JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A10, 1283, doi: /2000ja000190, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A10, 1283, doi: /2000ja000190, 2002 Dynamic fluid kinetic (DyFK) simulation of auroral ion transport: Synergistic effects of parallel potentials, transverse ion heating, and soft electron precipitation X.-Y. Wu, J. L. Horwitz, and J.-N. Tu Center for Space Plasma, Aeronomy and Astrophysics Research, University of Alabama in Huntsville, Huntsville, Alabama, USA Received 25 May 2000; revised 24 January 2002; accepted 24 January 2002; published 9 October [1] Ion outflow processes along auroral field lines are simulated with a dynamic fluid kinetic (DyFK) model which couples a comprehensive fluid ionospheric ( km altitude) model to a semikinetic treatment for the topside through 3 R E region. Using a simplified electron description, large-scale extended parallel electrical fields driven by anisotropic hot plasma distributions have been incorporated in addition to the soft auroral electron precipitation and wave-driven ion-heating processes previously simulated [Wu et al., 1999]. Simulations show that auroral ionospheric ion outflows involve initial evacuation, ion-heating, and replenishment phases. The ionospheric ion supply is effectively elevated by the soft electron precipitation to topside altitudes, where the wave-driven transverse ion heating pumps ions upward. The altitude distribution and duration of wave heating and potential drop largely affect the pressure cooker ion trap formation. With comparable and persistent downward potential drop and wave heating, the pressure cooker produces slow and dense suprathermal ion outflows. The ion velocity distribution evolves in an extended ion trap from bowl and counterstreaming suprathermal conic distributions at lower altitudes into mirrored conics and finally toroidal distributions at the top of the pressure cooker. The wave heating is less effective for H + ions, owing partly to their fast transit through the wave-heating region. The H + ion trap tends to be lower but more extended in altitudinal extent than the O + ion trap. H + flux and total flow are about a third to half of those of O +. Some of the toroidal distributions and ion species variations of beam and conic energies in these simulations qualitatively resemble satellite observations of such ion distributions. INDEX TERMS: 2407 Ionosphere: Auroral ionosphere (2704); 2451 Ionosphere: Particle acceleration; 7859 Space Plasma Physics: Transport processes; 7843 Space Plasma Physics: Numerical simulation studies; KEYWORDS: Outflow, upflow, auroral ionosphere, ion transport, wave heating, hybrid simulation Citation: Wu, X.-Y., J. L. Horwitz, and J.-N. Tu, Dynamic fluid kinetic (DyFK) simulation of auroral ion transport: Synergistic effects of parallel potentials, transverse ion heating, and soft electron precipitation, J. Geophys. Res., 107(A10), 1283, doi: /2000ja000190, Introduction [2] High-latitude field-aligned ionospheric outflow is a significant source of magnetospheric plasma (see recent reviews by Horwitz and Moore [1997] and Moore et al. [1999]). The ionospheric outflow responses to the magnotospheric energy dissipation, and the ouflowing ions in turn can be a significant and variable load on the magnetospheric dynamic system. Global MHD simulations are moving toward incorporating the ionosphere-magnetosphere coupling to prove more realistic lower boundary conditions [e.g., Winglee et al., 1999]. Accurate outflow parameterization is thus indispensable to self-consistent global modeling. [3] Observations suggest that auroral field lines are the main channels for the ionospheric heavy ions to be energized Copyright 2002 by the American Geophysical Union /02/2000JA and flow toward the magnetosphere. At high altitudes, the ion outflow flux is much larger in the cusp and the auroral oval than in the polar cap [cf. Loranc et al., 1991; Wu et al., 2000] and statistically increases exponentially with Kp index [Yau and Andre, 1997]. Topside ionospheric upflows were found to be closely correlated with the soft auroral electron precipitation [e.g., Seo et al., 1997]. Suprathermal ion distributions in forms of conics, bowls, rings, and beams of up to tens or hundreds of ev are frequently present along active auroral field lines [e.g., Klumpar et al., 1984; Hirahara et al., 1998]. It has been generally regarded that most of transversely heated ion conics are due to the cyclotron-resonant interaction with the broadband electrostatic ELF waves [Norqvist et al., 1998]. Knudsen et al. [1998] reported that ion-heating events were strongly correlated with suprathermal electron bursts below 500 ev. Over the auroral zone, satellite observations [e.g., Marklund et al., 1997; Carlson et al., 1998; McFadden et al., 1999] show SIA 5-1

