AIAAAAS Astrodynamics Specialist Conference, 21 24 Aug 2006,

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Satellite Drag Variability at Earth, Mars and
Venus Due to Solar Rotation
Jeffrey M. Forbes Department of Aerospace
Engineering Sciences, UCB 429 University of
Colorado, Boulder, Colorado Sean Bruinsma
Department of Terrestrial and Planetary Geodesy,
Centre Nationale D'Etudes Spatiales, Toulouse,
France Frank G. Lemoine Planetary Geodynamics
Laboratory Code 698, NASA Goddard Space Flight
Center, Greenbelt, Maryland, USA Bruce R.
Bowman Space Analysis Division/A9A, US Air force
Space Command, Colorado Springs, Colorado
80914 Alex Konopliv Jet Propulsion Laboratory,
California Institute of Technology, Pasadena, CA.
Objective Utilize thermosphere densities deduced
from precise orbit determination (POD) of the
Mars Global Surveyor (MGS), Pioneer Venus Orbiter
(PVO) and Magellan satellites, and 6
Earth-orbiting satellites during contemporaneous
time periods, to perform a comparative analysis
of the satellite drag environments of Earth, Mars
and Venus due to the rotation of the Sun.
See papers by Lemoine (this session), Bowman
(previous Astrodynamics conferences), and
Konopliv on specific POD methodologies for Mars,
Earth, Venus, respectively
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• Motivations
• In the context of Thermosphere General
  Circulation Models (TGCMs)
• (e.g., Bougher et al.), comparative
  thermosphere data analyses can help to constrain
  the poorly-known rate coefficient for O CO2
  -- O CO2
• (2) Improved thermosphere density models are
  needed for aerobraking, re-entry,
• satellite ephemeris prediction, and mission
  planning.

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Our study focuses on thermosphere density
variations related to rotation of the Sun.
The Suns atmosphere rotates with a period of
25 days near the equator and 35 days near the
poles, with an average rate of 27 days.
This differential rotation causes magnetic field
lines to twist, resulting in the formation of
active regions that release enhanced solar energy
in various forms, including the extreme
ultraviolet (EUV) radiation responsible for
heating the hot outermost region of a planetary
upper atmosphere, the thermosphere (ca. 100 km
for Earth, Mars, Venus).
19.5 nm EUV emission
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The rotation of solar active regions produces
quasi-27-day periodicities in EUV flux emanating
from the Sun and subsequently absorbed by
planetary thermospheres.
Relatively little is known about the response
of Mars neutral thermosphere to short-term
solar flux variations. Since EUV solar
radiation can only be measured from space, the
10.7 cm solar flux that has been observed from
the ground for several decades is often used as
a proxy for the EUV flux. In our study, the 10.7
cm solar flux (F10.7) measured from Earth is
adjusted to Mars and Venus taking into account
relative angle and relative distance with respect
to the Sun.
Quasi-27-day cycle
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March 11, 2003
19.5 nm EUV emission
March 21, 2003
March 21, 2003
10.7 cm radio flux
Day in 2003
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Data

• Mars
• Daily density values at 390 km were
  inferred from (POD) of MGS during two intervals
  of particularly pronounced quasi-27-day
  variability of solar flux days 75-150 and
  270-365 of 2003.
• Venus
• Exosphere temperatures from drag analyses of
  Pioneer Venus Orbiter (PVO) were obtained
  directly from the NASA Planetary Data System, for
  two daytime intervals days 100-220, 1979, and
  days 320-75 of 1980-1981.
• Daily density data from one daytime interval
  (days 250-365 of 1992) was obtained from POD of
  the Magellan satellite.
• Earth
• Drag analyses of the following 6
  satellites, covering all of the above periods

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Methodology
In order to compare relative responses, density
variations are converted to equivalent
temperature variations using empirical models of
each thermosphere DTM-Mars, Hedin (1983) Venus
Model, and J70. Phasing varies from cycle to
cycle so that it is difficult to find a single
linear coefficient that simply relates the solar
and temperature variations. To circumvent this
problem in the interest of obtaining some
quantitative results, we considered each positive
and negative excursion of temperature (?T) and
F10.7 (?F10.7) as a pair, and calculated the
corresponding value of ?T/?F10.7. For this
purpose the average of all 6 Earth-orbiting
satellites were used to obtain a single data
point for each ?T/?F10.7 calculation.
DT
DF10.7
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Thermosphere response of Earth, Mars and Venus to
changes in solar flux due to the Suns rotation.
?F change in 10.7 cm solar radio flux (F10.7)
received at the planet ?T change in exospheric
temperature (K).
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Conclusions concerning the thermosphere responses
of Earth, Mars and Venus to quasi-27-day
variations in solar flux due to rotation of the
Sun

• Marss thermosphere response is approximately
  1/3rd that of Earth.
• Venuss response is barely discernible,
  approximately 1/10th that of Earth.
• The above differences are likely due to the
  differing efficiencies of CO2 cooling
• in these upper atmospheres. Our results can
  therefore be used to constrain
• planetary atmosphere models that seek to
  self-consistently and inter-consistently
• simulate the thermospheres of these planets.
• Our tabulated data might be used for that
  purpose. However, additional insight
• might be gained by attempting to model the
  actual experimental data, as this
• better retains the value of contemporaneity. In
  particular, different effective local
• times and latitudes correspond to each
  illustrated data set, and numerical
• models attempting to emulate these results may
  need to similarly sample the
• model output to optimize the fidelity of the
  comparison.

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• The results presented here should prove valuable
  in validating and/or updating
• the parameterization of short-term solar flux
  variations in empirical models of
• Earths, Mars, and Venus thermospheres,
  especially for the purposes of
• specifying or predicting atmospheric drag on
  satellites.

Future Work

• The effects of long-term solar flux changes on
  the thermospheres of these
• planets, using expanded data sets, i.e., MGS,
  Odyssey, MRO, Magellan, Venus
• Express, etc..
• Improved proxies for EUV solar variability
  effects, such as E10.7, MgII,
• Soho EUV, etc.
• Incorporation into DTM-Mars

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