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 PressureMagnetic 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