2 SIA 5-2 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT evidence of ion conics and beams associated with upward and downward quasi-static parallel electric fields at altitudes of several thousand kilometers. [4] Among the many important processes affecting the auroral ion outflow are soft electron precipitation in the upper-f region and accompanying parallel potential drops and wave-driven ion heating at higher altitudes. These three major auroral processes result in complicated behavior of the ionospheric plasma transport. As pointed out by Loranc et al. [1991] and Horwitz and Moore [1997], the topside F- region ionosphere actively supplies thermal ions to higher altitude regions, where part of these ions are further energized beyond escape energies and transformed into outflowing beam and conic distributions by various auroral acceleration and transport mechanisms. Their potential synergistic effects on outflowing ions have not been fully investigated with a model treating the auroral ion flow from its F-region ionospheric source outward to the higher magnetospheric altitudes. [5] In most previous simulations of transport on highlatitude flux-tubes the ionospheric upflow drivers and various high-altitude outflow acceleration processes have been investigated separately. They have either been based on momentbased treatments [e.g., Schunk and Sojka, 1989; Mitchell and Palmadesso, 1983] which can not fully treat non-maxwellian distributions in the higher altitude regions, or hybrid [e.g., Wilson, 1992; Brown et al., 1995] and fully kinetic [e.g., Schriver, 1999] treatments which are difficult and inefficient to apply to the dense collisional F-region ionosphere. [6] A combination of downward electric field and wave heating can create energetic conics through the pressure cooker effect [Gorney et al., 1985; Barakat and Barghouthi, 1994; Jasperse, 1998]. The electric potential barrier would trap the upflowing ions until the ions gain sufficient energy to escape the barrier. The pressure cooker process explains observation of conics of a few hundred ev without presence of high levels of wave power. Brown et al. [1995] conducted a semikinetic simulation of the synergistic effects of the potential drops and wave heating on the ionospheric ion outflow. Their results revealed complex ion velocity distribution along the flux tube. In their simulation, however, ion-neutral and ion-ion collisions were neglected, and a fixed lower ionospheric boundary was assumed at 1500 km altitude. [7] To treat the multiple scale ionospheric plasma transport self-consistently and efficiently, different approaches have been tried to construct a model that couples a momentbased treatment for the collision-dominated ionosphere to a particle or distribution function-based code for the highaltitude transition and collisionless regions. Lemaire [1972], Lie-Svendsen and Rees [1996], and Su et al. [1998] have used hydrodynamic, generalized transport, and semikinetic treatments for the high-altitude region, respectively. Recently, a new dynamic coupled fluid kinetic (DyFK) model has been developed and used to demonstrate the synergistic effects of soft electron precipitation-induced heating and ionization together with the wave-induced heating [Estep et al., 1999; Wu et al., 1999]. Wu et al. [1999] showed that F-region O + ions produced and heated by the precipitating auroral soft electrons gradually attain the altitudes of active wave-driven ion perpendicular heating. The enhanced ion plumes are effectively wave heated and pumped up by the mirror magnetic field. Conics and bowl velocity distributions fully develop for a relatively weak ion wave along the field lines. The H + ion transport is less affected. For the characteristic parameters used, the net escaping flux of O + conics of tens of ev ions may enhance by a factor of ten above the escaping flux with only one or the other process operative. [8] In this report, we have incorporated hot magnetospheric plasma-driven potentials in a way similar to Brown et al. [1995] but now have included transition region collisional effects as well as realistic ionospheric coupling (rather than the high-altitude injection boundary used by Brown et al.) with the effects examined by Wu et al. [1999]. Such simulations are highly desirable to understand the synergistic effects of multiple scale auroral ion outflow drivers and accelerators. For instance, such simulations can elucidate the significant species differences as observed, which could not be fully resolved in the previous simulation by Brown et al. [1995] without the realistic ionospheric responses. We will describe the extended DyFK model and parameters used in the simulations. The simulation results of auroral ion transport for different wave heating and potential profiles are presented. The effects of the three auroral processes on the ionospheric O + and H + ion transport will be discussed qualitatively. 2. Dynamic Fluid Kinetic (DyFK) Model and Simulation Parameters [9] The DyFK model is a one-dimensional dynamic flux tube model which joins the field line interhemispheric plasmasphere (FLIP) treatment [Richards and Torr, 1990] and the generalized semikinetic (GSK) model [Wilson, 1992; Brown et al., 1995; Ho et al., 1997]. The truncated version of FLIP model includes comprehensive dynamics and chemistry for the ionospheric 120 km to 1100 km altitude region. Auroral precipitation is described through a two-stream electron transport model [Richards, 1995]. The GSK treatment is a type of hybrid simulation model for the high-altitude region from 800 km to 3 R E. Ion gyro-centers are advanced subject to the effects of parallel electric fields, gravity, and the geomagnetic mirror force. In addition, the ions experience ion-ion, and ion-neutral collisions. Ions are injected across low-altitude boundary and are also produced in the simulation region through the solar illumination ionization processes acting on the neutral atmosphere, and chemical reactions such as accidentally resonant charge exchange of ions with neutrals. In the present simulation region, approximately O + and H + super-particles, representing ionospheric ions, are advanced. [10] To join the fluid and hybrid regions, an overlap region and 8-s leap-frogging advance method is employed. Flow bulk parameters from the 800 km location in the fluid zone supply the lower boundary conditions to drive the next advance of the GSK treatment. Conversely, parameters from the 1100 km altitude cell in the GSK zone form upper boundary conditions which drive the next response of the lower fluid region [Estep et al., 1999; Wu et al., 1999]. [11] Within the GSK region, the description of the thermal electrons is of a massless, neutralizing fluid with a drift velocity ensuring the current conservation. Coulomb heat transfer from the electrons to ions is included; however, the electron temperature itself is not directly affected by

