July 1991 

National Aeronautics and 

Space Administration

NASA—CR-191 377 

THERMOSPHERE-IONOSPHERE-MESOSPHERE 

ENERGETICS AND DYNAMICS 

(TIMED)___________________ 

/(rCf rL r 

THE TIMED MISSION AND SCIENCE PROGRAM 

REPORT OF THE SCIENCE DEFINITION TEAM

VOLUME I: EXECUTIVE SUMMARY 

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"the Earth's mesosphere and lower thermosphere/ionosphere are the least

explored regions in the Earth's atmosphere..." 

Space Science in the Twenty-First Century: Imperatives for the Decades 1995-2015,

Space Science Board, National Academy of Sciences, 1988. 

it series of space observations is needed to advance understanding of the

interesting dynamical, chemical and radiative processes in the mesosphere,

stratosphere and thermosphere. One low and one high-inclination spacecraft

are first needed to establish basic atmospheric properties and their

geographic, diurnal, and seasonal dependencies" 

Solar-Terrestrial Research for the 1980's, Committee on Solar-Terrestrial Research,

National Academy of Sciences, 1981.
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j TABLE OF CONTENTS 

Preface . iv 

Science Definition Team ...................................................................... V 

Summary....................................................................................... V 

I. Introduction .............................................................................1 

II. Scientific Objectives.................................................................... 

III. TIMED Mission Requirements.......................................................9 

111.1 Measurement Requirements.................................................9 

111.2 Orbital Requirements.........................................................12 

IV. Implementation Plan.................................................................... 12 

IV. 1 Mission Scenario and Orbit Selection...................................... 12 

IV.2 Strawman Instrument Complements ....................................... 14 

IV.3 Spacecraft Design............................................................. 14 

IV.4 Data System................................................................... 16 

V. Scientific Merit.......................................................................... 17

LIST OF FIGURES 

Frontispiece ......................................................................................... II 

1 Schematic illustration of the regions in the neutral atmosphere and 

ionosphere for investigation...........................................................4 

2 Plot of the number of nights per year, N, on which noctilucent clouds 

were reported from north-west Europe ..............................................7 

3 Apogee and perigee scenarios for TIMED-H and -L ..............................13 

4 TIMED conceptual spacecraft configuration ......................................... 16 

5 Distributed data system for TIMED ..................................................17 

LIST OF TABLES 

1 List of National Advisory Reports calling for MLTI exploration.................3 

2 Characteristics of the 60-180 km transition region .................................6 

3 Overall Objectives of the TIMED Mission.

 ........................................... 8 

4 Measurement Requirements...........................................................10 

5 Core Strawman Instruments for TIMED-H and -L.................................15 
PREFACE 

The work described in this report was performed by the Thermosphere-IonosphereMesosphere Energetics and Dynamics (TIMED) Science Definition Team under the 

cognizance of the Ionospheric Physics Branch of the Space Physics Division of NASA's 

Office of Space Science and Applications.

lv 
THERMOSPHERE-IONOSPHERE-MESOSPHERE ENERGETICS AND DYNAMICS

(TIMED) 

Science Definition Team 

T. L. Killeen (Chairman) University of Michigan 

L. H. Brace University of Michigan 

J. M. Forbes Boston University 

D. C. Fritts University of Alaska 

R. A. Heelis University of Texas at Dallas 

E. J. Llewellyn University of Saskatchewan, Canada 

H. G. Mayr NASA Goddard Space Flight Center 

C.I. Meng Applied Physics Laboratory 

R. G. Roble National Center for Atmospheric Research 

D. W. Rusch University of Colorado 

J. M. Russell ifi NASA Langley Research Center 

E. P. Szuszczewicz Science Applications International Corporation 

M. R. Torr NASA Marshall Space Flight Center 

U. von Zahn Bonn University, Germany

Several individuals have contributed importantly to the development of this report. These 

include: Lee-Ann Adams, Arthur C. Aikin, Daniel N. Baker, Charles A. Barth, Chein-San 

Cheng, J. Dodgen, David Eakman, Jack Ercol, Joseph Grebowski, Fred Hager, Isadore 

Harris, Paul B. Hays, Alan E. Hedin, Walter Hoegy, Robert A. Hoffman, William 

Hoggard, Rainer Illing, Charles H. Jackman, Roberta M. Johnson, John Kelly, Bryan C. 

Kennedy, Stamatios M. Krimigis, Joan Lavallee, Kaichi Maeda, Nelly de Maldonado, Ian 

McDade, Martin Mlyncsac, Larry Mooney, John J. O'Brien, Larry Paxton, George T. 

Roach, Rao Raghavarao, Gerald Romick, Mark R. Schoeberl, George Siscoe, David 

Siskind, James A. Slavin, Stanley C. Solomon, Malcom J. Tarlton, Jr., Bob Taylor, 

William Thompson, Robert Vincent, Lawrence E. Wharton, Lori Winter, Jeng-Hwa Yee, 

V 

Al Yetman.
SUMMARY 

I A Science Definition Team was established in December 1990 by the Space Physics 

Division, NASA, to develop a satellite program to conduct research on the energetics, 

dynamics, and chemistry of the mesosphere and lower thermosphere/ionosphere. This 

two-volume publication describes the TIMED (Thermosphere-Ionosphere-Mesosphere, 

Energetics and Dynamics) mission and associated science program. The report outlines the 

scientific objectives of the mission, the ,program requirements, and the approach towards 

meeting these requirements. - 

TIMED will carry Out the first comprehensive space-borne investigation of the physical and 

chemical processes acting in the terrestrial mesosphere and lower thermosphere/ionosphere 

(MLTI) between —60 and —180 km. This region represents a "crossroads" in the flow of 

mass, momentum, and energy through the coupled atmosphere-ionosphere-magnetosphere 

system. It is a region that is poorly understood due, in part, to its inherent complexity and 

due, in part, to the difficulty of carrying out in situ measurements at these altitudes. It is a 

"transition" region where atmospheric temperatures and thermal gradients reach extreme 

values, where turbulent flow changes to diffusive molecular flow, where the composition 

changes from molecular to atomic, where complex photochemical and electrodynamical 

processes become predominant, and where these combined effects challenge theoretical 

description. 

TIMED will be an exploratory mission to characterize and understand the interplay of 

composition, energetics, radiation and dynamics of this region. It will complement the 

NASA UARS (stratospheric) and ISTP (magnetospheric) missions and contribute to the 

international STEP program. TIMED will also contribute to the study of anthropogenicallyinduced changes in the atmosphere. Specifically, the mesosphere is sensitive to increasing 

levels of methane and carbon dioxide and may be an important harbinger of global change. 

There is already strong evidence that the mesosphere may be changing on relatively short 

time scales (-10's of years), and hence it is critical to establish a baseline global description 

of the region at the earliest possible date. 

The satellite and instrument requirements for the TIMED mission are derived from 

prioritized requirements calling for measurements of the energy and momentum sources, 

state variables, dynamics, and the related ionospheric structure and electrodynamics. The 

resulting mission comprises two identical spacecraft in different orbits to provide extensive

vi 
coverage of MLTI parameters in latitude, altitude, local time, and season. The spacecraft 

are launched on separate Delta-class expendable launch vehicles into orbits having 

inclinations of 95° and 490• Both spacecraft have on-board propulsion capabilities to allow 

for circular and eccentric orbits. The operating lifetime of each spacecraft is 48 months. 

The TIMED mission requires a dedicated ground-based data system and a science team 

organization that ensure strong interactions between experiments and theory and 

appropriate use of the data resource. As such, TIMED represents a natural evolution from 

previous NASA missions, such as AE, SME, DE, and UARS. It will enable a quantitative 

exploration of a critical transition region of the Earth's upper atmosphere that has 

previously proved resistant to experimental study. 

The TIMED mission concept was a product of the 1990 Strategy-Implementation Study that 

brought together scientists from the disciplines in the ionosphere-thermosphere-mesosphere 

(ITM) community. This document is the final report of the Science Definition Team for the 

TIMED pre-Phase-A study. Volume I provides an executive summary, while Volume II 

provides the detailed findings of the team. 

With the presentation of this pre-Phase-A study report, the TIMED mission and supporting 

technology are sufficiently well defined so that, with a one-year Phase-A study, TIMED 

will be ready as a new-start candidate in the intermediate-class cost category.

vii 
I. INTRODUCTION 

While our knowledge of the near-Earth space environment has increased enormously since 

the start of the Space Age, many significant gaps in this knowledge exist and our ability to 

model the variability of the system as a whole remains rudimentary at best. Furthermore, 

certain regions of the solar-terrestrial system remain largely unexplored and important 

physical, chemical, dynamical, electrodynamical, and radiative coupling processes which 

act upon and between different regions have been identified but not investigated in detail. 

The desire to investigate and understand our "local" space environment is not only driven 

by intellectual curiosity, but also by practical societal motivations. In particular, there has 

been increasing concern and interest in recent years about the variability, both natural and 

artificial, of the Earth's upper atmosphere and ionosphere. These concerns have been 

emphasized by the discoveries of anthropogenically-induced changes in the ozone 

abundance and potential global warming related to increased atmospheric loading of 

radiatively active minor constituents. In addition, long-term variations in solar activity and 

large solar storms have led to instances of premature re-entry of orbital vehicles and 

disruptions in power supply and global communications. These important issues and their 

societal effects have heightened our appreciation of the complexity of the Earth's upper 

atmosphere and ionosphere and its sensitivity to external influences. The potential threats 

to the stability .of the near Earth space environment, and the possibility of additional and yet 

unidentified changes, call for an enhanced long-term program of scientific research directed 

toward improving our knowledge of the Earth's atmosphere. 

The region of the atmosphere about which the least is currently known is the mesosphere 

and the MLTI. This region, between roughly 60 and 180 km altitude, is too high for 

probing by balloons and too low for direct in situ measurements by long-lived satellites 

without on-board propulsion. As a consequence, it has received much less attention than 

the more readily accessible regions above and below. In many respects, the MLTI region 

represents an uncharted "frontier" for upper atmospheric research. 

