scholarly journals Interpretation of the power spectrum of the quiet Sun photospheric turbulence

2020 ◽  
Vol 499 (4) ◽  
pp. 5363-5365
Author(s):  
Itzhak Goldman
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Near Infrared ◽  
Infrared Imaging ◽  
Spatial Scales ◽  
Power Spectra ◽  
Effective Width ◽  
Turbulent Layer ◽  
Quiet Sun ◽  
Magnetic Field Turbulence

ABSTRACT Observational power spectra of the photospheric magnetic field turbulence, of the quiet-sun, were presented in a recent paper by Abramenko & Yurchyshyn. Here, I focus on the power spectrum derived from the observations of the Near InfraRed Imaging Spectrapolarimeter operating at the Goode Solar Telescope. The latter exhibits a transition from a power law with index −1.2 to a steeper power law with index −2.2, for smaller spatial scales. This paper presents an interpretation of this change. Furthermore, this interpretation provides an estimate for the effective width of the turbulent layer probed by the observations. The latter turns out to be practically equal to the depth of the photosphere.

scholarly journals Analysis of quiet-sun turbulence on the basis of SDO/HMI and goode solar telescope data

2020 ◽  
Vol 497 (4) ◽  
pp. 5405-5412 ◽  
Author(s):  
Valentina I Abramenko ◽  
Vasyl B Yurchyshyn
Keyword(s):  
Near Infrared ◽  
Infrared Imaging ◽  
Magnetic Energy ◽  
Spatial Scales ◽  
Power Spectra ◽  
Solar Telescope ◽  
Magnetic Field Generation ◽  
Quiet Sun ◽  
Field Generation ◽  
Turbulent Dynamo

ABSTRACT We analysed line-of-sight magnetic fields and magnetic power spectra of an undisturbed photosphere using magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamic Observatory and the Near InfraRed Imaging Spectrapolarimeter (NIRIS) operating at the Goode Solar Telescope of the Big Bear Solar Observatory. In the NIRIS data, we revealed thin flux tubes of 200–400 km in diameter and of 1000–2000 G field strength. The HMI power spectra determined for a coronal hole, a quiet sun, and a plage areas exhibit the same spectral index of −1 on a broad range of spatial scales from 10–20 Mm down to 2.4 Mm. This implies that the same mechanism(s) of magnetic field generation operate everywhere in the undisturbed photosphere. The most plausible one is the local turbulent dynamo. When compared to the HMI spectra, the −1.2 slope of the NIRIS spectrum appears to be more extended into the short spatial range until the cut-off at 0.8–0.9 Mm, after which it continues with a steeper slope of −2.2. Comparison of the observed and Kolmogorov-type spectra allowed us to infer that the Kolmogorov turbulent cascade cannot account for more than 35 per cent of the total magnetic energy observed in the scale range of 3.5–0.3 Mm. The energy excess can be attributed to other mechanisms of field generation such as the local turbulent dynamo and magnetic superdiffusivity observed in an undisturbed photosphere that can slow down the rate of the Kolmogorov cascade leading to a shallower resulting spectrum.

scholarly journals Interplanetary Scintillation Power Spectra

1976 ◽  
Vol 29 (3) ◽  
pp. 201 ◽  
Author(s):  
RG Milne
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Scattering Theory ◽  
Power Spectra ◽  
Inverse Power ◽  
Radio Sources ◽  
Interplanetary Scintillation ◽  
Inverse Power Law ◽  
Power Law Index ◽  
Strong Scattering

Power spectrum measurements of interplanetary scintillation at 408 MHz show that an inverse power law spectrum provides the best description for all scintillating radio sources. The inverse power law index is reasonably constant at ~ 2�4 for solar elongation angles 8 > 10�, and this agrees well with spacecraft observations. For 8 < 10� the index apparently decreases with decreasing 8, and this appears to be consistent with recent strong scattering theory. A Bessel analysis attempted in order to detect Fresnel structure proved unsuccessful because of noise on the power spectra.

scholarly journals Magnetic power spectrum in the undisturbed solar photosphere

2020 ◽  
Vol 1 (1) ◽  
pp. 1-5
Author(s):  
Valentina Abramenko ◽  
Olga Kutsenko
Keyword(s):  
Power Spectrum ◽  
Power Spectra ◽  
Active Regions ◽  
Reliable Estimate ◽  
Small Scale ◽  
Solar Photosphere ◽  
Dynamo Action ◽  
Quiet Sun ◽  
Turbulent Dynamo ◽  
Wide Range

