Solar Activity and VLF

Prepared by Sheila Bijoor and Naoshin HaqueStanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME

Network

2

Outline

Solar wind Sun’s magnetic topology Transients: CIRs, CMEs, Solar flares Earth’s magnetosphere/ionosphere VLF activity

3

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)

4

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

5

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

6

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)

7

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.

8

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.

9

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.

10

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.”

11

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-

12

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

13

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.

14

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

15

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

16

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

17

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

18

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

19

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

20

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.”