Ion and Electron Characteristics in Auroral Density Cavities Associated with Ion Beams: No Evidence for Cold Ionospheric Plasma

Low density cavities associated with upgoing ion beams were identified in the auroral regions over two decades ago. In order to understand the waves, double layers, and solitary structures observed within these cavities, accurate measurements of the plasma distribution function are required. Although measurements by DE-1 indicated that these cavities were composed primarily of hot plasma in the form of ion beams, plasma sheet ions, and inverted-V electrons, later reports from Viking showed these cavities contained a cold plasma component whose density was an order of magnitude larger than the hot component. Recent measurements by the FAST satellite contrast sharply with the Viking results and support the earlier DE-1 observations. Regions of upgoing ion beams observed by FAST are shown to contain little or no cold plasma. The hot electron densities (>100 eV) and the combined plasma sheet ion and upgoing ion beam densities (>30 eV) agree remarkably well. Furthermore, no cold (0-30 eV) ions are measured at low energies by the mass spectrometer which precludes the presence of significant (>20%) cold electrons to preserve charge neutrality. Characteristics of the plasma for 11 ion beam events are tabulated.

The auroral upward current regions contain low density cavities associated with upgoing ion beams (Persoon et al., 1988; Strangeway et al., 1998). These cavities are thought to be caused by the evacuation of plasma by field aligned potential drops at lower altitudes (Mozer et al., 1977). Accurate measurement of the particle distributions are required in order to characterize the waves, double layers and solitary structures observed in these density cavities (Kintner et al., 1978; Temerin et al., 1982; Koskinen et al., 1990). Accurate particle measurements are also necessary to understand how the plasma is able to support the parallel electric fields. This paper will identify the primary particle distributions observed in these ion beam regions or density cavities. Throughout this paper the term density cavity and ion beam region can be considered synonymous, however we primarily use the latter since historically these regions have been identified by the ion sensors.

In an extensive study using both wave and particle data from DE-1, Persoon et al. (1988) concluded that the thermal (<18 eV) electrons and ions were minor components of the plasma in density cavities associated with upgoing ion beams. Later observations from the Viking spacecraft (Koskinen et al., 1990) indicated that ion beam regions contained a cold (few eV) ion population that was approximately an order of magnitude denser than the energetic ion beam. Observations near their instrument sensitivity threshold also indicated that the cold ions were drifting up the field line. No mass separation was performed and the cold ions were assumed to be hydrogen. A cold (few eV) electron component with roughly the same density was inferred from Langmuir probe measurements (Bostrom et al., 1988; Koskinen et al., 1989) that could provide charge neutrality. These Viking observations were made at 8000-11000 km altitudes, in the afternoon-evening sector, and in aurora with a total potential drop of about 1 kilovolt.

During the first year of the FAST mission, numerous ion beams were identified at altitudes >2500 km, typically in the nightside winter hemisphere. The FAST observations described below were selected from a set of orbits near midnight, in the northern winter hemisphere, at altitudes near 4000 km. Below we present detailed observations from three ion beam regions and general properties of the 11 events studied. Selection criteria for the events included ion beams lasting >10 seconds, valid mass spectrometer data, and low amplitude (<100 mV/m) electric fields. We will show that the hot ion and electron densities agree within measurement uncertainties (~20%), and that no evidence for cold plasma is observed. These results support the observations reported by Persoon et al. (1988) and contrast sharply with Viking results.

The FAST satellite contains top hat design, (Carlson et al., 1983; Carlson and McFadden., 1998a) electrostatic analyzers to measure the ion (3 eV - 25 keV) and electron (4 eV - 30 keV) distributions. The analyzers employ microchannel plate detectors in chevron configuration and biased to produce narrow pulse height distributions well above the preamplifier thresholds. The spacecraft is in a reverse cartwheel orbit and oriented relative to the magnetic field such that plasma analyzers measure full pitch angle distributions continuously. Analyzers include deflectors at their entrance aperture that steer their field of view by +/-10^oto always include both the parallel and anti-parallel magnetic field line. Both analyzers form a 48 energy x 32 angle measurement each 78 ms. This study uses "survey data" formed by onboard averaging software which provides continuous coverage across the auroral regions. The electrostatic analyzers provide the bulk of the measurements used in this study, so we include a discussion of instrument calibration and measurement uncertainties below. A discussion of false counts in ion sensors (background, UV contamination, scattered electrons) is included in the "Observations" section where these sources of error are obvious from the data.

