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T.G. Moran (CUA)
We have detected the presence of ion cyclotron waves at the proton gyrofrequency at the base of a polar coronal hole using ultraviolet spectroscopic observations of minor ions. The SOHO/SUMER ultraviolet spectrograph was used to observe 22 coronal ions with masses ranging from 14 to 56 AMU and charge states ranging from + 4 to +11. The line widths of 33 UV lines radiated by these ions were measured using long exposure, low scatter, off-limb measurements of the line profiles in the the region between 20,000 and 60,000 km altitude in a coronal hole in a search for signatures of ion cyclotron waves. In the case of ion cyclotron waves at the proton gyrofrequency dominating the wave field, the minor ions act as probes of the wave field, and we expect an approximately linear relationship between ion velocity and charge-to-mass ratio. This linear relationship derives from the equation of motion for an ion moving in a magnetic field under the influence of an oscillating wave electric field. The measured linewidths, and therefore mean speeds show this linear relationship with gyrofrequency. Velocity considerations preclude waves at electron cyclotron or plasma frequencies, indicating the presence of ion cyclotron waves. In addition, the relative amplitudes of ion velocities compared with 106 K proton velocities indicate the wave frequency is near the proton gyrofrequency. The measurements also indicate a turbulent or bulk wave velocity of 15-20 km/sec, in addition to the gyrating ion velocities. This investigation was funded by NASA through Grant No. NAG5-9888 and carried out at the NASA/GSFC SOHO Experimental Analysis Facility.
The author(s) of this abstract have provided an email address for comments about the abstract: tmoran@cspsw4.nascom.nasa.gov
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.