Observations of nightside magnetic reconnection during substorm growth and expansion phases

Abstract

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.