Ionospheric patch formation: Direct measurements of the origin of a polar cap patch

We present the first direct measurements documenting the origin of an ionospheric patch, emerging into the polar cap from the noon dark cusp region of the dayside auroral oval. A series of such patches were observed. The observed patch‐formation signatures, in plasma density and velocity, ion and electron temperature, and optical transient signatures, are all compared to those required of the Lockwood and Carlson [1992] transient reconnection mechanism for patch formation. Conformity of the observed signatures, to the stringent signatures demanded of these five independent parameters by the tested mechanism, is taken as strong substantiation of the applicability of this mechanism. The data excludes two alternate candidate mechanisms.