FilterK: A new outlier detection method for k-means clustering of physical activity.

In this paper, a new algorithm denoted as FilterK is proposed for improving the purity of k-means derived physical activity clusters by reducing outlier influence. We applied it to physical activity data obtained with body-worn accelerometers and clustered using k-means. We compared its performance with three existing outlier detection methods: Local Outlier Factor, Isolation Forests and KNN using the ground truth (class labels), average cluster and event purity (ACEP). FilterK provided comparable gains in ACEP (0.581 â†’ 0.596 compared to 0.580-0.617) whilst removing a lower number of outliers than the other methods (4% total dataset size vs 10% to achieve this ACEP). The main focus of our new outlier detection method is to improve the cluster purities of physical activity accelerometer data, but we also suggest it may be potentially applied to other types of dataset captured by k-means clustering. We demonstrate our method using a k-means model trained on two independent accelerometer datasets (training n = 90) and re-applied to an independent dataset (test n = 41). Labelled physical activities include lying down, sitting, standing, household chores, walking (laboratory and non-laboratory based), stairs and running. This type of clustering algorithm could be used to assist with identifying optimal physical activity patterns for health.

Coronal and heliospheric magnetic flux circulation and its relation to open solar flux evolution.

Solar cycle 24 is notable for three features that can be found in previous cycles but which have been unusually prominent: (1) sunspot activity was considerably greater in the northern/southern hemisphere during the rising/declining phase; (2) accumulation of open solar flux (OSF) during the rising phase was modest, but rapid in the early declining phase; (3) the heliospheric current sheet (HCS) tilt showed large fluctuations. We show that these features had a major influence on the progression of the cycle. All flux emergence causes a rise then a fall in OSF, but only OSF with foot points in opposing hemispheres progresses the solar cycle via the evolution of the polar fields. Emergence in one hemisphere, or symmetric emergence without some form of foot point exchange across the heliographic equator, causes poleward migrating fields of both polarities in one or both (respectively) hemispheres which temporarily enhance OSF but do not advance the polar field cycle. The heliospheric field observed near Mercury and Earth reflects the asymmetries in emergence. Using magnetograms, we find evidence that the poleward magnetic flux transport (of both polarities) is modulated by the HCS tilt, revealing an effect on OSF loss rate. The declining phase rise in OSF was caused by strong emergence in the southern hemisphere with an anomalously low HCS tilt. This implies the recent fall in the southern polar field will be sustained and that the peak OSF has limited implications for the polar field at the next sunspot minimum and hence for the amplitude of cycle 25.

A statistical survey of ultralow-frequency wave power and polarization in the Hermean magnetosphere.

We present a statistical survey of ultralow-frequency wave activity within the Hermean magnetosphere using the entire MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data set. This study is focused upon wave activity with frequencies <0.5 Hz, typically below local ion gyrofrequencies, in order to determine if field line resonances similar to those observed in the terrestrial magnetosphere may be present. Wave activity is mapped to the magnetic equatorial plane of the magnetosphere and to magnetic latitude and local times on Mercury using the KT14 magnetic field model. Wave power mapped to the planetary surface indicates the average location of the polar cap boundary. Compressional wave power is dominant throughout most of the magnetosphere, while azimuthal wave power close to the dayside magnetopause provides evidence that interactions between the magnetosheath and the magnetopause such as the Kelvin-Helmholtz instability may be driving wave activity. Further evidence of this is found in the average wave polarization: left-handed polarized waves dominate the dawnside magnetosphere, while right-handed polarized waves dominate the duskside. A possible field line resonance event is also presented, where a time-of-flight calculation is used to provide an estimated local plasma mass density of ∼240 amu cm-3.