On the cause of a magnetospheric flux transfer event

Abstract

Abstract. We present a detailed investigation of a magnetospheric flux transfer event (FFE) seen by the Active Magnetospheric Tracer Explorer (AMPTE) UKS and IRM satellites around 1046 UT on October 28, 1984. This event has been discussed many times previously in the literature and has been cited as support for a variety of theories of FTE formation. We make use of a model developed to reproduce ion precipitations seen in the cusp ionosphere. The analysis confirms that the FTE is well explained as a brief excursion into an open low-latitude boundary layer (LLBL), as predicted by two theories of magnetospheric FTEs, namely, that they are bulges in the open LLBL due to reconnection rate enhancements or that they are indentations of the magnetopause by magnetosheath pressure increases (but in the presence of ongoing steady reconnection). The indentation of the inner edge of the open LLBL that these two models seek to explain is found to be shallow for this event. The ion model reproduces the continuous evolution of the ion distribution function between the sheath-like population at the event center and the surrounding magnetospheric populations; it also provides an explanation of the high-pressure core of the event as comprising field lines that were reconnected considerably earlier than those that are draped over it to give the event boundary layer. The magnetopause transition parameter is used to isolate a field rotation on the boundaries of the core, which is subjected to the tangential stress balance test. The test identifies this to be a convecting structure, which is neither a rotational discontinuity (Pal)) nor a contact discontinuity, but could possibly be a slow shock. In addition, evidence for ion reflection off a weak RD on the magnetospheric side of this structure is found. The event structure is consistent in many ways with features predicted for the open LLBL by analytic MHD theories and by MHD and hybrid simulations. The de HoffinanTeller velocity of the structure is significantly different from that of the magnetosheath flow, indicating that it is not an indentation caused by a high-pressure pulse in the sheath but is consistent with the motion of newly opened field lines (different from the sheath flow because of the magnetic tension force) deduced from the best fit to the ion data. However, we cannot here role out the possibility that the sheath flow pattern has changed in the long interval between the two satellites observing the FTE and subsequently emerging into the magnetosheath; thus this test is not conclusive in this particular case. Analysis of the fitted elapsed time since reconnection shows that the core of the event was reconnected in one pulse and the event boundary layer was reconnected in a subsequent pulse. Between these two pulses is a period of very low (but nonzero) reconnection rate, which lasts about 14 rains. Thus the analysis supports, but does not definitively verify, the concept that the FTE is a partial passage into an open LLBL caused by a traveling bulge in that layer pr...

Plasma structure within poleward-moving cusp/cleft auroral transients: EISCAT Svalbard radar observations and an explanation in terms of large local time extent of events

“…As discussed in section 1, the motion and occurrence of these events have been well explained in terms of pulsed magnetopause reconnection. The repeat periods found here are also consistent with those of FTEs at the magnetopause (Lockwood and Wild, 1993) and deduced from cusp ion steps , events which are also well explained in terms of the e ects of reconnection pulses (see reviews by Hapgood, 1998 andrespectively). The ESR data presented here give us our ®rst opportunity to study the ®eld-parallel pro®les of plasma parameters in and around these events and, in particular, to compare with the predictions made for the ESR by Davis and Lockwood (1996 ± hereafter referred to as D&L) for periods when the magnetopause reconnection rate is pulsed.…”

“…This result is consistent with analysis of the magnetopause signatures of pulsed reconnection, provided one uses the 2-dimensional reconnection pulse model of these``¯ux transfer events'' (Lockwood et al, 1995b). Detailed analysis of FTE signatures also strongly supports this two-dimensional model (Lockwood and Hapgood, 1998). Lockwood and Davis (1996) also show that longitudinal passes through cusp ion steps (see below) reveal that events cover considerable ranges of MLT.…”

“…Davis and Lockwood modelled poleward-moving 630 nm events using the same formulation of the e ects of pulsed reconnection as used by Lockwood and Davis (1996) successfully to model both meridional and longitudinal passes of low-altitude spacecraft through cusp ion steps and also used by to model simultaneous steps in the up-and down-going ions in the mid-altitude cusp. The concepts of this model were also used by Onsager et al (1993), Lockwood (1997) andFederov et al (1999) to model steady-state cusp ion distribution functions at low, middle and high altitudes, respectively, and by Lockwood and Hapgood (1998) to model the ions in a magnetopause FTE.…”

Two-dimensional electric field measurements in the ionospheric footprint of a flux transfer event

“…They suggested that the newly opened¯ux is of the form of ā ux rope of 1 R E in diameter. Lockwood and Hapgood (1998) later demonstrated that the``2D reconnection'' models of Saunders (1983) and Southwood et al (1988), in which reconnection could occur along a length of the magnetopause possibly several R E long, were in better accordance with observation. In the present study, we have found that the PIF maps out to an area in the dipole ecliptic plane of approximately 0.5 R E radially by 4 R E in azimuth.…”

A statistical survey of dayside pulsed ionospheric flows as seen by the CUTLASS Finland HF radar

“…Theoretical considerations of FTEs have resulted in suggestions for the possible cause of episodic bursts of reconnection at the magnetopause and in a description of their behaviour, both at the reconnection site and at their ionospheric footprint (Cowley, 1984;Siscoe and Huang, 1985;Cowley et al, 1991Lockwood, 1993;Lockwood et al, 1995;Lockwood and Hapgood, 1998). Hypotheses regarding the triggering mechanism, which predict separation times for FTEs from several to a few tens of minutes, include IMF B z¯u ctuations (Lockwood et al, 1989), spontaneous FTEs (Lockwood and Wild, 1993), and intrinsic magnetospheric system control (Kuo et al, 1995).…”