Transcript Ionospheric Research at NRL - Joe Lazio

Long-Wavelength Radio
Science
Astronomical and Ionospheric
Imaging
Joseph Lazio
A. Cohen (NRL), C. Coker (Praxis Inc.), J. Condon (NRAO),
W. Cotton (NRAO), N. Kassim (NRL), W. Lane (NRL), P. Loughmiller
(Cornell), J. Makela (NRL/U. Illinois), R. Perley (NRAO), &
S. Thonnard (NRL)
The VLA as an Ionospheric
Probe
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VLA measures differential
total electron content along
all combinations of
antenna pairs
27 antennas in 3 arms
Arms extend up to 20 km
from the array center
Pietown link extends
separations to 70 km
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High precision
1013 electrons/m2 at the lowest
frequency 74 MHz
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Small scale ionospheric
structure
Sub-km to 10s of km

Array
Center
Refractive Wander
t=0
arcminute
1 minute
sampling
intervals
arcminute
The large-scale ionospheric refraction over 60-min is shown at the left by reference to
L band data. Refractive wander at 1-min intervals shows considerable variability.
Self-calibration
VLA A-configuration
VLA + Pie Town
(T. Delaney, L. Rudnick)
Self-calibration
VLA+PT: Cgynus A at
74 MHz
• 10” resolution
• Still not high enough
resolution
• Want to be able to do
more than just the
brightest sources in the
sky
(Lazio et al. 2006)
Challenge
Ionospheric Remediation
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Self-calibration useful for correcting in a given direction (or over a small field).
Field of view of antennas sees a large, and different, portion of the ionosphere.
Ionosphere
<5 km
Correlation
Preserved
> 5 km
Correlation
Destroyed
(antennas, not stations)
Differential Ionospheric
Refraction
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These nine radio sources
were selected from a deep 74
MHz image.
The individual 30-second
maps were compiled as
animations of the nine hour
measurement, running from
nighttime through two hours
past sunrise.
The variations in position,
peak intensity, and sidelobe
structure show the effects of
differential ionospheric
refraction across the field.
Field-based Calibration
Take snapshot images of bright sources in the field and
compare to known positions.
 Fit to a 2nd order Zernike polynomial phase delay screen for
each time interval.
 Apply time variable phase delay screen to produce corrected
image.

