Thermal Ion Dynamics Experiment Research Articles

Simulation of the formation of O+ density troughs at 6000 km altitude in the polar cap ionosphere‐magnetosphere region

Spacecraft‐based in situ observations have indicated considerable variability in O+ densities in the polar cap ionosphere‐magnetosphere region near 6000 km altitude, from above 10 O+ cm−3 to lower than 0.01 O+ cm−3. Regions with densities in the low range have been referred to as O+ density troughs or ion depletion zones at these altitudes. In this paper, we use the UT Arlington Dynamic Fluid‐Kinetic (DyFK) ionospheric plasma transport model to simulate O+ density profiles under various conditions, in order to reproduce O+ densities which were observed by the Thermal Ion Dynamics Experiment (TIDE) on the Polar spacecraft on 20 August 1998 near Polar perigee. Using solar wind parameters and IMF conditions as inputs driving the time‐varying high‐latitude electric potential model and incorporating auroral processes involving the effects of soft electron precipitation and wave‐driven transverse ion heating, we simulate the ionosphere‐magnetosphere plasma transport and the associated O+ bulk parameter profiles within four flux tubes convecting in the high‐latitude region. The principal competing processes controlling the O+ density levels at these altitudes are the auroral processes, which elevate the O+ densities in the ∼6000 km altitude region, and the effects of F region O+‐electron recombination in darkness, which decrease the O+ densities at higher altitudes through draining to the reduced F region ionosphere. Depending chiefly upon the flux tube convection trajectories and their relationship to estimated auroral oval locations, both elevated and trough‐like densities are seen at ∼6000 km altitudes in the simulations, as observed by Polar/TIDE in the polar cap magnetosphere.

Self-organized maps in scientific data analysis

The Thermal Ion Dynamics Experiment (TIDE) investigates low energy (0 - 500 eV) plasma in the Earth's magnetosphere, especially in the polar regions. It is part of NASA's larger POLAR mission. Shortly after the POLAR spacecraft was in orbit, TIDE's mass spectrometer functionality failed. However, data from other instruments can be used to create energy vs. time spectrograms. The number of peaks in these spectrograms relates to the composition of the plasma, so determining the number of peaks in the spectrogram effectively regains the mass spectrometer functionality. Kohonen self-organized maps (SOMs,) a type of neural network, are particularly suited to this problem due to the amount of data that needs to be analyzed and the algorithm's ability to find patterns within data. The algorithm leads to clustering of similar data points on the map. Ultimately, the location of the input data point on the map allows for determination of how many peaks the data point contains, and thus the composition of the plasma at that time. The SOM correctly classified 99% of the input data, making it a viable solution to the problem. Further research is planned, namely the possibility of extending this concept to investigate energetic neural atom (ENA) images in order to determine the source of these atoms.

Observations of the ion signatures of double merging and the formation of newly closed field lines

Observations from the Thermal Ion Dynamics Experiment (TIDE) on the Polar spacecraft, taken during a period of northward interplanetary magnetic field (IMF) show magnetosheath ions within the magnetosphere with velocity distributions resulting from two widely separated (∼30 RE) merging sites along the same field lines. These results are consistent with the hypothesis that double merging can produce closed field lines populated by solar wind plasma under northward IMF. While the merging sites cannot be unambiguously located, the directions of the flowing magnetosheath populations and the convection of merged field lines are consistent with locations of two merging sites northward of the spacecraft and the analyses favor one site poleward of the northern cusp and a second site at lower latitudes equatorward of the northern cusp.

