Cluster observations of quasi‐periodic impulsive signatures in the dayside northern lobe: High‐latitude flux transfer events?

Abstract

[1] We report on a series of quasi-periodic reversals in GSM B Z observed by the four Cluster spacecraft in the northern dayside lobe poleward of the cusp on 23 February 2001. During an interval of about 35 min, multiple reversals (negative to positive) in B Z of approximately 1-min duration with an approximate 8-min recurrence time were observed. The individual structures do not resemble low-latitude flux transfer events (FTE) but the 8-min recurrence frequency suggests that intermittent reconnection may be occurring. Measurements (appropriately lagged) of the solar wind at ACE show that the IMF was southward-oriented with a strong B X and that a modest dynamic pressure increase occurred as the events started. The multi-point observations afforded by the Cluster spacecraft were used to infer the motion (direction and speed) of the observed magnetic field reversals. The associated currents were also calculated and they are consistent with the spatial confinement of the observed magnetic field reversals. We propose that the observed reversals are due to flux tubes reconnecting with closed field lines on the dayside. Ancillary data from the Cluster Ion Spectrometry (CIS) and Plasma Electron And Current Experiment (PEACE) instruments were used to develop a physical picture of the reversals.

Pulsed flows at the high-altitude cusp poleward boundary, and associated ionospheric convection and particle signatures, during a Cluster - FAST - SuperDARN- Søndrestrøm conjunction under a southwest IMF

“…Our study was based on just one event, reserving an analysis of many events for future work. We note, however, that in a very recent work Thompson et al (2004) observed the same type of impulsive events, though they discussed them only from the Cluster perspective. The focus there was on a sequence of reversals of the GSM B z field component observed downstream of the cusp, each reversal being ascribed to currents flowing perpendicular to the field.…”

Flux transfer events on the high-latitude magnetopause: Interball-1 observations

“…The event exhibited near monopolar signatures in the X and Z components of the magnetic field. Finally, Thompson et al (2004) interpreted Cluster observations of a series of transient events that occurred poleward of the northern cusp just prior to local noon as evidence for FTEs. A reversal in B z , but no magnetic field strength increase or bipolar signature normal to the nominal magnetopause, marked each event.…”

“…A reversal in B z , but no magnetic field strength increase or bipolar signature normal to the nominal magnetopause, marked each event. As they are not classic examples of FTEs, we did not include events of the type reported by Berchem et al (1995), Sonnerup et al (2004), or Thompson et al (2004) in our study.…”

Motion of flux transfer events: a test of the Cooling model

“…Wang et al (2005) suggested that this was because FTEs were more likely to extend further out into the magnetosheath than they extend into the magnetosphere; recent analysis of some individual FTEs observed by Cluster has confirmed that this may be the case (Hasegawa et al, 2006). However, a further possible explanation is that the magnetospheric signatures of FTEs at high latitudes, particularly in the vicinity of the cusp, may often be more complicated than low latitude signatures, possibly leading to an underestimate of the number of magnetospheric events (Thompson et al, 2004;Fear et al, 2005a).…”