Solar Activity and VLF Prepared by Sheila Bijoor and Naoshin Haque Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network
Outline • Solar wind • Sun’s magnetic topology • Transients: CIRs, CMEs, Solar flares • Earth’s magnetosphere/ionosphere • VLF activity
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)
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
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
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)
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.
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.
What is the Magnetosphere? Solar Wind • 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.
Magnetosphere and solar activity Magnetosphere before, during, and after storm • Borovsky, Joseph E. et al. “The ‘calm before the storm’ in CIR/magnetosphere interactions.”
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-
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
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.
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 Moon’s Two Shadows
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 Solar Eclipses and the Ionosphere
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
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
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 Eclipse Signatures
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 Findings from Solar Eclipse Studies Fleury, Lassudrie-Duchesne 2000
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.”