Using the magnetic field data obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), an investigation of magnetic power spectra in the undisturbed solar photosphere was performed. The results are as follows. 1) To get a reliable estimate of a magnetic power spectrum from the uniformly distributed quiet-sun magnetic flux, a sample pattern of no less than 300 pixels length should be adopted. With smaller patterns, energy on all observable scales might be overestimated. 2) For patterns of different magnetic intensity (e.g., a coronal hole, a quiet-sun area, an area of supergranulation), the magnetic power spectra in a range of (2.5-10) Mm exhibit very close spectral indices of about -1. The observed spectrum is more shallow than the Kolmogorov-type spectrum (with a slope of -5/3) and it differs from steep spectra of active regions. Such a shallow spectrum cannot be explained by the only direct Kolmogorov’s cascade, but it can imply a small-scale turbulent dynamo action in a wide range of scales: from tens of megameters down to at least 2.5 Mm. On smaller scales, the HMI/SDO data do not allow us to reliably derive the shape of the spectrum. 3) Data make it possible to conclude that a uniform mechanism of the small-scale turbulent dynamo is at work all over the solar surface outside active regions.

scholarly journals Joint estimation of the Epoch of Reionization power spectrum and foregrounds

2019 ◽  
Vol 488 (2) ◽  
pp. 2904-2916 ◽  
Author(s):  
Peter H Sims ◽  
Jonathan C Pober
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Spatial Scales ◽  
Model Complexity ◽  
Scale Model ◽  
Spectral Structure ◽  
Synchrotron Emission ◽  
Power Law Model ◽  
Data Set ◽  
Spectral Scale

ABSTRACT The power spectrum of redshifted 21 cm emission brightness temperature fluctuations is a powerful probe of the Epoch of Reionization (EoR). However, bright foreground emission presents a significant impediment to its unbiased recovery from interferometric data. We estimate the power spectrum within a Bayesian framework and demonstrate that incorporating a priori knowledge of the spectral structure of foregrounds in the large spectral scale component of the data model enables significantly improved modelling of the foregrounds without increasing the model complexity. We explore two astrophysically motivated parametrizations of the large spectral scale model: (i) a constant plus power-law model of the form $q_{0}+q_{1}(\nu /\nu _{0})^{b_{1}}$ for two values of b1: b1 = 〈β〉GDSE and b1 = 〈β〉EGS, the mean spectral indices of the Galactic diffuse synchrotron emission and extragalactic source foreground emission, respectively; and (ii) a constant plus double power-law model of the form $q_{0}+q_{1}(\nu /\nu _{0})^{b_{1}}+q_{2}(\nu /\nu _{0})^{b_{2}}$ with b1 = 〈β〉GDSE and b2 = 〈β〉EGS. We estimate the EoR power spectrum from simulated interferometric data consisting of an EoR signal, Galactic diffuse synchrotron emission, extragalactic sources, and diffuse free–free emission from the Galaxy. We show that, by jointly estimating a model of the EoR signal with the constant plus double power-law parametrization of the large spectral scale model, unbiased estimates of the EoR power spectrum are recoverable on all spatial scales accessible in the data set, including on the large spatial scales that were found to be contaminated in earlier work.

scholarly journals Spatial power spectra of dust across the Local Group: No constraint on disc scale height

2019 ◽  
Vol 492 (2) ◽  
pp. 2663-2682 ◽  
Author(s):  
Eric W Koch ◽  
I-Da Chiang (江宜達) ◽  
Dyas Utomo ◽  
Jérémy Chastenet ◽  
Adam K Leroy ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Large Scale ◽  
Power Spectra ◽  
Far Infrared ◽  
Infrared Emission ◽  
Scale Height ◽  
Turbulent Motion ◽  
Spatial Power Spectrum ◽  
30 Doradus

ABSTRACT We analyse the 1D spatial power spectra of dust surface density and mid to far-infrared emission at $24\!-\!500\, \mu$m in the LMC, SMC, M31, and M33. By forward-modelling the point spread function (PSF) on the power spectrum, we find that nearly all power spectra have a single power-law and point source component. A broken power-law model is only favoured for the LMC 24 μm MIPS power spectrum and is due to intense dust heating in 30 Doradus. We also test for local power spectrum variations by splitting the LMC and SMC maps into 820 pc boxes. We find significant variations in the power-law index with no strong evidence for breaks. The lack of a ubiquitous break suggests that the spatial power spectrum does not constrain the disc scale height. This contradicts claims of a break where the turbulent motion changes from 3D to 2D. The power spectrum indices in the LMC, SMC, and M31 are similar (2.0–2.5). M33 has a flatter power spectrum (1.3), similar to more distant spiral galaxies with a centrally-concentrated H2 distribution. We compare the power spectra of H i, CO, and dust in M31 and M33, and find that H i power spectra are consistently flatter than CO power spectra. These results cast doubt on the idea that the spatial power spectrum traces large scale turbulent motion in nearby galaxies. Instead, we find that the spatial power spectrum is influenced by (1) the PSF on scales below ∼3 times the FWHM, (2) bright compact regions (30 Doradus), and (3) the global morphology of the tracer (an exponential CO disc).

