Plasma Flux Tube Research Articles

Relative solar and auroral contribution to the polar F region: Implications for National Space Weather Program

When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the auroral precipitation is not subject to diurnal or seasonal dependences in the same predictable manner; the auroral precipitation can almost be viewed as stochastic within certain geomagnetic coordinate constraints. In this study we use a physical model to separate the effects of solar EUV and auroral precipitation. We find that the auroral contribution does provide a far‐from‐negligible “baseline” level of polar cap F region plasma, upon which is superimposed the UT and seasonally dependent TOI. This baseline level of ionization is very difficult to predict or forecast since it is determined by plasma flux tube histories through extended regions of the auroral oval over several hours. This result raises the need for more advanced auroral precipitation modeling in order to obtain improved space weather specification. The inclusion of soft auroral precipitation is especially important since it can be a significant source of F region plasma.

A theoretical model study of F-region response to high latitude neutral wind upwelling events

Neutral wind upwelling events are well documented in both the northern and southern high latitude ionosphere. The vertical winds in the F-region frequently exceed 100 m/ s , and winds in excess of 200 m/ s have been observed. These upwelling events occur in a latitudinal band of 4°–6° width that extends in local time in the midnight-morning sector; this band always lies poleward of the auroral precipitation. Using the time dependent ionospheric model (TDIM), a series of sensitivity simulations are carried out, based on observational constraints provided by the spectacular upwelling event seen at both the Mawson and Davis stations in the Antarctic on 08 June 1997 (Innis et al., 1999). The model simulations indicate that the F-layer density at any given point may either increase or decrease during an upwelling event, depending upon the past history of the plasma flux tube. Because this past history of the F-layer convection is unknown for the specific upwelling events a detailed case study cannot be undertaken. Instead a series of sensitivity simulations based upon a range of possible convection histories will be studied to determine the relative effect of the upwelling. The absolute density is not dependent upon solar EUV production because of winter conditions, but is sensitive to the auroral electron precipitation. The best F-layer indicator of the upwelling is the height of the layer, h m F 2. For upwelling events with vertical drifts of 100 m/ s h mF 2 can be increased by 100 km in 10 min . Upon leaving an upwelling region, the h m F 2 almost as rapidly decreases to its normal height. Resulting from this lifting of the O + layer is the reduction in O + recombination and 630-nm emission; this latter consequence is observed as a standard feature of the upwelling events. In the topside ionosphere the electron density is responsive to the upwelling. The total electron content (TEC) is not, in general, sensitive to the uplifting events, however, low elevation slant path GPS TEC measurements might well detect the rapid uplifting of the F-layer. The upwelling event observations are insufficient to constrain our understanding of their impact upon the ionosphere. This model study does imply that upwelling events can modify the F-layer height severely. Such layer height modification can have measurable effects on radio frequency ray paths through the ionosphere. To quantify such effects a fuller description of the upwelling events as well as the past history of ionospheric plasma is needed. Experiments with higher time resolution of both the neutral parameters and F-region at multiple locations are necessary to unravel these complex events.

Seasonal and solar cycle variations of the polar wind

Until recently, theoretical models of the polar wind were one‐dimensional. In these models, reasonable but ad hoc boundary conditions were fed to a mathematical model, which was then solved to provide either a steady state or time‐dependent description of the plasma along a particular field line. These models were inherently unable to address the three‐dimensional structure of the polar wind. Now, a truly global model of the polar wind has been developed. This model is a three‐dimensional, time‐dependent, fluid model that self‐consistently couples the ionosphere and the polar wind. It describes the plasma dynamics in a large number of high‐latitude flux tubes as they move under the combined influence of corotational and convection electric fields. The model incorporates realistic models of the high‐latitude convection and auroral electron precipitation. In this study, the global model of the ionosphere and polar wind was used to illuminate the seasonal and solar cycle variability of the polar wind's three‐dimensional structure. In particular, the three‐dimensional distribution of both H+ and O+ ions in the polar wind was examined for both solar maximum and minimum conditions and for both summer and winter solstices, as well as for various levels of geomagnetic activity. Over 1000 flux tubes of plasma are modeled for each set of geophysical conditions considered in this study, providing a horizontal resolution that is much finer than in previous applications of the global model. The modeling results are compared with relevant observational databases.

Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations

Multiple and sporadic time‐of‐flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy‐dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection‐induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy‐dispersed ion beams.

