Observations

Auroral absorption recorded by the 16-beam prototype HAARP imaging riometer on March 14, 1995 is shown in Figure 5. The data are for the entire UT (Universal Time) day and are presented as decibels (dB) of absorption, that is, the amount of signal attenuation relative to the quiet (i.e., unperturbed) signal level expected at that sidereal time. Several weeks of data near the time of interest are used to calculate the reference quiet-day levels. Note that magnetic local time (MLT) for Gakona is MLT = UT-11 hours. Thus, magnetic midnight at Gakona is approximately at the center of the plots.


  
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Figure 5: Left Panel: Auroral absorption recorded through the eight north-pointing beams (left) of the prototype array on March 14, 1995. Beam 1 is most inclined to the north while beam 8 is most nearly overhead (see Figure 4). Right Panel: Auroral absorption recorded through the eight south-pointing beams of the prototype array on March 14, 1995. Beam 9 is most nearly overhead while beam 16 is most inclined to the south.

Each panel of Figure 5 corresponds to one of the 16 riometer beams. The beams are numbered from 1 (most poleward) through 16 (most equatorward), following the pattern depicted in Figure 4. Thus, beams 1-8 cover an area from nearly overhead (beam 8) to the north, while beams 9-16 cover an area from nearly overhead (beam 9) to the south.

The absorption activity on March 14, 1995, typifies the range of activity often seen with this instrument, with the exception of the large, spiky enhancements occurring near 0400 UT. For example, the absorption between 1000 and 1400 UT (2300-0300 MLT) over the entire array, but diminishing in amplitude to the south, is commonly observed near midnight in association with magnetic substorms. The fast rise, slow decay absorption beginning near 2000 UT (0900 MLT) and lasting about two hours is referred to as a slowly-varying absorption event. This type of event is thought to be a dayside manifestation of nightside substorm activity, caused by the precipitation of eastward-drifting energetic electrons injected from the magnetotail into the nightside substorm source region.

While midnight and midday absorption are fairly common features, occurring on many days throughout a month, the large, spiky activity near 0400 UT (1700 MLT) close to the dusk meridian is much less common, occurring on only a few days of the month. These events are often characterized by large amplitudes (reaching as much as 6 dB) and short durations (as small as 5 minutes). They can be localized to only a portion of the riometer field of view as, for example, the sharp initial spike at 0310 UT seen only with beams 2-7, or cover a wider area, as for the broader spike at 0400 UT.


  
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Figure 6: Left Panel: A 2-hour plot (0300-0500 UT) showing details of the absorption spike activity occurring near the dusk meridian on March 14, 1995, for the eight north-pointing beams. Right Panel: A 2-hour plot (0300-0500 UT) showing details of the absorption spike activity occurring near the dusk meridian for the eight south-pointing beams. Note that the sharp spike at 0310 UT is confined to the northern half of the array and that both spikes exhibit equatorward drift of the peak amplitudes.

Details of the spike activity on March 14, 1995, are shown with better time resolution in Figure 6 which covers the 2 hours from 0300-0500 UT in the same format as for Figure 5. The two features at 0310 and 0400 UT show evidence of equatorward drift, that is, the peak amplitudes occur later in time as one moves further to the south. This is seen especially clearly for the latter peak in Figure 6 and is typical of the other duskside absorption spikes thus far examined. Such behavior could be interpreted as due to the equatorward drift of L-shell aligned auroral (absorption) arcs, although other geometrical configurations and motions may be possible. The one-dimensional nature of the present instrument limits further study of the spatial and temporal morphology of the spike phenomena.

An alternative display of the latitude-time-intensity variations of Figure 6 in the riogram format is presented in Figure 7. The riogram is analogous to an optical keogram, where a meridional slice through an all-sky camera is displayed. In Figure 7, the ordinate is depicted as distance at a 90 km altitude range (D-region) with north (south) at the top (bottom), UT time is along the abscissa, and color as given by the bar is proportional to absorption in dB. The ordinate can also be represented by latitude, also depicted in the Figure. For typical auroral substorm related activity, riometer absorption often results from deposition of 10-20 keV electrons at $\sim 90$ km. Such deposition can lead to increased D-region conductivity and enhanced current flows in this region, usually leading to large magnetic variations. However, more energetic electrons can penetrate to lower altitudes and not greatly increase the conductivity of the lower ionosphere. For example, a 100 keV electron would penetrate to $\sim 64$ km. Significant magnetometer activity would not be expected for particle deposition at such low altitudes.

In general, Gakona magnetometer data typically show only weak magnetic signatures during spike events, less than 100 nT variations in H or Z at the times of the spikes (J.V. Olson, personal communication). Therfore, it is not unreasonable to assume that such intese absorption spikes, often occurring within +/- 2 hours of the dusk meridian and at lower L-shells ($L \sim 4-5$), might be the result of very energetic electron precipitation. Further studies are underway to determine the hardness of the electron spectrum during spike events.

It is also interesting to note that, at least for the March 14, 1995, event discussed above, no significant substorm activity and/or associated riometer absorption was observed near 0400 UT by the CANOPUS geophysical network in Canada, which was closer to midnight at this time (M.L. Lessard, personal communication). Thus, the relationship between dusk spikes and substorms is not clear. The role of plasmaspheric dynamics in the duskside bulge region [Carpenter et al. 1993] is also unclear and under investigation. Similar spike events have also been observed in Siple Station ($L \sim 4.2$) and Halley Bay ($ L \sim 4.4$)Antarctic data sets, and may have been observed by Ranta et al. [1983] in a study of absorption events in the afternoon-early evening sector. However, the the Ranta et al. [1983] study focused on substorm development.


  
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Figure 7: A keogram-style display of the latitude-time-intensity variations of auroral absorption for the data of Figure 5. The display ranges from north (beam 1) at the top to south (beam 16) at the bottom. The level of absorption in dB is given by the color bar. The spatially localized and broad absorption spikes near dusk ($\sim 0600$ UT) are evident. Note that the sharp spike at 0310 UT is confined to the northern half of the array and that both spikes exhibit equatorward drift of the peak amplitudes.
Allan T. Weatherwax
12/9/1998