Plasma Mantle Research Articles

Ionospheric footprint of magnetosheathlike particle precipitation observed by an incoherent scatter radar

We have examined Sondrestrom incoherent scatter radar observations of ionospheric plasma density and temperature distributions and measurements of F region ion drifts that were made during a prenoon pass of the DMSP‐F7 satellite through the radar field of view. The spacecraft traversed a region of intense electron precipitation with a characteristic energy below approximately 200 eV. Particles with such low characteristic energies are believed to be directly or indirectly of magnetosheath origin. The precipitation region had a width of about 2° invariant latitude and covered the low‐latitude boundary layer (LLBL), the cusp, and the equatorward section of the plasma mantle (PM). The corotating radar observed a patch of enhanced electron density and elevated electron temperature in the F2 region between about 10.5 and 12 magnetic local time in the same invariant latitude range where DMSP‐F7 detected the soft‐electron flux. The ion drift pattern, also obtained by radar, shows that it is unlikely that the plasma patch was produced by solar radiation and advected into the radar field of view. We suggest that the radar observed modifications of the ionospheric plasma distribution, which resulted from direct entry of magnetosheath electrons into the magnetosphere and down to ionospheric altitudes. Model calculations of the ionospheric response to the observed electron precipitation support our interpretation. The spectral characteristics of the electron flux in the LLBL, cusp, and equatorward section of the PM were in this case too similar to allow to distinguish between them by using incoherent scatter radar measurements only.

Radiative edges under control by impurity fluxes

Experimental results from TEXTOR are presented to provide strong evidence for the feasibility of the 'cold radiative plasma mantle', a concept which might be a possible solution for the energy exhaust problem in a fusion reactor. The concept is compared with the high density divertor. The compatibility to other constraints, limitations and open problems are discussed, in particular the issues of stationarity (feed-back control, thermal instabilities, q=2), energy confinement, He-exhaust and fuel dilution.

Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8, and Søndrestrøm incoherent scatter radar observations

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft and the Søndrestrøm incoherent scatter radar coincide in space. Such coincidence offers a unique opportunity to study temporal vs spatial variations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP satellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distribution and line-of-sight plasma velocities. We combine these data sets to construct a two-dimensional map of a possible auroral pattern above Søndrestrøm. It is characterized by the following properties. No difference is seen between the gross precipitation patterns measured along the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic east-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells. However, F6 and F8 observed minor differences in the small-scale structures. More significant differences are found between small-scale features in the DMSP precipitation measurements and in radar observations of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4°, in magnetic longitude, and 0–40 s in time along the spacecraft trajectories (varying with magnetic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide. The combined data suggest that the plasma flow reversal delineates the polar cap boundary, that is, the boundary between precipitation characteristic for the plasma mantle and for the plasma sheet boundary layer.

The low‐latitude boundary layer at mid‐altitudes: Relation to large‐scale Birkeland currents

Data from the hot plasma instruments and the magnetometer on board Viking (apogee at 13,530 km) acquired in the high latitude pre‐dawn to pre‐noon region are combined to study the relations between the low‐latitude boundary layer (LLBL) projection and large‐scale field‐aligned currents (FAC). The projection of the LLBL is defined as the region with precipitation of ions with magnetosheath origin [Woch and Lundin, 1993]. The cusp and plasma mantle regions are excluded. The study focuses on magnetically quiet to moderately disturbed conditions. On the vast majority of passes the R1 FAC is found to be totally confined within the LLBL projection. This can be regarded as direct evidence of a LLBL source for the dayside R1 FAC. The latitudinal width of the LLBL (and consequently also that of the R1 FAC) decreases considerably going from pre‐dawn to pre‐noon MLT sectors. The current intensity increases only slightly towards pre‐noon. The R1 current density maximum known to exist in the pre‐noon region is thus primarily due to the LLBL projection converging towards local noon.

Galileo observations of the motions of ion and electron plasmas in the magnetotail

We report the first fully three‐dimensional determinations of the bulk motions of ion and electron plasmas in Earth's magnetotail at geocentric radial distances near the lunar orbit. The two regions that are examined are the plasma sheet and the plasma mantle. The magnitudes and directions of the bulk velocities are the same for the ion and electron plasmas in the plasma mantle. However, in the plasma sheet the electron bulk speeds are often substantially greater than the ion bulk speeds. This situation arises because the component of electron flow along the magnetic field is greater than the ion parallel flow. Most of the bulk flow of the electrons is often attributable to the presence of field‐aligned currents, not E × B convection of plasmas. Hence previously reported two‐dimensional electron velocity distributions do not give a reliable determination of mass transport and convection in the plasma sheet at these distances in the magnetotail.