3 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-3 interaction with the ions. In this report, the thermal electron temperature is assumed to be isotropic and consistent with a heat conductivity-dominated altitude profile with a fixed temperature of 8000 K at 3 R E altitude [cf. Ho et al., 1997]. The present description of the electrons and zero-current assumption in the GSK region does place limits on the interpretation of the electron behavior itself as well as its influence on other factors such as the electric potential. [12] Assuming that the gyro-frequency is much larger than the collision frequency and the ion gyro-radius is much smaller than the wave perpendicular wavelength, the quasilinear cyclotron resonant wave-driven ion heating is included in the GSK region. The wave heating rate is calculated via the anomalous transverse velocity-diffusion coefficients [Crew et al., 1990]. The DyFK treatment involves the same techniques for representing the stochastic perpendicular ion heating used by Brown et al. [1991]. According to Freja observations by Knudsen et al. [1998], ion heating may also occur via non-resonant sloshing [Lundin et al., 1990] in wave fields below the ion gyrofrequency, but the heating rate for this process is believed to be less than the gyro-resonant heating. [13] How a parallel electrical field along auroral field lines is maintained and distributed in a collisionless plasma has not been entirely established [e.g., Borovsky, 1993], although recent FAST and other spacecraft observations have stimulated new theoretical progress (see below). Possible mechanisms include anomalous resistivity, weak double layers, magnetic mirror force, fast solitary waves, etc. One of the candidate mechanisms for producing parallel electric fields is the differences between the temperature anisotropies of hot magnetospheric plasma [e.g., Alfvén and Fälthammar, 1963]. The difference in the electron and proton pitch angle distributions dictates a parallel electric field in order to maintain ion-electron charge balance otherwise the ions and electrons would have different mirror point locations and thus unrealistically large space charge densities would develop. As done by Brown et al. [1995], this report will consider this mechanism for the parallel electric field effects, in part as a convenient proxy for the myriad types of mechanisms that may produce the potential drops along auroral field lines. [14] We of course realize that the differential hot proton/ electron anisotropies considered here may not be the only or even the primary mechanism for creating auroral parallel electric fields. It should be noted that recent FAST measurements by Ergun et al. [1998] and simulations [Ergun et al., 2000; Jasperse, 1998] have pointed, for example, to relatively thin potential layers associated with field-aligned currents, which are not included here. Nevertheless, our approach lends itself readily to incorporate a geophysically realistic treatment of the effects of extended parallel electric fields on the ionospheric outflow, together with the effects of soft electron precipitation and transverse heating. [15] The anisotropic magnetospheric protons and electrons are included in GSK region via Liouville mapping [Olsen et al., 1994]. A constraint in the present simulation as in Brown et al. [1995] is that the ionospheric and magnetospheric components are independently quasi-neutralized. The magnetospheric components are assumed to be non-drifting bi-maxwellians and experience only adiabatic variations. Thus the parallel temperatures of the hot plasma populations T k remain constant along the flux tube [Olsen et al., 1994]. The simple approach is not intended as a substitute for full self-consistent dynamic model, but rather set a preliminary baseline to provide further insight into the complex processes and guide future studies. The density n and perpendicular temperature T? of species s under the potential drop j along the flux tube are mapped from the upper boundary altitude, r 0,as[Olsen et al., 1994] n s ðþ¼ r n sðr 0 Þexp ej=k B T sk ðr 0 Þ ð1þ T s? ðr 0 Þ Ts? r0 þ 1 ð Þ Br0 ð Þ T s? ðþ¼ r T sk ðr 0 Þ Br ð 0 Þ Br ðþ þ T sk ðr 0 Þ Br ðþ T s? ðr 0 Þ Br0 1 ð Þ Ts? ðr 0 Br ðþ Þ T sk ðr 0 Þ where k B is Boltzmann constant and B is the geomagnetic field strength. [16] The difference of the anisotropies of the magnetospheric components is critical to the potential drop formed and is measured by the differential anisotropy ratio (DAR) as defined by Brown et al. [1995] at the upper boundary of the simulation region, DAR ¼ T H þ? T ek = TH þ k T e? DAR > 1 results in a downward electric field while DAR <1 produced an upward one. The electric field under multifluid approximation throughout the GSK region is calculated from the bulk parameters as [Mitchell and Palmadesso, 1983]: E k ¼ 1 S X s q s þ n s T sk n sv 2 s A gan r þ k B m n st sk where S ¼ P An s e 2 m s, A is the flux tube cross-sectional area, s J is the current density which is assumed to be zero for the present simulation, and s runs over all species including the ionospheric and magnetospheric components. Field-aligned currents (which may produce potential drops as noted above) [cf. Ganguli et al., 1994a] have been set to zero in our simulations. [17] We simulated the plasma transport along a flux tube with foot point at geographic 80.4 N latitude and E longitude. The base neutral atmosphere and ionosphere are based on solar minimum conditions with F 10.7 = 90, and magnetic activity with Ap = 18. The fluid region from km altitude is divided into 65 cells. The kinetic region from 800 km to 3 R E consists of 173 cells. The flux tube was initially evolved for 36 hours in geophysical time to achieve a steady state baseline polar wind prior to initiating an auroral event at local time [18] Knudsen et al. [1998] show a correlation between the wave intensity and the soft electron flux. Statistically, indications of core ion heating appear as wave powers exceed a heating threshold of 10 3 mv 2 m 2 Hz 1 and the electron fluxes exceed 10 7 cm 2 sr 1 s 1. The waves observed on auroral field lines by many satellites were usually associated both with field-aligned currents and upflowing ion beams. The generation mechanism for these ELF waves is unclear, and limited by the available data resolution. The leading candidate for the ELF wave generation is instabilities driven by the auroral current (mostly ð2þ ð3þ

