Magnetic Power Spectra Research Articles

Spectrum of kinetic plasma turbulence at 0.3-0.9 astronomical units from the Sun.

We investigate spectral properties of turbulence in the solar wind that is a weakly collisional astrophysical plasma, accessible to in situ observations. Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence. At such small scales, we show that magnetic power spectra between 0.3 and 0.9 AU from the Sun have a generic shape ∼f^{-8/3}exp(-f/f_{d}), where the dissipation frequency f_{d} is correlated with the Doppler shifted frequency f_{ρe} of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with f_{d}=f_{ρe}/1.8. Our results indicate that the electron gyroradius plays the role of the dissipation scale and marks the end of the electromagnetic cascade in the solar wind.

Baryogenesis, magnetogenesis and the strength of anomalous interactions

The production of the hypermagnetic gyrotropy is investigated under the assumption that the gauge coupling smoothly evolves during a quasi-de Sitter phase and then flattens out in the radiation epoch by always remaining perturbative. In the plane defined by the strength of the anomalous interactions and by the rate of evolution of the gauge coupling the actual weight of the pseudoscalar interactions turns out to be always rather modest if major deviations from the homogeneity are to be avoided during the inflationary phase. Even if the gauge power spectra are related by duality only in the absence of anomalous contributions, an approximate duality symmetry constrains the late-time form of the hypermagnetic power spectra. Since the hypermagnetic gyrotropy associated with the modes reentering prior to the phase transition must be released into fermions later on, the portions of the parameter space where the obtained baryon asymmetry is close to the observed value are the most relevant for the present ends. For the same range of parameters the magnetic power spectra associated with the modes reentering after symmetry breaking may even be of the order of a few hundredths of a nG over typical length scales comparable with the Mpc prior to the collapse of the protogalaxy.

Scale-dependent anisotropy of electric field fluctuations in solar wind turbulence

ABSTRACT We study the variation of average powers and spectral indices of electric field fluctuations with respect to the angle between average flow direction and the mean magnetic field in solar wind turbulence. Cluster spacecraft data from the years 2002 and 2007 are used for the present analysis. We perform a scale-dependent study with respect to the local mean magnetic field using wavelet analysis technique. Prominent anisotropies are found for both the spectral index and power levels of the electric power spectra. Similar to the magnetic field fluctuations, the parallel (or antiparallel) electric fluctuation spectrum is found to be steeper than the perpendicular spectrum. However, the parallel (or antiparallel) electric power is found to be greater than the perpendicular one. Below 0.1 Hz, the slope of the parallel electric power spectra deviates substantially from that of the total magnetic power spectra, supporting the existence of Alfvénic turbulence.

On the possibility of probing the flare productivity of an active region in the early stage of emergence

ABSTRACT Prediction of the future flare productivity of an active region (AR) when it is in the early-emergence stage is a longstanding problem. The aim of this study is to probe two parameters of the photospheric magnetic field, both derived during the emergence phase of an AR, and to compare them with the flare productivity of a well developed AR. The parameters are: (i) the index of the magnetic power spectrum (the slope of the spectrum) at the stage of emergence, and (ii) the flux emergence rate. Analysis of 243 emerging ARs showed that the magnetic power index increases from values typical of quiet-Sun regions to those typical of mature ARs within a day, while the emergence proceeds for several days; frequently, after the increase, the value of the power index oscillates around some mean value with the fluctuations being several times smaller than the growth of the power index during the emergence onset. For a subset of 34 flare-productive ARs we found no correlation between the power spectrum index at the stage of emergence and the flare index derived from the entire interval of the AR’s presence on the disc. At the same time, the flux emergence rate correlates well with the flare index (Pearson’s correlation coefficient is 0.74). We conclude that a high flux emergence rate is a necessary condition for an AR to produce strong flares in the future; thus the flux emergence rate can be used to probe the future flare productivity of an AR.

