Table 2: Summary of ionization bulge parameters from observations.

date	range N	range S	minimum	height N	height S
1996.43	23.9	-26.7	-1.4	1.7	1.5
1996.93	27.6	-33.3	-2.9	1.6	1.6
1997.43	27.8	-29.5	-0.9	1.8	1.6
1997.93	28.3	-34.2	-2.9	1.8	1.6
1998.43	39.5	-32.5	3.5	1.6	1.5
1998.93	40.6	-42.3	-0.9	1.9	1.6
1999.43	43.5	-59.4	-7.9
1999.93	33.9	-58.1	-12.1