Magnetosheath Contamination - Data Visualisation and Presentation

4.3 Data Visualisation and Presentation

4.3.2 Magnetosheath Contamination

It has been noted that very high velocity plasma flow vectors are present in the raw data set, especially at distances greater where

GSM +ZGSM2 > 12 RE. After some consideration, it was decided that these vectors were likely due to the spacecraft entering and perhaps passing through the magnetosheath and into the solar wind. A series of data cutoffs were established in order to remove the contaminant data. In order to specify which cutoff were needed to only remove sheath data while preserving as much useful data as possible, the Cluster Science Archive quick-look plots were utilised to establish the plasma behaviour within the various magnetospheric regions.

It can be seen in first pane of Figure 4.8 that the magnetic field strength is fairly low. In the top corner of the same figure, it is clear that the spacecraft are situated close to

Figure 4.8: A quick-look plot from the Cluster Science Archive showing a period for

which the Cluster Spacecraft are situated in the terrestrial plasma sheet.

the middle of the X-Z plane. In the second pane of Figure 4.8, it can be seen that the bulk velocity of the ions is fast, around 500 km s−1, and they are also hot, looking at the penultimate panel of Figure 4.8, it can be seen that the average ion energy (related to temperature) is above 10 keV. The electrons in this region are also hot (seventh pane), showing temperature between 1 - 10 keV (Walsh et al., 2011a). From this analysis, it was decided that these properties best represent the plasma sheet (Baumjohann et al., 1989).

It can be seen in plane 6 of Figure4.9that the ion temperatures are roughly alternating between 1 keV and 10 keV at the same time as alternating 100 eV and 1 keV electrons (pane 7). This motion suggests that the spacecraft is alternating between being situated in the sheath and the magnetosphere (Kletzing et al.,2003; Walsh et al., 2011a). The

Figure 4.9: A quick-look plot from the Cluster Science Archive showing a period

for which the Cluster Spacecraft are alternating between being situated in the magnetosheath and magnetosphere.

ion density in pane 5 supports this idea as it is high during periods of low temperature regimes, showing the spacecraft is in the sheath (Baumjohann et al.,1989).

In can be seen in the last plane of Figure 4.10 that between 18:00 and 20:40, sub 100 eV electrons are present at the same time as a fast narrow ion beam (plane 6). This is evidence of the spacecraft being situated in the solar wind. Looking at the latter part of the same two panes, it can be seen that the electron temperatures are a little higher at around 100 eV and the ion distribution is much wider. Couple this with spatial positioning as seen in the top right corner, it is clear that the spacecraft is switching from being in the solar wind to the sheath.

Figure 4.10: A quick-look plot from the Cluster Science Archive showing a period

for which the Cluster Spacecraft are alternating between being situated in the magnetosheath and the solar wind.

From this investigation, the cutoffs to remove the contaminant sheath and solar wind data were selected, all of which must be satisfied:

• Ion density>0.5 particles cm−1

• Ion temperature <0.5 keV

• qY2

GSM +ZGSM2 > 12 RE

It can be seen in Figure4.11, panes A, B, E and F, that the overall ion counts in the plane centred about Z = 3 RE for both the tailward (from panes A - E) and the earthward (from panes B - F) are reduced. The average tailward ion velocity has also decreased

Figure 4.11: A series of plots for plane Z = 3 RE showing the how applying the

magnetosheath removal algorithm affects the results. Panes A-B: Show the tailward and earthward counts respectively calculated before the algorithm is implemented. Panes E- F: Show the tailward and earthward counts respectively calculated after the algorithm is implemented. Panes C-D: Show the tailward and earthward ion velocity vectors respectively calculated before the algorithm is implemented. Panes G-H: Show the tailward and earthward ion velocity vectors respectively calculated after the algorithm

Figure 4.12: A series of plots for plane Z = -9 RE showing the how applying the

from 46.5 km s−1(pane C) to 28.2 km s−1(pane G). Many of the faster flank ion velocity vectors have also been removed too, reducing the maximum bin-average velocity from close to 300 km s−1 to below 100 km s−1. Looking at the earthward ion velocity vectors, it is clear that the fact the two earthward averages are similar (plane-average velocity of 37.7 km s−1 in pane D and 36.7 km s−1 in pane H), indicates that the method has over- whelmingly worked in this plane, though the slight difference between these two figures indicates that a small number of earthward flows (which will not be magnetosheath) have erroneously been removed. The algorithm however is not perfectly accurate, as can be seen in Figure4.12. Looking at panes C and G is is clear that the very large velocity vectors have not been removed. This was most likely due to the spatial parameter as these extremely fast flows occurred within the parameter and as such would not trigger its removal. Decreasing the distance of the spatial parameter was considered, but since this was the only plane for which particularly large vectors were not removed, it was decided it was not worth risking the removal of potentially good data closer in to the Earth. Looking at panes D and H of Figure4.12, a slight increase in the plane-average ion velocity was also found. It is also worth noting that the maximum velocity in pane G actually increases. This is because there was a 2000 km s−1 velocity cutoff originally implemented which was deemed irrelevant once the new magnetosheath removal algorithm was implemented. Overall, the it was deemed successful and as such was used throughout the study.

In document Magnetospheric plasma dynamics:investigating ion and electron flow in the magnetotail (Page 98-104)