Optical signatures and sources of prenoon auroral precipitation

Optical observations from Ny Ålesund, Svalbard, of prenoon auroral structures are compared with simultaneous measurements of energetic particle precipitation and plasma drifts taken during a nearby overpass of the DMSP‐F9 satellite near the 0830 MLT meridian. Close to the time of this pass, a stable band of red‐line emissions was detected from south of Ny Ålesund. Very weak, transient auroral activity was observed from the sky above and north of the station. An automatic classification algorithm applied to the DMSP‐F9 particle‐precipitation data, identified source regions in the central plasma sheet, the boundary plasma sheet, the plasma mantle, and the polar rain. We interpreted the poleward and equatorward parts of the red auroral band as being collocated with regions of mantle and boundary plasma sheet precipitation on tailward and sunward convecting field lines, respectively. However, the mantle part of the red auroral band, as interpreted by the automated algorithm, is difficult to reconcile with simultaneously measured, high‐latitude convection distributions. Another interpretation of the data, which includes magnetic mapping considerations and constraints imposed by the global electric field pattern, suggests that the entire band of red‐line emissions from south of Ny Ålesund is excited by electrons originating in the nightside, boundary plasma sheet. Although neither interpretation is definitive, resolving dilemmas that they pose is critical for understanding this common and simple form of dayside aurora.

On the instability and energy flux of lower hybrid waves in the Venus plasma mantle

Waves generated near the lower hybrid resonance frequency by the modified two stream instability have been invoked as a possible source of energy flux into the topside ionosphere of Venus. These waves are observed above the ionopause in a region known as the plasma mantle. The plasma within the mantle appears to be a mixture of magnetosheath and ionospheric plasmas. Since the magnetosheath electrons and ions have temperatures of several tens of eV, any instability analysis of the modified two stream instability requires the inclusion of finite electron and ion temperatures. Finite temperature effects are likely to reduce the growth rate of the instability. Furthermore, the lower hybrid waves are only quasi‐electrostatic, and the energy flux of the waves is mainly carried by parallel Poynting flux. The magnetic field in the mantle is draped over the ionopause. Lower hybrid waves therefore cannot transport any significant wave energy to lower altitudes, and so do not act as a source of additional heat to the topside ionosphere.

The low‐latitude boundary layer at mid‐altitudes: Identification based on Viking hot plasma data

On passes through the pre‐dawn to pre‐noon auroral oval the Viking spacecraft (apogee at 13,400 km) encounters often a generally well‐defined precipitation region of ions with characteristic energies of several hundred eV, with neither cusp nor plasma mantle characteristics. Furthermore, these ions are clearly separable from ions originating from the plasma sheet. Partial ion number densities in this plasma precipitation region strongly correlate with solar wind densities, supporting the view that it constitutes the mid‐altitude projection of the magnetospheric low‐latitude boundary layer (LLBL). The LLBL projection thus identified is observed with high probability in the MLT range from near‐noon to at least 6 MLT but occasionally extends toward 4 to 3 MLT. In the dawn to pre‐noon local time sector auroral ion precipitation pattern can be divided into precipitation originating from the plasma sheet and LLBL, i.e. generally no region was encountered with properties suggesting that it projects to the magnetospheric plasma sheet boundary layer (PSBL).

Relationship between Birkeland current regions, particle precipitation, and electric fields

Data from eight DMSP F7 satellite passes coincident with Sondrestrom radar observations have been examined to determine how the large‐scale dayside Birkeland currents are related to the particle precipitation regions and to the convection pattern. The classification schemes recently developed from DMSP particle data were adopted. The observations were limited to the prenoon local time hours and led to the following conclusions: (1) The local time of the mantle currents (which were traditionally called cusp currents) is not limited to the longitude of the cusp proper, but covers a larger local time extent. (2) The mantle currents flow entirely on open field lines (where “open field lines” is defined as a region where the ion precipitation and electron precipitation have the characteristics of plasma mantle, cusp, or polar rain.) This confirms and extends to all local times similar results obtained from other observations. (3) About half of region 1 currents flow on open field lines. This is consistent with the assumption that the region 1 currents are generated by the solar wind dynamo and flow within the surface that separates open and closed field lines. (4) More than 80% of the Birkeland current boundaries do not correspond to particle precipitation boundaries. Region 2 currents extend beyond the plasma sheet poleward boundary; region 1 currents flow in part on open field lines; mantle currents and mantle particles are not coincident. (5) On most passes when a triple current sheet is observed (region 2, region 1, and mantle currents), the convection reversal is located on closed field lines. When only two current sheets are observed (either region 2/region 1, or region 1/mantle currents), the convection reversal is on open field lines. (6) The data appear to be more consistent with a topology such that mantle currents are an extension of region 1 currents, rather than a separate system located poleward of the region 1 current system.