Magnetospheric convection and thermal ions in the dayside outer magnetosphere

We have surveyed 3.5 years of Polar Thermal Ion Dynamics Experiment (TIDE) data between 1 January 2000 and 30 June 2003, when Polar apogee paths (sections of orbits with geocentric distances r > 5 RE) were in the dayside outer magnetosphere, to study the spatial distribution of thermal ions and the magnetospheric convection paths of the thermal ions, as a function of interplanetary magnetic field (IMF) orientation. We have found a dawn‐dusk asymmetry in the occurrence of detectable thermal ions above the instrument threshold. The occurrence rate was significantly higher at the duskside. The probability of observing thermal ions, particularly at 1300–1600 local time (LT) near the magnetopause, was >50%, compared with <30% at the dawnside. We interpret the thermal ion events as the result of plasmaspheric drainage plumes, as observed by IMAGE spacecraft or geosynchronous orbiters. The episodic appearance of the thermal ions in the outer magnetosphere could be a significant factor for the dynamo process of global magnetospheric convection. The variation of the convection pattern due to the IMF orientation is consistent with equatorward and poleward reconnection scenarios that superimpose dayside convection driven by reconnection on top of the background convection driven by a viscous interaction at the magnetopause, together with the corotation of the magnetospheric plasma with the ionosphere.

An examination of the process and magnitude of ionospheric plasma supply to the magnetosphere

The contribution of ionospheric plasma to the Earth's magnetosphere has been recognized for more than 3 decades. The magnitude of this contribution has become more evident over that same time period with the observed magnitude of the low‐energy ionospheric supply increasing as the measurement techniques improved. Estimates based on Dynamics Explorer measurements in the mid‐1980s suggested that the ionospheric plasma supply is sufficient to populate the plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Recent measurements from the Thermal Ion Dynamics Experiment on the Polar spacecraft have been used in conjunction with an ion trajectory model to reexamine the process and magnitude of the ionospheric supply of magnetospheric plasma. These measurements reveal the energy, pitch angle, and flux characteristics of the upward flowing polar wind over broad regions of the high‐latitude ionosphere. Onboard measurement of spacecraft potential is found to be a fundamental element in interpreting the measured ion outflow. Newly derived polar wind fluxes are determined to be near 6.0 × 107 ions cm−2 s−1 at 5000 km altitude during local winter and magnetically quiet conditions. Using the measured ionospheric source characteristics in combination with the trajectory code reveals the nature of the ionospheric/magnetosphere filling process and shows that the ionospheric source is sufficient to supply the observed densities and energies of the plasma sheet and magnetotail lobes. Many of the ionospheric particles are further transformed to ring current energies and locations after circulation through the plasma sheet. This measurement/calculation approach is able to show which regions of the high‐latitude ionosphere are important for plasma sheet/ring current filling. The ionospheric sources used in the calculations include the dominant polar wind, the cleft ion fountain, and the auroral zone.

Occurrence statistics of cold, streaming ions in the near‐Earth magnetotail: Survey of Polar‐TIDE observations

Results are presented from a survey of cold ion observations in the near‐Earth magnetotail using data from the Polar Thermal Ion Dynamics Experiment (TIDE). During the interval from July to December of 2001, Polar had its apogee (∼9.5RE) near the equatorial plane in the tail region of the magnetosphere. It is shown that a lobal wind is ubiquitous in the inner tail, with low‐energy (<300 eV) ions streaming from the ionosphere downtail. These lobal winds often pass through the plasma sheet, forming bidirectional streams, in addition to the unidirectional beams seen at higher magnetic latitudes. The observance of bidirectional streams is inversely, although weakly, correlated with geomagnetic activity. Bidirectional streams are interpreted as indicating the minimum size of the closed flux tube region. The reduced frequency of bidirectional streams with activity level times is consistent with the thinning of the plasma sheet during these times. It is inferred from the universality of these observations during Polar's passage through the inner tail region that the ionosphere is a continuous supplier of plasma to the near‐Earth magnetosphere. The high occurrence rate of these streams means that during geomagnetic disturbances, it is not necessary to wait for outflow and magnetospheric circulation in order to supply the inner magnetosphere with ionospheric ions; these cold streams are an immediately available supply of ionospheric‐origin particles.