This lack of knowledge and understanding of the MLTI region has been recognized by the 

various national advisory groups. For example, the Space Science Board of the National 

Academy of Sciences has stated that: 

"The Earth's mesosphere and lower thermosphere are the least explored regions of 

the Earth's atmosphere. They are influenced by varying solar extreme ultraviolet,

1 
ultraviolet, and X-ray radiation, auroral particles and fields, and upward 

propagating waves and tides from the lower atmosphere. There are strong 

interactions between the chemistry, dynamics, and radiation of both the neutral and 

ionized constituents of this region. It is known that the global structure of this 

region of the atmosphere can be perturbed during stratospheric warinings and solarterrestrial events (magnetospheric substorms, solar flares, etc.), but the overall 

structure and dynamic responses to these effects and even the basic controlling 

physical and chemical processes of these effects are not understood" 

To study this system, the National Academy's Committee for Solar-Terrestrial Research 

pointed out that: 

"A series of space observations is needed to advance understanding of the 

interesting dynamical, chemical, and radiative processes in the mesosphere, 

stratosphere, and thermosphere. One low and one high inclination spacecraft are 

first needed to establish basic atmospheric properties and their geographic, diurnal, 

and seasonal dependencies" 

A list of recent Academy reports calling for the exploration and quantitative investigation of 

the MLTI region is given in Table 1. 

The recent Strategy-Implementation Study (SIS) of the Space Physics Division also 

recommended a phased program of NASA missions devoted to a comprehensive study of 

the lonosphere-Thermosphere—Mesosphere (ITM) system. The first component of this 

program identified during the SIS is the Thermosphere—Ionosphere—Mesosphere, 

Energetics and Dynamics (TIMED) mission, which is designed to address the MLTI 

region and is in the "intermediate class" ($ 200 - 300 M) cost category. This TIMED 

mission is an important new effort aimed at improving our knowledge and understanding 

of the atmosphere and ionosphere above the stratosphere and below the upper 

thermosphere/ionosphere and magnetosphere. In this region, complex physical, chemical, 

dynamic, radiative, and electrodynamic processes all undergo profound transitions and 

combine to drive a rich thermal, dynamical, and compositional global morphology. 

Through a careful balance between comprehensive measurements, theoretical studies of the 

basic processes, and model analysis, it is now clear that substantial progress can be made 

in solving the major outstanding problems of this region.

2 
Table 1

National Advisory Reports calling for the exploration of the Mesosphere,

Lower-Thermosphere/Ionosphere System 

(1) Solar-Terrestrial Research for the 1980's, Committee on Solar-Terrestrial 

Research, National Academy of Sciences, 1981. 

(2) A Strategy for the Explorer Program for Solar and Space Physics, National 

Academy Press, Washington D.C.,1984. 

(3) An Implementation Plan for Priorities in Space Physics, Space Science Board, 

National Academy of Sciences, 1985. 

(4) Strategic Plan, NASA Office of Space Sciences, 1988. 

(5) Solar-Terrestrial Sciences, Report from the Science Strategy Workshop, NASA 

Space Physics Division, 1988. 

(6) Space Science in the Twenty-First Century, Solar and Space Physics report of the 

Study Steering Group, Space Science Board, National Academy of Sciences, 

1988. 

(7) Earth System Science, A Closer View, Report of the Earth System Sciences 

Committee, NASA Advisory Council, 1988. 

(8) Long-term Solar-Terrestrial Observations, Panel on long-term observations, 

Committee for Solar-Terrestrial Research, National Academy of Sciences, 1988. 

(9) Space Physics Strategy-Implementation Study Workshop, Executive Summary - 

Report of the Mission Integration and Divisional Science Plan, 1990. 

This report (Volume I of the pre-Phase-A study) provides a brief summary of the findings 

of the TIMED Science Definition Team. Full details of the study may be found in Volume 

II. The following sections briefly describe the TIMED science rationale and 

implementation plan.

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II. SCIENTIFIC OBJECTIVES 

The scientific objectives for the TIMED mission were developed after a thorough review of 

the current state of knowledge of the field and the development of a prioritized set of key 

scientific questions. After this review, and in accordance with the mandates of both the 

Academy of Sciences and the Strategy-Implementation Study, the Science Definition Team 

adopted the following Primary Objective for TUvIED: 

To perform an exploratory study of the physical and chemical processes

acting within and upon the coupled mesospheric, thermospheric, and

ionospheric system between —60 and 180 km. 

Figure 1 illustrates the basic thermal and plasma density structure of the MLTI region, 

together with an indication of the altitude regimes of interest for the UARS and ISTP 

missions as well as those for balloon and some ground—based experiments.

4 

Ground Based Gas Temperature Ionospheric 

Density (log) 

Figure 1. Schematic illustration of the atmospheric thermal structure and electron (ion) 

content. The primary altitude regimes of scientific investigations for TIMED - 

as well as those for UARS, ISTP, ballon and some ground—based experiments 

- are indicated. 
The region to be investigated by TIMED is home to a large number of fundamental 

transitions in atmospheric processes. Perhaps the most fundamental one is the change from 

turbulent mixing to molecular diffusion. However, there are also many others. The energy 

balance is marked by a change from radiative control to thermal conductive control. The 

composition changes from molecular to atomic. The available solar radiation changes 

spectrally, with EUV important at higher altitudes and UV important at lower altitudes. 

The dynamics of the region changes from geostrophic control to ageostrophic control, with 

viscosity and ion drag playing more important roles at higher altitudes. The highly variable 

heating rates, due to solar EUV/UV absorption, Joule heating, particle heating, tidal, and 

gravity wave dissipation, all maximize in the MLTI transition region. Yet, the lowest 

temperatures anywhere in the Earth's atmosphere occur near the summer polar mesopause. 

The region also houses the primary atmospheric, ionospheric, and electrodynamical 

interactions. The influence of the magnetosphere becomes significant, and ionospheric 

conductivities and currents maximize. Gravity wave breaking and plasma instabilities 

affect the energy and momentum balances, as turbulence becomes a primary factor in mass, 

momentum, and energy transfer processes at the lower boundary. Finally, in this region of 

the Earth's atmosphere, the largest temperature gradients are found - leading to a rapid 

change in atmospheric stability. 

Table 2 summarizes the fundamental and dramatic changes that characterize this transition 

region. Some of the processes responsible for these effects are schematically illustrated in 

the frontispiece. All of these processes occur within the MLTI region that has never been 

the subject of a comprehensive satellite investigation. It is the general purpose of TIMED 

to provide such a comprehensive and global study of the interplay between dynamics, 

energetics, chemistry, radiation, and electrodynamics in this region. 

In addition to the exploratory and investigative motivations for TIMED, there is an 

important additional need for early measurements of the MLTI region. It is expected that 

anthropogenic changes will cause the terrestrial mesosphere to experience first-order 

changes in its chemistry, composition, temperature distribution, and dynamics within the 

next two to three decades. The projected cooling of the mesosphere from increased 

amounts of carbon dioxide, for example, will affect the chemistry of important minor 

constituents. This cooling will also allow high altitude ice clouds (Polar Mesospheric

5 
Clouds) to form at lower attitudes than normally possible. This equatorward spread may 

influence the climate down to the Earth's surface. It seems very likely that the mesosphere. 

Table 2

Characteristics of the 60-180 km Transition Region 

(1) Turbulent mixing gives way to molecular diffusive separation and transport. 

(2) Radiative processes dominate the thermal balance below and thermal conduction 

dominates above. 

(3) Composition ceases to be dominated by molecules and becomes increasingly 

dominated by atoms. 

(4) Solar EUV is important at higher altitudes and solar UV is important at lower 

altitudes. 

(5) Heating rates due to solar EUV/UV absorption, Joule heating, and particle heating 

all maximize. 

(6) The dynamics of the region change from geostrophic control to ageostrophic 

control. 

(7) The coupling processes between the atmosphere and the magnetosphere become 

significant, conductivities maximize, and the charged particle population plays an 

important role in atmospheric energetics. 

(8) Gravity wave breaking and plasma instabilities play significant roles in the energy 

and momentum balances. 

(9) The thermal structure shows a fundamental change in lapse rate, leading to rapid 

changes in atmospheric stability. 

(10) Local thermodynamic equilibrium ceases to apply. 

(11) The charged particle population changes from one that has positive and negative 

cluster ions at lower altitudes, to one that has electrons and chemically controlled 

molecular ions, to one that has electrons and diffusively controlled atomic ions at 

higher altitudes. 

(12) The dynamics of charged particles is controlled by collisions with the neutral 

atmosphere at lower altitudes and by diffusion and electric as well as magnetic 

fields at higher altitudes.
of the future will be quite different from what it is now. Due to this high degree of 

senitivity to anthropogenic effects in temperature and composition, the mesosphere proves 

to be an ideal region for the early detection of related long-term atmospheric changes. 

The anticipated changes can be appreciated from a straight—forward assessment of the 

effects of a growing abundance of atmospheric carbon dioxide, methane, and water vapor. 

CO2, increasing at the rate of 0.5%/yr, will lead to a far stronger cooling of the middle 

atmosphere than to a warming of the Earth's surface. The expected changes in temperature 

from a doubling of carbon dioxide will be in the 10's of degrees in the middle atmosphere, 

compared to fractions of a degree at the surface. Methane is increasing at the rate of 1 - 

2%/yr in the middle atmosphere. Its oxidation to water vapor in the middle 

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Figure 2. Plot of the number of nights per year, N, on which noctilucent clouds 

were reported from north—west Europe, with the effect of solar activity 

removed. This plot (from Gadsden, 1990) provides evidence for secular 

increases in mesospheric noctilucent cloud occurrence. 

atmosphere will drive a stronger odd—hydrogen chemistry, probably resulting in a decrease 

in the density of ozone in this region. This sensitivity of the mesosphere is exemplified in 

Figure 2 by the observed increase in noctilucent cloud occurrence, which accounts for no 

less than a factor of —2 increase over the past 20 years and has been attributed to the man 

induced methane increase or, alternately, to the secular decrease in mesopause temperature.