Cusp/cleft region as observed by the Viking UV imager

The cusp/cleft region is composed of direct entry magnetosheath and magnetospheric boundary layer plasma. Much effort has recently been put forth in distinguishing the individual particle signatures of these regions. This report uses the Viking UV imager to show variable optical signatures of the cusp/cleft region. The region has been classified into four categories via their particle signatures: the cusp proper, cusp, cleft, and plasma mantle. Time series plots of intensity profiles along the Viking footprint are used to show the evolution of the cusp/cleft region during different auroral morphologies. These profiles establish that the observed emissions are located at the footprint of the cleft region. In addition, the region defined as the cusp was found poleward of this continuous emission. These data are also compared to recent particle studies of the cusp/cleft region and to appropriate topological models.

The auroral distribution and its relation to magnetospheric processes

The auroral distribution has several aspects which can be naturally related to magnetospheric processes. Dayside asymmetries in intensity and location of emissions enable one to visualize the implied convection and thus the geometry of dayside boundary regions. For B y > 0 (B y < 0) northern hemisphere polar arcs tend to be confined to the dusk (dawn) sector and the dusk (dawn) auroral distribution tends to lie poleward of the dawn (dusk) emissions near noon. These observations are consistent with the orientation of the convection throat and its dependence on IMF B y. High latitude dayside features, poleward of the particle cusp, are also dependent on B y and confirm that there is indeed a separate auroral system (and presumably current system) in the particle plasma mantle region (as defined by DMSP). The distribution of the auroras in the midnight sector during both quiet and disturbed times allows one to make important inferences about the mapping of these forms into the magnetotail. Fundamental to this is the observations of a continuous oval throughout the night sector, the location of substorm onset relative to this oval and the subsequent development of a “double oval” during substorm recovery. A unique set of observations are given illustrating how the different portions of the “double oval” develop during a substorm expansion phase. These observations support the view that on some occasions the expansion phase dominantly affects the central plasma sheet and leaves the inner and outer edges of the magnetotail relatively undisturbed. New observations concerning the dynamics of arc development westward of the auroral surge supports an hypothesis of Alfven waves modulating currents which in turn give rise to magnetic pulsations and multiple auroral arc systems.

Shaping of the magnetotail from the mantle: Global and local structuring

This paper discusses kinetic modeling of the properties of magnetotail formation from a plasma mantle source and develops a unified view of the structure of the central part of the magnetotail plasma sheet as well as the structure of its boundary layer. Trajectories of mantle protons in the presence of a uniform dawn‐dusk electric field have been traced using the Tsyganenko magnetic field model for quiet periods of magnetospheric activity. The most important portion of the particle trajectories is each particle's first interaction with the sharp reversal of the magnetic field in the tail midplane, because this interaction results in particle energization and chaotic scattering. The closer this interaction takes place to the X line, the larger a particle's energy will become. The intensity of the chaotic scattering in the tail midplane depends also on the position in the tail at which it occurs. The energization and scattering result in a significant restructuring of the tail ion distributions, both in space and in velocity coordinates. Our model shows the evolution of the global structure of the tail with a clearly defined central plasma sheet and plasma sheet boundary layer developing from its beginnings as a plasma “nucleus” in the distant tail current sheet. This large‐scale restructuring is accompanied by the creation of small‐scale features in the particle distribution functions. For example, the model not only correctly reproduces the spatial distribution and velocity dispersion of the fast ion beams moving both earthward and tailward in the plasma sheet boundary layer, but also indicates that these beams should be highly structured spatially into 5‐6 smaller beamlets with distinct velocities. In addition the model shows that complementary ring distribution structures should also exist in the central plasma sheet. Our analysis indicates that the ion distribution functions in the central plasma sheet should take a rather specific form in velocity space with loss regions oriented predominantly orthogonal to the magnetic field. Our results also emphasize the importance of counterstreaming populations, not only in the boundary layer, but also in the central part of the plasma sheet. Analytical calculations indicate that the properties of chaotic scattering in the magnetotail under realistic conditions (x dependence of the normal magnetic field and dawn‐dusk electric field) are quite different from those predicted by earlier simple xindependent models. Finally the model results are compared with recent observations of ion distribution functions and their moments for various regions of the magnetotail, and quantitative estimates from the model are shown to be in good agreement with observations. Small‐scale structuring and the presence of counterstreaming are also discussed, as well as their possible importance in explaining the observed intermittency in the plasma sheet bulk flows.

