Cluster observation of few-hour-scale evolution of structured plasma in the inner magnetosphere

Abstract

Using Cluster Ion Spectrometry (CIS) data from the spacecraft-4 perigee traversals during the 2001-2006 period (nearly 500 traversals after removing those that are highly contaminated by radiation belt particles), we statistically examined the local time distribution of structured trapped ions at sub- to few-keV range as well as inbound-outbound differences of these ion signatures in intensities and energy-latitude dispersion directions. Since the Cluster orbit during this period was almost constant and approximately north-south symmetric at nearly constant local time near the perigee, inbound-outbound differences are attributed to temporal developments in a 1-2 h timescale. Three types of structured ions at sub- to few keV range that are commonly found in the inner magnetosphere are examined:

- Energy-latitude dispersed structured ions at less than a few keV,
- Short-lived dispersionless ion stripes at wide energy range extending 0.1-10 keV,
- Short-lived low-energy ion bursts at less than a few hundred eV.

The statistics revealed that the wedge-like dispersed ions are most often observed in the dawn sector (60% of traversals), and a large portion of them show significant enhancement during the traversals at all local times. The short-lived ion stripes are predominantly found near midnight, where most stripes are significantly enhanced during the traversals and are associated with substorm activities with geomagnetic AL < -300 nT. The low-energy bursts are observed at all local times and under all geomagnetic conditions, with moderate peak of the occurrence rate in the afternoon sector. A large portion of them again show significant enhancement or decay during the traversals.