Observations of nightside magnetic reconnection during substorm growth and expansion phases
The temporal and spatial variability of nightside magnetic reconnection is described using two-dimensional ionospheric measurements during the growth phase and early expansion phase of a single substorm. Two techniques (A and B) are used to address both the localized (across ∼15° longitude at ∼2300 magnetic local time (MLT)) and the large-scale reconnection rate, using ionospheric data that provides the component of the electric field tangential to the polar cap boundary (PCB) in the stationary boundary frame. Technique A uses localized high-resolution measurements derived from 630 nm all-sky imager data at Rankin Inlet and ionospheric convection vectors obtained from line-of-sight velocity data from the Kapuskasing and Saskatoon Super Dual Auroral Radar Network (SuperDARN) HF radars. Technique B uses lower resolution global measurements, obtained by combining Polar VIS imager data and a velocity field derived using SuperDARN global convection mapping. A third technique (C) estimates the nightside reconnection potential from the dayside reconnection potential and the variation in the polar cap area. Technique A reveals standing wave-like variation of period ∼16 min in the reconnection rate in the late growth phase (spatial noncontinuity in the x-line). The localized measurements of the reconnection rate vary between 0 and 50 mV/m during both the growth and expansion phases. Technique B shows the expansion of the x-line toward the duskside during the first 15 min of the expansion phase, from a width of ∼4 hours of MLT to ∼7 hours MLT.