POLAR PLASMA WAVE INVESTIGATION: SCIENCE NUGGETS

1. COHERENT STRUCTURES OBSERVED BY THE POLAR PLASMA WAVE INSTRUMENT
   
   The waveform interferometry data from the POLAR Plasma Wave Instrument have been used to estimate the typical parallel scale sizes (100-1000 m), velocities (1000 km/s) and the bi-directional propagation of the coherent electric field structures previously observed by the GEOTAIL and FAST spacecraft in magnetospheric regions with bursty broadband electrostatic noise spectral signatures. These structures are found to correspond to regions of positive charge at high altitudes in the earth's polar magnetosphere. The determination of scale size and velocity estimates now make it possible to quantitatively test theories of coherent structures.
   
   Franz, Jason R., Paul M. Kintner and Jolene S. Pickett, POLAR observations of coherent electric field structures, Geophys. Res. Lett., Vol. 25, 1277, April 15, 1998.
   
   
   
2. SOLAR WIND DYNAMIC PRESSURE AS A DRIVER FOR CHORUS EMISSIONS
   
   Coherent electromagnetic chorus emissions, the most intense of all naturally-generated plasma waves in the earth's magnetosphere, have been observed to be turned on/off in direct association with sudden fluctuations in solar wind dynamic pressure and southward turnings of the interplanetary magnetic field (IMF) associated with the impact of a magnetic cloud on the magnetosphere. High resolution waveform data from the POLAR Plasma Wave Instrument have shown a fast and direct response of the chorus to sudden magnetospheric compression/relaxation and to small, quick variations in the IMF on short (<60 second) time scales. These findings cast doubt on previously held mechanisms for chorus generation and requires that new and faster operating mechanisms may be responsible for the emission of these intense plasma waves, which drive morningside pulsating and diffuse aurorae.
   
   Lauben, D.S., U.S. Inan, T.F. Bell, D.L. Kirchner, G.B. Hospodarsky, and J.S. Pickett, VLF chorus emissions observed by POLAR during the January 10, 1997 magnetic cloud, Geophys. Res. Lett., Vol. 25, 2995, August 1, 1998.
   
   
   
3. BROADBAND PLASMA WAVES IN THE POLAR CAP BOUNDARY LAYER
   
   Broadband plasma waves have been found to bound the dayside polar cap magnetic field lines. These emissions occur on nearly every apogee Polar pass with a 96% occurrence rate and are well-correlated with enhanced fluxes of 10 to 100 eV protons, doubly charged helium and singly charged oxygen ions. In the noon sector there is a strong relationship between the enhanced fluxes of energetic ionospheric and magnetosheath ions and intense wave energy, suggesting that these waves may be responsible for ion heating near the cusp region. The greater power in the low frequency end of the wave spectra indicates that these waves will preferentially energize oxygen ions over protons, causing the heating of ionospheric oxygen on these field lines. If the interplanetary magnetic field turns southward, interconnection with the earth's magnetic field will occur, leading to oxygen ion transport into the earth's magnetotail. Subsequent injection into the nightside magnetosphere could lead to the delayed enhancement of oxygen ions in the ring current. Ion heating by these broadband waves provides one possible mechanism to explain plasmasheet ion composition during a storm main phase.
   
   Tsurutani, B.T., G.S. Lakhina, C.M. Ho, J.K. Arballo, C. Galvan, A. Boonsiriseth, J.S. Pickett, D.A. Gurnett, W.K. Peterson, and R.M. Thorne, Broadband plasma waves observed in the polar cap boundary layer: Polar, J. Geophys. Res., Vol. 103, 17,351, August 1, 1998.
   
   
   
4. EQUATORIAL SOURCE LOCATION OF CHORUS EMISSIONS
   
   Whistler-mode chorus is generated within a few degrees of the geomagnetic equator. A spatial survey of Poynting flux measurements computed using high-resolution electric and magnetic field waveform data from the Polar Plasma Wave Instrument provides the first direct and unambiguous evidence that chorus is generated near the geomagnetic equator. Chorus is shown without exception to propagate away from the geomagnetic equator. No chorus emissions were observed to propagate toward the equator as might be expected from high-latitude magnetospheric reflections. The absence of a reflected component indicates that chorus is absorbed before reflection. These results indicate that previously suggested chorus generation mechanisms requiring multiple passes through the source region are not valid.
   
   LeDocq, M. J., D. A. Gurnett, and G. B. Hospodarsky, Chorus source locations from VLF Poynting flux measurements with the Polar spacecraft, Geophys. Res. Lett., Vol. 25, 4063, November 1, 1998.