Self-Calibration
Field-Based Calibration
Cosmic Evolution
The First Black Holes
VLSS FIELD 1700+690
~80”,  ~50 mJy
~20o
What About GPS?
3C 461
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GPS provides high
precision TEC
measurements over
large scales (~ 1000
km).
Of only limited use for
small-scale structures
(< 100 km; Erickson et
al. 2001, A&A, 366,
1071)
500
1000
1500
2000
2500
Magnetospheric
Observations
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Jacobson and Erickson (1992a, 1992b) observed electron
density structures in the plasmasphere.
Plasma trapped in Earth’s magnetic field lines
VLA 307.5 and 333 MHz data collections
Oscillations with 1–3 min period
Magnitude ~0.01 TECU (~1014 electrons/m2) across 20-km
baseline
Apparent velocity across VLA line-of-sight due to corotation of trapped plasma
–
–
–
–
magnetically eastward directed
0.1–1.5 km/s
2000 to 10,000 km altitude
L-shells L=2-3
Simultaneous Radio and
Optical Observations
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August 2003 (AC677)
Three 8-hr data collects on 74
MHz
Nighttime optical measurements
–
–
–
–
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630.0 nm F-region (Ne & height)
777.4 nm F-region (Ne2)
557.7 nm Mesosphere
Oxygen Hydroxyl (broadband)
Mesosphere
Objective
Identify nighttime ionospheric
structures affecting 74 MHz VLA
Ionospheric Layers
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Sporadic-E Layers
–
–
–
–
–
– Altitude 150–1000
km
– Dominant layer
– Solar EUV
photoionization
– Magnetic storms
– TIDs
– Instabilities
Near 100-120 km
Thin, 1-3 km in altitude
Variable Density
Tied to Wind Shears
Theories on E-Layer
Patches
» K-H Turbulence
» Plasma Instabilities
» Gravity Waves
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Intermediate Layers
– Near Sunset
– Break-Off from F-layer
– Descend to a Stable
Altitude Above Strong ERegion
F-Layer
F
E
E-Layer
– Altitude 95-140 km
– Secondary Layer
– Solar EUV and Xray photoionization
– Solar zenith angle
dependent
– Vanishes at night
– Solar flares
F-Layer Wedge
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Nighttime wedge evident in VLA data
– 1–5 UT decreasing ionosphere from NW to SE
– 6–9 UT wedge not apparent
– 20-km radius horizontal scale
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360o @ 74-MHz
Wedge
Optical camera observed same effect over longer
baselines
– 200-km radius
360o
N to E gradient during
evening hours as
ionosphere decays
2003-08-25 0215 UT
midnight
360o
Mesospheric Waves
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Complex mesospheric waves observed by optical camera
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Mesosphere - neutral atmosphere, 50–85 km altitude
Attributed to gravity waves
Suggests possibility of Sporadic-E plasma clouds at ~100-km
altitude
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Sporadic-E Clouds
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Es clouds observed
in TX and CA
Ionosonde measures
electron density and
height of cloud
TEC estimated
assuming 1–3 km
cloud thickness
Typical nighttime
fluctuations due to
Es clouds
– 0.01–0.05 TECU
– 60°–360° @ 74 MHz
Sporadic-E 2003-08-25
0.12
0.11
Pt Arguello
Dyess
0.10
0.09
360o @
74 MHz
0.08
0.07
TEC
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0.06
0.05
0.04
0.03
0.02
0.01
0.00
0:00
1:00
2:00
3:00
4:00
5:00
UT
6:00
7:00
8:00
9:00
10:00
VLA Observed Sporadic-E?
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There is evidence that VLA
observed Es
Estimated Es scale sizes
match fluctuations in
August 2003 VLA data
– 50 km horizontal
– 50–150 m/s velocity
– 5–15 min fluctuations
– 60º–360º @ 74 MHz
Es
15 min
360o
Es
5 min
F-layer structure
360o @ 74-MHz
 Optical
camera observed faint F-layer
structure
– 7–8 UT
– Elongated structure NW-SE
– Motion from SW to NE
 Largest
effect expected along path of wave
– West arm largest (+)
– North arm second largest (-)
– East arm least affected (-)
 Consistent
–
N arm
with VLA observations
– Details masked by
– continued presence
– of Sporadic-E
+
W arm
E arm
Climatology
Summer Night
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Nighttime Sporadic-E peaks in
Summer
– 80% Summer nights
– 40% Equinox nights
– 20% Winter nights
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Sporadic-E
360o
25 August 2003
Summer nighttime VLA data
impacted by Sporadic-E
Winter night significantly less
impact
Mitigate through scheduling
UT Hours
Winter Night
19 January 2001
360o
UT Hours
Sporadic-E
and F-layer
structure
Long Wavelength Imaging
Scheme
FoV
Maximum
Baseline
Comments
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Self-calibration
Full field
< 5 km
Treats all flux in primary beam
as “single bright source”—Clark
Lake, VLA C & D
configurations

Self-calibration
“small”
> 50 km
Limited to single bright source
at phase center—VLA+PT
Field-based calibration
Full field
< 10 km
VLA B configuration (VLSS); in
good weather works to longer
baselines
Aperture plane
coverage/alternate basis
function/ionospheric
modeling/???
Full field
Unlimited (~
400 km)
future
Summary
Present
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Limits to wide-field
imaging at longwavelengths being reached
with current arrays.
LWA, SKA, etc. will be even
more challenging.
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Existing methods do have
regimes of applicability.
– Self-calibration for strong
sources.
– Field-based calibration for
intermediate baselines.
Future
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Investigate small scale ionospheric
phenomena at high precision using VLA and
LWA
– Sporadic-E clouds?
– F-layer wave
– F-layer evening wedge
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Collaborative instruments to identify
ionospheric structures?
– Limitations of camera
– Ionosonde
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Aid radio astronomy ionospheric mitigation?
– Climatology (planning)
– Morphology (algorithm development)
– Weather (ops impact, algorithm
selection/tuning)
Basic research in radio astronomy at the NRL is supported by the Office of Naval Research.