km is especially sensitive to these concentrations.  Measurement of the
composition and density of these constituents is essential for the mission
(measured by GUVI).  In addition, profile measurements of these
constituents also provide the temperature profile above 120 km because of
the connection between temperature and scale height (provided by GUVI).

Integration of data from the individual instruments will be facilitated by
the six TIMED Interdisciplinary Scientists.  The provision of supporting
ground-based measurements, the exact form of which is still be discussed,
will also be important to the success of this program.


Core Mission Experimental Investigations:

GUVI  - Global Ultraviolet Imager
        Dr Andrew Christensen/.Aerospace Corporation

SABER - Sounding of the Atmosphere Using Broadband Emission Radiometry
        Dr James Russell/NASA Langley

SEE   - Solar EUV and XUV Spectral Irradiance Experiment for the TIMED
        Mission
        Dr Thomas Woods/University of Colorado

TIDI  - A TIMED Doppler Interferometer
        Dr Timothy Killeen/University of Michigan


Core Mission Interdisciplinary Scientists:

Dr Guy Brasseur/National Center for Atmospheric Research 
Dr Jeffrey Forbes/University of Colorado 
Dr David Fritts/University of Colorado
Dr Janet Kozyra/University of Michigan 
Dr Hans Mayr/NASA Goddard Space Flight Center 
Dr Stanley Solomon/University of Colorado