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1] Long-range, discrete, radio echo traces observed in the magnetosphere by the Radio Plasma Imager (RPI) on IMAGE have been interpreted as signals guided along geomagnetic field lines. During IMAGE traversals of the plasmapause and near-equatorial plasmasphere, multiple echo traces, attributed to signals reflected successively between conjugate he...
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... their ionospheric counter- part [Dyson and Benson, 1978], echoes reflected succes- sively between the local and conjugate hemispheres can form epsilon signatures. Figure 4a shows epsilons observed by two measurement programs (different frequency ranges and gain settings) at nearly identical locations. In each case, trace 1 results from direct echoes from near regions (path 1). ...
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... Conjugate end-point conditions will vary with iono- spheric conditions and can influence the epsilon appearance. Figure 4b shows a diffuse, but recognizable, epsilon with both the local and conjugate signals appearing coherently scattered or range-spread. Although the epsilons in Figures 4a and 4b were observed in nearly the same magnetic meridian, the nightside ones (22.0 MLT, Figure 4a) appear sharper than the one in the morning sector (10.6 MLT, Figure 4b). ...
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... 4b shows a diffuse, but recognizable, epsilon with both the local and conjugate signals appearing coherently scattered or range-spread. Although the epsilons in Figures 4a and 4b were observed in nearly the same magnetic meridian, the nightside ones (22.0 MLT, Figure 4a) appear sharper than the one in the morning sector (10.6 MLT, Figure 4b). The cause of the differences in the dayside and nightside epsilons is not yet known. ...
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... 4b shows a diffuse, but recognizable, epsilon with both the local and conjugate signals appearing coherently scattered or range-spread. Although the epsilons in Figures 4a and 4b were observed in nearly the same magnetic meridian, the nightside ones (22.0 MLT, Figure 4a) appear sharper than the one in the morning sector (10.6 MLT, Figure 4b). The cause of the differences in the dayside and nightside epsilons is not yet known. ...
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... are widespread in the magnetosphere. While discrete guided echoes can occur inside, near, and outside the plasmaspause, epsilon signatures (Figure 4) tend to occur mainly inside the plasmasphere. Conjugate echoes are rarely seen far outside the plasmapause (Figure 2) as the trough tends to be ''smoother'' by comparison [Carpenter et al., 2002]. ...
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... Figure 4 shows the opposite case where conjugate signals appear weaker than the local-hemisphere signals, suggesting wave energy dissipation over the long conjugate path [Fung et al., 2000]. Figure 4b also indicates the presence of conjugate end point irregularities resulting in the fuzzy epsilon. ...
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... Figure 4 shows the opposite case where conjugate signals appear weaker than the local-hemisphere signals, suggesting wave energy dissipation over the long conjugate path [Fung et al., 2000]. Figure 4b also indicates the presence of conjugate end point irregularities resulting in the fuzzy epsilon. ...
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Citations
... A network of such pairs means multiple LOS through the plasmaspheric and ionospheric edensity to enable tomographic reconstruction of density to complement EUV, and as a standalone capability. [Reinisch et al 2000], the notional instrument scans within 10 kHz -1 MHz, and 3-axis dipole antennas perform active sounding (transmitting radio pulses, observing echoes) of free-space and Z/whistler modes to get rapid field-aligned electron density [Fung et al 2003[Fung et al , 2008Sonwalker et al 2014, 2011a, b, Reinisch et al 2004. Because RPS needs dipole antennas on a spinning SC, it is probably best accommodated on an in situ observatory (Table 2.1). ...
... Measurements of ducted whistlers around L = 3-5 provided estimates of FAIs with sizes of 68-1,260 km and background density variations of 6%-40% (Sonwalkar, 2006). The radio plasma imager on the IMAGE satellite detected discrete echoes caused by FAI ducting (Fung et al., 2003) and spreaded echoes owing to scattering by density irregularities (Carpenter et al., 2002), with the irregularities having cross-field sizes of ∼200 m to 10 km and background density variations of approximately 10% (Carpenter et al., 2002). Disturbances in the total electron content due to density irregularities were captured by radio antenna arrays through satellite beacons (Helmboldt, 2020;Jacobson et al., 1996) and celestial sources Loi et al., 2015). ...
