Sound Waves from the Northern

Lights

Photo courtesy of Bjorn Jorgensen, 18 January 2005, near Tromsø, Norway

http://www.spaceweather.com/aurora/images2005/17jan05/Jorgensen1.jpg

Jada Maxwell

• How are aurorae created?

• Sound waves from the aurorae

• Wave transformation in the Sun’s atmosphere

• Wave transformation in Earth’s atmosphere

Can magnetic waves in the aurorae transform into sound

waves?

How are aurorae created?

Chain of EventsThe aurora is created by a chain of

events, beginning with energy from the sun.

Image courtesy of Tom Michalikhttp://faculty.rmwc.edu/tmichalik/solarwind.htm

The Magnetosphere

Image courtesy of Minnesota Technologhttp://technolog.it.umn.edu/technolog/novdec97/cover.htm

The Ring Current

Image: M of E: Ring Current, 2005

2.5 to 7 RE

Coronal Mass Ejection (CME)

Illustration by Steele Hill, courtesy of NASAhttp://sohowww.nascom.nasa.gov/localinfo/steele.html

South-North Magnetic Orientation

of CME

North-South Magnetic Orientation

of CME

Magnetic Reconnection on Dayside of Earth

Magnetic Field Lines are Drawn Back to Magnetotail

Magnetotail is Compressed

Magnetic Reconnection in Magnetotail

Substorm Current Wedge & Field-aligned Currents

Ring current

Particles Spiral Down Field Lines

Image: Fundamentals of Physics, 2005

Particles 2

• Collide with atomic and molecular oxygen and nitrogen

• Emit energy as light

Particles Spiral Down Field Lines

This Creates the

Aurorae

Image courtesy of Shawn Malonehttp://www.lakesuperiorphoto.com

• Northern Lights = Aurora Borealis

• Southern Lights = Aurora Australis

Nearly Mirror Images

Aurora Borealis

Aurora Australis

Images courtesy of NOAAhttp://sec.noaa.gov/pmap/

Can we hear the aurorae?

Anecdotal evidence of auroral sounds

• hissing, popping, crackling, swooshing

• corresponds with motions of light

6 minutes• Sound would take at least 6 minutes to travel from the aurora to the ground

• No recordings of audible aurorae

• Psychological

• Freezing Breath

• “Brush discharge”

Explanations of Audible Sounds

• Humans can hear between 20 and 20,000 Hertz (Hz)

• Infrasound is below 20 Hz

• Currently being investigated

• Gas expansion

Evidence of Infrasound from Aurorae

The corona of the Sun is hotter than the surface!

Image: Universe, 6th ed., 2002

Sound Waves on the Sun Created by Convection

Sound Waves Propagate through Sun’s Atmosphere

Sun

Energy of Sound Waves is Transformed into Magnetic

Waves

Sun

• Alfvén waves (s-mode)

• Magnetosonic waves (p-mode)

Types of Magnetic Waves

S-mode image courtesy of Georgia State Universityhttp://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html

BBC animations

Can magnetic waves in the aurorae

transform into sound waves?

My Question

Magnetic Waves

Earth

Transform into Sound Waves

Earth

Beta is a Clue

Beta = β =

Gas Pressure

Magnetic Field Pressure

Plasma is like…

β = 1 is important

Resonance

• All material has a frequency that it vibrates at

• Matching the frequency allows the material to absorb energy

• Singing wine glass

Resonance in the Ionosphere

When β = 1,

• Energy from magnetic waves can be absorbed by the atmosphere

Mechanism for Transformation

y (B0, k)

z (E1)

x (v1, B1)

y (B0)

x (v1, B1, k)

z (E1)

Alfvén waves Magnetosonic waves

Mechanism for Transformation

y (B0, k)

z (E1)

x (v1, B1)

y (B0)

x (v1, B1, k)

z (E1)

Alfvén waves toAcoustic waves

Magnetosonic waves to Acoustic waves

E1 p E1 p

Mechanism for Transformation

y (B0, k)

z (E1)

x (v1, B1)

y (B0)

x (v1, B1, k)

z (E1)

Alfvén waves toAcoustic waves

Magnetosonic waves to Acoustic waves

v1 p v1 p

E1 p E1 p

When Beta = 1β = 1 in Earth’s atmosphere?

CMEs

Aurorae

MHD Waves

Infrasound

?

observed observed

observed

What have we learned?

Future Plans

• Gather data from satellite observations of resonant acoustic and Alfvén waves in a single CME induced geomagnetic event

• Evaluate how wave velocities, frequencies and wavelengths change as altitude and β changes

• Use data of auroral infrasound observed at Earth’s surface to extrapolate speeds in the ionosphere and compare to magnetic wave speeds

Thanks to:

• Dr. E.J. Zita for her guidance, input and helpful discussions

• Paul Lessard for helpful discussions

Image Sources:▪ NOAA (National Oceanic & Atmospheric Administration)http://sec.noaa.gov/pmap/

▪ NASAhttp://sohowww.nascom.nasa.gov/localinfo/steele.html

▪ SpaceWeather.com http://www.spaceweather.com/aurora/images2005.htm

▪ Shawn Malonehttp://www.lakesuperiorphoto.com

▪ Tom Michalikhttp://faculty.rmwc.edu/tmichalik/solarwind.htm

▪ Minnesota Technologhttp://technolog.it.umn.edu/technolog/novdec97/cover.htm

▪ Halliday/Resnick/Walker. Fundamentals of Physics, John Wiley & Sons, 2005, p 746

▪ Freedman, R.A. and W.J. Kaufman. Universe, 6th ed. W.H. Freeman & Co., 2002, p 404

▪ Wolf, R. Magnetosphere of Earth: Ring Current. Encyclopedia of Aston. & Astrophys. 2005

http://academic.evergreen.edu/m/maxjad02