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Next: October 3, 1995 Up: Latitudinal dynamics of auroral Previous: Instrumentation

Data Presentation

The colocation of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometers and the Dartmouth PSFRs allows the relationship between auroral currents and auroral roar emissions to be studied continuously. For this study we selected 5 days (3 days in 1995 and one each in 1997 and 1998) during which all five PSFRs were operational, auroral roar was seen at several stations, and the emissions either displayed significant time and latitude variations or occurred during a FAST conjunction. The range in magnetic local time (MLT) that is included in this study for each of the five selected days is shown in Figure 1.

Figure 1. The magnetic local time (MLT) range of the auroral roar events studied in this paper, showing the tendency of these events to occur in the premidnight sector.
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To provide a larger-scale context for these selected events, the daily average planetary $K$ indices for 30 days before and after each of the five selected study days are shown in Figure 2.

Figure 2. The average daily $Kp$ index for $\sim$30 days before and after the events studied in this paper. Vertical lines mark the days chosen for this study.
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Figures 3-7

Figure 3. Programmable stepped-frequency receiver (PSFR) data at (a) Taloyoak, Northwest Territories, (b) Baker Lake, Northwest Territories, (c) Arviat, Manitoba, (d) Churchill, Manitoba, and (e) Gillam, Manitoba from 2345 UT on October 3, 1995, to 0345 UT on October 4, 1995. (f) The latitude of the peak PSFR intensity and the electrojet boundaries inferred from the CANOPUS magnetometer data for the same period.
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/95276-2345-0345.eps, width=17.3cm}}
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Figure 4. Programmable stepped-frequency receiver (PSFR) data at (a) Taloyoak, Northwest Territories, (b) Baker Lake, Northwest Territories, (c) Arviat, Manitoba, (d) Churchill, Manitoba, and (e) Gillam, Manitoba from 2300 UT on October 4, 1995, to 0430 UT on October 5, 1995. (f) The latitude of the peak PSFR intensity and the electrojet boundaries inferred from the CANOPUS magnetometer data for the same period.
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/95277-2300-0430.eps, width=17.3cm}}
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Figure 5. Programmable stepped-frequency receiver (PSFR) data at (a) Taloyoak, Northwest Territories, (b) Baker Lake, Northwest Territories, (c) Arviat, Manitoba, (d) Churchill, Manitoba, and (e) Gillam, Manitoba from 2300 UT on November 5, 1995, to 0200 UT on November 6, 1995. (f) The latitude of the peak PSFR intensity and the electrojet boundaries inferred from the CANOPUS magnetometer data for the same period.
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/95309-2300-0200.eps, width=17.3cm}}
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Figure 6. Programmable stepped-frequency receiver (PSFR) data at (a) Taloyoak, Northwest Territories, (b) Baker Lake, Northwest Territories, (c) Arviat, Manitoba, (d) Churchill, Manitoba, and (e) Gillam, Manitoba from 0230 to 0900 on May 2, 1997. (f) The latitude of the peak PSFR intensity and the electrojet boundaries inferred from the CANOPUS magnetometer data for the same period. The vertical dashed lines indicate the times of the Fast Auroral Snapshot (FAST) overflight of each station, and the solid vertical line indicates the time FAST passes into the precipitation region (see Figure 10).
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/97121-0230-0900.eps, width=17.3cm}}
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Figure 7. Programmable stepped-frequency receiver (PSFR) data at (a) Taloyoak, Northwest Territories, (b) Baker Lake, Northwest Territories, (c) Arviat, Manitoba, (d) Churchill, Manitoba, and (e) Gillam, Manitoba from 2300 UT on February 17, 1998 to 0700 UT on February 18, 1998. (f) The latitude of the peak PSFR intensity and the electrojet boundaries inferred from the CANOPUS magnetometer data for the same period. The vertical dashed lines indicate the times of the FAST overflight of each station, and the solid vertical line indicates the time FAST passes into the precipitation region (see Figure 11).
\begin{figure*}\figbox*{\hsize}{}{\epsfig{file=figs/98048-2300-0700.eps, width=17.3cm}}
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display the PSFR and magnetometer data for each of the five selected days. Figures 3-7 contain grayscale spectrograms of the PSFR data from each of the five sites (Figures 3a-3e, 4a-4e, 5a-5e, 6a-6e, and 7a-7e) and a time series plot of the invariant latitude of the maximum integrated PSFR power (Figures 3f, 4f, 5f, 6f, and 7f). The gray level in the PSFR spectrograms represents the received signal strength, where darker gray pixels represent stronger signals as described in section 2. A given gray level in each plot corresponds to a particular antenna signal level to within $\sim$3 dB, as determined from calibration signals described above. The difference in darkness of the background in these plots is due primarily to differences in the background noise level at the stations.

