Ion Density Depletions Research Articles

Giant ionospheric density hole near the 2022 Hunga-Tonga volcanic eruption: multi-point satellite observations

A giant ionospheric hole was simultaneously detected in the in situ measurements of FORMOSAT-7/COSMIC-2 (F7/C2), Ionospheric Connection Explorer (ICON), Swarm missions, and ground-based total electron content (TEC) by global navigation satellite system receivers, and F7/C2 Global Ionosphere Specification (GIS) data near Tonga, following the explosive volcano eruption on 15 January 2022. The TEC maps displayed the huge depletions that developed near Tonga after the eruption and gradually evolved. The ICON IVM, F7/C2 IVM and Swarm-LP detected large depletions not only near Tonga, but also in the EIA trough region. The GIS observations clearly show the ionospheric hole that extends spatially near Tonga, especially strongly south/southward. The simultaneous observations showed that the ionosphere hole near Tonga combined with the EIA trough and finally evolved into a giant ionosphere hole around 07 UT. The ionospheric hole, which occurred at 05 UT near Tonga, extended over a wide area of 160°-200°E and 25°S-20°N and lasted for about 11 h. The F7/C2 and ICON satellites overpasses showed large ion density depletions by the hole at orbit altitudes, accompanied by enhancements in ion temperature and field-aligned and perpendicular ion drift. Such a long-lasting giant ionospheric hole by a seismic event has not been reported earlier, creating a unique ionospheric environment near Tonga after the eruption. The strong successive impulses by multiple volcano eruptions, together with O/N2 decrease in the summer hemisphere, interhemispheric wind, and water vapor injection into high altitudes apparently yielded such a giant ionospheric hole, 4–6 times larger than that observed during the Tohoku earthquake.Graphical

Spatial and Temporal Confinement of the Ionospheric Responses During the St. Patrick's Day Storm of March 2015

AbstractThe present study provides a multi‐instrument analysis of the ionospheric response to the effects of the St. Patrick's Day storm of 17–18 March 2015. Simultaneous observations from 85 global positioning system stations and multiple satellites along the past observations from the literature survey are utilized to ascertain the spatio‐temporal confinements between −20°E and 150°E longitudes. The main results include longitudinal differences in the episodic equatorward expansions of the auroral oval which is shown to relate with varying high‐latitude Joule heating and equatorward wind surges at different local times. The zonal movement of equatorial ion density depletions from initially east to west followed by a sluggish westward pattern on the night of 17 March is observed. The zonal movement is found to be better explained by the longitudinally decreasing impact of the disturbance dynamo in the nightside. The enhancements in total electron content during morning on 17 March are observed over mid‐latitudes, which are found to move equatorward in the noon time. These enhancements could be associated with the enhanced O density bulge resulting from the thermospheric expansion during the main phase. A daytime sustained and confined electron density depletion between about 100°E and 170°E is observed on 18 March which is being discussed in detail for the first time. Interestingly, large longitudinal differences are observed in the response of the disturbance dynamo, including no observable effect during morning (−20°E–20°E) but dominant afternoon and westward progression during the day of 18 March.

On the distribution of ion density depletion along magnetic field lines as deduced using C/NOFS

To investigate ion density depletion along magnetic field lines, we compare in situ-measured ion density fluctuations as seen from C/NOFS and compare them to the field-line-integrated depletion of the whole bubble as inferred from electric field measurements. Results show that, within C/NOFS' range, local measurement of the normalized density depletion, (Delta)n/n(sub 0), near the apex may be far less than at other points on the same field line. We argue that the distribution of (Delta)n/n(sub 0) is a weighted distribution concentrated at latitudes of the Appleton anomalies and becomes more heavily weighted the closer the field-aligned bubble rises to the peak of the anomalies. A three-dimensional simulation of an ionospheric bubble verifies our arguments.

Occurrence of Equatorial Plasma Bubbles during Intense Magnetic Storms

An important issue in low-latitude ionospheric space weather is how magnetic storms affect the generation of equatorial plasma bubbles. In this study, we present the measurements of the ion density and velocity in the evening equatorial ionosphere by the Defense Meteorological Satellite Program (DMSP) satellites during 22 intense magnetic storms. The DMSP measurements show that deep ion density depletions (plasma bubbles) are generated after the interplanetary magnetic field (IMF) turns southward. The time delay between the IMF southward turning and the first DMSP detection of plasma depletions decreases with the minimum value of the IMFBz, the maximum value of the interplanetary electric field (IEF)Ey, and the magnitude of the Dst index. The results of this study provide strong evidence that penetration electric field associated with southward IMF during the main phase of magnetic storms increases the generation of equatorial plasma bubbles in the evening sector.