Large electric fields along the spacecraft velocity vector (approximately north-south) can produce ExB drifts that move field aligned ion beams out of the analyzer's 360^ox6^o planar (or conical when deflectors are used) field of view. This deflection introduces an error in the density calculation of ion beams. The deflection is most pronounced for oxygen beams, where a 100 mV/m field can deflect a 1 keV oxygen beam by about 5^ofor magnetic fields at 4000 km. Although the deflection of hydrogen beams is 1/4 that of oxygen beams at the same energy, hydrogen beams tend to be narrower (~10^o FWHM) in pitch angle than oxygen beams (~30^o FWHM) so the effect on the pitch angle distribution measurement is similar. A 5^o deflection of a 1 keV proton beam requires a 400mV/m field at 4000 km altitudes, and reduces the calculated ion density to about 0.7 of the actual density for a 10^o FWHM beam.

East-west electric fields also affect the ion density calculation by shifting the ion beam off the magnetic field line. This error increases the calculated beam density since a larger phase space volume is assumed in the density calculation. Although east-west electric fields tend to be smaller, errors caused by these fields can be larger. For the same 5^o deflection of a 1 keV proton beam described above, the calculated ion beam density is about 1.6 times the actual density.

To reduce errors in the ion density calculation, slowly varying ion beam events with small electric fields (<100 mV/m during most of the ion beam) were selected for the study. Errors introduced by these fields are believed to be the major source of the small ~20% variations in the ratio of ion to electron densities seen below. It may be possible to calculate the ion density by correcting for large (>>100 mV/m) electric fields seen in the more turbulent ion beams, however this type of modeling is beyond the scope of this paper.

The ion and electron analyzer absolute geometric factors were estimated pre-flight from computer simulation, known grid transparencies, and published values of microchannel plate efficiency. The sensors were checked for uniformity using a Ni63 beta source and found to vary <20% around the analyzer field of view. The electron sensor was also tested in-flight for uniformity and small corrections in sensitivity with look direction are included in the calibration. The absolute geometric factor was tested in-flight by comparing the measured magnetic deflection across inverted-V arcs with the expected deflection determined from the measured electron flux and found to agree nearly exactly. (For examples see McFadden et al., 1998; Carlson et al., 1998b.)

The ion sensor calibration was tested by comparing the hot (>100 eV) electron and hot (>30 eV) ion densities in a selected ion beam (see Figure 1 and orbit 1616 in Table 1). A 35% adjustment to the pre-flight estimated sensitivity was required to increase the calculated ion density to that of the measured hot electron density. If cold electrons were present, a larger correction factor would be required. We attribute this correction to uncertainties in our knowledge of the absolute MCP efficiency for ions. Data from orbit 1616 were chosen because the plasma was primarily hydrogen (mass effects were nearly negligible on the density calculation) and because low level background counts due to scattered electrons in the ion sensor (see Observations section) could be ignored. In addition, no cold (<30 eV) ions were observed in the mass spectrometer, so the primary error in this normalization comes from a lack of knowledge of the cold (<100 eV) electron density (see Appendix: Cold Electron Measurements). The use of orbit 1616 data to normalize the ion sensor geometric factor would appear to invalidate this orbit as test for agreement between ion and electron densities. However, all 10 additional orbits showed good agreement between the hot ion and electron densities and a similar correction to the ion spectrometer's geometric factor would have been calculated for any of the 11 orbits. Since it is unlikely that the ratio of cold to hot electrons was the same for all 11 ion beam events, we have confidence that our normalization method is sound.

Finally, an additional source of error can arise from microchannel plate efficiency dependence upon energy and mass (Burrous et al., 1967; Gao et al., 1984). The FAST sensors include post-analyzer accelerations of ~450 V for electrons and ~1800 V for ions which should reduce these efficiency variations to the <10% level.

The FAST satellite also contains a Time-of-flight Energy-Angle imaging Mass Spectrometer (TEAMS) which measures the 3-D ion distribution each spin (Moebius et al., 1998a). These data, typically available with 5 second resolution, are used to obtain the relative H+, He+, and O+ densities across the ion beams. Pre-flight TEAMS calibrations are used for these data with estimated errors in relative sensitivity versus mass of <20% and with absolute sensitivity knowledge of ~50%. Since our calculations only use relative density versus mass to correct the upgoing ion beam density for mass, and since these corrections are typically a 25% density correction, TEAMS calibration errors should introduce <5% errors.