The Uranian corona as a charge exchange cascade of plasma sheet protons

Models of magnetospheric convection and interparticle interactions are used in a study of the collisional interactions between atmospheric neutral hydrogen and magnetospheric charged particles observed by Voyager to be convecting through the Uranian magnetosphere. The most important reactions are e−‐H collisional ionization and p+‐H charge exchange with the atmosphere. The e−‐H collisional ionization process, continually reenergized by compressional heating of the electrons as they drift toward Uranus, produces a cascade of new plasma. This process has been suggested by many authors as the source of the warm (10 eV at L = 5) plasma and is found by this work to continue in a cascade to even cooler and more abundant plasma. This newly created plasma consists almost entirely of electrons and protons because He and H2 are nearly absent from the very upper layers of the atmosphere. The densities computed for the warm plasma are within an order of magnitude of the values observed by Voyager. The cold plasma should occur only near or sunward from Uranus, where it is too cold to have been detected by Voyager on its inbound trajectory. Low‐altitude p+‐H charge exchange between the atmosphere and the new plasma produces a significant flux of hot H. This hot H may comprise the Uranian H corona, and its production serves as a high‐altitude thermospheric heat source, both of which are implied by Voyager UVS observations (although this heat source is apparently inadequate to produce the high thermospheric temperature that was observed). The generation of significant new plasma near Uranus leads to the following speculation: If this plasma crosses the dayside magnetopause and mixes with magnetopause boundary layers such as the plasma mantle, there to be swept back along the magnetotail, reincorporated into the magnetotail by the same processes postulated for solar wind plasma entry, and reenergized in the magnetotail current sheet, it would constitute an important source for the hot plasma (1 keV at L = 5) observed by Voyager.

On Current Fluctuations in Near‐Earth Space Plasma with Lower‐Hybrid‐Drift Turbulence

AbstractElectron and ion current fluctuations caused by lower‐hybrid‐drift turbulence are estimated within nonlinear theory for the plasma of the ionospheric F‐layer, as well as for the plasma mantle and the plasma sheet boundary layer of the tail of the earth's magnetosphere. They are found to be of the order of 10−14 — 10−11 A/m2 and 10−13 — 10−9 A/m2, respectively.

Plasma observations near Neptune: Results from Voyager 2

Results from observations made by the Plasma Science experiment on Voyager 2 at Neptune are reviewed. The magnetosphere of Neptune is filled with a tenuous plasma, which consists of at least two components: a light ion, probably H + and a heavy ion, probably N +. Triton's atmosphere or ionosphere is thought to be the source of both heavy and light ions. Much of the low energy plasma in the inner magnetosphere is concentrated near the magnetic equator and near closest approach to the planet. The large tilt of the magnetic dipole axis from the rotation axis produces a dynamic magnetosphere which goes from an “earth-like” configuration to a “pole-on” configuration and back every 16 hours. The polar cusp regions change location and size as the planet rotates; at the time of the inbound magnetopause crossing, the phase of Neptune's rotation was such that the spacecraft entered the magnetosphere through the southern polar cusp region. Outbound from Neptune observations made in the magnetosheath show a possible signature of diurnal oscillation of the plasma mantle that grows, shrinks, and rocks, in a diurnal cycle. After the encounter with Neptune's magnetosphere, an upstream wave event was observed when the interplanetary magnetic field connected to the bow shock. The low frequency waves observed appear to be a mixture of Alfvénic and/or fast mode waves propagating away from the planet.