Simulating the cleft ion fountain at polar perigee altitudes

A dynamic fluid semi-kinetic (DyFK) model is used to simulate the cleft ion fountain. Ion field-aligned flows are modeled for a flux tube convecting along an empirical model specified convection trajectory across the polar ionosphere from the cusp/cleft region. In this DyFK model, the collision-dominated portion of the flux tube is treated with a moment-based fluid model for altitudes from 120 to 1100 km, while a generalized semi-kinetic model is used for the 800 km to 3 R E altitude region. Convection-driven frictional ion heating and the effects of cusp/cleft soft electron precipitation in the F region/topside ionosphere, and centrifugal acceleration of ions and wave-driven transverse ion heating at high altitudes, are incorporated into the present simulations of the ion field-aligned transport. The modeled evolution of the O + flow parameters at 5500 km altitude exhibits typical patterns for the cleft ion fountain: O + field-aligned flows are upward over the dayside cusp/cleft and remain upward for 5 or more degrees latitude into the polar cap, with decreasing magnitude towards the pole; the flows turn downward at about 86° invariant latitude and tend to increase in magnitude (downward) across the polar cap from dayside to nightside; the O + density also displays apparent day–night asymmetry with higher density on the dayside. The simulated field-aligned flow pattern is in qualitative agreement with the observations from the thermal ion dynamics experiment (TIDE) during a Polar satellite southern perigee pass. The simulated ion densities and field-aligned fluxes are in general consistent with those observed. It is also shown in systematic simulations that the day–night asymmetry of the O + density across the polar cap from dayside to nightside may be directly controlled by the cleft ion fountain, while the H + density asymmetry is probably caused by day–night variations in solar illumination.

The O+ density trough at 5000 km altitude in the polar cap

At altitudes near 5000 km over the southern polar cap region of the terrestrial magnetosphere/ionosphere, the Thermal Ion Dynamics Experiment (TIDE) on board the Polar satellite has observed O+ ion density trough regions, in which the O+ densities were at least one order of magnitude lower than the surrounding O+ densities. In the O+ density trough regions, the estimated O+ densities were generally lower than 0.01 per cc. The boundaries between normal density level regions and the trough density regions were usually abrupt transitions. From 1 December 1997 to 30 November 1998, polar cap O+ troughs in Polar/TIDE observations occurred at a frequency of about 48%. Statistical examination of the Polar perigee observations from 1 December 1997 to 30 November 1998 shows that the Polar perigee passes evenly covered the southern polar cap region, while the O+ density trough was always located on the nightside portion of the polar cap magnetosphere/ionosphere, and that invariant latitude spans of such troughs could be as large as 23° in extent. The trough occurrence displayed a strong seasonal dependence; in the winter season (e.g., for July in the Southern Hemisphere) the O+ ion density trough occurrence frequency ranged up to 92%, while in the summer season (e.g., for January in the Southern Hemisphere) it decreased to as low as 15%. Our statistical results show that the trough occurrence was generally anticorrelated with solar wind dynamic pressure in the solar wind dynamic pressure range 0.8–2.6 nanopascal. The O+ ion density trough occurrence appeared relatively independent of the geomagnetic Kp index, IMF Bz, and By conditions. However, as suggested by the seasonal dependence, the O+ ion density trough occurrence was strongly related to the solar zenith angle (SZA). In the SZA range 50° to 125°, the trough occurrence increased monotonically with SZA. In addition, we sought to determine consistent density and velocity signatures at lower altitudes associated the O+ ion density trough at 5000 km by examining the near‐simultaneous O+ densities and vertical velocities observed by the DMSP satellite group orbiting at 840 km altitude. However, consistent correlations between the dual altitudes were not reliably established from the present examinations.

