SEÑALIZACIÓN VERTICAL

The methodology for this chapter’s study, as well as the organisation and visualisation of the data, is predominantly the same as for the first study described in Chapter 4. As well as the figures described in Chapter 4, a set of alternatively binned plots were created and an additional substorm-time parameter was included to remove all data not collected during a substorm interval. The methods used to select the appropriate intervals and to create the new plots are described below.

5.2.1 Determining Substorm Phases

Substorm onset times were obtained from the SuperMAG substorm database (Gjerloev, 2012). The database is a comprehensive list of substorm onset times which have been derived using an automated algorithm used to identify substorm expansion phase onsets from the SML index, the SuperMAG equivalent of the AL index. There are a number of methods which can be used to select substorm onset times. Kissinger et al. (2012) selects onsets visually from auroral indices characterised by a sharp drop in the AL index (Hsu and McPherron, 2012). More recently, Forsyth et al. (2015) provides a quantitative method to determine the times and durations of substorm expansion and recovery phases, as well as possibly determining substorm growth phases too, based on percentiles of the rate of change of auroral electrojet indices.

Substorm event selection was carried out using several temporal parameters from the Kissinger et al. (2012) study. Only isolated substorms were selected, and so in order to ensure no overlapping events were selected, it was required that onset times could not be within ±2.5 hours of another onset time. In the tail seasons between 2001 and 2006, there were found to be 656 isolated substorm events available for analysis. The three substorm phases could then be identified: growth, expansion and recovery. Their phase duration times were again taken from Kissinger et al.(2012). The growth phase interval was set to begin 30 minutes before onset and end of the interval as the onset time. The expansion phase intervals were set to begin at onset and end 30 minutes later.

The recovery phase interval was set to begin 45 minutes after onset and end 120 minutes after the onset time.

5.2.2 Data Selection

As previously mentioned, 656 isolated substorm events were selected for analysis during the six year data window the study planned to use. The relatively low number of events (in comparison to the 8600 substorm events Kissinger et al. (2012) used) combined with the short duration of each substorm phase meant that the amount data available was severely limited. For example, there was a total of 19680 minutes of substorm expansion phase covered by the study, just 0.62% of the total time available, and data could only be collected if they occurred during the four months of the tail season each year, further limiting its availability. From this, it was decided to expand the time frame being used from six years to fourteen years, spanning the entire data availability for the CIS instruments on the Cluster mission. Data for the Cluster 3 spacecraft was collected until the 11 November 2009 under normal operations and was switched off following this date. For the Cluster 1 spacecraft, data was collected under normal operations until April 2011 when it was restricted to only magnetospheric modes. From November 2012, the instrument was reduced to one hour segments of operation, with HIA moment and quality flag data being available until the end of 2014. The inclusion of these additional years of data in the study allowed 1430 substorm events to be analysed, almost three times the original count. Finally it must be noted that the study presented in this chapter is a statistical study and as such, the dataset does contain Bursty Bulk Flow events and other such perturbations.

Interestingly, when the new figures were created using the 14 years of data, the amount of data available for analysis in each plane did not increase proportionally, increasing at most by about 1000 counts, but most planes only saw a very small increase. An investigation was carried out to establish why this was the case, and it turned out that when reducing the data quality flag threshold by one (allowing any data through with a value of two or more) increased the amount of data available substantially, see Table5.1. It was tempting to utilise the lower quality flag value in order to maximise the amount of data available, however, after reading more about the data flags in the Cluster Science Archive (Dandouras and the CIS Team, 2017), where data with a quality flag value of less than three was described to be of inadequate quality, it was decided to maintain the use of data with quality flag values of three or more. However, it was also decided to include the additional years of data into the study.

Table 5.1: The number of days with available data for each substorm phase, collected

by Cluster 1 and 3 when varying the quality flag (QF) value.

Substorm Phase C1. QF >2 C1. QF >1 C3. QF >2 C3. QF> 1

Growth 81 1147 173 898

Expansion 87 1157 175 889

Recovery 97 1129 178 868

Total [days] 265 3433 526 2655

As with Chapter4the magnetosheath removal procedure was also applied to the analysis here in order to remove as much of the contamination from the Earth’s magnetosheath as possible. This study utilises the same cutoff values for the ion density, temperature and distance from the Earth and as such all that is discussed in Section 4.3.2 is also relevant in this chapter.

5.2.3 Alternative Substorm Plots

The limited amount of data available for this study led to a substantial reduction in the spatial coverage available (when compared with the coverage available in Chapter 4), especially for the growth and expansion phases, which have a shorter duration than the recovery phase. This was likely due to a combination of each substorm phase only occurring for a short period of time as well as it being an artefact of the spacecraft orbital trajectory. In order to improve the data coverage show in each of the figures, the original 33RE bins were averaged together in groups of three (z-direction for the XY plane and the y-direction for the XZ plane), creating a new set of bins with the dimensions 3×3× 9 RE. An alternative way to describe this method binning is a collapse of the plots in the z-direction to fill out gaps in the x-y distribution. Three sets of figures were created |Z| ≤13.5 RE and |Y| ≤13.5 RE for the XY and XZ planes respectively, with each figure spanning 9 RE in the relevant direction.

The new spatial parameters used for the alternative substorm plots allow for improved visual clarity, placing more bins within the figures, in essence compressing the region visually.

As the spatial coverage is so limited, many of the bin positions are only occupied in one of the three original planes, and as such the flow velocity and direction is not diluted, and so applying the larger spatial parameters acts to compress the region spatially, allowing for improved visual clarity. Of course there are some bin positions which are doubled up and in these cases, the multiple bins are averaged together. When this occurs, it can become difficult to infer average flow directions accurately since each bin covers such a large area (9 RE out of the plane of view) and as such, the results in these cases should

Phase Plane (Z = nRe) Nearthward Ntailward Ntotal % EF % TF 9 1731 3223 4954 34.9 65.1 Growth 0 10681 5690 16371 65.2 34.8 -9 1862 2048 3910 47.6 52.4 Total 14274 10961 25235 56.6 43.4 9 1636 3799 5435 30.1 69.9 Expansion 0 12693 7564 20257 62.7 37.3 -9 2343 2537 4880 48.0 52.0 Total 16672 13900 30572 54.5 45.5 9 2307 5217 7524 30.7 69.3 Recovery 0 26441 11349 37790 70.0 30.0 -9 4080 4496 8576 47.6 52.4 Total 32828 21062 53890 60.9 39.1

Table 5.2: Sample counts distribution for the XY plane across 3 cuts in the z-direction for each substorm phase. The Z value given is the position on which the plane is centred.

EF represents earthward flow and TF represents tailward flow.

be treated with care, looking at the three constituent planes averaged together to create the alternative substorm plots.