The relative ion densities vary slowly across arcs (Moebius et al., 1998b) and average ion density ratios were used to correct the higher time resolution ion spectrometer data for mass in the density calculation. The density ratio variations were small for most of the selected events, and we estimate <10% error introduced by changes in composition across these ion beams. However, two of the ion beams (orbits 1761 and 1793, Table 1) showed a larger variation in composition, which resulted in ~20% errors in the calculation during portions of these beams. A more sophisticated composition correction would have reduced this error, but was deemed unnecessary to improve the conclusions. The mass spectrometer data are also used to show that low level background counts in the ion spectrometer are due to internally scattered energetic electrons.

Figure 1 shows a section of an inverted-V arc (orbit 1616) containing ion beams that stretch over ~0.5o of invariant latitude. The top four panels show electron energy, electron pitch angle, ion energy and ion pitch angle spectrograms, respectively. The electrons have a spectral peak varying slowly between 3 and 1 keV and are isotropic in pitch angle except for the loss cone. Dropouts in the atmospheric secondary electrons (<100 eV, panel a) can be seen coincident with the appearance of the ion beam (panels c and d). The ion beam varies slowly in energy between 30 eV and 1 keV, and is collimated to within about 10^o of the magnetic field. Panel e shows the hot (>100 eV) electron density calculated from the measured distributions. The low energy (<100 eV) electron measurement is dominated by spacecraft photoelectrons and scattered secondary electrons precluding any reliable measurement of this density (see Appendix: Cold Electron Measurements). Panels f and g show the hot ion beam (>30 eV, 150^o-180^o pitch angle) and plasma sheet ion (>30 eV, 0o-150o pitch angle) densities calculated from the measured distributions. As discussed below, the mass spectrometer shows that no cold (<30 eV) ions are present in the ion beam region. The density calculation requires a mass assumption (n ~ (count rate) x

) since the ion spectrometer is not mass resolving. The plasma sheet ion density is calculated assuming hydrogen, which is consistent with the mass spectrometer data. The ion beam density has been calculated using a mass,

, given by:

where n_p, n_h, n_o are the relative densities of hydrogen, helium, and oxygen, determined by the mass spectrometer and averaged over the ion beam. For this event, the above correction is

. Panel h shows the ratio of hot ion to hot electron densities which shows remarkably good agreement between the densities inside the ion beam even though the beam density (panel f) varies by a factor of ~5 and the electron density (panel f) varies by a factor of ~2.

Now consider the measured ion distributions and why we introduced a cutoff of 30 eV in the ion density calculation. Figure 2a shows a contour plot of the ion differential energy flux (proportional to sensor count rate) for a 10 second average during the ion beam in Figure 1. The beam at 180^o pitch angle, and the hot, nearly isotropic, plasma sheet ions are easily identified. The average counts per energy-angle bin below 100 eV is about 1 in the contour plot. The slight enhancement in low energy counts at pitch angles between 180^oand 270^o is due to scattered ultraviolet into the sensor. Figure 2b and 2c show the differential flux and count rate for this event, summed over pitch angle. Note that the count rate (2c) is roughly constant, or independent of energy, below 50 eV and forms a differential flux spectra (2b) proportional to 1/energy. Figure 2d shows the partial ion density at each energy step assuming H+ (blue line,

, where E_i is the center energy of each energy step and

is the energy width), and the partial integral of the density starting at the highest step (green line,

, where Emax is the highest energy step). One can see that most of the density resides in the hot component, however as much as 20% appears to exist in the cold ions (4-30 eV). These low energy counts are about an order of magnitude above the microchannel plate noise levels. However, as shown below, these counts are due to a small number of scattered energetic electrons and a small amount of solar ultraviolet and do not represent actual cold plasma.

In addition to the ion spectrometer, the FAST satellite has a mass spectrometer which covers roughly the same energy range. The overall geometric factor of the mass spectrometer is about one half that of the ion spectrometer, producing count rates that differ by a factor of ~2 between the instruments. Figure 3a shows the count rate, summed over angle, from the mass spectrometer for the same interval as Figure 2. Total counts from the ion beam are smaller by a factor of ~50 for the mass spectrometer since it samples the ion beam only twice per spin whereas the ion spectrometer samples the beam continuously. A better comparison uses the nearly isotropic plasma sheet ion count rates (>5 keV) which differ by ~2 as expected for the two instruments.

