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100 <br />�o <br />d <br />LL <br />10 <br />C <br />W <br />01 <br />SB Acela <br />—+�-° SB Regional <br />NB Regional <br />—+—SB Acela <br />--e—SB Regional <br />— - Reference <br />a� <br />a <br />10 100 <br />Distance From SS Track Centerline (m) <br />-e 20. Maximum Measured Magnetic Fields as a Function of Position for Each of <br />Five Train Passes. For Reference, the Bottom Line Shows Attenuation <br />that is Inversely Proportional to the Distance Squared <br />Figure 21 is a surface plot of ELF resultant magnetic field measured at the 16.4 ft (5 m) position <br />during the first train pass, a southbound Acela Express. This detail plot shows magnetic field <br />magnitude as a function of time and frequency. In this plot, the 60 Hz component is dominant, <br />but third and fifth harmonics are significant. Harmonics above 2 kHz are also visible in this plot. <br />In Fi ure 21, the time- variation of magnetic fields does not correspond to the position of the train <br />(i.e., maximum does not occur when the train is at the sensor location). Rather, the magnetic <br />field depend on the currents in the OCS and rails, and these current levels depend on the <br />operation of the train (for example, acceleration) and the location of the autotransformers that <br />supply electric. power. <br />TRA <br />47 <br />