14 April 2006Photo: Daryl Pedersen

Crow Pass, AK

14 April 2006Photo: Dave Parkhurst

Matanuska-Susitna Valley, AK

Can magnetic waves in the auroral region transform into acoustic waves?

Jada Maxwell, E.J. ZitaThe Evergreen State College

Images: http://www.spaceweather.com/aurora/images2005.htm

Abstract

Aurorae are caused by geomagnetic storms created by magnetic storms from the Sun (Akasofu, 1991).  These storms drive magnetic waves in the magnetosphere (Cornilleau-Wehrlin, 2000).  Infrasonic waves have

been observed to emanate from aurorae (Wilson and Olson, 2005).  This suggests that magnetic waves in Earth’s upper atmosphere may drive infrasound in Earth’s lower atmosphere.  Similar processes have been

demonstrated in reverse in the Sun’s atmosphere (Johnson, et al., 2002; Bogdan, et al., 2000, 2002, 2003).  Using techniques from solar

magnetohydrodynamics (MHD), we have shown that atmospheric pressure and magnetic pressure are comparable (plasma beta = 1) at 120 km, well within the auroral region, above Fairbanks, AK (Maxwell and

Zita, 2005).  This is an important condition for MHD wave transformations to occur (Bogdan, et al., 2003).  We have also proposed mechanisms for the creation of infrasonic waves from electromagnetic waves (Maxwell and Zita, 2005).  Now, we investigate evidence and

data from satellite and ground-based instruments to test our hypotheses.

• How are aurorae created?

• Magnetic Waves

• Sound waves from the aurorae

• Wilson model

• Wave transformation in the Sun’s atmosphere

• Wave transformation in Earth’s atmosphere

• Mechanisms for transformation

• Future work

Overview

Solar storms cause geomagnetic storms

Image by Steele Hillhttp://scijinks.jpl.nasa.gov/en/educators/gallery/spa

ceweather/solar_wind_comp_L.jpg

Particles Spiral Down Field Lines

Image: Fundamentals of Physics, 2005

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

This Creates Aurorae

• Alfvén waves (s-mode)

Types of Magnetic Waves

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

• Magnetosonic waves (p-mode)

Can we hear aurorae? Anecdotal evidence

• hissing, popping, crackling, swooshing

• corresponds with motions of light

• Sound takes about 6 minutes to travel from aurora to the ground

• No recordings of audible aurorae

• Freezing Breath

• “Brush discharge”

• Psychological

Explanations of Audible Sounds

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

• Infrasound is below 20 Hz

• Currently being investigated

Evidence of Infrasound from Aurorae

Figure 1. Bow wave model of the generation of AIW by auroral electrojet supersonic motion at a height h above the x,y ground plane. The aurora arc, shown in red, moves supersonically in the direction Va creating a bow wave, shown in purple, that propagates to the surface at line ab causing the AIW pressure wave to be observed if the infrasonic array is in the Front Shock Region.

From Inframatics, No. 10, June 2005, C.R. Wilson

Charles Wilson’s Auroral Infrasonic Wave Model

fv

SUN

N

Convection

Sound waves

Sound waves on the Sun

SUN

N

Mag

netic

fiel

d

Convection

Sound waves

SUN

N

Magnetic waves

Can the inverse happen in Earth’s atmosphere?

Earth

Plasma is like…

Acoustic and magnetic waves couple where β ≈ 1

Beta = β =

Gas Pressure

Magnetic Field Pressure

β = 1 in Earth’s 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

Where will this transformation happen?

1

02 2/

B

nkT

P

P

B

G

Beta ≈ 1 at altitude lower than 120 km

2

22

A

S

V

C

*Wilson – origin of waves 110 km

*From Inframatics, No. 10, p 1, June 2005, C.R. Wilson

fv Wavelength should increase

What’s next?

• Apply temperature data

• Access satellite data (FAST)• Magnetic and electric field fluctuations• Magnetic waves

• Access infrasound data

• Compare ground and auroral waves ( )fv ,,

Acknowledgements:

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

• Dr. Charles Wilson for providing resources

• Dr. Kristine Sigsbee for her generous insight regarding Polar satellite data

• Dr. Charles Carlson for direction to useful FAST satellite data

• All of my classmates in Physics of Astronomy for their good questions and suggestions

• The Evergreen State College Foundation for Activity Grant

Sources & References• Akasofu, S.-I. “Auroral Phenomena.” In: Meng, C.-I., M.J. Rycroft, and L.A. Frank (editors). Auroral Physics. Cambridge Univ. Press, 1991

• Cornilleau-Wehrlin, N. “Magnetosphere of Earth: Waves.” Encyclopedia of Astronomy & Astrophysics [online], Nature Publishing Group, 2001/IoP Publishing, 2005

• Bogdan, T.J., et al. “Waves in the Magnetized Solar Atmosphere. II. Waves from Localized Sources in Magnetic Flux Concentrations.” Astrophys. J., 599, 626-660, 2003

• Johnson, M.C., S. Petty-Powell, and E.J. Zita. “Energy Transport by MHD Waves Above the Photosphere Numerical Simulations.” 17 Oct 2002, http://192.211.16.13/z/zita/students/matt/researchmatt.html

• Israelevich, P.L. and L. Ofman. “Parallel Electric Field in the Auroral Ionosphere: Excitation of Acoustic Waves by Alfvén Waves.” Ann. Geophys., 22, 2797-2804, 2004

• Maxwell, J.F. and E.J. Zita. “Can magnetic waves in aurorae transform into acoustic waves?” APS NW Section Meeting, 13 May 2005

• Wilson, C.R. "Infrasound from Auroral Electrojet Motions at I53US." Inframatics, 10, 1-13, 2005

• Wilson, C.R. and J.V. Olson. “Auroral Infrasound Observed at I53US at Fairbanks, Alaska.” American Geophysical Union, Fall Meeting, 2003

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

Photo: Eskil Olsenhttp://www.spaceweather.com/aurora/gallery_01apr06.htm

3 April 2006, Aldersundet, Norway