D region radio measurements at the magnetic equator
Daytime D region electron density profiles at magnetic equator from ARCAS flights
Kane, J. A.↗
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Shape of a solar wind disturbance on July 9, 1966, inferred from radio signal delay to Pioneer 6
Solar wind free electron number density measurement by Pioneer 6 spacecraft after solar proton flare, constructing plasma cloud models
Croft, T. A.↗
Effects of a finite plasma temperature on electron-cyclotron maser emission
Auroral kilometric radiation, Jupiter's decametric radio emission, and microwave spike bursts have all been attributed to the semirelativistic maser instability. The effect of a finite plasma temperature on the emission from this instability is investigated. Temperature effects reduce the frequency of the x mode and thereby enable fundamental x-mode radiation to occur at higher omega sub p/Omega sub e (where omega sub p is the plasma frequency and Omega sub e is the electron-cyclotron frequency). When the plasma frequency is sufficiently high to suppress x-mode growth, z-mode growth then dominates. The z-mode radiation is, however, subject to electron-cyclotron damping, and this damping can cause heating of the plasma in the vicinity of the source region. In this case, x-mode radiation can be generated even though initial conditions might favor z-mode growth.
Winglee, R. M.↗
Remote radio observations of solar wind parameters upstream of planetary bow shocks
Radio emission is frequently produced at twice the electron plasma frequency 2fp in the foreshock region upstream of the terrestrial bow shock. Observations of this emission provide a remote diagnostic of solar wind parameters in the foreshock. Using ISEE-3 radio data, we present the first evidence that the radio intensity is proportional to the kinetic energy flux and to other parameters correlated with solar wind density. We provide a qualitative explanation of this intensity behavior and predict the detection of similar emission at Jupiter by the Ulysses spacecraft.
Macdowall, R. J.↗
Rapid Succession of Sep Events Associated With A Series of Euv Jets: Solar Orbiter, Stereo-A and Near-Earth Spacecraft Observations
A series of near-relativistic solar electron events was observed on November 9 to 15, 2022 by STEREO-A at 0.96 au, by near-Earth spacecraft such as ACE, Wind and SOHO at ⇠0.99 au, and by Solar Orbiter at heliocentric distances ranging from 0.59 to 0.66 au. At least 22 electron intensity enhancements at energies >10 keV were clearly distinguishable in the particle data collected by the Energetic Particle Detector (EPD) suite of instruments on board Solar Orbiter, with 12 of these events occurring on 11 November 2022. Several of these electron events were accompanied by <10 MeV proton and <⇠2 MeV/n heavy ion intensity enhancements. The origin of this rapid succession of particle events is a series of jet-like eruptions detected in extreme ultraviolet (EUV) observations from the vicinity of active region AR 13141 that were closely associated with type III bursts in the decametric-hectometric range. We find a close association between the EUV jets, type III radio bursts and the release of near relativistic electrons. The events on 9 November and late on 14 November are particularly interesting in that they were just evident at electron energies below ⇠30 keV, were rich in heavy ions with energies below ⇠1 MeV/n, associated with type III radio bursts at frequencies below ⇠1.0 MHz, and originated at EUV solar eruptions lacking a clear jet. By combining remote and in-situ data from the three viewpoints (Solar Orbiter and STEREO-A were ⇠20 degrees and ⇠15 degrees east of Earth, respectively), we analyze the origin of this sequence of events, determine the interplanetary context where they were observed, and investigate the interplanetary transport of electrons to the different spacecraft.
D Loyo↗
Solar He-3-rich events and non-relativistic electron events: A new association
In 15 months of observation by the ISEE-e spacecraft, it was found that virtually all solar greater than or approximately equal to 1.3 MeV/nucleon He-3-rich events are associated with impulsive 2 to approximately 100 keV electron events, although many electron events were not accompanied by detectable He-3 increases. Both the He-3 and the electrons exhibit nearly scatter-free propagation in the interplanetary medium, and the times of onset and maximum for the He-3 and electron increases are closely related by velocity dispertion. The electron events and their related type III solar radio bursts provide, for the first time, identification of the flares which produce He-3-rich events. He-3 appears to be accelerated at the flash phase of solar flares along with nonrelativistic electrons.