7 
In response to a series of key scientific questions formulated by the Science Definition 

Team, the five overall objectives for the TIMED mission have been developed and are 

given in Table 3:

Table 3 

TIMED Overall Mission Objectives 

(1) To investigate and quantitatively understand the important physical and chemical 

processes responsible for the energy and momentum budgets of the MLTI region 

and the electrodynamic as well as chemical interactions between the neutral and 

plasma components of the region. 

(2) To investigate and quantitatively understand the coupling between chemistry, 

radiation, waves (tides, gravity waves, and planetary waves), unorganized 

motions, and the large-scale circulation, and the resultant effects on momentum, 

energy, and mass (species) transfer within, to, and from the mesosphere and 

lower thermosphere. 

(3) To investigate and quantitatively understand the lower ionosphere: to elucidate its 

role in the global electric circuit, to understand the chemistry and energy budget of 

this region, and to determine how these processes affect the coupling between the 

lower thermosphere and mesosphere. 

(4) To investigate and quantitatively understand the odd oxygen, odd nitrogen, and 

odd hydrogen chemistry within the Mesosphere and Lower Thermosphere 

(5) To investigate anthropogenically induced changes in the mesosphere and lower 

thermosphere and establish a baseline for future investigations. 

These objectives, together with the measurement oriented science questions, in turn, were 

used to derive the measurement requirements, leading to an implementation strategy. Full 

details of this process can be found in Volume 11 of this report.

8 
III. TIMED MISSION REQUIREMENTS 

111.1 Measurement Requirements 

The primary measurements of TIMED include observations of the global and temporal 

variations in the atmosphere and ionosphere, which are controlled by radiative, chemical, 

dynamic, thermodynamic, and electrodynamic processes. Measurements of the dynamics 

of neutral and ionized species will serve to describe, for the first time, the global-scale 

circulation of the MLTI region, including global wind systems, planetary waves, solar and 

lunar tides, and a broad spectrum of gravity waves. They will allow fundamental 

investigations of turbulence and wave breaking, which are known to affect profoundly the 

mass, momentum, and energy budgets of the region. The measurements will also enable 

the electrodynamics of the most highly conducting part of the atmosphere to be 

investigated, leading, in turn, to deeper insights concerning the global electrical circuit, 

including magnetospheric components. 

Measurements of the radiative and corpuscular sources of energy, together with 

observations of airglow emissions and neutral and plasma temperature distributions, will 

serve to determine the energy balance of the region. Such observations and associated 

theoretical work will quantitatively determine, for example, the causes of the extremely low 

temperatures found in the summer mesopause - the coldest region of the atmosphere. 

Measurements of the relative abundances of the major neutral and ionized constituents of 

the region, together with observations of the altitude profiles of a panoply of 

photochemically active ionized and neutral trace constituents will enable the first 

comprehensive and detailed compositional global inventory to be assembled. Such 

measurements will provide a new window on the extremely complex chemical and radiative 

processes known to couple the odd oxygen, odd hydrogen, and odd nitrogen chemical 

families, which collectively control atmospheric ozone. 

Table 4 summarizes the needed observations related to the key scientific questions (ref. 

Volume II), without regard to the ease of measurement. These measurements are required 

to accomplish the five overall scientific objectives.
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111.2 Orbital Requirements 

The three principal orbital requirements for the mission are: 

(1) The need for global coverage, including in particular the high-latitude 

regions. 

(2) The need for rapid local time precession of the orbit to enable tidal structures 

to be mapped with a resolution of (at a minimum) 24 hrs LT per month. 

(3) The need for remote measurements down into the mesosphere and in situ 

measurements down into the lower thermosphere and ionosphere. 

These requirements point to the need for 3-axis stabilization of the vehicles (for accurate 

remote sensing), propulsion capability (for phased orbital adjustments), and a combination 

of elliptical orbits (to provide low altitude in situ measurement capability) and circular orbits 

(to optimize the remote sensing measurement capability). A scenario designed to meet the 

measurement and orbital requirements is discussed below. 

IV. IMPLEMENTATION PLAN 

IV.1 Mission Scenario and Orbit Selection 

No single orbit is suitable for separating the spatial and temporal variations of the 

mesosphere and lower thermosphere. A configuration with two spacecraft (TIMED-H and 

-L) is required to provide the complementary coverage. TIMED-H will be placed into in a 

high—inclination, nearly sun-synchronous 95°orbit, while TIMED-L will be in a 490 orbit. 

The slowly precessing orbit of H will provide essentially full global coverage, while the 

rapidly precessing orbit of L will provide full diurnal coverage each month at middle and 

low latitudes. From these orbits, a combination of remote and in situ devices will cover the 

entire altitude region of interest in both latitude and local time.

12 
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Figure 3. Apogee and perigee scenarios for TIMED-H and -L 

A strawman phased mission scenario is illustrated in Figure 3. The length of each mission 

phase will be selected to provide optimum geophysical coverage by combining the 

measurements from the two spacecraft. TIMED-L begins in circular orbit at altitudes 

between 300 and 400 km (Phase I) where both remote and in—situ global coverage will be 

obtained. TIMED-H starts in a high elliptical orbit (Phase I) that favors "deep diving" into 

the lower thermosphere and ionosphere where in—situ measurements will be stressed. In 

Phase II, the roles of TIMED-H and -L will be reversed. TIMED-L will be maneuvered 

into an elliptical orbit for deep diving, while the orbit of TIMED-H will be circularized 

through the use of aerodynamic drag. Onboard propulsion capability will provide the 

sampling advantages of both elliptical and circular orbits. A 4-year mission lifetime is 

adequate to permit full separation of latitude, local time, and seasonal variations in the 

mesosphere and lower thermosphere.
Ii 

14

IV.2 Strawman Instrument Complements 

Table 5 identifies a total of 15 instruments for the Core Strawman Payload of the two 

spacecraft, although each spacecraft would carry only a subset of these. Most of these 

instruments are required on both spacecraft, but a few are restricted to one. The infrared 

limbscanning is best done from the lower inclination TIMED-L orbit because it precesses 

rapidly enough to permit measurement of the very important diurnal and sernidiurnal tides 

in the mesosphere. The fields and particle measurements are more appropriate for the 

higher inclination orbit of TIMED-H. 

IV.3 Spacecraft Design 

The instrument complements and mission plans of the two spacecraft led to a common 

spacecraft design, and Figure 4 shows a suggested configuration. 

Both spacecraft require a substantial onboard propulsion capability to permit survival for 4 

years in the relatively high drag environment to which they will be exposed. There must be 

adequate fuel capacity to attain and maintain the desired orbital eccentricities and altitudes 

given in the strawman mission scenario. They must also provide the capability to acquire 

and store data at any remote point about the orbit and to dump those data once or twice a 

day during passages over a NASA tracking station. The battery capacity should be 

adequate to permit continuous operation of all instruments for at least one full orbit, and its 

solar cell capacity should be adequate to recharge the battery quickly enough to permit full 

orbit operation for one out of every four orbits. In an elliptical orbit, the spacecraft should 

be designed to operate for at least 25 minutes during every perigee passage. 

The spacecraft thermal design and the layout of the solar cells must accommodate both high 

and low inclination orbital geometries with widely varying Sun angles. The spacecraft 

must also support simultaneous in-Situ and remote measurements from both orbits. This 

means that they must employ a method of attitude stabilization which simultaneously points 

the limb scanners at the limb, the ram lookers into the velocity vector, the airglow imagers 

appropriately downward to view the underlying atmosphere, and the Solar EUV 

Spectrometer at the Sun. The deep diving requirement precludes the use of a deployed 

solar array which would add unacceptably to the total spacecraft drag coefficient. Such
arrays are also incompatible with the need to avoid the scattering of light and particles into 

the sensors, and to avoid producing a wake that could affect the in—situ measurements at 

certain combinations of solar and velocity directions. 

Table 5

Core Strawman Instruments for TIMED-H and -L 

No * Instruments Acronym Measured Parameters Spacecraft 

H L 

A. Remote Sensing 

6 UV Spectrometer UVS 03, N0, and other minor X X 

constituents 

14 Near Infrared NIRSF 020A), (03), OH rot., and H20 X I 

Spectrometer/Photometer 

1 Fabty-Perot Interfemmeter FPI Wind, temperature, and emission X X 

rate profiles 

7 Imaging Photometer EP Wave structures X X 

12 Global UV Airgiow Imager GUVI Global Imaging of atmospheric X 1 

UV emissions 

9 Infrared Limb Sounder2 IRLS Mesospheric CO2,03, OH, and 1 X 

NO 

11 Solar EUV Spectrometer3 SEUVS Spectral Solar EUV and X-ray X - 

fluxes 

B. In situ 

2 Neutral Mass Spectrometer/Wind NMSWT Local gas composition, winds, X X 

and Temperature and temperature 

3 Ion Mass Spectrometer IMS Ion composition X X 

4 Langmuir Probe LP Electron/ion density, and electron X X 

temperature 

10 Energetic Particles Analyzer EPA Auroral electron and ion energy X 1 

fluxes 

5 Ion Drift Meter Retarding IDM/RPA Ion drift/electric fields, and ion X X 

Potential Analyzer temperature 

17 Magnetometer MAG Currents X I 

8 AccelemmeteA ACC Total air density X X 

15 Electric Field Detector/Plasma EFD/PWE Three-axis AC and DC electric X 1 

Wave Experiment5 fields

The Fast Electron Spectrometer (13) and Energetic Particle Spectrometer (16) are secondary instruments. 