In contrast to the ion spectrometer, the mass spectrometer measures no counts below about 40 eV. This is because the mass spectrometer is not susceptible to scattered electrons (or sunlight) due to coincidence detection in the time-of-flight electronics and due to a large (-20 kV) post acceleration between the analyzer and time-of-flight detector (Moebius et al., 1998a). Examination of ion beam events has shown that the ion spectrometer's low energy count rate increases with electron flux and electron energy, whereas the mass spectrometer observes no counts at these energies. For the event in Figure 1, false counts due to scattered electrons are rather small because most of the inverted-V electrons are repelled by the -1800 V bias on the front of the ion spectrometer's microchannel plate. In energetic arcs (~10 keV), the scattered electron count rate can be an order of magnitude larger. For these reasons, density calculations using the ion spectrometer do not include the low energy (<30 eV) counts due to scattered electrons, which is consistent with no counts in the mass spectrometer data at these energies.

Figure 3b uses the same format as Figure 2d to display the partial ion density at each energy step assuming H+ (blue line), and the partial integral of the density starting at the highest step (green line) for the TEAMS measurement. Here we have summed the counts at all masses and assumed H+ to mimic the ion spectrometer measurement, but have used the full 3-D TEAMS measurement. A comparison of the two density calculations show quite good agreement (only 20% difference when the <30 eV counts are ignored) between instruments with different sensitivities, field-of-views, and detection systems.

Before proceeding with additional examples, a final word should be said about the possible presence of cold ions whose energy is below the mass spectrometer's lower energy threshold of 2-5 eV (which depends upon spacecraft potential), and the possibility of cold electrons below our 100 eV cutoff. First, it would be quite remarkable if this plasma existed, since it would require both the cold electron and cold ion populations to be present with the same density, independent of hot plasma densities. However, the proof comes from the presence of large (~1 V/m) perpendicular electric fields in the stronger ion beams not used in this study. The ExB drifts from these fields would cause cold hydrogen (oxygen) to have energies of tens (hundreds) of eV in the spacecraft frame. Low energy cold ions moving perpendicular to B would be easily detected by FAST and have not been observed during these strong electric fields. The lack of cold ions precludes the presence of significant cold electrons to preserve charge neutrality. In addition, Strangeway et al. (1998) has shown that the hot electron density is consistent with measured plasma wave cutoffs, and Ergun et al. (1998) has shown that the AKR spectrum is consistent with a hot, relativistic electron population, providing further proof that the cold plasma is a small fraction of the total density.

Figure 4 shows an example of an ion beam event from orbit 1676 with the same format as Figure 1. The top 2 panels show an isotropic inverted-V electron distribution whose energy is just above 1 keV. The ion beam (panels c and d) varies between 100 eV and 2 keV. Low energy ion conics are also observed at 10:05:41-45 and after 10:05:49. Panel e shows the hot (>100 eV) electron density and panels f and g show the ion beam (>30 eV, 150o-180o pitch angle) and plasma sheet ion (>30 eV, 0^o-150^o pitch angle) densities. The ion beam density calculation used

as determined from the mass spectrometer data. The jump in plasma sheet ion density (panel g) at 10:05:41-:45 and after 10:05:49 is caused by ion conics included in the integral. Panel h shows that the ratio of hot ion to hot electron density is about 1 when the ion beam is present.

The smaller flux of inverted-V electrons in Figure 4a, as compared to Figure 1a, resulted in a factor of 3 reduction in scattered electrons into the ion sensor. Figure 5 shows a 10 second average of the ions in a format identical to Figure 2. Figure 5c shows a relatively constant count rate below 50 eV, similar to that in Figure 2c but with a lower rate. Mass spectrometer data for this beam shows no ions below 40 eV indicating these counts are instrumental. As can be seen from Figure 5d, the counts resulting from scattered electrons would have resulted in only a ~20% error for the ion density.

Figure 6 shows a third example of an ion beam event, this time from orbit 1771. The format is the same as Figures 1 and 4. The top 4 panels show an isotropic inverted-V electron population with an energy of ~1 keV, and an ion beam varying from about 100 eV to 1 keV. The mass spectrometer gave a beam mass of

for the beam density calculation (panel f). The plasma sheet ion density (panel g) is almost negligible (~0.05 cm^-3) for this event. Although the ion beam density (panel f) varies by nearly a factor of 8, panel h shows that the ratio of hot ion to hot electron densities remains near one across the event.

