Measurements

The Hydra instrument on the Polar spacecraft consists of two separate particle measurement systems, the DuoDeca Electron Ion Spectrometer (DDEIS) and the Parallel Plate Analyzer (PPA). The DDEIS measures electron and ion spectra in broad angular bins ($8^\circ \times 8^\circ$) at twelve look directions in 1.15 s energy sweeps from 12 eV to 18 keV. Alternate sweeps simultaneously measure all twelve directions for electrons and then ions, yielding a time resolution for either species of 2.3 s. As the spacecraft rotates, complete coverage of all pitch angles is obtained. The PPA measures electrons only and provides fine angular resolution ($0.7^\circ$ pixels over a $32 \times 32$ grid). A half spin is required for complete pitch angle coverage, yielding a time resolution of$\sim$25 s for an eight-point energy sweep. PPA pitch angles are sorted on-the-fly as the spacecraft spins and only the resultant pitch angle-energy information is telemetered. Complete details of the instrumentation can be found in Scudder et al.(1995).

We have surveyed 90 orbits of Hydra data from April-May of 1996 and 1997 when Polar passed through the high latitude plasma sheet. These orbits provide coverage of the evening auroral zone over an MLT range of 2000-2400 at altitudes well above the auroral acceleration region (4-7.5 R_E geocentric). The Polar orbit is such that it is always in this MLT region and altitude range at the same time of year. The electron anisotropy results presented are therefore limited to this MLT range and season.

Figure 1 shows Hydra data for a Polar passage across auroral field lines on April 29, 1996. As time increases, the spacecraft is moving equatorward (toward lower invariant latitude) and first encounters the plasma sheet just before 1340 UT. The middle three panels labeled (b)-(d) show electron spectrograms for pitch angle ranges of 0-30$^\circ$ (downward), 75-105$^\circ$ (perpendicular), and 150-180$^\circ$ (upward) respectively. Panels (e) and (f) show measures of electron pitch angle structure as a function of energy. The first, in panel (e), called skew, shows the difference between field-aligned electrons moving upward (0-30$^\circ$) and downward (150-180$^\circ$) along the magnetic field. The second in panel (f), called anisotropy, shows the difference between the field-aligned (average of 0-30$^\circ$ and 150-180$^\circ$) electrons and those that are trapped (75-105$^\circ$). In both cases, these differences are normalized by the expected standard deviation for the difference. In this way, skew and anisotropy represent signal to noise statistics. Yellow-red corresponds to positive values and blue-purple to negative values. Normalized differences of less than $1\sigma$, which are not significant, are plotted as black.

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Figure 1: Electron data from a northern latitude plasma sheet crossing by Polar on 29 April, 1996. The top panel shows selected distribution functions as the spacecraft moves equatorward (left to right). The magnetic field direction is downward and each axis covers $\pm 25000$ km/s. Panels (b)-(d) show three pitch angle ranges of 0-30$^\circ$, 75-105$^\circ$,150-180$^\circ$, respectively. The lower two panels show measures of pitch angle structure in the opposing field-aligned directions (e) and the field-aligned direction vs. 90$^\circ$.
 
 

The skew (Figure 1e) shows that at the poleward edge of the plasma sheet between 1338 and 1405 UT, fluxes are generally enhanced in the field aligned direction with a weak (blue) preference for the downward ($\mathbf b$)direction. After 1404 UT, there is little significant structure to the skew. The anisotropy shown in Figure 1f, however, reveals considerable detail over the entire plasma sheet. At the poleward edge, alternating regions of highly field-aligned electrons (red) and enhanced 90$^\circ$electrons (blue) are seen.

The contoured distribution functions in Figure 1a show further detail of the electron anisotropy for selected times. In each plot,$v_\parallel$ is along the vertical axis with the arrowhead at the bottom of each plot indicating the direction of the magnetic field and $v_\perp$ is along the horizontal axis. Both axes cover a range of $\pm 25000$ km/s. The first and third distribution function plots (from left) in Figure 1a show an enhancement of flux in the anti-field-aligned direction, that is, upward out of the ionosphere. The second and fourth plots in Figure 1a show a loss-cone indicative of electrons that have precipitated into the ionosphere. In both of these latter two examples, there is no enhancement at lower energies indicative of backscattered electrons (degraded primaries and ionospheric secondaries). This data is consistent with a field-aligned potential drop below the spacecraft which accelerates electrons downward and prevents backscattered electrons which have energies less than the potential drop from reaching Polar altitudes.

The interpretation of these observations of electron anisotropy is shown schematically to the left of the vertical dotted line in Figure 2. corresponds to the second and fourth distributions of Figure 1a that are consistent with an upward pointing electric field below the spacecraft which accelerates electrons into the ionosphere, creating an ionospheric population of hot, accelerated electrons, cold ionospheric electrons, and intermediate energy backscatter electrons as shown at the bottom of Figure 2. At Polar only the loss cone distribution is observed because the upward electric field prevents backscattered electrons from reaching Polar. The leftmost illustration corresponds to the first and third distributions shown in Figure 1a that are consistent with a downward pointing electric field below the spacecraft which accelerates the cold background ionospheric electrons (shown as set of small, circular contours at the bottom of the figure) to create an upward-directed ($- \mathbf b$)electron beam at Polar. The middle illustration of Figure 2 

 

Figure 2: Schematic illustration of anisotropic electron distributions observed on Polar and their relationship to discrete (accelerated) and diffuse (unaccelerated) auroral processes below the spacecraft.
 
