Ion Flows Away From the Equatorial Plane

This short section discusses the behaviour of earthward and tailward directed ion flows in the outer planes of the three substorm phases.

Away from the equatorial plane, there is generally less data available in the northern hemisphere and as such the data spatial coverage is generally reduced, although in the growth phase it is more balanced. This is almost certainly due to the Cluster space- craft spending less time in this region due to the orbital trajectory and its progression throughout the mission. In general, as previously mentioned, due to the lack of spatial coverage, it is very difficult to infer any specific flow patterns for any of the substorm phases. It is also worth noting that earthward flow is greatly reduced in these regions because there are periods in which the spacecraft lies out of the plasma sheet, where the outer planes are dominated by tailward flow data.

The northern hemisphere (positive Y values) of the growth phase shows a fairly sparse spatial distribution of earthward and tailward plasma flows (Figure 5.7). Earthward flows are mostly deflected towards the dawn flank in the dawn hemisphere. Within a radial distance of about 5 RE of Earth, flows tend to be directed more earthward along the NMM, while there are slight dawnward/duskward deflections on the dawn and dusk sides respectively. Looking at the tailward flows in the same plane, the dawn flank

Figure 5.8: Ion velocity plots displaying tailward (left) and earthward (right) plasma

flow during the substorm growth phase in the XY plane where Z = -9±4.5 RE .

resents deflections directed towards the NMM with the two bins centred on Y = -6.5 RE directed dawnward (yellow box). In the southern hemisphere (Figure 5.8), earthward flows in the dawn sector are mostly deflected dawnward. At local midnight flows are predominantly earthward and in the dusk hemisphere, flows are mostly directed towards the NMM with one bin directed duskward.

In the northern hemisphere of the expansion phase (Figure5.9), there is almost no data available in the dusk hemisphere and not a great deal in the dawn hemisphere too. Within a radial distance of about 5 RE from Earth, ion flows are roughly earthward at the NMM and again dawn and duskward deflections are present in their respective sides. In the dawn hemisphere flows are exclusively dawnward directed with the outermost vectors deflected very steeply. Looking at the tailward flows, within a radial distance of about 5 RE from Earth, flows are almost parallel with the NMM with only very minor deflections towards the NMM. In the dawn hemisphere, ion flows are almost all deflected towards the NMM with the exception of one bin. The southern hemisphere (Figure5.10) provides slightly more spatial coverage than the northern hemisphere. The earthward flows in the dawn hemisphere are gently deflected dawnward. At local midnight ion is almost exactly earthward. At the dusk terminator, deflections are strongly directed to the dusk, although at greater distances from earth, the flow pattern is directed towards the NMM. The tailward flows in the more distant regions of both the dawn and dusk hemispheres show fairly strong deflections towards the NMM while closer in, ions flows

Figure 5.9: Ion velocity plots displaying tailward (left) and earthward (right) plasma flow during the substorm expansion phase in the XY plane where Z = 9±4.5 RE .

Figure 5.10: Ion velocity plots displaying tailward (left) and earthward (right) plasma

Figure 5.11: Ion velocity plots displaying tailward (left) and earthward (right) plasma

flow during the substorm recovery phase in the XY plane where Z = 9±4.5 RE .

appear to be orientated almost parallel to the NMM, with only minimal deflections away from this.

The northern hemisphere of the recovery phase (Figure 5.11) provides very little data beyond 10 RE. Earthward flows at local midnight are near parallel to the NMM with the exception of one bin. In the dusk hemisphere, ion flows are deflected duskward. There is only one bin populated in the dawn hemisphere and its vector is directed dawnward. Tailward flows at local midnight are near parallel to the NMM with the exception of one bin. The rest of the vectors in both the dawn and dusk hemispheres are deflected towards local midnight. The southern hemisphere (Figure 5.12) provides much better coverage, although almost no data is present beyond Y = 10 RE. On either side of local midnight, earthward flows throughout are generally directed towards the flanks with the exception of one bin. Tailward flows again show strong deflections towards local midnight throughout again with the exception of one bin.

Overall when looking at the outer planes, the take-home message is that across all the substorm phases earthward flows consistently show deflections towards the dawn and dusk flanks while tailward flows tend to present deflections towards local midnight.

Figure 5.12: Ion velocity plots displaying tailward (left) and earthward (right) plasma

flow during the substorm recovery phase in the XY plane where Z = -9±4.5 RE .