The solid line in Figures 3f, 4f, 5f, 6f, and 7f displays the latitude of the peak emission intensity as measured by integrating the PSFR power over a range of frequencies which includes the auroral roar emission but excludes certain frequencies which are dominated by anthropogenic signals or interference lines. The invariant latitude of each PSFR site is indicated on the right side of Figures 3f, 4f, 5f, 6f, and 7f by the appropriate site abbreviation. The latitude of the peak emission intensity is determined by setting a threshold at the measured power level of the site with the highest or second highest noise level (see discussion below) and discerning the site with the largest emission power above this threshold. Gaps in the line indicate times when no emissions occur above the threshold level, and vertical bars indicate transitions when the peak intensity changes rapidly from one site to another. The coarse spacing of the PSFRs is evident in the large jumps in latitude of this line, but in reality the latitude variations are probably much smoother.

The electrojet boundaries, determined every 5 min from magnetometer data using the method described above, are also shown in Figures 3f, 4f, 5f, 6f, and 7f as dotted lines. Along the bottom of Figures 3-7 a grayscale strip represents the strength of the electrojet currents with darker gray indicating stronger currents. The two grayscale colorbars above Figures 3a, 4a, 5a, 6a, and 7a indicate the intensity of auroral roar emissions in Figures 3a-3e, 4a-4e, 5a-5e, 6a-6e, and 7a-7e and the strength of the electrojet at the bottom of Figures 3f, 4f, 5f, 6f, and 7f, respectively.

The large variation in the intensity of the background noise detected at the five sites requires that the signals be compared carefully in order to assure that the latitude of maximum wave intensity is not determined by the background noise at any station and that the site of maximum wave intensity is not identified in cases where another site, with higher background noise, could reasonably have harbored more intense but undetectable emission signals. For two days, May 2, 1997, and February 17, 1998, the threshold level for determining the latitude of maximum auroral roar emission was set at the background level of the Churchill receiver (4.5$\times$ 10$^{-10}$ V$^2$/m$^2$), which during this time had the least sensitivity because of the installation of a smaller antenna there in 1997-1998 for polarization measurements [see][]Shepherd:97a. For the other 3 days the threshold level was set at the background level of the Arviat receiver (2.0$\times$ 10$^{-11}$ V$^2$/m$^2$), in effect ignoring the data from the Gillam receiver at the southern extreme of the chain, which had a significantly higher background noise level, except for a short time during the October 4, 1995, event when the emissions are clearly most intense at Gillam. By choosing the lower threshold and ignoring the Gillam data when the auroral roar activity was clearly centered at the poleward stations, a much clearer picture of the latitude variations of the emissions is obtained than would be possible if the only emissions studied were those that exceeded the rather high noise level of the Gillam receiver. In all 5 days some data points for the solid line, representing the latitude of the peak emissions, have been removed to eliminate spurious peaks due to intermittent anthropogenic signals, atmospherics, or other radio noise.



Subsections
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Next: October 3, 1995 Up: Latitudinal dynamics of auroral Previous: Instrumentation


Simon Shepherd 2002-06-05