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Radio images at a single frequency give
us two-dimensional
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information,
but no height
information
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Different frequencies refer to different
layers of constant
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magnetic
field: multiple
frequencies therefore reveal different
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heights (peel
away layers), but the scale is not measured
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Can in principle use div.B = 0 but
we do not measure
field
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direction
(although information from comparing x and o from
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same
layer?) and we do not measure weak fields, so we
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cannot solve this problem with radio data alone (?)
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But in conjunction with extrapolations, which also have
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their
own limitations, progress may be possible
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