2.5 Results
2.5.5 Varying the driving current
A faster rise in current corresponds to a rapid build up of expansion forces and is expected to accelerate the plasma quickly. A slower rise in current corresponds to the slower build-up of expansion forces. By adjusting Lextra and main bank charging voltage, it is possible to modify the slope of the driving current while preserving
(a) (b)
Figure 2.14: (a) Dierent current proles for the NS conguration. Blue represents time before the plasma reaches the magnetic probe and green represents when the plasma is at the magnetic probes. (b) Average velocities before the magnetic probes (dark blue) and average velocities during the magnetic probe (dark green). The light green represents velocity just before the magnetic probes assuming the plasma starts in quasi-static equilibrium and experiences constant acceleration
peak current. Figure 2.14 (a) shows the NS current prole for three dierent Lextra congurations: None, Medium, Maximum5.
Since the magnetic probe array comprises four clusters at known locations, the average velocity of the plasma can be obtained by tracking persistent probe features (Sec. G.2 and Figs. 2.10 (d) and (e)). If the initial height of the plasma apex is known, the average velocity from plasma formation to the rst magnetic probe at z = 17.5 cm can be also calculated. Camera images show that application of strapping eld results in a smaller initial plasma than if no eld were applied, consistent with the estimates using inductance. We use z = 6 cm and z = 4 cm as the initial plasma apex position for NS and IS congurations, respectively. Since the plasma starts at in quasi-static equilibrium, and Eq. 2.24 predicts constant acceleration, the velocity at the rst probe is approximately double the average velocity.
Figure 2.14 (b) shows the average velocities for None, Medium, and Max Lextra congurations when no strapping eld is applied. When no extra inductance is ap- plied, the average plasma velocity before the rst magnetic probe (dark blue) is greater than the corresponding average velocities when inductance is added. Nevertheless, all three congurations exhibit the same average velocity at the probes (green). This sug-
5The data from the previous sections are taken with the MaximumLextra conguration.
(a) (b)
Figure 2.15: (a) Dierent current proles for the IS conguration. Blue represents time before the plasma reaches the magnetic probe and green represents when the plasma is at the magnetic probes. (b) Average velocities before the magnetic probes (dark blue) and average velocities during the magnetic probe (dark green). The light green represents velocity just before the magnetic probes assuming the plasma starts in quasi-static equilibrium and experiences constant acceleration
gests that the Maximum and Medium congurations undergo weaker acceleration over longer periods of time. The estimated velocities using Eq. 2.24 (light blue) match the Maximum conguration quantitatively, but are too large for the Medium and None conguration, suggesting that the constant acceleration assumption eventually breaks down. One of the requirements for constant acceleration is linearly increasing current. The current traces in Fig. 2.14 (a) is colored blue before the plasma reaches the magnetic probe, green when the plasma is at the four magnetic probes, and black when the plasma passes the magnetic probes. The current in the None conguration is falling when the plasma reaches the probes, so Eq. 2.24 no longer applies. Simi- larly, the current for the Medium conguration is about to peak so Eq. 2.24 may not work as well. Equation 2.24 is expected to apply for the Maximum conguration up to 8µs, consistent Fig. 2.12 (c), which shows decreased acceleration at 8 µs and even deceleration at later times.
Figure 2.15 shows the threeLextracongurations when IS strapping eld is applied.
The strapping eld slows down the plasma so that the current traces (Fig. 2.15 (a)) are past the peak by the time the plasma reaches the magnetic probe. Figure 2.15 (b) shows the average velocities before and at the probe. Both Medium and Maximum Lextra congurations suggest a slow rise to fast eruption conguration. As described
earlier, the enhanced acceleration is attributed to the decay of the strapping eld and the peaking of the current trace when the plasma moves past the peak strapping eld region. The current rises too quickly in the None conguration, accelerating the plasma past the strapping coils. While there is an increase in velocity at the probe, this conguration does not reproduce a slow rise follow by rapid acceleration.
The plasma velocities at the probes in Fig. 2.15 (b) are also larger than the calculated velocities reported for IS conguration in Fig. 2.12. This suggests that the magnetic ux rope may be moving faster in IS conguration than the bright high density plasma. Previous works [17, 98] suggest that the magnetic ux rope (the current channel) is wider than the bright high density plasma from camera images.
When no strapping eld is applied, both the high density region and the ux rope are expected to move with comparable velocities. The IS conguration may accelerate the magnetic ux rope structure to higher velocities than apparent in visible images. This is compatible with CMEs where the bright prominence represents the high-density plasma (the core in Fig. 1.8) located within (but at the bottom of) a larger erupting magnetic ux rope (the cavity in Fig. 1.8).