Publication Date:
2015-05-19
Description:
Based on Cluster observations, the propagation velocities and normal directions of hot flow anomaly (HFA) boundaries upstream the Earth's bow shock are calculated. Twenty-one young HFAs, which have clear leading and trailing boundaries, were selected and multi-spacecraft timing method considering errors was employed for the investigation. According to the difference in the propagation velocity of the leading and trailing edges, we categorized these events into three groups, namely, contracting, expanding and stable events. The contraction speed is a few tens of kilometers per second for the contracting HFAs, and the expansion speed is tens to more than one hundred kilometers per second for expanding events. For the stable events, the leading and trailing edges propagate at almost the same speed within the error range. We have further investigated what causes them to contract, expand or stay stable by carefully calculating the thermal pressure of the young HFAs which have two distinct ion populations (solar wind beam and reflected flow). It is found that the extreme value of the sum of the magnetic and thermal pressure inside the HFAs compared with that of the nearest point outside of the leading edges is higher for expanding events and lower for contracting events, and there is no significant difference for the stable events, and the total pressure (sum of thermal, magnetic and dynamic pressure) variation has a significant effect on the evolution for most (70%) of the HFAs, which implies that the pressure plays an important role in the evolution of young HFAs.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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