Average Thickness of Magnetosheath Upstream of Magnetic Clouds at 1 AU versus Solar Longitude of Source

Abstract

Starting with a large number (N = 100) of Wind magnetic clouds (MCs) and applying necessary restrictions, we find a proper set of N = 29 to investigate the average ecliptic plane projection of the upstream magnetosheath thickness as a function of the longi- tude of the solar source of the MCs, for those cases of MCs having upstream shock waves. A few of the obvious restrictions on the full set of MCs are the need for there to exist a driven upstream shock wave, knowledge of the MC's solar source, and restriction to only MCs of low axial latitudes. The analysis required splitting this set into two subsets according to av- erage magnetosheath speed: slow/average (300 - 500 km s −1 ) and fast (500 - 1100 km s −1 ) speeds. Only the fast set gives plausible results, where the estimated magnetosheath thick- ness (�S) goes from 0.042 to 0.079 AU (at 1 AU) over the longitude sector of 0° (adjusted source-center longitude of the average magnetic cloud) to 40° off center (East or West), based on N = 11 appropriate cases. These estimates are well determined with a sigma ( σ) for the fit of 0.0055 AU, where σ is effectively the same as √ (chi-squared) for the appro- priate quadratic fit. The associated linear correlation coefficient forS versus |Longitude| was very good (c.c. = 0.93) for the fast range, andS at 60° longitude is extrapolated to be 2.7 times the value at 0°. For the slower speeds we obtain the surprising result thatS is typically more-or-less constant at 0.040 ± 0.013 AU at all longitudes, indicating that the MC as a driver, when moving close to the normal solar wind speed, has little influence on magnetosheath thickness. In some cases, the correct choice between two candidate solar- source longitudes for a fast MC might be made by noting the value of the observedS just