14.2 Merged data on 5 August 2002
14.2.1 Single radar observations
Each radar in the SuperDARN network is able to measure the line-of-sight velocity of the irregularities in the beam. The velocity of the radar is given as
Vr =Vi·r=Vicos(θi−θr) (14.1) where Vr is the velocity measured by the radar, Vi is the velocity of ionospheric irregu- larities, ris a unit vector in the direction of the bearing of the line-of-sight of the radar cell measured east-of-north where θr is the bearing angle of the radar cell and θi is the direction of the irregularity motion.
If the radar measures a uniform motion across its field of view then the measured ve- locities in each cell will vary according to the cell bearing. Some authors, [51] and [28], have tried to model this variation from single radar measurements but did not include any possible amplitude and phase variation of the irregularity motion when making the fit. Ponomarenko et al, [46], took a typical field line resonant behaviour, i.e., a peak in amplitude and a 180◦ phase change across the resonance, and tried to fit a single radar set of observations to his model. They were able to improve on the uniform motion fit using equation 14.1 and model the main features observed in the data but there were still
10 20 30 40 50 AACGM Longitude/[Degrees]
-74 -72 -70 -68 -66 -64 -62 -60
AACGM Latitude/[Degrees]
0 1
2 3
4 5
6 7
8 9 10 11 12 13 14
0 2
4 6
8 1012
14 16
18 20
22 24
26 28 15 30 0
1 2
3 4 5 6 7 8 9 10 11 12 13 14
0 2
4 6
8 1012
14 16
18 20
22 24
26 28
30 15
M83/348 M84/336
M81/338
Figure 14.15: SANAE and Halley Bay radar field of view in AACGM coordinates. The high lighted cells from the two radars correspond to lines of constant longitude and latitude. Also shown are the local BAS LPM sites.
differences between the model and the data.
A latitude and longitude cross of overlapping radar cells in the Halley and SANAE radar was chosen along lines of 27.5◦ AACGM longitude and −69.5◦ AACGM latitude. The cross with the cells from each radar is shown in figure 14.15.
A electric field of field line resonant pulsation will vary along a line of longitude as is outlined in section 5.3.2. The radar will be able to measure the E×B drift from these electric fields. A radar cell that is directed poleward will observe theEφperturbation which is indicative of the fast mode while toroidal directed cells will observe theEν perturbation that is associated with the Alfv`en field line resonant pulsation.
Longitude line
A contour map of the pulsations that occurred in each radar for the longitude line is shown in figure 14.16. The bearing angles for each of the cells of the longitude line are shown in figure 14.17
Figure 14.16 shows that both Halley and SANAE have peaks as a function of latitude with
CHAPTER 14. SOME EVENTS IN DETAIL 123
Figure 14.16: Pulsations from cells along a line of constant longitude for Halley Bay and SANAE as a function of time.
Halley Longitude Line
-70 -69 -68 -67
Latitude/[Degrees]
-174 -173 -172 -171 -170 -169
Bearing/[Degrees]
SANAE Longitude Line
-70.0 -69.5 -69.0 -68.5 -68.0 -67.5 -67.0 Latitude/[Degrees]
-136 -134 -132 -130 -128 -126 -124
Bearing/[Degrees]
Figure 14.17: The bearing angles for the cells in the Halley Bay and SANAE longitude line.
Figure 14.18: Pulsations from cells along a line of constant latitude for Halley Bay and SANAE as a function of time.
the peaks occurring at different latitudes The dip in figure 14.16 of the SANAE pulsation at−68.5◦, which is from cells with a bearing of approximately −130◦, indicate that these cells are directed orthogonal to the irregularity motion. From this the irregularity motion has a bearing of 140◦. The Halley pulsation has a bearing between−168◦ and−175◦ and the amplitude of the pulsation peaks in more eastward directed cells that is, the cells are close to poleward. This indicates that the irregularity motion is east of south similar to the SANAE results.
The dotted line at 9 UT in figure 14.16 indicates phase change as a function of latitude.
There is no marked change in phase in the data. This is attributed to the fact that the cells are directed poleward thus they are not observing the motion due toEν. The SANAE data, however, does indicate a phase change. This is due to the fact that the cells are more toroidally directed, that is between −123◦ and −137◦ and have a greater toroidal component. associated with field line resonance.
Latitude line
A contour map of the pulsations that occurred in each radar for the latitude line is shown in figure 14.18. The bearing angles for each of the cells of the latitude line are shown in figure 14.19.
CHAPTER 14. SOME EVENTS IN DETAIL 125
Halley Latitude Line
22 24 26 28 30
Longitude/[Degrees]
-190 -180 -170 -160
Bearing/[Degrees]
SANAE Latitude Line
20 22 24 26 28 30 32
Longitude/[Degrees]
-145 -140 -135 -130 -125 -120
Bearing/[Degrees]
Figure 14.19: The bearing angles for the cells in the Halley Bay and SANAE latitude line.
The Halley bearings in figure 14.19 have been unwrapped to show the trend. The dotted line indicating the poleward direction. The pulsation amplitude for the Halley measure- ments falls off the more westward the cells are directed similar to the longitude line analysis but the SANAE pulsation shows a peak at 26◦ longitude which is in a direction of approx- imately−130◦ which is in contrast with the findings of the longitude line analysis.
A full two dimensional pattern of the irregularity velocities are needed to resolve these differences. Single radars are unable to account for all the variations as was found in the fitting done by Ponomarenko et al [46]. The merge process of chapter 12 makes no assumption of the motion of the irregularities but rather removes the radar’s directional characteristics from the data. The resulting merged data are then only a function of the irregularity velocity dynamics.