Reames, D. V.↗
Solar radio emission
For this review, a selection has been made of a number of topics which are current active areas of both observational and theoretical research. Observations of Type III bursts are examined, taking into account ground-based observations (above approximately 8 MHz), spacecraft observations (below 1 MHz), and Langmuir waves and electron streams. Microwave bursts are considered along with Type II bursts, moving Type IV bursts, and Type I noise storms. The theory of Type III radio bursts (radio emission from electron streams) is discussed, giving attention to quasi-linear theory, induced scatter of ions, wave-wave effects of the nonlinear refractive and self-focusing variety, the second harmonic emission from Langmuir waves, fundamental emission from Langmuir waves, and density irregularities and ion-acoustic waves. Aspects of radio emission from shock waves and current sheets are also studied.
Goldman, M. V.↗
The participation of nuclei in type-III-related electron streams
We study 27 increases of the flux of 300-800 keV electrons on board HELIOS A or B, associated with intense type III radio bursts close to perihelion passages of the two spacecraft, during the solar minimum. Electrons can be detected inside cones with an angular width between 30 deg and 60 deg. Though only intense type III bursts are associated with recognizable electron events in space, such an association does not exist for all of them; this fact and great differences in fluxes of the individual events indicate that, apart from the intensity, also some other characteristic of the type III burst acceleration or propagation process determines the resulting flux of electrons in space; the energy spectrum of the accelerated electrons is one of the likely candidates. A comparison of the electron flux in these events with the flux of 1.7-3.7 MeV/nucl helium reveals very large variations of the helium/electron flux ratio, by a factor of at least 15 and possibly much higher. We demonstrate that these variations are not caused by propagation effects in interplanetary space. Therefore, they must be due either to propagation effects in the solar corona, or more likely, to intrinsic variations in the relative production of electrons and nuclei in the type III burst process. An extrapolation of the observed fluxes to 1 AU shows that in only 7 of the 27 electron events studied might a marginal greater than 1.7 MeV helium flux be recognized at the Earth distance.
Kallenrode, May-Britt↗
The Ionospheric Three-Dimensional Electron Density Variations Induced by the 21 August 2017 Total Solar Eclipse by Using Global Ionospheric Specification
Global Ionospheric Specification (GIS) is based on the Gauss–Markov Kalman filter to assimilate the slant total electron content (TEC) observed from ground-based GPS receivers and space-based radio occultation instrumentations in order to reconstruct three-dimensional (3D) ionospheric electron density structure, and it can remotely sense and monitor the weather condition in space. In this study, five minutes of high temporal resolution GIS is implemented in order to reconstruct the 3D electron density structure on the 21 August 2017 total solar eclipse and analyze the variations induced by the moon’s shadow. To obtain more information of the ionosphere, from the extend 2200 GPS stations on the continental United States, are added for assimilation. The results show the ionosphere peak height (hmF2) uplift was 30–50 km altitude in latitude 25–40°N, and that the electron density depletion at higher altitudes (400 km) has a more noticeable time delay than at low altitudes (200 km), especially in low-latitude regions.
Ionosphere↗
Improved Chemistry and Attenuation Models for Communication Blackout Simulation During Mars 2020 Entry
As a blunt body enters a planetary atmosphere, a plasma forms in the hypersonic shock layer and attenuates radio communication causing signal blackout for some duration of the entry sequence. In our previous work,1 computational fluid dynamics (CFD) was applied to model the entry flow around the Mars 2020 spacecraft, including ionization and electron density throughout the flow field, and predict ultra-high frequency (UHF) radio wave attenuation due to electrons. In total, 17 chemical species and their spatial profiles are modelled around the Mars 2020 spacecraft at 11 different points in time during entry. Although the simulation predicted the onset of attenuation well, the timing of the end of the predicted blackout window significantly preceded the end time observed during the 2021 landing. The present work seeks to improve the attenuation model by accounting for the fact that electrons undergo collisions with heavier species in the flow, which is an effect that was neglected in previous analyses. It is determined that including electron collisions increases the overall magnitude of attenuation predicted especially towards the end of the measured attenuation period, improving qualitative agreement between predicted and measured attenuation to both spacecraft receiving the signal from Mars 2020. To explore the remaining uncertainty in signal attenuation predictions further, a sensitivity study is performed to investigate the impact of associative ionization and electron-impact ionization rate coefficients on the electron density predicted by CFD and on the resulting attenuation predictions. These coefficients are believed to contain up to order-of-magnitude uncertainty, and therefore may significantly affect the number density of electrons throughout the flow field. Variations in associative ionization coefficients demonstrate significant impact on the magnitude of attenuation due to variation in the electron density coming from associative ionization. However, the start and end times of the predicted signal attenuation period are only slightly impacted by said variation.
