Ion Density Enhancement Research Articles

Localized Enhancements of Fe+10Density in the Corona as Observed in Fexi789.2 nm during the 2006 March 29 Total Solar Eclipse

The first ever image of the full solar corona in the Fe XI 789.2 nm spectral line was acquired during the total solar eclipse of 2006 March 29. Several striking features stand out in the processed image: (1) The emission extended out to at least 3 R☉ in streamers. (2) A bubble-like structure, occupying a cone of about 45° and reaching out to 1 R☉ above the limb, was observed southward of a bright active region complex close to the limb. (3) Localized intensity enhancements were found in different parts of the corona at heights ranging from 1.2 to 1.5 R☉. (4) Striations extended out to the edge of the field of view above an almost north-south-oriented prominence. Comparison with the corresponding white-light image taken simultaneously during the eclipse showed no evidence for these localized enhancements, and the bubble-like structure and striations, while present, did not stand out in the same manner. The extent of the Fe XI emission is attributed to the dominance of radiative over collisional excitation in the formation of that spectral line. The localized intensity enhancements, observed only in Fe XI and not in white light, are a signature of localized increases in Fe+10 density relative to electron density. These are the first observations to show direct evidence of localized heavy ion density enhancements in the extended corona. They point to the importance of implementing observations of the Fe XI 789.2 nm line with existing or future coronagraphs for the exploration of the physical processes controlling the behavior of heavy ions in different source regions of the solar wind.

Satellite observations of electric fields in the South Atlantic anomaly region during the July 2000 magnetic storm

During the great magnetic storm of 15 July 2000 the ROCSAT‐1 low Earth‐orbiting satellite detected an unusually large region of density depletion at low latitudes (<35°) in the ionosphere in corotation with the South Atlantic anomaly (SAA). Adjacent to the region of density dropout was a region of density enhancement with irregularities. We report characteristics of convective electric fields deduced from analyzing measurements of ROCSAT‐1 drift meters and a retarding potential analyzer in the region of ion density structures during the event. Specifically, the ROCSAT‐1 satellite observed penetration of large convective electric fields at apex height below 5000 km (L = 1.78) in the low‐latitude nightside ionosphere. The zonal electric field component was eastward, corresponding to outward convection, and the radial electric field component on the magnetic meridian plane was outward, corresponding to westward convection. In general, both components of the observed convective electric fields were reduced and characterized with wave structures or fluctuations with a wavelength of the order of 100 km in the density enhancement region. In the density depletion region, electric fields increased steadily as density decreased. Distinctly, the observed electric field had a peak near the apex height of the magnetic field line connected to the boundary between ion density dropout and enhancement. The magnitude of the peak electric field was largest in the beginning of the storm main phase and decayed with time. We have conducted a modeling calculation to demonstrate that energetic electron precipitation in the SAA region could increase Hall conductivities in the E region. We have theoretically examined electric field patterns for a circular region of enhanced conductivity in the ionosphere in the presence of a background electric field. Theoretically, electric fields are reduced inside the high‐conductance region but are enhanced immediately outside the conductive region with radial decay from the conductive boundary. It is interpreted that the localized electric field signatures observed by ROCSAT‐1 are produced when ionospheric conductivities are enhanced in the density enhancement region and reduced in the density depletion region. On the basis of modeling results we speculate that energetic particle precipitation might occur in the density enhancement region near the SAA during magnetic superstorms. The obtained results suggest that ionospheric density variation in the SAA region during intense magnetic disturbances might play an important role in the low‐latitude ionosphere electrodynamics.

