solar activity and vlf prepared by sheila bijoor and naoshin haque stanford university, stanford, ca...
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Solar Activity and VLF
Prepared by Sheila Bijoor and Naoshin HaqueStanford University, Stanford, CA
IHY Workshop on Advancing VLF through the Global AWESOME
Network
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Outline
Solar wind Sun’s magnetic topology Transients: CIRs, CMEs, Solar flares Earth’s magnetosphere/ionosphere VLF activity
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The Solar Wind
Hot plasma (106 K) from the solar corona is the source of the solar wind
The coronal plasma is accelerated and flows radially outward from the sun, filling interplanetary 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 taken by STEREO spacecraft in ultraviolet light. (NASA)
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Sun’s magnetic topology
Sun’s magnetic topology strongly influences characteristics 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 by the solar wind: CIRs, CMEs, and Solar Flares
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Co-rotating Interaction Regions (CIRs)
Sun’s rotation causes fast (polar) and slow (equatorial) streams to interact
Produces compression (CIRs)
CIR leading edge propagates forward into solar wind
CIR trailing edge propagates back to Sun
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Coronal Mass Ejections (CMEs)
Large eruptions of coronal plasma
Originate from active regions in Sun associated 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 in Sun’s atmosphere
Spans EM frequencies from radio to X-ray
May be caused by release of energy stored in twisted magnetic field lines
Large increase in X-ray flux can affect satellites
Energy release accelerates protons in solar wind and cause disturbances in Earth’s 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 Earth’s magnetosphere and ionosphere.
Their effects can be severe enough to cause damage to satellites and power systems.
VLF is sensitive to changes in the ionosphere and magnetosphere, so it is ideal for studying the effects and characteristics of solar phenomena.
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What is the Magnetosphere?
Sol
ar W
ind
• Solar wind flows past Earth and is deflected around Earth’s 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 Earth—a region called the plasmasphere.
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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.”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 are associated with enhanced geomagnetic activity enhanced chorus (VLF) wave activity
They may be produced by resonant interactions with enhanced whistler-mode chorus emissions.
Full plasmasphere less chorus less relativistic e-
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Earth’s Ionosphere
Atmosphere above ~70km is partially ionized by Sun’s 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 lowest layer (D-layer) Increases D-layer ionization rate and electron
density
The D-layer ionosphere and the Earth’s surface form a waveguide that can propagate VLF signals over long distances If the D-layer electron density changes along
the path from a VLF transmitter to a receiver, amplitude and phase changes can be observed by the receiver.
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Moon’s Two Shadows
Shadow of the moon consists of:
1) Penumbra: Faint outer shadow
2) Umbra: Dark inner shadow
Total eclipse of Sun seen when umbral shadow sweeps across Earth’s surface
Path of Totality: track of this shadow across 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 in the ionosphere
Effects noticed on VLF radio waves that propagate in Earth-ionosphere waveguide between ground and D region of ionosphere
Solar eclipses represent localized D region disturbance on propagation of these waves
Rare opportunity of getting direct measurements of D region characteristics
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VLF Radio Paths
Studies of the effects of solar eclipses on amplitude and phases of waves use multiple transmitter networks
Use this to model propagation of VLF waves in Earth-ionosphere waveguide
Fleury, Lassudrie-Duchesne 2000
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Typical Field Spectrum
Example of measured spectrum during a total solar eclipse August 11, 1999
Peaks from various transmitters observed Clear variation of field strength during time of eclipse
Fleury, Lassudrie-Duchesne 2000
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Eclipse Signatures
Eclipse signatures have various shapes: 1 peak, drop in peak, 2 peaks
Use signatures to study effect of eclipse in Earth-ionosphere waveguide
Fleury, Lassudrie-Duchesne 2000
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Findings from Solar Eclipse Studies
Effect of eclipse: raises reflecting height of ionosphere toward its nighttime value
Height uniformly rises over entire radio path of VLF signal
Amount height increases is proportional to obscuration value: fraction of Sun covered by Moon
Fleury, Lassudrie-Duchesne 2000
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Bibliography
Cliverd, et al, “Total solar eclipse effects on VLF signals: Observations and modeling,” Radio Science, Volume 36, Number 4, 773-778, July/August 2001
Fleury, R. and P. Lassudrie-Duchesne, “VLF-LF Propagation Measurement During the 11 August 1999 Solar Eclipse,” HF Radio Systems and Techniques, Conference Publication No. 474, IEEE 2000
Borovsky, Joseph E. et al. “The ‘calm before the storm’ in CIR/magnetosphere interactions.”
Lyons, L.R. “Solar wind-magnetosphere coupling leading to relativistic electron energization during high-speed streams.”