4 SIA 5-4 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT carried by suprathermal electron beam). Other wave producing mechanisms include ion beam instabilities and velocity shear [Ganguli et al., 1994b]. [19] S3-3 [Gorney et al., 1981] and DE-1 [Yau et al., 1984] observations show statistically that the ion conics occurrence decreases below 2000 km and above km. This conic altitude distribution may indicate that local perpendicular energization occurs within a limited altitude range. Gyro-resonance wave heating may not occur for the frequent collisions at lower altitude and the large gyroradius relative to the electrostatic wavelength [Barghouthi, 1997]. As done by Wu et al. [1999], a constant broadband ELF wave distribution was assumed along the auroral field lines between altitude 1900 km 2 R E with a commonly observed power-law frequency spectrum with power index of 1.7 [e.g., Kintner et al., 1986; Knudsen et al., 1998]. The low-altitude cutoff was included to assure a smooth transition to the fluid treatment region that includes no wave heating. We used the same wave power spectral density of 10 2 mv 2 m 2 Hz 1 at the 1 R E O + ion gyro-frequency of 6.5 Hz as used by Brown et al. [1995]. The corresponding O + ion transverse heating rate is peaked at erg s 1 ( ev s 1 ) near 2 R E altitude. The wave amplitudes at H + gyrofrequencies are about 100 times weaker than those at O + gyrofrequencies. Due to mass dependent transit times and other effects, the resulting H + heating rate turns out to be approximately a factor of 10 smaller due to the power-law wave spectrum. [20] To examine the dependence on the wave profiles, we also run simulations with a statistical altitude-dependent profile based on fitting DE-1 plasma wave data with a power law spectrum as reported by Barghouthi [1997]. Barghouthi s [1997] profile presents much stronger heating rates increase with altitudes, especially for O +, compared with an altitude-independent wave profile. The real wave intensity varies along the auroral field lines as well as cross the field lines. Information on the detailed altitude profiles of waves and how they vary in space and time is sparse, and so again, the conclusions and results on the ion transport involved here must be viewed chiefly from a qualitative perspective. [21] Considering that ion transport heavily depends on the flux tube drift history and the wave, precipitation, and potential drop may vary rapidly in rather unknown ways, we have to set simulating scenarios with typical values and relatively simple temporal variations. No consideration of field-aligned currents or possible association of this precipitation with parallel electric fields and cyclotron waves is included here. The auroral precipitating electrons are assumed to have a Maxwellian distribution with a peak at 100 ev and an energy flux of 3 erg cm 2 s 1 at 800 km altitude. This energy flux is in the upper range of those measured with the low-altitude plasma instrument (LAPI) aboard DE-2 [Seo et al., 1997]. The directly relevant effects of the precipitating electrons in these simulations are to produce ionization and thermal plasma heating within the ionospheric fluid treatment region. [22] The hot magnetospheric components were taken to have a typical plasma sheet density of 1 cm 3 at 3 R E altitude with isotropic electrons and anisotropic protons, and T e ¼ T H þ k ¼ 1 KeV [Brown et al., 1995]. The temperature anisotropy at the upper boundary was held fixed during the simulation. Figure 1. Evolution of electric potential for DAR = 0.5 with wave heating and precipitation. Zero potential is defined at 800 km altitude. [23] The hot magnetospheric component, as well as the precipitation and wave heating, were turned off after 73 minutes into the simulation. Hence, we simulated the effects of a simple 73 min pulse of auroral processes on the ionospheric transport. This could represent, for instance, a relatively stationary flux tube with auroral processes pulsed locally, or the transport along a flux tube being convected through a relatively stationary cleft or auroral region, or some combination of temporal changes and such convection through different spatial regions. 3. Ionospheric Transport Under Upward Parallel Electric Field Conditions [24] The DAR in the first simulation case was chosen to be 0.5, which means the ions were more field-aligned than the electrons. Thus the parallel electric field must be upward to ensure quasi-neutrality of protons and electrons throughout the mirror field. Figure 1 shows the temporal evolution of potential profile in the GSK region. Figure 2 shows O + and H + ion density, velocity, perpendicular and parallel temperatures, ion flow, and accumulated flow in the altitude range of 120 km 3 R E. The definition of the flow is the net particles transport for a unit flux tube of 1 cm 2 crosssectional area at the foot altitude of 200 km.

5 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-5 a) Figure 2. Temporal evolution of ionospheric ion bulk flow for the simulation case shown in Figure 1 of (a) O + and (b) H + ion density, velocity, perpendicular and parallel temperatures, ion flow, and accumulated flow in a flux tube of 1 cm 2 cross-sectional area at 200 km. The auroral event starts at t =0 and ends at 73 min. Positive velocity value indicates upward flows. [25] As shown in Figure 1, at the introduction of the hot anisotropic plasma a potential drop of 110 V developed in the altitude range of 7500 km 3 R E. The potential drop increased to 170 V at 10 min into the simulation. From 20 min, the potential drop started to decrease gradually with time. At 73 min, it decayed to 50 V and lower edge retreated to 9000 km. As will be discussed later, the initial potential increase indicates the ionospheric ion evacuation, and the later potential drop decrease or erosion results from the increasing ionospheric plasma shielding. [26] The outward O + ion bulk outflow in Figure 2 increased rapidly following the turn-on of the auroral event, reaching 45 km s 1 at 3 R E altitude, and gradually dropped to 25 km s 1 after 60 min. Occurring mainly above altitude 7000 km, the flow acceleration showed a temporal variation in accordance with that of the potential drop. The O + ion densities in altitudes below 1 R E increased with time gradually. The O + population above 7000 km altitude first decreased and started to increase 20 min after the turn-on. By 60 min, the O + outflow density increased by a factor of 5. Perpendicular O + temperatures increased drastically in first 5 min reaching a peak of 6 8 ev stationed around peak wave heating altitude of 2 R E. The O + parallel temperatures cooled with increasing altitude. However, the gradient of the O + parallel temperatures reversed near 30 min, showing heating process in the altitude range ,000 km. The O + ions formed conics and bowl (elevated conics) velocity distributions as indicated in Figure 3a with ratios of T? / T k 10. [27] In the baseline polar wind, H + ions experienced a large acceleration between altitude km. After the auroral processes turn-on, the H + ion flow above altitude of 8000 km showed a similar but faster velocity variation compared with O + ion flow (Figure 2). A steady outflow velocity ratio of v H þ=v O þ 3.5 was reached, which implies additional O + acceleration other than the simple acceleration through the electric potential drop. Below the main potential