1 Secondary Instrument 

2 Low duty cycle due to high power requirements 

3 Not needed on TIMED if done simultaneously on other mission(s) 

4 Non-PI class instrument. 

5 Contingent on feasibliry of booms at low perigee altitude 

* Numbers referred to in Figure 4. 

15 
'OUTLINE OF RADIATIF 

SURFACE (BOTH END 

14

-. 3

4 

ANTEI 

(BOIl

8 

17 

4 

nne 

13- / 

PROPULSION TANKS J 

FRONT VIEW

DIRECTION OF FLIGHT 

END VIEW 

VELOCITY VECIOR 

S

Instrument numbers are defined in Table 5 

Figure 4. TIMED conceptual spacecraft configuration 

IV.4 Data System 

The TIMED project will employ a distributed data system that will connect the Principal 

Investigator teams with both the project at GSFC and with ground-based investigators 

around the world. The system builds on the experience gained in the AE and DE missions 

by providing rapid access and exchange of processed geophysical data among the 

investigators. Figure 5 shows the system as it is currently envisioned. It will consist of a 

Spacecraft Control Center node and a Data Distribution Center node (both part of Code 500 

at GSFC), and investigator nodes at the home institutions of the Principal Investigators. 

The NSSDC will also become a node for acquiring and disseminating final geophysical 

data. These units will be networked to: (1) facilitate communication of the information 

needed to operate the spacecraft and instruments; (2) permit rapid transmission of telemeliy 

and orbital data to the investigator sites; (3) support the early and continuous exchange of 

geophysical data between the TIMED investigators; and (4) provide for access and the 

exchange of correlative data from Guest Investigators and other scientists indirectly 

connected to the Tll's4ED project.
OD/)Ys (Yata 

"111111h, 

tä 

Investigator Investigato 

Node 5) Node6 etc. 

Preliminary J - ;;;;;-;;------- mi 

arcnlve 5/ arcnive 6

ççc\ 

Co 1 O \ Ii 

SflX\1i 

\rC

NSI/DECnet 

lnvestlgato\ Investigato Investigator 

INode Node2 Node 3 U

CInvestigator

Preliminary PeIiminary Preliminary 

arcnlve a-cnive2 arcnive 3 CL

17 

TDRS housekee,ng data 

TIMED 

TIMED Data Distribution TIMED

Center (GSFC Code 500)

(f 01,

Control Center 

(GSFC Code 500) 

Collaborative I I NSSDC 

Measurements 1TIMED final geophysical data base (aroundbasecj, rockets)

Guest Investigators 

and other users 

Figure 5. Distributed data system for TIMED 

V. SCIENTIFIC MERIT 

Scientific Objectives and Significance 

The key scientific issues addressed by the TIMED mission have been briefly described 

above. The mission will provide the first comprehensive spaceborne investigation of an 

important region of the Earth's near-space environment - a complex transition region that 

may well be susceptible to rapid anthropogenic change. Since the mission has a real 

exploratory aspect, new discoveries are to be expected. It is possible, however, to identify 
18 

various significant scientific issues that will definitely be resolved by the mission. 

Specifically, TIMED will: 

• Determine the temperature and wind structure of the mesosphere and 

lower thermosphere, delineating the seasonal/latitudinal variations and 

tidal components. 

• Determine the relative importance of the major sources and sinks of 

energy for the thermal structure of the mesosphere and lower 

thermosphere. 

• Examine anthropogenically induced changes in the mesosphere and lower 

thermosphere. 

• Determine the roles of CO2 and NO cooling in the lower thermosphere 

and the importance of 0 in enhancing these cooling rates. 

• Determine the roles of exothermic chemical reactions, involving HOx and 

O '

 for the energetics and radiative budget of the mesosphere and lower 

thermosphere. 

• Advance our understanding of the radiative and dynamical processes that 

produce the cold and warm mesopause temperatures in the summer and 

winter hemispheres, respectively. 

• Shed light on the effects of albedo changes and radiation associated with 

noctilucent and polar mesospheric clouds and their importance for the 

energy balance of the mesosphere. 

• Determine the global distribution of the major constituents N2, 02, and 0 

in the upper mesosphere and lower thermosphere as well as that of CO2, 

which is important for the energy budget of the region. 

• Determine the spatial and temporal variations in odd oxygen [0 and 031, 

odd hydrogen [OH, H20, Cl-f4, and H021, and odd nitrogen compounds 

[NO, NO2, N(2D), and N(4S)J.
• Determine how the large-scale circulation in the mesosphere and 

thermosphere affects the distribution of chemically active and radiatively 

important species (such as F120, G02, CO. NO, and 0). 

• Advance our understanding of the photochemical, dynamical, and 

radiative processes that control the basic structure of the mesosphere and 

lower thermosphere. 

• Determine the spatial and temporal variations of radiatively significant 

minor species in the mesosphere and lower thermosphere [G02, NO, 03, 

OH, 02(1D), and 02(1S)]. 

• Determine the mean global circulation (meridional and zonal) in the 

mesosphere and lower thermosphere in response to annual forcing cycles 

and its geographic and short-term variability. 

• Determine the diurnal tidal structures, their annual and height variations, 

and their effects on the mean large-scale circulation. 

• Determine the sources and magnitudes of gravity waves and planetary 

waves in the mesosphere and thermosphere, and their spatial and temporal 

variability. 

• Determine the effects of gravity waves and planetary waves on the largescale circulation, thermal structure, and composition. 

• Provide an understanding of the interactions that occur among different 

wave motions (tidal, gravity, and planetary) and the extent, causes, and 

consequences of their variability. 

• Provide data to study the effects of gravity wave and tidal dissipation on 

turbulent transport of energy, mass, and momentum. 

• Determine the dynamical effects of magnetospheric coupling and the depth 

to which they penetrate.

19 
• Help delineate the electric field components of the magnetosphere, the 

disturbance dynamo, and the tidal dynamos that control the 

electrodynamics at middle and equatorial latitudes. 

• Help to determine the roles of neutral winds and electric fields affecting 

the small- and large-scale variability in the ionospheric density and 

composition. 

• Contribute significantly to our understanding of the coupling between Eand F- region plasma structures. 

• Contribute significantly to our understanding of the ionization sources and 

the metastable ion chemistry responsible for the molecular ion inventory in 

the lower ionosphere. 

• Significantly advance our understanding of the ion and neutral chemistry 

in the lower thermosphere which is important for the partitioning of 

energy. 

• Significantly advance our understanding of the global composition, 

temperatures, and transport velocities of neutrals and ions in the lower 

thermosphere by providing, for the first time, simultaneous observations 

of these variables. 

• Significantly advance our understanding of tides and related dynamo 

electric fields, as well as gravity waves and traveling ionospheric 

disturbances, by performing simultaneously in situ and remote 

measurements in the thermosphere, ionosphere, and mesosphere. 

• Significantly advance our understanding of magnetic storm effects in the 

neutrals and ions by performing simultaneous measurements of 

temperatures, densities, transport velocities, and electric as well as 

magnetic fields.

20 
Generality of Interest 

These scientific issues addressed by TIMED are of considerable general interest, since they 

are related to important sub-disciplines. These include: gas kinetics; turbulence; plasma 

physics; fluid dynamics; photochemistry; spectroscopy; atomic and molecular collision 

physics; thermodynamics; etc. Issues of practical concern include: satellite drag; shuttle reentry; aerobraking in planetary atmospheres; communication—disruption; and global 

change. 

The adapted technology used for the TIMED program will also be of considerable general 

interest. New stereoscopic remote sensing techniques will be developed to probe planetary 

atmospheres, and new tomographic techniques will be used to interpret the results. In—situ 

sensors will be used in the challenging low altitude regime. The combination of remote and 

in—situ measurements from two closely coordinated spacecraft will provide for new 

methods of resolving temporal and spatial ambiguities. New numerical models and global 

time dependent simulation techniques will be developed. 

TIMED has great potential for new discoveries, because the MLTI regime represents a 

unique laboratory for the study of transitional fluid dynamics and gas kinetics. Unexpected 

atmospheric linkages that couple the lower atmosphere to the MLTI and magnetosphere, 

may be discovered. Important new chemical schemes may be disovered, others 

elucidated, involving constituents or compounds that are key species for atmospheric 

evolution. New insights may be gained into the role played by turbulent processes in 

controlling the natural world. 

TIMED bridges a major gap in Solar-Terrestrial Science, connecting lower and upper 

atmospheric physics, magnetospheric physics and planetary aeronomy. Global 

understanding of the MLTI region requires a spaceborne measurement program - the 

knowledge to be provided by TIMED could not be gained using ground-based or suborbital techniques. 

Feasibility and Readiness 

The results of the pre-Phase-A study have shown that the TIMED mission is feasible and 

cost effective. The scientific community is ready for this mission, having anticipated it for 

some time. TIMED is built on the strong foundation of successful previous missions, such 

as AE, SME, and DE. The community has demonstrated its ability to coordinate efforts to 

21 

'I
22 

solve global-scale problems, as evidenced by the successful NSF CEDAR program. Strong J

international participation, both in the mission development phase and in the data analysis 

phase, is anticipated. Two international colleagues (one from Germany and one from 

Canada) participated in the pre-Phase-A study and their respective national agencies have 

already expressed an interest in the mission. TIMED is well poised to contribute LI

importantly to the international STEP program. 

The scale of the TIMED mission is well suited for participation by a spectrum of 

institutions; including universities, NASA centers, and industry. 

Finally, and perhaps most importantly, we reiterate that TIMED will explore the 

mesosphere and lower thermosphere/ionosphere, termed by the Academy of Sciences, "the 

least understood region in the Earths atmosphere." It is thought that this region of our 

near Earth space environment, which is closer to us than Baltimore is to Washington, may 

be highly susceptible to global change. The TIMED Science Definition Team considers it to 

be important that this broad, exciting, and well balanced research program is carried Out at 

the earliest possible date.
"the Earth's mesosphere and lower thermosphere/ionosphere are the least

explored regions in the Earth's atmosphere..." 