Plain Language Summary
We performed a statistical study on small‐scale (≤1,000 km) density fluctuations in near‐Earth space using 7 years of density data from two satellites (Van Allen Probes) near the magnetic equator. We focus on fluctuations that are responsible for guiding whistler‐mode wave propagations and therefore have sizes within the order of 1,000 km across field lines. Density fluctuations were investigated in the spatial distributions of the magnetic local times and distances toward the Earth, as well as their relations with geomagnetic conditions. In the dense plasmasphere, we found that density fluctuations are primarily distributed in the night and dawn sectors. These fluctuations occur deep inside the plasmasphere during quiet geomagnetic conditions and show no significant changes during active times. Outside the plasmasphere and further away from the Earth, density fluctuations occur in the post‐midnight sector during quiet times and extend to all nighttime sectors during active times. The density fluctuation levels at the outer boundary of the plasmasphere are much higher than those at the other locations. We discuss connections of density fluctuations deep inside plasmasphere to ionospheric electric field disturbances and those in and near the outer boundary of the plasmasphere to interchange instabilities.
... For a lower limit, with a reduced frequency resolution or spatial resolution along the satellite track, a nominal 0.5 Mb/s data rate in the subsurface sounding mode can be used. For magnetospheric sounding, the strongest echoes propagate along the field [24,31,32]. The choice of the adjustable pulse width in the 1 µs to 3.2 ms range depends on the scale size of the sampled region, i.e. 500 km resolution is sufficient for sounding along Jovian magnetospheric field lines, while kilometer resolution is needed for moon magnetic field regions, e.g., for Ganymede's magnetosphere. ...
... If the signal power decreases with 1/r 2 (specular reflection), a transmitter of 30~120 kW would be required. Since the terrestrial observations have revealed field-aligned propagation [24,31,32], where the echo power drops more slowly with distance than 1/r 2 , a transmitter power of 30 W to 10 kW should be adequate for Jovian magnetospheric duct sounding. ...
... Numerical studies suggest that tubular structures can also be effective waveguides [Smith, 1961;Platt and Dyson, 1989]. The existence of tubular structures with inferred widths of 10-100 km has been established through satellite in situ measurements [Angerami, 1970;Sonwalkar et al., 1994;Décréau et al., 2005], satellite remote sensing [Fung et al., 2003;Darrouzet et al., 2009;Woodroffe et al., 2013], ground-based whistler spectrograms [Hayakawa and Tanaka, 1978;Singh et al., 1998;Altaf and Ahmad , 2013], and ground-based radio interferometric arrays [Jacobson and Erickson, 1993;Hoogeveen and Jacobson, 1997a;Helmboldt and Intema, 2012]. Recent interferometric observations support the idea that tubular FAIs may in some instances be confined to a narrow sheet-like layer on a single magnetic shell [Loi et al., 2015a, b]. ...
Electron density irregularities in the ionosphere are known to be magnetically anisotropic, preferentially elongated along the lines of force. While many studies of their morphology have been undertaken by topside sounding and whistler measurements, it is only recently that detailed regional-scale reconstructions have become possible, enabled by the advent of widefield radio telescopes. Here we present a new approach for visualising and studying field-aligned irregularities (FAIs), which involves transforming interferometric measurements of TEC gradients onto a magnetic shell tangent plane. This removes the perspective distortion associated with the oblique viewing angle of the irregularities from the ground, facilitating the decomposition of dynamics along and across magnetic field lines. We apply this transformation to the dataset of Loi et al. [2015a], obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent FAIs. We study these FAIs in the new reference frame, quantifying field-aligned and field-transverse behaviour, examining time and altitude dependencies, and extending the analysis to FAIs on sub-array scales. We show that the inclination of the plane can be derived solely from the data, and verify that the best-fit value is consistent with the known magnetic inclination. The ability of the model to concentrate the fluctuations along a single spatial direction may find practical application to future calibration strategies for widefield interferometry, by providing a compact representation of FAI-induced distortions.
