Ionospheric altitude profiles in the main ionospheric trough as observed by field-aligned EISCAT incoherent scatter radar observations

► Electron density profiles in the main trough were studded using EISCAT observations. ► A hmF2 increase in the trough and a hmF2 decrease in the polar wall were observed. ► Such variations are typical of narrow troughs but not of wide ones. ►The topside effective scale height of Ne(h) increases in the trough minimum. ► Flat and rounded Ne(h) in narrow troughs are due to strong upward flow of plasma.