The Electron Reflectometer (ER) on board Mars Global Surveyor measures the energy and angular distributions of solar wind electrons and ionospheric photoelectrons. These data can be used in conjunction with magnetometer data to probe Mars' crustal magnetic field and to study Mars' ionosphere and solar wind interaction. During aerobraking, ionospheric measurements were obtained in the northern hemisphere at high solar zenith angles (SZAs, typically ~78¿). The ionopause was crossed at altitudes ranging from 180 km to over 800 km, with a median of 380 km. The 400-km-altitude polar mapping orbit allows observations at SZAs from 25¿ to 155¿ in both the northern and southern hemispheres. The near-planet ionosphere and magnetotail structure of the night hemisphere is dominated by the presence of intense crustal magnetic fields, which can exceed 200 nT at the spacecraft altitude. Closed field lines anchored to highly elongated crustal sources form magnetic cylinders, which exclude solar wind plasma traveling up the magnetotail. When the spacecraft passes through one of these structures, the ER count rate falls to the instrumental background, representing an electron flux drop of at least two orders of magnitude. A map of these flux dropouts in longitude and latitude closely resembles a map of the crustal magnetic sources. When the crustal magnetic cylinders rotate into sunlight, they fill with ionospheric plasma. Since many of these crustal fields are locally strong enough to stand off the solar wind to altitudes well above 400 km, the ionosphere can extend much higher than would otherwise be possible in the absence of crustal fields. Even weak crustal fields may locally bias the median ionopause altitude, which provides an indirect method of detecting crustal fields using ER observations. ¿ 2001 American Geophysical Union