Magnetospheric flux transport in the Dungey cycle during 2010

We quantify the contributions of different convection states to the magnetic flux throughput of the magnetosphere during 2010. To do this we provide a continuous classification of convection state for the duration of 2010 based upon observations of the solar wind and interplanetary magnetic field, geomagnetic indices, and field-aligned currents measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Convection states are defined as 1) quiet and 2) weak activity, substorm 3) growth, 4) expansion, and 5) recovery phases, 6) substorm driven phase (when relatively steady magnetospheric convection occurs), 7) recovery bays (when recovery phase is accompanied by a negative excursion of the AL electrojet index), and 8) periods of multiple intensifications (storm-time periods when continuous short-period AL activity occur). The magnetosphere is quiet for 46% of the time, when very little convection takes place. The majority of convection occurs during growth and driven phases (21% and 38%, respectively, of open magnetic flux accumulation by dayside reconnection). We discuss these results in the context of the expanding/contracting polar cap model of convection, and describe a framework within which isolated substorms and disturbances during periods of more continuous solar wind-magnetosphere driving can be understood.

troduce the term driven phase to describe this aspect of the substorm cycle.

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In Section 2 we describe the observables we use in this study and the convection 147 states that we identify. Section 3 presents an analysis of the occurrence of different states 148 and the sequences of states that represent substorms and other forms of geomagnetic ac-149 tivity. Finally, we conclude and describe future directions for research in Section 4. We determine magnetospheric convection state continuously for the duration of 2010.

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A few data gaps are present in the data, and the total period of analysis comprises just 153 over 360 full days. Figure 1 shows a 60 h interval from May, which we discuss below. This  for understanding magnetospheric convection, though in general are difficult to measure 161 accurately. As described below we use proxies, F P C , Φ D , and Φ P C , for three of these 162 parameters; Φ N can be inferred from these using eqs.

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Additional parameters are included in the analysis, but are not used to determine   (c) The associated cumulative occurrence distribution, showing that the median Λ ≈ 17.5 • , or the median F P C ≈ 0.4 GWb.
As will be discussed below, F P C overestimates the true value of F P C when a sig- 213 Figure 3 tests the relationship between Φ D and F P C expected from eq. (1), using has been converted to F P C using eq. (4). Panel (c) shows Φ D evaluated at 2 min cadence.

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Multiple data gaps in V SW create gaps in Φ D , and where these are less than 10 min in 220 duration we have linearly interpolated over the missing values.

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The transport of magnetic flux within the magnetosphere leads to ionospheric con-245 vection during growth, expansion, driven, and recovery phases. From eq. 2 we expect the 246 cross-polar cap potential during each substorm phase to be: growth, during the typical growth-expansion-recovery phase sequence of a substorm Φ P C will be  Fig. 1(d)). Our proxy for the cross-polar cap potential is then Φ P C (kV) ≈ 17 PCN. 255 We note that during strong northward IMF conditions PCN can respond to polar     and percentage of the whole year, and the average duration of each event. Table 2    growth phase (also apparent as a simultaneous positive excursion of SYM-H). We also 515 note that more weakly and more strongly driven cases are on average associated with 516 lower (350 km s −1 ) and higher (500 km s −1 ) solar wind speed, respectively.

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In many of the substorms identified in Fig. 3, F P C continues to grow for 20 mins 518 or so after expansion phase onset. This behaviour is also seen in some of the superposed 519 epoch analyses of Fig. 6. On one hand, in most cases Φ D remains high after onset, so 520 open flux continues to be accumulated even after nightside reconnection has commenced, 521 and if Φ D > Φ N then F P C will continue to grow. On the other hand, the assumption 522 that the polar cap is circular, used to calculate F P C , is likely to break down at these times  Driven phases are periods of quasi-balanced dayside and nightside reconnection, 534 Φ N ≈ Φ D and F P C ≈ const, that is, periods during which the magnetotail has adjusted 535 itself to release magnetic flux at the same rate that it is being accumulated on the day-536 side. However, Φ D responds promptly to changes in the solar wind, whereas Φ N appears 537 to respond more slowly. For instance, an abrupt northwards turning of the IMF can lead 538 to a sudden decrease in Φ D but nightside reconnection can continue unabated, result-539 ing in a decrease in F P C (which we define as a recovery phase).

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Driven phase onsets appear to be the response to more gradual changes in Φ D , specif-541 ically moderate increases. Fig. 8 shows that on average 2 h prior to each onset Φ D ≈ 542 Φ P C , but that a slight increase in Φ D occurs approximately 1 h before. Φ P C remains 543 unchanged at this time, suggesting that Φ N also continues uniformly. Dayside and night-544 side reconnection are now slightly unbalanced leading to an increase in F P C (Λ). Even-545 tually this situation can no longer be supported and onset occurs: Λ decreases and Φ P C perposed epoch analyses (e.g., Caan et al., 1977;Lyons, 1995). 557 We have argued that classic substorms are those that occur within an hour or so