On the cessation of magnetic reconnection

Abstract. Kinetic simulations of collisionless magnetic reconnection are used to study the effect on the reconnection rate of ion density enhancements in the inflow region. The goal of the investigation is to study a candidate mechanism for the slow-down of magnetic reconnection. The calculations involve either proton or oxygen additions in the inflow region, initially located at two distances from the current sheet. Protons are found to be much more tightly coupled into the evolution of the reconnecting system and, therefore, they effect an immediate slowdown of the reconnection process, as soon as the flux tubes they reside on become involved. Oxygen, on the other hand, has, within the limits of the calculations, a much less pronounced effect on the reconnection electric field. The difference is attributed to the lack of tight coupling to the magnetic field of the oxygen populations. Last, a study of proton and oxygen acceleration finds that protons respond primarily to the reconnection electric field, whereas the main oxygen electric field is achieved by Hall-type electric fields at the plasma sheet boundary. Key words. Space plasma physics (magnetic reconnection; numerical simulation studies; numerical simulation studies)

Evolution of perturbation in charge-varying dusty plasmas

The nonstationary problem of the evolution of perturbation and its transformation into nonlinear wave structure in dusty plasmas is considered. For this purpose two one-dimensional models based on a set of fluid equations, Poisson’s equation, and a charging equation for dust are developed. The first (simplified) model corresponds to the case [Popel et al., Phys. Plasmas 3, 4313 (1996)] when exact steady-state shock wave solutions can exist. This simplified model includes variable-charged dust grains, Boltzmann electrons, and inertial ions. The second (ionization source) model takes into account the variation of the ion density and the ion momentum dissipation due to dust particle charging as well as the source of plasma particles due to ionization process. The computational method for solving the set of equations which describe the evolution in time of a nonlinear structure in a charge-varying dusty plasma is developed. The case of the evolution of an intensive initial nonmoving region with a constant enhanced ion density is investigated on the basis of these two models. The consideration within the ionization source model is performed for the data of the laboratory experiment [Luo et al., Phys. Plasmas 6, 3455 (1999)]. It is shown that the ionization source model allows one to obtain shock structures as a result of evolution of an initial perturbation and to explain the experimental value of the width of the shock wave front. Comparison of the numerical data obtained on the basis of the ionization source model and the simplified model shows that the main characteristic features of the shock structure are the same for both models. Nevertheless, the ionization source model is much more sensitive to the form of the initial perturbation than the simplified model. The solution of the problem of the evolution of perturbation and its transformation into shock wave in charge-varying dusty plasmas opens up possibilities for description of the real phenomena like supernova explosions as well as of the laboratory and active space and geophysical experiments.

Observations in the polar middle atmosphere by rocket-borne Rayleigh lidar: First results

We present a new rocket instrument which measures total atmospheric density with great precision and resolution by Rayleigh scattering of infrared light. Comparison with a ground-based lidar shows: (a) both instruments measure the same physical parameters, even though they have different integration times and volumes. (b) The observed density structures change little over the course of an hour and the horizontal distance of 14 km. The rocket instrument has a basic vertical resolution of approximately 8 m and a number density precision of 4.2 · 1019 m−3. Above 56 km we integrate over an increasing vertical range, reaching 80 m at 70 km. The measured number density profile shows remarkable alternations between very stable layering and metastable layering (adiabatic lapse rate) in the atmosphere between 52 km and 71 km. Comparison with the hodograph of the horizontal wind profile measured by a falling sphere 29 minutes later shows that the metastable height regions coincide with height regions where the hodograph deviates from an ideal spiral. The observation is tentatively interpreted as a gravity wave that is saturating (or encountering a critical level) in these height regions. The comparison of the fine-scale neutral number density observations with measurements of ion density by electrostatic skin probes on board the same vehicle shows a number of ion density enhancements in the stably-layered height regions. With one exception out of six cases, these enhancements occur where the vertical gradient of the meridional wind would collect positive ions as in sporadic E layers. This may be the first observation of such ion density enhancements in the height region 55 to 70 km.

Magnetohydrodynamic processes in the inner coma of comet Halley

A two‐fluid, two‐dimensional, magnetohydrodynamical (MHD) model is used to study the dynamics of the inner coma of comet Halley. Ion production, electron‐ion recombination, charge transfer collisions, and ion‐neutral friction are included. The model adopts a cylindrical grid with nonuniform radial spacing extending from a cometocentric distance of 1000 km out to 107 km. A magnetic barrier and a diamagnetic cavity both appear in the model results, and the plasma flow is directed around the cavity. Electron temperature profiles have recently been suggested for the inner coma in which large temperature enhancements (Te ≈ 25,000 K) are present at a radial distance of about 10,000 km and have been used to explain, via reduced recombination, the ion density enhancement observed by the Giotto ion and neutral mass spectrometers at this location. Using our MHD model as well as analytical methods we find that large electron temperature enhancements in the inner coma result in large reductions in the magnetic field strength which were not observed by the Giotto magnetometer. We suggest that perhaps the electron temperature is only enhanced to a value of about 10,000 K rather than to 25,000 K at the ion pileup.

