Centrifugal acceleration in the magnetotail lobes

Abstract

Abstract. Combined Cluster EFW and EDI measurements have shown that cold ion outflow in the magnetospheric lobes dominates the hydrogen ion outflow from the Earth's atmosphere. The ions have too low kinetic energy to be measurable with particle instruments, at least for the typical spacecraft potential of a sunlit spacecraft in the tenuous lobe plasmas outside a few R E . The measurement technique yields both density and bulk velocity, which can be combined with magnetic field measurements to estimate the centrifugal acceleration experienced by these particles. We present a quantitative estimate of the centrifugal acceleration, and the velocity change with distance which we would expect due to centrifugal acceleration. It is found that the centrifugal acceleration is on average outward with an average value of about of 5 m s −2 . This is small, but acting during long transport times and over long distances the cumulative effect is significant, while still consistent with the relatively low velocities estimated using the combination of EFW and EDI data. The centrifugal acceleration should accelerate any oxygen ions in the lobes to energies observable by particle spectrometers. The data set also put constraints on the effectiveness of any other acceleration mechanisms acting in the lobes, where the total velocity increase between 5 and 19 R E geocentric distance is less than 5 km s −1 .

Shock-driven variation in ionospheric outflow during the 11 October 2001 moderate storm

“…[36] Over the prolonged period and long distance of the transport of ions from the polar ionosphere through lobe/ mantle to plasma sheet, the cumulative effect of the centrifugal acceleration is significant [e.g., Cladis, 1986;Nilsson et al, 2010]. Of course, it has also been shown by Cladis et al [2000] that centrifugal acceleration is an important ion acceleration term over the polar cap even during a short time period such as the shock passage.…”

“…The mechanism of centrifugal acceleration is that charged particles are accelerated along a "rotating" magnetic field line due to E × B drift along the direction of change of the field direction [Northrop, 1963;Cladis, 1986;Horwitz et al, 1994]. Normally, only the cumulative effect of centrifugal acceleration is significant, i.e., over a long spatial distance and during a long transport time [Cladis, 1986;Nilsson et al, 2010]. However, using observations from the Polar spacecraft over the polar cap, Cladis et al [2000] showed an example where O + ions acquired their energies in a short time primarily through centrifugal acceleration in the shock event on 24 September 1998.…”

Relative outflow enhancements during major geomagnetic storms – Cluster observations

“…Studies by Engwall et al (2006) and Haaland et al (2012) studied cold plasma and found that around 10 25 ions s −1 of the outflowing cold ions are lost to the solar wind. A study by Nilsson et al (2010) indicated that these cold ions are made up of protons and not oxygen. Moreover, geomagnetic disturbances lead to significant enhancement of the outflow but also strong convection towards the plasma sheet (Haaland et al, , 2015.…”

Outflow of low-energy O<sup>+</sup> ion beams observed during periods without substorms

“…We have mapped the footprint of Cluster 3 using Tsyganenko 89 and 96 models to show that the spacecraft are located in the auroral oval. By comparing our results with those in Nilsson et al (2010) and with the discussion in Nilsson et al (2013), one may find that not much heating and centrifugal acceleration is expected along these outflow paths in the near-Earth lobes to the plasma sheet. The auroral ions have similar and different features from the ions in the polar wind, upwelling ions (cleft ion fountain) and polar cap (Yan and Maggiolo et al, 2006Maggiolo et al, , 2011Nilsson et al, 2006).…”

“…The polar cap ions include the polar wind (Engwall et al, 2009;Li et al, 2012) and cusp origin O + Liao et al, 2010Liao et al, , 2012. Cold polar cap ions sometimes consist of mainly H + ions and can dominate the lobe outflow fluxes (Engwall et al, 2009;Nilsson et al, 2010). Ions flowing out with transpolar arcs (TPAs, also known as theta auroras) are field-aligned and observed when the interplanetary magnetic field (IMF) is northward (Kullen, 2012).…”