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ACE News #88 - Mar 9, 2005
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Field line merging at the heliospheric current sheet (HCS) and the resulting disconnection of heliospheric magnetic field (HMF) lines from the Sun, has been controversial. The SWEPAM and MAG experiments on ACE have now obtained the first direct evidence for such merging and disconnection from the Sun. As illustrated by the figure on the left above, reconnection at the HCS should create closed field lines sunward and disconnected (from the Sun) field lines anti-sunward of the reconnection site. Near an away/toward HCS a spacecraft to the east of the reconnection site should successively sample: 1) unmerged field lines of away (from the Sun) polarity; 2) merged field lines of away polarity that lie outside the reconnection exhaust (defined by the red dashed lines marked A1 and A2 that pass through the kinks on merged field lines; see ACE News #85); 3) the reconnection exhaust containing the accelerated plasma flow and the field polarity reversal; 4) merged field lines of toward (the Sun) polarity that lie outside the reconnection exhaust; and 5) unmerged field lines of toward polarity. The figure also illustrates the expected changes in solar wind suprathermal electron pitch angle distributions (PADs) in the vicinity of the HCS. Usually those PADs consist of a strahl directed anti-sunward along the HMF and a more isotropic halo , as illustrated at points 1 and 5 in the figure, where we have assumed strahls and halos of unequal intensity on opposite sides of the HCS. From points 2 to 4 the field lines are disconnected from the Sun so that the strahl should be absent there and the halo should be asymmetric, consisting of electrons that originally were all sunward-directed on opposite sides of the HCS.
At least 42 reconnection exhausts in the solar wind have been detected in the ACE data, but only one such event occurred at the HCS. ACE crossed the HCS on 17 September 1998 and from ~03:17:33 to ~03:20:29 UT observed the accelerated plasma flow characteristic of a reconnection exhaust (see ACE News #85). The figure on the right above shows how the suprathermal electron PADs evolved in the vicinity of the HCS on that date. As expected, the strahl switched from being field-aligned to being anti-field-aligned at the HCS. Notably, however, the strahl disappeared within the field reversal region (denoted by red in the figure) and was reduced in intensity for a couple of minutes thereafter. Moreover, the halo population within the exhaust was asymmetric, the PAD between 0° and 90° being essentially identical to that at those PAs after the HCS crossing and the PAD between 90° and 180° being essentially identical to that at those PAs prior to the crossing, as predicted for disconnected field lines. ACE observations of accelerated plasma flow and suprathermal electron PADs during this HCS crossing thus firmly demonstrate that reconnection does at least occasionally occur at the HCS in the solar wind and that such reconnection produces field lines disconnected from the Sun.
Contributed by Jack Gosling and Ruth Skoug of Los Alamos National Laboratory, Dave McComas of Southwest Research Institute and Chuck Smith of the University of New Hampshire. Address comments or questions to firstname.lastname@example.org.
Last modified 9 Mar 2005, by