Initiation Criteria For the Onset Of Geomagnetic Substorms Based on Auroral Observations And Electrojet Current Signatures

In recent years, there have been several substorm onset initiation criteria that have been developed, either from auroral observations (many authors), or from auroral electrojet features e.g. Forsyth et. al. 2015, Partamies et. al. 2013, Newell and Gjerloev, 2011, Maimaiti et. al. 2019. We investigate the different criteria using a low order physics model of the magnetosphere called WINDMI (Spencer et. al. 2019). We will compare the model variables with the criteria for substorm onset proposed through examining the AL index, d(SML)/dt, or from observations of auroral brightenings and enhancements.

The WINDMI model uses solar wind and IMF measurements from the ACE spacecraft as input into a system of 8 nonlinear ordinary differential equations. The state variables of the differential equations represent the energy stored in the geomagnetic tail, central plasma sheet, ring current and field aligned currents. The output from the model is the ground based geomagnetic westward auroral electrojet (AL) index, and the Dst index. We can constrain the WINDMI model to trigger substorm events. By forcing the model to be consistent with satellite electric and magnetic field observations, we are able to track the magnetotail energy dynamics, the field aligned current contributions, energy injections into the ring current, and ensure that they are within allowable limits.

In this work, we will show how the WINDMI model is used to analyze isolated substorms and storm time substorms when driven by solar wind activity. The model is constrained to trigger the substorms and establish the magnetospheric conditions that influence substorm dynamics. The timing of onset for each event, the model parameters and the model intermediate state space variables are examined and analyzed. We will compare the model variables and solar wind driven dynamics with the auroral observations and electrojet features.

References:

Forsyth, C., I. J. Rae, J. C. Coxon, M. P. Freeman, C. M. Jackman, J. Gjerloev, and A. N. Fazakerley (2015), A new technique for determining Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), J. Geophys. Res. Space Physics, 120, 10,592–10,606, doi:10.1002/2015JA021343.

N. Partamies, L. Juusola, E. Tanskanen and K. Kauristie, Statistical properties of substorms during different storm and solar cycle phases, Ann. Geophys., 31, 349–358, 2013 www.ann-geophys.net/31/349/2013/ doi:10.5194/angeo-31-349-2013

Newell, P. T., and J. W. Gjerloev (2011), Substorm and magnetosphere characteristic scales inferred from the SuperMAG auroral electrojet indices, J. Geophys. Res., 116, A12232, doi:10.1029/2011JA016936.

Maimaiti, M., Kunduri, B., Ruohoniemi, J. M., Baker, J. B. H., & House, L. L. (2019). A deep learning‑based approach to forecast the onset of magnetic substorms. Space Weather, 17, 1534 1552. https://doi.org/10.1029/2019SW002251.

Spencer, E. A., Srinivas, P., & Vadepu, S. K. (2019). Global energy dynamics during substorms on 9 March 2008 and 26 February 2008 using satellite observations and the WINDMI model. Journal of Geophysical Research: Space Physics, 124, 1698– 1710. https://doi.org/10.1029/2018JA025582