We present plasma and electric field data obtained by the S3‐3 satellite over the polar caps. We demonstrate that: (1) plasma signatures in the polar cap arc formation region near 5000 km altitude show clear intrusions of plasma sheet ( ∼ keV) and magnetosheath ( ∼ 100 eV) plasma into a background of low‐energy polar cap plasma; (2) the combined plasma and electric field signatures (electron inverted‐V, ion beam and ▽ •E→0) are exactly the same as in the evening discrete arc. We interpret this equivalence of polar cap and evening discrete arc signatures as indication that their formation processes are identical. The spatial structures of polar cap electric fields and the associated plasma signatures are consistent with the hypothesis that plasma intrusion into the polar cap takes the form of multiple cellular eddies. This hypothesis provides a unifying view of arc formation and arc
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