Abstract. On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, threedimensional ion distribution of the major magnetospheric ions (H + , He + , He ++ , and O + ) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5 • ) angular resolution, and a Hot Ion AnalCorrespondence to: H. Rème (Henri.Reme@cesr.fr) yser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6 • ) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range.

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A model of flux transfer events (FTEs) at the dayside magnetopause is proposed, which is based on non‐stationary reconnection along a single X line over a large longitudinal segment. An individual field line coming from the magnetospheric side constitutes after reconnection a loop‐like structure in the magnetopause current layer and leaves into the magnetosheath. In the presence of a By component of the magnetic field in the transition region of the magnetopause each loop is twisted in the y direction. Whereas traditionally FTEs are considered to be localized flux tubes with a helical field inside, in the present model field lines come from the magnetospheric side over a large longitudinal segment, have a loop‐like structure in the magnetopause resulting in the FTE signatures, and leave over the same segment in longitude into interplanetary space.

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[1] One-dimensional (1-D) full particle simulations of almost perpendicular supercritical collisionless shocks are presented. The ratio of electron plasma frequency w pe to gyrofrequency ce , the ion to electron mass ratio, and the ion and electron b (b = plasma to magnetic field pressure) have been varied. Due to the accumulation of specularly reflected ions upstream of the shock, ramp shocks can reform on timescales of the gyroperiod in the ramp magnetic field. Self-reformation is not a low w pe / ce process but occurs also in (w pe / ce ) 2 ) 1, low b simulations. Self-reformation also occurs in low ion b runs with an ion to electron mass ratio m i /m e = 1840. However, in the realistic mass ratio runs, an electromagnetic instability is excited in the foot of the shock, and the shock profile is considerably changed compared to lower mass ratio runs. Linear analysis based on three-fluid theory (incident ions, reflected ions, and electrons) indicates that the instability is a modified two-stream instability between the decelerated solar wind electrons and the solar wind ions on the whistler mode branch. In the reforming low ion b shocks, part of the potential drop occurs at times across the foot, and part of the potential ($40%) occurs over a few ($4) electron inertial lengths in the steepened up ramp. Self-reformation is a low ion b process and disappears for a Mach 4.5 shock at/or above b i % 0.4. It is argued that the ion thermal velocity has to be an order of magnitude smaller than the shock velocity in order for reformation to occur. Since according to these simulations only part of the potential drop occurs for relatively short times over a few electron inertial lengths, it is concluded that coherent shock surfing is not an efficient acceleration mechanism for pickup ions at the low b heliospheric termination shock.INDEX TERMS: 2139 Interplanetary Physics: Interplanetary shocks; 2154 Interplanetary Physics: Planetary bow shocks; 7851 Space Plasma Physics: Shock waves; 7843 Space Plasma Physics: Numerical simulation studies; K EYWORDS : collisionless shocks, shock potential, particle acceleration Citation: Scholer, M., I. Shinohara, and S. Matsukiyo, Quasi-perpendicular shocks: Length scale of the cross-shock potential, shock reformation, and implication for shock surfing,