Chih-Ping Wang

Global auroral responses to abrupt solar wind changes : Dynamic pressure, substorm, and null events

[1] Global auroral images are used to investigate how specific types of solar wind change relate to the resulting type of auroral-region disturbance, with the goal of determining fundamental response types. For not strongly southward IMF conditions (B z ≥ -5 nT), we find that IMF changes that are expected to reduce the convection electric field after ≥30 min of negative IMF B z cause typical substorms, where expansion phase auroral activity initiates within the expected location of the Harang electric field reversal and expands in ∼10 min to cover ∼5 hours ofMLT. For not strongly southward IMF conditions, solar wind dynamic pressure (P dyn ) enhancements compress the entire magnetosphere, leading to a global auroral enhancement with no evidence for substorm bulge-region aurora or current wedge formation. Following prolonged strongly southward IMF (B z ≤ -8 nT), an IMF change leading to convection electric field reduction gives a substorm disturbance that is not much different from substorms for less strongly southward IMF conditions, other than the expansion phase auroral bulge region seems to expand somewhat more in azimuth. However, under steady, strongly southward IMF conditions, a P dyn enhancement is found to cause both compressive auroral brightening away from the bulge region and a Harang-region substorm auroral brightening. These two auroral enhancements merge together, leading to a very broad auroral enhancement covering ∼10-15 hours of MLT. Both current wedge formation and compressive energization in the inner plasma sheet apparently occur for these events. We also find that interplay of effects from a simultaneous IMF and P dyn change can prevent the occurrence of a substorm, leading to what we refer to as null events. Finally, we apply the plasma sheet continuity equation to the IMF and pressure driven substorm responses and the null events. This application suggests that solar wind changes cause substorm onset only if the changes lead to a reduction in the strength of convection within the inner plasma sheet.

Modeling the quiet-time inner plasma sheet protons

In order to understand the characteristics of the quiet time inner plasma sheet protons, we use a modified version of the Magnetospheric Specification Model to simulate the bounce averaged electric and magnetic drift of isotropic plasma sheet protons in an approximately self-consistent magnetic field. Proton differential fluxes are assigned to the model boundary to mimic a mixed tail source consisting of hot plasma from the distant tail and cooler plasma from the low latitude boundary layer (LLBL). The source is local time dependent and is based on Geotail observations and the results of the finite tail width convection model. For the purpose of self-consistently simulating plasma motion and a magnetic field, the Tsyganenko 96 magnetic field model is incorporated with additional adjustable ring-current shaped current loops. We obtain equatorial proton flow and midnight and equatorial profiles of proton pressure, number density, and temperature. We find that our results agree well with observations. This indicates that the drift motion dominates the plasma transport in the quiet time inner plasma sheet. Our simulations show that cold plasma from the LLBL enhances the number density and the proton pressure in the inner plasma sheet and decreases the dawn-dusk asymmetry of the equatorial proton pressure. From our approximately force-balanced simulations the magnetic field responds to the increase of pressure gradient force in the inner plasma sheet by changing its configuration to give a stronger magnetic force. At the same time, the plasma dynamics is affected by the changing field configuration and its associated pressure gradient force becomes smaller. Our model predicts a quiet time magnetic field configuration with a local depression in the equatorial magnetic field strength at the inner edge of the plasma sheet and a cross-tail current separated from the ring current, results that are supported by observations. A scale analysis of our results shows that in the inner plasma sheet the magnitude of the Hall term in the generalized Ohms law is not small compared with the quiet time electric field. This suggests that the frozen-in condition E = −v×B is not valid in the inner plasma sheet and that the Hall term needs to be included to obtain an appropriate approximation of the generalized Ohms law in that region.

RCM‐E simulation of bimodal transport in the plasma sheet

Plasma sheet transport is bimodal, consisting of both large-scale adiabatic convection and intermittent bursty flows in both earthward and tailward directions. We present two comparison simulations with the Rice Convection Model—Equilibrium (RCM-E) to investigate how those high-speed flows affect the average configuration of the magnetosphere and its coupling to the ionosphere. One simulation represents pure large-scale slow-flow convection with time-independent boundary conditions; in addition to the background convection, the other simulation randomly imposes bubbles and blobs through the tailward boundary to a degree consistent with observed statistical properties of flows. Our results show that the bursty flows can significantly alter the magnetic and entropy profiles in the plasma sheet as well as the field-aligned current distributions in the ionosphere, bringing them into much better agreement with average observations.

Revisit of relationship between geosynchronous relativistic electron enhancements and magnetic storms

We find evidence that magnetic storms are not only unnecessary for geosynchronous relativistic electron enhancements but also not directly relevant to the electron enhancements even if the enhancements are accompanied by magnetic storms. What is crucial for electron enhancements at geosynchronous orbit are sustained south-oriented or north-south fluctuating interplanetary magnetic field (IMF) Bz that drives sufficiently large substorm activity and small solar wind density Nsw that likely leads to low loss rate of relativistic electrons to the ionosphere and/or to the magnetopause for an extended time period. Specifically, almost all the abrupt, large electron increases in our data set took place under the condition of average AE > 235 nT and average Nsw ≤ 5 cm−3. Examination of detailed time profiles clearly shows that electron flux starts to increase quite immediately with arrival of the right IMF and solar wind conditions, regardless of a magnetic storm, leaving the accompanied magnetic storms merely coincident.

