14 April 2006 Photo: Daryl Pedersen Crow Pass, AK

1. 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

2. 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.

3. 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

4. Solar storms cause geomagnetic storms Image by Steele Hillhttp://scijinks.jpl.nasa.gov/en/educators/gallery/spaceweather/solar_wind_comp_L.jpg

5. This Creates Aurorae Particles Spiral Down Field Lines Image courtesy of Shawn Malonehttp://www.lakesuperiorphoto.com Image: Fundamentals of Physics, 2005

6. 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

7. 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

8. Explanations of Audible Sounds • Freezing Breath • “Brush discharge” • Psychological

9. Evidence of Infrasoundfrom Aurorae • Humans can hear between 20 and 20,000 Hertz (Hz) • Infrasound is below 20 Hz • Currently being investigated

10. 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

11. Sound waves on the Sun N Sound waves Convection SUN

12. Magnetic field N Sound waves Convection SUN

13. Magnetic waves N SUN

14. Can the inverse happen in Earth’s atmosphere? Earth

15. Plasma is like… Gas Pressure Beta = β = Magnetic Field Pressure Acoustic and magnetic waves couple where β ≈ 1

16. β= 1 in Earth’s atmosphere?

17. y (B0, k) y (B0) x (v1, B1) x (v1, B1, k) z (E1) z (E1) Mechanism for Transformation Alfvén waves Magnetosonic waves

18. 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

19. 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

20. Where will this transformation happen?

21. *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

22. 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 ( )

23. 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

24. 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

25. http://academic.evergreen.edu/m/maxjad02 3 April 2006, Aldersundet, Norway Photo: Eskil Olsen http://www.spaceweather.com/aurora/gallery_01apr06.htm