Outflow from the ionosphere in the vicinity of the cusp

Low‐energy ion outflows from the ionosphere are known to be an important source of plasma for the magnetosphere. The objective of this study is to describe the spatial relationships between low‐energy ionospheric outflows and the cusp. The analysis is similar to that used in superposed epoch analyses, except that a spatial reference is used instead of a temporal one. Data from the Thermal Ion Dynamics Experiment (TIDE) supported by data from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) and Hydra, during low‐altitude, perigee passes over the South Pole by the Polar spacecraft are used. The presence of isotropic ions from the magnetosheath is used to define the equatorward boundary of the cusp. By using this magnetospheric boundary as a reference point instead of invariant latitude, the temporal variability of the cusp location with respect to the Earth is removed. Poleward of this boundary in the cusp proper, observations show ionospheric outflow in addition to the isotropic magnetosheath ions. A technique used to separate the ion distributions originating from the two sources is presented and applied in order to calculate moments of the outflowing ion distribution. Ionospheric outflow is found to occur in the cleft region that extends ≈1.5° equatorward of the low‐latitude cusp boundary and in the cusp that extends ≈5° poleward of this boundary. Both the parallel and perpendicular temperatures of the outflowing ions are found to be approximately a factor of 3 larger on the equatorward side of this cusp boundary, i.e., in the cleft.

Polar observations of topside field‐aligned O+ flows and auroral forms

Measurements of thermal O+ ion densities, field‐aligned velocities, and fluxes from the Thermal Ion Dynamics Experiment (TIDE) on Polar obtained near 5000 km altitude over the Southern Hemisphere are compared with auroral images from the Ultraviolet Imager (UVI). Three passes were selected for analysis in this paper based on data availability from the TIDE and UVI instruments. Results indicate upward O+ flows in the cleft region but downward O+ flows in the polar cap region. Also, the O+ ion density follows a decreasing trend from the poleward side of the cusp region into the nightside aurora region. The magnitude of the downward O+ parallel velocities increases from dayside to nightside across the polar cap boundary. The upflows tend to occur over or near auroral forms, while the downflows are seen in relatively dark regions, such as the polar cap. These results are consistent with a cleft ion fountain source for the polar cap O+ ions. In the nightside polar cap, the results indicate a transition from downward to upflowing field‐aligned O+ ions near boundaries of bright auroral arcs.

Numerical model of the plasma sheath generated by the plasma source instrument aboard the Polar satellite

The plasma sheath generated by the operation of the Plasma Source Instrument (PSI) aboard the Polar satellite is studied by using a three‐dimensional particle‐in‐cell (PIC) code. When the satellite passes through the region of low‐density plasma, the satellite charges to positive potentials as high as 40–50 V, owing to the photoelectron emission. In such a case, ambient core ions cannot accurately be measured or detected. The goal of the onboard PSI is to reduce the floating potential of the satellite to a sufficiently low value so that the ions in the polar wind become detectable. When the PSI is operated, ion‐rich xenon plasma is ejected from the satellite, such that the floating potential of the satellite is reduced and is maintained at ∼2 V. Accordingly, in our three‐dimensional PIC simulation we considered that the potential of the satellite is 2 V as a fixed bias. Considering the relatively high density of the xenon plasma in the sheath (∼10–103 cm−3), the ambient plasma of low density(<1 cm−3) is neglected. In the simulations the electric fields and plasma dynamics are calculated self‐consistently. We found that an “apple”‐shape positive potential sheath forms surrounding the satellite. In the region near the PSI emission a high positive potential hill develops. Near the Thermal Ion Dynamics Experiment detector away from the PSI, the potentials are sufficiently low for the ambient polar wind ions to reach it. In the simulations it takes only about a couple of tens of electron gyroperiods for the sheath to reach a quasi steady state. This time is approximately the time taken by the heavy Xe+ ions to expand up to about one average Larmor radius of electrons from the satellite surface. After this time the expansion of the sheath in directions transverse to the ambient magnetic field slows down because the electrons are magnetized. Using the quasi steady sheath, we performed trajectory calculations to characterize the detector response to a highly supersonic polar wind flow. The detected ions' velocity distribution shows significant deviations from a shifted Maxwellian in the ambient polar wind population. The deviations are caused by the effects of electric fields on the ions' motion as they traverse the sheath.