The ECOMA 2007 campaign: rocket observations and numerical modelling of aerosol particle charging and plasma depletion in a PMSE/NLC layer

Abstract. The ECOMA series of rocket payloads use a set of aerosol particle, plasma, and optical instruments to study the properties of aerosol particles and their interaction with the ambient plasma environment in the polar mesopause region. In August 2007 the ECOMA-3 payload was launched into a region with Polar Mesosphere Summer Echoes (PMSE) and noctilucent clouds (NLC). An electron depletion was detected in a broad region between 83 and 88 km, coincident with enhanced density of negatively charged aerosol particles. We also find evidence for positive ion depletion in the same region. Charge neutrality requires that a population of positively charged particles smaller than 2 nm and with a density of at least 2×108 m−3 must also have been present in the layer, undetected by the instruments. A numerical model for the charging of aerosol particles and their interaction with the ambient plasma is used to analyse the results, showing that high aerosol particle densities are required in order to explain the observed ion density depletion. The model also shows that a very high photoionisation rate is required for the particles smaller than 2 nm to become positively charged, indicating that these may have a lower work function than pure water ice.

Weak Dust Ion-Acoustic and Dust-Acoustic Solitons With Absorption of Ions, Ionization, and Ion Drag

The novelty of the approach lies in a self-consistent treatment of the ion density depletion due to absorption on dust grains, of dust charge variations, ionization, and the ion-dust drag friction. For dust ion-acoustic solitons, it is shown that the validity of the Korteweg-de Vries (KdV) equation which is usually used is restricted to some range of amplitudes (which does not always exist). For dust-acoustic solitons, an additional dispersion term responsible both for the ion-dust drag friction and the ion density depletion due to absorption of ions is derived. It is found that two types of dust-acoustic solitons are possible, namely, when the new dispersion term is small and when the new dispersion term dominates. For the second type of solitons, the equation is not the KdV equation. Solitary wave solutions are represented in analytical form and discussed. Weak dust-acoustic solitons of both types are found to be compressive with the same range of amplitudes but with different ranges of spatial width. Estimates are made to see the possibility of experimental observation of the calculated soliton structures.

Longitudinal ionospheric effects in the South Atlantic evening sector during solar maximum

Large‐scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10°S–20°S, longitudes 50°W–10°E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450‐km altitude show vertically downward ion drift velocities exceeding 120 m s−1 and depleted regions where the ion density is around 2 × 104 cm−3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE‐E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.

Plasma voids (holes) in a dusty plasma

A theory for plasma voids (large amplitude plasma holes) in a dusty plasma is presented. Specifically, it is proposed that the plasma voids are self-consistent solutions of Poisson's equation in which the electron density response is Boltzmannian and the ion density response is non-Maxwellian due to the ion trapping in large amplitude plasma potentials. Poisson's equation, with appropriate electron and ion number densities, is then reduced to an energy integral which provides criteria for the existence of the plasma voids. The latter are characterized as regions of significant ion and electron density depletions in association with negative plasma potentials. The voids shrink when the dust grains are added into the plasma, and they tend to disappear if the dust number density is sufficiently high.

Storm time plasma irregularities in the pre‐dawn hours observed by the low‐latitude ROCSAT‐1 satellite at 600 km altitude

Large scale ion density depletions were detected in the nighttime sector by ROCSAT‐1 for over 10 hours during the 22 October 1999 geomagnetic storm. Prominent depletion structures (bubbles) that are characterized by large‐amplitude density decrease (N/No ≤ 1%) with rapid horizontal ion drift (600 ∼ 800 m/s) are found to cluster in the 03:00 ∼ 04:30 local time sector and at magnetic latitudes 14° ∼ 20° S when the storm was in its early recovery phase. These presunrise bubbles are positively correlated to the enhanced eastward electric fields of greater than 1 ∼ 2 mV/m, which were in response to the storm‐time disturbances resulting from the in‐phase contributions of the prompt penetration magnetospheric and the long lasting ionospheric disturbance dynamo electric fields. Further analyses of the field‐aligned and cross‐field ion drifts within the depletions reveal that bubble plasma were driven by the eastward polarization electric fields to move upward, but these upward velocities were compensated by large downward field‐aligned diffusive motions. These features confirm that the disturbance electric fields produced during a great magnetic storm can significantly affect the occurrence timing and spatial extent of severe plasma irregularities in low‐latitude ionosphere. The spatial dimensions of the pre‐sunrise irregularities may exceed the large region observed by the 35° inclined circular orbiting ROCSAT‐1.