6 SIA 5-6 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT b) Figure 2. (continued) drop altitude, however, the H + drift velocity decreased with time. H + ion population and flow flux in the higher altitudes decreased in the first 5 10 min by a factor of more than 100 but increased gradually afterwards. The initial beam consisted of nearly equal O + and H + ion composition and higher H + flux. In the following stage, the H + ion density remained about one order of magnitude lower than the O + ion density above 1 R E altitude. However, between about km altitude, the n H þ : n O þ ratio increased with time by a factor of 10. By 60 min, the H + outflow density and flow were up to four times the initial baseline values. The H + ion perpendicular and parallel temperatures also increased by about four times and showed a peak of ev which shifted upward between km altitude with time. The temperature profile below the altitude of 4000 km saturated within 10 min. Above the temperature peak, H + ions cooled quickly with increasing altitude, forming a cold beam at high altitudes. The H + velocity distributions shown in Figure 3b are clearly much less anisotropic than those for the O + distributions in Figure 3a, showing smaller perpendicular heating for the H + ions. The H + ions had much smaller perpendicular temperatures than did O + but had comparable parallel temperatures to the O + ions. The low speed tail seen in the H + velocity distribution in the altitude range km increased local parallel temperatures, which was thought due to O + H + collisions and ion production from non-drifting H atoms in the transition region [Wilson, 1994]. [28] After the hot magnetospheric plasma components, wave heating, and electron precipitation were turned off at 73 min, the overall potential drop in Figure 1 suddenly decreased to 6 V. Both O + and H + drift velocities and perpendicular temperatures in Figure 2 declined quickly

7 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-7 a) b) Figure 3. (a) O + and (b) H + ion velocity phase space plots at different altitudes at 60:20 for the simulation case shown in Figure 2. over all altitudes. The bulk outflow velocity ratio v H þ=v O þ dropped to about 2 and then recovered to more than 3.5 in 30 min. The ion parallel temperatures, however, continued to increase after the turn-off. The restoring phase after the turn-off of the auroral event featured increasing T H þ?=t O þ? Ion densities continued to increase at higher altitudes. H + densities increased to about one third of the O + densities in 10 min and then decreased to about one tenth of the O + densities. The O + densities at lower altitudes and the H + densities in the mid-altitude range started to decrease about 10 min after the turn-off. The accumulated flux tube ion number outflow during the simulated auroral event was approximately of order of ions cm 2 normalized to the ionospheric base altitude of 200 km. 4. Ionospheric Transport Under Downward Parallel Electric Field Conditions [29] To probe the transport under conditions involving downward parallel electric fields, the hot proton temperature

8 SIA 5-8 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT Figure 4. Evolution of electric potential for DAR = 2 with wave heating and precipitation. Zero potential is defined at 1000 km altitude. anisotropy was chosen such that DAR = 2. The proton velocities were more perpendicularly aligned, and thus led to a downward parallel electric field. As shown in Figure 4, the potential drop was larger (200 V) and was confined to higher altitudes (above the altitude 2 R E ). As additional ionospheric plasma entered the higher altitude regions, the potential drop was almost completely shielded by 60 min. Below 9000 km altitude, a stable upward ambipolar potential drop of 5 V was maintained throughout the simulation period. [30] As shown in Figure 5, as the potential drop was developing, initial polar wind ionospheric ion along the auroral flux tube was evacuated downward from the potential drop region. Downward H + and O + ion beams developed were accelerated up to bulk velocity of about 50 km s 1 for H + and 10 km s 1 for O +, respectively. These initially down-flowing ionospheric ion beams slowed and were partially reflected by the magnetic mirror force at lower altitudes. The H + beam reached an altitude of 1000 km in about 5 min at approximately 20 km s 1 (2 ev). The downgoing O + beam reached 1000 km altitude later in min after the start of the event. The possible effects of such ion precipitation on the lower ionosphere is an interesting subject for future work. [31] During the rest of event, the front of the ionospheric plasma plume propagated upward at a speed of 2 kms 1. Ion densities in the higher altitude ranges increased with time gradually toward about 10 times those in the first case with upward electric field. The upward O + plumes penetrated deeper into the potential barrier than the H + plumes. Above 5000 km altitude, the H + density decreased with time and was slightly smaller than the O + density. Below 5000 km altitude, the dominant ion was O +, but the H + composition increased continuously as in the first case. The H + and O + bulk flows displayed rather smooth and stable velocity profiles with altitude. The H + bulk flow velocity had its peak velocity of 8 kms 1 around 1 R E altitude, while the O + outflow velocity reached a peak of about 5 km s 1 above 2 R E altitude. The perpendicular ion temperatures displayed two peaks in the altitude profile, a relatively stable one of 10 ev for O + and 2.5 ev for H + near the maximum heating altitude of 2 R E, and a larger sharp peak of ev for O + and 3 8 ev for H + at their corresponding upflowing plume fronts. At the H + front altitude, the O + conics had slightly larger perpendicular energies than did the H + conics. The parallel temperatures showed a single peak slightly below the perpendicular temperature peak at the front. The bottom of the pulse of heated ionospheric ion also moved upward with time. The counter streaming beams, as shown in Figure 6, effectively widened the velocity distribution in the parallel direction, producing the high parallel temperatures in the high-altitude region in Figure 5. The reflected ions also counterbalanced the contribution of upflowing ions to the net outflow, leading to the decrease in the overall bulk drift velocity. [32] As shown in the velocity and phase distribution plots in Figures 6 and 7, multiple beams were formed in the flux tube. Two or more H + beams were formed that were faster and bounced over a larger altitude extent than the O + streams. Complex non-maxwellian velocity distributions as simulated by Brown et al. [1995] developed at high altitudes (Figure 7). Generally, as will be discussed later, with increasing altitude, counter-streaming conics, bowl, mirrored conics, and toroidal distributions formed in that order, with different time scales for the different species. The conics elevated and folded with altitude. Toroidal velocity distributions were present at the front of the upgoing ionospheric ion plumes. [33] After the potential drop, wave-heating, and precipitation were all terminated, large ion outflows followed, owing to the relaxation of the magnetic moments, with temperatures cooling rapidly and velocities jumping to 7 kms 1 for O + and 20 km s 1 for H +, respectively, as shown in Figure 5. The H + outflow density decreased and the O + outflow density continued to increase for at least 20 min after the auroral event cessation. In the lower km transition altitude region, the O + density started to decrease slowly while the H + density continued to increase. The accumulated flux tube ion outflow during the auroral event was of the order of ions cm 2, and the O + flow was about twice the H + flow. [34] To elicit the roles played by the wave heating and the soft electron precipitation under the conditions of hot plasma-driven potential drops, we also ran three cases with each process separately and both being turned off, respectively. For the case with the downward potential barrier but no precipitation and no wave heating (Figure 8), a