Space Science in the Twenty-First Century: Imperatives for the Decades 1995-2015,

Space Science Board, National Academy of Sciences, 1988. 

"a series of space observations is needed to advance understanding of the

interesting dynamical, chemical and radiative processes in the mesosphere,

stratosphere and thermosphere. One low and one high-inclination spacecraft

are first needed to establish basic atmospheric properties and their

geographic, diurnal, and seasonal dependencies" 

Solar-Terrestrial Research for the 1980's, Committee on Solar-Terrestrial Research,

National Academy of Sciences, 1981.
_ r 

4

0 1. 

COLOR PHOTOGRAPH
TABLE OF CONTENTS 

Preface . 1V 

Science Definition Team . V 

Summary....................................................................................... Vi 

1. Introduction .............................................................................1 

II. Scientific Objectives....................................................................4 

III. TIMED Mission Requirements.......................................................9 

111.1 Measurement Requirements.................................................9 

111.2 Orbital Requirements.........................................................12 

IV. Implementation Plan....................................................................12 

IV.1 Mission Scenario and Orbit Selection......................................12 

IV.2 Strawman Instrument Complements .......................................14 

IV.3 Spacecraft Design.............................................................14 

IV.4 Data System...................................................................16 

V. Scientific Merit..........................................................................17 

LIST OF FIGURES 

Frontispiece......................................................................................... 11

1 Schematic illustration of the regions in the neutral atmosphere and 

ionosphere for investigation...........................................................4 

2 Plot of the number of nights per year, N, on which noctilucent clouds 

were reported from north-west Europe ..............................................7 

3 Apogee and perigee scenarios for TIMED-H and -L ..............................13 

4 TIMED conceptual spacecraft configuration.........................................16 

5 Distributed data system for TIMED ..................................................17 

LIST OF TABLES 

1 List of National Advisory Reports calling for MLTI exploration.................3 

2 Characteristics of the 60-180 km transition region .................................6 

3 Overall Objectives of the TIMED Mission...........................................8 

4 Measurement Requirements...........................................................10 

5 Core Strawman Instruments for TIMED-H and -L.................................15

III 
PREFACE 

The work described in this report was performed by the Thermosphere-IonosphereMesosphere Energetics and Dynamics (TIMED) Science Definition Team under the 

cognizance of the Ionospheric Physics Branch of the Space Physics Division of NASA's 

Office of Space Science and Applications.

lv 
THERMOSPHERE-IONOSPHERE-MESOSPHERE ENERGETICS AND DYNAMICS

(TIMED) 

Science Definition Team 

T. L. Killeen (Chairman) University of Michigan 

L. H. Brace University of Michigan 

J. M. Forbes Boston University 

D. C. Fritts University of Alaska 

R. A. Heelis University of Texas at Dallas 

E. J. Llewellyn University of Saskatchewan, Canada 

H. G. Mayr NASA Goddard Space Flight Center 

C.I. Meng Applied Physics Laboratory 

R. G. Roble National Center for Atmospheric Research 

D. W. Rusch University of Colorado 

J. M. Russell ifi NASA Langley Research Center 

E. P. Szuszczewicz Science Applications International Corporation 

M. R. Torr NASA Marshall Space Flight Center 

U. von Zahn Bonn University, Germany

Several individuals have contributed importantly to the development of this report. These 

include: Lee-Ann Adams, Arthur C. Aikin, Daniel N. Baker, Charles A. Barth, Chein-San 

Cheng, J. Dodgen, David Eakman, Jack Ercol, Joseph Grebowski, Fred Hager, Isore 

Harris, Paul B. Hays, Alan E. Hedin, Walter Hoegy, Robert A. Hoffman, Williami 

Hoggard, Rainer Illing, Charles H. Jackman, Roberta M. Johnson, John Kelly, Bryan C. 

Kennedy, Stamatios M. Krimigis, Joan Lavallee, Kaichi Maeda, Nelly de Maldonado, Ian 

McDade, Martin Mlyncsac, Larry Mooney, John J. O'Brien, Larry Paxton, George T. 

Roach, Rao Raghavarao, Gerald Romick, Mark R. Schoeberl, George Siscoe, David 

Siskind, James A. Slavin, Stanley C. Solomon, Malcom J. Tarlton, Jr., Bob Taylor, 

William Thompson, Robert Vincent, Lawrence E. Wharton, Lori Winter, Jeng-Hwa Yee, 

AlYetman.

VA 
vi

-P . . SUMMARY 

A Science Definition Team was established in December 1990 by the Space Physics 

Division, NASA, to develop a satellite program to conduct research on the energetics, 

dynamics, and chemistry of the mesosphere and lower thermosphere/ionosphere. This 

two-volume publication describes the TIMED (Thermosphere-Ionosphere-Mesosphere, 

Energetics and Dynamics) mission and associated science program. The report outlines the 

scientific objectives of the mission, the program requirements, and the approach towards 

meeting these requirements. '-

TIMED will carry out the first comprehensive space-borne investigation of the physical and 

chemical processes acting in the terrestrial mesosphere and lower thermosphere/ionosphere 

(MLTI) between —60 and —180 km. This region represents a "crossroads" in the flow of 

mass, momentum, and energy through the coupled atmosphere-ionosphere-magnetosphere 

system. It is a region that is poorly understood due, in part, to its inherent complexity and 

due, in part, to the difficulty of carrying out in situ measurements at these altitudes. It is a 

"transition" region where atmospheric temperatures and thermal gradients reach extreme 

values, where turbulent flow changes to diffusive molecular flow, where the composition 

changes from molecular to atomic, where complex photochemical and electrodynamical 

processes become predominant, and where these combined effects challenge theoretical 

description. 

TIMED will be an exploratory mission to characterize and understand the interplay of 

composition, energetics, radiation and dynamics of this region. It will complement the 

NASA UARS (stratospheric) and ISTP (magnetospheric) missions and contribute to the 

international STEP program. TIMED will also contribute to the study of anthropogenicallyinduced changes in the atmosphere. Specifically, the mesosphere is sensitive to increasing 

levels of methane and carbon dioxide and may be an important harbinger of global change. 

There is already strong evidence that the mesosphere may be changing on relatively short 

time scales (-10's of years), and hence it is critical to establish a baseline global description 

of the region at the earliest possible date. 

The satellite and instrument requirements for the TIMED mission are derived from 

prioritized requirements calling for measurements of the energy and momentum sources, 

state variables, dynamics, and the related ionospheric structure and electrodynamics. The 

resulting mission comprises two identical spacecraft in different orbits to provide extensive
coverage of MLTI parameters in latitude, altitude, local time, and season. The spacecraft 

are launched on separate Delta-class expendable launch vehicles into orbits having 

inclinations of 950 and 490 Both spacecraft have on-board propulsion capabilities to allow 

for circular and eccentric orbits. The operating lifetime of each spacecraft is 48 months. 

The TIMED mission requires a dedicated ground-based data system and a science team 

organization that ensure strong interactions between experiments and theory and 

appropriate use of the data resource. As such, TIMED represents a natural evolution from 

previous NASA missions, such as AE, SME, DE, and UARS. It will enable a quantitative 

exploration of a critical transition region of the Earth's upper atmosphere that has 

previously proved resistant to experimental study. 

The TIMED mission concept was a product of the 1990 Strategy-Implementation Study that 

brought together scientists from the disciplines in the ionosphere-thermosphere-mesosphere 

(ITM) community. This document is the final report of the Science Definition Team for the 

TIMED pre-Phase-A study. Volume I provides an executive summary, while Volume II 

provides the detailed findings of the team. 

With the presentation of this pre-Phase-A study report, the TIMED mission and supporting 

technology are sufficiently well defined so that, with a one-year Phase-A study, TIMED 

will be ready as a new-start candidate in the intermediate-class cost category.

vu 
I. INTRODUCTION 

While our knowledge of the near-Earth space environment has increased enormously since 

the start of the Space Age, many significant gaps in this knowledge exist and our ability to 

model the variability of the system as a whole remains rudimentary at best. Furthermore, 

certain regions of the solar-terrestrial system remain largely unexplored and important 

physical, chemical, dynamical, electrodynamical, and radiative coupling processes which 

act upon and between different regions have been identified but not investigated in detail. 

The desire to investigate and understand our "local" space environment is not only driven 

by intellectual curiosity, but also by practical societal motivations. In particular, there has 

been increasing concern and interest in recent years about the variability, both natural and 

artificial, of the Earth's upper atmosphere and ionosphere. These concerns have been 

emphasized by the discoveries of anthropogenically-induced changes in the ozone 

abundance and potential global warming related to increased atmospheric loading of 

radiatively active minor constituents. In addition, long-term variations in solar activity and 

large solar storms have led to instances of premature re-entry of orbital vehicles and 

disruptions in power supply and global communications. These important issues and their 

societal effects have heightened our appreciation of the complexity of the Earth's upper 

atmosphere and ionosphere and its sensitivity to external influences. The potential threats 

to the stability of the near Earth space environment, and the possibility of additional and yet 

unidentified changes, call for an enhanced long-term program of scientific research directed 

toward improving our knowledge of the Earth's atmosphere. 

The region of the atmosphere about which the least is currently known is the mesosphere 

and the MLTI. This region, between roughly 60 and 180 km altitude, is too high for 

probing by balloons and too low for direct in situ measurements by long-lived satellites 

without on-board propulsion. As a consequence, it has received much less attention than 

the more readily accessible regions above and below. In many respects, the MLTI region 

represents an uncharted "frontier" for upper atmospheric research. 

