  The solar active region photospheric magnetic field evolves rapidly during
major eruptive events, suggesting appreciable feedback from the corona.
Previous studies of these "magnetic imprints" are mostly based on line-of-sight
only or lower-cadence vector observations; a temporally resolved depiction of
the vector field evolution is hitherto lacking. Here, we introduce the
high-cadence (90~s or 135~s) vector magnetogram dataset from the Helioseismic
and Magnetic Imager (HMI) that is well suited for investigating the phenomenon.
These observations allow quantitative characterization of the permanent,
step-like changes that are most pronounced in the horizontal field component
($B_h$). A highly structured pattern emerges from analysis of an archetypical
event, \texttt{SOL2011-02-15T01:56}, where $B_h$ near the main polarity
inversion line increases significantly during the earlier phase of the
associated flare with a time scale of several minutes, while $B_h$ in the
periphery decreases at later times with smaller magnitudes and a slightly
longer time scale. The dataset also allows effective identification of the
"magnetic transient" artifact, where enhanced flare emission alters the Stokes
profiles and the inferred magnetic field becomes unreliable. Our results
provide insights on the momentum processes in solar eruptions. The dataset may
also be useful to the study of sunquakes and data-driven modeling of the
corona.
