It shows the location of all polar cap patches identified on a given day in magnetic latitude/longitude coordinates. The top panel shows the Bz component of the interplanetary magnetic field (IMF), as determined by the OMNI service. Vertical dashed lines mark the times when polar cap patches were encountered; the encounters are numbered in order to match locations on the maps in the lower panels with IMF conditions. On the maps, the polar cap patches are colored: the leading edge (plasma density gradient anti-parallel to the background ion velocity) is colored green, the patch proper is colored yellow, and the trailing edge (plasma density gradient parallel to the background ion velocity) is colored red.
For each crossing of the northern and southern polar cap (above 70 degree magnetic latitude), one such plot is produced. In the title the date and the Swarm satellite are identified. In the top right corner the satellite track across the polar region is shown in a magnetic local time, magnetic latitude coordinate system. The Sun (12 MLT) is to the top. The gray circle is located at 77 degree magnetic latitude, which is currently used as the boundary for the polar cap region. The letter inside the polar cap is "S" if the southern hemisphere is crossed and "N" for the northern hemisphere. In the following plots the times when the satellite crossed into and out of the polar cap are marked by vertical dashed lines. Only data inside the polar cap are searched for signatures of polar cap patches.
The main panels show, from top to bottom:
- The polar cap patch flag (see table below).
- The foreground (black line) and background (cyan solid line) plasma density measured by the Langmuir probe. The broken cyan line shows twice the background density. A polar cap patch is defined as a density increase by at least a factor of two, i.e., when the solid black line rises above the broken cyan line. There are a few other conditions (the patch must be larger than a minimum size; the density must drop below a threshold towards the edges of the patch) such that sometimes when the foreground rises above twice the background, no patch is identified.
- The ion drift speed as measured by the thermal ion imager (if data is available) as a black line. The gray line indicates the angle between spacecraft velocity and ion drift velocity. Here, "+||" indicates parallel velocities, and "-||" are anti-parallel velocities.The factor between background and foreground density, i.e., foreground divided by background.
- The linear growth time of the gradient drift instability. This value is only defined within polar cap patches. Black crosses indicate a positive growth time (instability growth), whereas negative growth times (stable density gradient) are shown in gray. The distinction between a stable and an unstable situation is made according to the angle between the ion drift velocity and the spacecraft velocity (the direction of the density gradient).
- The electron temperature measured by the Langmuir probe.
The example, then, shows a noon-midnight pass of the Swarm A satellite across the northern polar cap. It crossed into the polar cap (above 77 degree magnetic latitude) around 00:19 UT and out of the polar cap just after 00:25 UT. Between 00:20 and 00:24 UT the satellite encountered two plasma density increases at least a factor of two above the background (cyan solid line in the third panel); these increases are identified as polar cap patches.
The patch proper is that region of the polar cap patch, where the ratio foreground/background is above 2; its extent is indicated by a polar cap patch flag value of 2 (labeled P.P.). In the case of the later patch encounter, there are regions after the initial patch proper where inc again rises above 2, we do not count these as part of the patch proper because in between the ratio dropped below 2. For a definition of the polar cap patch flag, see the table below.
Once we have identified the patch proper, we find the edges. First, we calculate the average background density during the patch proper. From the start of the patch proper we then walk backwards in time until we find a point where the foreground plasma density drops below 1.3 times the average background plasma density during the patch proper; this marks the start of the left edge. The right edge is found in the same manner but moving forward in time. From its position in the polar cap in magnetic local time/magnetic latitude coordinates we then define the sunward edge of the patch as the trailing edge, whereas the anti-sunward edge is defined as the leading edge.
The second to last panel shows that, assuming linear theory, the growth time of the gradient-drift instability was of the order of a few tens of seconds.