Adaptive identification and characterization of polar ionization patches

Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and characterize polar cap “ionization patches”, loosely defined as large‐scale (>100 km) regions where the F region plasma density is significantly enhanced (≳ 100%) above the background level. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the identification and characterization of these structures, as a function of scale‐size and density enhancement, using data from the retarding potential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The results indicate that in the altitude region from 240 to 950 km ion density enhancements greater than a factor of 3 above the background level are relatively rare. Further, the ionization patches show a preferred horizontal scale size of 300–400 km. There exists a clear seasonal and universal time dependence to the occurrence frequency of patches with a northern hemisphere maximum centered on the winter solstice and the 1200–2000 UT interval.

Modeling of O+ ions in the plasmasphere

Heavy ion (O+, O++, and N+) density enhancements in the outer plasmasphere have been observed using the retarding ion mass spectrometer instrument on the DE 1 satellite. These are seen at L shells from 2 to 5, with most occurrences in the L = 3 to 4 region; the maximum L shell at which these enhancements occur varies inversely with Dst. It is also known that enhancements of O+ and O++ overlie ionospheric electron temperature peaks. It is thought that these enhancements are related to heating of plasmaspheric particles through interactions with ring current ions. This was investigated using a time‐dependent one‐stream hydrodynamic model for plasmaspheric flows, in which the model flux tube is connected to the ionosphere. The model simultaneously solves the coupled continuity, momentum, and energy equations of a two‐ion (H+ and O+) quasi‐neutral, currentless plasma. This model is fully interhemispheric and diffusive equilibrium is not assumed; it includes a corotating tilted dipole magnetic field and neutral winds. First, diurnally reproducible results were found assuming only photoelectron heating of thermal electrons. For this case the modeled equatorial O+ density was below 1 cm−3 throughout the day. The O+ results also show significant diurnal variability, with standing shocks developing when production stops and O+ flows downward under the influence of gravity. Numerical tests were done with different levels of electron heating in the plasmasphere; these show that the equatorial O+ density is highly dependent on the assumed electron heating rates. Over the range of integrated plasmaspheric electron heating (along the flux tube) from 8.7 to 280×109 eV/s, the equatorial O+ density goes like the heating raised to the power 2.3.

One‐dimensional hybrid simulations of the diamagnetic cavity boundary region of comet Halley

A one‐dimensional hybrid technique (particle ions and fluid electrons) is used to study the cometary diamagnetic cavity surface (CS). This hybrid study is unique in that it includes the effects of binary ion‐ion Coulomb collisions, an important process in the dense inner coma. The equilibrium location of the CS is maintained by a force balance mainly between the ion‐neutral drag force and the magnetic pressure gradient force. However, the detailed structure of the CS layer also depends on properties of the plasma such as the thermal pressure. Significant variations of the ion density, ion flow speed, and magnetic field strength take place across the CS boundary layer. Our hybrid code description of the CS structure compares favorably with the data from experiments onboard the Giotto spacecraft. When compared to the magnetohydrodynamical (fluid) results of Cravens (1989), there is good agreement on the "core" width of the plasma density enhancement and on the width of the current layer associated with the magnetic field gradient, but a large discrepancy exists in the width of the ion flow speed transition because of the failure of the fluid model to discern particle effects. Related to this, the hybrid code ion density enhancement is not symmetric as a result of a magnetically reflected, backstreaming ion population within the cavity. The core width of this enhancement (Δn) is highly dependent upon the dissociative recombination rate coefficient, and the hybrid results agree to within 20 percent with the fluid model results of Cravens (1989). The width of the velocity transition (Δν), or the "tail" of the density enhancement, is determined by the collision time for the backstreaming ions. The effect of Coulomb collisions is to decrease (Δν) by a factor of 2. The magnetic field transition has a width (ΔB) that is of the order of a few ion gyroradii. Disrupting the ion gyration by including the effects of binary ion‐ion Coulomb collisions alters the role of the ions as the dominant current carriers within the current layer and slightly reduces ΔB.

