THE NEAR EARTH SPACE ENVIRONMENT

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THE NEAR EARTH SPACE ENVIRONMENT

• The near-Earth space environment is defined as
  that in the region of space that includes our
  Earth, but extending out to the orbit of our Moon
  (and to somewhat larger distances, in the
  direction opposite from the Sun).
• In addition to the light and heat emitted by the
  Sun, other forms of electromagnetic radiation
  emitted by the Sun which do not penetrate our
  atmosphere to the Earths surface, such as
  far-ultraviolet and X-ray radiation, are present
  in this region.
• The near-Earth space environment also includes
  electrically charged particles, primarily
  electrons and protons, which are influenced by
  Earths magnetic field.
• These particles are produced, directly or
  indirectly, by the Sun or by its influences on
  Earths upper atmosphere.

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THE NEAR EARTH SPACE ENVIRONMENT

• The ionosphere, the electrically charged
  component of Earths upper atmosphere, is
  produced by solar far-ultraviolet and X-ray
  radiation effects on the upper atmosphere, and
  (indirectly) by solar charged particle emissions.
• The ionosphere is of great practical importance,
  because it makes possible long-distance radio
  communications, due to its reflection of radio
  waves around the Earth.
• The ionosphere is also significantly influenced
  by Earths magnetic field at higher altitudes,
  giving rise to the plasmasphere.

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THE NEAR EARTH SPACE ENVIRONMENT

• The solar wind is a low density, high temperature
  flow of ionized gas outward from the Sun (an
  extension of the solar corona) which travels
  throughout the solar system, guided by
  interplanetary magnetic field lines, which also
  originate in the Sun.
• The interaction of the solar wind with Earths
  magnetic field gives rise to the magnetosphere.
• Because the ionosphere, plasmasphere, and
  magnetosphere (and the interplanetary space
  environment, in general) are affected by solar
  activity, these have given rise to the new
  subject of space weather.

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THE SUN AND ITS EFFECTS ON THE NEAR-EARTH
SPACE ENVIRONMENT

• The Sun, which provides us with the essential
  heat and light we utilize on Earths surface,
  also has (much more variable) effects on Earths
  upper atmosphere, and on the near-Earth and
  interplanetary space environments.
• These effects are produced by high-energy
  (far-ultraviolet and X-ray) electromagnetic
  radiation, and also by highly energetic charged
  particles emitted by the Sun.
• The far-UV and X-ray radiations, and (to a great
  extent) charged particle emissions, cannot be
  observed from Earths surface because Earths
  atmosphere absorbs all of the solar far-UV and
  X-ray radiations, and scatters visible light from
  the solar disk, which makes it very difficult or
  impossible to observe the corona and coronal mass
  ejections (except during the extremely rare total
  eclipses of the Sun by the Moon).

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THE SUN AND ITS EFFECTS ON THE NEAR-EARTH
SPACE ENVIRONMENT

• These primarily affect humans and space vehicles
  traveling above Earths lower atmosphere, but
  also can affect long-distance radio
  communications and, in extreme cases, electric
  power transmission, on or near Earths surface.
• These effects also are becoming of greater
  interest as we plan future human and robotic
  explorations of the Moon and other planets in our
  solar system.
• Charged particles emitted by the Sun, especially
  the highly energetic particles emitted in coronal
  mass ejections, can be hazardous to the health of
  astronauts, and damaging to electronic equipment,
  in vehicles traveling in regions of space beyond
  the near-Earth space environment.

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SOLAR FAR-UV AND X-RAY EMISSIONS

• The Suns output of far-ultraviolet and X-ray
  emissions are much more variable with solar
  sunspot activity, than is its output of visible
  light.
• Solar flares and coronal mass ejections are most
  common during the times when the Sun has its
  greatest (average) number of visible dark
  sunspots (which varies over an 11-year cycle,
  from maximum to minimum and back to maximum).
• Active regions on or near the solar surface, such
  as solar flares, can best be detected and
  monitored in the far ultraviolet and x-ray
  wavelength ranges, because these zones are of
  much higher temperature, but (typically) much
  lower gas density, than the visible-light
  photosphere.