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Electron density irregularities in the ionosphere are known to be magnetically anisotropic, preferentially elongated along the lines of force. While many studies of their morphology have been undertaken by topside sounding and whistler measurements, it is only recently that detailed regional-scale reconstructions have become possible, enabled by the advent of widefield radio telescopes. Here we present a new approach for visualising and studying field-aligned irregularities (FAIs), which involves transforming interferometric measurements of TEC gradients onto a magnetic shell tangent plane. This removes the perspective distortion associated with the oblique viewing angle of the irregularities from the ground, facilitating the decomposition of dynamics along and across magnetic field lines. We apply this transformation to the dataset of Loi et al. [2015a], obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent FAIs. We study these FAIs in the new reference frame, quantifying field-aligned and field-transverse behaviour, examining time and altitude dependencies, and extending the analysis to FAIs on sub-array scales. We show that the inclination of the plane can be derived solely from the data, and verify that the best-fit value is consistent with the known magnetic inclination. The ability of the model to concentrate the fluctuations along a single spatial direction may find practical application to future calibration strategies for widefield interferometry, by providing a compact representation of FAI-induced distortions.
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Key wordsionosphere-plasmasphere-electron density reconstruction-ionospheric scale height-total electron content
... Radar echoes from remote plasma structures appear as traces on plasmagrams (dark lines observed above 250 kHz in the left panel of Fig. 17). Discrete thin traces correspond to field-aligned propagation (FAP) of signals that are guided with little attenuation along a field line (Reinisch et al. 2001; Fung et al. 2003). Plasmagram traces are intermixed with vertical signatures corresponding to the locally excited plasma resonances (e.g., intensification near 200 kHz in the left panel of Fig. 17) and various natural emissions propagating in space. ...
Ground-based instruments and a number of space missions have contributed to our knowledge of the plasmasphere since its discovery half a century ago, but it is fair to say that many questions have remained unanswered. Recently, NASA's IMAGE and ESA's CLUSTER probes have introduced new observational concepts, thereby providing a non-local view of the plasmasphere. IMAGE carried an extreme ultraviolet imager producing global pictures of the plasmasphere. Its instrumentation also included a radio sounder for remotely sensing the spacecraft environment. The CLUSTER mission provides observations at four nearby points as the four-spacecraft configuration crosses the outer plasmasphere on every perigee pass, thereby giving an idea of field and plasma gradients and of electric cur-rent density. This paper starts with a historical overview of classical single-spacecraft data interpretation, discusses the non-local nature of the IMAGE and CLUSTER measurements, and emphasizes the importance of the new data interpretation tools that have been developed to extract non-local information from these observations. The paper reviews these innova-tive techniques and highlights some of them to give an idea of the flavor of these methods. 8 J. De Keyser et al. In doing so, it is shown how the non-local perspective opens new avenues for plasmaspheric research.
... Multifrequency echoes can form distinct traces in a plasmagram. It has been found that when discrete traces are observed they correspond to reflected signals that propagate along the magnetic field line threading the satelliteFung et al., 2003;Fung and Green, 2005]. Applying a new inversion algorithm, we can derive the N e distribution along a field line almost instantaneously (in 1 min). ...
Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron number density distributions along magnetic field lines in the polar cap magnetosphere during an intense magnetic storm. It is shown that electron densities along magnetic field lines in the polar cap magnetosphere were greatly enhanced on both the dayside and nightside during the storm, compared to the electron density profiles measured during periods of lower geomagnetic activities. The electron density enhancements were observed extending to 7 Earth radii (R E) in altitude on the dayside, with the electron density value reaching about 10 cm À3 at 7 R E altitude. The observed density enhancements were likely due to the enhanced cleft ion fountain during the storm although some of nightside density enhancements might be caused by the increased ion outflows locally in the polar cap. The strongest electron density enhancements observed on the dayside are possibly further associated with storm-time transport of plasma from the midlatitude ionosphere and plasmasphere to high latitudes, which manifests as a plasma plume intruding to dayside high latitudes as seen from total electron content (TEC) maps. With an enhanced source population supplied by the plasma plume, acceleration and heating processes in the dayside cusp/auroral region may produce a large flux of outflowing plasma along magnetic field lines while the outflowing plasma is convected anti-sunward toward the polar cap. These processes lead to strongly enhanced cleft ion fountain and thus greatly raised electron densities at magnetospheric altitudes in the polar cap. The present study captures an event of a massive redistribution of the magnetospheric and ionospheric plasma during a geomagnetic storm caused by extreme solar wind/interplanetary magnetic field (IMF) conditions.