Solar wind influence on the oxygen content of ion outflow in the high‐altitude polar cap during solar minimum conditions

We correlate solar wind and interplanetary magnetic field (IMF) properties with the properties of O+ and H+ during early 1996 (solar minimum) at altitudes between 5.5 and 8.9 RE geocentric using the Thermal Ion Dynamics Experiment (TIDE) on the Polar satellite. Throughout the high‐altitude polar cap we observe H+ to be more abundant than O+. O+ is found to be more abundant at lower latitudes when the solar wind speed is low (and Kp is low), while at higher solar wind speeds (and high Kp), O+ is observed across most of the polar cap. The O+ density and parallel flux are well organized by solar wind dynamic pressure, both increasing with solar wind dynamic pressure. Both the O+ density and parallel flux have positive correlations with both VswBIMF and Esw. No correlation is found between O+ density and IMF Bz, although a nonlinear relationship with IMF By is observed, possibly due to a strong linear correlation with the dynamic pressure. H+ is not as highly correlated with solar wind and IMF parameters, although H+ density and parallel flux are negatively correlated with IMF By and positively correlated with both VswBIMF and Esw. In this solar minimum data set, H+ is dominant, so that contributions of this plasma to the plasma sheet would have very low O+ to H+ ratios.

Quantitative modeling of modulated ion injections observed by Polar‐Thermal Ion Dynamics Experiment in the cusp region

On May 13, 1996, as the Polar spacecraft was traveling at high invariant latitudes (∼78°–79°) in the prenoon sector (∼1050 magnetic local time), the Thermal Ion Dynamics Experiment on board recorded successive injections of protons with clear energy‐time dispersion. These dispersion structures spread over several minutes and extend from several hundreds of eV down to a few tens of eV. During this pass, simultaneous measurements from the Wind spacecraft revealed little variation of the solar wind dynamical pressure but a gradual turning of the interplanetary magnetic field (IMF) from an essentially dawn‐to‐dusk orientation (i.e., predominant positive BY component and slightly negative BZ) to a north‐to‐south one (predominant negative BZ). We show that the observed injections result from magnetosheath particle entry at higher and higher latitudes in the dawn sector. Using test particle calculations in a simple model of reconnected interplanetary and magnetospheric field, we show that the injection modulation likely follows from changes in the dynamical regime experienced by the ions upon traversal of the magnetopause current sheet. That is, as the IMF gradually rotates, the time‐varying BY and BZ lead to changes in the adiabaticity parameter κ in the region of entry and affect particle access to the Polar location. In the morning sector where magnetosheath plasma accelerates downtail, such an access to the inner magnetosphere requires magnetic moment damping and is thus favored during nonadiabatic episodes. The flux variations obtained numerically are in qualitative agreement with those observed, both in terms of characteristic energy and overall time evolution. This supports our interpretation of the modulated ion injections in terms of intermittent nonadiabatic entry from the magnetosheath followed by time of flight dispersion between the magnetopause and the spacecraft.

Relationship of O + field-aligned flows and densities to convection speed in the polar cap at 5000 km altitude

Measurements of thermal O + ion number fluxes, densities, field-aligned velocities, and convection speeds from the Thermal Ion Dynamics Experiment (TIDE) on POLAR obtained near 5000 km altitude over the Southern hemisphere are examined. We find that the O + parallel velocities, densities, and number fluxes are strongly related to the convection speeds. The polar cap densities decrease rapidly with convection speed, with a linear least square fit formula to bin averaged data giving the relationship log N O + =−0.33∗V conv +0.07 , with a correlation coefficient of r=−0.96. The parallel bulk flow velocities are on average, slightly downward (0–2 km/s) for V conv < 2.5 km/s, but tend to be upward (0–4 km/s) for average V conv > 2.5 km/s. We also find that the downward number flux is strongly related to convection speed by log Flux =−0.54V conv +5.14 , with a correlation coefficient of r=−0.98. We interpret these relationships in terms of the Cleft Ion Fountain paradigm. The density decline with convection speed may result from increased spreading and resulting dilution from the restricted cleft source over the polar cap area with convection speed. The parallel velocities tend to be downward for low convection speeds because at such speeds, the ions fall earthward at shorter anti-sunward distances into the polar cap. At the higher convection speeds, the initially-upward flows are transported further into the polar cap and thus occupy a larger area of the polar cap.