A comparison of ion beams on the three orbits (Figures 1, 4, and 6) shows a number of differences and similarities between the ion beams. The density was highest for orbit 1616, varying from about 1 to 3 cm^-3, whereas the beam density on orbit 1676 remained below 0.6 cm^-3. Orbit 1771 showed the largest variations in density swing from 0.2 to 1.1 cm-3. Slightly more oxygen was present during orbits 1676 and 1771, as seen from the beam mass correction or Table 1. The additional oxygen also shows up in contour plot 5a as the wider pitch angle portion of the beam at higher energies. In all three events the plasma sheet ions have a nearly constant density across the event, whereas the ion beam shows large density variations. To first order the beam density variations reflect conservation of flux, where a relatively constant flux of upgoing ions into the acceleration region should show a density inversely proportional to the

Table 1 summarizes characteristics of the electrons and ions for 11 ion beam events investigated in detail. All these events give good agreement between the hot ion and hot electron densities. Orbit 1623 was the lowest energy event and the lower limit for the ion beam calculation was dropped to 15 eV for this event. Most of the ion beam events had densities less than 1 cm^-3, and the lowest observed density was 0.13 cm^-3. Variations in the O+/H+ and He+/H+ beam density ratios were significant (greater than a factor of 2) for many of the events, especially orbit 1761 where a factor of 5 variation in O+/H+ was observed across the ion beam. However, the mass correction to the beam density for this event,

, varied by only +21% to -22% across the event due to additional changes in He+/H+, and was relatively constant across most of the event giving good agreement between ion and electron densities. A more sophisticated program that uses a time varying,

, across the beams could have produced limited improvement in a couple of the density ratio calculations but was unnecessary for this study. Table 1 provides theorists a range of parameters that can be used to model the ion beam regions.

The results presented above show convincing evidence that the hot electron and hot ion densities in auroral ion beam regions provide the bulk of the total density. The measurements were made by high resolution electron and ion spectrometers, with mass corrections provided by a time-of-flight mass spectrometer. The primary errors in this measurement appear to be introduced by large, low frequency electric fields, or electrostatic shocks, that deflect the ion beam transverse to the field line due to ExB drifts. These deflections result in <20% variations in the calculated density ratio on short time scales, however the density ratio averaged over the events agree to better than 10%. The cold ion (<30 eV) measurement is difficult with a spectrometer because a small fraction of energetic electrons can scatter through the analyzer producing false counts that corrupt the low energy measurement. This contamination is not present in the mass spectrometer data, and these data were used to show that no measurable ion fluxes were present below 30 eV. The detection of small fluxes of cold (<100 eV) electrons is very difficult to make with a spectrometer due to spacecraft photoelectrons and secondary electrons produced in the analyzer aperture (see Appendix: Cold Electron Measurements), so only the hot electron density was calculated. However, to preserve charge neutrality, we infer that no significant cold electron population is present during ion beams. This result is consistent with ion beam regions being produced by quasi-static parallel electric fields, which prevent cold electrons from penetrating into the density cavities associated with ion beams. In addition, wave observations by Strangeway et al. (1998) and Ergun et al. (1998) have shown that plasma wave cutoffs are also consistent with a hot (>100 eV) electron population providing the bulk of the total density.

The evidence for a lack of cold plasma in the density cavity supports the DE-1 results presented by Persoon et al. (1988), but differs from previous reports of Viking data. Koskinen et al. (1990) report evidence of a low energy, cold ion component moving up the field line during ion beam events. The density of the cold ion component was estimated to be an order of magnitude larger than the hot ion beam. Figure 6 of the Koskinen et al. (1990) paper shows ion spectra that resulted in the identification of cold plasma. The spectra below 100 eV varies as 1/energy, which implies a constant count rate similar to Figures 2c and 5c of this paper. Koskinen et al. (1990) conclude that the measurement is valid since the low energy counts are significantly above their instrument sensitivity. However, no discussion of possible sources of false counts is included in the paper, specifically the instrument's sensitivity to scattered energetic electrons. The Viking measurement of the low energy ion flux is an order of magnitude larger than the false count rate in the FAST ion spectrometer, and 2 orders of magnitude above the sensitivity of the FAST mass spectrometer which saw no flux at these energies.

To provide charge neutrality, Koskinen et al. (1990) report that a cold electron component was also present during the ion beams with about the same density as the cold ion component. This result was based upon Langmuir probe sweeps near the ion beams which inferred a 1-10 eV temperature for the electrons (Bostrom et al., 1988). However, Hilgers et al. (1992) shows that the Viking Langmuir probe current is dominated by photoelectrons from the wire boom within ion beam regions. They conclude that only an upper limit to the thermal electron density can be determined and that cold electrons are at most of the order of the energetic electron density.