Effects of time‐dependent photoionization on interstellar pickup ions

We present an analytical model for the density variations of interstellar pickup ions in the solar wind due to a time‐dependent variation in the photoionization rate of the interstellar atoms. Following a sudden increase in the photoionization rate, our model predicts a pickup ion density enhancement lasting for a time of the order of the duration of the increase plus the solar wind convection time to the observation point. If the photoionization rate returns to its initial value, this enhancement is followed by a decreased pickup ion density resulting from a depleted interstellar neutral particle density. In the absence of further variations in the photoionization rate, the pickup ion density recovers on a time which scales as the radial position of the observation point divided by the inflow speed of the neutral particles. Gradual variations in the photoionization rate result in a pickup ion density which tends to track the ionization rate, though the density variations are smoothed and delayed in time due to the solar wind convection of ions picked up at points closer to the Sun.

The study of a 2.45 GHz plasma source as a plasma generator for the SCECR electron cyclotron resonance ion source

The 2.45 GHz plasma source is a multicusp electron cyclotron resonance source with a tuned single mode microwave cavity. This is a bright plasma source with a well characterized operating mode and plasma parameters [A. K. Srivastava, et al., Rev. Sci. Instrum. 63, 2556 (1992); A. K. Srivastava and J. Asmussen, J. Vac. Sci. Technol. A 11, 1307 (1993)]. The superconducting electron cyclotron resonance (SCECR) is a fully superconducting, hexapole stabilized tandem mirror ion source with a multimode microwave cavity. It is a high charge state ion source with extensive beam diagnostics [T. A. Antaya, Nucl. Instrum. Methods B 40/41, 1024 (1989)]. The coupling of these two sources permits a number of fundamental studies important to both classes of ion sources. The 2.4 GHz plasma source will be used to study the effects of ion or electron (or both) density enhancement on the charge state distribution, total extracted current, and ion energy distributions of the SCECR. The SCECR in turn will be used as a test stand to measure directly the ion distribution and beam characteristics of the 2.45 GHz plasma source. The first results with this hybrid ion source will be presented at this conference.

Electric fields and cold electrons in the vicinity of comet Halley

The structure and dynamics of the plasma environment on the sunward side of comet Halley are investigated using measurements during the Vega inbound encounters from a double probe antenna and two Langmuir probes (the APV‐V experiment), within a cometocentric distance of ∼850,000 km. Based upon the behavior of the electric fields and the cold electrons, three regions can be identified in the cometosheath (in particular during the Vega 1 approach): transition layers are passed through at ∼780,000 km (R1) and ∼360,000 km (R2). The outer cometosheath (near and beyond R1) is characterized by large‐scale variations in the cold electron density and the electric field, peaking at ∼1 mHz. The R2 crossing is detected in the plasma wave data as enhanced fluctuations at ∼15 mHz. About 25,000 km downstream of R2, the spacecraft traverses a current layer (thickness ∼10,000 km) indicated by a sharp gradient in the dc electric field and the cold electron density. The continuous, relatively rapid increase in the intensity of plasma waves begins at a cometocentric distance of ∼200,000 km. Simultaneously, the cold electron density starts growing in accordance with the r−2 law. The cometopause crossing at ∼160,000 km produces only moderate effects in the APV‐V measurements. In the cometary plasma region, some interesting features are examined, such as a current layer passed by Vega 1 at 54,000 km and sudden ion density enhancements observed by Vega 2 at ∼35,000 km. On several occasions, the APV‐V measurements are heavily influenced by dust particles striking the spacecraft structure. From these events we can conclude that the dust particles are strongly charged and partially coupled to cometary plasmas (or vice versa).