The adequacy of the ionospheric source in supplying magnetospheric plasma

More than 30 years after the prediction of the polar wind outflow from the high latitude ionosphere, the exact magnitude and ultimate fate of the ionospheric plasma supply remains unknown. Estimates made more than a decade ago suggested that the polar ion outflow might well be of sufficient strength to populate the different regions of the Earth’s magnetosphere. Direct measurements in the high altitude magnetosphere became possible only with the launch of the Polar spacecraft. The combination of the Thermal Ion Dynamics Experiment and the Plasma Source Instrument has revealed the presence of low energy (<10 eV) ions moving through the polar regions and into the lobes of the magnetotail. These ions would have been invisible to previous un-neutralized satellites because of the high positive spacecraft potentials. Through the use of a recently developed single particle trajectory and energization code, the movement and energy transformation of these measured particles can be estimated. They are found to move into the plasma sheet region and to be energized to typical plasma sheet energies. The magnitude of the flux of the highly variable out-flowing ions mapped to 1000 km altitude is 1 − 3 × 10 8 ions/cm 2 s in agreement with the original estimates. Future observations by the TIDE/PSI instruments will be required to determine the extent of the total ionospheric contribution.

Evidence of component merging equatorward of the cusp

The Polar spacecraft passed through a region near the dayside magnetopause on May 29, 1996, at a geocentric distance of ∼8 RE and high, northern magnetic latitudes. The interplanetary magnetic field (IMF) was northward during the pass. Data from the Thermal Ion Dynamics Experiment revealed the existence of low‐speed (∼50 km s−1) ion D‐shaped distributions mixed with cold ions (∼2 eV) over a period of 2.5 hours. These ions were traveling parallel to the magnetic field toward the Northern Hemisphere ionosphere and were convecting primarily eastward. The D‐shaped distributions are distinct from a convecting Maxwellian and, along with the magnetic field direction, are taken as evidence that the spacecraft was inside the magnetosphere and not in the magnetosheath. Furthermore, the absence of ions in the antiparallel direction is taken as evidence that low‐shear merging was occurring at a location southward of the spacecraft and equatorward of the Southern Hemisphere cusp. The cold ions were of ionospheric origin, with initially slow field‐aligned speeds, which were accelerated upon reflection from the magnetopause. These observations provide significant new evidence consistent with component magnetic merging sites equatorward of the cusp for northward IMF.

Polar wind survey with the Thermal Ion Dynamics Experiment/Plasma Source Instrument suite aboard POLAR

In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 RE geocentric distance apogee in the northern hemisphere and 1.8 RE perigee in the southern hemisphere. The Thermal Ion Dynamics Experiment (TIDE) on POLAR has allowed sampling of the three‐dimensional ion distribution functions with excellent energy, angular, and mass resolution. The Plasma Source Instrument (PSI), when operated, allows sufficient diminution of the electric potential to observe the polar wind at very high altitudes. In this paper, we describe the results of a survey of the polar wind characteristics for H+, He+, and O+ as observed by TIDE at ∼5000 km and ∼8 RE altitudes over the polar cap during April–May 1996. At 5000 km altitude, the H+ polar wind exhibits a supersonic outflow, while O+ shows subsonic downflow, which suggests a cleft ion fountain origin for the O+ ions in the polar cap region. Dramatic decreases of the 5000 km altitude H+ and O+ ion densities and fluxes are seen as the solar zenith angle increases from 90° to 100° for the ionospheric base, which is consistent with solar illumination ionization control. However, the polar cap downward O+ flow and density decline from dayside to nightside in magnetic coordinates suggest a cleft ion fountain origin for the polar cap O+. Cleft ion fountain origin O+ density plumes could also be partially responsible for a similar day‐night asymmetry in H+, owing to the charge‐exchange reaction. At 8 RE altitude, both H+ and O+ outflows are supersonic and H+ is the highly dominant ion species. The average bulk ion field‐aligned velocities are in the typical ratio V0+ ∶ VHe+ ∶ VH+ ∼ 2 ∶ 3 ∶ 5, which may suggest a tendency toward comparable energy gains, such as via an electric potential layer.