It may be possible that differences between the FAST and Viking observations are due to other effects. Both data sets were measured at similar local times, with Viking observations in the afternoon-evening sector and FAST observations at ~22 MLT. Viking measurements were made at higher altitudes (8000-11000 km), typically above most of the acceleration region, whereas FAST observations (~4000 km) are near the bottom of the acceleration region. However, there are no reported observations or theories that suggest the plasma density should increase with altitude inside the ion beam regions, with the lowest densities at the bottom of the density cavities. The Viking observations were made near solar minimum, as were the FAST observations. Koskinen et al. (1990) state that the events were from "relatively weak acceleration events associated with low geomagnetic activity". As seen from Table 1, most of the FAST events were also during low activity with relatively weak acceleration (~1-2 keV ion beams). In both cases the O+ beams tended to have a higher energy and broader pitch angle width than the H+ beams. Koskinen et al. (1990) report a relatively small (<30%) heavy ion density in the upflowing ion beams. As seen from Table 1, the FAST observations differ in that the sum of the He+ and O+ densities typically exceeded the H+ density for the 11 events. These are typical of ion beams reported by Moebius et al. (1998b) in a statistical study of FAST ion beam events. The Viking events were also selected for a study of solitary waves. The 11 FAST events were examined and solitary wave structures were found in all the beams. From the above analysis and comparison, which suggests measurements of similar phenomena under similar conditions, we conclude that the interpretation of Viking measurements as observations of cold plasma in ion beam regions are in error.

The new FAST observations, which find only hot plasma in the ion beam regions, have strong implications for the density profile and plasma waves in the upward current regions. Without cold plasma, certain wave modes will disappear and other modes grow without heavy damping by the cold electrons. Studies of wave modes and solitary structures within ion beam regions that assumed a cold background plasma (Marchenko and Hudson, 1995, and references therein) need to be revisited in light of these new measurements. The need to postulate anomalous resistivity to prevent current runaway may also be unnecessary since the parallel acceleration region appears to contain only hot plasma. The measurements are consistent with simple acceleration by parallel electric fields which prevent cold ionospheric and secondary electrons from penetrating into the acceleration region, and accelerate all the cold ions (or cool ion conics) into energetic ion beams. The density profile along the magnetic field line appears to be a simple function of the parallel electric field profile, the high altitude plasma distribution, and the ionospheric plasma distributions. It may be possible to determine the electric field profile directly from the low and high altitude plasma distributions, combined with charge neutrality and current continuity in a region carrying an upward current. It is hoped that these new measurements will allow a consistent theory to be developed to explain the parallel electric fields and waves in the upward current regions.

Low energy electron measurements in the auroral zone are extremely difficult due to spacecraft photoelectron and secondary electron contamination. This appendix is included to demonstrate the difficultly in this measurement, to identify the dominant sources of low energy electron counts, and to justify the 100 eV cutoff that was used. Figure 7 shows contour plots of the electron differential energy flux (proportional to count rate) both inside (7a) and outside (7b) the ion beam of Figure 1. There are 6 primary sources of counts below 100 eV: background microchannel plate noise, solar ultraviolet scattering into the sensor, spacecraft generated photoelectrons, internally scattered energetic electrons from the auroral electron beam, trapped plasma electrons outside the loss/antiloss cone, and atmospheric secondary electrons in the loss/antiloss cone. Only the last two sources of counts represent plasma that should be used in the density calculation. Microchannel plate background noise caused by radioactive decay and penetrating radiation are negligible. Orientation of the analyzer and blackened, scalloped analyzer hemispheres reduce solar ultraviolet to negligible levels also. Spacecraft photoelectrons can be seen in both distributions near 90^oand 270^o pitch angles. These electrons disappear when the spacecraft is in shadow, but dominate the low energy measurement in sunlight. Scattered electrons from the primary auroral electron beam dominate the count rate during ion beams (7a) at pitch angles near 0^o and 180^o. Atmospheric secondary and trapped electrons dominate the count rate when the spacecraft is below the ion beam regions (7b), except at the lowest energies and near 90^o pitch angles where spacecraft photoelectrons always dominate.

From the above example, one can see that a quantitative low energy electron measurement is difficult at best. Since low energy counts can dominate the density calculation due to weighting by 1/

in the density calculation, a spectrometer determined density measurement that includes low energy electrons requires close scrutiny. Figure 7c, formed from the distribution in 7a within the ion beam, shows the partial electron density at each energy step (blue line,

, where E_i is the center energy of each energy step and

is the energy width), and the partial integral of the density starting at the highest step (green line,

, where Emax is the highest energy step). The contribution of photoelectrons and internally scattered secondary electrons to the density clearly becomes significant below 50 eV. The plateau of the green curve between 50 and 300 eV suggests minimal contribution of real electrons to the total density at these low energies. This can be contrasted with Figure 7d, formed from the distribution in 7b, which shows that atmospheric secondary electrons between 50 and 300 eV contribute significantly to the total local density outside ion beams.