9 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-9 a) Figure 5. Temporal evolution of ionospheric ion bulk flow for the simulation case shown in Figure 4 of (a) O + and (b) H + ion density, velocity, perpendicular and parallel temperatures, ion flow, and accumulated flow in a flux tube of 1 cm 2 cross-sectional area at 200 km. The auroral event starts at t =0 and ends at 73 min. potential drop of 210 V developed and penetrated down to the altitude of 10,000 km. The potential drop reduced slightly afterwards. The resulting down-going ion beams produced small local O + ion density enhancements that propagated down to 4000 km altitude (Figure 8a). The O + concentration and perpendicular temperatures below 8000 km increased slightly with time. The H + bulk parameters in Figure 8b showed larger variations. In contrast to the previous case shown in Figure 5, the H + ion plume reached a higher density cutoff altitude than the O + concentration. The H + densities increased by a factor of 10 in 60 min and the cutoff altitude was elevated by 2000 km up to near 12,000 km. Oscillating patterns were seen in the bulk drift velocities, and reduced to near rest throughout the region. The H + ion temperatures display an increased and up-shifting peak, showing an oscillating pattern similar to that of the bulk velocities. After the removal of the hot plasma-driven potential barrier, the trapped ionospheric ions started to flow upwards at 30 km s 1 for H + and 3 km s 1 for O + ions. The fast upflowing H + ions cooled rapidly. [35] For the case including the electron precipitation but not wave heating, increased upper F-region O + ionization, and ion temperatures, as well as upflow enhancements, were seen. The potential drop was reduced to 150 V at 70 min. The H + displayed similar temperature variations to those for the no precipitation case; however, the peak propagated upwards at a much faster rate of 1 kms 1. For the case including wave heating but no precipitation, the highaltitude potential drop were eroded slower than that in case shown in Figure 5 with the precipitation plus wave heating. The H + bulk flows behaved generally similarly to case shown in Figure 5. The O + bulk velocities were mostly 0 5 km s 1 upward, and the temperatures at higher

10 SIA 5-10 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT b) Figure 5. (continued) altitudes were increased as in the precipitation plus waveheating case. The O + ion outflow flux for precipitation only was about one third to half of the case with both precipitation and wave heating. [36] To examine the sensitivity of the results to the adopted wave profile, we performed simulations with Barghouti s [1997] wave profile model, which has a much stronger heating rate at higher altitudes, especially for O + ions (about 1000 times larger O + heating at 2 R E altitude than the cases presented above). The results indicate that the general ion transport features at lower altitudes remain the same. However, the produced electric potential did not trap ions at high altitudes due to the strong local wave heating leading to O + conics with energies of 10 KeV and H + conics of 200 ev at top altitude. The initial downgoing O + beam in the previous case was quickly reversed, while the downgoing H + beam persists. The outflow O + bulk velocity and ion perpendicular temperatures attained relatively stable profiles involving strong increases above 10,000 km within 5 min. O + perpendicular energies were about 50 times those of H + and parallel bulk velocities about 5 times those of H +. After the turn-off of heating, there was a similar enhanced H + outflow burst. The O + flow decreased quickly in the restoring or replenishment phase. [37] The ion heating and regional transit time depend strongly on the species-dependent heating rate and the potential drop. To elicit the effects of different potential profiles, we performed simulation with steady prescribed downward parallel electric field of 0.01 mvm 1, i.e., a linear potential drop, in the region above 2 R E altitude. This electric field is similar to that adopted in the simpler but relevant calculations by Lund et al. [1999]. With the same truncated weak wave profile for the cases shown in Figures 5 7, the evolution of ion transport as shown in Figures 9 10 was similar to that previously described. The ion conics had larger energies and H + ions were steadily trapped below the 2 R E because of the persistent potential

