JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. A2, PAGES 1431-1447, FEBRUARY 1, 1992 Signatures of Transient Boundary LayerProcesses Observed With Viking J. Wocn ANO R. LVNOI• Swedish Institute o.fSpacePhysics, Kiruna Transient penetration of plasma with magnetosheath origin is frequently observed with the hot plasma experimenton board the Viking satellite at auroral latitudes in the dayside magnetosphere. The injected magnetosheath ions exhibit a characteristic pitch angle/energydispersion pattern, earlier reported for solar wind ions accessing the magnetosphere in the cusp regions. In contrast to the continuous plasma entry in the cusp, the events discussed here show temporal features which suggest a connectionto transient processes at or in the vicinity of the magnetospheric boundary. A single event study confirmspreviously published observations that the injected ions flow essentiallytailward with a velocity comparable to the magnetosheath flow and that the energy spectra inferred for the source population resemble magnetosheath spectra. Those ion injection structures, which were resolved by the Viking mass spectrometer, consist of protons. Based on a statistical study, it is found that these events are predominantly observed around 0800 and 1600 MLT, in a regionpopulated both by ring current/plasmasheetparticles and by particles whose source is the magnetosheath plasma. Magnetic field line tracing based on the Tsyganenko magneticfield model yields a scatter of the source locationsaround the mid-latitude region of the magnetospheric boundary. The probability for theseeventsto occur is highest when the interplanetary magnetic field (IMF) is confined to the eclipticplane. The event occurrence frequency showsa dawn-dusk asymmetry depending on the azimuthal direction of the IMF. The occurrence frequency is independent of the solar wind velocity but increaseswith increasing solar wind pressure. For radially directed IMF the probability for observing the events is generally higher than for azimuthally directed IMF. The differenceis especially pronounced during times when the solar wind pressure is comparatively low. Some of the most prominent events are obviouslyassociated with significantchanges in solar wind plasma density. The connectionof the eventsto transient impulsive solar wind/magnetosphere interaction processes, suchas transient reconnection (FTE), impulsive plasmatransfer,Kelvin Helmholtzinstabilities,and solar wind pressure pulses, is discussed. A relation with transient reconnection can be excluded. 1. INTP•O DU CTION It is well established that the subsolar region of the mag- netosphere is a site where the shocked solar wind and the magnetosphere strongly and impulsively interact. One be- lieves that these interaction processes drive or at least con- tribute significantly to the magnetospheric convectionsys- tem. Furthermore,the frontside magnetopause is considered one of the prime regions for magnetosheath plasma transfer to the magnetospheric boundary layers. The plasma entry may occur in a more or less continuousway, as in the polar cusp regions, or take place associated with impulsive inter- action processes. These transient processes are most often discussed in terms of sporadic intermittent reconnection of magnetosheath and magnetospheric flux tubes, as proposed originaJly by Russell andElphic[1979]. As an alternative ex- planation, impulsive penetration of magnetosheath plasma filamentswith excess momentumhas been suggested, where reconnection of flux tubes is at least not the primary process [Lemaire, 1977; Schindler, 1979; Heikkila, 1982]. However, the importance of the flanks of the magneto- sphere asa sitefor energy, momentum, and/or mass transfer from the solar wind to the magnetosphere is lesswell estab- lished. It is still unclear whether the flank boundary layers are primarily a transport region, or whether also here, as in the subsolar region, the solar wind and magnetosphere inter- act strongly and impulsively. Only a few in situ observations of dynamical features in the flank boundary layer plasma Copyright 1992 by the American Geophysical Union. Paper number 91JA02490. 0148-0227/92/91 JA-02490505.00 have been presented. Sckopke et al. [1981]observed the intermittent appearanceof high-density regionsof magne- tosheathplasmain an ISEE pass through the dawnside low- latitude boundary layer ("pulsed boundary layer"). These observations were and still are controversial and have been discussed as boundary layer thicknessmodulations due to a Kelvin-Helmholtz instability (KHi) operating at the inner edge of the low-latitude boundary layer (LLBL) [Sckopke et al., 1981],as plasma transferassociated with impulsive re- connection [Paschmann et al., 1982;Cowley 1982; Saunders, 1983] or withoutinvoking open fieldlines[Lundin and Du- binin, 1984], and assolarwind pressure pulse induced mag- netopause disturbances [Sibeck et al., 1989].Evidence for di- rect magnetosheath plasma injection into the high-latitude boundary layer has been presentedby Lundin and Aparicio [1982] and Lundinand Vubinin [1984]. The expectation that any process at the magnetospheric boundary will produce an ionospheric signature at or above auroral latitudes has stimulated an eagersearch for these sig- natures. Conclusiveevidencefor ionosphericsignaturesof solar wind/magnetosphere interaction at the subsolar region of the magnetopause is reported from ground-based optical and radar observations [e.g.,Goertz et al., 1985;Lockwood et al., 1986, 1989; Sandholt et al., 1986, 1990, and references therein]. These axe short-lived transientauroralintensifi- cations at the equatorward boundary of the persistent cusp emissions ("midday auroral breakup events") or sporadic en- hancements of the ionospheric ion drift ("ion drift events") close to the convection reversal boundary. With the moni- toring capability of ground-based observations it was estab- lished that these events propagate poleward into the open flux tube region and that the propagationin the azimuthal 1431