Iain J. Coleman

An ionospheric convection signature of antiparallel reconnection

This paper sets out a critical test of the antiparallel merging hypothesis. For the conflicting theories of antiparallel and subsolar reconnection, we model the location of reconnection regions on the dayside magnetopause, their ionospheric footprints, and the resulting ionospheric convection patterns. We show that antiparallel reconnection, under particular seasonal and solar wind conditions, gives rise to a distinctive ionospheric convection signature. Specifically, around midwinter with a quasi-steady solar wind and IMF Bz < 0 and |By | ∼ |Bz |, we predict equatorward flow in the noon sector with poleward flow either side of noon if the antiparallel merging hypothesis is correct. In contrast, we predict poleward flow in the noon sector in midwinter under these solar wind conditions if the subsolar reconnection hypothesis is correct and in other seasons under both hypotheses. We go on to present radar and spacecraft data for an interval which satisfies the above seasonal and solar wind criteria, demonstrating that the convection signature of antiparallel merging is present. This is not consistent with the subsolar merging hypothesis.

Some comments on transient and steady‐state reconnection at the dayside magnetopause

Reconnection at the dayside magnetopause is the principal method by which energy is transferred from the solar wind into the magnetosphere-ionosphere system. There is still considerable uncertainty as to whether reconnection is transient or quasi-steady. Here we discuss some of the implications of assuming that reconnection occurring in regions where the magnetosheath flow is super-Alfvenic must be transient. We use a simple magnetospheric model to illustrate where on the magnetopause transient reconnection is most likely to occur, and show that the location of these regions is dependent critically upon the dipole tilt angle and the interplanetary magnetic field orientation. Although our idealised examples do not take into account temporal variations of the solar wind conditions, or the influence of the bow shock or magnetosheath, we believe that they demonstrate interesting features. For example, the results suggest that reconnection for northward IMF is almost always likely to be transient.

Fractal reconnection structures on the magnetopause

Simple laminar flow models of the magnetosheath plasma lead to predictions of large-scale reconnection structures on the dayside magnetopause. There is observational evidence from the ionosphere that such structures exist. However, in situ observations of the magnetosheath plasma near the magnetopause indicate that the fluid is highly disordered, with the magnetic field orientation in particular being extremely noisy. This raises the question of how coherent large-scale reconnection structures can be created and persist. This paper shows that, if the apparent noise in the magnetosheath field has the characteristics of fractal turbulence, large-scale structures can persist even when the apparent noise levels in the draped magnetic field are very large. Furthermore, the common observation of reconnection transients is naturally explained by this model.

Anti‐Parallel Reconnection at the Dayside Magnetopause: Ionospheric Signatures and Implications for the Low Latitude Boundary Layer

Reconnection at the dayside magnetopause is of fundamental importance in determining the dynamics and composition of the magnetosphere and the high latitude ionosphere. There are two competing hypotheses for such reconnection, sub-solar and anti-parallel. In this paper, evidence is provided that suggests that anti-parallel reconnection occurs, at least under some circumstances. Some of the consequences of anti-parallel reconnection are considered. These include the absence of a low latitude boundary layer during southward IMF, the relative timing of reconnection events as observed in the ionosphere, the time-dependence of ion outflow and the implications for the composition of the magnetosheath, and the transient nature of reconnection. Some suggestions for tests that would allow differentiation between the anti-parallel and sub-solar reconnection are also made.

Reply to comment by S. M. Petrinec and S. A. Fuselier on “An ionospheric convection signature of antiparallel reconnection”

: magnetic reconnectionmagnetopause, ionosphere, antiparallelCitation: Coleman, I. J., G. Chisham, M. Pinnock, and M. P. Freeman, Reply to comment by S. M. Petrinec and S. A. Fuselier on‘‘An ionospheric convection signature of antiparallel reconnection’’by I. J. Coleman et al., J. Geophys. Res., 108(A5), 1178,doi:10.1029/2002JA009568, 2003.