11 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-11 a) Figure 6. Reduced phase space plots for (a, b) O + and (c, d) H + ions at different times for the simulation case shown in Figure 5. drop. The ion velocity distributions in Figure 10 showed similar field-aligned evolution as in Figure 7. In another simulation case with stronger electrical field, a steady field increasing linearly from 0 at 2 R E to 0.2 mvm 1 at 3 R E altitude was superposed to the ambipolar electric field, which produced a potential drop over 400 V. The wave profile was 100 times larger in amplitude than those of cases of Figures 5 7, which is the same as used by Crew

12 SIA 5-12 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT b) Figure 6. (continued) et al. [1990] but with the altitude truncations. As shown in Figures 11 12, 5 min into the simulation the H + trap still persisted with the ions attaining perpendicular energies of less than 100 ev. But within 2 min into the simulation O + ions above 3000 km altitude escaped the trap. The O + conics with energies of several hundreds of ev developed in 2 min and increased with altitude. Along the flux tube, an initial downward electric field driven downgoing O +

13 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-13 c) Figure 6. beam was produced at low altitudes while rapidly heated upgoing O + conics appeared at high altitudes. Again, the velocity distribution evolution along the flux tube was similar to previous cases except for the large asymmetry between the upgoing and downgoing O + distributions at the top altitude. (continued) 5. Discussion [38] Indirect measurements [Marklund et al., 1994] have indicated that most of the auroral parallel potential drops were inferred to be between 3000 km and 15,000 km, while the average field aligned electric field points upward above

14 SIA 5-14 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT d) Figure km and often downward below 4000 km. The electrical potentials referred from observations are highly variable, generally in the range between several hundred to a few thousand volts. The potential drops in our simulation are about 200 V. Theoretically, the anisotropic plasma (continued) induced field-aligned potential without ionospheric components is proportional to the parallel temperature [Alfvén and Fälthammar, 1963; Olsen et al., 1994], which is expected to be about 500 V for the simulation case. The difference is due to ionospheric plasma electric shielding. We also ran a

15 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-15 [39] Again we reiterate that the limitations of the potential calculation in this transport calculation (assumption of zero-current and isotropic ionospheric electron and incomplete description of electron dynamics) mean that the quantitative distribution and evolution of hot plasma driven potential drops in our simulation results is not expected to be accurate. For example, the extended potential would be established faster because of quick electron response. As with the more limited simulations of Lund et al. [1999], however, the simulations presented here do show qualitaa) b) Figure 7. (a) O + and (b) H + ion velocity phase space plots at different altitudes at 60:20 for the simulation case shown in Figure 5. simulation case with the hot plasma density being five times larger, and the potential drop increased by 25% and extended to lower altitudes. In this treatment, the large gradients in the potential profiles are mainly controlled by the transition between the ionospheric plasma-dominated region and the magnetospheric plasma-dominated region. At later stage into the simulation, as the ionospheric plasma slowly (partly due to the wave cut-off above 2 R E ) approaching the upper boundary, the large field is localized near the upper boundary too.

16 SIA 5-16 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT a) Figure 8. Temporal evolution of ionospheric ion bulk flow for the case with DAR = 2 but without wave heating and precipitation of (a) O + and (b) H + ion density, velocity, perpendicular and parallel temperatures, ion flow, and accumulated flow in a flux tube of 1 cm 2 cross-sectional area at 200 km. The auroral event starts at t = 0 and ends at 73 min. tively the general behavior of species-dependent ion transport under the influences of persistent extended potential drops. Depending on the polarity of the large-scale parallel electric fields, the polar wind plasma initially present in the flux tubes are accelerated either up or down along the flux tube, forming fast initial streams. The faster H + ions are evacuated earlier, producing short-lived beams with large densities and fluxes. At lower altitudes, the electric field due to the hot-plasma temperature anisotropy is partially shielded by the ambipolar electric field since the dense cold-plasma pressure imbalance becomes dominant (cf. Equation (3)). [40] The initially down-going beams coming from the upper altitudes in this simulation set up produce a small density enhancement as shown in Figure 8. These ions are transversely heated and reflected due to the enhanced upward geomagnetic mirror force F = mrb. Increased collisions at the lower altitudes further contribute to the slowing and heating of the initial down-going beams. A fraction of the ionospheric ion population is trapped between the upper potential barrier and the magnetic mirror points, producing the bouncing ion populations shown in Figure 8. The trapped bouncing ions can also involve oscillating temperatures. The formation of pressure cooker is largely determined by the relative strength of the wave-heating and the electric field along the field line. It may again be stated that a more accurate and improved description of pressure cooker phenomena could well involve current-driven wave descriptions and calculations of current-involved potential distributions. However, it should be noted that the Ergun et al. [2000] and Jasperse [1998] treatments, though relevant, are not immediately applicable to the dynamic transport investigated here, because they are steady-state and contain other simplifications. [41] The effects of precipitation and wave heating alone were previously simulated by Wu et al. [1999]. Under the

