1. A statistical study of Cluster and ground-based observations of Pi1B pulsations at substorm onset
Marc Lessard1, Eric Lund1, Christopher Mouikis1, Yasong Ge1 and Mark Engebretson2
 1University of New Hampshire, Space Science Center, 8 College Rd, Durham, NH USA
2Augsburg College, Physics Dept Riverside Ave. Minneapolis, MN USA
Sponsored by NSF
Introduction
Beyond geosynchronous orbit - March 19, 2009
Statistical study of fast flows in the tail (Cluster) and Pi1B on the ground (Antarctica)
Pi1B definition
For this event, an excellent conjunction exists between all of the THEMIS satellites, GOES 10 and 12 and South Pole Station on the ground, with all of these platforms in the pre-midnight region. Multiple signatures of fast flows are observed by THEMIS that coincide with Pi1B pulsations on the ground (see the section to the right for an illustration of the fast flow magnetic signature, which is P1iB-like).
While we have identified a few clear examples that show an association of fast flows with Pi1B pulsations, statistical confirmation has been lacking. Here, we present results of a statistical study comparing observations of fast flows by the Cluster spacecraft to ground (Antarctic) observations of Pi1B pulsations. Substorms were identified based on injections observed by LANL spacecraft; a majority of these events were confirmed to be substorms by the IMAGE spacecraft (i.e., whenever data were available). Restricting the range of Cluster observations to be within 3 hours of magnetic midnight, we then compared signatures of fast flows to P1iB on the ground. Cluster spacecraft were located at approximately 19 RE downtail at this time.
P: "pulsations"
i: "irregular"
1: periods from 1 to 40 seconds
B: "Bursty" as opposed to "c", which means continuous
Pi1B
PiC
From top to bottom, these are Bz and Vx data from THB, THC, THD, THE and THA. The locations of these spacecraft range from nearly 19 to 7 Re in GSE X. Specific locations are included on the right side of the plots.
The passage of the flows are consistently associated with magnetic perturbations in Bz. In this case, three flows are clearly observed.
Two examples are shown to the right, with Cluster data in the upper plots and induction coil data in the lower plots. The Cluster plots show fast flow signatures (in the 5th panel) as abrupt increases in velocities. Note the corresponding Pi1B signatures in the 7th and 8th panels.
There are two factors that can confuse the interpretation. One is that flows may not be detected by Cluster because they simply miss the spacecraft (thus, a Pi1B may be seen on the ground with no flow observed). On the other hand, Pi1B signatures are ducted in the ionosphere, so a station (even at high latitudes) may respond to multiple flows arriving at lower latitudes, without discrimination.
These two events are shown because the flows are observed following a relatively quiet interval – thus they are “clean” events. The passage of the flows coincides with substorm onset, as well as Pi1B on the ground (onsets are marked with red vertical lines), although earlier (and weaker) Pi1B signatures may be present.
Of 202 purported substorm onsets with Cluster data, 157 were associated with observations of Pi1B signatures on the ground. Of these, 67 also included Cluster observations of fast flows. There were 4 events where Cluster observed fast flows without a corresponding Pi1B signature on the ground.
Both types are observed simultaneously or nearly so. The example above shows a classic signature, in the form of dynamic spectra of induction coil magnetometer data. PiC has been cited as a possible signature of an Alfven resonator.
Evidence for a magnetospheric origin
Arnoldy et al [JGR, 103 (10) 1998] studied 20 Pi1B events, 8 of which had corresponding GOES data. Of these,
1) 7 showed dipolarization and 5 showed Pi1B signatures at GOES.
2) GOES observations with no Pi1B signature were some distance from midnight.
3) Pi1B at GOES were more limited in frequency than on the ground, though this could be an instrumental effect.
4) Attributed the evolution of Pi1B/PiC to magnetospheric waves, related to ionospheric currents and resonant cavities (for PiC waves).
Ionospheric ducting of Pi1B pulsations
Posch et al [PSS, 2007] observed Pi1B in 88% of substorms, within +/-2 hours of local time and 7 degrees of magnetic latitude of the initial auroral brightening location as observed by IMAGE FUV. Note that this study addressed the initial onset, which is presumably the ducted signature.
