The dawn‐dusk length of the X line in the near‐Earth magnetotail: Geotail survey in 1994–2014

The dawn‐dusk length of the X line is investigated for magnetic reconnection in association with substorms in the near‐Earth magnetotail on the basis of the 21 year Geotail plasma sheet observations. The X line is identified as a simultaneous plasma flow V x and magnetic field B z reversal inside the ion‐electron decoupling region of magnetic reconnection. Forty‐four X lines can be found at X GSM  = −20 to −31  R E in the magnetotail. The X line length is estimated on the basis of occurrence of magnetic reconnection. Characteristics of flows and fields are investigated for magnetic reconnection and tailward flow events to verify the obtained X line length. The dawn‐dusk length of the X line is most likely 6  R E with its center in the premidnight sector for moderate substorms. Hence, the magnetic reconnection site is mapped to approximately 1 h local time sector in the auroral ionosphere. The dawn‐dusk length of the X line is extended mainly dawnward for larger substorms.