How Much Flux Does a Flux Transfer Event Transfer?

Abstract

Flux transfer events are bursts of reconnection at the dayside magnetopause, which give rise to characteristic signatures observed by a range of magnetospheric/ionospheric instrumentation. One outstanding problem is that there is a fundamental mismatch between space‐based and ionospheric estimates of the flux that is opened by each flux transfer event—in other words, their overall significance in the Dungey cycle. Spacecraft‐based estimates of the flux content of individual flux transfer events (FTEs) correspond to each event transferring flux equivalent to approximately 1% of the open flux in the magnetosphere, whereas studies based on global‐scale radar and auroral observations suggest this figure could be of the order of 10%. In the former case, flux transfer events would be a minor detail in the Dungey cycle, but in the latter they could be its main driver. We present observations of two conjunctions between flux transfer events observed by the Cluster spacecraft and pulsed ionospheric flows observed by the Super Dual Auroral Radar Network (SuperDARN) network. In both cases, a similar number of FTE signatures were observed by Cluster and one of the SuperDARN radars, but the conjunctions differ in the azimuthal separation of the spacecraft and ionospheric observations (i.e., the distance of the spacecraft from the cusp throat). We argue that the reason for the existing mismatch in flux estimates is due to implicit assumptions made about FTE structure, which tacitly ignore the majority of flux opened in mechanisms based on longer reconnection lines. If the effects of such mechanisms are considered, a much better match is found.

The Contribution of Flux Transfer Events to Mercury's Dungey Cycle

“…Since the size of Mercury's polar cap is estimated to be about 4–6 MWb (Alexeev et al, ), this would suggest that an individual FTE could open flux corresponding to ∼1–5% of the open flux content of Mercury's magnetosphere. This compares with previous estimates, from spacecraft observations, of individual FTEs opening between 0.1–1% of Earth's polar cap (see summary by Fear et al, ). Factoring in the high repetition rate, Imber et al () concluded that FTEs could provide at least ∼30% of the flux transport needed to drive Mercury's substorm cycle, compared with less than 2% at Earth.…”

“…We have applied arguments from a recent study into the flux content of flux transfer events at Earth (Fear et al, ) to Mercury's magnetosphere. Using a statistically‐determined mean flux rope radius for “large” Hermean FTEs (Imber et al, ), upper values for the magnetic perturbation due to the passage of the FTE flux rope as reported during a period of extreme solar wind driving at Mercury (Slavin et al, ) and making the assumption that, at its largest extent, the reconnection process could occur coherently along a single reconnection line across the entire dayside magnetosphere (12 hr in magnetic local time), we place a sensible upper limit for the total flux content of a large flux transfer event at Mercury to be ∼1 MWb.…”

“…The normal procedure employed by spacecraft studies is to estimate the cross‐sectional area of the flux rope, and then calculate the total amount of flux mapping through that cross section: where B axial is the component of the magnetic field strength parallel to the axis of the flux rope, and r is the flux rope radius. Fear et al () argue that this calculation tacitly assumes the Russell and Elphic (, ) scenario, as it is only in this mechanism that Φ rope is representative of the total amount of magnetic flux opened in the burst of reconnection. In the Lee and Fu () multiple X‐line scenario, it is important to distinguish between the “flux transfer event” (in the sense of the reconnection burst as a whole) and the flux rope; the burst of reconnection gives rise to the flux rope observed by the spacecraft, but in doing so opens a potentially much greater amount of magnetic flux which does not map through the flux rope, but does map to the ionosphere (see Figure b).…”

“…Furthermore, in the Southwood et al ()/Scholer () single X‐line scenario, the structure which gives rise to the in situ signatures is not a flux rope, and hence calculating Φ rope is not appropriate. Instead, Fear et al () argue that for either of the latter two (longer X‐line) scenarios, the total amount of magnetic flux opened in a burst of reconnection is more completely represented by the flux which maps through the gray surfaces in Figure , that is, where B N is, as above, the component of the magnetic field within the FTE that is normal to the magnetopause, r is again the FTE radius and L is the length of the reconnection line (and hence the FTE) at the magnetopause. Fear et al () examined two case studies for which they reported simultaneous observation of in situ signatures of FTEs at Earth's magnetopause and their ionospheric counterpart in radar data.…”

“…Imber et al (2014) carried out a survey of large Figure 1. A cut-away illustration of the three main FTE mechanisms, reproduced from Fear et al (2017). The panels show a view, from normal to the magnetopause plane, of the open field lines in (a) the Russell and Elphic (1978), flux tube model, (b) the Lee and Fu (1985) multiple X-line model, and (c) the Southwood et al (1988) /Scholer (1988) single X-line model.…”