Modern phase-based ionosonde as a thermospheric neutral wind profiler
June 2010
Boulder CO
N. Zabotin S. Vadas
1 University
1,
2
of Colorado at Boulder, 2 NWRA/CoRA Division
Capabilities of modern ionosondes
Modern digital ionosondes have advanced their capabilities to
measure accurately both the group time of propagation and the
directions of arrival for each ionogram echo. The ‘Stationary Phase
Group Range’ (SPGR) method provides accuracy down to few tens
meters.
PFISR’s measurements of gravity waves and of neutral winds as
a prototype
As a gravity wave (GW) propagates upwards in the thermosphere,
its amplitude grows rapidly until it reaches its dissipation altitude,
whereupon its amplitude decays rapidly with altitude and time.
Although a GW’s horizontal wavelength remains constant with
altitude if the winds and temperatures change only with altitude, its
vertical wavelength changes with altitude as a result of changing
winds, temperatures, and dissipation due to kinematic viscosity and
thermal diffusivity. If a GW’s horizontal wavelength, period, and
vertically varying vertical wavelengths are known along with the
background temperatures, then the background, horizontal neutral
winds along the GW propagation direction can be calculated in the
thermosphere using the next-generation GW dissipative theory
developed in [Vadas and Fritts, JGR, v. 110, D15103, 2005]. This
approach was implemented recently with PFISR electron density
data by Vadas and Nicolls, [GRL, v. 35, L02105, 2008; JASTP, v. 71,
p. 744, 2009].
8-channel fully digital
VIPIR system at Wallops
Flight Facility (left) and
example of a 3-D
distribution of measured
ionogram echoes (right).
NeXtYZ: 3-D plasma density inversion
Inversion algorithm NeXtYZ performs the recovery of parameters of a
parameterized model that describes locally both the vertical and
horizontal gradients of ionospheric plasma
density. The "Wedge-Stratified Ionosphere"
(WSI) model is the appropriate substitution
for the former "Plane-Stratified Ionosphere"
h
f
model. In the WSI model, the plasma density
i
surfaces are represented locally for small
f
increments in plasma frequency fp at a
f
sequence of ranges hi along the vertical axis,
by slanted sections of “frame” planes; the
slope of each frame plane is characterized
by the two horizontal components nx i, ny i of
its normal vector. The normal to the plasma
density surface determines the local direction of the total gradient in
the layer. Ranges hi and the components nx i, ny i are found by
iterative ray tracing to match the observed SPGR and echolocations
of echoes reflected within the current wedge. [Zabotin et al., Radio
Sci., 41, RS6S32, 2006].
Testing 3-D inversion with simulated data sets
(θi,φi)
i+1
i
3
2
→
n
Z
h i +1
hi
h i -1
h i -2
fp i+1
1
fp i
fp i -1
fp i 2
Dynasonde
nx and ny
-0.1
-0.05
0
0.05
model nx
NeXtYZ nx
model ny
NeXtYZ ny
model Fp
300
300
nx and ny
-0.1 -0.05 0 0.05 0.1
250
200
h, km
250
h, km
Measured parameters of ionospheric plasma for 3 of the 10 PFISR’s
beams: electron densities (first row); band pass filtered relative
electron density perturbations (second row); line-of-sight ion
velocities (third row); band-pass filtered velocity perturbations (forth
row). The left column is up the B-field beam, the middle column is for
the vertically-pointed beam. [Nicolls and Heinselman, GRL, 34,
L21104, 2007].
200
175
150
125
150
1
2 3 4 5 67
fp, MHz
Inversion of the PFISR data to GW and to the neutral wind
parameters: a) Measured profiles of vertical GW wavelength in the
vertical beam every 10 min. Profiles are offset by 1000 km. b)
Extracted background, horizontal, neutral winds every 10 minutes in
the direction of GW propagation (southeastward) in the vertical
beam. Profiles are offset by 300 m/s. c) Time dependence of
horizontal winds in neighboring beams at z=190 km.
We are demonstrating how similar technique can be applied to data
from modern phase-based ionosondes.
NeXtYZ nx
NeXtYZ ny
model nx
model ny
100
1
2 3 4 5 6789
fp, MHz
Results of NeXtYZ inversion with a data set simulated for night-time
conditions (left panel; only a parabolic F layer is present), and for
day-time conditions (right panel). The WSI model used for simulation
is represented by solid lines without data markers. The NeXtYZ
vertical profile fp(h) and altitude dependence of two components of
the normal vector n are shown by lines with filled circles. For the
day-time case inversion was done without accurate modeling of the
valley, under the assumption of monotonic profile. The NeXtYZ
solution remains stable even with the extremely inadequate valley.
NeXtYZ inversion is now a standard part of ionogram analysis
The insert in the standard ionogram images shows the local tilt
angles in the magnetic meridian (red) and zonal (blue) planes.
Processing NeXtYZ results to determine parameters of TID/GW
We demonstrate this procedure using realistic GW-like disturbance
model:
,
,
where fE, zmE, hE and fF, zmF, hF are critical plasma frequencies, peak
heights and thickness parameters of the E, F layers, δ(z) is the GWassociated TID amplitude, zm and hGW are parameters characterizing
altitude span of the TID, kx,y,z are the wavevector components, and φ
is the TID’s phase at the coordinate origin.
Simulated vertical profiles of the
tilts in the bottomside ionosphere (left panels) compared
to results of NeXtYZ inversion
of sample recordings obtained
at Jicamarca (right panels):
(top) when horizontal components of the GW wavevector
have opposite signs, tilts have
opposite signs too;
(mid) when horizontal components of the GW wavevector
have the same sign, tilts vary “in
phase” with each other;
(bottom) shorter vertical wavelength (20 km instead of 40
km) results in shorter scales of
the tilt variations.
For small tilts tx,ty, (low amplitude δ(z)) the following relationships
take place between them and the parameters of the above model:
These explain the simulation results. In each considered case a
counterpart of the simulated behavior of the tilts may be found in the
results of processing of the real data. This suggests an inversion
scheme where layer-by-layer specification of the wave disturbance,
including all three components of its wavevector, is done based on
the NeXtYZ results without any model assumptions.
Several key parameters of TID are immediately available after a
straightforward processing of a single NeXtYZ output. After a
detrending that removes large-scale tilts of the ionospheric layer,
positions of zeroes and amplitudes of the remaining oscillations in
tx,ty, yield
and two auxiliary functions
The background plasma density profile characteristics (f0,f0z′) are
also provided by NeXtYZ. The TID amplitude δ(z) may be
immediately available from a straightforward processing of the
vertical profile or from the Doppler characteristics of Dynasonde
echoes. Processing a short time series (20-30 min) of NeXtYZ
outputs yields the sign of kz and the observed frequency ω(z). The
described procedure essentially provides the same TID
characteristics that were measured by PFISR and allows one to
proceed with inversion to the background neutral wind velocity.
Conclusion
Application of described technique to data of modern phase-based
ionosondes will expand dramatically our ability to monitor the
background, horizontal neutral wind characteristics and to quantify
heating/body forcing effects caused by dissipation of gravity waves.