A long-lived band of plasma density enhancement at mid-latitudes during the 2003 Halloween magnetic storm

Abstract

During the magnetic storm recovery phase between 08 and 13 UT on 30th October 2003 (so-called Halloween Storm), CHAMP, GRACE and TIMED satellites encountered an unusual post-midnight streak of high plasma density. The streak was latitudinally narrow (∼0.5° GLAT), zonally elongated (>10° GLON), and vertically extended for several hundred kilometers. Inside the streak, as observed by CHAMP, the deviation of the magnetic field from the background is greater in the zonal component than in the meridional component. This observation is consistent with the zonally extended nature of the streak, as confirmed by TIMED/GUVI optical observations. The field-aligned current (FAC) direction is generally upward (downward) at the equatorward (poleward) edge of the streak. We discuss the time history of observed features. We suggest the following scenario of the streak formation. A storm-enhanced density (SED) was generated before 19 h local time by a sub-auroral polarization stream (SAPS). The SED gradually separated from the poleward-retreating region of strong westward plasma drift and turned into an electron density enhancement fossil (EDEF). This EDEF at mid-latitude (about 40°S MLAT) practically co-rotated with the Earth for more than 10 h through the night till dawn. We suggest that the enhanced plasma pressure was counter-balanced by magnetic pressure gradient and kept stable over many hours.