An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow

Nilsson, Hans; Waara, Martin; Marghitu, O.; Yamauchi, M.; Lundin, R.; Rème, H.; Sauvaud, J. A.; Dandouras, I.; Lucek, E.; Kistler, L. M.; Klecker, B.; Carlson, C. W.; Bavassano-Cattaneo, M. B.; Korth, A.

doi:10.5194/angeo-26-145-2008

Cited by 43 publications

(72 citation statements)

References 35 publications

(63 reference statements)

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“…Another mechanism that could be responsible for observed heating is centrifugal acceleration (Cladis, 1986). We have calculated the centrifugal acceleration as in Nilsson et al (2008) Assuming for simplicity that centrifugal acceleration varies only with altitude, we have summed up the centrifugal acceleration, as observed along the orbit, from the lowest altitude at 19:00 UT up to the highest altitude we use (17:45 UT). The cumulative sum of the centrifugal acceleration relative lowest altitude is only 1.5 km/s.…”

Section: Discussionmentioning

confidence: 99%

Oxygen ion energization observed at high altitudes

et al. 2010

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Abstract. We present a case study of significant heating (up to 8 keV) perpendicular to the geomagnetic field of outflowing oxygen ions at high altitude (12 R E ) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2-0.4 R E in altitude). This is a clear example of local ion energization at much higher altitude than usually reported. In contrast to many events at lower altitudes, it is not likely that the locally observed wave fields can cause the observed ion energization. Also, it is not likely that the ions have drifted from some nearby energization region to the point of observation. This suggests that additional fundamentally different ion energization mechanisms are present at high altitudes. One possibility is that the magnetic moment of the ions is not conserved, resulting in slower outflow velocities and longer time for ion energization.

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Section: Discussionmentioning

confidence: 99%

Oxygen ion energization observed at high altitudes

et al. 2010

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“…We combine the analysis method and data set used by Engwall et al (2009a,b) with the method to estimate centrifugal acceleration used by Nilsson et al (2008a).…”

Section: Methodsmentioning

confidence: 99%

“…As a first approximation one can expect protons to obtain a similar acceleration (the centrifugal acceleration being mass independent, but part of the centrifugal acceleration, as well as the time available for acceleration, is dependent on the parallel velocity of the particles which may vary for different populations). Nilsson et al (2008a) have calculated the centrifugal acceleration of outflowing ions in the high altitude (5-12 R E geocentric distance) polar cap/mantle. The study used four-spacecraft Cluster magnetometer measurements to estimate the gradient of the magnetic field, and Cluster ion spectrometer (Rème et al, 2001) ion measurements to estimate the parallel and perpendicular ion velocities, thus obtaining all terms determining the centrifugal acceleration from direct measurements.…”

Section: Introductionmentioning

confidence: 99%

Centrifugal acceleration in the magnetotail lobes

et al. 2010

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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 .

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“…momentum transfer from the solar wind H + to the escaping O + . Liao et al (2015) also showed extra O + acceleration between the cusp and nightside magnetosphere other than the centrifugal acceleration (Cladis, 1986;Nilsson et al, 2008). We first estimate the energy conversion rate using the density ratio in a similar way as Yamauchi and Slapak (2017).…”

Section: Amount Of Converted Kinetic Energy By the Escaping O +mentioning

confidence: 99%

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

Yamauchi¹,

Slapak²

2018

Ann. Geophys.

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Abstract. By conserving momentum during the mixing of fast solar wind flow and slow planetary ion flow in an inelastic way, mass loading converts kinetic energy to other forms -e.g. first to electrical energy through charge separation and then to thermal energy (randomness) through gyromotion of the newly born cold ions for the comet and Mars cases. Here, we consider the Earth's exterior cusp and plasma mantle, where the ionospheric origin escaping ions with finite temperatures are loaded into the decelerated solar wind flow. Due to direct connectivity to the ionosphere through the geomagnetic field, a large part of this electrical energy is consumed to maintain field-aligned currents (FACs) toward the ionosphere, in a similar manner as the solar wind-driven ionospheric convection in the open geomagnetic field region. We show that the energy extraction rate by the mass loading of escaping ions ( K) is sufficient to explain the cusp FACs, and that K depends only on the solar wind velocity accessing the mass-loading region (u sw ) and the total mass flux of the escaping ions into this region (m load F load ), as K ∼ −m load F load u 2 sw /4. The expected distribution of the separated charges by this process also predicts the observed flowing directions of the cusp FACs for different interplanetary magnetic field (IMF) orientations if we include the deflection of the solar wind flow directions in the exterior cusp. Using empirical relations of u 0 ∝ Kp+1.2 and F load ∝ exp(0.45Kp) for Kp = 1-7, where u 0 is the solar wind velocity upstream of the bow shock, K becomes a simple function of Kp as log 10 ( K) = 0.2 · Kp + 2 · log 10 (Kp + 1.2) + constant. The major contribution of this nearly linear increase is the F load term, i.e. positive feedback between the increase of ion escaping rate F load through the increased energy consumption in the ionosphere for high Kp, and subsequent extraction of more kinetic energy K from the solar wind to the current system by the increased F load . Since F load significantly increases for increased flux of extreme ultraviolet (EUV) radiation, high EUV flux may significantly enhance this positive feedback. Therefore, the ion escape rate and the energy extraction by mass loading during ancient Earth, when the Sun is believed to have emitted much higher EUV flux than at present, could have been even higher than the currently available highest values based on Kp = 9. This raises a possibility that the ion escape has substantially contributed to the evolution of the Earth's atmosphere.

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An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow

Cited by 43 publications

References 35 publications

Oxygen ion energization observed at high altitudes

Oxygen ion energization observed at high altitudes

Centrifugal acceleration in the magnetotail lobes

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

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