Inverse energy transfer in decaying, three-dimensional, non-helical magnetic turbulence due to magnetic reconnection

ABSTRACT It has been recently shown numerically that there exists an inverse transfer of magnetic energy in decaying, non-helical, magnetically dominated, magnetohydrodynamic turbulence in three dimensions (3D). We suggest that magnetic reconnection is the underlying physical mechanism responsible for this inverse transfer. In the two-dimensional (2D) case, the inverse transfer is easily inferred to be due to smaller magnetic islands merging to form larger ones via reconnection. We find that the scaling behaviour is similar between the 2D and 3D cases, i.e. the magnetic energy evolves as t−1, and the magnetic power spectrum follows a slope of k−2. We show that on normalizing time by the magnetic reconnection time-scale, the evolution curves of the magnetic field in systems with different Lundquist numbers collapse on to one another. Furthermore, transfer function plots show signatures of magnetic reconnection driving the inverse transfer. We also discuss the conserved quantities in the system and show that the behaviour of these quantities is similar between the 2D and 3D simulations, thus making the case that the dynamics in 3D could be approximately explained by what we understand in 2D. Lastly, we also conduct simulations where the magnetic field is subdominant to the flow. Here, too, we find an inverse transfer of magnetic energy in 3D. In these simulations, the magnetic energy evolves as t−1.4 and, interestingly, a dynamo effect is observed.

Imprints of the post-recombination dissipation of helical magnetic field on the Cosmic Microwave Background Radiation

Astrophysical magnetic fields decay primarily via two processes, namely ambipolar diffusion and turbulence. Constraints on the strength and the spectral index of nonhelical magnetic fields have been derived earlier in the literature through the effect of the above-mentioned processes on the cosmic microwave background (CMB) radiation. A helical component of the magnetic field is also produced in various models of magnetogenesis, which can explain larger coherence length magnetic field. In this study, we focus on studying the effects of post-recombination decay of maximally helical magnetic fields through ambipolar diffusion and decaying magnetic turbulence and the impact of this decay on CMB. We find that helical magnetic fields lead to changes in the evolution of baryon temperature and ionization fraction which in turn lead to modifications in the CMB temperature and polarization anisotropy. These modifications are different from those arising due to nonhelical magnetic fields with the changes dependent on the strength and the spectral index of the magnetic field power spectra.

Energy Supply for Heating the Slow Solar Wind Observed by Parker Solar Probe between 0.17 and 0.7 au

Abstract Energy supply sources for the heating process in the slow solar wind remain unknown. The Parker Solar Probe (PSP) mission provides a good opportunity to study this issue. Recently, PSP observations have found that the slow solar wind experiences stronger heating inside 0.24 au. Here for the first time we measure in the slow solar wind the radial gradient of the low-frequency breaks on the magnetic trace power spectra and evaluate the associated energy supply rate. We find that the energy supply rate is consistent with the observed perpendicular heating rate calculated based on the gradient of the magnetic moment. Based on this finding, one could explain why the slow solar wind is strongly heated inside 0.25 au but expands nearly adiabatically outside 0.25 au. This finding supports the concept that the energy added from the energy-containing range is transferred by an energy cascade process to the dissipation range, and then dissipates to heat the slow solar wind. The related issues for further study are discussed.

Geomagnetically Induced Currents and Harmonic Distortion: High Time Resolution Case Studies

AbstractHigh time resolution (1–5 s) magnetometer, geomagnetically induced current (GIC), and mains harmonic distortion data from the Halfway Bush substation in Dunedin, New Zealand, are analyzed. A recently developed technique using very low frequency (VLF) radio wave data provides high‐resolution measurements of mains harmonic distortion levels. Three case studies are investigated, each involving high rates of change of local geomagnetic field, but with different timescales of magnetospheric driver mechanisms, and different substation transformer configurations. Two cases of enhanced GIC during substorm events are analyzed, and one case of a storm sudden commencement. Time delays between magnetic field fluctuations and induced transformer currents are found to be ~100 s for substorm events, but only ~20 s for the storm sudden commencement containing higher‐frequency variations. Boxcar averaging of the magnetic field fluctuations using running windows of ±2 min leads to spectral power profiles similar to those of GIC profiles, with reduced power at frequencies >0.003 Hz (periods <5 min). Substantially lower mains harmonic distortion levels were observed after the removal of the single phase bank transformer, HWB T4, from the high‐voltage configuration at Halfway Bush. No systematic time delay was found between GIC variations and mains harmonic distortion levels. The power spectra of magnetic field fluctuations and GIC variations during the sudden storm commencement with no harmonic distortion showed low levels of low‐frequency power (<0.003 Hz, periods >5 min). This low‐frequency component of the magnetic field power spectrum appears necessary for mains harmonic distortion to occur.

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

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.

Primordial magnetogenesis in a bouncing universe

We investigate primordial magnetogenesis and the evolution of the electromagnetic field through a quantum bounce, in a model that starts in the far past from a contracting phase where only dust is present and the electromagnetic field is in the adiabatic quantum vacuum state. By including a coupling between curvature and electromagnetism of the form $RF_{\mu \nu}F^{\mu \nu}$, we find acceptable magnetic field seeds within the current observational constraints at 1 mega-parsec (Mpc), and that the magnetic power spectrum evolves as a power-law with spectral index $n_B=6$. It is also shown that the electromagnetic backreaction is not an issue in the model under scrutiny.

Magnetic power spectrum in the undisturbed solar photosphere

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.

On the Fourier Contribution of Strong Current Sheets to the High‐Frequency Magnetic Power SpectralDensity of the Solar Wind

AbstractThe hypothesis is explored that the high‐frequency magnetic spectral power of the solar wind is associated with the spatial profiles of strong current sheets (directional discontinuities) in the solar wind plasma. This hypothesis is based on previous findings about current sheets and the solar wind's magnetic power spectra (1) that the amplitude distribution and waiting time distribution of strong current sheets determines the amplituic composition and the electron strade and spectral slope of the “inertial range” of frequencies and (2) that the thicknesses of strong current sheets in the solar wind determine the breakpoint frequency that ends the inertial range. Solar wind current sheets are collected from the WIND 0.09375‐s and MMS 7.8 × 10−3‐s magnetic data sets, and the current sheets are individually Fourier transformed. At frequencies above the solar wind breakpoint (1) the shape of the magnetic power spectra of the current sheets resembles the shape of the magnetic power spectra of the solar wind and (2) the current sheets occur frequently enough to account for the magnetic power of the solar wind above the breakpoint. This has implications for the physics underlying the high‐frequency magnetic power spectral density of the solar wind, supplementing the energy‐cascade description of the high‐frequency spectrum.

Спектр мощности магнитного поля в невозмущенной фотосфере Солнца

Проведено исследование спектров мощности магнитного поля в невозмущенной фотосфере Солнца по данным инструмента Helioseismic and Magnetic Imager (HMI) на борту станции Solar Dynamics Observatory (SDO). Результаты можно сформулировать следующим образом. 1) Для достоверной оценки спектра мощности в обширной зоне квазиравномерного распределения поля следует выбирать участок длиной не менее 300 пикселей. По оценкам по меньшему участку, мощность на всех доступных частотах оказывается завышенной. 2) Для магнитных зон разной интенсивности, а именно для корональной дыры, спокойного Солнца и супергрануляционной сетки, спектр мощности в диапазоне (2.5-10) Мм проявляет одинаковый спектральный индекс, близкий к -1. Наблюдаемый спектр более пологий, чем колмогоровский (с наклоном -5/3), и отличается от более крутых спектров активных областей. Такой пологий спектр нельзя объяснить только прямым колмогоровским каскадом. Его можно объяснить дополнительной накачкой магнитной энергии за счет мелкомасштабного турбулентного динамо, работающего в широком диапазоне масштабов: от десятков мегаметров до, по крайней мере, 2.5 Мм. На масштабах менее 2.5 Мм SDO/HMI-данные не позволяют адекватно оценить форму спектра. 3) Данные позволяют заключить, что однообразный механизм мелкомасштабного турбулентного динамо работает по всей поверхности Солнца вне активных областей.

Anisotropic turbulence of kinetic Alfvén waves and heating in solar corona

Understanding solar coronal heating has been one of the unresolved problems in solar physics in spite of the many theories that have been developed to explain it. Past observational studies suggested that kinetic Alfvén waves (KAWs) may be responsible for solar coronal heating by accelerating the charged particles in solar plasma. In this paper, we investigated the transient dynamics of KAWs with modified background density due to ponderomotive force and Joule heating. A numerical simulation based on pseudo-spectral method was applied to study the evolution of KAW magnetic coherent structures and generation of magnetic turbulence. Using different initial conditions in simulations, the dependence of KAW dynamics on the nature of inhomogeneous solar plasma was thoroughly investigated. The saturated magnetic power spectra follow Kolmogorov scaling of k−5/3 in the inertial range, then followed by steep anisotropic scaling in the dissipation range. The KAW has anisotropy of , , and depending on the kind of initial conditions of inhomogeneity. The power spectra of magnetic field fluctuations showing the spectral anisotropy in wavenumber space indicate that the nonlinear interactions may be redistributing the energy anisotropically among higher modes of the wavenumber. Therefore, anisotropic turbulence can be considered as one of the candidates responsible for the particle energization and heating of the solar plasmas.

Numerical simulations to study turbulent magnetic field amplification by nonlinear interaction of high-power laser and kinetic Alfvén waves in laboratory and astrophysical plasmas

The model contouring the dynamics of transient nonlinear interaction between the high-frequency extraordinary-elliptically polarized laser (HFXPL) and low-frequency kinetic Alfvén wave (LFKAW) dynamics in the magnetized plasma is the focal point of the present investigation. The quasistatic ponderomotive force driven by the HFXPL pump induces density cavitation and humps in the low-frequency kinetic Alfvén wave. In order to study the intricate localized structures of HFXPL pump waves that evolve with time, the requisite dimensionless equations of the coupled system (HFXPL and LFKAW) are evaluated by using numerical methods in the nonlinear stage. The typical scale sizes of these structures in the early phase are ∼9 μm, and the typical time to grow is ∼10 ps. The ensemble-averaged magnetic power spectra are also presented, indicating energy cascade. The rendered investigations follow direct relevance to the experimental observations [Chatterjee et al., Rev. Sci. Instrum. 85, 013505 (2014); Romagnani et al., Phys. Rev. Lett. 122, 025001 (2019); Tzeferacos et al., Nat. Commun. 9, 591 (2018); Phys. Plasmas 24, 041404 (2017); Meinecke et al., Proc. Natl. Acad. Sci. 112, 8211 (2015); Nat. Phys. 10, 520–524 (2014); Mondal et al., Proc. Natl. Acad. Sci. 109, 8011 (2012); Chatterjee et al., Nat. Commun. 8, 15970 (2017)] and are imperative in understanding turbulence in astrophysical scenarios.

Radio Noise Storms and the Connection with the Reorganization of Photospheric Magnetic Fields

This work presents the analysis of a Type-I radio noise storm (RNS) recorded by the CALLISTO spectrometers network. Considering that RNSs are associated with the reorganization of the photospheric magnetic field of active regions, we have analyzed the temporal evolution of the magnetic power spectra and the dissipation spectra of the photospheric magnetic field of two active regions (NOAA 11542 and NOAA 11543). This analysis was used to identify for which wave-numbers and time intervals this reorganization occurs. The Type-I emission is supposedly generated by the fundamental emission of plasma, and the reorganization of the line-of-sight component of the photospheric magnetic field was investigated as a possible mechanism responsible for the maintenance process of the RNS. The analysis of the magnetic power spectra and the dissipation spectra showed that the RNS was probably generated and maintained by the reorganization of the magnetic field of NOAA 11542. The reorganization of the magnetic-energy spectra occurs mainly for wave-numbers smaller than $0.2~\mbox{Mm}^{-1}$ and the dissipation spectrum increases for wave-numbers around $1~\mbox{Mm}^{-1}$.

Magnetic Power Spectra of Emerging Active Regions

Magnetic field data provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory were utilized to explore the changes in the magnetic energy of four active regions (ARs) during their emergence. We found that at the very early stage of the magnetic flux emergence, an abrupt steepening of the magnetic power spectrum takes place leading to rapid increase of the absolute value of the negative spectra power index $\alpha $ in $E(k)\sim k^{\alpha }$ . As the emergence proceeds, the energy increases at all scales simultaneously implying that elements of all sizes do appear in the photosphere. Meanwhile, the energy gain at scales larger than ${\approx}\,10~\mbox{Mm}$ prevails over that at smaller scales. Both direct (i.e., fragmentation of large structures into smaller ones) and inverse (i.e., merging of small magnetic features into larger elements) cascades are readily observed during the emergence. However, in the case of inverse cascade, the total energy gained at large scales exceeds the energy loss at smaller scales assuming simultaneous appearance of large-scale magnetic entities from beneath the photosphere. We conclude that most of the time the energy may grow at all scales. We also cannot support the point of view regarding the dominant role of the inverse cascade in the formation of an AR. Although coalescence of small magnetic elements into larger pores and sunspots is observed, our analysis shows that the prevailed energy contribution to an AR comes from emergence of large-scale structures.