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SPACE OBSERVATIONS OF SOLAR VISIBLE LIGHT
EMISSIONS

• The solar corona, and coronal mass ejections, can
  be observed in visible light, but this can be
  done from the ground only during total eclipses
  of the Sun by the Moon because of the extreme
  visible-light brightness of the Suns disk, and
  the scattering of this light by Earths
  atmosphere.
• Coronagraphs in space avoid these problems, and
  can be used to observe the Suns corona
  continuously, 24 hours a day, 7 days per week!
• The simultaneous monitoring of the Sun, using
  both far-UV and X-ray imaging of the solar disk,
  and visible-light monitoring of its corona, is of
  great importance to both the scientific research
  on, and the early warning of, potentially
  hazardous solar eruptions.

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Solar Corona Observed in a Total Eclipse
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Solar Chromospheric Prominence, in H? Light
(656.3 nm) as Observed in a Total Eclipse
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Temperature vs. Altitude Above Solar Photosphere
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The Solar Visible and Ultraviolet Spectrum
Solar Visible and Near-IR Spectrum Dashed Line
Black Body Equivalent Spectrum
Note transition from an absorption line spectrum
in the visible, to an emission line spectrum in
the far- and extreme-ultraviolet. Note also, the
emission lines in the far- and extreme-UV are
produced in the chromosphere and corona of the
Sun, where temperatures are much higher than the
5800 K temperature of the visible-light
photosphere.
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The Sun as Viewed at Different Wavelengths
Visible (White) Light
Extreme UV (30.4 nm)
X-Rays (lt10 nm)
Extreme UV (19.5 nm)
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Effects of Solar Far-UV and X-Ray Emissions on
Earths Ionosphere and Exosphere

• The major constituents of Earths upper
  atmosphere, atomic and molecular oxygen and
  molecular nitrogen, are subject to ionization by
  solar far- and extreme-ultraviolet radiation and
  X-rays, constituting the region known as the
  ionosphere.
• In addition, the neutral gases in the upper
  atmosphere (mostly atomic oxygen and molecular
  nitrogen), the region known as the exosphere, are
  heated by these radiations, and thereby increase
  the atmospheric neutral density at high altitudes
  (hence increasing the atmospheric drag on
  satellites in low Earth orbits).
• Since the solar ionizing radiation is much more
  variable than the solar visible, near-UV, and
  middle-UV radiations, but cannot be observed from
  the ground, long-term space-based observations
  are necessary to monitor these radiations, and to
  compare these measurements with radio
  measurements and other studies of the ionosphere.

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Typical Density vs. Altitude of Atmospheric
Constituents in the Earths Upper Atmosphere
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Intensity and Solar-Cycle Variability vs.
Wavelength in the Solar Spectrum
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SOLAR PARTICLE EMISSIONS

• Another way in which the Sun affects the solar
  system space environment is by its ejection of
  charged particles (mostly electrons and protons),
  emanating from its upper atmosphere, or corona.
• These are, primarily, a relatively constant
  outflow of relatively low energy particles, known
  as the solar wind.
• These particles travel outward along solar
  magnetic field lines, which permeate the entire
  solar system, along spiral paths (due to the
  Suns rotation on its axis).
• However, on occasions (especially during times of
  high solar activity), explosive eruptions from
  the Sun, such as solar flares, send much higher
  energy and density gas clouds out into the solar
  system, known as coronal mass ejections.

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SOLAR PARTICLE EMISSIONS

• The Suns corona can be seen in visible light
  from the ground only in total eclipses of the Sun
  by the Moon, because it is much fainter the
  Earths bright daytime sky.
• However, coronagraphs in space, which are free of
  Earths daytime atmospheric skyglow, can monitor
  the Suns corona 24 hours a day, 7 days a week,
  and give early warning of coronal mass ejections.
  
