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 n VLA measures differential total electron content along all combinations of antenna pairs n 27 antennas in 3 arms n Arms extend up to 20 km from the array center Pietown link extends separations to 70 km n High precision electrons/m 2 at the lowest frequency 74 MHz n Small scale ionospheric structure Sub-km to 10s of km Array Center 

Refractive Wander t = 0 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)

n Self-calibration useful for correcting in a given direction (or over a small field). n Field of view of antennas sees a large, and different, portion of the ionosphere. Challenge Ionospheric Remediation Ionosphere Correlation Preserved Correlation Destroyed > 5 km <5 km (antennas, not stations)

Exquisite Sensitivity to Fine- Scale Ionospheric Phenomena Scintillation Refractive wedge At dawn Quiescence ‘Midnight wedge’ TIDs Phase variation on three 8-km VLA spacings at 3 different azimuths Wide range of ionospheric phenomena seen Some of the ionospheric phase fluctuations arise from the sporadic E-layer of the ionosphere? (Perley)

n These nine radio sources were selected from a deep 74 MHz image. n The individual 30-second maps were compiled as animations of the nine hour measurement, running from nighttime through two hours past sunrise. n The variations in position, peak intensity, and sidelobe structure show the effects of differential ionospheric refraction across the field. Differential Ionospheric Refraction

Self-CalibrationField-Based Calibration Take snapshot images of bright sources in the field and compare to known positions. Fit to a 2 nd order Zernike polynomial phase delay screen for each time interval. Apply time variable phase delay screen to produce corrected image. Field-based Calibration

~20 o VLSS FIELD  ~80”,  ~50 mJy Cosmic Evolution The First Black Holes

What About GPS? n 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) C 461

Magnetospheric Observations n Jacobson and Erickson (1992a, 1992b) observed electron density structures in the plasmasphere. n Plasma trapped in Earth’s magnetic field lines n VLA and 333 MHz data collections n Oscillations with 1–3 min period n Magnitude ~0.01 TECU (~10 14 electrons/m 2 ) across 20-km baseline n Apparent velocity across VLA line-of-sight due to co- rotation 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 n August 2003 (AC677) n Three 8-hr data collects on 74 MHz n Nighttime optical measurements –630.0 nm F-region (N e & height) –777.4 nm F-region (N e 2 ) –557.7 nm Mesosphere –Oxygen Hydroxyl (broadband) Mesosphere n Objective Identify nighttime ionospheric structures affecting 74 MHz VLA

F E Ionospheric Layers n Sporadic-E Layers –Near 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 n Intermediate Layers –Near Sunset –Break-Off from F-layer –Descend to a Stable Altitude Above Strong E- Region n F-Layer –Altitude 150–1000 km –Dominant layer –Solar EUV photoionization –Magnetic storms –TIDs –Instabilities n E-Layer –Altitude km –Secondary Layer –Solar EUV and X- ray photoionization –Solar zenith angle dependent –Vanishes at night –Solar flares

F-Layer Wedge n 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 n Optical camera observed same effect over longer baselines –200-km radius Wedge 360 o MHz N to E gradient during evening hours as ionosphere decays midnight UT

Mesospheric Waves n Complex mesospheric waves observed by optical camera n Mesosphere - neutral atmosphere, 50 – 85 km altitude n Attributed to gravity waves n Suggests possibility of Sporadic-E plasma clouds at ~100-km altitude

Sporadic-E Clouds n Es clouds observed in TX and CA n Ionosonde measures electron density and height of cloud n TEC estimated assuming 1 – 3 km cloud thickness n Typical nighttime fluctuations due to Es clouds –0.01 – 0.05 TECU –60° – 74 MHz MHz

VLA Observed Sporadic-E? n There is evidence that VLA observed Es n Estimated Es scale sizes match fluctuations in August 2003 VLA data –50 km horizontal –50 – 150 m/s velocity –5 – 15 min fluctuations –60º – 74 MHz 15 min 5 min Es 360 o

F-layer structure n Optical camera observed faint F-layer structure –7–8 UT –Elongated structure NW-SE –Motion from SW to NE n Largest effect expected along path of wave –West arm largest (+) –North arm second largest (-) –East arm least affected (-) n Consistent with VLA observations –Details masked by –continued presence –of Sporadic-E MHz N arm W arm E arm – +

Climatology Summer Night UT Hours Sporadic-ESporadic-E and F-layer structure 360 o Winter Night UT Hours 360 o n Nighttime Sporadic-E peaks in Summer –80% Summer nights –40% Equinox nights –20% Winter nights n Summer nighttime VLA data impacted by Sporadic-E n Winter night significantly less impact n Mitigate through scheduling 19 January August 2003

Long Wavelength Imaging SchemeFoVMaximum Baseline Comments Self-calibrationFull field< 5 kmTreats all flux in primary beam as “single bright source”—Clark Lake, VLA C & D configurations Self-calibration“small”> 50 kmLimited to single bright source at phase center—VLA+PT Field-based calibrationFull field< 10 kmVLA B configuration (VLSS); in good weather works to longer baselines Aperture plane coverage/alternate basis function/ionospheric modeling/??? Full fieldUnlimited (~ 400 km) future

Summary Present n Limits to wide-field imaging at long- wavelengths being reached with current arrays. LWA, SKA, etc. will be even more challenging. n Existing methods do have regimes of applicability. –Self-calibration for strong sources. –Field-based calibration for intermediate baselines. Future n Investigate small scale ionospheric phenomena at high precision using VLA and LWA –Sporadic-E clouds? –F-layer wave –F-layer evening wedge n Collaborative instruments to identify ionospheric structures? –Limitations of camera –Ionosonde n Aid radio astronomy ionospheric mitigation? –Climatology (planning) –Morphology (algorithm development) –Weather (ops impact, algorithm selection/t uning) Basic research in radio astronomy at the NRL is supported by the Office of Naval Research.