Abstract. Using data from the four Cluster spacecraft we study the separatrix regions of magnetic reconnection sites at the dayside magnetopause under conditions when reconnection is occurring in the magnetopause current layer which separates magnetosheath plasma from the hot magnetospheric plasma sheet. We define the separatrix region as the region between the separatrix -the first field line opened by reconnection -and the reconnection jet (outflow region). We analyze eight separatrix region crossings on the magnetospheric side of the magnetopause and present detailed data for two of the events. We show that characteristic widths of the separatrix regions are of the order of ten ion inertial lengths at the magnetopause. Narrow separatrix regions with widths comparable to a few ion inertial lengths are rare. We show that inside the separatrix region there is a density cavity which sometimes has complex internal structure with multiple density dips. Strong electric fields exist inside the separatrix regions and the electric potential drop across the regions can be up to several kV. On the magnetosheath side of the region there is a density gradient with strong field aligned currents. The observed strong electric fields and currents inside the separatrix region can be important for a local energization of ions and electrons, particularly of ionospheric origin, as well as for magnetosphere-ionosphere coupling.

[1] We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency. Citation: Lindstedt, T., Y. V. Khotyaintsev, A. Vaivads, M. André, H. Nilsson, and M. Waara (2010), Oxygen energization by localized perpendicular electric fields at the cusp boundary, Geophys. Res. Lett., 37, L09103,

Stasiewicz, Nordblad, and Lindstedt Reply: In the preceding Comment [1], Vranjes, Poedts, and Pandey claim that propagation of Alfvén waves involves gyromotion of the particles, and that, as a consequence, these modes cannot develop in highly collisional plasmas such as the solar photosphere. This in turn would imply that the mechanism of coronal heating by alfvenons (dissipative Alfvén solitons) proposed in [2] cannot work.We would like to point out that gyromotion is not in itself a condition for the occurrence of Alfvén waves. In the standard approach, Alfvén modes are obtained as solutions of ideal magnetohydrodynamic or twofluid equations, entirely without reference to gyromotion or other single particle concepts. Accordingly, the waves can develop whenever such descriptions are valid. In fact, Alfvén waves were first detected in liquid mercury [3], where one can hardly speak of gyromotion.However, it is certainly true that collisional processes will cause damping of Alfvén waves. One must therefore examine the balance between driving power and dissipative processes to see how far such waves can propagate, taking into account, for instance, that waves traveling in tubes of concentrated magnetic flux are less prone to damping. Although this analysis is complicated by the great complexity of conditions in the Sun's atmosphere, theory as well as observations support the claim that Alfvén waves observed in the corona do emanate from the underlying layers [4 -7].Briefly, Alfvén wave excitation in the photosphere can be understood from the fact that these waves represent a natural means of adjusting transverse motion of plasma in different places along the same magnetic field line. Thus, they can be generated by variable motion of ions driven by thermal convection at the photospheric footprints. Collisional transfer of momentum from neutrals to ions is not an obstacle, but a necessary part of this process.As for damping of the modes, at subcoronal heights, this is mainly due to ion-neutral collisions [8]. The associated dissipation and heating is presumably partly responsible for the creation of the transition layer. However, those Alfvén waves which do penetrate the transition region, or are excited in this region, may propagate into the corona and evolve into nonlinear alfvenons, as described in [2]. Since collisions become increasingly rare at high altitudes, collisional terms should not be needed to model this process.Nonetheless, it is possible to extend the validity of the model by including local dissipation related to the resistivity term in the generalized Ohm's law, thereby accounting for electron-ion collisions. Then, Eqs. (2), (3), and (5) of [2] are modified to read