Distant tail behavior during high speed solar wind streams and magnetic storms

Abstract

We have examined the ISEE 3 distant tail data during three intense (Dst < −100 nT) magnetic storms and have identified the tail response to high‐speed solar wind streams, interplanetary magnetic clouds, and near‐Earth storms. The three storms have a peak Dst ranging from −150 to −220 nT and occur on January 9, February 4, and August 8, 1983. During the storm onsets, the fast solar wind and magnetic field dynamic pressure (B2/8π+ΣnikTi) fluctuations moved the tail across the spacecraft multiple times. The magnetotail is strongly compressed by the outside sheath pressure. The lobe field strength can usually be predicted by the pressure balance. The strongest lobe field magnitude detected is 37 nT during storm main phase on January 10, which is higher than the sheath field by 5 – 10 nT. The sheath plasma pressure accounts for the higher lobe field strengths. However, for the February 4 storm, we find that three tail lobe encounters are not in static balance with sheath pressure. During the storm times, the field magnitudes of the lobe and plasma sheet increase by a factor of 3–5 relative to the quiet time. The temperature and density in both regions also increase by factors of 2–3, but with little plasma β changes, as one would expect. Under the assumption of tail flux conservation, increased sheath pressure implies a reduced tail size. Besides the tail size changes, the location of the nominal tail axis is controlled by solar wind flow orientation. This study shows that more than 70% of tail in‐and‐out events are predicted by either of these external mechanisms (changes of tail size due to the external pressure and the solar wind directional changes). Nine tail plasma sheet jettings and 12 slow‐mode shocks have been detected during the three storms. One remarkable feature of the jettings is very strong earthward flow (up to 1200 km/s) and tailward flow (up to 1500 km/s). The solar wind speed for these events was only ∼900 km/s. Both tail flow events have the highest speeds found to date. The preponderance of such a strong earthward flow indicates that during magnetic storms, magnetic reconnection occurs at locations well beyond the distance of ISEE 3. Through the interface of slow‐mode shocks between the tail lobe and the plasma sheet/boundary layer, magnetic energy is being converted into plasma thermal and kinetic energy by the magnetic merging process. The predicted downstream plasma jetting speed (978 km/s) is consistent with the observations (1000 km/s) in the boundary layer. One surprising feature is that this reconnection process seems to be quite prominent during the storm recovery phase. One possible suggestion is that the dynamics of the distant tail are not at all related to magnetic storms and substorms but is an aftereffect, releasing extra magnetic tail energy by field sloughing via these reconnection events, accompanying the plasma sheet expansion.