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Next: 3. From Dispersion Characteristics Up: 2. From Measured Spectra Previous: 2.3. The Experimental Dispersion

2.4. Some Limitations of the Method

Let us now discuss two physical limitations of our determination of tex2html_wrap_inline1094 as a function of frequency:
1. A straightforward consequence of our fitting method is that it is impossible to determine very small values of tex2html_wrap_inline1094 (i.e., tex2html_wrap_inline1426 ), because in this case the antenna response is a weak function of tex2html_wrap_inline1094 except for a multiplicative factor independent of tex2html_wrap_inline1096 . Owing to the shape of the dispersion curves (Figure B1), this restriction corresponds to observing frequencies a little below the gyroharmonics (in addition, in this case tex2html_wrap_inline1250 is not small compared to tex2html_wrap_inline1314 so that (1) is no longer valid).
2. Since the antenna moves with respect to the plasma (this motion is mainly the corotation of the IPT with Jupiter), the received waves are Doppler shifted. How far does that effect bias the determination of tex2html_wrap_inline1094 ? This point was qualitatively analyzed by [Meyer-Vernet, Hoang and Moncuquet, 1993] concerning the frequency shift, but there is also a Doppler effect on the shape of the antenna response function tex2html_wrap_inline1292 given by (2). We further discuss that point in appendix A. We find that our tex2html_wrap_inline1094 determination is unaffected by the Doppler shift as long as tex2html_wrap_inline1442 , but its value is in most cases underestimated for larger tex2html_wrap_inline1314 . Again, this occurs for frequencies close to the gyroharmonics but this time above them.


Michel Moncuquet
Tue Nov 18 19:18:28 MET 1997