What do Scintillations tell us about the Ionized ISM ? - PDF Document

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  1. What do Scintillations tell us about the Ionized ISM ? Barney Rickett UC San Diego SINS Socorro May 2006

  2. Electron density and its spectrum The radio scintillation of pulsars and AGNs probe the fine structure in the interstellar electron density ne(s s,z) versus transverse position s s and distance z from the Earth. From observations one can estimate the structure function of density versus a transverse separation σ σ Dne(σ σ) = ∫0 This is the density structure function (integrated over los). It’s related to the Power spectrum of density versus transverse wavenumber κ κ Dne(σ σ) = ∫P3ne(κ κ,κz=0) {1-eiκ κ. .σ σ}d2κ L<[ne(s,z) - ne(s+σ σ,z)]2 > dz (for a pulsar at distance L) For scales far from the inner and outer scales in an isotropic Kolmogorov spectrum P3ne(κ) = Cn2κ−(2+α) ; κinner > κ > κouter{κinner = 1/linner ; κouter =1/louter} Dne(σ) ∝σαwithα=5/3 linner < σ < louter P (κ) 3ne κ-(3.67) Notice that a Kolmogorov density spectrum only suggests turbulence (no V or B obs) The scales probed by ISS are 100km to 30 AU Tiny for ISS is 100 km louter > 10AU log(κ) κouter κinner

  3. Structure Function with Inner scale Log[ Dne(σ) ] σα σ2 Log[ σ] linner For scales smaller than the inner scale the structure function follows a square law. Thus scattering effects are still present on scales below the inner scale. Note that there is not much difference between α=1.667 and 2, so the ISS effects of an inner scale are subtle.

  4. What can we learn from ISS? Scintillation can probe the form of the spectrum => inner & outer scales, anisotropy for one line of sight And explore its rms amplitude versus Galactic coords => Cn2 or more accurately Scattering Measure SM = ∫ Cn2 dz versus latitude, longitude and scattered path length The first order description is an isotropic Kolmogorov spectrum that is pervasive throughout the Galaxy - with some volume filling factor. This is the conceit of ISS studies that the small scale structure of the ionized ISM could be decribed by three parameters: exponent (5/3), level (Cn2) & filling factor Pulsar dispersion measures combined with independent distance estimates suggest that the volume average <ne> ~ 0.03 cm-3 Estimates of the volume filling factor are 0.1-0.25 and so imply local electron densities in the ionized regions 0.1-0.3cm-3

  5. Variation of Dispersion Measure psr B1937+21 Ramachandran et al 2006: 3x1014 cm-2 (2.2x1018 cm-2)

  6. Structure Function of phase at 1.4 GHz psr B1937+21 The solid line gives the best fit line for the data in the time interval of 10 days to 5000 days. The derived values of the the power law index α =1.66 ±0.04. Remarkably close to Kolmogorov value 5/3 If extrapolated down to where Dφ = 1 rad2 the diffractive scale is predicted to be 7x106 m. Compare with the scale estimated fromτsc. It is 2 times larger which they suggest is due to an inner scale of 1.3x109 m (~0.01 AU) which is larger than the diffractive scale ~0.001 AU Slope = 1.66

  7. Pulse broadening vs DM Rickett MNRAS 1970 γ γ Bhat et al. ApJ, 2004 (scaled to 1 GHz) Ramachandran et al. MNRAS 1997 τsc(msec) ~ Lkpc SM1.2νGHz-4.4if uniform SM ∝ DM ∝ L => τsc∝ DM2.2

  8. DM dependence 2 The uniform Kolmogorov model predicts:τsc∝ DM2.2 But the observations show a much steeper dependence on DM. (first noticed by Sutton 1971). They imply that at larger DMs, (larger distances through the electron layer) there is an increasing chance of encountering regions of large density and large density variance ie higher turbulence (?). Note that SM is column of density-variance (it's related to emision measure). We expect δne ~ ne So ISS picks out the highest densities along a line of sight. We model this by a phase screen. Boldyrev and Gwinn model the plot by proposing non-Gaussian statistics for the interstellar electron density n ne e - specifically a Levy flight distribution, in which its probability distribution has a power law tail n ne e- -β β. They find β ~ 0.7, which is a rather extreme distribution for which both the mean and variance diverge. This is a novel description of the extra scattering deep into the Galaxy. More traditionally….

  9. DM dependence 3 SM∝DM3 Cordes & Lazio used their electron model to convert the observed τd to SM. For uniform turbulence we expect SM ∝ DM, but the observations are much steeper => an enormous increase in “plasma turbulence” toward the inner Galaxy. SM ∝ DM Cordes et al 1991 describe this as electron “clouds” which fill a fraction of the volume and are turbulent internally (with a Kolmogorov spectrum) such that contributions to SM are: dSM ∝ F ne2 dz, where ne is local average density and F is a fluctuation parameter. NE2001 models have mean density increasing by 3 and F increasing from 0.2 to 110 toward the inner (thinner) Galactic electron disk. Here F ~ (δne2/<ne>2 ) /(f louter2/3) where f is the volume filling factor for ionized ISM and louter is the outer scale. The 550 factor increase in F implies a 23 times increase in the fractional fluctuations of density or a 550 times decrease in filling factor or a 104 decrease in outer scale… or more reasonably some combination of these. We have not found what are these sparsely distributed dense ionized regions

