Large-Amplitude Electric Fields Associated with Bursty Bulk Flow

Braking in the Earth’s Plasma Sheet R. E. Ergun et al., JGR (2014)

Speaker: Zhao Duo

Main point

• Report observations of large-amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. • High-time resolution waveforms reveal nonlinear structures such as

electron phase-space holes and double layers, which suggest strong field-aligned currents or electron beams. • These large-amplitude electric fields are almost always accompanied by

enhanced magnetic field fluctuations. Shear Alfvén waves are participating in these events. • Intense currents or electron beams, generated by kinetic Alfvénic waves

that result from a turbulent cascade in BBF braking region, may be an energy source for large-amplitude electric fields.

An Example of a Large-Amplitude Electric Field Event• fast flows (>100 km/s), primarily in the GSM X and Y

direction. • The ion flows suggest that the large-amplitude E events

may be associated with BBF braking. • electron energy fluxes temporarily decrease • Poynting flux is primarily in the GSM X direction • S|| (black trace in Panel 1e) is traveling anti-parallel to

B. Poynting flux is Alfvénic • physical size of the regions of high Poynting fluxes

ranges from ~30 km to 700 km• Mapped to the ionosphere, these regions would have a

thickness of ~1 km to ~25 km, which is entirely consistent with the widths of the Alfvénic regions at low-altitudes

An Example of a Large-Amplitude Electric Field Event

• (left) A snapshot of E captured at 8196 samples/s as part of a wave burst

• Field-aligned coordinates • The high-frequency wave

signals are dominated by E|| and have the clear signature of electron phase-space holes

• Electron holes are observed repeatedly in many of the events

• (right) displays an expanded view of a possible double layer captured at 128 samples/s.

Large-Amplitude Electric Field Event Statistical Properties• 3.1. Large-Amplitude Electric Field Events• This study concentrates on BBF braking region over the two-year period

of 2008-2009• The first step of the study isolated moderately active and active periods.• A large-amplitude E event is required to have a minimum amplitude of 50

mV/m in the particle burst data • The duration of the event is calculated by requiring that the |E| > 25

mV/m in successive 10 s intervals surrounding the >50mV/m peak• the average duration of the events is approximately two minutes

(standard deviation of ~30 s)

3.2. Event Amplitudes and Locations

• The location of the large-amplitude E events in the GSM X-Y plane• Most of the events are inside

12 RE, consistent with the BBF braking region

3.2. Event Amplitudes and Locations

• peak values of |E||| and | E⊥ | (128 samples/s) for each event.

• In most cases, E⊥ is larger• examination of the wave burst data (8196 samples/s),

when available, indicates that the high-frequency E||

amplitudes often dominate over those of E⊥

• Electron phase space holes were present for at least 90% of the events.

• Double layers were identified in ~20% of the events that had wave burst data.

• Electron phase-space holes and double layers are known to be generated in regions of strong field-aligned currents in space plasmas

• These observations suggest possible nonlinear, kinetic interactions in strong field-aligned currents.

E and B Properties • Dramatic difference between the quiet-time

baseline and the magnetic field behavior during large-amplitude E events.

• 98% of the large-amplitude E events have enhanced B fluctuations

• The large-amplitude E events are strongly associated with enhanced B fluctuations.

• Enhanced B fluctuations can be routinely observed outside of large-amplitude E events. This property indicates that the large-amplitude E events can be caused by the B fluctuations

• The ratio increases by at least 10% in 156 of the 171 events (91%), indicating that the electric field events are observed primarily in conjunction with magnetic field dipolarization

E and B Properties • The steepening of the B spectral index

(Kolmogorov ) at frequencies above the ion cyclotron frequency (fci), whereas the E spectral index does the opposite, it flattens above fci• below fci, the |E2|/|B2| ratio is just

above VA2

E and B Properties

• The probability distribution of the direction of δS with respect to the average magnetic field is plotted

• Enhancement of parallel Poynting flux indicating an Alfvénic component.

• THEMIS orbits are more often in the Southern part of the plasma sheet where Bx is negative, the higher probability to be anti-field aligned indicates that δS is primarily earthward.

• The amplitude of δS is often greater than 0.1 mW/m, which, when mapped to auroral altitudes, is enough power to result in visible aurora.

3.4. Ion Velocities and Electron and Ion Energy Fluxes

• (left) The peak velocities in the GSM X and Y directions measured during each of the large-amplitude E events.

• The BBFs are significantly slowed • (right) all of the ion velocities

measured during all of the events. • While VX does have a small positive

bias, the velocities are often in the –X direction and VY has a strong component.

• These data are consistent turbulent or vortical flow in the BBF braking region.

• The electron and ion energy fluxes and temperatures vary dramatically during the events and mostly increase.

Discussion • BBF braking -> strong, turbulent Alfvénic fluctuations -

> strong field-aligned, currents perpendicular currents, or electron beams-> large-amplitude E and the nonlinear structures including electron phase-space holes and double layers

Nonlinear electric field structures in the inner

magnetosphereD. M. Malaspina et al., JGR (2014)

Speaker: Zhao Duo

Van Allen Probe Observations

• Active time: AE~600nT• High speed solar wind stream,

plasmapause eroded to L~4.4• Nonlinear electric field structures• Electron energy flux increase• Magnetic field dipolarization• Spacecraft potential, abrupt

dencity variation

Van Allen Probe Observations

• Geomagnetic storm: AE~870nT• High speed solar wind stream,

plasmapause eroded to L~3.1• Nonlinear electric field structures• Electron energy flux increase• Wave power strongest at the onset

of the electron flux increase

Van Allen Probe Observations

• Nonlinear electric field structures: electron-acoustic double layers, phase space holes, and strong double layers• Electron-acoustic double layers• The E structures are nearly ∥

unipolar, indicating a net potential across each structure and making them candidates for particle acceleration

Van Allen Probe Observations• Figures are consistent with a double layer and associated

phase space holes.• The structures (Figures 4c–4d) between 0.2 s and 0.85 s are

phase space holes generated by two-stream instability, and the unipolar structure near 0.9 s is the double layer

• Figure 4c shows a well-formed phase space hole relatively far from the double layer.

• Figure 4d shows a poorly formed hole closer to the double layer

• Figure 4e shows that coherent phase space holes have not yet formed close to the double layer

• Figure 4f shows the electric field power spectra for this event• Figure 4g shows the measured monopole voltages, An

electric field parallel to B (positive) is encountered first, followed by an anti-parallel (negative) electric field.

• consistent with expectations for a positive charge surrounded by negative charge moving parallel along B, identifying this structure as an electron phase space hole.

Van Allen Probe Observations• To quantify the spatial distribution of

nonlinear electric field structures, and to qualitatively explore their connection to dipolarizations

• Mean nonlinear electric field structure amplitudes are strongest at high L and weaken closer to Earth, similar to the bulk flows associated with dipolarizations

• A concentration of high amplitudes is observed premidnight

• Yet few strong earthward flows are expected near dawn, suggesting that earthward flows are not the only source of nonlinear electric field structures in the inner magnetosphere.

Discussion and Conclusions

• Van Allen Probe observations demonstrate that :• (1) nonlinear electric field structures are observed in the inner

magnetosphere • (2) these structures are observed over a range of radial distances and

magnetic local times• (3) nonlinear electric field structures can be closely associated with

magnetic dipolarizations and enhancements of 1–30 keV electron energy flux.