(Click here for full-sized image)
Event Description:
This event is chosen to study the mapping of particle, wave, field and
emission
boundaries from POLAR to the Halley SuperDARN radar Field of View for a
nightside period when Bz is strongly positive. POLAR has its perigee in
the south with the ground track traversing equatorward to poleward through
the radar FOV from 2330 UT to 2355 on 28 May. The radar records a stable
backscatter feature with a distinct boundary picked out in the spectral
width. This boundary is associated with boundaries in waves, particles and
E on POLAR. The backscatter region overlaps the region of luminosity
observed by VIS field camera. Later (around 03 to 04UT on 29 May) there is
a sharp step in the Scatter boundary in the noon midnight plane - possibly
associated with BX,By changes. This latter period shows how the nightside
is reconfiguring whilst POLAR is flying through the reconnection region on
the northern dayside.
Event Description:
It is frequently observed by the HYDRA instrument on-board Polar spacecraft
that enhanced fluxes of hot electrons and ions appear in the middle of the
polar cap, isolated from the cusp/mantle and the auroral oval particles. These
particles are characterized as plasma sheet plasmas with ion average energies
above 1 keV and electron energies much higher than the values in the polar cap.
They often associate with the transpolar arcs in the theta-auroral patterns at
the ionosphere height, confirmed by the auroral images from the VIS Earth
Camera. An example of such events in the northern hemisphere can be found on
96/11/01. HYDRA observed plasma sheet like electrons and ions at 2:30, 4:10,
4:55, and 5:15 UT near the pole while Polar was traveling from midnight to noon.
Also, in the southern hemisphere on 96/05/07, they were observed at 1:09 UT.
Moreover, when HYDRA observes such enhanced plasmas within the polar cap, TIMAS
and CEPPAD IPS instruments also observe ions of different compositions and much
higher energies, respectively. Usually, the IMF By and/or Bz components change
sign before the theta-auroral patterns develop. However, they sometimes do not
appear in the same configuration suggested by Newell and Meng [1995].
Event Description:
Observations from Hydra and from the Electric Field Experiment are
being used to investigate the direct entry of magnetosheath plasma
into the LLBL, the cusp, and the mantle. The entering magnetosheath
plasma is observed to have large-scale variability, over time scales
of tens of minutes, and small-scale variability, over time scales
of several minutes. The large-scale structure consists of multiple
entries and exits from the dispersed cusp ions that are expected to
result from variations in the dayside reconnection rate. These
entries and exits are seen to correspond to variations in the southward
component of the interplanetary magnetic field observed by the
WIND MFI instrument. Superimposed on the large-scale variability
is a small-scale structure in the particle dispersion that similarly
is consistent with variations in the reconnection rate.
We are interested in identifying charged particles that travel between pairs of spacecraft. If such particles can be identified, then they can be used for a number of geophysical investigations. We have been looking at a magnetically quiet period from 1700-1900 UT on August 19, 1996, with the aim of finding the magnetospheric electrostatic potential difference between the locations of the Polar and Geotail spacecraft.
We make use of the fact that magnetospheric magnetic field models are in much better shape than electric field models. We are using the Tsyganenko 96 magnetic field model to calculate the change in pitch-angle (PA) of particles seen by each spacecraft as a function of distance (s) along the field lines going through each spacecraft. We also calculate the modified longitudinal invariant K(s). Imagine a particle at some PA travelling up the field line from Polar. When its PA reaches 90 degrees it mirrors, but the particle mirror point stays on a constant K surface as the particle drifts from one field line to another. If a particle reaches the field line of Geotail, we know from its value of K what the value of B is at its mirror point, and therefore what its PA will be at Geotail which is at some other point on that field line.
Since the magnetic moment (mu) and longitudinal invariant (K) are conserved during the particle motion, we can use particle data from each spacecraft to compare measured phase space densities as a function of (mu, K). Liouville's theorem tell us that (with no scattering) the phase space densities (f) should be equal for particles seen at both spacecraft. We are using Hydra data on Polar and CPI data on Geotail. We hunt for matches in f knowing that such a match should have a characteristic shape in the (mu, K) plane. The difference in energy of the particles as seen at the two spacecraft is a measure of the potential difference between the two regions, and should be the same along the matched f(mu, K). We are also looking at electric field data from the two spacecraft (EFI and EFD) to see if the potential differences that we infer are consistent with their measured electric fields. We also make use of the magnetic field data on each spacecraft (MFE and MGF).
The figure illustrates how a particle from Polar spirals up the field-line (in blue), then how its mirror point drifts (purple curve) across to the Geotail field line, and then spirals down the Geotail field line to the Geotail spacecraft.
