Abstract
AbstractFlux 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.
Highlights
It was first proposed by Dungey (1961) that the dynamics of Earth’s magnetosphere were primarily driven by magnetic reconnection
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
1 MWb 77 ± 26, 46 ± 25 MWbd aAssuming a typical polar cap flux content of 600 MWb (Milan et al, 2007; Huang et al, 2009). bIncludes reanalysis of an flux transfer events (FTEs) reconstructed by Sonnerup et al (2004), who determined a flux of 2 MWb. cCalculated from quoted value of area of polar cap opened by each event and ionospheric magnetic field strength of 5 × 104 nT. dValues refer to the radar and spacecraft estimates for the FTEs in Event 1
Summary
It was first proposed by Dungey (1961) that the dynamics of Earth’s magnetosphere were primarily driven by magnetic reconnection. Conjugate studies have shown the link between spacecraft and ionospheric signatures of FTEs (Amm et al, 2005; Elphic et al, 1990; McWilliams et al, 2004; Wild et al, 2001, 2003, 2005, 2007), between optical and radar FTE signatures (Moen et al, 1995; Milan, Cowley, et al, 1999), and in occasional case studies, simultaneous observations of all three (Neudegg et al, 2001). Some of the case studies examined by Neudegg et al (2000) and in subsequent studies (e.g., Wild et al, 2001, 2003) found a clearer one-to-one correlation between individual in situ FTE signatures and their ionospheric counterparts
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