Large increase in stratospheric NO2 content has been observed during the 2003–2004 Arctic winter. The first one, in early November 2003 is well documented and is due to a strong solar protons event. A second event occurred on January 22, 2004, leading to a large amount of NO2 in the lower mesosphere. This second event can be analyzed using data from nighttime satellite measurements performed by the GOMOS and MIPAS instruments onboard Envisat, and by ground based column measurements performed by SAOZ. It seems that in-situ production of NO2 is located at an altitude of around 60 km associated with the precipitation of electrons with energy of a few hundred keV. These electrons originated from the high latitudes magnetosphere, and are associated with a solar coronal mass ejection. Therefore, a particular nighttime chemistry in the lower mesosphere is proposed to explain the measurements.