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IESO
Space Science
Part 1
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Solar Activity and VLF
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Outline
Solar wind
Suns magnetic topology
Transients: CIRs, CMEs, Solar flares
Earths magnetosphere/ionosphere VLF activity
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The Solar Wind
Hot plasma (106 K) from the solar corona isthe source of the solar wind
The coronal plasma is accelerated and flowsradially outward from the sun, fillinginterplanetary space
Solar wind properties at Earth (1 AU): Speed ~400 km/s
Speed range ~200-700 km/s
Number density ~ 7 cm-3
Magnetic field ~ 5 nT
Electron temperature ~ 105 K
Proton temperature ~3 x 104 K Image of solar corona takenby STEREO spacecraft inultraviolet light. (NASA)
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Suns magnetic topology
Suns magnetic topology strongly influencescharacteristics of solar wind
Slow streams at streamers (equator)
Fast streams at coronal holes (poles)
Field is well-ordered at solar minimum
Field is complicated at solar maximum
Magnetic topology causes transients that are carried bythe solar wind: CIRs, CMEs, and Solar Flares
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Co-rotating Interaction Regions (CIRs)
Suns rotation causes fast (polar) andslow (equatorial) streams to interact
Produces compression (CIRs)
CIR leading edge propagates forwardinto solar wind
CIR trailing edge propagates back toSun
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Coronal Mass Ejections (CMEs)
Large eruptions of coronal plasma
Originate from active regions in Sunassociated with solar flares
Solar minimum: ~1/week Coronal streamer belt near the solar
magnetic equator
Solar maximum: ~ 2-3 /day Active regions, latitudinal distribution is
more homogeneous.Coronal mass ejection.
Image shows the sun in
ultraviolet light. (NASA)
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Solar Flares
Solar flare is a violent explosion inSuns atmosphere
Spans EM frequencies from radio toX-ray
May be caused by release of energystored in twisted magnetic field lines
Large increase in X-ray flux can affectsatellites
Energy release accelerates protons insolar wind and cause disturbances inEarths magnetic field. An X-ray image of an intense X9 flare taken from
the GOES-13 satellite. The flare was actually
intense enough to damage the imager.
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Solar Transients Affect Earth
CMEs, CIRs, and solar flares can affect the Earthsmagnetosphereand ionosphere.
Their effects can be severe enough to cause damage tosatellites and power systems.
VLF is sensitive to changes in the ionosphere andmagnetosphere, so it is ideal for studying the effects andcharacteristics of solar phenomena.
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What is the Magnetosphere?
SolarWind
Solar wind flows past Earth and is deflectedaround Earths magnetic field.
The solar wind compresses the magnetic
field on the sun-side, creating a boundary
termed the magnetopause at ~10 RE.
On the night side, the solar wind-dipole field
interaction results in a tail up to~60 RE.
The magnetosphere is the region within the
magnetopause, from ~10 RE on the sun side to~60 RE on the night side.
Plasma within ~4 6 RE rotates with the
Eartha region called theplasmasphere.
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Magnetosphere and solar activity
Magnetosphere before, during, and after storm
Borovsky, Joseph E. et al. The calm before the storm in CIR/magnetosphere interactions.
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Solar Activity and VLF
Increases in relativistic electron fluxes in outer radiation belt areassociated with
enhanced geomagnetic activity
enhanced chorus (VLF) wave activity
They may be produced by resonant interactions with enhancedwhistler-mode chorus emissions.
Full plasmasphere less chorus less relativistic e-
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Earths Ionosphere
Atmosphere above ~70km is partially ionized by Suns radiation
Ionosphere extends up and merges with Magnetosphere
Low frequency (< 30kHz) are reflected from D region
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Solar Activity and the Ionosphere
X-ray radiation during solar flares penetrate into the lowestlayer (D-layer)
Increases D-layer ionization rate and electron density
The D-layer ionosphere and the Earths surface form a
waveguide that can propagate VLF signals over longdistances If the D-layer electron density changes along the path from a VLF
transmitter to a receiver, amplitude and phase changes can beobserved by the receiver.
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Moons Two Shadows
Shadow of the moon consists of:
1) Penumbra: Faint outer shadow
2) Umbra: Dark inner shadow
Total eclipse of Sun seen whenumbral shadow sweeps acrossEarths surface
Path of Totality: track of this shadowacross the Earth
Must be inside this path of totality to
see the total eclipse of the Sun
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Solar Eclipses and the Ionosphere
Solar eclipses cause disturbances inthe ionosphere
Effects noticed on VLF radio wavesthat propagate in Earth-ionospherewaveguide between ground and Dregion of ionosphere
Solar eclipses represent localized Dregion disturbance on propagationof these waves
Rare opportunity of getting directmeasurements of D region
characteristics