MIST

By Andy Smith (Mullard Space Science Laboratory - University College London)

Large variability in the Earth’s magnetic field can induce anomalous and damaging currents in power systems and pipelines.  It is crucial that we understand and can predict the processes responsible.  In this work we quantified how Sudden Commencements (SCs) contribute to creating large rates of change of the surface magnetic field.  SCs are caused by the impact of solar wind pressure pulses, e.g. interplanetary shocks, on the Earth’s magnetosphere.  They represent one of the more reliably forecastable forms of space weather, where the driving solar wind structure can be observed upstream of the Earth prior to its arrival.

We found that SCs are related to enhanced rates of change of the ground magnetic field (R).  The Figure below shows the fraction of R in excess of 50 nTmin-1 that is related to SCs, as a function of latitude.  The top panel shows the percentage observed during the SCs themselves.  This maximises at around 20 – 30% at low latitudes, but drops to <1% by around 55° as other processes become fractionally more important at generating large R.

SCs often precede further magnetospheric activity, such as geomagnetic storms and substorms.  The lower panel below shows the statistics for the time period during SCs, while also including the 24 hours that follow.  This extended period can be seen to account for around 75% of large R (in excess of 50 nTmin^-1) at locations below ~55°.

This work has shown that SCs are an important source of potentially hazardous magnetic field perturbations, and proportionally they are more important at mid-to-low latitudes.  Usefully, SCs also provide a 24 hour window within which the majority (~75%) of large rates of change of the field are observed, below ~55 degrees latitude.

Figure 1: The percentage of observations of R ≥ 50 nT min⁻¹ (1996 - 2016) that can be related to SCs as a function of magnetic latitude. The rows represent the data obtained during the SCs themselves (i), 
and the data inclusive of 24 hours following the SC (ii).

Please see the paper for full details:

Smith, A. W., Forsyth, C., Rae, I. J., Rodger, C. J., & Freeman, M. P. (2021). The Impact of Sudden Commencements on Ground Magnetic Field Variability: Immediate and Delayed Consequences. Space Weather, 19, e2021SW002764. https://doi.org/10.1029/2021SW002764

By Laura Fryer (University of Southampton)

The coupling between the Interplanetary Magnetic Field (IMF) and the magnetosphere has been extensively studied over the last few decades. This has been facilitated by the launch of multiple spacecraft, such as ESA’s Cluster mission (Escoubet et al 2001), which probes different regions of the Earth's magnetosphere. There have been many studies dedicated to understanding the response of the magnetosphere during more turbulent southward orientated IMF conditions, however, there is still great uncertainty in our understanding of how the magnetosphere, particularly the magnetotail, responds to northward IMF. In general, the lobes in the Earth's magnetotail are typically described as having cool, low energy and often low density plasma populations and therefore hot plasma observations are unexpected in these regions of the magnetosphere. Despite this, there have been a small number of studies reporting energetic plasma populations in the lobes during northward IMF conditions (Huang et al 1987, Shi et al 2013, Fear et al 2014).

We present three case studies which show hot plasma embedded in the lobes of the magnetosphere. For two of these case studies, simultaneous observations of the plasma sheet confirmed that the energies observed within the lobe were directly comparable in magnitude to the populations in the plasma sheet. In addition to this, we observed plasma characteristics which indicated that the plasma is likely to be on closed field lines (evidenced by electron pitch angle distributions and variation in motion of the spacecraft). Tracing the footprint of these field lines to ionospheric altitudes revealed that in each Event, the footprint intersected with a transpolar arc. An example of this for Event 2 can be seen in Figure 1, which shows the footprint of each of the Cluster spacecraft in the tetrahedron, intersecting with a transpolar arc. This provided further evidence to suggest that the energetic plasma was likely to form on closed field lines and could be explained well by the result of recent magnetotail reconnection during northward IMF conditions, a mechanism proposed by Milan et al 2005 to explain the formation of transpolar arcs.

Figure 1: SSUSI (DMSP-F16) FUV auroral observations from the Northern Hemisphere. The panels show the images taken from 16:06 UT to 19:28 UT for Event 2. The data is plotted in AACGM coordinates (magnetic latitude, MLT). The top three images are repeated in the bot-tom row but overplotted with the footprints of Cluster 1, 2, 3 and 4. This has been traced using the T96 model (Tsyganenko, 1996) to an altitude of 120km and are represented by black, red, green and blue circles respectively.

