K.-J. Hwang

Wave‐particle interactions during a dipolarization front event

We present Cluster observations of wave-particle interactions during an earthward propagating dipolarization front (DF) and associated fast plasma bulk flows detected at the central current sheet in Earths magnetotail. During this period, flux tubes behind the DF frequently contain more energetic or hotter ions than did the preexisting flux tubes ahead of the DF. On the other hand, electrons within the DF flux tubes heat less, or are even colder, than were the preexisting populations and are often accompanied by superposed isolated beams. At the same time, electrostatic emissions are strongly enhanced over a wide range of frequencies (up to several times the electron cyclotron frequency) behind the DFs. This low-frequency electrostatic wave power is well correlated with ion energization. From linear theory, we find two wave modes: a high-frequency beam mode and a low-frequency whistler mode that are associated with the electron beam component. We attribute the generation of whistlers to electron beams that persist for a while before undergoing rapid thermalization. The existence of isolated beam components behind DFs detected during the 4 s Cluster spin period indicates that DFs either provide a continuous source of electron beams or facilitate a physical process that maintains the beams against rapid thermalization. Our analysis suggests that the earthward motion of the DF flux tube, via Fermi acceleration as the magnetic field lines behind the DF shorten, can lead to the persistent electron beams that generate whistler mode waves, which in turn can heat ions. This scenario, by which free energy in electron beams generates waves that then heat ions, accounts for the Cluster observations of different energization behaviors between electrons and ions behind DFs.

A tailward moving current sheet normal magnetic field front followed by an earthward moving dipolarization front

A case study is presented using measurements from the Cluster spacecraft and ground-based magnetometers that show a substorm onset propagating from the inner to outer plasma sheet. On 3 October 2005, Cluster, traversing an ion-scale current sheet at the near-Earth plasma sheet, detected a sudden enhancement of Bz, which was immediately followed by a series of flux rope structures. Both the local Bz enhancement and flux ropes propagated tailward. Approximately 5 min later, another Bz enhancement, followed by a large density decrease, was observed to rapidly propagate earthward. Between the two Bz enhancements, a significant removal of magnetic flux occurred, possibly resulting from the tailward moving Bz enhancement and flux ropes. In our scenario, this flux removal caused the magnetotail to be globally stretched so that the thinnest sheet formed tailward of Cluster. The thinned current sheet facilitated magnetic reconnection that quickly evolved from plasma sheet to lobe and generated the later earthward moving dipolarization front (DF) followed by a reduction in density and entropy. Ground magnetograms located near the meridian of Clusters magnetic foot points show two-step bay enhancements. The positive bay associated with the first Bz enhancement indicates that the substorm onset signatures propagated from the inner to the outer plasma sheet, consistent with the Cluster observation. The more intense bay features associated with the later DF are consistent with the earthward motion of the front. The event suggests that current disruption signatures that originated in the near-Earth current sheet propagated tailward, triggering or facilitating midtail reconnection, thereby preconditioning the magnetosphere for a later strong substorm enhancement.

Dense plasma and Kelvin‐Helmholtz waves at Earth's dayside magnetopause

Spacecraft observations of boundary waves at the dayside terrestrial magnetopause and their ground-based signatures are presented. Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measured boundary waves at the magnetopause while ground-based HF radar measured corresponding signatures in the ionosphere indicating a large-scale response and tailward propagating waves. The properties of the oscillations are consistent with linear phase Kelvin-Helmholtz waves along the magnetopause boundary. During this time period multiple THEMIS spacecraft also measured a plasmaspheric plume contacting the local magnetopause and mass loading the boundary. Previous work has demonstrated that increasing the density at the magnetopause can lower the efficiency of reconnection. Extending this further, present observations suggest that a plume can modulate instability processes such as the Kelvin-Helmholtz instability and allow them to form closer to the subsolar point along the magnetopause than without a plume. The current THEMIS observations from 21 September 2010 are consistent with a theory which predicts that increasing the density at the boundary will lower the Kelvin-Helmholtz threshold and allow waves to form for a lower velocity shear.

The global context of the 14 November 2012 storm event

From 2 to 5 UT on 14 November 2012, the Van Allen Probes observed repeated particle flux dropouts during the main phase of a geomagnetic storm as the satellites traversed the post-midnight to dawnside inner magnetosphere. Each flux dropout corresponded to an abrupt change in the magnetic topology, i.e., from a more dipolar configuration to a configuration with magnetic field lines stretched in the dawn-dusk direction. Geosynchronous GOES spacecraft located in the dusk and near-midnight sectors and the LANL constellation with wide local time coverage also observed repeated flux dropouts and stretched field lines with similar occurrence patterns to those of the Van Allen Probe events. THEMIS recorded multiple transient abrupt expansions of the evening-side magnetopause ∼20–30 min prior to the sequential Van Allen Probes observations. Ground-based magnetograms and all sky images demonstrate repeatable features in conjunction with the dropouts. We combine the various in situ and ground-based measurements to define and understand the global spatiotemporal features associated with the dropouts observed by the Van Allen Probes. We discuss various proposed hypotheses for the mechanism that plausibly caused this storm-time dropout event as well as formulate a new hypothesis that explains the combined in situ and ground-based observations: the earthward motion of magnetic flux ropes containing lobe plasmas that form along an extended magnetotail reconnection line in the near-Earth plasma sheet.