Interplanetary scattering effects in the jovian bKOM radio emission observed by Ulysses

The scattering of radio waves by the interplanetary medium (subsequently referred to simply as `scattering') imposes limitations on measurements of fine temporal and angular structure at low radio frequencies. The jovian broad-band kilometric radiation (bKOM) is highly structured and observations of this emission, by the Unified Radio and Plasma Experiment (URAP) on board ULYSSES, have been used to study scattering at extremely low frequencies. Structures observed in the bKOM during 1995-6, when the spacecraft was at distances greater than 5 AU from the planet, are compared with observations made in 1991 when the spacecraft was relatively close to Jupiter (∼ AU). Measured values of the e-folding time are compared with theoretical values of temporal broadening and with solar wind electron density measurements made by the SWOOPS experiment on Ulysses. In general, we find that temporal broadening increases with source-observer distance and decreasing frequency, as expected from standard broadening theory, although a few inordinately low values were also observed at distances of 5 to 7 AU. At frequencies close to 45 kHz, an upper limit of about 8 min is clearly visible in observations of the temporal broadening made about 1 AU from Jupiter. Average values of the e-folding time at these same frequencies show a well-defined increase with distance from Jupiter of about 8 min AU-1. We interpret this as `saturated scattering,' where all ray paths between source and observer are explored, giving an upper limit to the temporal broadening which the medium can introduce. These observations establish limits for scattering in the interplanetary medium (IPM), which are relevant to all low frequency radio observations

Plasma Double Layers at the Boundary between Venus and the Solar Wind

International audienceThe solar wind is slowed, deflected, and heated as it encounters Venus's induced magnetosphere. The importance of kinetic plasma processes to these interactions has not been examined in detail, due to a lack of constraining observations. In this study, kinetic‐scale electric field structures are identified in the Venusian magnetosheath, including plasma double layers. The double layers may be driven by currents or mixing of inhomogeneous plasmas near the edge of the magnetosheath. Estimated double layer spatial scales are consistent with those reported at Earth. Estimated potential drops are similar to electron temperature gradients across the bow shock. Many double layers are found in few high cadence data captures, suggesting that their amplitudes are high relative to other magnetosheath plasma waves. These are the first direct observations of plasma double layers beyond near‐Earth space, supporting the idea that kinetic plasma processes are active in many space plasma environments

Catalogue of solar type II radio bursts observed from September 1990 to December 1993 and their statistical analysis

Solar type II radio bursts represent the radio signature of shock waves travelling through the solar corona. They are associated with flares, coronal mass ejections (CME's) and interplanetary shocks. Type II radio bursts appear as emission stripes slowly drifting from high to low frequencies in dynamic radio spectra. The spectral features of all solar type II radio bursts observed by the new radiospectrograph of the Astrophysikalisches Institut Potsdam in Tremsdorf during the time period from September 1, 1990 to December 31, 1993, i.e., during the first part of the ULYSSES spacecraft mission, are summarized and statistically investigated. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The ALFA medium explorer mission

The frequency range below a few tens of MHz is unexplored with high angular resolution due to the opacity of Earth's ionosphere. An interferometer array in space providing arcminute angular resolution images at frequencies of a few MHz would allow a wide range of problems in solar, planetary, galactic, and extragalactic astronomy to be attacked. These include the evolution of solar radio emissions associated with shocks driven by coronal mass ejections and searches for coherent radio emission from supernova remnants and relativistic jets, in addition, it is likely that unexpected objects or emission processes will be discovered by such an instrument, as has always happened when high resolution astronomical observations first become possible in a new region of the electromagnetic spectrum. The Astronomical Low Frequency Array (ALFA) mission will consist of 16 identical small satellites forming an aperture synthesis array. The satellites will cover the surface of a spherical region approximate to 100 km in diameter, thus providing good aperture plane coverage in all directions simultaneously. The array will operate in two modes: 1) "snapshot" imaging of strong, rapidly changing sources such as solar radio bursts and 2) long-term aperture synthesis observations for maximum sensitivity, high dynamic range imaging, in both cases a large number of array elements is needed