• These instruments are usually accompanied, on the
  same spacecraft, with ones which can observe the
  Suns far-ultraviolet and X-ray emissions
  (without using coronagraphs) simultaneously with
  the coronagraph visible-light observations of the
  solar corona and coronal mass ejections.

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Coronal Mass Ejection Observed with the
Large-Angle Solar Coronagraph (LASCO) on the
Solar and Heliospheric Observatory (SOHO)
White circles indicate size of the visible-light
solar disk, hidden from direct view by the
coronagraphs occulting disks.
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The Solar Wind
The apparent directions of the solar wind and
interplanetary magnetic field lines, as seen from
Earth, are not from the direction of the Sun, but
at an angle, due to the Suns rotation on its
axis and Earths revolution around the Sun.
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THE SOLAR WIND
Because of the Suns magnetic field and its
rotation on its axis, solar wind particles travel
outward at much higher speeds along the magnetic
polar directions than along its magnetic
equatorial plane.
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Motions of Charged Particles in Magnetic Fields

• The effects of magnetic fields on charged
  particles are not simple attraction or repulsion.
• The force acts only on particles moving in a
  direction perpendicular to both the velocity of
  the particle, and the direction of the magnetic
  field. In vector notation,
• F q v x B
• In the simple case of a uniform magnetic field,
  and velocity perpendicular to the magnetic field,
  the magnetic force causes the particle to travel
  in a circular orbit around the magnetic field
  lines, with centripetal force equal to the
  magnetic force
• mv2/r qvB
• The particles orbit radius (called the
  cyclotron radius) rc mv/qB.
• In actuality, the trajectories of charged
  particles are rarely perpendicular to the
  magnetic fields, and the magnetic fields
  themselves are not uniform in geometry and
  intensity, so the actual situations are
  considerably more complex.

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The Earths Magnetic Field
The current loop model is considered to be the
closest approximation to the actual source of
Earths magnetic field.
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Interactions of Solar Particles with Earths
Magnetosphere

• The magnetosphere is defined as the region of
  near-Earth space in which Earths magnetic field
  has significant influence on the motions of solar
  wind and coronal mass ejection particles.
• The size and configuration of Earths
  magnetosphere are highly variable with solar
  activity, due to the variations in both energy
  and number of solar particles.
• Solar wind and solar flare particles can interact
  with Earths magnetosphere to produce a variety
  of phenomena.
• Energetic particles can be trapped in
  donut-shaped regions surrounding Earth, giving
  rise to the Van Allen radiation belts.
• These particles can be hazardous to the health of
  astronauts and to electronic equipment in
  near-Earth space.

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Interactions of Solar Particles with Earths
Magnetosphere

• Charged particles (mostly of lower energy than
  those in the radiation belts) can also be
  directed, by complex processes in the
  magnetosphere, downward into Earths atmosphere
  in regions near the magnetic poles, producing
  auroras.
• The zones in which auroras occur most frequently
  are rings surrounding the magnetic poles, called
  the auroral zones.
• The sizes of the auroral zones, and the
  intensities of auroral displays, are strongly
  influenced by solar activity.

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Note, solar wind particles cannot cross into the
magnetosphere directly.
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Note that solar wind particles interact with
Earths magnetic field down wind of Earth, and
can then return to Earths vicinity along regions
of magnetic field reversal (reconnection).
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Earths Inner Magnetosphere and Particle
Interactions
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Charged Particle Trapping in Earths Radiation
Belts
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Earths Radiation Belts
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EARTHS POLAR AURORAS

• The polar auroras are the most visible, and
  longest known, examples of the interaction of
  solar wind particles with Earths atmosphere.
• The particles that produce the auroras typically
  have much lower energies than those that produce
  the radiation belts.
• The frequency, brightness, and latitude of
  observance of auroras are highly dependent on
  solar activity.
• The colors of auroras are representative of the
  spectral emissions of the upper atmospheric
  species involved, and (to some extent) the nature
  and energy of the incoming particles.
• Spectroscopic measurements of the auroras, as
  well as measurements of the altitudes at which
  the emissions occur, provide information about
  the energetics of the auroral particles, and
  whether they are due to electrons or protons.
• Observations from spacecraft, and with the Hubble
  Space Telescope, have shown that other planets
  (such as Jupiter and Saturn) also have polar
  auroras.