Further results referring to the neutral density depletions attributed to plasma bubbles

The interaction between the ionized and neutral components of the upper atmosphere (ion drag, air drag) is not limited to the large-scale motions, but it works also among the small-scale motions. This is demonstrated by neutral density depletions (NDD) revealed by us in the neutral density measurements of great time resolution of the San Marco V satellite. The morphological and statistical investigations indicate that NDDs and plasma bubbles have similar characteristics. On the other hand, according to modelling, depletions of the total neutral density discovered by us below a height of about 350 km (collision-dominated case) in the vicinity of the equator might be really due to equatorial plasma bubbles. The relation of the NDDs to plasma bubbles (ion density depletions) is studied partly by direct comparison and modelling, partly indirectly by the distribution of the occurrence of NDDs according to local solar time, to season, to height and to longitude. All of them are arguing in favour of the plasma bubble origin of the NDDs.

Singular plasma disturbances in the low‐latitude F region

We describe here a new phenomenon characterized by unusual patterns of ion drifts inside ion density depletion regions observed by the AE‐E satellite in the low‐latitude F region. In about 30 depletions, vertical ion drift relative to the background was upward on the western sides, downward on the eastern sides, and zero near the middle where the density depletion was greatest. These drift characteristics are distinct from those observed in plasma bubble depletions. The structures reported here were observed on circular orbits below 300 km altitude and had density depletions of up to 2 orders of magnitude or more below the ambient ion density. The upward and downward drift excursions were up to 200 m/s relative to the background. Almost all these structures were observed over oceans or near coasts and largely between ±10° and ±30° dip latitude. The structures were observed mostly as isolated, single depletion regions with the majority of them about 250 km wide in the east–west direction. They occurred during quiet magnetic conditions with near‐equal occurrence frequencies in the premidnight and postmidnight periods. The characteristic density and drift signatures indicate westward propagating disturbances in which the bottomside F layer is first lifted and then returned back to its original position, leaving the ionosphere undisturbed after the disturbance passes by. The estimated speed of these disturbances is of the order of 200 m/s. These unique solitary plasma disturbances, which we designate as singular plasma disturbances, are associated with a propagating source of E×B drift, not driven by neutral perturbations at the altitude of observation.

Morphology of equatorial plasma bubbles

Some occurrence characteristics of plasma bubbles based on data from the ion drift meter aboard the Atmosphere Explorer E satellite are presented. Of 5200 bubbles detected between 250 and 475 km altitude, 65% had ion density depletions of less than 1 order of magnitude below the ambient ion density, and the occurrence frequency decreased with increasing depletion depth. Depletion depth decreased with local time, probably due to diffusive decay of bubbles. Bubble activity above 300 km altitude had its onset around 1900 and peaked around 2130 LT. Late, shallow bubbles appeared to be associated with frequency spread F, and early, deep bubbles with range spread F. Increase in magnetic activity inhibits bubble generation except near sunrise, at which time bubble activity has been found to increase with magnetic disturbance. Bubble activity is restricted largely to a belt within about ±20° dip latitude.

Detection of an equatorial spread-F bubble by ground-based photometers and the San Marco 5 satellite

Abstract In the Fall of 1988, San Marco 5, instrumented with ion density and electric field experiments, made passes through the equatorial F-region in the Brazilian sector, where scanning photometers have been operated at Cachoeira Paulista, Brazil, for many years. We have cross-checked these large data sets and report here on one night of active spread-F with good data from both San Marco 5 and Cachoeira Paulista. As expected, a correlation is observed between airglow bubbles and ion density depletions encountered by San Marco 5. Additional airglow bubbles can be linked to enhanced AC electric field fluctuations, similar to those previously suspected to characterize satellite passes through pinched off spread-F bubbles. These data complement existing airglow-satellite correlations made in other longitude sectors and provide information relevant to the interpretation of satellite and airglow data.

