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FAST Conjunctions

Besides the magnetometer array which provides a continuous but low-spatial resolution monitor of the auroral currents, on occasions satellite overflights provide higher-resolution measurements. Since the launch of the FAST satellite in August 1996, there have been hundreds of orbits where the footprint of FAST (mapped to 100 km) has passed within 3$^\circ$ of one or more of the PSFR ground stations. Eight such conjunctions have been identified during which auroral roar is observed at one or more of these sites. During two of these the PSFRs at all five ground stations were operational: May 2, 1997 and February 17-18, 1998. PSFR data from these two nights are shown in Figures 6 and 7 and discussed above.

The footprint of FAST is computed by mapping the location of the spacecraft down the field line to a point in the Earth's ionosphere (100 km altitude) that is magnetically connected to the satellite location in space. The IGRF model with extrapolated secular variation IGRF:85 is used for the Earth's magnetic field in this computation. Dark lines in Figures 8 and 9

Figure 8. The footprint of the FAST satellite during orbit 2745. The geographic location of the Dartmouth College northern hemisphere PSFRs (circles) and the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometers (crosses) are shown for reference. The dawn-dusk terminator is also shown.
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bb=120 300 460 550,clip}}
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Figure 9. The footprint of the FAST satellite during orbit 5903. The geographic location of the Dartmouth College northern hemisphere PSFRs (circles) and the CANOPUS magnetometers (crosses) are shown for reference. The dawn-dusk terminator is also shown.
\begin{figure}\figbox*{\hsize}{}{\epsfig{file=figs/orb5903.ps, width=8.4cm,
bb=120 300 460 550,clip}}
\end{figure}

show the footprint of FAST relative to the ground stations during the two conjunctions on May 2, 1997, and February 17, 1998, respectively. The motion of the satellite is equatorward (down) in both cases. The lighter line in these plots indicates the dawn-dusk terminator. FAST covers the entire latitude range of the PSFRs (67$^\circ$-79$^\circ$) within 5-10 min, in effect providing a snapshot of auroral properties along a cut through the oval at the conjunction time.

Figures 10 and 11 display data from the

Figure 10. Data from electrostatic analyzers aboard FAST during orbit 2745 showing electron energy flux versus energy for (a) 0$^\circ$-30$^\circ$ and (b) 150$^\circ$-180$^\circ$ pitch angle ranges. (c) Electron energy flux versus pitch angle are shown for energies $>$1 keV. (d) Integrated PSFR power averaged for 4 min when FAST crosses the polar cap boundary. The altitude of the spacecraft and the invariant latitude of its footprint are indicated below Figure 10d.
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Figure 11. Data from electrostatic analyzers aboard FAST during orbit 5903 showing electron energy flux versus energy for (a) 0$^\circ$-30$^\circ$ and (b) 150$^\circ$-180$^\circ$ pitch angle ranges. (c) Electron energy flux versus pitch angle are shown for energies $>$1 keV. (d) Integrated PSFR power averaged for 4 min when FAST crosses the polar cap boundary. The altitude of the spacecraft and the invariant latitude of its footprint are indicated below Figure 11d.
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/5903_thresh.ps, width=12cm}}
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electrostatic analyzers aboard FAST during the two overflights. Figures 10a, 10b, 11a, and 11b show electron energy flux versus energy for the pitch angle ranges 0$^\circ$-30$^\circ$ and 150$^\circ$-180$^\circ$, respectively, where 0$^\circ$ corresponds to earthward (downgoing) and 180$^\circ$ corresponds to anti-earthward (up-going) directions. Figures 10c and 11c show electron energy flux versus pitch angle for energies exceeding 1 keV. No wave data above 2 MHz are available for these orbits.

Figures 10d and 11d represent the power of the auroral roar emissions detected at each of the ground stations during the FAST overpass. The measured power is averaged for a 4 min interval centered on the time when FAST passes over the polar cap boundary, which is inferred from the sudden increase in the measured electron flux at energies capable of producing visible aurorae ($>$0.1 keV). The solid line indicates the integrated and averaged power at each station, excluding Churchill. Since the noise level at Churchill is $\sim$10 dB higher than that for any of the other PSFRs because of the low sensitivity of the polarization antenna, a straight comparison of power is biased by the differing noise levels and therefore is not very insightful. In order to better represent the relative intensity of the observed emissions between sites, the dotted line in Figures 10d and 11d represents the power above the Churchill noise level ($\sim$4.0$\times$10$^{-10}$ V$^2$/m$^2$), integrated over the frequency range specified and averaged for 4 min around the FAST polar cap boundary crossing. From these two lines the real power at the stations can be envisioned by continuing the solid line to include Churchill using the dotted line as an estimate of the relative intensities. A power level of $\le$5$\times$10$^{-12}$ V$^2$/m$^2$ indicates there were no observed emissions above the Churchill noise level. Tick marks at the bottom of Figures 10d and 11d indicate when the footprint latitude of FAST coincides with that of each ground station.



Subsections
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Next: May 2, 1997 Up: Latitudinal dynamics of auroral Previous: February 17, 1998


Simon Shepherd 2002-06-05