Eve Papajak↗
Improved Chemistry and Attenuation Models for Communication Blackout Simulation During Mars 2020 Entry
As a blunt body enters a planetary atmosphere, a plasma forms in the hypersonic shock layer and attenuates radio communication causing signal blackout for some duration of the entry sequence. In our previous work,1 computational fluid dynamics (CFD) was applied to model the entry flow around the Mars 2020 spacecraft, including ionization and electron density throughout the flow field, and predict ultra-high frequency (UHF) radio wave attenuation due to electrons. In total, 17 chemical species and their spatial profiles are modelled around the Mars 2020 spacecraft at 11 different points in time during entry. Although the simulation predicted the onset of attenuation well, the timing of the end of the predicted blackout window significantly preceded the end time observed during the 2021 landing. The present work seeks to improve the attenuation model by accounting for the fact that electrons undergo collisions with heavier species in the flow, which is an effect that was neglected in previous analyses. It is determined that including electron collisions increases the overall magnitude of attenuation predicted especially towards the end of the measured attenuation period, improving qualitative agreement between predicted and measured attenuation to both spacecraft receiving the signal from Mars 2020. To explore the remaining uncertainty in signal attenuation predictions further, a sensitivity study is performed to investigate the impact of associative ionization and electron-impact ionization rate coefficients on the electron density predicted by CFD and on the resulting attenuation predictions. These coefficients are believed to contain up to order-of-magnitude uncertainty, and therefore may significantly affect the number density of electrons throughout the flow field. Variations in associative ionization coefficients demonstrate significant impact on the magnitude of attenuation due to variation in the electron density coming from associative ionization. However, the start and end times of the predicted signal attenuation period are only slightly impacted by said variation.
Eve Papajak↗
Constraints on continuous beam models of the jet in 3C 273
The one-sidedness, 'superluminal' motion, and detailed spectra observed for 3C 273 put significant constraints on continuous beam models for the source of the extended emission in 3C 273 A. The one-sidedness of the extended emission implies that the age of the radio-frequency cloud is probably about 3-5 Myr. The 'superluminal' motion observed with VLBI constrains the bulk Lorentz factor of the jet to be more than about 10. Finally, the observed X-ray emission from the jet provides a method for deciding the question of beam composition: for a given beam power, an electron-positron beam would produce a much higher (beamed) inverse-Compton X-ray flux off extended-jet optical photons than an electron-proton beam would, thus making the observed weak jet X-ray flux a strong indicator of a proton positive component to the jet. Remaining experimental uncertainties in the case of 3C 273 make it impossible to decide the question of beam composition at present, but the next generation of experiments may make this possible for 3C 273 and other 'superluminal' objects. The above arguments have the advantage of not depending on the details of the mechanism for producing the high-energy radio-cloud electrons.
Roberts, D.↗
Model ionospheres of Jupiter
The principal concepts presently involved in modeling the Jovian ionosphere are reviewed. A model ionosphere is developed on the basis of our present knowledge of atmospheric composition, relevant chemical and ion-molecule reactions, with their associated rate constants. The shortcomings of this model are discussed when it is compared with the electron density profile obtained from the Pioneer 10 radio occultation data. It is demonstrated that the apparent great extent of the observed topside ionosphere may imply a hot thermosphere, as if Jupiter sustained a corona. Some of the layers observed in the electron density profile may be due to sporadic-E like clustering of protons and other ions.