... Multifrequency echoes can form distinct traces in a plasmagram. It has been found that when discrete traces are observed they correspond to reflected signals that propagate along the magnetic field line threading the satellite Fung et al., 2003; Fung and Green, 2005]. Applying a new inversion algorithm , we can derive the N e distribution along a field line almost instantaneously (in 1 min). ...
Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron number density distributions along magnetic field lines in the polar cap magnetosphere during an intense magnetic storm. It is shown that electron
densities along magnetic field lines in the polar cap magnetosphere were greatly enhanced on both the dayside and nightside during the storm, compared to the electron density profiles measured during periods of lower geomagnetic activities. The electron density enhancements were observed extending to 7 Earth radii (RE) in altitude on the dayside, with the electron density value reaching about 10 cm�3 at 7 RE altitude. The observed density enhancements were likely due to the enhanced cleft ion fountain during the storm although some of nightside density enhancements might be caused by the increased ion outflows locally in the polar cap. The strongest electron density enhancements observed on the dayside are possibly further associated with storm-time transport of plasma from the midlatitude ionosphere and plasmasphere to high latitudes, which manifests as a plasma plume intruding to dayside high latitudes as seen from total electron content (TEC) maps. With an enhanced source population supplied by the plasma plume, acceleration and heating processes in the dayside cusp/auroral region may produce a large flux of outflowing plasma along magnetic field lines while the outflowing plasma is convected anti-sunward toward the polar cap. These processes lead to strongly enhanced cleft ion fountain and thus greatly raised electron densities at magnetospheric altitudes in the polar cap. The present study captures an event of a massive redistribution of the magnetospheric and ionospheric plasma during a geomagnetic storm caused by extreme solar wind/interplanetary magnetic field (IMF) conditions.
... However, recent IMAGE satellite work shows that ducts are more prevalent than previously realized, whether they are involved in whistler-mode or free-space X-and O-mode propagation. The RPI radio sounder has shown that when a pulse is transmitted at high altitudes within or near the plasmasphere, any discrete echoes received are predominantly the result of guided X-and O-mode propagation along geomagneticfield-aligned paths to reflection points (e.g., Fung et al., 2003). This finding is consistent with and is also an extension of ducting observations from the ISIS-series topside sounders (e.g., Loftus et al., 1966; Muldrew and Hagg, 1969). ...
Chung Park (1938–2003) was a true pioneer of magnetosphere–ionosphere coupling research. During a short career at Stanford University that began in 1970 and ended in 1981, he wrote seminal papers on several topics. Using ground-based whistler data, he was the first to demonstrate experimentally that day-side upward ion flow from the mid-latitude ionosphere was sufficient to maintain the night-time ionosphere. He made the only measurements to date of longitudinally localized drainage of significant quantities of plasmaspheric plasma into the underlying ionosphere during a period of enhanced convection activity. He pioneered in demonstrating the presence at ionospheric heights of geophysically important electric fields that originate in the troposphere in thunderstorm centers. He cooperated in a unique study of the guidance of whistler-mode waves by field-aligned density irregularities (ducts) in the magnetosphere. Park provided unique observational data on nonlinear wave–particle interaction processes such as: (i) the development of sidebands during the injection of whistler-mode waves from Siple, Antarctica, and (ii) the mysterious whistler precursor phenomenon. Today, in spite of the several decades that have elapsed since his work, Park's early findings remain cornerstones of our understanding of magnetosphere–ionosphere coupling processes. Some of his later studies of non-linear magnetospheric wave–particle interaction phenomena have stirred lively debate, and today remain relevant to a number of topics in space plasma wave research.
