J. Liao

Statistical study of O+ transport from the cusp to the lobes with cluster CODIF data

[1]xa0Ionospheric origin O+ accelerated in the cusp/cleft region convects over the polar cap and flows along open field lines to the tail lobes. Some O+ in the lobes enters the near-Earth plasma sheet where it is then convected to the inner magnetosphere, while some O+ ends up in the distant tail where it is lost. In order to understand the transport of ionospheric O+ to the plasma sheet so as to understand its contribution to the formation of geomagnetic storms, we have determined the occurrence frequency of cusp source O+ over the polar caps and in the lobes to determine where and when it is observed. The results show that the probability of observing O+ along the transport path is high even during nonstorm times, although, as expected, the highest probability is found during storm times. It was also found that when interplanetary magnetic field (IMF) By is positive, O+ from the northern cusp/cleft tends to stream toward the dawnside tail lobe while O+ from the south are observed on the duskside. The transport path for negative IMF By is more symmetric, but shows some evidence for a reversed asymmetry when IMF By is strongly negative. IMF Bz has little influence on the asymmetry. The asymmetry for positive By and lack of mirror symmetry between positive and negative By most likely result from the combination of convection driven by the solar wind and coupling with the ionosphere. Similar asymmetries have been observed in the convection patterns over the polar caps, which are attributed to a day-night ionospheric conductivity gradient adding to the IMF By effect. However, there are some disagreements between the asymmetries observed in polar cap potential patterns and the asymmetries observed in the O+ spatial distribution, indicating there may be other causes for the symmetry breaking, in addition to the day-night conductivity gradient.

Solar cycle dependence of the cusp O + access to the near-Earth magnetotail

[1]xa0Ionospheric origin O+ accelerated in the cusp/cleft region is one source for the O+ observed in the plasma sheet. This O+ is convected over the polar cap, flowing along open field lines to the tail lobes, and finally entering the plasma sheet. In this paper we use Cluster/CODIF data to study how the occurrence of cusp origin O+ in the polar cap and tail lobe changes over the solar cycle. Our study shows that the probability to observe cusp origin O+ decreases steeply during the declining phase of solar cycle 23 and starts to increase again at the start of the solar cycle 24. The decrease is much greater in the tail lobes than in the polar cap. A detailed analysis reveals that the O+ on the dominant transport path moves to higher latitudes in the Northern Hemisphere and more sunward in the Southern Hemisphere during the declining phase of the solar cycle (from the solar maximum to the solar minimum), so that the O+no longer reaches the near-Earth (<20RE) tail lobes. This change in transport is consistent with a significantly reduced convection velocity and a northward shift of the cusp region as the solar activity decreases, so that the O+ moves much further along the field line before it is convected into the tail. It will therefore enter the plasma sheet much further down the tail.

Acceleration of O+ from the cusp to the plasma sheet

Heavy ions from the ionosphere that are accelerated in the cusp/cleft have been identified as a direct source for the hot plasma in the plasma sheet. However, the details of the acceleration and transport that transforms the originally cold ions into the hot plasma sheet population are not fully understood. The polar orbit of the Cluster satellites covers the main transport path of the O+ from the cusp to the plasma sheet, so Cluster is ideal for tracking its velocity changes. However, because the cusp outflow is dispersed according to its velocity as it is transported to the tail, due to the velocity filter effect, the observed changes in beam velocity over the Cluster orbit may simply be the result of the spacecraft accessing different spatial regions and not necessarily evidence of acceleration. Using the Cluster Ion Spectrometry/Composition Distribution Function instrument onboard Cluster, we compare the distribution function of streaming O+ in the tail lobes with the initial distribution function observed over the cusp and reveal that the observations of energetic streaming O+ in the lobes around −20 RE are predominantly due to the velocity filter effect during nonstorm times. During storm times, the cusp distribution is further accelerated. In the plasma sheet boundary layer, however, the average O+ distribution function is above the upper range of the outflow distributions at the same velocity during both storm and nonstorm times, indicating that acceleration has taken place. Some of the velocity increase is in the direction perpendicular to the magnetic field, indicating that the Eu2009×u2009B velocity is enhanced. However, there is also an increase in the parallel direction, which could be due to nonadiabatic acceleration at the boundary or wave heating.

The relationship between sawtooth events and O+ in the plasma sheet

In order to study the relationship between sawtooth events and the composition of the plasma sheet, we perform a superposed epoch analysis (SEA) of the O+ concentration inside the near-Earth plasma sheet during sawtooth events and substorms sorted by different geomagnetic storm phases, using Cluster/Composition Distribution Function data. The SEA shows that the O+ content increases during sawtooth growth phase, regardless of storm phase, and reaches 20% around the onset of dipolarization. For storm main phase events, the plasma sheet O+ concentration during sawtooth events is only slightly higher than that observed during substorm events. However, for storm recovery phase and nonstorm time events, there is significantly more O+ within the plasma sheet during sawtooth events than during substorm events. No difference is found in the comparison between the O+/H+ density ratio changes during the first tooth and the subsequent teeth in a series of a sawtooth interval. Hence, there is no evidence to support the hypothesis that due to the higher O+ inside the plasma sheet, subsequent teeth will lead to a closer near-Earth X line and then a wider magnetic local time response. Finally, despite the association between sawtooth events and high O+/H+ ratio, there are times when the O+/H+ density ratio is high in the plasma sheet but no sawtooth event is observed, and there are sawtooth events when the O+/H+ ratio is low. This indicates that enhanced O+ is neither a necessary nor a sufficient condition but is likely one of many factors that play a role in triggering sawtooth events.