next up previous
Next: Data Presentation Up: Latitudinal dynamics of auroral Previous: Introduction


The Dartmouth College PSFRs are located at the sites listed in Table 1 and shown in Figures 8 and 9. These record 30-5000 kHz spectra 20 hours/day at 2 s time resolution. The antennas are sensitive to the magnetic component of the auroral roar emissions, and the power spectral density is computed assuming the waves propagate at the speed of light. Effort has been made to synchronize the stations with Greenwich mean time and with each other to within 2 s. In case studies this synchronization has been verified using anthropogenic signals detected simultaneously with multiple receivers. The different PSFRs use nearly identical antennas, which are $\sim$10 m$^2$ vertical loops. The orientations of the antennas differ at each site since they are adjusted to null local sources of interference. The orientation of the loops does not greatly affect the sensitivity to auroral roar emissions, which are more circularly rather than linearly polarized Shepherd:97a. The gains and offsets of the different PSFRs are designed to be as identical as possible. However, in order to compare signals received at the different sites, calibration signals are injected at the preamplifier of each PSFR once each day. Using two calibration signals, one 20 dB below the other, both the gain and the offset of each PSFR can be determined. The resulting signals can be compared to within an accuracy of $\sim$3 dB.

Table 1. Locations in the Northern Hemisphere With Geomagnetic Instrumentation
    Geographic Invariant UT of
ID Site Location Lat (N) Lon (E) Lat (N) Lon (E) 2400 MLT
TA $^\mathrm{a, b}$ Taloyoak, NWT 69.54 266.45 79.1 328.5 0646
BL$^\mathrm{a}$ Baker Lake, NWT 64.32 263.97 74.2 327.1 0651
RI $^\mathrm{b, c}$ Rankin Inlet, NWT 62.82 267.89 73.1 334.5 0626
AR $^\mathrm{a, b}$ Arviat, NWT 61.11 265.95 71.4 331.6 0636
CH $^\mathrm{a, b}$ Churchill, Manitoba 58.76 265.92 69.1 332.1 0634
GI $^\mathrm{a, b, c}$ Gillam, Manitoba 56.38 265.36 66.8 331.6 0636
IL$^\mathrm{b}$ Island Lake, Manitoba 53.86 265.34 64.4 332.0 0635
PI $^\mathrm{b, c}$ Pinawa, Manitoba 50.20 263.96 60.7 330.5 0640
International Geomagnetic Reference Field (IGRF) Epoch 1998 magnetic model is used. Abbreviations are as follows: ID, site identification; Lat, latitude; Lon, longitude; NWT, Northwest Territories; and MLT, magnetic local time.
$^\mathrm{a}$ Dartmouth programmable stepped-frequency receiver (PSFR) is present.
$^\mathrm{b}$ CANOPUS magnetometer is present.
$^\mathrm{c}$ CANOPUS meridian scanning photometer (MSP) is present.

The magnetometer array whose coordinates are listed in Table 1 and which is shown in Figures 8 and 9 is operated by the Canadian Space Agency. These sites are equipped with a three-component ring core magnetometer which provides data at a sampling rate of 5 s. A more detailed description of the capabilities of the instruments at these sites is given by Rostoker:95. The magnetometers are sensitive to currents in the ionosphere, such as the eastward or westward auroral electrojets. By plotting the $Z$ component of the magnetic disturbance measured by magnetometers along a rough magnetic meridian, the poleward and equatorward borders, as well as the strength, of the auroral electrojet crossing this meridian can be estimated. An east (west) electrojet current, for example, causes a positive (negative) perturbation in the $Z$ component of the magnetic field at stations north of the current and negative (positive) perturbations at stations south of the current. Deconvolving the observed magnetic signatures with a model including the electrojet and the associated field-aligned currents, which electrically connect the ionosphere and magnetosphere, reveals that the extrema in the latitudinal profile of the $Z$ component of the disturbance magnetic field provide a good measure of the boundaries of the electrojet. The strength of the electrojet is estimated from the difference between the measured extrema Kisabeth:72, where an electrojet strength of 1 MA uniformly distributed across 5$^\circ$ of latitude produces a perturbation of $\sim$700 nT Rostoker:98b. The poleward edge of the auroral electrojet calculated this way is a proxy for the polar cap boundary. The University of Alberta provides a standard analysis of electrojet boundaries and activity level at a 5 min sample rate. They use the Polar Anglo-American Conjugate Experiment (PACE) invariant coordinate system Baker:89, Gustafsson:92, which agrees with the International Geomagnetic Reference Field (IGRF) coordinates to within $\sim$0.5$^\circ$.

NASA's FAST satellite was launched August 21, 1996, into an 83$^\circ$ inclination elliptical orbit of 350 km by 4175 km with an orbit period of 2.4 hours and a 1.45$^\circ$ per year westward precession of the orbit plane. A 6$^\circ$ wide region of the northern auroral zone is thus overflown by FAST on four consecutive days every $\sim$25 days. The precessional period of the perigee latitude is $\sim$204 days giving a wide sampling of auroral zone altitudes during the year. Exploiting multiple particle and field sensors including a set of 16 electrostatic analyzers, FAST measures unobstructed 360$^\circ$ electron pitch angle distributions over an energy range of 4 eV to 30 keV at 17 ms time resolution. The high-resolution spatial and temporal measurements in the low-altitude auroral acceleration region allow the study of small-scale plasma interactions at a level unachieved by previous spacecraft Carlson:98a.

next up previous
Next: Data Presentation Up: Latitudinal dynamics of auroral Previous: Introduction

Simon Shepherd 2002-06-05