scholarly journals A POWER-LAW BREAK IN THE NEAR-INFRARED POWER SPECTRUM OF THE GALACTIC CENTER BLACK HOLE

2009 ◽  
Vol 694 (1) ◽  
pp. L87-L91 ◽  
Author(s):  
L. Meyer ◽  
T. Do ◽  
A. Ghez ◽  
M. R. Morris ◽  
S. Yelda ◽  
...  
Keyword(s):  
Black Hole ◽  
Power Spectrum ◽  
Power Law ◽  
Near Infrared ◽  
Galactic Center

Inherent power‐law behavior of magnetic field power spectra from a Spector and Grant ensemble

Geophysics ◽  
1997 ◽  
Vol 62 (4) ◽  
pp. 1143-1150 ◽  
Author(s):  
Maurizio Fedi ◽  
Tatiana Quarta ◽  
Angelo De Santis
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Depth Distribution ◽  
Power Spectra ◽  
General Rule ◽  
Power Law Decay ◽  
Depth Determination ◽  
Rate Of Decay ◽  
Fractal Character ◽  
Block Sizes

The Spector and Grant method, which has been in use for 25 years, relates average depths to source to rate of decay of the magnetic power spectra. This method, which assumes a uniform distribution of parameters for an ensemble of magnetized blocks, leads to a depth‐dependent exponential rate of decay. We show that also inherent in this model is a power‐law rate of decay that is independent of depth. For most cases, except for extreme depths and small block sizes, the observed power spectrum should be corrected for a power law decay rate of β∼3. If the depth distribution of the magnetic blocks is Gaussian, then the observed power spectrum should be corrected for both a depth independent power law and exponential decay. This power‐law decay is very similar to the scaling behavior, supposed as a fractal character, of observed magnetic fields in North America. As a general rule, when β∼3, further information is needed to discriminate between a fractal or Spector and Grant model. However, it is becoming quite clear that magnetic power spectra should be corrected for a power law decay before applying the Spector and Grant method for depth determination.

scholarly journals Angular power spectrum of supernova remnants: effects of structure, geometry and diffuse foreground

2020 ◽  
Author(s):  
Samir Choudhuri ◽  
Preetha Saha ◽  
Nirupam Roy ◽  
Somnath Bharadwaj ◽  
Jyotirmoy Dey
Keyword(s):  
Power Spectrum ◽  
Power Law ◽  
Supernova Remnants ◽  
Shell Thickness ◽  
Three Dimensional ◽  
Power Spectra ◽  
Input Power ◽  
Dimensional Structure ◽  
Small Scale ◽  
Two Dimensional

Abstract The study of the intensity fluctuation power spectrum of individual supernova remnants (SNRs) can reveal the structures present at sub-pc scales, and also constrain the physical process that generates those structures. There are various effects, such as the remnant shell thickness, projection of a three-dimensional structure onto a two-dimensional observational plane, and the presence of diffuse “foreground” emission, which causes the observed power spectrum to deviate from the intrinsic power spectrum of the fluctuations. Here, we report results from a systematic study of these effects, using direct numerical simulations, in the measured power spectrum. For an input power-law power spectrum, independent of the power-law index, we see a break in the observed power law at a scale which depends on the shell thickness of a shell-type SNR, and the three-dimensional turbulence changes to two-dimensional turbulence beyond that scale. We also report how the estimated power spectrum is expected to deviate from the intrinsic SNR power spectrum in the presence of additional diffuse Galactic synchrotron emission (DGSE) around the remnant shell. For a reasonable choice of the parameters, if the intrinsic SNR power spectrum is shallower than the DGSE power spectrum, the SNR contribution dominates at small angular scales of the estimated power spectra. On the other hand, if the SNR power spectrum is relatively steeper, the original power spectra is recovered only over a small window of angular scales. This study shows how detailed modeling may be used to infer the true power spectrum from the observed SNR intensity fluctuations power spectrum, which in turn can be used to constrain the nature of the turbulence that gives rise to these small scale structures.

Log-log scale roughness spectroscopy of surfaces

1993 ◽  
Vol 51 ◽  
pp. 224-225
Author(s):  
P. Fraundorf ◽  
B. Armbruster
Keyword(s):  
Power Spectrum ◽  
Autocorrelation Function ◽  
Power Spectra ◽  
Scanning Force Microscopy ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Force Microscopy ◽  
Scanning Force ◽  
Lateral Structure ◽  
Scale Roughness

Optical interferometry, confocal light microscopy, stereopair scanning electron microscopy, scanning tunneling microscopy, and scanning force microscopy, can produce topographic images of surfaces on size scales reaching from centimeters to Angstroms. Second moment (height variance) statistics of surface topography can be very helpful in quantifying “visually suggested” differences from one surface to the next. The two most common methods for displaying this information are the Fourier power spectrum and its direct space transform, the autocorrelation function or interferogram. Unfortunately, for a surface exhibiting lateral structure over several orders of magnitude in size, both the power spectrum and the autocorrelation function will find most of the information they contain pressed into the plot’s origin. This suggests that we plot power in units of LOG(frequency)≡-LOG(period), but rather than add this logarithmic constraint as another element of abstraction to the analysis of power spectra, we further recommend a shift in paradigm.

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