 

At lower latitudes, after 1415 UT, the spacecraft moves onto field lines with electron signatures consistent with diffuse aurora below the spacecraft. The anisotropy pattern in Figure 1f appears stacked as a function of energy; higher energy electrons with perpendicular anisotropy (blue) appear above lower energy regions of field-aligned anisotropy (red). As the average energy of the electrons decreases (see Figure 1b-d), the energy range of field-aligned electrons also decreases. These low energy field-aligned electrons are identified as backscattered electrons produced by precipitation of the higher energy electrons of the diffuse aurora. Because no field-aligned potential drops exist for the diffuse aurora, these electrons escape the ionosphere to be observed on Polar and are field-aligned due to the effect of the magnetic mirror force. Although they are emitted from the ionosphere over all upward directions, conservation of the first adiabatic invariant focuses these electrons into a narrow beam at the Polar altitude of 6-7 R_E. This is shown schematically in rightmost illustration of Figure 2. Because the backscattered electrons have energies less than the primaries, it is expected that as the primary spectrum decreases in energy, so too will the backscattered electron spectrum.

A second example from May 8, 1997 is shown in Figure 3. As can be seen by comparing with Figure 1, the basic types and organization of features is the same as in the previous example. At the poleward edge, field-aligned and loss-cone features alternate until the band of diffuse aurora is reached with loss-cone distributions stacked in energy above field-aligned distributions. Regions of discrete and diffuse aurora alternate until becoming purely diffuse at 1125 UT. For this event, the most poleward part of the plasma sheet is less clearly structured than the April 29, 1996 crossing with a mix of types of anisotropy as a function of energy as shown in Figure 3f. The first of the plots in Figure 3a shows that at the most poleward edge, the distribution function is substantially more enhanced in the downward ($\mathbf b$) direction than the upward direction. This may indicate that the poleward edge of the plasma sheet has moved poleward and that the plasma has not yet had time to mirror and return up the field line. The remaining distributions in Figure 3a are very similar to those in Figure 1a.

 

Figure 3: Electron data during a northern latitude plasma sheet crossing by Polar on May 8, 1997 in the same format as Figure 1.
 
 

For both events, the poleward electron observations that we have associated with discrete aurora are often accompanied by electrostatic shocks and field-aligned ion distributions (not shown). Neither shows a simple relationship to the electron features. It is not clear that a direct association between electron and ion features is expected because the much slower parallel velocities of the ions will cause them to drift substantially relative to the electrons at Polar altitudes. We focus here on the electron features only and leave inclusion of these other data for future work.

The identification of the upward field-aligned electrons observed on Polar as the downward-current carrying electrons observed at lower altitudes implies that they should be extremely field aligned at Polar altitudes. At the altitudes sampled by FAST, these electrons are already field-aligned within a few degrees of $180^\circ$ [Carlson et al.(1998)]. At Polar altitudes, the mirror force will cause them to be even more field aligned. By conservation of the first adiabatic invariant, a loss cone distribution should show an angular width given by the mapping of $90^\circ$ pitch angle electrons at ionospheric altitudes to Polar altitudes. Backscattered electrons emitted due to the the precipitating population will obey the same mapping. In both cases, these are very narrow angular ranges which are not resolved by the measurements in Figures 1 and  3.

These narrow pitch angle features can be resolved in the data from the PPA which has angular resolution of 1.4$^\circ$ for angles near the field-aligned direction. No PPA data for the April 29, 1996 event are available. Figure 4a shows a pitch angle-energy spectrogram of PPA data corresponding to the third pitch angle plot (from left) in Figure 3a. The distribution function plot shows a loss cone at higher energy and field-aligned electrons at lower energies, both in the upward direction. The PPA data (Figure 4a) show the same features but are are now clearly resolved to have a width of $\sim$5$^\circ$. To guide the eye, a dashed line has been placed at 175$^\circ$. Computation of the expected width of the loss cone gives a value of 4$^\circ$, in excellent agreement with these results.

 

Figure 4: High angular resolution (1.4$^\circ$ bins) electron data from the northern latitude plasma sheet crossing on May 8, 1997. Panel (a) shows a pitch-angle-energy spectrogram when Polar was above a region of unaccelerated plasma sheet precipitation into the ionosphere indicated by the narrow loss cone near 180$^\circ$ and enhanced backscattered electrons over the same angular range at lower energy. Panel (b) shows electrons which have been upwardly accelerated at the ionosphere resulting in an extremely field-aligned beam. In both panels a dashed line marks 175$^\circ$ pitch angle.
 
 

Figure 4b shows data taken $\sim$4 minutes later during the intense region of upward field-aligned electron fluxes shown in the fourth distribution function plot of Figure 3. The high angular resolution PPA data also show strong field-aligned fluxes. The most field-aligned pitch angle bin shows a count level of the order of 2-3 times greater than the next adjacent pitch angle bin which suggests that this beam is narrower than the 1.4$^\circ$ width of the most field-aligned bin. This observation agrees very well with the interpretation that these are the same highly field-aligned fluxes observed at lower altitudes by spacecraft such as FAST [Carlson et al.(1998)].

3/5/1999