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Can magnetic waves in the auroral region transform into acoustic waves?. Jada Maxwell, E.J. Zita The Evergreen State College. 14 April 2006 Photo: Dave Parkhurst Matanuska-Susitna Valley, AK. 14 April 2006 Photo: Daryl Pedersen Crow Pass, AK.

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slide1

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

Jada Maxwell, E.J. Zita

The Evergreen State College

14 April 2006

Photo: Dave Parkhurst

Matanuska-Susitna Valley, AK

14 April 2006

Photo: Daryl Pedersen

Crow Pass, AK

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

slide2

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.

slide3

Overview

  • 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
slide4

Solar storms cause geomagnetic storms

Image by Steele Hillhttp://scijinks.jpl.nasa.gov/en/educators/gallery/spaceweather/solar_wind_comp_L.jpg

slide5

This Creates Aurorae

Particles Spiral Down Field Lines

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

Image: Fundamentals of Physics, 2005

slide6

Magnetosonic waves (p-mode)

Types of Magnetic Waves

  • Alfvén waves (s-mode)

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

slide7

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
slide8

Explanations of Audible Sounds

  • Freezing Breath
  • “Brush discharge”
  • Psychological
slide9

Evidence of Infrasoundfrom Aurorae

  • Humans can hear between 20 and 20,000 Hertz (Hz)
  • Infrasound is below 20 Hz
  • Currently being investigated
slide10

Charles Wilson’s Auroral Infrasonic Wave Model

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

slide11

Sound waves on the Sun

N

Sound waves

Convection

SUN

slide12

Magnetic field

N

Sound waves

Convection

SUN

slide15

Plasma is like…

Gas Pressure

Beta = β =

Magnetic Field Pressure

Acoustic and magnetic waves couple where

β ≈ 1

slide17

y (B0, k)

y (B0)

x (v1, B1)

x (v1, B1, k)

z (E1)

z (E1)

Mechanism for Transformation

Alfvén waves

Magnetosonic waves

slide18

Mechanism for Transformation

y (B0, k)

y (B0)

x (v1, B1)

x (v1, B1, k)

E1 p

E1 p

z (E1)

z (E1)

Alfvén waves to

Acoustic waves

Magnetosonic waves to

Acoustic waves

slide19

Mechanism for Transformation

y (B0, k)

y (B0)

v1 p

v1 p

x (v1, B1)

x (v1, B1, k)

E1 p

E1 p

z (E1)

z (E1)

Alfvén waves to

Acoustic waves

Magnetosonic waves to

Acoustic waves

slide21

*Wilson – origin of waves 110 km

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

Beta ≈ 1 at altitude lower than 120 km

Wavelength should increase

slide22

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 ( )
slide23

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
slide24

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
slide25

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

3 April 2006, Aldersundet, Norway

Photo: Eskil Olsen

http://www.spaceweather.com/aurora/gallery_01apr06.htm

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