Plasma Waves near the Electron Cyclotron Frequency in the Near-Sun Solar Wind

International audienceData from the first two orbits of the Sun by Parker Solar Probe reveal that the solar wind sunward of 50 solar radii is replete with plasma waves and instabilities. One of the most prominent plasma wave power enhancements in this region appears near the electron cyclotron frequency (f ce). Most of this wave power is concentrated in electric field fluctuations near 0.7 f ce and f ce, with strong harmonics of both frequencies extending above f ce. At least two distinct, often concurrent, wave modes are observed, preliminarily identified as electrostatic whistler-mode waves and electron Bernstein waves. Wave intervals range in duration from a few seconds to hours. Both the amplitudes and number of detections of these near-f ce waves increase significantly with decreasing distance to the Sun, suggesting that they play an important role in the evolution of electron populations in the near-Sun solar wind. Correlations are found between the detection of these waves and properties of solar wind electron populations, including electron core drift, implying that these waves play a role in regulating the heat flux carried by solar wind electrons. Observation of these near-f ce waves is found to be strongly correlated with near-radial solar wind magnetic field configurations with low levels of magnetic turbulence. A scenario for the growth of these waves is presented, which implies that regions of low-turbulence near-radial magnetic field are a prominent feature of the solar wind structure near the Sun

Observations of Heating along Intermittent Structures in the Inner Heliosphere from PSP Data

International audienceThe solar wind proton temperature at 1 au has been found to be correlated with small-scale intermittent magnetic structures, i.e., regions with enhanced temperature are associated with coherent structures, such as current sheets. Using Parker Solar Probe data from the first encounter, we study this association using measurements of the radial proton temperature, employing the partial variance of increments (PVI) technique to identify intermittent magnetic structures. We observe that the probability density functions of high PVI events have higher median temperatures than those with lower PVI. The regions in space where PVI peaks were also locations that had enhanced temperatures when compared with similar regions, suggesting a heating mechanism in the young solar wind that is associated with intermittency developed by a nonlinear turbulent cascade in the immediate vicinity

Parker Solar Probe observations of He/H abundance variations in SEP events inside 0.5 au

Aims. The Parker Solar Probe (PSP) orbit provides an opportunity to study the inner heliosphere at distances closer to the Sun than previously possible. Due to the solar minimum conditions, the initial orbits of PSP yielded only a few solar energetic particle (SEP) events for study. Recently during the fifth orbit, at distances from 0.45 to 0.3 au, the energetic particle suite on PSP, Integrated Science Investigation of the Sun (IS⊙ IS), observed a series of six SEP events, adding to the limited number of SEP events studied inside of 0.5 au. Variations in the H and He spectra and the He/H abundance ratio are examined and discussed in relation to the identified solar source regions and activity. Methods. IS⊙ IS measures the energetic particle environment from ~20 keV to >100 MeV/nuc. Six events were selected using the ~1 MeV proton intensities, and while small, they were sufficient to calculate proton and helium spectra from ~1 to ~10 MeV/nuc. For the three larger events, the He/H ratio as a function of energy was determined. Using the timing of the associated radio bursts, solar sources were identified for each event and the eruptions were examined in extreme ultraviolet emission. Results. The largest of the selected events has peak ~1 MeV proton intensities of 3.75 (cm2 sr s MeV)-1. Within uncertainties, the He and H spectra have similar power law forms with indices ranging from -2.3 to -3.3. For the three largest events, the He/H ratios are found to be relatively energy independent; however, the ratios differ substantially with values of 0.0033 ± 0.0013, 0.177 ± 0.047, and 0.016 ± 0.009. An additional compositional variation is evident in both the 3He and electron signatures. These variations are particularly interesting as the three larger events are likely a result of similar eruptions from the same active region. © ESO 2021.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]