  10. Local Density Spectrum (Armstrong Rickett & Spangler, ApJ 1995) Note that PSR B1937+21 confirms the Kolmogorov spectrum over scales from 0.001AU to 100AU => a remarkable confirmation of the Kolmogorov model all on one sight line. κ-4 κ-11/3 However, note the relatively minor difference between the Kolmogorov spectrum κ-11/3 and κ-4, which corresponds to a random superposition of abrupt density steps such as due to shocks, discontinuities etc. 1 pc 1 AU 100km

  11. Discontinuity model (Lambert & Rickett 2000) We tested the discontinuity model against observations of refractive scintillation index (mR) [especially 610 MHz data from Stinebring et al. 1998] relative to the observed diffractive decorrelation bandwidth. 0.3pc 0.1AU The solid lines are predictions for the discontinuity model for various “outer scales” (0.1 AU TO 0.3 pc). They do not follow the 610 MHz data for either a screen or an extended medium calculation. The dashed line is for a simple Kolmogorov model. It does better in the trend but shows an excess in mR. => could be due an inner scale.

  12. AGN ISS versus Hα emission NRL monitored 150 AGNs for 10-year project using the Green Bank interferometer at 2 & 8 GHz. 121 of them exhibited ISS at 2 GHz on time scales of 5-50 days. Their scintillation index (rms/mean) m2 is plotted vs the WHAM brightness of Hα emission (in Rayleighs). Dashed line has the expected slope assuming that the brightness is proportional to the emission measure. m2 is strongly limited by angular diameters of the AGNs Flatter spectrum (smaller diameter) sources are shown by squares.

  13. WHAM Haffner Reynolds et al, 2003 ApJS, 149,405

  14. Inner Scale Estimates Spangler & Gwinn ApJ 1990: Angular broadening measurements of strongly scattered extra-galactic sources. They measured the precise shape of the visibility function Found inner scales ~ 100 km Suggested the ion inertial scale as the cut-off for the density spectrum lioninertial = Alfven speed/(ion larmor frequency) = (ne cm-3 )-0.5 230 km => ne ~ 5 cm-3 If ISS occurs where ne ~ 0.2 cm-3 we expect inner scale ~ 500 km The shape of the far-out tail of scatter-broadened pulses provides another diagnostic.

  15. Scattered pulse shape for PSR J1644-45 observed at 660 MHz at Parkes Rickett, Johnston and Tomlinson, 2004 x x Cauchy-Levy dist ∝ t-1.5 x x x Inner scale < 10km Inner scale > 1000km Isotropic Kolmogorov models (with inner scale) fitted from 1/e point outward; Conclude for screen: linner < 100 km or for extended scattering medium: linner ~ 76 km Allowing for possible anisotropy makes these values lower limits

  16. Anisotropy There has been increasing evidence in recent years that the scattering plasma often shows evidence for anisotropy. Scattered images can appear elongated Axial ratio A~1.2-2 • Rapid ISS of quasars (IHV) appears to be “oscillatory” B0405-385 => A>4 • Quasar J1819+38 has annual changes in its ISS timescale with a 6-mo and 12-mo periods that require anisotropy A~6 (maybe source influence) • Scintillation arcs are prominent A ~2-5 • Correlated ISS of the two pulsars in J0737-3039 requires anisotropy A>4 •

  17. Scintillation in local ISM The variation of a few very compact radio sources on times of 1 hour or shorter (IHV) must be caused by scintillation at a distance of 3-30 pc. There are only 4 IHV sources. (MASIV scintillating time scales are 1day or more) => The local turbulent regions must have a low covering fraction. • It will be important to find which of the local interstellar clouds are responsible • The turbulence in these local clouds is anisotropic •

  18. Scintillation Arcs PSR B1133+16 at Arecibo (Stinebring et al.) δtd δνd “Secondary Spectrum” (S2) with three scintillation arcs Primary Dynamic Spectrum

  19. The Puzzle of the “Arc-lets” Hill, Stinebring et al. (2005) showed this example of the arcs observed for pulsar B0834+06. In addition to the main forward arc (following the dotted curve) there are “reverse arclets”. Those labelled a-d are particularly striking. They followed these over 25 days and found that they moved in the secondary spectrum plots, and that the movement was due to the known pulsar proper motion and was consistent with scattering from isolated structures that were stationary in the ISM and survived for at least 20 days.

  20. The Puzzle of the “Arc-lets” 2 Predicted for plasma refraction The right plot shows how the ft values vary with observing frequency. Open circles at 334 MHz and filled circles at 321 MHz. The left plot shows the angular position of the structures (in mas) responsible for each reverse arclet, mapped from the Doppler frequency ft . The lines have the slope expected for the known pulsar proper motion. Remarkably this shows that the spatial location of the scatterers is independent of frequency. They DO NOT show the expected shift due to the dispersive nature of plasma refraction.

