Chao Yue

Current reduction in a pseudo‐breakup event: THEMIS observations

Pseudo-breakup events are thought to be generated by the same physical processes as substorms. This paper reports on the cross-tail current reduction in an isolated pseudo-breakup observed by three of the THEMIS probes (THEMIS A (THA), THEMIS D (THD), and THEMIS E (THE)) on 22 March 2010. During this pseudo-breakup, several localized auroral intensifications were seen by ground-based observatories. Using the unique spatial configuration of the three THEMIS probes, we have estimated the inertial and diamagnetic currents in the near-Earth plasma sheet associated with flow braking and diversion. We found the diamagnetic current to be the major contributor to the current reduction in this pseudo-breakup event. During flow braking, the plasma pressure was reinforced, and a weak electrojet and an auroral intensification appeared. After flow braking/diversion, the electrojet was enhanced, and a new auroral intensification was seen. The peak current intensity of the electrojet estimated from ground-based magnetometers, ~0.7 × 105 A, was about 1 order of magnitude lower than that in a typical substorm. We suggest that this pseudo-breakup event involved two dynamical processes: a current-reduction associated with plasma compression ahead of the earthward flow and a current-disruption related to the flow braking/diversion. Both processes are closely connected to the fundamental interaction between fast flows, the near-Earth ambient plasma, and the magnetic field.

The relationship between the macroscopic state of electrons and the properties of chorus waves observed by the Van Allen Probes

Plasma kinetic theory predicts that a sufficiently anisotropic electron distribution will excite whistler mode waves, which in turn relax the electron distribution in such a way as to create an upper bound on the relaxed electron anisotropy. Here using whistler mode chorus wave and plasma measurements by Van Allen Probes, we confirm that the electron distributions are well constrained by this instability to a marginally stable state in the whistler mode chorus waves generation region. Lower band chorus waves are organized by the electron β∥e into two distinct groups: (i) relatively large-amplitude, quasi-parallel waves with β∥e ≳0:025 and (ii) relatively small-amplitude, oblique waves with β∥e ≲0:025. The upper band chorus waves also have enhanced amplitudes close to the instability threshold, with large-amplitude waves being quasi-parallel whereas small-amplitude waves being oblique. These results provide important insight for studying the excitation of whistler mode chorus waves.

Pitch angle evolutions of oxygen ions driven by storm time ULF poloidal standing waves

We present the first systematic observational study on the pitch angle evolutions of O(+) ions associated with ULF Pc5 poloidal standing waves excited during geomagnetic storms. The O(+) ion measurements are made on board the CLUSTER satellites with the Composition Distribution Function (CODIF) instrument, which covers energies from 1 to 40 keV, a low-energy portion of the ring current. We find that the nature of the ion flux oscillation strongly depends on the magnetic latitude of observation. Near the magnetic equator, the flux oscillation appears only around 0 degrees and 180 degrees pitch angles with no phase delay, which can result from wave-particle interactions in a fundamental mode standing wave with a strong poloidal component. Away from the equator, however, the flux oscillation appears in a wide range of pitch angles with strong pitch angle dispersion that reverses sign from the Southern Hemisphere to the Northern Hemisphere. The latitude dependence of the dispersion signature is explained by combining the ion energy modulation near the equator and the time of flight effect of ion bounce motion. The analysis technique shown in this study can be used to diagnose the field line mode structure of ULF waves.