Please see the paper for full details:

Fryer, L. J., Fear, R. C., Coxon, J. C., & Gingell, I. L. (2021). Observations of closed magnetic flux embedded in the lobes during periods of northward IMF. Journal of Geophysical Research: Space Physics, 126, e2021JA029281. https://doi.org/10.1029/2021JA029281 

By Maria-Theresia Walach (Lancaster University)

Geomagnetic storms are a global electrodynamic phenomenon, during which the ring current is loaded with energy. The coupled magnetospheric/ionospheric system responds during a geomagnetic storm and the dynamics of the system can be monitored using measurements of the high-latitude ionosphere. Ordering data from the Super Dual Auroral Radar Network (SuperDARN) by geomagnetic storm phase, we produce convection maps for a geomagnetic storm, which allow us to discern changes that occur in association with the development of the storm phases. We utilize principal component analysis to identify and quantify the primary electric potential morphologies during geomagnetic storms. Along with information on the size of the patterns, the first six eigenvectors provide over ∼80% of the variability in the morphology, providing us with a robust analysis tool to quantify the main changes in the patterns. Studying the first six eigenvectors and their eigenvalues shows that the primary changes in the morphologies with respect to storm phase are the convection potential enhancing and the dayside throat rotating from pointing toward the early afternoon sector to being more sunward aligned during the main phase of the storm. We find that the ionospheric electric potential increases through the main phase and then decreases once the recovery phase begins. Furthermore, we find that a two‐cell convection pattern is dominant throughout and that the dusk cell is overall stronger than the dawn cell.

The figure shows the mean electrostatic potential, followed by the first six eigenvectors. These show the most common components that can be used to make up the ionospheric convection patterns during geomagnetic storms.

Figure Caption: Ionospheric electric field component patterns showing the mean for geomagnetic storms (top left), followed by the patterns corresponding to the first six eigenvectors of the Principal Component Analysis. Each pattern is centered on the geomagnetic pole, with 12:00 magnetic local time pointing toward the top of the page, and dusk toward the left. Lines of geomagnetic latitudes are indicated from 40° to 90° by the dashed gray circles.

Animation Caption: Average convection patterns from SuperDARN data for geomagnetic storm main phase

Please see the paper for full details: Walach, M.‐T.,  Grocott, A., &  Milan, S. E. (2021).  Average ionospheric electric field morphologies during geomagnetic storm phases. Journal of Geophysical Research: Space Physics,  126, e2020JA028512. https://doi.org/10.1029/2020JA028512

By Lauren Orr (University of Warwick)

Geomagnetic substorms are a global magnetospheric reconfiguration, during which energy is abruptly transported to the ionosphere. Central to this are the auroral electrojets, large-scale ionospheric currents that are part of a larger three-dimensional system, the substorm current wedge. Many, often conflicting, magnetospheric reconfiguration scenarios have been proposed to describe the substorm current wedge evolution and structure. We have used well-established network science techniques to analyze data from >100 ground-based magnetometers operated by the SuperMAG collaboration¹.  We translated this data into a time-varying directed network, based on canonical cross-correlation of the vector magnetic field perturbations measured at each magnetometer pair^2,3 and performed community detection on the network⁴. Communities are locally dense but globally sparse groups of connections in the network, identifying emerging coherent patterns in the current system as the substorm evolves. Analysis of 41 substorms exhibit robust structural change from many small, uncorrelated current systems before substorm onset, to a large spatially-extended coherent system, between 10-20 minutes after onset. We interpret this as strong indication that the auroral electrojet system during substorm expansions is inherently a large-scale phenomenon and is not solely due to many meso-scale wedgelets.

Figure 1: The community structure of a substorm on the 16/03/1997. The abscissa of all panels is normalized time, where t'=0 is onset (dashed green line) and t'=30 (dashed purple line) is the time of maximum auroral bulge expansion. Panels 1-2 plots individual communities as circles where the size of the circle reflects the  number of connections within the community. The ordinate plots the mean MLT/MLAT of the community, and the color indicates the proportion of connections with each time lag, |τ_c|. The dashed lines overplotted are the edges of the auroral bulge (MLT) and the onset location (MLAT), found from auroral images. Panel 3 shows snapshots of the community structure at time points t’=0 (onset), t’=10 and t’=20, overplotted on maps provided by SuperMAG¹. There is a clear shift from many, small uncorrelated communities before onset to one large correlated system half way through the expansion phase.

Please see the paper for full details:

Orr, L., Chapman, S.C., Gjerloev, J.W. et al. Network community structure of substorms using SuperMAG magnetometers. Nat Commun 12, 1842 (2021). https://doi.org/10.1038/s41467-021-22112-4