Transcript GPS and Solar Radio Burst Forensics Brady O`Hanlon, Paul Kintner

Cascades2: Quantifying Shears in the Auroral Electric Field
Erik Lundberg, Paul Kintner, Cornell University
Kristina Lynch, Meghan Mella, Dartmouth College
Marc Lessard, University of New
On March, 20th 2009 the Cascades2 sounding rocket was launched from
Poker Flat Alaska reaching an apogee of 564km. Two electric/magnetic
field subpayloads were ejected at a high velocity parallel to the local
magnetic field and included a suite of 3-axis fluxgate magnetometers and
crossed dipole electric field antennas with receivers from DC to HF. On
the downleg, at altitudes near ~450km, the array of Cascades2 payloads
encountered a Poleward Boundary Intensification (PBI) and strong
Alfvenic activity with perturbations in the DC electric field of
>200mV/m. In this region the two electric field subpayloads were
separated by ~6km with a 5 degree angle between their separation
vector and the local magnetic field. Initial analysis of the despun DC
electric field data differences between payloads of 20 mV/m that are
primarily oriented in the East-West direction which at our separation
distance of 6km correspond to shears of .0033 mV/m^2. Coupling of
these shears to other wave modes is investigated and their role as a
possible driver of ion heating is discussed.
Figure 7:Raw electric and
magnetic fields for each payload
(FWD blue, AFT green).
Figure 3: Unfiltered Electric Fields from The AFT payload overlaid on 0-1KeV
electrons.
Figure 6: Cross correlation of the FWD and AFT
North DC E signals. Lag indicates by how much time
the AFT signal needs to be offset to match the FWD
signal (a correlation peak at a positive lag indicates a
signal that arrives at the FWD payload first). The
inter-payload separation in the North direction is
410m.
Quantification of shear: Figures 4 presents electric field data from the AFT payload
showing vortex structures. The second panel is a series of 3 hodograms showing the
sense of rotation of the electric field in three different time sections, first is counter
clockwise around B, second clockwise and third counter clockwise again. The next
set of panels present hodograms of the differences between the two payloads during
these field reversals. We can convert these differences to velocity through E cross B
and then convert those velocities to shear by dividing by the separation between the
payloads. This quantification of the shear is exasperated by the need to interpret the
differences at spatial or temporal, and, if assuming the differences are spatial whether
they are parallel or transverse to B. The space-time ambiguity is discussed later in
this poster while different possible values for the shear are presented in Table 1. By
assuming the differences are cause by the separation perpendicular field we observe
shear frequencies that are appreciable compared to the local O+ cyclotron frequency,
fshear/fo+ ~ .3.
Lag
VplasmaVrocket
Vphase
(d/lag-Vrel)
601.8
-.067 s
2520 m/s
-8534 m/s
603.2
.032 s
2660 m/s
8103 m/s
605.9
-.069 s
2155 m/s
-8140 m/s
Table 2: Phase velocity estimates
derived from cross correlation
analysis
Correlation Analysis: Figure 6 presents cross correlation analysis, at 3 times spaced
before, in, and after the regions of highest shears. The DC electric field is bandpass
filtered between .1Hz and 1.5Hz to isolate the lowest frequency waves from DC
Electric field. A similar analysis of the east signal has broader peaks at a nearly
constant lag of ~.005s (not shown). The plasma velocity is estimated by applying a
.1Hz low-pass filter to the DC electric field data and calculating E x B. While realizing
this .1Hz cutoff is an arbitrary separation between wave fields and bulk flows, we can
calculate the velocity implied by the correlation peaks and the separation between the
payloads (dsep/tlag). By subtracting from this the plasma velocity, can calculate a phase
velocity. These results are tabulated in Table 2. Notably, the phase velocity in each
region is similar in magnitude, but oppositely directed, which we can interpret as either
opposite crests of a ~1Hz plane wave, or as the reflection of a plane wave from below
the rocket.
Spectral Analysis: Figure 7 presents E
over B spectral ratios for the period of
interest between 2 and 20Hz. These
data are limited by our current
magnetometer data reduction which
requires high pass filtering of the
magnetometer data above 2Hz and is
limited by noise above 20Hz. The
magnitude of these spectra give us an
estimate of the parallel phase velocity of
the waves of 1-2*106 m/s which we can
extrapolate to lower frequencies, further
work
is
necessary
to
nondimensionalize this for comparison to
theory, however, we are cautious of
using
the
traditional
conversion
between space and time (k=omega/vr).
Figure 1: Schematic diagram of the CASCADES-2 payloads after all instruments
have been deployed.