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.

Properties of low‐latitude mantle plasma in the Earth's magnetotail: ARTEMIS observations and global MHD predictions

The Earths plasma mantle is one of the major suppliers of particles for the plasma sheet. To understand its plasma characteristics, spatial distributions, and dependencies on interplanetary magnetic field (IMF) direction, we statistically analyzed the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moons Interaction with the Sun (ARTEMIS) observations in the low-latitude magnetotail (~10 RE above and below the current sheet) and investigated the predictions from global Block Adaptive Tree Solar wind-Roe-Upwind Scheme MHD simulations. The mantle plasma flows tailward along magnetic field lines (~50–200 km/s) and at the same time drifts toward midnight and toward the current sheet. The mantle plasma has similar temperature (~0.05–0.2 keV) to the magnetosheath plasma but has lower density (~0.1–1 cm−3). The mantle appearance is dawn-dusk asymmetric depending mainly on the IMF By direction. The occurrence rates, density, and V|| all decrease with decreasing |Y|. This density cross-tail profile suggests that the low-latitude mantle plasma mainly comes from the magnetosheath entering through the tail magnetopause along the open field lines. Density is highly and positively correlated with V||. These observations are qualitatively consistent with the MHD results. The simulations indicate that as IMF By becomes dominant, the source locations at the magnetopause for the mantle move to lower latitudes and become dawn-dusk asymmetric, and the tail cross section also becomes distorted with the magnetopause shape elongating and the current sheet tilting significantly. Degrees of these changes also vary with the downtail distances and IMF Bz direction. The source location change leads to the dawn-dusk asymmetric mantle appearance. The tail cross-section change alters the distance from the sources to the current sheet and thus the resulting mantle density distributions just outside the plasma sheet.

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.

Plasma sheet Pi2 pulsations associated with bursty bulk flows

There is a debate as to whether magnetic Pi2 pulsations (0.006–0.025 Hz) observed in the plasma sheet and on the ground are triggered by the earthward moving bursty bulk flows (BBFs) from midtail or distant tail or are caused by the substorm plasma instabilities initiated in the near-Earth plasma sheet. To resolve this debate, we conducted case and statistical studies of enhanced plasma sheet Pi2 pulsations using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from X~−30 RE to X~−8 RE. The case studies show that for both substorm-related and quiet time BBFs, enhanced Pi2 oscillations are observed in front of BBFs from midtail to the near-Earth plasma sheet. The oscillations intensify 1–4 min prior to the substantial flow enhancement, and usually initiate within several narrow discrete frequency bands, with lower frequencies intensifying earlier. We further examined the wave properties using low-frequency MHD fitting method and found that one or more discrete waves for each event are magnetosonic waves. Our statistical study of 364 BBFs shows that Pi2 pulsations are commonly associated with BBFs throughout the plasma sheet. The lowest-frequency oscillations usually appear below 0.01 Hz, and the strongest power oscillations appear at ~0.01 Hz or higher frequencies. We found that over 75% of the discrete oscillations are magnetosonic waves, and that 70%–75% of them are slow mode. These suggest that plasma sheet Pi2 pulsations are likely triggered by BBFs in the entire plasma sheet regardless of substorm onsets.

Source and structure of bursty hot electron enhancements in the tail magnetosheath: Simultaneous two-probe observation by ARTEMIS

Bursty enhancements of hot electrons (≳0.5 keV) with duration of minutes sometimes occur in the tail magnetosheath. In this study we used the unique simultaneous measurements from the two Acceleration Reconnection Turbulence and Electrodynamics of Moons Interaction with the Sun probes to investigate the likely sources, spatial structures, and responsible processes for these hot electron enhancements. The enhancements can be seen at any distance across the magnetosheath, but those closer to the magnetopause are more often accompanied by magnetosheath density and flow magnitudes changing to more magnetosphere-like values. From simultaneous measurements with the two probes being on either side of magnetopause or both in the magnetosheath, it is evident that these hot electrons come from the magnetosphere near the current sheet without further energization and that the enhancements are a result of bursty lateral magnetosphere intrusion into the magnetosheath, the enhancements and changes in the magnetosheath properties becoming smaller with increasing outward distance from the intrusion. From limited events having specific separation distances and alignments between the probes, we estimated that a single isolated enhancement can have a thin and elongated structure as narrow as 2 RE wide in the X direction, as long as over 7 RE in the Y direction, and as thin as 1 RE in the Z direction. We propose that Kelvin–Helmholtz perturbations at the magnetopause and subsequent magnetosphere-magnetosheath particle mixing due to reconnection or diffusion can plausibly play an important role in generating the bursty magnetosphere intrusion into the magnetosheath and the hot electron enhancements.