On the termination of the closed field line region of the magnetotail

The structure of the distant magnetotail, where the closed field line region terminates, is investigated in the framework of several models and on the basis of laboratory simulations of the interaction of a flowing magnetized plasma with a dipole field. From our simulations we find that this region for northward “interplanetary” field (IMF) resembles that of a quasi‐static tail structure, in which a flaring tail transits into a nonflaring current sheet configuration at a location where the internal pressure is balanced by the external plasma and magnetic pressures only. This structure is associated with a Y‐type magnetic neutral line. The magnetic tail apparently includes an open field line region, despite the fact that this magnetosphere did not have a history of southward IMF, which could have opened the dipole field through frontside reconnection. This is a strong indication of the occurrence of high‐latitude nightside reconnection, suggested by Dungey (1963). For southward model IMF the closed field line region appears to terminate at an X‐type magnetic neutral line, as in the standard open magnetosphere model of Dungey (1961). This configuration is also in qualitative agreement with the simple superposition of a dipole field and a uniform IMF and with the presence of a plasma mantle which expands through drifts from the magnetosphere boundary toward the plasma sheet and becomes partially trapped through magnetic reconnection at the distant X line.

Determining the source region of auroral emissions in the prenoon oval using coordinated polar BEAR UV‐imaging and DMSP particle measurements

The Polar Beacon Experiment and Auroral Research (Polar BEAR) satellite included the capability for imaging the dayside auroral oval in full sunlight at several wavelengths. We compare particle observations from the DMSP F7 satellite during dayside auroral oval crossings with approximately simultaneous Polar BEAR 1356‐Å images to determine the magnetospheric source region of the dayside auroral oval. The source region is determined from the Defense Meteorological Satellite Program (DMSP) particle data, according to recent work concerning the classification and identification of precipitation source regions. The close DMSP/Polar BEAR coincidences all occur when the former satellite is located between 0945 and 1000 MLT. We found instances of auroral arcs mapping to each of several different regions, including the boundary plasma sheet, the low‐latitude boundary layer, and the plasma mantle. However, our results indicate that about half the time the most prominent auroral arcs are located at the interfaces between distinct plasma regions, at least at the local time studied here.

Incoherent Scatter Radar Observations of Ionospheric Signatures of Cusp-Like Electron Precipitation.

We present a case study of spatially and temporally coincident DMSP-F7 electron and ion precipitation measurements and Sondrestrom incoherent scatter radar observations of ionospheric plasma parameters. The particle flux data allow us to determine the boundaries between the magnetospheric regions central plasma sheet (CPS), plasma sheet boundary layer (PSBL), cusp proper, and plasma mantle (PM), mapped down to the DMSP orbit (835km altitude). The PSBL, cusp, and the equatorward section of the PM are characterized by intense low energy (<400eV) electron precipitation. The radar measured enhanced plasma density and elevated electron temperature at 350km altitude at the same invariant latitude interval where DMSP-F7 detected the intense low-energy electron flux. The ion temperature was unaffected, as were plasma density and electron and ion temperatures at 200km altitude. The plasma convection pattern inferred from radar Doppler shift measurements suggests that the heated electron cloud observed in the F region was locally produced by the soft electron precipitation in and near the cusp proper rather than convected into the radar field of view. This study demonstrates that soft electron precipitation, which is typical for magnetosheath-like plasma entry observed in particular in the cusp, can under certain conditions be identified by its thermal ionospheric plasma signatures.

Physical processes in the plasma mantle of Venus

The Phobos‐2 observations established the presence of a turbulent region on the dayside, called the mantle, just outside the magnetic barrier at Mars. This mantle is the interaction region of cold Martian ions and the shocked solar wind. This work presents the results of a study, which analyzed data from ten Pioneer Venus orbits, in order to see whether similar wave particle interaction processes also exist in the corresponding region around Venus. The first conclusion is that the apparent physical processes in the mantle are indeed similar around Venus and Mars. The planetary thermal O+ ions outside the ionopause interact with the shocked solar wind and excite electrostatic waves close to the lower hybrid frequency. These wave propagate inwards, heating first the electron and deeper down in the ionosphere the thermal ion population. The observed superthermal ions are believed to be the product of this wave particle interaction process. Our second conclusion is that the wave energy transferred to the thermal electrons is of the right magnitude (∼4×109 eV cm−2 s−1) to provide the supplemental heat source necessary to reconcile observed and calculated electron temperatures in the ionosphere.

Regular and chaotic aspects of charged particle motion in a magnetotail‐like field with a neutral line

The motion of charged particles is studied numerically and analytically using a test particle approach. Acceleration near the X‐type neutral line is described in terms of adiabatic invariants and separatrix crossings. The relation between regular and chaotic properties of the process is discussed for a realistic source in the plasma mantle.