On an energy‐latitude dispersion pattern of ion precipitation potentially associated with magnetospheric EMIC waves

Ion precipitation mechanisms are usually energy dependent and contingent upon magnetospheric/ionospheric locations. Therefore, the pattern of energy-latitude dependence of ion precipitation boundaries seen by low Earth orbit satellites can be implicative of the mechanism(s) underlying the precipitation. The pitch angle scattering of ions led by the field line curvature, a well-recognized mechanism of ion precipitation in the central plasma sheet (CPS), leads to one common pattern of energy-latitude dispersion, in that the ion precipitation flux diminishes at higher (lower) latitudes for protons with lower (higher) energies. In this study, we introduce one other systematically existing pattern of energy-latitude dispersion of ion precipitation, in that the lower energy ion precipitation extends to lower latitude than the higher-energy ion precipitation. Via investigating such a “reversed” energy-latitude dispersion pattern, we explore possible mechanisms of ion precipitation other than the field line curvature scattering. We demonstrate via theories and simulations that the H-band electromagnetic ion cyclotron (EMIC) wave is capable of preferentially scattering keV protons in the CPS and potentially leads to the reversed energy-latitude dispersion of proton precipitation. We then present detailed event analyses and provide support to a linkage between the EMIC waves in the equatorial CPS and ion precipitation events with reversed energy-latitude dispersion. We also discuss the role of ion acceleration in the topside ionosphere which, together with the CPS ion population, may result in a variety of energy-latitude distributions of the overall ion precipitation.

On the parameter dependence of the whistler anisotropy instability

The evolution of the whistler anisotropy instability relevant to whistler-mode chorus waves in the Earths inner magnetosphere is studied using kinetic simulations and is compared with satellite observations. The electron distribution is constrained by the whistler anisotropy instability to a marginal stability state and presents an upper bound of electron anisotropy, which agrees with satellite observations. The electron beta β∥e separates whistler waves into two groups: (i) quasi-parallel whistler waves for β∥e≳0.02 and (ii) oblique whistler waves close to the resonance cone for β∥e≲0.02. Landau damping is important in the saturation and relaxation stage of the oblique whistler wave growth. The saturated magnetic field energy of whistler waves roughly scales with the electron beta β∥e2, shown in both simulations and satellite observations. These results suggest the critical role of electron beta β∥e in determining the whistler wave properties in the inner magnetosphere.

Empirical modeling of 3‐D force‐balanced plasma and magnetic field structures during substorm growth phase

Accurate evaluation of the physical processes during the substorm growth phase, including formation of field-aligned currents (FACs), isotropization by current sheet scattering, instabilities, and ionosphere-magnetosphere connection, relies on knowing the realistic three-dimensional (3-D) magnetic field configuration, which cannot be reliably provided by current available empirical models. We have established a 3-D substorm growth phase magnetic field model, which is uniquely constructed from empirical plasma sheet pressures under the constraint of force balance. We investigated the evolution of model pressure and magnetic field responding to increasing energy loading and their configurations under different solar wind dynamic pressure (PSW) and sunspot number. Our model reproduces the typical growth phase evolution signatures: plasma pressure increases, magnetic field lines become more stretched, current sheet becomes thinner, and the Region 2 FACs are enhanced. The model magnetic fields agree quantitatively well with observed fields. The magnetic field is substantially more stretched under higher PSW, while the dependence on sunspot number is nonlinear and less substantial. By applying our modeling to a substorm event, we found that (1) the equatorward movement of proton aurora during the growth phase is mainly due to continuous stretching of magnetic field lines, (2) the ballooning instability is more favorable during late growth phase around midnight tail where there is a localized plasma beta peak, and (3) the equatorial mapping of the breakup auroral arc is at X~−14 RE near midnight, coinciding with the location of the maximum growth rate for the ballooning instability.

A 2‐D empirical plasma sheet pressure model for substorm growth phase using the Support Vector Regression Machine