Asymmetric ion ray enhancement of Comet P/Halley by a southward jet

An asymmetric ion ray of Comet P/Halley was found on December 12 of 1985 when a strong outburst with a southward jet was observed in the near-nucleus region. There were two principal ion structures: a strong southward ray, which is the main focus of this paper, and a northern structure, which is either the main tail or a folding tail ray asymmetrically paired with the southern ray. If we regard the northern ion structure as the main tail, the ion density in the southward ray reached eight times as much as that of the northward ion ray at the distance of 7–8 × 10 5 km from the nucleus. On the other hand, if we consider the northern structure as the folding ray, the ion density enhancement in the southward ray is less than 10%. In this case, the enhancement is limited to ∼ 4 × 10 5 km from the nucleus. The ion increase in the southward ray in either case may be a first observational example that the inhomogeneous mass ejection-like jet phenomena directly causes the asymmetric enhancement of the ion ray.

Pickup ions in the unshocked solar wind at comet Halley

The Tünde‐M experiment on board the VEGA 1 spacecraft detected energetic cometary ions as far as 10 million kilometers from comet Halley. The measured ion fluxes increased with decreasing radial distance from the nucleus, in agreement with theoretical expectations. Sharp enhancements spaced at approximately 4‐hour intervals were superimposed on the general increase of the ion fluxes on the inbound leg of VEGA 1. Periodic neutral structures originating from gas production by active areas on the nucleus are suggested to explain the recurrent intensity peaks. These neutral density enhancements would then produce the ion density enhancements seen by Tünde‐M as the spacecraft traversed the structures. This explanation requires a considerably larger neutral expansion velocity (≈6 km/s) than was actually measured near the nucleus in order to allow the rotation period of the nucleus to generate the observed period of the enhancements. No obvious correlations of the ion fluxes with the limiting energy for pickup were found. However, at a distance of about 107 km from the comet, a weak anticorrelation was found between the ion flux and the angle between the interplanetary magnetic field and solar wind direction. Cometary ion distribution functions in the solar wind reference frame were derived from measured counting rates. These distributions are approximately Maxwellian for energies between about 100 keV and 150 keV. The derived temperatures are between about 5 and 8 keV for most cometocentric distances, although in a region between 10 and 8 million km from the nucleus the ion spectra were harder, with temperatures ranging from 10 to 20 keV.

Heavy ion density enhancements in the outer plasmasphere

A region of density enhancements of thermal heavy ions (O+, O++, and N+) has been observed on numerous occasions by the retarding ion mass spectrometer on the Dynamics Explorer 1 satellite. Outer plasmasphere densities of heavy ions are often observed to be up to 2 orders of magnitude higher than equatorward densities within an orbital pass. This phenomenon is almost always observed in the region of the plasmasphere just inside the plasmapause and has been seen at all local times. A statistical study of these heavy ion density enhancements, covering almost 600 passes of DE 1 through the plasmasphere, shows that O+ and O++ enhancements occur over about 64% of the observed passes, with the highest frequency of occurrence being found in the late evening and morning regions. O+ enhancements tend to be seen more frequently in the morning, while O++ enhancements are more likely to be observed in the evening. O+ appears to have a higher outer plasmasphere density enhancement than O++ in all local time regions. Evidence showing that the L shell of the heavy ion density enhancement peaks is dependent upon Dst is presented. Finally, mechanisms for the creation of the heavy ion density enhancements are briefly discussed, including the “geomagnetic mass spectrometer” and plasmapause‐associated electron heating of the topside ionosphere.

Height‐integrated Pedersen and Hall conductivity patterns inferred from the TIROS‐NOAA satellite data

