Automated Bow Shock and Magnetopause Boundary Classification At Saturn
Using Statistics of Magnetic Fields and Particle Flux
Abstract
Statistical studies of the properties of different plasma regions, such
as the magnetosheath and outer magnetosphere found near the boundaries
of planetary magnetospheres, require knowledge of boundary (bow shock
and magnetopause) crossings for purposes of classification. These are
commonly detected by visual inspection of the magnetic field and / or
particle data sampled by the relevant spacecraft. Automation of this
type of activity would thus improve the efficiency of boundary and
region studies, which benefit from large crossing datasets, and could
also have implications for future development of onboard data-processing
protocols in the pre-downlink stage. The Cassini mission at Saturn
(2004-2017) provided an invaluable dataset for testing the viability of
automated boundary classification. The training dataset consists of BS
and MP crossings for the time period 2004 to 2016 (Jackman et al.
(2019)). We have employed a series of techniques which involve
pre-processing the calibrated magnetometer data, unsupervised training
of a LSTM recurrent neural network on magnetometer data to filter
magnetosheath regions where crossings are most likely to be found,
isolating large rotations in magnetic field using minimum variance
analysis (MVA), feature engineering such as magnetic field strength
ratio either side of the field rotation to form a ‘feature vector’ for
each candidate, and finally applying a gradient-boosting
decision-tree-based algorithm to predict the probability that a given
interval of data contains the signature of a bow shock (BS), a
magnetopause (MP), or None (not a boundary crossing). The resulting
model performs better on bow shock events, with a precision (fraction of
true events in the retrieved sample) and recall (fraction of the total
true events which were retrieved) of ~86% and
~90% respectively, as compared to ~50%
and ~68% for the MP. The ongoing work focuses on
augmenting the feature space for improved classification of MP, based on
a magnetic pressure model of MP crossings derived using a local pressure
balance condition (e.g. Pilkington et al. 2015) and using the distinct
energetic particle flux changes across the MP in MIMI data (e.g. Liou et
al. 2021). We expect that these promising new features will help us to
better constrain the retrieval of candidate events which are true MP
crossings.