The plasma sheet pressure and its spatial structure during the substorm growth phase are crucial to understanding the development and initiation of substorms. In this paper, we first statistically analyzed the growth phase pressures using Geotail and Time History of Events and Macroscale Interactions during Substorms data and identified that solar wind dynamic pressure (PSW), energy loading, and sunspot number as the three primary factors controlling the growth phase pressure change. We then constructed a 2-D equatorial empirical pressure model and an error model within r ≤ 20 RE using the Support Vector Regression Machine with the three factors as input. The model predicts the plasma sheet pressure accurately with median errors of 5%, and predicted pressure gradients agree reasonably well with observed gradients obtained from two-probe measurements. The model shows that pressure increases linearly as PSW increases, and the PSW effect is stronger under lower energy loading. However, the pressure responses to energy loading and sunspot number are nonlinear. The pressure increases first with increasing loading or sunspot number, then remains relatively constant after reaching a peak value at ~8000 kV min loading or sunspot number of ~80. The loading effect is stronger when PSW is lower and the pressure variations are stronger near midnight than away from midnight. The sunspot number effect is clearer at smaller r. The pressure model can also be applied to understand the pressure changes observed during a substorm event by providing evaluations of the effects of energy loading and PSW, as well as the temporal and spatial effects along the spacecraft trajectory.

Current sheet scattering and ion isotropic boundary under 3‐D empirical force‐balanced magnetic field

To determine statistically the extent to which current sheet scattering is sufficient to account for the observed ion isotropic boundaries (IBs) for <30 keV ions, we have computed IBs from our 3-D empirical force-balanced magnetic field, identified IBs in FAST observations, and investigated the model-observation consistency. We have found in both model and FAST results the same dependences of IB latitudes on magnetic local time, ion energy, Kp, and solar wind dynamic pressure (PSW) levels: IB moves to higher latitudes from midnight toward dawn/dusk and to lower latitudes as energy increases and as Kp or PSW increases. The model predicts well the observed energy dependence, and the modeled IB latitudes match fairly well with those from FAST for Kp = 0. As Kp increases, the latitude agreement at midnight remains good but a larger discrepancy is found near dusk. The modeled IBs at the equator are located around the earthward boundary of highly isotropic ions observed by Time History of Events and Macroscale Interactions during Substorms at midnight and postmidnight, but with some discrepancy near dusk under high Kp. Thus, our results indicate that current sheet scattering generally plays the dominant role. The discrepancies suggest the importance of pitch angle scattering by electromagnetic ion cyclotron waves, which occur more often from dusk to noon and are more active during higher Kp. The comparison with the observed IBs is better with our model than under the nonforce-balanced T89, indicating that using a forced-balanced model improves the description of the magnetic field configuration and reinforces our conclusions regarding the role of current sheet scattering.

Proton auroral intensification induced by interplanetary shock on 7 November 2004

We report a shock-induced auroral intensification event observed by the IMAGE spacecraft on 7 November 2004. The comparison of simultaneous auroral snapshots, obtained from FUV-SI12 and FUV-SI13 cameras onboard IMAGE spacecraft, indicates the dominance of proton precipitation (rather than electron precipitation) throughout the auroral oval region. The proton aurora in the postnoon sector showed the most significant intensification, with luminosity increasing by 5 times or more. We describe the main characteristics of interplanetary parameters observed by the ACE and Geotail satellites and plasma parameters within the mapped precipitation region detected by the Los Alamos National Laboratory 1990-1995 satellite. The generation mechanism of postnoon proton auroral intensification is further investigated on the basis of these observations. The estimated increase of loss cone size was not enough to produce the required proton auroral precipitation enhancement. The expected oxygen band electromagnetic ion cyclotron waves (no available observation), in the highly fluctuating density region during the shock period, might contribute to the enhanced precipitation of auroral protons. Our new finding is that the shock-driven buildup of 1-10 keV proton fluxes could account for the observed proton auroral intensification.

Low-energy (< 200 eV) electron acceleration by ULF waves in the plasmaspheric boundary layer: Van Allen Probes observation

We report observational evidence of cold plamsmaspheric electron (< 200 eV) acceleration by ultra-low-frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 minute which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10-20 keV) bi-directional pitch angle signatures observed during the event are suggestive of the drift-bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15 and MMS 1 observations suggest ULF waves in the event were strongest on the dusk-side magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump-on-tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift-bounce resonant instability, with the free energy supplied by substorm-injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere.