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Spectra of the non-thermal radio radiation from the galactic polar regions
Abstract
Results are presented for measurements of the galactic radio background
at frequencies of 5.2, 9.0, 15.6, and 23.0 MHz, which were made by using
half-wave dipoles directed toward + and - 43 deg declination. It is
found that the temperatures of the regions closest to the galactic poles
for these declinations agree to within 10% and that the data are
compatible with recent satellite spectra and southern measurements. A
straight spectrum from 6 to 100 MHz is shown to yield a total background
spectral index of 0.55 + or - 0.03. It is concluded that the observed
polar spectra can be explained in terms of a simple two-component model
consisting of a disk of uniform emission and absorption as well as an
extragalactic component responsible for about 18% of the total emission
at 10 MHz.
... The flux curve for Miranda is dashed because it is currently uncertain whether the beam pattern illuminates it or not. The sky background noise flux (data and parametrization from Cane (1979)) is included for comparison. Menietti et al. (1990) performed a ray tracing study to determine the southern source region of the smooth high-frequency nightside Uranus kilometric radiation. ...
... This work uses publicly available data from a variety of sources. Figure 3 uses UKR average flux density spectrum is from Zarka (1998) and sky background noise spectral density is from Cane (1979). Figure 4 uses icy moon radii and orbital distances from https:// ssd.jpl.nasa.gov/sats/phys ...
We present a feasibility study for passive sounding of Uranian icy moons using Uranian Kilometric Radio (UKR) emissions in the 100 - 900 kHz band. We provide a summary description of the observation geometry, the UKR characteristics, and estimate the sensitivity for an instrument analogous to the Cassini Radio Plasma Wave Science (RPWS) but with a modified receiver digitizer and signal processing chain. We show that the concept has the potential to directly and unambiguously detect cold oceans within Uranian satellites and provide strong constraints on the interior structure in the presence of warm or no oceans. As part of a geophysical payload, the concept could therefore have a key role in the detection of oceans within the Uranian satellites. The main limitation of the concept is coherence losses attributed to the extended source size of the UKR and dependence on the illumination geometry. These factors represent constraints on the tour design of a future Uranus mission in terms of flyby altitudes and encounter timing.
... In the Supplementary Material we provide a simple model for an RLC circuit that allows to broaden the frequency response up to ∆ν ∼ MHz. The circuit we describe, and the value of its parameters, are similar to those of very old radio missions [77,78]. The result of this broad frequency response is that in order to scan an e-fold in DM mass t e , an integration time at a given frequency of t int ∼ t e min (1, 2 π ∆ν/m α ) is required. ...
We consider resonant wave-like dark matter conversion into low-frequency radio waves in the Earth's ionosphere. Resonant conversion occurs when the dark matter mass and the plasma frequency coincide, defining a range $m_{ \text{DM} } \sim 10^{-9} - 10^{-8}$ eV where this approach is best suited. Owing to the non-relativistic nature of dark matter and the typical variational scale of the Earth's ionosphere, the standard linearized approach to computing dark matter conversion is not suitable. We therefore solve a second-order boundary-value problem, effectively framing the ionosphere as a driven cavity filled with a positionally-varying plasma. An electrically-small dipole antenna targeting the generated radio waves can be orders of magnitude more sensitive to dark photon and axion-like particle dark matter in the relevant mass range. The present study opens up a promising way of testing hitherto unexplored parameter space which could be further improved with a dedicated instrument.
... Figure 3 uses UKR average flux density spectrum is from Zarka (1998). The sky background noise spectral density is from Cane (1979). Figure 4 uses icy moon radii and orbital distances from https://ssd.jpl.nasa.gov/sats/phys_par/ ...
We present a feasibility study for passive sounding of Uranian icy moons using Uranian Kilometric Radio (UKR) emissions in the 100–900 kHz band. We provide a summary description of the observation geometry, the UKR characteristics, and estimate the sensitivity for an instrument analogous to the Cassini Radio Plasma Wave Science (RPWS) but with a modified receiver digitizer and signal processing chain. We show that the concept has the potential to directly and unambiguously detect cold oceans within Uranian satellites and provide strong constraints on the interior structure in the presence of warm or no oceans. As part of a geophysical payload, the concept could therefore have a key role in the detection of oceans within the Uranian satellites. The main limitation of the concept is coherence losses attributed to the extended source size of the UKR and dependence on the illumination geometry. These factors represent constraints on the tour design of a future Uranus mission in terms of flyby altitudes and encounter timing.
... The antenna receives not only a useful signal but also external noise (radiated galactic and extragalactic), the intensity of which can be described by the following formula, which is valid for the considered frequency range: Equation (8) was obtained in [46] based on the results of the study of the polar regions of the sky and corrected in [47] and [48] by increasing g I by 30%, which makes it possible to use it to estimate the average B T over the entire sky sphere. ...
This work is concerned with a detailed study of an active antenna, which can serve as a prototype of a phased array antenna element for a future lunar very-low-frequency (VLF) radio telescope operating in the frequency range of 1–30 MHz. The antenna consisted of a ribbon symmetrical dipole and a low-noise preamplifier. A dipole 10 m long lay on a flat surface of lunar soil, consisting of a layer of regolith 10 m thick and solid bedrock. The dipole parameters were determined by full-wave simulation using the Altair Feko 2022 software. The preamplifier was a single-stage HEMT low-noise amplifier, whose parameters were determined using the Advanced Design System 2023 software. An active antenna was analyzed using a model created to calculate all electrical and noise parameters. Special attention was paid to the analysis of the antenna sensitivity in terms of the System Equivalent Flux Density (SEFD) and Sky Noise Dominance (SND), taking into account changes in the lunar ambient temperature from 100 K to 400 K. It was shown that the frequency dependencies of many active antenna parameters, in particular, radiation efficiency, directivity, effective area, SND, and SEFD had noticeable oscillatory components. The radiation pattern of the antenna was also subject to cyclical changes that occur in sync with the changes in directivity. These properties of the active antenna, caused by the presence of two-layer soil, should be considered when developing future lunar VLF radio telescopes.
... The frequency range of the RAD1 receiver is such that background radio emission is present due to both the local plasma environment and non-thermal emission from the galactic center or disk. At lower frequencies (below ∼ 300 kHz) the thermal motion of charged particles in the plasma surrounding the spacecraft creates quasi-thermal noise (QTN) (Meyer-Vernet and Perche, 1989) while emission from the galaxy dominates at higher frequencies (Novaco and Brown, 1978;Cane, 1979). Previous examination of measurements across the entire Waves frequency range has consolidated the measured galactic spectrum with previously derived functional forms (Dulk et al., 2001) and more recent measurements across the RAD1 receiver show agreement with a spectrum that falls off between 100-200 kHz (Hillan et al., 2010). ...
Auroral Kilometric Radiation (AKR) is Earth’s most powerful, naturally-emitted radio emission. It is a direct result of the dynamics of energetic electrons along high latitude magnetic field lines in the magnetosphere-ionosphere coupling region and is useful for inference of the spatial morphology and activation of the auroral acceleration region. In this thesis, a novel technique is developed to retrieve calibrated AKR observations from the Wind spacecraft, without the presence of solar and other contaminating radio emissions. Wind houses a remote-sensing radio observatory and has been active since 1995, providing the longest dataset of AKR from many viewing positions around the Earth. For AKR, which is anisotropically beamed, this breadth of data allows the effect of the observer position to be characterised. Subsequently, the data are used to explore AKR dynamics and corresponding changes in the auroral acceleration region during substorms and other disturbed periods. Firstly, the AKR selection is applied to a 30 day period during the Earth flyby of the Cassini spacecraft for a multipoint observation. Comparing the temporal modulation of the AKR power from both spacecraft during this period, it is shown that a diurnal modulation is likely due to the geometrical effect of the rotating magnetic dipole, but also sees intrinsic source modulation common to both hemispheres. Secondly, the selection is applied to 10 years of observations that overlap with published lists of substorm onsets. After accounting for viewing, the average AKR power in low and high frequency bands was examined during the substorm timeline. This demonstrated that higher altitude AKR sources provide a greater contribution during substorm onset on average, and that this relative contribution scales with the level of disturbance. Finally, dayside observations of AKR were used during the arrival of a coronal mass ejection (CME) at Earth. While showing that Wind’s dayside observations can be used as a metric for magnetospheric activity, coincident observations of the UV aurora were used to constrain the auroral origin, namely intense, dayside auroral dynamics related to an ongoing substorm.
Radar sounder profiles are essential for imaging the subsurface of planetary bodies and the Earth as they provide valuable geological insights. However, the limited availability of high-resolution radargrams poses challenges. This paper proposes a novel method based on generative models to super-resolve radargrams. Our approach addresses the ill-posed and ill-conditioned nature of the super-resolution problem by training a neural network to learn the correlation between radargrams at different scales. The network learns a proxy for the mapping function between ambiguous low-resolution radargrams and more detailed high-resolution ones, considering the data’s geological and statistical properties. The mapping function enables the super-resolution of previously unseen low-resolution radargrams acquired in comparable conditions to those in the training and imaging similar underlying geology. To achieve this, we adopt a generative Cycle-Consistent Adversarial Network (CycleGAN), explicitly designed to match properties between low- and high-resolution radargrams, accounting for variations in dimensions and radiometric properties. Furthermore, we enhance the network performance by incorporating skip connections, a ResNet module, and attention mechanisms. The proposed method is validated using MCoRDS3 radargrams acquired in Greenland and Antarctica as high-resolution data. As low-resolution data, we used simulated radargrams representing what is expected by an Earth-orbiting low-resolution radar sounder to have a controlled experiment. The results are evaluated qualitatively and quantitatively, focusing on the areas with reflections with complex shapes that may generate artifacts and unrealistic geological features.
Context. Arrays of radio antennas have proven to be successful in astroparticle physics with the observation of extensive air showers initiated by high-energy cosmic rays in the Earth’s atmosphere. Accurate determination of the energy scale of the primary particles’ energies requires an absolute calibration of the radio antennas for which, in recent years, the utilization of the Galactic emission as a reference source has emerged as a potential standard.
Aims. To apply the “Galactic calibration” a proper estimation of the systematic uncertainties on the prediction of the Galactic emission from sky models is necessary, which we aim to quantify on a global level and for the specific cases of selected radio arrays. We further aim to determine the influence of additional natural radio sources on the Galactic calibration.
Methods. We compared seven different sky models that predict the full-sky Galactic emission in the frequency range from 30 to 408 MHz. We made an inventory of the reference maps on which they rely and used the output of the models to determine their global level of agreement. We subsequently took typical sky exposures and the frequency bands of selected radio arrays into account and repeated the comparison for each of them. Finally, we studied and discuss the relative influence of the quiet Sun, the ionosphere, and Jupiter.
Results. We find a systematic uncertainty of 14.3% on the predicted power from the Galactic emission, which scales to approximately half of that value as the uncertainty on the determination of the energy of cosmic particles. When looking at the selected radio arrays, the uncertainty on the predicted power varies between 11.7% and 21.5%. The influence of the quiet Sun turns out to be insignificant at the lowest frequencies but increases to a relative contribution of ~30% around 400 MHz.
Aims. We applied the quasi-thermal noise (QTN) method to Parker Solar Probe (PSP) observations to derive the total electron temperature ( T e ). We combined a set of encounters to make up a 12-day period of observations around each perihelion from encounter one (E01) to ten (E10), with E08 not included. Here, the heliocentric distance varies from about 13 to 60 solar radii ( R ⊙ ).
Methods. The QTN technique is a reliable tool to yield accurate measurements of the electron parameters in the solar wind. We obtained T e from the linear fit of the high-frequency part of the QTN spectra acquired by the RFS/FIELDS instrument. Then, we provided the mean radial electron temperature profile, and examined the electron temperature gradients for different solar wind populations (i.e. classified by the proton bulk speed, V p , and the solar wind mass flux).
Results. We find that the total electron temperature decreases with the distance as ∼ R −0.66 , which is much slower than adiabatic. The extrapolated T e based on PSP observations is consistent with the exospheric solar wind model prediction at ∼10 R ⊙ , Helios observations at ∼0.3 AU, and Wind observations at 1 AU. Also, T e , extrapolated back to 10 R ⊙ , is almost the same as the Strahl electron temperature, T s (measured by SPAN-E), which is considered to be closely related to or even almost equal to the coronal electron temperature. Furthermore, the radial T e profiles in the slower solar wind (or flux tube with larger mass flux) are steeper than those in the faster solar wind (or flux tube with smaller mass flux). The more pronounced anticorrelation of V p – T e is observed when the solar wind is slower and located closer to the Sun.
Observations of the distribution of brightness at intermediate latitudes in the galaxy and of the edge-on spiral galaxy, NGC 891, indicate that the emissivity extends to heights of several kpc perpendicular to the plane. In several galaxies, the angular distributions of neutral hydrogen and nonthermal emission are roughly coextensive and show similar features such as spiral structure. If radio galaxies and normal galaxies with strong nuclear radio sources are excluded, there appears to be a proportionality between their total H(I) content and their nonthermal radio luminosity.
High-resolution maps of brightness distribution for a 'complete' sample
of more intense extragalactic radio sources over the range from 0.5 to
400 MHz are used to investigate the origin of the isotropic component of
the observed radio background radiation. The low-frequency turnover in
the spectrum of the isotropic component is attributed to synchrotron
self-absorption in the spectra of the discrete sources that comprise the
background. The overall source spectrum in the indicated frequency range
is derived, and Cyg A is studied as a test case. The integrated spectrum
is calculated first for a model in which all sources have the spectrum
of Cyg A and then in a more general manner by assuming that a 'complete'
sample of 80 intense sources is typical of the whole source population;
the variation of cosmological evolution with radio luminosity is taken
into account in these computations. The results show that: (1) the
superposition of extragalactic radio sources results in a predicted
extragalactic background that makes up a large fraction of that derived
from observations in the frequency range considered; (2) low-luminosity
sources inadequately represented by the 'complete' sample may make a
comparable contribution to the background at low frequencies; and (3) a
large fraction of the background originates in sources at moderate
redshifts.
The relationship between nonthermal radio emission of the Galaxy and the
cosmic ray electron spectrum is examined emphasizing a direct comparison
between the local cosmic ray electron flux deduced from measurements on
earth and the average synchrotron emissivity observed in the spiral arm
of the Galaxy. It is concluded that (1) the nonthermal emissivity of the
Galaxy varies in a systematic way related to the structure of the spiral
arms, (2) the local emissivity near the sun may be a factor of up to
roughly 4 less than typical emissivities in our own spiral arm, (3) the
electron spectrum producing the nonthermal emission may itself flatten
at energies less than 1 GeV, thus producing a flattening in the
emissivity spectrum, and (4) solar modulation effects at low energies
may be much less than previously thought
Cosmic radio background noise spectrum near north galactic pole based on Radio Astronomy Explorer satellite experiments
The Radio Astronomy Explorer 2 (RAE 2) lunar-orbiting satellite has provided measurements of the nonthermal galactic radio emission at frequencies below 10 MHz. Measurements of the emission spectra are presented for the center, anticenter, north polar, and south polar directions at 22 frequencies between 0.25 and 9.18 MHz. Survey maps of the spatial distribution of the observed low-frequency Galactic emission at 1.31, 2.20, 3.93, 4.70, 6.55, and 9.18 MHz are presented. The observations were obtained with the 229-m traveling-wave V antenna on this lunar-orbiting spacecraft. The improved frequency coverage offers additional insights into the structure of the local Galactic neighborhood.