Atreya, S. K.↗
Solar He-3-rich events and nonrelativistic electron events - A new association
In 15 months of observation by the ISEE-e spacecraft, it was found that virtually all solar greater than or approximately equal to 1.3 MeV/nucleon He-3-rich events are associated with impulsive 2 to approximately 100 keV electron events, although many electron events were not accompanied by detectable He-3 increases. Both the He-3 and the electrons exhibit nearly scatter-free propagation in the interplanetary medium, and the times of onset and maximum for the He-3 and electron increases are closely related by velocity dispersion. The electron events and their related type III solar radio bursts provide, for the first time, identification of the flares which produce the He-3-rich events. He-3 appears to be accelerated at the flash phase of solar flares along with nonrelativistic electrons.
Reames, D. V.↗
The X-ray structure and spectrum of NGC 6251
Observations of NGC 6251 in the low-energy X-rays with the Position Sensitive Proportional Counter on board ROSAT are reported. No X-ray emission is seen from the 4.5-arcmin-long radio jet of this galaxy, suggesting that the electron spectrum is cut off for electron energies greater than 10 exp 12 eV, but a bright X-ray source is coincident with the center of the galaxy. The power-law spectral indexes is close to the HF radio spectral index of the VLBI core, supporting a synchro-self-Compton model for this component of the X-ray emission. The infall of material into the nucleus is sufficiently rapid to power the mini-AGN component, if the mass of the central black hole exceeds 50,000 solar masses.
Birkinshaw, M.↗
Probing the Solar Corona with Radio Ranging Measurements
An asymmetry in the radial variation of electron density above the east and west limbs of the Sun was inferred from centimeter wavelength ranging measurements conducted by Voyager 2 during its 1985 solar conjunction. The Voyager 2 ranging measurements are compared with the Mauna Loa Solar Observatory white-light coronagraph measurements of the underlying corona. Corona probing abilities are summarized.
sun solar corona radio ranging coronagraph white-l↗
Optimal Estimation Inversion of Ionospheric Electron Density from GNSS-POD Limb Measurements: Part I-Algorithm and Morphology
GNSS-LEO radio links from Precise Orbital Determination (POD) and Radio Occultation (RO) antennas have been used increasingly in characterizing the global 3D distribution and variability of ionospheric electron density (N e ). In this study, we developed an optimal estimation (OE) method to retrieve N e profiles from the slant total electron content (hTEC) measurements acquired by the GNSS-POD links at negative elevation angles (ε < 0°). Although both OE and onion-peeling (OP) methods use the Abel weighting function in the N e inversion, they are significantly different in terms of performance in the lower ionosphere. The new OE results can overcome the large N e oscillations, sometimes negative values, seen in the OP retrievals in the E-region ionosphere. In the companion paper in this Special Issue, the HmF2 and NmF2 from the OE retrieval are validated against ground-based ionosondes and radar observations, showing generally good agreements in NmF2 from all sites. Nighttime hmF2 measurements tend to agree better than the daytime when the ionosonde heights tend to be slightly lower. The OE algorithm has been applied to all GNSS-POD data acquired from the COSMIC-1 (2006–2019), COSMIC-2 (2019–present), and Spire (2019–present) constellations, showing a consistent ionospheric N e morphology. The unprecedented spatiotemporal sampling of the ionosphere from these constellations now allows a detailed analysis of the frequency–wavenumber spectra for the N e variability at different heights. In the lower ionosphere (~150 km), we found significant spectral power in DE1, DW6, DW4, SW5, and SE4 wave components, in addition to well-known DW1, SW2, and DE3 waves. In the upper ionosphere (~450 km), additional wave components are still present, including DE4, DW4, DW6, SE4, and SW4. The co-existence of eastward- and westward-propagating wave4 components implies the presence of a stationary wave4 (SPW4), as suggested by other earlier studies. Further improvements to the OE method are proposed, including a tomographic inversion technique that leverages the asymmetric sampling about the tangent point associated with GNSS-LEO links.
GNSS constellation↗
Probing the Solar Corona with Radio Ranging Measurements
An asymmetry in the radial variation of electron density above the east and west limbs of the Sun was inferred from centimeter wavelength ranging measurements conducted by Voyager 2 during its 1985 solar conjunction. These older data are compared with white- light coronagraph measurements of the underlying corona collected by the Mark III K-coronameter at the Mauna Loa Solar Observatory.
solar corona Voyager coronameter coronagraph radio↗