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Ground-Based Image of an Aurora
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Auroral Color Variations
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Observation of Aurora from a Space Shuttle
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Far-UV Image of Aurora from High Altitude
Satellite
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EARTHS POLAR AURORAS

• The polar auroras are not produced by solar wind
  particles entering Earths upper atmosphere
  directly, along Earths magnetic field lines, but
  by a more complex and indirect route which
  involves both the interplanetary and Earths
  magnetic fields.
• As shown in earlier charts, the solar wind
  particles are brought into the magnetosphere by
  magnetic field interactions well beyond Earths
  distance from the Sun, and then can approach the
  polar regions from the rear.
• The magnetic field lines inside the auroral ovals
  are open, that is, extend into interplanetary
  space, whereas those outside of the auroral oval
  are closed, that is, connect with the opposite
  magnetic hemisphere of Earth.

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EARTHS POLAR AURORAS

• The auroras tend to be more intense, and the
  auroral zones tend to extend further from the
  magnetic polar regions, with higher solar
  activity (which increases both the flux and
  energy of incoming solar wind particles).
• Auroral particles are typically much less
  energetic than those in the Van Allen radiation
  belts, and normally cannot enter the atmosphere
  to altitudes of less than about 100 km.
• However, intense auroral activity can cause
  communications interference, and may induce
  electric currents in the ground and in power
  lines which can cause major power outages.

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Far-UV Images of Auroras on Jupiter and
Saturn Obtained with Hubble Space Telescope
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Vacuum Chamber Setup for Aurora Simulation at
Naval Research Laboratory
The auroral simulation equipment included a
hollow stainless steel float ball, 4 inches in
diameter, inside of which was placed a
vertically-oriented bar magnet. The negative high
voltage electrode is in the tube extending from
the main chamber in the left image (to the left
in the right image).
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Aurora Simulation in Vacuum Chamber at NRL
Electron Beam Direction
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THE IMAGE SPACE MISSION

• The Imager for Magnetopause-to-Aurora Global
  Exploration (IMAGE) is the first NASA mission
  dedicated to imaging Earths magnetosphere.
• IMAGE was launched into a highly eccentric polar
  orbit (1000 km perigee, 45,871 km apogee).
• With the exception of the auroras, most
  magnetospheric particles and phenomena cannot be
  directly observed using conventional remote
  sensing techniques based on the emission or
  absorption of electromagnetic radiation.
• In addition to direct imaging in the far- and
  extreme-ultraviolet spectral ranges, IMAGE makes
  use of new technologies, for Neutral Atom
  imaging, and Radio Plasma Imaging.
• The High, Medium, and Low Energy neutral atom
  imagers (HENA, MENA, and LENA) detect the neutral
  atoms produced when energetic plasma particles
  are converted into neutral atoms by capturing an
  electron.
• The advantage of being able to observe energetic
  neutral atoms is that their trajectories are not
  affected by Earths (or the interplanetary)
  magnetic fields hence their directions of origin
  can be accurately determined from a distance.

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Electron and Proton Auroras Observed by the IMAGE
Satellite
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Time Variations of Polar Aurora, Observed in
Far-UV by IMAGE Satellite
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Extreme-UV Images of Earths Plasmasphere
Obtained by the IMAGE Satellite
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IMAGE Image of Earths Magnetosphere in Energetic
Neutral Particles
Sun Direction