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 Earth’s
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 Earth’s magnetic field.
• These particles are produced, directly or indirectly, by the
Sun or by its influences on Earth’s upper atmosphere.
THE NEAR EARTH SPACE ENVIRONMENT
• The ionosphere, the electrically charged component of Earth’s
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 Earth’s
magnetic field at higher altitudes, giving rise to the
plasmasphere.
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 Earth’s 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”.
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 Earth’s surface, also has (much more variable) effects
on Earth’s upper atmosphere, and on the near-Earth and
interplanetary space environments.
• These effects are produced by high-energy (far-ultraviolet and Xray) 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 Earth’s surface
because Earth’s 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).
THE SUN AND ITS EFFECTS ON THE NEAR-EARTH
SPACE ENVIRONMENT
• These primarily affect humans and space vehicles traveling
above Earth’s lower atmosphere, but also can affect longdistance radio communications and, in extreme cases,
electric power transmission, on or near Earth’s 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.
SOLAR FAR-UV AND X-RAY EMISSIONS
• The Sun’s 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.
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 Sun’s disk, and the
scattering of this light by Earth’s atmosphere.
• Coronagraphs in space avoid these problems, and can be used
to observe the Sun’s 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.
Solar Corona Observed in a Total Eclipse
Solar Chromospheric Prominence, in H Light (656.3 nm)
as Observed in a Total Eclipse
Temperature vs. Altitude Above Solar Photosphere
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 farand 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.
The Sun as Viewed at Different Wavelengths
Visible (White) Light
Extreme UV (30.4 nm)
Extreme UV (19.5 nm)
X-Rays (<10 nm)
Effects of Solar Far-UV and X-Ray Emissions on
Earth’s Ionosphere and Exosphere
• The major constituents of Earth’s upper atmosphere, atomic
and molecular oxygen and molecular nitrogen, are subject to
ionization by solar far- and extreme-ultraviolet radiation and Xrays, 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.
Typical Density vs. Altitude of Atmospheric
Constituents in the Earth’s Upper Atmosphere
Intensity and Solar-Cycle Variability vs. Wavelength
in the Solar Spectrum
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 Sun’s 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.
SOLAR PARTICLE EMISSIONS
• The Sun’s 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 Earth’s bright daytime sky.
• However, coronagraphs in space, which are free of Earth’s
daytime atmospheric skyglow, can monitor the Sun’s
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 Sun’s farultraviolet and X-ray emissions (without using
coronagraphs) simultaneously with the coronagraph
visible-light observations of the solar corona and coronal
mass ejections.
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.
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 Sun’s rotation on its axis and Earth’s revolution around the Sun.
THE SOLAR WIND
Because of the Sun’s 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.
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=qvxB
• 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 particle’s 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.
The Earth’s Magnetic Field
The “current loop” model is considered to be the closest approximation to the actual
source of Earth’s magnetic field.
Interactions of Solar Particles with
Earth’s Magnetosphere
• The magnetosphere is defined as the region of near-Earth
space in which Earth’s magnetic field has significant influence
on the motions of solar wind and coronal mass ejection
particles.
• The size and configuration of Earth’s 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 Earth’s
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.
Interactions of Solar Particles with
Earth’s 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 Earth’s
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.
Note, solar wind particles cannot cross into the magnetosphere directly.
Note that solar wind particles interact with Earth’s magnetic field “down wind”
of Earth, and can then return to Earth’s vicinity along regions of magnetic field
reversal (“reconnection”).
Earth’s Inner Magnetosphere and Particle Interactions
Charged Particle Trapping in Earth’s Radiation Belts
Earth’s Radiation Belts
EARTH’S POLAR AURORAS
• The polar auroras are the most visible, and longest known,
examples of the interaction of solar wind particles with Earth’s
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.
Ground-Based Image of an Aurora
Auroral Color Variations
Observation of Aurora from a Space Shuttle
Far-UV Image of Aurora from High Altitude Satellite
EARTH’S POLAR AURORAS
• The polar auroras are not produced by solar wind particles
entering Earth’s upper atmosphere directly, along Earth’s
magnetic field lines, but by a more complex and indirect route
which involves both the interplanetary and Earth’s magnetic
fields.
• As shown in earlier charts, the solar wind particles are brought
into the magnetosphere by magnetic field interactions well
beyond Earth’s 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.
EARTH’S 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.
Far-UV Images of Auroras on Jupiter and Saturn
Obtained with Hubble Space Telescope
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).
Aurora Simulation in Vacuum Chamber at NRL
Electron Beam
Direction
THE IMAGE SPACE MISSION
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The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is
the first NASA mission dedicated to imaging Earth’s 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 Earth’s (or the interplanetary) magnetic
fields; hence their directions of origin can be accurately determined from a
distance.
Electron and Proton Auroras Observed by the
IMAGE Satellite
Time Variations of Polar Aurora, Observed in Far-UV by IMAGE Satellite
Extreme-UV Images of Earth’s Plasmasphere
Obtained by the IMAGE Satellite
IMAGE Image of Earth’s Magnetosphere in
Energetic Neutral Particles
Sun
Direction