  Line ratios for different transitions of the same molecule have long been
used as a probe of gas temperature. Here we use ALMA observations of the N2H+
J~=~1-0 and J~=~4-3 lines in the protoplanetary disk around TW Hya to derive
the temperature at which these lines emit. We find an averaged temperature of
39~K with a one sigma uncertainty of 2~K for the radial range 0.8-2'',
significantly warmer than the expected midplane temperature beyond 0.5'' in
this disk. We conclude that the N2H+ emission in TW Hya is not emitting from
near the midplane, but rather from higher in the disk, in a region likely
bounded by processes such as photodissociation or chemical reprocessing of CO
and N2 rather than freeze out.
