Magnetotail reconnection rate during magnetospheric substorms

The magnetotail reconnection rate during substorms is studied using data taken by the Sondrestrom incoherent scatter radar during an ensemble of 24 substorms. A superposed epoch analysis indicates that magnetotail reconnection increases marginally significantly above its average level near the time of substorm expansion phase onset. Our data set shows this increase with 87% confidence within +/- 5 min of onset and with greater than 99% confidence at 10 +/- 5 min after onset. However, the median reconnection rate measurement does not begin to increase until 20 min after onset. This effect is shown to be due to a spatial variation in the development of magnetotail reconnection during substorms; magnetotail reconnection increases very soon after onset in a small magnetic local time sector near midnight, while over most of the separatrix, the increase in the reconnection rate is delayed. The average delay in the local time sectors greater that 1.5 hours from midnight is approximately 20 min. We assign functional forms to the average dependence of the magnetotail reconnection rate on magnetic local time, on the interplanetary magnetic field, and on substorm activity, and analyze the generalized linear combination of these functions. The resulting expression consists of the sum of the average interplanetary magnetic field and substorm dependence of the reconnection rate gated by the average spatial variation and accounts for 36% of the variance in the reconnection rate (0.55 linear correlation), which is most of the low-frequency (f < 0.5 h(-1)) variation. The residual variance in the reconnection rate is mostly noise at a level consistent with the reconnection rate measurement uncertainty.