We present measurements of the density of the Martian atmosphere between 170- and 180-km altitude above the high northern latitudes over a 6-month period in 1998, when the solar cycle was beginning to rise out of its activity minimum. These measurements were made from the observed orbital decay of the Mars Global Surveyor (MGS) spacecraft during its Science Phasing Orbits (SPO) (April to September 1998) using X band Doppler tracking observations. The densities that we retrieve are comparable to model values given by Culp and Stewart <1984>, Stewart <1987>, Mars-GRAM 3.7 <Justus et al., 1996>, and recent Mars Thermospheric Global Circulation Model (MTGCM) simulations <Bougher et al., 2000>. However, the SPO period can be divided into two time periods (separated at LS≈355¿~0¿) that are characterized by significantly different orbit-to-orbit variability that is not predicted by these earlier models. The first time period corresponds to the time during which the MGS orbit perifocus moved toward the north pole while the local solar time was 1000--1100; during this period, orbit-to-orbit variability is 50--70%, and our average measured density at 175 km is 0.018¿0.007 kg km-3 (between 67¿ and 72 ¿N and LS=315¿ to 320¿). The second time period corresponds to the time during which the orbit perifocus moved south from the north pole and the local time was 1700--1730; during this period , orbit-to-orbit variability is 40-20%, and our average measured density at 175 km is 0.024¿0.004 kg km-3 (between 62¿ and 69 ¿N and LS=17¿ to 28¿). For both time periods the observed latitudinal gradient of density on a constant altitude surface exhibited a factor of 3--4 decrease between 60¿ and 90 ¿N. This gradient is comparable to that expected by the polar vortex (high-latitude wind) effect modeled by the MTGCM for solar medium conditions at southern summer solstice <Bougher et al., 2000>. A southern hemisphere dust storm that the MGS Thermal Emission Spectrometer (TES) observed at LS=309¿ is distinguishable in our data set as a 100% rise in density at 180 km above the 70¿ northern latitudes 7 days later (LS=313¿). ¿ 2001 American Geophysical Union