  In the standard model of magnetic reconnection, both ions and electrons
couple to the newly reconnected magnetic field lines and are ejected away from
the reconnection diffusion region in the form of bidirectional burst ion and
electron jets. Recent observations propose a new model: electron only magnetic
reconnection without ion coupling in electron scale current sheet. Based on the
data from Magnetospheric Multiscale (MMS) Mission, we observe a long extension
inner electron diffusion region (EDR) at least 40 di away from the X line at
the terrestrial Magnetopause, implying that the extension of EDR is much longer
than the prediction of the theory and simulations. This inner EDR is embedded
in an ion scale current sheet (the width of 4 di, di is ion inertial length).
However, such ongoing magnetic reconnection was not accompanied with burst ion
outflow, implying the presence of electron only reconnection in ion scale
current sheet. Our observations present new challenge for understanding the
model of standard magnetic reconnection and electron only reconnection model in
electron scale current sheet.