The series of polar‐orbiting National Oceanic and Atmospheric Administration spacecraft TIROS, NOAA 6, and NOAA 7 have been monitoring the particle influx into the atmosphere since late 1978. This data base has been used to construct statistical global patterns of height‐integrated Pedersen and Hall conductivities for a discrete set of auroral activity ranges. The observations of energy influx and “characteristic electron energy” have been binned in a 1° latitude and 2° magnetic local time grid and ordered by an auroral activity index. This index is an estimate of the energy deposited into a single hemisphere by incident particles, a parameter generated directly from the particle observations and, therefore, internally consistent with the statistical patterns that are constructed. An average electron spectrum is associated with each characteristic energy, which enables a height profile of ionization rate in the upper atmosphere to be determined. The use of a pressure coordinate system insures that the normalized ionization rate profiles are independent of atmospheric model parameters. To create the statistical pattern of height‐integrated conductivities, however, vertical profiles of atmospheric temperature and composition are assumed, and the ion density enhancements are evaluated from a chemical balance between ion production and recombination based on an “effective” recombination coefficient. The data base can also provide the statistical pattern of particle heating rates and ionization rates over a three‐dimensional grid suitable as input to more sophisticated ionospheric and neutral thermospheric codes.

A two‐dimensional, high‐resolution, nested‐grid model of the thermosphere: 2. Response of the thermosphere to narrow and broad electrodynamic features

Using a two‐dimensional, nested‐grid model of the thermosphere, with high‐latitude resolution, the dynamic, energy, and composition response to a number of electrodynamic features has been considered. The forcing is characterized by a Gaussian latitude distribution of the convective electric field across the dawn or dusk auroral ovals with scale sizes ranging from 1° to 5° latitude full width at half maximum, together with an enhancement of E region ion density characteristic of the diffuse aurora. An extreme case for long‐lived features of the 100 mV m−1 electric field is considered, together with a more realistic magnitude of 50 mV m−1, coexisting with a 3×1011 m−3 ion density enhancement at low altitudes. The dynamic response is shown to be critically dependent on the magnitude of the winds across the source channel, either self‐induced or imposed from external sources. Self‐induced cross winds result from the action of the Coriolis force and, in addition, in the lower thermosphere, by the action of Hall drag. Crosswinds are shown to create plumes of momentum downwind and inhibit the peak neutral wind response that would otherwise exist. The combination of increased self‐induced cross wind and increasing width of the source creates a surprisingly similar peak in the zonal neutral wind response at high altitudes. The crosswinds induced by the Hall drag imply that a greater lower thermosphere zonal neutral wind is created by the 50 mV m−1 electric field compared with the 100 mV m−1 source. During the recovery a lower thermosphere jet is shown to persist for many hours with a slight preference for the smaller‐scale features. A stronger response is predicted in the dusk sector due to an inertial balance that exists, which implies that small‐scale features existing in the dusk sector are more likely to have an observable neutral response. The temperature perturbation is shown to be dependent on a self‐consistent treatment of composition and to increase in magnitude as the width of the source increases.

A wave of ion density enhancement following sheath contraction

The method of characteristics is used to study disturbances generated by the contraction of ion-rich sheaths. It is shown that an enhancement of ion density propagates into the plasma at the local ion acoustic speed. This complements the density rarefactions associated with expanding sheaths.

The Effects of Ions on vlf and elf Propagation in an Abnormally Ionized Atmosphere

The paper deals with the propagation of elf and vlf waves in abnormally ionized lower atmosphere. The absorption of these waves in whistler mode propagation has been calculated for three disturbed conditions (PCA event, solar flares and nuclear explosion) and ambient condition of the lower ionosphere. It is found that due to enhanced ion density, elf waves suffer more absorption than that suffered by vlf waves.

Polar ionosphere: Measured ion density enhancements and soft electron precipitation

The experiment complement aboard a polar satellite having a perigee near 160 km and initial apogee near 1800 km included an ion trap and sensors for low-energy electrons and protons. The data from these two experiments have been correlated for the period August 13–18, 1968, during which time the perigee invariant latitude was ≥70°. Results of these studies for the polar region are the following. (a) The large ion density enhancements observed are correlated with simultaneously observed enhancements in the precipitating fluxes of low-energy electrons. (b) The trough and ionospheric enhancements are shown to occur below as well as above the F region maximum density height. (c) It is concluded that the position of the midlatitude trough minimum should not be expected to show a simple relationship with magnetic indices since the shape depends on (1) the local precipitation (ionization) patterns; (2) the total energy input, its time and spatial extent; (3) diffusion rates of the ionized species; (4) universal time; and (5) altitude of the observation.