Relationship of topside ionospheric ion outflows to auroral forms and precipitation, plasma waves, and convection observed by Polar

The POLAR satellite often observes upflowing ionospheric ions (UFIs) in and near the auroral oval on southern perigee (∼5000 km altitude) passes. We present the UFI features observed by the thermal ion dynamics experiment (TIDE) and the toroidal imaging mass angle spectrograph (TIMAS) in the dusk‐dawn sector under two different geomagnetic activity conditions in order to elicit their relationships with auroral forms, wave emissions, and convection pattern from additional POLAR instruments. During the active interval, the ultraviolet imager (UVI) observed a bright discrete aurora on the duskside after the substorm onset and then observed a small isolated aurora form and diffuse auroras on the dawnside during the recovery phase. The UFIs showed clear conic distributions when the plasma wave instrument (PWI) detected strong broadband wave emissions below ∼ 10 kHz, while no significant auroral activities were observed by UVI. At higher latitudes, the low‐energy UFI conies gradually changed to the polar wind component with decreasing intensity of the broadband emissions. V‐shaped auroral kilometric radiation (AKR) signatures observed above ∼200 kHz by PWI coincided with the region where the discrete aurora and the UFI beams were detected. The latitude of these features was lower than that of the UFI conies. During the observations of the UFI beams and conies, the lower‐frequency fluctuations observed by the electric field instrument were also enhanced, and the convection directions exhibited large fluctuations. It is evident that large electrostatic potential drops produced the precipitating electrons and discrete auroras, the UFI beams, and the AKR, which is also supported by the energetic plasma data from HYDRA. Since the intense broadband emissions were also observed with the UFIs, the ionospheric ions could be energized transversely before or during the parallel acceleration due to the potential drops.

The Thermal Ion Dynamics Experiment and Plasma Source Instrument

The Thermal Ion Dynamics Experiment (TIDE) and the Plasma Source Instrument (PSI) have been developed in response to the requirements of the ISTP Program for three-dimensional (3D) plasma composition measurements capable of tracking the circulation of low-energy (0–500 eV) plasma through the polar magnetosphere. This plasma is composed of penetrating magnetosheath and escaping ionospheric components. It is in part lost to the downstream solar wind and in part recirculated within the magnetosphere, participating in the formation of the diamagnetic hot plasma sheet and ring current plasma populations. Significant obstacles which have previously made this task impossible include the low density and energy of the outflowing ionospheric plasma plume and the positive spacecraft floating potentials which exclude the lowest-energy plasma from detection on ordinary spacecraft. Based on a unique combination of focusing electrostatic ion optics and time of flight detection and mass analysis, TIDE provides the sensitivity (seven apertures of ∼ 1 cm2 effective area each) and angular resolution (6°×18°) required for this purpose. PSI produces a low energy plasma locally at the POLAR spacecraft that provides the ion current required to balance the photoelectron current, along with a low temperature electron population, regulating the spacecraft potential slightly positive relative to the space plasma. TIDE/PSI will: (a) measure the density and flow fields of the solar and terrestrial plasmas within the high polar cap and magnetospheric lobes; (b) quantify the extent to which ionospheric and solar ions are recirculated within the distant magnetotail neutral sheet or lost to the distant tail and solar wind; (c) investigate the mass-dependent degree energization of these plasmas by measuring their thermodynamic properties; (d) investigate the relative roles of ionosphere and solar wind as sources of plasma to the plasma sheet and ring current.