Experiment Setup/Instrumentation: The CASCADES-2 Sounding rocket
consisted of 5 instrumented sub-payloads pictured in Figure 1.
Two
electric/magnetic field sub-payloads are ejected at high velocity (~15 m/s) parallel
to the local magnetic field line achieving a parallel separation distance of 8km by
the end of flight (Figure 2). These payloads consist of 2 pairs of 12m tip-to-tip wire
boom double probes directed radially from the payload’s spin axis which detect
plasma waves from DC to 2.4MHz. Two small sub-payloads are ejected
perpendicular to the local magnetic field which consist of a single top hat electron
detectors designed to observe electrons between .01-5keV along with their pitchangle distributions. The main payload contains a suite of particle detectors; a pitch
angle resolving electron detector observing electrons between .01-5keV, a pitch
angle resolving ion detector observing ions between .1-1keV, a field aligned
electron detector observing electrons between .01-1keV with very high temporal
resolution. All 5 payloads have 3-axis science magnetometers and GPS receivers.
Data presented are from the two electric/magnetic field sub-payloads and the
electron and ion detectors on the main payload. The relative position between the
FWD and AFT subpayloads is presented in figure two. During the time of interest
the FWD subpayload is ~6500m above AFT and they’re separation perpendicular
to the magnetic field is ~400m (both East and West).
Figure 4: The top panel is a quiver plot of the DC electric field measured for
the AFT payload. The next panel is 3 hodograms spaced throughout the
largest field reversal. The third panel displays hodograms of the differences
between the FWD and AFT payload.
Time
Max North
Difference
Assuming
Perp Shear
Assuming Parallel
Shear
602.2-602.5
.052 V/m
3.714 Hz
.186 Hz
602.5-602.8
.054 V/m
5.5 Hz
.19 Hz
602.8-603.3
.148 V/m
10.57 Hz
.528 Hz
Time
Max East
Difference
Assuming
Perp Shear
Assuming Parallel
Shear
602.2-602.5
.077 V/m
5.5 Hz
.275 Hz
602.5-602.8
.093 V/m
6.64 Hz
.332 Hz
602.8-603.3
.065 V/m
4.57 Hz
.232 Hz
Table 1: Shear frequency estimates for
different assumptions about derived from
differencing the measured electric fields,
converting to velocity through ExB and
dividing by separation difference (both
parallel and perpendicular).
Figure 2: Separation distance between the two electric field subpayloads in VerticalEast-North coordinates where vertical is transformed to be parallel to local B. The
first panel covers the entire flight while the second and third panels focus on the
along B and perpendicular to B separations during the period of interest.
Time of
flight
Figure 5: The top two panels are quiver plots of the AFT DC electric field and
the differences between the FWD and AFT electric fields respectively. The
third plot is a frequency-time spectrogram of the VLF12 channel of the AFT
payload indicated enhanced BBELF waves and enhanced emissions near the
local hydrogen gyrofrequency (~535 Hz).
Figure 4: E/B spectral ratios for two
the two orthogonal directions for the
AFT payload.
Coupling to other plasma wave
modes: The 3rd panel of figure 5
presents a spectrogram of electrostatic
plasma waves between 20 and 1000Hz.
During this active period we observed
elevated levels of BBELF noise
including emission near the local
hyrdogen cyclotron frequency. We note
that the largest shears (indicated in the
second panel of Figure 5) do not
directly line up with the most intense
plasma wave emissions, but instead
seem to lag slightly behind them. This
suggests that the shears and enhanced
wave emissions are correlated rather
than the shears causing the waves.
Conclusions: These multipoint measurements of the electric field indicate appreciable
shears in the electric field which are maximized on the edges of vortical structures.
Shear frequencies of up to 10Hz (corresponding to sheared flows of .37 mV/m2) are
observed. During the most active period of flight enhanced BBELF plasma waves are
observed along with emission near the local hydrogen gyrofrequency. However, these
enhanced plasma waves don’t necessarily correlate with the periods of the highest
shear. The temporal evolution of these waves are studied via cross correlation
analysis, which reveals oscillating wave fields that reach up to 8000km/s for the
frequency range between .1-1.5Hz.
Parallel phase velocity is estimated to be 12*106m/s from |E/deltaB|, which is consistent the temporal delay in the Eastward
electric field signals (not shown). Further work is needed to elucidate the connections
between structured precipitating electrons and electric fields, the observed waves and
fields and accelerated ions.
Contact:
Erik Lundberg
[email protected]
301 Rhodes Hall
Cornell University
Ithaca, NY 14853