Structure and dynamics of inner cometary plasmas

The inner cometary coma is a weakly ionized plasma, and its structure and dynamics are governed mainly by ionization due to solar radiation and solar wind electrons and losses due to the radiative processes, recombination, and transport. At comet Halley a narrow ion density depletion region was observed by spacecraft as well as ground‐based instruments and has been linked to the dynamics of the plasma and radiation. A model of the cometary plasma consisting of water group ions, bulk electrons, and energetic electrons produced mainly by photoionization is presented. The dominant losses in the inner coma are the radiation from the excitation of rotational and vibrational levels of water molecules and the recombination of the plasma. The electron energy losses due to these processes peak near 4000 K, and at temperatures higher than this value a localized cooling leads to further cooling arising from increased radiation loss and consequently to a thermal instability. The resulting increase in recombination leads to an ion density depletion, and the estimates for this depletion at comet Halley agree with the observations. This instability is sensitive to the plasma conditions and the transport processes, that is, diffusion and thermal conductivity. There is no direct measurement of the electron temperature in this range, and the electron temperature profile from MHD simulations has been used to develop a model of the inner cometary plasma, which yields the localization of the thermal instability and, hence, the observed ion density depletion region. The resulting electron temperature profile is also consistent with that obtained from the temperature dependence of the electron‐ion recombination rate.

Thermal instability and transport effects in comets

The localized ion density depletion region observed at comet Halley can be understood in terms of a thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules. The electron energy losses due to these processes peak near 4000 K and at higher temperatures a localized cooling leads to the thermal instability due to the increased radiation loss. The resulting increase in recombination leads to the ion density depletion and the estimates for this depletion at comet Halley agree with the observations. The transport effects at the density depletion region due to the diffusion and thermal conductivity are found have time scales larger than the instability time scale and does not affect the formation of the depletion region.

Thermal instability in the inner coma of a comet

The spacecraft and ground based observations of comet Halley inner coma showed a localized ion density depletion region whose origin is not well understood. Although it has been linked to a thermal instability associated with negative ions, the photodetachement lifetime of negative ions (∼ 1 sec) is too short compared to the electron attachment time scale (∼ 100 sec) for this process to have a significant effect. A mechanism for the ion density depletion based on the thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules is proposed. The electron energy losses due to these processes peak near 4000 K (0.36 eV) and at temperatures higher than this value a localized cooling leads to further cooling (thermal instability) due to the increased radiation loss. The resulting increase in recombination leads to an ion density depletion and the estimates for this depletion at comet Halley agree with the observations.

Two dimensional collapse of Langmuir waves in electron beam-plasmas

Interactions between an electron beam and argon plasma, in the presence of a strong axial magnetic field, have resulted in the formation of standing Langmuir waves together with spatially localized ion density depletions of up to 40%. On typical ion motion time scales, the standing waves collapse to axial and radial dimensions of approximately 20 lambda D and a few thermal electron Larmor radii respectively. Both accelerated and decelerated components of the electron beam have been observed to couple with the plasma generating additional features in the wave spectrum.

Equatorial plasma bubbles: Vertically elongated wedges from the bottomside F layer

We address the question regarding the two‐dimensional shape of equatorial plasma bubbles in the plane transverse to the geomagnetic field. By comparing the east‐west spatial relationship of ion‐density depletions measured in‐situ by the Atmospheric Explorer E (AE‐E) satellite to backscatter plumes measured by the ALTAIR radar, we show that plasma bubbles are vertically elongated depletions that extend upward from the bottom side of the F layer, in the form of tilted wedges, rather than more isotropically shaped but isolated structures. The shape of plasma bubbles is inferred from (1) ion density depletions that exceeded 99% in the ‘neck’ regions of plumes and (2) the eastward drift velocities of the plumes. The expected electrodynamics of vertically elongated plasma bubbles are consistent with the observations of large eastward drift velocities of plumes that are comparable to F region plasma drift measurements made at Jicamarca and to F region neutral wind measurements made at Kwajalein. The results also reveal that the west wall of large‐scale altitude modulations of the bottomside F layer that produces the primary plumes and bubbles becomes structured, and evolves with the generation of secondary plumes and bubbles.