On the ion-inertial-range density-power spectra in solar wind turbulence

Abstract. A model-independent first-principle first-order investigation of the shape of turbulent density-power spectra in the ion-inertial range of the solar wind at 1 AU is presented. Demagnetised ions in the ion-inertial range of quasi-neutral plasmas respond to Kolmogorov (K) or Iroshnikov–Kraichnan (IK) inertial-range velocity–turbulence power spectra via the spectrum of the velocity–turbulence-related random-mean-square induction–electric field. Maintenance of electrical quasi-neutrality by the ions causes deformations in the power spectral density of the turbulent density fluctuations. Assuming inertial-range K (IK) spectra in solar wind velocity turbulence and referring to observations of density-power spectra suggest that the occasionally observed scale-limited bumps in the density-power spectrum may be traced back to the electric ion response. Magnetic power spectra react passively to the density spectrum by warranting pressure balance. This approach still neglects contribution of Hall currents and is restricted to the ion-inertial-range scale. While both density and magnetic turbulence spectra in the affected range of ion-inertial scales deviate from K or IK power law shapes, the velocity turbulence preserves its inertial-range shape in the process to which spectral advection turns out to be secondary but may become observable under special external conditions. One such case observed by WIND is analysed. We discuss various aspects of this effect, including the affected wave-number scale range, dependence on the angle between mean flow velocity and wave numbers, and, for a radially expanding solar wind flow, assuming adiabatic expansion at fast solar wind speeds and a Parker dependence of the solar wind magnetic field on radius, also the presumable limitations on the radial location of the turbulent source region.

Localized Structures and Turbulent Spectra in the Magnetopause

AbstractKinetic Alfvén wave (KAW) and whistler wave play a key role in the framework of turbulence and reconnection. There are lot of observations of these wave in the magnetopause region. The present paper deals with the nonlinear evolution of KAW and a weak whistler wave through the preexisting magnetic reconnection site. For this study the dynamical evolution equations are derived by taking into account the ponderomotive force driven density modification and magnetic field fluctuations due to shear field modeled by the Harris sheet. Furthermore, these equations have been solved numerically as well as semianalytically. For numerical integrations we have used the pseudospectral method and finite difference method and for semianalytically Runge Kutta method. Simulated results have shown the evolution of coherent structures or current sheets, which are capable to energy transfer efficiently. These structures have scale size around ion gyroradius as well as electron inertial length as calculated analytically. At a later time the chaotic structures arise in this reconnection site. This gives the signature of turbulence generation. Therefore, the magnetic power spectrum with scaling is also presented in this manuscript and their relevance with the observed spectrum (calculated from the Cluster data (Chaston, 2008, https://doi.org/10.1029/2008GL033601)) is also pointed out.

Magnetic buoyancy in simulated galactic discs with a realistic circumgalactic medium

We present simulations of isolated disc galaxies in a realistic environment performed with the Tree-SPMHD-Code Gadget-3. Our simulations include a spherical circum-galactic medium (CGM) surrounding the galactic disc, motivated by observations and the results of cosmological simulations. We present three galactic models with different halo masses between 10e10 Msol and 10e12 Msol, and for each we use two different approaches to seed the magnetic field, as well as a control simulation without a magnetic field. We find that the amplification of the magnetic field in the centre of the disc leads to a biconical magnetic outflow of gas that magnetizes the CGM. This biconical magnetic outflow reduces the star formation rate (SFR) of the galaxy by roughly 40 percent compared to the simulations without magnetic fields. As the key aspect of our simulations, we find that small scale turbulent motion of the gas in the disc leads to the amplification of the magnetic field up to tens of 10e-6 G, as long as the magnetic field strength is low. For stronger magnetic fields turbulent motion does not lead to significant amplification but is replaced by an alpha-omega dynamo. The occurance of a small scale turbulent dynamo becomes apparent through the magnetic power spectrum and analysis of the field lines' curvature. In accordance with recent observations we find an anti-correlation between the spiral structure in the gas density and in the magnetic field due to a diffusion term added to the induction equation.