Nugget: Are steady magnetospheric convection events prolonged substorms?

By Maria-Theresia Walach, Department of Physics and Astronomy, University of Leicester, Leicester, UK

The large scale convection of magnetic flux within the Earth’s magnetosphere due to reconnection, also known as the Dungey cycle [Dungey, 1961; 1963], is partially driven by the solar wind. During southward IMF reconnection at the subsolar magnetopause opens flux, which is then added to the magnetotail. Depending on the strength of solar wind-driving, the magnetospheric response can be delayed, episodic or prolonged, also known as “magnetospheric modes” [e.g. Pulkkinen et al., 2007].

Walach and Milan [2015] produced a statistical analysis of the event progression of steady magnetic convection events (intervals where the dayside reconnection is balanced by nightside reconnection [e.g. DeJong et al., 2008]), substorms (dominant dayside reconnection is followed by a delayed interval of dominant nightside reconnection [e.g. Baker et al., 1996]), and sawtooth events (signatures appearing to be quasi-periodic and quasi-global substorms [e.g. Henderson, 2004]). Superposed epoch analyses show that 58% of the studied steady magnetospheric convection events are part of prolonged substorms, where dayside reconnection is at first dominant. Then nightside reconnection is initiated as part of a substorm, but as the solar wind-driving continues the Earth’s magnetosphere then progresses into a state of steady magnetospheric convection, after which the substorm recovery continues.

Walach, M.-T., S. E. Milan (2015), J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020631.

walach nugget

Superposed epoch analysis of substorms (red), sawtooth events (orange), steady magnetospheric convection events with preceding substorms (blue) and steady magnetospheric convection events without preceding substorms (green). The onset of the steady magnetospheric convection events with preceding substorms has been shifted to match the preceding substorm onset. The time of the event duration for the steady magnetospheric convection events in superposed epoch analyses in the right column has been normalised.

Nugget: Statistical characterisation of the growth and spatial scales of the substorm onset arc

By Nadine Kalmoni, UCL Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, UK

During southward IMF reconnection on the dayside leads to a build up of magnetic energy in the tail. As flux is piled into the tail the configuration becomes unstable leading to an explosive release in magnetic energy, termed a substorm. The rearrangement of the magnetic field is accompanied by highly dynamic substorm aurora.

The relatively high temporal and spatial resolution of the THEMIS mission All Sky Imagers have allowed recent observations of small scale azimuthal structures, auroral beads, which form in the minutes leading to auroral onset [e.g. Rae et al., 2009]. Conjugate observations in the Northern and Southern hemisphere suggest that the beads have a common magnetotail driver and are the ionospheric signature of a magnetospheric instability [Motoba et al., 2012].

Kalmoni et al. [2015] statistically analyse the growth and spatial scales of clear signatures of auroral beads observed in the minutes leading to substorm onset. The statistical observations are compared with the Shear-Flow Ballooning Instability (SFBI) [Voronkov et al., 1997] and the Cross-Field Current Instability [Lui, 2004 and references therein] which have both been proposed to play a role in substorm onset. Our observations conclude that the SFBI initiated in the near-Earth plasma sheet is the most likely explanation.

Kalmoni, N. M. E., I. J. Rae, C. E. J. Watt, K. R. Murphy, C. Forsyth, and C. J. Owen (2015), J. Geophys. Res. Space Physics, 120, doi:10.1002/2015JA021470.

kalmoni nugget

Normalised growth rate as a function of spatial scale in comparison to (a) the Cross-Field Current Instability for inner-edge and mid-tail plasma sheet parameters and (b) the Shear-Flow Ballooning Instability for varying shear-flow widths.