1. 14 April 2006 Photo: Daryl Pedersen Crow Pass, AK 14 April 2006 Photo: Dave Parkhurst Matanuska-Susitna Valley, AK Can magnetic waves in the auroral region transform into acoustic waves? Jada Maxwell, E.J. Zita The Evergreen State College 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. 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

4. Solar storms cause geomagnetic storms Image by Steele Hill http://scijinks.jpl.nasa.gov/en/educators/gallery/spacewe ather/solar_wind_comp_L.jpg

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

6. Alfvén waves (s-mode) Types of Magnetic Waves S-mode image courtesy of Georgia State University http://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html Magnetosonic waves (p-mode)

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

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

10. 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

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

12. SUN N Magnetic field Convection Sound waves

13. SUN N Magnetic waves

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

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

16. β = 1 in Earth’s atmosphere?

17. Mechanism for Transformation y (B 0, k) z (E 1 ) x (v 1, B 1 ) y (B 0 ) x (v 1, B 1, k) z (E 1 ) Alfvén wavesMagnetosonic waves

18. Mechanism for Transformation y (B 0, k) z (E 1 ) x (v 1, B 1 ) y (B 0 ) x (v 1, B 1, k) z (E 1 ) Alfvén waves to Acoustic waves Magnetosonic waves to Acoustic waves E 1 p

19. Mechanism for Transformation y (B 0, k) z (E 1 ) x (v 1, B 1 ) y (B 0 ) x (v 1, B 1, k) z (E 1 ) Alfvén waves to Acoustic waves Magnetosonic waves to Acoustic waves v 1 p E 1 p

20. Where will this transformation happen?

21. Beta ≈ 1 at altitude lower than 120 km *Wilson – origin of waves 110 km *From Inframatics, No. 10, p 1, June 2005, C.R. Wilson 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 Photo: Eskil Olsen http://www.spaceweather.com/aurora/gallery_01apr06.htm 3 April 2006, Aldersundet, Norway