This lack of knowledge and understanding of the MLTI region has been recognized by the 

various national advisory groups. For example, the Space Science Board of the National 

Academy of Sciences has stated that: 

"The Earth's mesosphere and lower thermosphere are the least explored regions of 

the Earth's atmosphere. They are influenced by varying solar extreme ultraviolet,

1 
ultraviolet, and X-ray radiation, auroral particles and fields, and upward 

propagating waves and tides from the lower atmosphere. There are strong 

interactions between the chemistry, dynamics, and radiation of both the neutral and 

ionized constituents of this region. It is known that the global structure of this 

region of the atmosphere can be perturbed during stratospheric warmings and solarterrestrial events (magnetospheric substorms, solar flares, etc.), but the overall 

structure and dynamic responses to these effects and even the basic controlling 

physical and chemical processes of these effects are not understood" 

To study this system, the National Academy's Committee for Solar-Terrestrial Research 

pointed out that: 

"A series of space observations is needed to advance understanding of the 

interesting dynamical, chemical, and radiative processes in the mesosphere, 

stratosphere, and thermosphere. One low and one high inclination spacecraft are 

first needed to establish basic atmospheric properties and their geographic, diurnal, 

and seasonal dependencies" 

A list of recent Academy reports calling for the exploration and quantitative investigation of 

the MLTI region is given in Table 1. 

The recent Strategy-Implementation Study (SIS) of the Space Physics Division also 

recommended a phased program of NASA missions devoted to a comprehensive study of 

the Ionosphere-Thermosphere—Mesosphere (ITM) system. The first component of this 

program identified during the SIS is the Thermosphere—lonosphere—Mesosphere, 

Energetics and Dynamics (TIMED) mission, which is designed to address the MLTI 

region and is in the "intermediate class" ($ 200 - 300 M) cost category. This TIMED 

mission is an important new effort aimed at improving our knowledge and understanding 

of the atmosphere and ionosphere above the stratosphere and below the upper 

thermosphere/ionosphere and magnetosphere. In this region, complex physical, chemical, 

dynamic, radiative, and electrodynamic processes all undergo profound transitions and 

combine to drive a rich thermal, dynamical, and compositional global morphology. 

Through a careful balance between comprehensive measurements, theoretical studies of the 

basic processes, and model analysis, it is now clear that substantial progress can be made 

in solving the major outstanding problems of this region.

2 
Table 1

National Advisory Reports calling for the exploration of the Mesosphere,

Lower-Thermosphere/Ionosphere System 

(1) Solar-Terrestrial Research for the 1980's, Committee on Solar-Terrestrial 

Research, National Academy of Sciences, 1981. 

(2) A Strategy for the Explorer Program for Solar and Space Physics, National 

Academy Press, Washington D.C.,1984. 

(3) An Implementation Plan for Priorities in Space Physics, Space Science Board, 

National Academy of Sciences, 1985. 

(4) Strategic Plan, NASA Office of Space Sciences, 1988. 

(5) Solar-Terrestrial Sciences, Report from the Science Strategy Workshop, NASA 

Space Physics Division, 1988. 

(6) Space Science in the Twenty-First Century, Solar and Space Physics report of the 

Study Steering Group, Space Science Board, National Academy of Sciences, 

1988. 

(7) Earth System Science, A Closer View, Report of the Earth System Sciences 

Committee, NASA Advisory Council, 1988. 

(8) Long-term Solar-Terrestrial Observations, Panel on long-term observations, 

Committee for Solar-Terrestrial Research, National Academy of Sciences, 1988. 

(9) Space Physics Strategy-Implementation Study Workshop, Executive Summary - 

Report of the Mission Integration and Divisional Science Plan, 1990. 

This report (Volume I of the pre-Phase-A study) provides a brief summary of the findings 

of the TIMED Science Definition Team. Full details of the study may be found in Volume 

II. The following sections briefly describe the TIMED science rationale and 

implementation plan.

3 
400 

100 

300 

E -x 

4:

II. SCIENTIFIC OBJECTIVES 

The scientific objectives for the TIMED mission were developed after a thorough review of 

the current state of knowledge of the field and the development of a prioritized set of key 

scientific questions. After this review, and in accordance with the mandates of both the 

Academy of Sciences and the Strategy-Implementation Study, the Science Definition Team 

adopted the following Primary Objective for TIMED: 

To perform an exploratory study of the physical and chemical processes

acting within and upon the coupled mesospheric, thermospheric, and

ionospheric system between —60 and 180 km. 

Figure 1 illustrates the basic thermal and plasma density structure of the MLTI region, 

together with an indication of the altitude regimes of interest for the UARS and ISTP 

missions as well as those for balloon and some ground—based experiments. 

4 

0 200 400 600 1000 

Ground Based Gas Temperature Ionospheric 

Density (log) 

Figure 1. Schematic illustration of the atmospheric thermal structure and electron (ion) 

content. The primary altitude regimes of scientific investigations for TIMED - 

as well as those for UARS, ISTP, ballon and some ground—based experiments 

- are indicated.
The region to be investigated by TIMED is home to a large number of fundamental 

transitions in atmospheric processes. Perhaps the most fundamental one is the change from 

turbulent mixing to molecular diffusion. However, there are also many others. The energy 

balance is marked by a change from radiative control to thermal conductive control. The 

composition changes from molecular to atomic. The available solar radiation changes 

spectrally, with EUV important at higher altitudes and UV important at lower altitudes. 

The dynamics of the region changes from geostrophic control to ageostrophic control, with 

viscosity and ion drag playing more important roles at higher altitudes. The highly variable 

heating rates, due to solar EUV/UV absorption, Joule heating, particle heating, tidal, and 

gravity wave dissipation, all maximize in the MLTI transition region. Yet, the lowest 

temperatures anywhere in the Earth's atmosphere occur near the summer polar mesopause. 

The region also houses the primary atmospheric, ionospheric, and electrodynamical 

interactions. The influence of the magnetosphere becomes significant, and ionospheric 

conductivities and currents maximize. Gravity wave breaking and plasma instabilities 

affect the energy and momentum balances, as turbulence becomes a primary factor in mass, 

momentum, and energy transfer processes at the lower boundary. Finally, in this region of 

the Earth's atmosphere, the largest temperature gradients are found - leading to a rapid 

change in atmospheric stability. 

Table 2 summarizes the fundamental and dramatic changes that characterize this transition 

region. Some of the processes responsible for these effects are schematically illustrated in 

the frontispiece. All of these processes occur within the MLTI region that has never been 

the subject of a comprehensive satellite investigation. It is the general purpose of TIMED 

to provide such a comprehensive and global study of the interplay between dynamics, 

energetics, chemistry, radiation, and electrodynamics in this region. 

In addition to the exploratory and investigative motivations for TIMED, there is an 

important additional need for early measurements of the MLTI region. It is expected that 

anthropogenic changes will cause the terrestrial mesosphere to experience first—order 

changes in its chemistry, composition, temperature distribution, and dynamics within the 

next two to three decades. The projected cooling of the mesosphere from increased 

amounts of carbon dioxide, for example, will affect the chemistry of important minor 

constituents. This cooling will also allow high altitude ice clouds (Polar Mesospheric

5 
Clouds) to form at lower attitudes than normally possible. This equatorward spread may 

influence the climate down to the Earth's surface. It seems very likely that the mesosphere. 

Table 2

Characteristics of the 60-180 km Transition Region 

(1) Turbulent mixing gives way to molecular diffusive separation and transport. 

(2) Radiative processes dominate the thermal balance below and thermal conduction 

dominates above. 

(3) Composition ceases to be dominated by molecules and becomes increasingly 

dominated by atoms. 

(4) Solar EUV is important at higher altitudes and solar UV is important at lower 

altitudes. 

(5) Heating rates due to solar EUVIUV absorption, Joule heating, and particle heating 

all maximize. 

(6) The dynamics of the region change from geostrophic control to ageostrophic 

control. 

(7) The coupling processes between the atmosphere and the magnetosphere become 

significant, conductivities maximize, and the charged particle population plays an 

important role in atmospheric energetics. 

(8) Gravity wave breaking and plasma instabilities play significant roles in the energy 

and momentum balances. 

(9) The thermal structure shows a fundamental change in lapse rate, leading to rapid 

changes in atmospheric stability. 

(10) Local thermodynamic equilibrium ceases to apply. 

(11) The charged particle population changes from one that has positive and negative 

cluster ions at lower altitudes, to one that has electrons and chemically controlled 

molecular ions, to one that has electrons and diffusively controlled atomic ions at 

higher altitudes. 

(12) The dynamics of charged particles is controlled by collisions with the neutral 

atmosphere at lower altitudes and by diffusion and electric as well as magnetic 

fields at higher altitudes.
of the future will be quite different from what it is now. Due to this high degree of 

senith'ity to anthropogenic effects in temperature and composition, the mesosphere proves 

to be an ideal region for the early detection of related long-term atmospheric changes. 

The anticipated changes can be appreciated from a straight—forward assessment of the 

effects of a growing abundance of atmospheric carbon dioxide, methane, and water vapor. 

CO2. increasing at the rate of 0.5%/yr, will lead to a far stronger cooling of the middle 

atmosphere than to a warming of the Earth's surface. The expected changes in temperature 

from a doubling of carbon dioxide will be in the 10's of degrees in the middle atmosphere, 

compared to fractions of a degree at the surface. Methane is increasing at the rate of 1 - 

2%/yr in the middle atmosphere. Its oxidation to water vapor in the middle 

50 0 

40 0 

N

20 0 

ii,]

1970 1980 1990 

YEAR 

Figure 2. Plot of the number of nights per year, N, on which noctilucent clouds 

were reported from north—west Europe, with the effect of solar activity 

removed. This plot (from Gadsden, 1990) provides evidence for secular 

increases in mesospheric noctilucent cloud occurrence. 

atmosphere will drive a stronger odd—hydrogen chemistry, probably resulting in a decrease 

in the density of ozone in this region. This sensitivity of the mesosphere is exemplified in 

Figure 2 by the observed increase in noctilucent cloud occurrence, which accounts for no 

less than a factor of —2 increase over the past 20 years and has been attributed to the man 

induced methane increase or, alternately, to the secular decrease in mesopause temperature.

