First simultaneous observation of medium-scale traveling ionospheric disturbances using an airglow imager and the Arase satellite in the inner magnetosphere

Medium-scale traveling ionospheric disturbance (MSTID) is a phenomenon widely and frequently observed over the ionosphere from high to low latitudes. The spatial scale of MSTID is ~100-1000 km. The MSTIDs generated by the E-region-F-region coupling and Perkins instabilities have mirrored structures at the magnetic conjugate point in both hemispheres [e.g., Otsuka et al., GRL, 2014]. In this case, MSTIDs are caused by the polarization electric field in the ionosphere. However, propagation of this polarization electric field to the magnetosphere have not been identified yet. The MSTIDs observed at high latitude also have oscillating features [e.g., Shiokawa et al., JGR, 2012]. Here we report an observation of the polarization electric field and density fluctuation of a nighttime MSTID in the magnetosphere for the first time. The MSTID event was observed by an airglow imager at Gakona (geographical latitude: 62.39°N, geographical longitude: 214.78°E, magnetic latitude: 63.2°N), Alaska. The Arase satellite passed over the MSTID at 0530-0800 UT (1930-2200 LT) on 3 November 2018 in the inner magnetosphere. This MSTID was observed in 630-nm airglow images propagating westward, and it had a north-south phase surface, a horizontal wavelength of ~165 km, a phase velocity of ~80 m/s, and a period of ~1 hour. These features and values are typical at high latitudes. The footprint of the Arase satellite to the ionosphere crossed almost perpendicularly to the MSTID phase surface. The electric field and electron density observed by the Arase satellite showed a periodic variation associated with the MSTID structure with amplitudes of ~2 mV/m and ~150 cm^-3, respectively. The electric field variations projected to the ionosphere are mainly eastward and westward and are consistent with the direction of the polarization electric field that is expected from the MSTID growth by E×B drift under an eastward ionospheric current. The present observation indicates that the polarization electric field associated with the MSTID in the ionosphere is projected to the magnetosphere and causes plasma density fluctuation in the magnetosphere.