The twin STEREO spacecraft provide a unique tool to study the temporal evolution of the solar-wind properties in the ecliptic since their longitudinal separation increases with time. We derive the characteristic temporal variations at ~ 1 AU between two different plasma parcels ejected from the same solar source by excluding the spatial variations from our datasets. As part of the onboard IMPACT instrument suite, the SWEA electron experiment provides the solar-wind electron core density at two different heliospheric vantage points. We analyze these density datasets between March and August 2007 and find typical solar minimum conditions. After adjusting for the theoretical time lag between the two spacecraft, we compare the two density datasets. We find that their correlation decreases as the time difference increases between two ejections. The correlation coefficient is about 0. 80 for a time lag of a half day and 0. 65 for two days. These correlation coefficients from the electron core density are somewhat lower than the ones from the proton bulk velocity obtained in an earlier study, though they are still high enough to consider the solar wind as persistent after two days. These quantitative results reflect the variability of the solar-wind properties in space and time, and they might serve as input for solar-wind models.
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Acknowledgements The authors acknowledge contribution from the entire STEREO group, especially the IMPACT and PLASTIC experiment teams. Another thank goes to the colleagues working with SWAVES for cross-calibration. We thank for discussions with Milan Maksimovic, Alexis Rouillard, David J. McComas, Teresa Nieves-Chinchilla, Ioannis Zouganelis and Karim Meziane. This work was supported by a CNES Grant for CESR (CNRS-UPS).