Although we neglect the low energy (<100 eV) electrons in the density calculation due to instrumental reasons, this assumption can be justified if the ion beams are formed by quasi-static parallel electric fields. In this case cold plasma should be excluded from the ion beam regions. One can also estimate the density of neglected atmospheric secondary and trapped electrons with energy <100 eV within an ion beam by folding an electron distribution measured outside the ion beam through a 1 kilovolt potential. The potential eliminates portions of the initial distribution below 1 keV, and adiabatically maps the measured distribution between 1 and 1.1 keV to the 0 to 100 eV energies. The calculated <100 eV density for this mapped distribution is only a few percent of the hot (>100 eV) electron density. Since the observed hot electron density agrees with the ion density as shown in Figures 1, 4, and 6, our assumption of negligible cold (<100 eV) plasma appears to be justified. We note that the assumption that the <100 eV electrons can be neglected is not valid when the ion beam has a low energy since significant secondary electrons can then penetrate the potential below the spacecraft. This effect can be seen in Figure 1 at 20:53:34, where the ion beam density (panel f) goes up, and is not balanced by the hot electrons (panel e).

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Moebius, E., L. M. Kistler, M. A. Popecki, K. N. Crocker, M. Granoff, Y. Jiang, E. Sartori, V. Ye, H. Reme, J. A. Sauvaud, A. Cros, C. Aoustin, T. Camus, J.-L. Medale, J. Rouzaud, C. W. Carlson, J. P. McFadden, D. Curtis, H. Heetderks, J. Croyle, C. Ingraham, B. Klecker, D. Hovestadt, M. Ertl, F. Eberl, H. Kastle, E. Kunneth, P. Laeverenz, E. Seidenschwang, E. G. Shelley, D. M. Klumpar, E. Hertzberg, G. K. Parks, M. McCarthy, A. Korth, H. Rosenbauer, B. Grave, L. Eliasson, S. Olsen, H. Balsiger, U. Schwab, M. Steinacher, The 3-D plasma distribution function analyzers with time-of-flight mass discrimination for Cluster, FAST, and Equator-S, AGU Monog. Meas. Techn. Space Plasmas, ed. R. Pfaff, in press, 1998a.

Moebius, E. L. Tang, L. M. Kistler, M. Popecki, E. J. Lund, D. Klumpar, W. Peterson, E. G. Shelley, B. Klecker, D. Hovestadt, C. W. Carlson, R. Ergun, J. P. McFadden, F. Mozer, M. Temerin, C. Cattell, R. Elphic, R. Strangeway, R. Pfaff, Species dependent energies in upward directed ion beams over auroral arcs as observed with FAST TEAMS, Geophys. Res. Lett., in press, 1998b.

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Strangeway, R. J., L. Kepko, R. C. Elphic, C. W. Carlson, R. E. Ergun, J. P. McFadden, W. J. Peria, G. T. Delory, C. C. Chaston, M. Temerin, C. A. Cattell, E. Moebius, L. M. Kistler, D. M. Klumpar, W. K. Peterson, E. G. Shelley, R. F. Pfaff, FAST observations of VLF waves in the auroral zone: evidence of very low plasma densities, Geophys. Res. Lett., submitted, 1998.

J. P. McFadden, C. W. Carlson, R. E. Ergun, Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450. (e-mail: mcfadden@ssl.berkeley.edu)

D. M. Klumpar, Lockheed Martin Palo Alto Research Laboratory, Palo Alto, CA 94304

The figure shows a comparison of the hot (>100 eV) electron and hot (>30 eV) ion densities across an ion beam. Panels a and b show electron energy and pitch angle spectrograms. Panels c and d show ion energy and pitch angle spectrograms. Panels e, f and g are the integrated hot electron density, ion beam density corrected for beam composition, and hot plasma sheet ion density. Panel h is the ratio of ion to electron density, which is nearly one across the ion beam.

Figure 2a is a contour plot of the ion differential energy flux (proportional to count rate) averaged over the beam in Figure 1. The 10 second average has broadened the ion beam's parallel energy. Isotropic plasma sheet ions, minus the loss cone, are seen at 10 keV. Counts below 30 eV are primarily due to internally scattered inverted-V electrons, along with some scattered solar ultraviolet (between 180^oand 270^o). These false counts have a constant count rate below 30 eV (see Figure 2c) and produce a spectra (Figure 2b) proportional to 1/energy. Figure 2d shows the partial density (blue line,

, where E_i is the center energy of each energy step and

is the energy width), and the partial integral of the density starting at the highest step (green line,

, where Emax is the highest energy step). From 2d it can be seen that the low energy false counts would produce a ~20% error in the total density if included.