5.3.3 Summary

When looking at all XY planes across all of the substorm phases, it was found that the dawn hemisphere generally has a greater spatial coverage than the dusk hemisphere in each plot. Across all planes and substorm phases, earthward flows had a higher occur- rence rate than that of tailward ion flows. When just considering the equatorial plane, earthward flow dominates across all substorm phases with the recovery phase possessing the largest number of sample counts, followed by the expansion phase and finally the growth phase. When looking at previous studies, these results agreed with Kissinger et al.(2012), but did not agree withJuusola et al.(2011) who found there to be slightly more fast earthward flows in the growth phase than the expansion phase. The difference in results between the three studies could well be to do with how the data has been sam- pled and presented. Juusolas study used smaller spatial bins and an aberrated coordinate system, where both this chapters study and Kissingers did not. When looking at the data distribution for each individual substorm phase, it was found that the growth phase had a larger proportion of earthward flow than tailward flow occurrence. This was due to the heightened rate of dayside reconnection prevalent throughout this phase, which creates a flux pileup in the near-Earth magnetotail. Tailward flows are proportionally less frequent due to the lower rate of nightside reconnection in comparison to the rate of dayside reconnection. The expansion phase is the most balanced phase with regards

to flow direction and has the largest proportion of tailward flows across all phases. This is likely due to the rapid increase of the rate of nightside reconnection, which acts to increase the rate of tailward flows as some of the newly reconnected magnetic field lines are returned to the solar wind. Finally, the recovery phase has the largest proportion of earthward flow in comparison with the other two substorm phases. The likely reason for this result is that in this phase, the mid-tail region plasma sheet suddenly thickens with fast plasma flows at the plasma sheet boundary a region which is almost certainly included in the wider bins utilised in this study.

Across all planes and substorm phases, velocity in the earthward direction dominates with Vxy across all planes in both flow directions is 1.5 km s−1 during the growth phase, 0.6 km s−1 during the expansion phase and 1.7 km s−1 during the recovery phase. In the equatorial plane for each substorm phase, the earthward and tailward velocities are generally higher than the outer planes, with weighted-mean earthward-flow velocity dominating; Vxy is 5.3 km s−1 during the growth phase, 3.9 km s−1 during the expansion phase and 7.0 km s−1 during the recovery phase. When looking at earthward and tailward flows separately across all planes and substorm phases, it was found that the average earthward flow velocity was generally smaller than the average tailward flow velocity in the expansion and recovery phases. The growth phase has the slowest average earthward flows (8.7 km s−1_{) and tailward flows (8.0 km s}−1_{), the expansion phase has}

slightly faster average earthward flows (9.5 km s−1 and tailward flows (10.0 km s−1) than the growth phase, and the recovery phase has equally fast average earthward flows (9.5 km s−1) but the fastest tailward flows (10.4 km s−1). When only considering the equatorial plane, the growth phase still has the slowest average earthward and tailward ion flow velocity (14.6 km s−1 and 12.0 km s−1 respectively), but now the expansion phase has the fastest average tailward ion flow (16.5 km s−1) and a faster average earthward ion flow velocity than the growth phase (16.0 km s−1_{), and the recovery}

phase has the fastest average earthward ion flow velocity (16.8 km s−1) and a slightly slower average tailward flow velocity (15.8 km s−1). These earthward average-velocity substorm-distribution results were in agreement with Kissinger et al. (2012). However they were slightly different to the results found byJuusola et al.(2011), who found that the expansion phase had the largest average earthward velocity, followed by the recovery phase and finally the growth phase. This disagreement could be due to the way that data was binned and organised as there was only 1% difference between the amount of slow flows Juusola found in the expansion and recovery phases. In general, all three studies agree on the very low average velocities throughout the substorm event, further highlighting that the explosive energetic nature of substorms mostly consist of slow ion flows, interspersed with short duration fast flow events.

The final area of discussion was the observation of the average ion flow directions at the equatorial and outer planes for each substorm phase. In the equatorial plane, the growth phase earthward-flow pattern presenting some dawn-dusk asymmetry as well as appearing to be the most disordered substorm phase. The expansion phase appeared to be a little more ordered than the growth phase and possessed further evidence of dawn-dusk asymmetry. There were more deflections in the dusk hemisphere than in the dawn hemisphere were flows were predominantly sunward. The results here were generally more ordered than the equivalent results in the Kissinger et al.(2012) study. The earthward flows during the recovery phase were the most ordered of the three substorm phases, with the dawn hemisphere presenting mostly sunward ion flows with slight dawnward deflections. The dusk also showed mostly sunward flows, but this time with slightly duskward deflections. Still looking a the equatorial plane, tailward flows in the growth phase were found to be gently deflected at the dawn flank towards local midnight. In the dusk hemisphere, within 15 RE from Earth, the flow is predominantly anti-sunward with only three bins showing deviation from this. Beyond 15 RE from Earth, flows quickly turned towards local midnight. The expansion phase again appeared to be more ordered than the growth phase, although in the dawn hemisphere, deflections of the tailward flows were consistently directed towards the local midnight. In the dusk hemisphere, again a mix of anti-sunward and strong deflections towards local midnight were observed. Finally, the recovery phase appeared to be the most ordered substorm phase. In the dawn hemisphere there is clear split between ions flows being somewhat parallel to the NMM and others deflected towards the NMM. At local midnight, the flow direction is mainly deflected towards the dawn flank beyond 15 RE. In the dusk hemisphere, strong deflections towards local midnight are present throughout. Looking at the outer planes, away from the equatorial plane, the spatial coverage was severely limited and as such it was difficult to make specific meaningful conclusions for each substorm phase. Overall when looking at the outer planes, across all the substorm phases earthward flows consistently show deflections towards the dawn and dusk flanks while tailward flows tend to present deflections towards local midnight.