17 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-17 b) Figure 8. influence of the soft electron precipitation, auroral F-region ion upflows are driven by sudden collisional thermal electron heating due to the precipitating electrons at the onset and the continuous ion heating due to the subsequent collisional heat transfer from the thermal electrons. Ion plumes expand upward through the neutral atmosphere. The topside H + ion gas expands upward and is accelerated by the enhanced ambipolar electric potential of 5 V over the extended altitude range, mostly produced by the electron temperature and density gradient. However, precipitationenhanced ambipolar electric potential of 0.2 V in the F- region/topside region is not sufficiently strong for most heavier O + upflows to overcome the gravitational barrier and the collisional drag with neutral particles. The topside upflows slow and cool with increasing altitude. [42] IntheF-region/topside altitude range up to 1000 km, H + is a minor ion species. The H + density is mainly controlled by the H O + accidentally-resonant charge exchange. H + reactions in response to the precipitation arises indirectly from the O + creation below. The topside H + density enhancement seen in the simulation is due to the (continued) enhanced H + production through charge exchange with the increased topside O + population. The topside O + enhancement is due to precipitation and even partly the deposition by the downgoing O + beam. As shown in Figures 2b and 5b and previously by Wilson [1994], the increased production of H + ion from non-drifting H atoms and the enhanced H + O + collisions reduce the net drift velocity of H + ions, spread the distribution in the parallel direction, and result in the increased H + temperature peaked around 4000 km. Early high-altitude H + density and velocity increases seen in Figure 5b before the local O + density increase result from the intensified topside H + flows due to the early enhanced ambipolar electric field and quicker transport of lighter H + ions. Both the H + density and temperature peaks shift upward with time because of increased H + production at increasing altitudes. [43] Under the downward electric field without wave heating, trapped H + ions in Figure 8b penetrate deeper into the potential barrier because of the smaller effect of gravity and lower collisions below the transition altitude for H + ions. The larger topside H + temperature seen in Figure 8

18 SIA 5-18 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT a) Figure 9. Reduced phase space plots for (a) O + and (b) H + ions at different times for the case with constant electrical field of 0.01 mvm 1. Other parameters are the same with the simulation case shown in Figure 5.

19 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-19 b) Figure 9. (continued)

20 SIA 5-20 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT a) b) Figure 10. (a) O + and (b) H + ion velocity phase space plots at different altitudes at 60 min for the simulation case shown in Figure 9. may indicate an effective energy transfer to H + ions from the O + ions and electrons. There is a marked difference in the behavior of the O + and H + ions during the restoring phase: The continuous large topside H + abundance and flow after the precipitation turn-off in our simulation are due to the enhanced ambipolar acceleration, while O + ions start to drop because of the cutoff of precipitation-driven ionization and heating. [44] Effects of wave heating on the ion transport have also been discussed previously [e.g., Temerin, 1986; Ganguli et al., 1994b; Barakat and Barghouthi, 1994; Brown et al., 1995; Barghouthi, 1997; Wu et al., 1999]. Competing effects of the cooling associated with first magnetic moment conservation and the wave-driven ion heating control perpendicular ion temperatures. The ion temperatures and bulk velocities stabilize due to the fact that the heating is limited by the speed of transit through the heating region. Competing heating and cooling processes lead to the formation and gradual expansion of the conic and bowl velocity distributions [e.g., Brown et al., 1995]. The weak heating level and rapid saturation in H + ion energy seen in Figure 5b mainly result from the quicker transit of the heating region experienced by the fast upflowing H + ions [Wu et al., 1999]. The O + ions experience stronger perpendicular heating from the waves partly because of their relatively slow transit of the heating region as evidenced in Figure 3.

21 WU ET AL.: SIMULATION OF AURORAL ION TRANSPORT SIA 5-21 a) Figure 11. Temporal evolution of ionospheric ion bulk flow for the case with 100 times stronger wave heating profile and electrical field increasing with altitude. Other parameters are the same with the simulation case shown in Figure 5. (a) O + and (b) H + ion density, velocity, perpendicular and parallel temperatures, ion flow, and accumulated flow in a flux tube of 1 cm 2 cross-sectional area at 200 km. [45] Recent FAST observations in the inverted-v region show a mass-dependent parallel ion beam energy indicating stronger acceleration for heavier ions [Mobius et al., 1998]. Differences in energies observed for the upflowing H +,He +, and O + beams in conjunction with auroral arcs suggests additional acceleration or energy transfer processes to the heavier ions. Ion beam instabilities and species-dependent wave heating have been proposed to explain the energy difference [Mobius et al., 1998]. However, their relative importance for the differential energization of ion beam species is not well understood. The simulations presented here involve transit time effects that involve preferential energization of heavier ions in wave heating regions as a partial explanation. [46] Using a semikinetic GSK simulation of auroral ion transport, Brown et al. [1995] found that the combination of wave heating and an upward electric field results in an order of magnitude increase in O + outflow compared to a case without wave heating. They also found that, under downward electric field conditions, the energy gained by the ions from the waves increased by a factor of 2 or 3 relative to the situation without potential drop. This was mainly due to the slower transit of the heating region for the downward electric field case. The simulations of Brown et al. [1995] also indicated that the potential drops were eroded by the upflowing ionospheric plasma. Brown et al. [1995] assumed a flux tube initially being empty, then the hot plasma and potentials being imposed. Thus the initial potential build-up and cold ion beams, and electron precipitation effects on the ion supply by ionosphere seen in our simulations were missing in their results. [47] In addition to the previous simulations of the synergistic effects of two processes on the auroral ion outflow [Brown et al., 1995; Wu et al., 1999], several important features are noted in this simulation. Due to the incorporation of precipitation, the outflow density is enhanced with varying composition, which produces upward motion of ion trap region and rapid erosion of the hot plasma-driven potential layer. The downward potential drop, combined with the precipitation and wave heating, produces slower