To the right are data from GOES 12 and 10 (eastward component). Although a Pi1B signature is not as clear as usual, wave activity is present that is clearly correlated with the fast flows seen on THEMIS. GOES 10 at 60 degrees W longitude and GOES 12 at 75 degrees W longitude.
Not
Pi1B
Beyond geosynchronous orbit – March 5, 2008
What does this look like in the big picture? The sketch to the right shows the idea. Based on the observations, it appears that fast flows propagate from the tail Earthward, carrying Pi1B wave power (e.g., as observed by various spacecraft). The wave power is spectral signature of Doppler-shifted fast flows, as observed by the spacecraft. As the flows reach the inner magnetosphere, wave power is coupled to the transverse mode (Lessard et al., GRL, 2006) and can excite Alfvenic aurora at the time of substorm onset (Lessard et al., JGR, 2011).
Induction coil data from the AGO P2 ground station in Antarctica, although only up to ~2:22. Pi1B signatures that are correlated with flow data are clear (the spectra only are shown to .5 Hz here).
Pi1B
Pi1B
Weak Pi1B
Weak Pi1B
Pi1B
Pi1B
Pi1B
The N-S and E-W components of induction coil data from the South Pole. Bursts of Pi1B pulsations are clear for the 1st and 3rd events. A weak signature is seen for the second event, with signatures near 1:08 and 2:30 UT, perhaps due to a flow that was not detected by the spacecraft.
Observations of Pi1B pulsations observed at the South Pole and THEMIS fast flows on March 5, 2008 from 0130 to 0300 UT. The upper panel shows induction coil spectra from the South Pole, with clear Pi1Bpulsations. The second panel shows dynamic spectra of the THEMIS-C magnetometer. The bottom panel shows ion flow velocities from THEMIS-C, which clearly correspond to the THEMIS observations of Pi1B, with corresponding pulsations seen on the ground.
Conclusion
The correlation of ground observations of Pi1B with GOES observations (as originally reported by R. Arnoldy) was confirmed in our study. Although a correlation does not prove a cause and effect relationship, it does imply that further study is warranted. That is, what is the magnetospheric origin of the Pi1B signature in the tail?
1. Using data from GOES 12 and THEMIS for two “clean” events, we show that 1) THEMIS observations of Pi1B spectra coincide with the passage of fast flows, implying that P1iB signatures (in space) are Doppler-shifted magnetic signatures of fast flows, and 2) that a plot of propagation delays from deep in the magnetotail to the ionosphere is consistent with the idea that fast flows may deliver the spectral energy to drive P1iB pulsations on the ground.
2. Of 202 substorms, 157 were associated with observations of Pi1B signatures on the ground. Of these, 67 also included Cluster observations of fast flows. There were 4 events where Cluster observed fast flows without a corresponding Pi1B signature on the ground. We emphasize that 1) we expect that Cluster will have missed some observations of fast flows because of their limited spatial extent and 2) Pi1B pulsations will be ducted in the ionosphere so ground stations will record those events that may have occurred in distant regions, presumably at lower latitudes. Both of these effects would contribute to the fact that Pi1B signatures were observed more often than fast flows.
This plot shows the propagation of Pi1B signatures for this date, from deep in the tail to the ionosphere, with time on the vertical axis and distance along the horizontal axis. For the THEMIS spacecraft, the Pi1B times correspond to onsets of fast flows (data from THB omitted).
This plot shows a general correspondence between fast flows observed in space and Pi1B pulsations observed on the ground, although the interpretation is complicated by at least three factors. First, the spatial size of the flows makes it difficult to guarantee that all flows are observed in the tail. Second, ionospheric ducting of Pi1B pulsations means that stations at high latitudes can often detect signatures that have ducted poleward from lower L shells. For example, the signature at 2:30 UT is clear in Pole data, but has no corresponding observations of flows. It is certainly possible that a flow arrived at a location such that it was undetected by the satellites. Third, the propagation paths from space to the ground can be expected to be complex, resulting in different propagation times for different flows.
Acknowledgement: Research at the University of New Hampshire was supported by NSF grants ANT , ANT