7 
In response to a series of key scientific questions formulated by the Science Definition 

Team, the five overall objectives for the TIMED mission have been developed and are 

given in Table 3:

Table 3 

TIMED Overall Mission Objectives 

(1) To investigate and quantitatively understand the important physical and chemical 

processes responsible for the energy and momentum budgets of the MLTI region 

and the electrodynamic as well as chemical interactions between the neutral and 

plasma components of the region. 

(2) To investigate and quantitatively understand the coupling between chemistry, 

radiation, waves (tides, gravity waves, and planetary waves), unorganized 

motions, and the large-scale circulation, and the resultant effects on momentum, 

energy, and mass (species) transfer within, to, and from the mesosphere and 

lower thermosphere. 

(3) To investigate and quantitatively understand the lower ionosphere: to elucidate its 

role in the global electric circuit, to understand the chemistry and energy budget of 

this region, and to determine how these processes affect the coupling between the 

lower thermosphere and mesosphere. 

(4) To investigate and quantitatively understand the odd oxygen, odd nitrogen, and 

odd hydrogen chemistry within the Mesosphere and Lower Thermosphere 

(5) To investigate anthropogenically induced changes in the mesosphere and lower 

thermosphere and establish a baseline for futurç investigations. 

These objectives, together with the measurement oriented science questions, in turn, were 

used to derive the measurement requirements, leading to an implementation strategy. Full 

details of this process can be found in Volume 11 of this report.

8 
III. TIMED MISSION REQUIREMENTS 

111.1 Measurement Requirements 

The primary measurements of TIMED include observations of the global and temporal 

variations in the atmosphere and ionosphere, which are controlled by radiative, chemical, 

dynamic, thermodynamic, and electrodynamic processes. Measurements of the dynamics 

of neutral and ionized species will serve to describe, for the first time, the global-scale 

circulation of the ML1'I region, including global wind systems, planetary waves, solar and 

lunar tides, and a broad spectrum of gravity waves. They will allow fundamental 

investigations of turbulence and wave breaking, which are known to affect profoundly the 

mass, momentum, and energy budgets of the region. The measurements will also enable 

the electrodynamics of the most highly conducting part of the atmosphere to be 

investigated, leading, in turn, to deeper insights concerning the global electrical circuit, 

including magnetospheric components. 

Measurements of the radiative and corpuscular sources of energy, together with 

observations of airglow emissions and neutral and plasma temperature distributions, will 

serve to determine the energy balance of the region. Such observations and associated 

theoretical work will quantitatively determine, for example, the causes of the extremely low 

temperatures found in the summer mesopause - the coldest region of the atmosphere. 

Measurements of the relative abundances of the major neutral and ionized constituents of 

the region, together with observations of the altitude profiles of a panoply of 

photochemically active ionized and neutral trace constituents will enable the first 

comprehensive and detailed compositional global inventory to be assembled. Such 

measurements will provide a new window on the extremely complex chemical and radiative 

processes known to couple the odd oxygen, odd hydrogen, and odd nitrogen chemical 

families, which collectively control atmospheric ozone. 

Table 4 summarizes the needed observations related to the key scientific questions (ref. 

Volume II), without regard to the ease of measurement. These measurements are required 

to accomplish the five overall scientific objectives.
10 

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11 
111.2 Orbital Requirements 

The three principal orbital requirements for the mission are: 

(1) The need for global coverage, including in particular the high-latitude 

regions. 

(2) The need for rapid local time precession of the orbit to enable tidal structures 

to be mapped with a resolution of (at a minimum) 24 hrs LT per month. 

(3) The need for remote measurements down into the mesosphere and in situ 

measurements down into the lower thermosphere and ionosphere. 

These requirements point to the need for 3-axis stabilization of the vehicles (for accurate 

remote sensing), propulsion capability (for phased orbital adjustments), and a combination 

of elliptical orbits (to provide low altitude in situ measurement capability) and circular orbits 

(to optimize the remote sensing measurement capability). A scenario designed to meet the 

measurement and orbital requirements is discussed below. 

IV. IMPLEMENTATION PLAN 

IV.! Mission Scenario and Orbit Selection 

No single orbit is suitable for separating the spatial and temporal variations of the 

mesosphere and lower thermosphere. A configuration with two spacecraft (TIMED-H and 

-L) is required to provide the complementary coverage. TIMED-H will be placed into in a 

high—inclination, nearly sun-synchronous 95°orbit, while TIMED-L will be in a 49° orbit. 

The slowly precessing orbit of H will provide essentially full global coverage, while the 

rapidly precessing orbit of L will provide full diurnal coverage each month at middle and 

low latitudes. From these orbits, a combination of remote and in situ devices will cover the 

entire altitude region of interest in both latitude and local time.

12 
13 

Phase I Phase II Extended Mission ? 

elliptical circular 

TIMED - H 

= 95 

4000

° 

6000

gee Alt 

(krn) 

2000

perigee (120-50km) 250-30Dk 53m 

0

1121 3 I 

Year 

Phase I I Phase II Phase III

elliptical circular Extended Mission 

2000 - TIMED - L 

Alt apogee 1 49 

(km) 

1000

300-400 km cee 3OOk 

1 2 3 0

4 5 I i 

Year 

Figure 3. Apogee and perigee scenarios for TIMED-H and -L 

A strawman phased mission scenario is illustrated in Figure 3. The length of each mission 

phase will be selected to provide optimum geophysical coverage by combining the 

measurements from the two spacecraft. TIMED-L begins in circular orbit at altitudes 

between 300 and 400 km (Phase I) where both remote and in—Situ global coverage will be 

obtained. TIMED-H starts in a high elliptical orbit (Phase I) that favors "deep diving" into 

the lower thermosphere and ionosphere where in —situ measurements will be stressed. In 

Phase II, the roles of TIMED-H and -L will be reversed. TIMED-L will be maneuvered 

into an elliptical orbit for deep diving, while the orbit of TIMED-H will be circularized 

through the use of aerodynamic drag. Onboard propulsion capability will provide the 

sampling advantages of both elliptical and circular orbits. A 4-year mission lifetime is 

adequate to permit full separation of latitude, local time, and seasonal variations in the 

mesosphere and lower thermosphere.
IV.2 Strawman Instrument Complements 

Table 5 identifies a total of 15 instruments for the Core Strawman Payload of the two 

spacecraft, although each spacecraft would carry only a subset of these. Most of these 

instruments are required on both spacecraft, but a few are restricted to one. The infrared 

limbscanning is best done from the lower inclination TIMED-L orbit because it precesses 

rapidly enough to permit measurement of the very important diurnal and semidiurnal tides 

in the mesosphere. The fields and particle measurements are more appropriate for the 

higher inclination orbit of TIMED-H. 

IV.3 Spacecraft Design 

The instrument complements and mission plans of the two spacecraft led to a common 

spacecraft design, and Figure 4 shows a suggested configuration. 

Both spacecraft require a substantial onboard propulsion capability to permit survival for 4 

years in the relatively high drag environment to which they will be exposed. There must be 

adequate fuel capacity to attain and maintain the desired orbital eccentricities and altitudes 

given in the strawman mission scenario. They must also provide the capability to acquire 

and store data at any remote point about the orbit and to dump those data once or twice a 

day during passages over a NASA tracking station. The battery capacity should be 

adequate to permit continuous operation of all instruments for at least one full orbit, and its 

solar cell capacity should be adequate to recharge the battery quickly enough to permit full 

orbit operation for one out of every four orbits. In an elliptical orbit, the spacecraft should 

be designed to operate for at least 25 minutes during every perigee passage. 

The spacecraft thermal design and the layout of the solar cells must accommodate both high 

and low inclination orbital geometries with widely varying Sun angles. The spacecraft 

must also support simultaneous in—situ and remote measurements from both orbits. This 

means that they must employ a method of attitude stabilization which simultaneously points 

the limb scanners at the limb, the ram lookers into the velocity vector, the airglow imagers 

appropriately downward to view the underlying atmosphere, and the Solar EUV 

Spectrometer at the Sun. The deep diving requirement precludes the use of a deployed 

solar array which would add unacceptably to the total spacecraft drag coefficient. Such

14 
arrays are also incompatible with the need to avoid the scattering of light and particles into 

the sensors, and to avoid producing a wake that could affect the in—Situ measurements at 

certain combinations of solar and velocity directions. 

Table 5

Core Strawman Instruments for TIMED-H and -L 

No* Instruments Acronym Measured Parameters Spacecraft 

H L 

15 

A. Remote Sensing 

6 UV Spectrometer UVS 

14 Near Infrared NIRSF 

Spectrometer/Photometer 

1 Fabry-Perot Interferometer FPI 

7 Imaging Photometer IP 

12 Global UV Airgiow Imager GUVI 

9 Infrared Limb Sounder2 IRLS

11 Solar EUV Spectrometer3 SEUVS 

B. In situ 

03, N0, and other minor X X 

constituents 

020/), (03), OH rot., and H20 X 1 

Wind, temperature, and emission X X 

rate profiles 

Wave structures X X 

Global Imaging of atmospheric X 1 

UV emissions 

Mesospheric CO2,03, OH, and 1 X 

NO 

Spectral Solar EUV and X-ray X - 

fluxes 

2 Neutral Mass Spectrometer/Wind NMSWT Local gas composition, winds, X X 

and Temperature and temperature 

3 Ion Mass Spectrometer IMS Ion composition X X 

4 Langmuir Probe LP Electron/ion density, and electron X X 

temperature 

10 Energetic Particles Analyzer EPA Auroral electron and ion energy X 1 

fluxes 

5 Ion Drift Meter Retarding IDM/RPA Ion drift/electric fields, and ion X X 

Potential Analyzer temperature 

17 Magnetometer MAO Currents X 1 

8 Accelerometer4 ACC Total air density X X 

15 Electric Field Detector/Plasma EFD/PWE Three-axis AC and DC electric X 1 

Wave Experiment5 fields

The Fast Electron Spectrometer (13) and Energetic Particle Spectrometer (16) are secondary instruments. 

