Transcript 2005 Evergreen Science Carnival Presentation

Sound Waves
from the
Northern Lights
Jada Maxwell
Photo courtesy of Bjorn Jorgensen, 18 January 2005, near Tromsø, Norway
http://www.spaceweather.com/aurora/images2005/17jan05/Jorgensen1.jpg
Can magnetic waves in the aurorae
transform into sound waves?
• How are aurorae created?
• Sound waves from the aurorae
• Wave transformation in the Sun’s
atmosphere
• Wave transformation in Earth’s
atmosphere
How are aurorae created?
The aurora is created by a chain of events,
beginning
with energy
from the sun.
Chain
of Events
Image courtesy of Tom Michalik
http://faculty.rmwc.edu/tmichalik/solarwind.htm
The Magnetosphere
Image courtesy of Minnesota Technolog
http://technolog.it.umn.edu/technolog/novdec97/cover.htm
The Ring Current
2.5 to 7 RE
Image: M of E: Ring Current, 2005
Coronal Mass Ejection (CME)
Illustration by Steele Hill, courtesy of NASA
http://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 Spiral Down Field Lines
Particles 2
• Collide with atomic and molecular
oxygen and nitrogen
• Emit energy as light
This Creates
the Aurorae
• Northern Lights =
Aurora Borealis
• Southern Lights =
Aurora Australis
Image courtesy of Shawn Malone
http://www.lakesuperiorphoto.com
Nearly Mirror Images
Aurora Borealis
Images courtesy of NOAA
http://sec.noaa.gov/pmap/
Aurora Australis
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
Explanations of Audible Sounds
• Psychological
• Freezing Breath
• “Brush discharge”
Evidence of Infrasound from
Aurorae
• Humans can hear between 20
and 20,000 Hertz (Hz)
• Infrasound is below 20 Hz
• Currently being investigated
• Gas expansion
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
Types of Magnetic Waves
• Alfvén waves (s-mode)
• Magnetosonic waves (p-mode)
BBC animations
S-mode image courtesy of Georgia State University
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html
My Question
Can magnetic waves in
the aurorae transform
into sound waves?
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)
y (B0)
x (v1, B1)
x (v1, B1, k)
z (E1)
z (E1)
Alfvén waves
Magnetosonic waves
Mechanism for Transformation
y (B0, k)
y (B0)
x (v1, B1)
z (E1)
E1
Alfvén waves to
Acoustic waves
p
z (E1)
x (v1, B1, k)
E1
p
Magnetosonic waves to
Acoustic waves
Mechanism for Transformation
y (B0, k)
y (B0)
v1
p
v1
x (v1, B1)
z (E1)
E1
Alfvén waves to
Acoustic waves
p
z (E1)
p
x (v1, B1, k)
E1
p
Magnetosonic waves to
Acoustic waves
β = 1 in Earth’s atmosphere?
When Beta = 1
What have we learned?
MHD Waves
observed
CMEs
?
Infrasound
observed
Aurorae
observed
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/
▪ NASA
http://sohowww.nascom.nasa.gov/localinfo/steele.html
▪ SpaceWeather.com http://www.spaceweather.com/aurora/images2005.htm
▪ Shawn Malone
http://www.lakesuperiorphoto.com
▪ Tom Michalik
http://faculty.rmwc.edu/tmichalik/solarwind.htm
▪ Minnesota Technolog
http://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