[Show abstract][Hide abstract] ABSTRACT: We present a comparative study of low frequency electric field spectral
densities and temperatures observed by the Cluster spacecraft in the
high altitude cusp/mantle region. We compare the relation between the
O+ temperature and wave intensity at the oxygen gyrofrequency
at each measurement point and find a clear correlation. The trend of the
correlation agrees with the predictions by both an asymptotic
mean-particle theory and a test-particle approach. The perpendicular to
parallel temperature ratio is also consistent with the predictions of
the asymptotic mean-particle theory. At times the perpendicular
temperature is significantly higher than predicted by the models. A
simple study of the evolution of the particle distributions (conics) at
these altitudes indicates that enhanced perpendicular temperatures would
be observed over many RE after heating ceases. Therefore,
sporadic intense heating is the likely explanation for cases with high
temperature and comparably low wave activity. We observe waves of
sufficient amplitude to explain the highest observed temperatures, while
the theory in general overestimates the temperature associated with the
highest observed wave activity, indicating that such high wave activity
is very sporadic.
[Show abstract][Hide abstract] ABSTRACT: We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions (O+) in the high altitude cusp and mantle. The ion energies considered are of the order 1000 eV and higher, observed above an altitude of 8 earth radii together with high wave power at the O+ gyrofrequency. We show that heating by waves can explain the observed high perpendicular energy of O+ ions, using a simple gyroresonance model and 25-45% of the observed wave spectral density at the gyrofrequency. This is in contrast to a recently published study where the wave intensity was too low to explain the observed high altitude ion energies. Long lasting cases (>10 min) of high perpendicular-to-parallel temperature ratios are sometimes associated with low wave activity, suggesting that high perpendicular-to-parallel temperature ratio is not a good indicator of local heating. Using multiple spacecraft, we show that the regions of enhanced wave activity are at least one order of magnitude larger than the gyroradius of the heated ions.
[Show abstract][Hide abstract] ABSTRACT: 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.
Geophysical Research Letters 01/2010; 37(9):L09103. · 3.98 Impact Factor
[Show abstract][Hide abstract] 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 RE) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2–0.4 RE 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.
[Show abstract][Hide abstract] ABSTRACT: The cusp ionosphere is an important source of heavy ions for the Earth's magnetosphere. The upflow in the cusp ionosphere is mainly associated with cusp precipitation and bursts in the convection electric field. The upflowing ions are gravitationally bound and further energization is needed above the ionosphere for the ions to reach escape velocity. It is not clear which process is the liming factor for the total outflow. We present measurements of these cusp origin oxygen ions when they have reached high altitudes above the polar cap, at about 5 to 12 RE altitude. In this region the outflowing ions can be observed for several hours at a time by the Cluster spacecraft. This allows us to study the oxygen ion number flux variability on time scales of minutes to tens of minutes. The number flux typically show sudden enhancements with a separation of 5 to 10 minutes. This is similar to reported ionospheric upflow events associated with flux transfer event signatures in the ionosphere. We therefore suggest that the ionospheric upflow, observable for example by incoherent scatter radars, is the limiting factor of the oxygen flux at high altitude. We also show how significant further energization of the outflowing oxygen ions take place in the high altitude polar cap region. The role of centrifugal acceleration and perpendicular heating due to wave particle interaction is assessed. The close coupling of the parallel velocity of outflowing proton and oxygen ion fluxes is shown.
[Show abstract][Hide abstract] ABSTRACT: Oxygen ion outflow associated with the cusp and cleft give rise to persistent oxygen ion beams which can be observed over the polar cap. For high altitude spacecraft such as Cluster these beams are often observed for several hours on each occasion. This allows for a study of typical temporal structures on the time scale of minutes. We have used 3 years of data from spring, January to May of years 2001 to 2003, for a study of the oxygen number flux variation in the polar cap ion outflow. The source of these oxygen ion beams is the cusp and cleft, and variations in ionospheric upflow on time scales of around 8 min have been reported from ground based studies using incoherent scatter radar. Such upflows typically do not reach escape velocity, and further energization above the ionosphere is required for outflow to occur. Our study shows that a typical time scale between sudden number flux enhancements observed by Cluster in a geocentric distance range of 5 RE to 12 RE is 5 to 10 min. A superposed epoch study does not reveal any significant convection velocity or temperature changes around the flux enhancement events. Sudden temperature enhancements occur with a typical time interval of about 4 min, A superposed epoch study does not reveal any number flux enhancements associated with the temperature enhancements. The clear modulation of the high altitude number flux in a manner which resembles the modulation of the ionospheric upflow indicates that this is the main limiting factor determining the total outflow. The process behind transient upflow events in the ionosphere is therefore important for the total ionospheric outflow. Subsequent heating above the ionosphere appears to be common enough in the cusp/cleft region that it does not significantly modulate the oxygen ion number flux.
[Show abstract][Hide abstract] ABSTRACT: The role of the centrifugal acceleration mechanism for ion outflow at high altitude above the polar cap has been investigated. Magnetometer data from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients. This is combined with ion moment data of the convection drift and the field-aligned particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration are directly obtained from observations. The temporal variation of the unit vector of the magnetic field is estimated by predicting consecutive measurement-points through the use of observed estimates of the magnetic field gradients, and subtracting this from the consecutively observed value. The calculation has been performed for observations of outflowing O+ beams in January to May for the years 2001–2003, and covers an altitude range of about 5 to 12 RE. The accumulated centrifugal acceleration during each orbit is compared with the observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration. Finally the observed spatial terms (parallel and perpendicular) of the centrifugal acceleration are compared with the results obtained when the magnetic field data was taken from the Tsyganenko T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may explain a large fraction of the parallel velocities observed at high altitude above the polar cap. The magnetic field model results underestimate the centrifugal acceleration at the highest altitudes investigated and show some systematic differences as compared to the observations in the lower altitude ranges investigated. Our results indicate that for altitudes corresponding to magnetic field values of more than 50 nT a test particle model with a steady state magnetic field model, a realistic convection model and an initial velocity of about 20 k m s−1 at 5 RE should be able to reproduce the main part of our observational results.
[Show abstract][Hide abstract] ABSTRACT: The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H<sup>+</sup>) and oxygen ions (O<sup>+</sup>) from 3 years (2001-2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5–12 R<sub>E</sub> geocentric distance. It was found that O<sup>+</sup> is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O<sup>+</sup> parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O<sup>+</sup> parallel bulk velocities in excess of 60 km s<sup>-1</sup> were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O<sup>+</sup> the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H<sup>+</sup> and O<sup>+</sup> was found to typically be close to the same throughout the observation interval when the H<sup>+</sup> bulk velocity was calculated for all pitch-angles. When the H<sup>+</sup> bulk velocity was calculated for upward moving particles only the H<sup>+</sup> parallel bulk velocity was typically higher than that of O<sup>+</sup>. The parallel bulk velocity is close to the same for a wide range of relative abundance of the two ion species, including when the O<sup>+</sup> ions dominates. The thermal velocity of O<sup>+</sup> was always well below that of H<sup>+</sup>. Thus perpendicular energization that is more effective for O<sup>+</sup> takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.