1 Secondary Instrument 

2 Low duty cycle due to high power requirements 

3 Not needed on TIMED if done simultaneously on other mission(s) 

4 Non-PI class instrument. 

5 Contingent on feasiblity of booms at low perigee altitude 

* Numbers referred to in Figure 4. 
v.,p I 

13— / 

PROPULSION TANKS 

FRONT VIEW

re

DIRECTION OF FLIGHT 

END VIEW 

'OUTLINE OF RADIATII 

SURFACE (BOTH ENE 

i— 14

,- 3 

ANTEI 

(BOIl 

4

8

4 

17 

VELOCITY VECTOR 

16 

Instrument numbers are defined in Table 5 

Figure 4. TIMED conceptual spacecraft configuration 

P/4 Data System 

The TIMED project will employ a distributed data system that will connect the Principal 

Investigator teams with both the project at GSFC and with ground-based investigators 

around the world. The system builds on the experience gained in the AE and DE missions 

by providing rapid access and exchange of processed geophysical data among the 

investigators. Figure 5 shows the system as it is currently envisioned. It will consist of a 

Spacecraft Control Center node and a Data Distribution Center node (both part of Code 500 

at GSFC), and investigator nodes at the home institutions of the Principal Investigators. 

The NSSDC will also become a node for acquiring and disseminating final geophysical 

data. These units will be networked to: (1) facilitate communication of the information 

needed to operate the spacecraft and instruments; (2) permit rapid transmission of telemety 

and orbital data to the investigator sites; (3) support the early and continuous exchange of 

geophysical data between the TIMED investigators; and (4) provide for access and the 

exchange of correlative data from Guest Investigators and other scientists indirectly 

connected to the TIMED project.
TIMED Data Distribution 

Center (GSFC Code 500)

TIMED

ConCenter 

(GSFC Code 500) 

5nvestigat

I

I

Investigator 

I

de4 

PreInrnary

arcnive4 

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TIMED 

ec 

ce 

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cata 

P, elimina, y Preliminary ar

etc. 

Collaborative I I NSSDC 

Measurements I TIMED final geophysical data base 

(aroundbased, rockets)

Guest Investigators 

and other users 

Figure 5. Distributed data system for TINED 

V. SCIENTIFIC MERIT 

Scientific Objectives and Significance 

The key scientific issues addressed by the TIMED mission have been briefly described 

above. The mission will provide the first comprehensive spaceborne investigation of an 

important region of the Earth's near-space environment - a complex transition region that 

may well be susceptible to rapid anthropogenic change. Since the mission has a real 

exploratory aspect, new discoveries are to be expected. It is possible, however, to identify 
various significant scientific issues that will definitely be resolved by the mission. 

Specifically, TIMED will: 

• Determine the temperature and wind structure of the mesosphere and 

lower thermosphere, delineating the seasonal/latitudinal variations and 

tidal components. 

• Determine the relative importance of the major sources and sinks of 

energy for the thermal structure of the mesosphere and lower 

thermosphere. 

Examine anthropogenically induced changes in the mesosphere and lower 

thermosphere. 

• Determine the roles of CO2 and NO cooling in the lower thermosphere 

and the importance of 0 in enhancing these cooling rates. 

• Determine the roles of exothermic chemical reactions, involving HOx and 

0x'

for the energetics and radiative budget of the mesosphere and lower 

thermosphere. 

• Advance our understanding of the radiative and dynamical processes that 

produce the cold and warm mesopause temperatures in the summer and 

winter hemispheres, respectively. 

• Shed light on the effects of albedo changes and radiation associated with 

noctilucent and polar mesospheric clouds and their importance for the 

energy balance of the mesosphere. 

• Determine the global distribution of the major constituents N2, 02, and 0 

in the upper mesosphere and lower thermosphere as well as that of CO2, 

which is important for the energy budget of the region. 

• Determine the spatial and temporal variations in odd oxygen [0 and 031, 

odd hydrogen [OH, k120, CH4, and H021, and odd nitrogen compounds 

[NO, NO2, N(2D), and N(4S)j.

18 
• Determine how the large-scale circulation in the mesosphere and 

thermosphere affects the distribution of chemically active and radiatively 

important species (such as H20, G02, CO, NO, and 0). 

• Advance our understanding of the photochemical, dynamical, and 

radiative processes that control the basic structure of the mesosphere and 

lower thermosphere. 

• Determine the spatial and temporal variations of radiatively significant 

minor species in the mesosphere and lower thermosphere [G02, NO, 03, 

OH, 02(1D), and 02(1S)]. 

• Determine the mean global circulation (meridional and zonal) in the 

mesosphere and lower thermosphere in response to annual forcing cycles 

and its geographic and short-term variability. 

Determine the diurnal tidal structures, their annual and height variations, 

and their effects on the mean large-scale circulation. 

• Determine the sources and magnitudes of gravity waves and planetary 

waves in the mesosphere and thermosphere, and their spatial and temporal 

variability. 

• Determine the effects of gravity waves and planetary waves on the largescale circulation, thermal structure, and composition. 

• Provide an understanding of the interactions that occur among different 

wave motions (tidal, gravity, and planetary) and the extent, causes, and 

consequences of their variability. 

• Provide data to study the effects of gravity wave and tidal dissipation on 

turbulent transport of energy, mass, and momentum. 

• Determine the dynamical effects of magnetospheric coupling and the depth 

to which they penetrate.

19 
• Help delineate the electric field components of the magnetosphere, the 

disturbance dynamo, and the tidal dynamos that control the 

electrodynamics at middle and equatorial latitudes. 

• Help to determine the roles of neutral winds and electric fields affecting 

the small- and large-scale variability in the ionospheric density and 

composition. 

• Contribute significantly to our understanding of the coupling between Eand F- region plasma structures. 

- onrribute significantly to our understanding of the ionization sources and 

the metastable ion chemistry responsible for the molecular ion inventory in 

the lower ionosphere. 

• Significantly advance our understanding of the ion and neutral chemistry 

in the lower thermosphere which is important for the partitioning of 

energy. 

• Significantly advance our understanding of the global composition, 

temperatures, and transport velocities of neutrals and ions in the lower 

thermosphere by providing, for the first time, simultaneous observations 

of these variables. 

• Significantly advance our understanding of tides and related dynamo 

electric fields, as well as gravity waves and traveling ionospheric 

disturbances, by performing simultaneously in situ and remote 

measurements in the thermosphere, ionosphere, and mesosphere. 

• Significantly advance our understanding of magnetic storm effects in the 

neutrals and ions by performing simultaneous measurements of 

temperatures, densities, transport velocities, and electric as well as 

magnetic fields.

20 
Generality of Interest 

These scientific issues addressed by TIMED are of considerable general interest, since they 

are related to important sub-disciplines. These include: gas kinetics; turbulence; plasma 

physics; fluid dynamics; photochemistry; spectroscopy; atomic and molecular collision 

physics; thermodynamics; etc. Issues of practical concern include: satellite drag; shuttle reentry; aerobraking in planetary atmospheres; communication—disruption; and global 

change. 

The adapted technology used for the TIMED program will also be of considerable general 

interest. New stereoscopic remote sensing techniques will be developed to probe planetary 

atmospheres, and new tomographic techniques will be used to interpret the results. In—situ 

sensors will be used in the challenging low altitude regime. The combination of remote and 

in—situ measurements from two closely coordinated spacecraft willnr rwide for new 

methods of resolving temporal and spatial ambiguities. New numerical models and global 

time dependent simulation techniques will be developed. 

TIMED has great potential for new discoveries, because the MLTI regime represents a 

unique laboratory for the study of transitional fluid dynamics and gas kinetics. Unexpected 

atmospheric linkages that couple the lower atmosphere to the MLTI and magnetosphere, 

may be discovered. Important new chemical schemes may be discovered, others 

elucidated, involving constituents or compounds that are key species for atmospheric 

evolution. New insights may be gained into the role played by turbulent processes in 

controlling the natural world. 

TIMED bridges a major gap in Solar-Terrestrial Science, connecting lower and upper 

atmospheric physics, magnetospheric physics and planetary aeronomy. Global 

understanding of the MLTI region requires a spaceborne measurement program - the 

knowledge to be provided by TIMED could not be gained using ground-based or suborbital techniques. 

Feasibility and Readiness 

The results of the pre-Phase-A study have shown that the TIMED mission is feasible and 

cost effective. The scientific community is ready for this mission, having anticipated it for 

some time. TIMED is built on the strong foundation of successful previous missions, such 

as AE, SME, and DE. The community has demonstrated its ability to coordinate efforts to

21 
solve global-scale problems, as evidenced by the successful NSF CEDAR program. Strong 

international participation, both in the mission development phase and in the data analysis 

phase, is anticipated. Two international colleagues (one from Germany and one from 

Canada) participated in the pre-Phase-A study and their respective national agencies have 

already expressed an interest in the mission. TIMED is well poised to contribute 

importantly to the international STEP program. 

The scale of the TIMED mission is well suited for participation by a spectrum of [1 

institutions; including universities, NASA centers, and industry. 

Finally, and perhaps most importantly, we reiterate that TIMED will explore the 

mesosphere and lower thermosphere/ionosphere, termed by the Academy of Sciences, "the 

least understood region in the Earth's atmosphere." It is thought that this region of our 

near Earth space environment, which is closer to us than Baltimore is to Washington, may 

be highly susceptible to global change. The TIMED Science Definition Team considers it to 

be important that this broad, exciting, and well balanced research program is carried out at 

the earliest possible date.