Figure 3a is the count rate from the FAST mass spectrometer (TEAMS) averaged over the same 10 second interval as Figure 2. No counts below 30 eV are measured by this instrument, proving that the constant count rate seen at low energies in Figure 2c is not real. The count rates for the nearly isotropic plasma sheet ions (>5 keV) differ by a factor of two between Figures 3a and 2c, as expected for the two instruments' relative geometric factors. The ion beam is not prominent in the mass spectrometer data since it samples the beam only twice per spin. The partial density calculation in Figure 3b is nearly identical to Figure 2d except for the error at low energy (<30 eV) caused by false counts in the spectrometer data.

The figure shows a comparison of hot (>100 eV) electron and hot (>30 eV) ion densities for orbit 1676 with the same format as Figure 1. Panels e, f, and g show the electron, ion beam and plasma sheet ion densities respectively. Panel h shows the ratio of ion to electron density is near one during the ion beam.

The figure shows ion characteristics during the ion beam from Figure 4. The format is the same as Figure 2 and displays many of the same characteristics. The ion beam and plasma sheet ions are easily seen in Figure 5a. A weaker flux of inverted-V electrons (see Figure 4) produces a smaller false count rate in Figure 5c at energies <30 eV than is seen in Figure 2c.

The figure shows a comparison of hot (>100 eV) electron and hot (>30 eV) ion densities for orbit 1771 with the same format as Figures 1 and 4. Panels e, f, and g show the electron, ion beam and plasma sheet ion densities respectively. Although the total density (panel e) varies by a factor of ~7 across this beam event, the ratio of ion to electron density remains near one (panel h).

Contour plots of the electron differential energy flux (proportional to count rate) both inside (Figure 7a) and outside (Figure 7b) the ion beam region of Figure 1. Figures 7c and 7d show the partial density (blue line,

, where Ei is the center energy of each energy step and

is the energy width), and the partial integral of the density starting at the highest step (green line,

, where Emax is the highest energy step) for the contour plots 7a and 7b, respectively. Photoelectrons (<100 eV near -90^o, 90^o, 270^o pitch angles) dominate the density calculation if included in the sum. Atmospheric secondary electrons are seen in Figure 7b at energies <200 eV and angles -50^o to 50^o and 130^o to 230^o, and secondary electrons produced in the analyzer are seen at these angles at <50 eV in Figure 7a.

* Averages over beam event. Factor of 2 to 5 variations may occur across a particular beam event. The largest variation occurred across orbit 1761 with O+/H+ changing from 0.5 to 2.5.

Orbit #	Ratio Ni/Ne	KP	Electron density Range (cm-3)	Peak Energy (kev)	Ion Beam Density (cm-3)	O+/H+ Ratio*	He+/H+ Ratio*	Energy range (keV)	PS Density (cm-3)	PS Temp (keV)
1525	1.13+/-0.11	1	.55-.85	1	.3-.6	0.28	0.67	.3-1.	.35-.45	10
1616	0.99+/-0.08	1	1.2-2.5	1-2	.4-2.2	0.36	0.65	.1-1.	.7-.8	5
1623	0.98+/-0.13	1-	.9-2.5	.8	.45-2.3	0.54	0.34	.1-.3	.3	2
1633	1.00+/-0.14	1	.4-.7	2-3	.15-.6	1.28	0.47	.1-3	.15	5
1676	0.93+/-0.09	1-	.33-.6	1.5	.15-.45	0.87	0.20	.1-2	.15-.2	3
1692	0.83+/-0.12	2+	.4-1.2	1-2	.15-.75	0.87	0.54	.3-2	.2-.35	6
1740	0.98+/-0.16	2-	.13-.40	1-2	0.08-.2	1.09	0.38	1-2	0.04-.1	5
1761	0.98+/-0.16	1-	.13-.28	2-3	.05-.1	1.44	0.46	1-5	0.06-.1	7
1771	0.88+/-0.13	1-	.2-1.2	1-2	.15-.9	0.54	0.57	.3-2	.05-.15	3
1793**	1.04+/-0.14	1	.25-1.0	3-10	.15-6.	1.42	0.23	.3-20.	.1-.25	10
1804	0.93+/-0.13	1	.2-1.0	1-10	.1-.5	1.11	0.31	.2-8	.05-.2	4