  21. The Puzzle of the “Arc-lets” 3 P My first thought on seeing these arclets was that they are due to a multipath condition in which four extra ray-paths through the irregular plasma exist at angular offsets further from the unscattered path than the angular width of the scattering disk. Such a multi-path could exist if there are large scale gradients and curvature across the scattering disk, with an amplitude higher than expected from a Kolmogorov medium ( in which the rms phase gradient on a scale σ decreases as σ-1/6). Lp D Lo ?r1 ?r2 O In other words the plasma could have an excess of power on scales larger than the scattering disk (~ AU). Such structures have been proposed previously in order to explain ESEs and fringes in pulsar dynamic spectra, which appear as “islands” in the arc plots. Excess power on AU scales has also been invoked for the higher than expected scintillation index for refractive scintillations mR.

  22. The Puzzle of the “Arc-lets” 4 BUT this idea is entirely incompatible with Dan’s result that the reverse arclets come from a fixed position in space - one that does not scale with frequency as do the stationary phase points that would govern multiple ray-paths in a plasma. I conclude that there must be isolated “tiny” structures that scatter or refract the waves through angles of the order of 10 mas at 330 MHz. But they must subtend an angle several times smaller than 10 mas or their signatures would overlap in the secondary spectrum. 10 mas at 300pc => 3 AU; so the ionized “clouds” have dimension a ≤ 0.5 AU say. As for ESEs there are two possibilities: pseudo-lens or scatterers: Consider a spherical lens of radius a and electron density nea ( see Hill et al. 2005) It refracts by an angle θr ~ (λ/2π) ∇φ Roughly ∇φ~ φ/a ~ reλ nea which is independent of scale a Substituting 10 mas for θr gives nea ~ 2πθr re−1λ−2 ~ 100 cm-3 ! If cloud is elongated along the propagation path by an axial ratio A => nea ~ 100/A cm-3 Even if A~10 nea ~ 10 cm-3 is uncomfortably high for a structure only ~0.5 AU

  23. The Puzzle of the “Arc-lets” 5 Now consider an elliptical scatterer of width a and length Aa with mean electron density nea Let it have a well developed Kolmogorov turbulence interior to a such that δnea ~ nea with an outer scale ~ a. Its scattering angle will be θsc ~ λ2.2 (re nea)1.2 a0.2 A0.6 Hence nea ~ 2πθsc re−1λ−2 {λ/(2πθsca)}1/6 A-.5 => with10 mas for θsc and a = 0.5 AU nea ~ 18 A-.5 cm-3 So for A=10 we need electron density ~ 6 cm-3 Are such small ionized dense (and turbulent) structures likely and what could they be? Stellar winds could be that small. But the chance of a 600pc line of sight passing within 3 AU of 4 stars is very small (~10-8). ie the space density of such clouds needs to be 108 times that of visible stars !

  24. The Puzzle of the “Arc-lets” 6 My best guess is that the arclets are caused by a random foam-like geometry when the line of sight happens to be tangential to a surface in the “foam” (preferentially picking out large A values favourably aligned). But what is this ionized foam with thin ionized walls with electron densities far above the equilibrium pressure at 104 K? Here’s where I truly step beyond my field of competence…. I hear talks about thermal instabilities in the heating/cooling of the ISM, such that ionized warm regions can cool and recombine. The instability may lead to sheet-like or filamentary cool condensations (whose life times I don’t know). These might reach neutral densities of 10 per cc. Is it possible that there could be thin "Stromgren skins" on these structures that would be sheet-like and that it is the rare edge-on alignment of these with a line of sight that cause the arclets ?

  25. Simulations of Turbulence Kritsuk, Norman et al, 2002 Box is 5 pc 2563 grid points (400AU steps) Starts at 106 K => cools and fragments into regions near 300 and 2x104 K The color coding is log particle density: Dense blobs at the intersections of the filaments, >60 cm-3 , are light blue; Stable cold phase, 6-60 cm-3 , is blue; Unstable density regime, 1.2-6 cm-3 is yellow to brown; Low-density gas, including the stable warm phase <1.2 cm-3, is a transparent red

  26. Simulations of Turbulence http://akpc.ucsd.edu/ThermalInstability/ Kritsuk & Norman 2002 Distribution of temperatures

  27. Summary 1. Kolmogorov spectrum for the interstellar electron density is only a first approximation for the various ISS phenomena. This suggests Plasma Turbulence but does not require it. ISS picks out the dense regions => The medium is very clumpy => denser and more turbulent regions becoming more common toward the inner Galactic Plane Local (<30pc) regions with small covering fraction need to be identified in other tracers of the ISM Inner scales consistent with the ion inertial scale are in the range 100- 500km Isolated regions may often be anisotropic with axial ratios A>2. Presumably this implies that the magnetic field controls the plasma Reverse arclets imply discrete structures (as do ESEs) that have very high electron densities on sub AU scales • Lens-like ne > 100/A cm-3. • Scatterers ne > 6 A-.5 cm-3 Are these the ionized equivalents of the TSAS n ~ 105 cm-3 ?? Ionization fraction 10-4 ??? 2. 3. 4. 5. 6. 7.