Theoretical developments on the causes of ionospheric outflow

It is now well known that there is a substantial outflow of ionospheric plasma from the terrestrial ionosphere at high latitudes. The outflow consists of light thermal ions (H +, He +) as well as both light and heavy energized ions (H +, He +, O +, N +, NO +, O 2 +, N 2 +). The thermal ion outflows tend to be associated with the classical polar wind, while the energized ions are probably associated with either auroral energization processes or nonclassical polar wind processes. Part of the problem with identifying the exact cause of a given outflow relates to the fact that the ionosphere continuously convects into and out of the various high-latitude regions (sunlight, cusp, polar cap, nocturnal oval) and the time-constant for outflow is comparable to the convection time. Therefore, it is difficult to separate and quantify the possible outflow mechanisms. Some of these mechanisms are as follows. In sunlit regions, the photoelectrons can heat the thermal electrons and the elevated electron temperature acts to increase the polar wind outflow rate. At high altitudes, the escaping photoelectrons can also accelerate the polar wind as they drag the thermal ions with them. In the cusp and auroral oval, the precipitating magnetospheric electrons can heat the thermal electrons in a manner similar to the photoelectrons. Also, energized ions, in the form of beams and conics, can be created in association with field-aligned auroral currents and potential structures. The cusp ion beams and conics that have been convected into the polar cap can destabilize the polar wind when they pass through it at high altitudes, thereby transferring energy to the thermal ions. Additional energization mechanisms in the polar cap include Joule heating, hot magnetospheric electrons and ions, electromagnetic wave turbulence, and centrifugal acceleration.Some of these causes of ionospheric outflow will be briefly reviewed, with the emphasis on the recent simulations of polar wind dynamics in convecting flux tubes of plasma.

The second pair in the INTERBALL quartet: Some main results

The INTERBALL-2 satellite (AURORAL PROBE) was launched on August 29, 1996, about a year after the INTERBALL-1 and MAGION-4 pair already worked on a higher orbit. It performed comprehensive measurements of magnetospheric plasmas and fields mainly at altitudes 2 – 3 R E during more than two years. Some of the main findings from these measurements are briefly reviewed, in particular: 1. Discovery of long-duration high-energy electron dispersion events above the polar edge of the post-midnight auroral oval. 2. Discovery of high-energy (> 20 keV, i.e. super-auroral energy) upward moving field-aligned electron beams accompanied by bi-directional electrons ∼ 1 keV, also in these auroral regions. 3. Mass-resolved velocity-dispersed ion structures (VDIS) extended down to superthermal energies injected in the plasma flux tubes at the auroral oval polar border in a wide range of local times. 4. Mid-altitude ion distribution functions in the cusp showing a combination of downward moving, and mirror reflected, magnetosheath ions, with upward moving thermal and superthermal H +, He +, and O + ions of the “cleft ion fountain” starting from eV energy range. 5. Thermal and superthermal plasma outflows of the classic polar wind type (only H + and He +, no O +) as contrasted to conditions when additional acceleration mechanisms are operating which accelerate thermal heavy ions also (O +, etc.).

Dynamic features of the polar wind in the presence of hot magnetospheric electrons

A time‐dependent macroscopic particle‐in‐cell (mac‐PIC) model was used to study the temporal evolution of the polar wind under the influence of a hot electron population. First, the steady state results of the mac‐PIC model were found for a wide range of hot/cold electron temperature ratios and compared with the results of the well‐established time‐independent semikinetic model, and excellent agreement was found. Second, simulations were conducted to study the temporal evolution of a plasma that was originally in a steady state condition, and then a hot electron population was suddenly introduced. The profiles of the plasma moments again displayed discontinuities, which oscillated with a decreasing amplitude until they reached their steady state values. As the hot electron temperature increased, the oscillation amplitude increased, and the altitude of the discontinuity decreased, while the period of oscillation and decay rate remained essentially unchanged. Third, simulations were conducted for plasma flux tubes as they drifted across the subauroral, cusp, polar cap, and auroral regions. It was found that as soon as the plasma entered the polar cap, the signatures of the hot electrons were observed. The strength of these signatures varied with time owing to the variation in the instantaneous values of the density and temperature of the thermal electrons. After the plasma exited the polar cap the signatures of the hot electrons persisted for a while, and a density bump formed. For more energetic hot electrons the signatures of the hot electrons became more pronounced in the polar cap and persisted longer after the flux tube left the polar cap. The results of this study were shown to explain some interesting features of the polar wind that were observed by the POLAR satellite.

Gradient drift instability growth rates from global‐scale modeling of the polar ionosphere

The winter polar ionosphere, under southward interplanetary magnetic field (IMF) conditions, experiences irregularity development leading to consequences such as scintillation on transionospheric communication links. These irregularities are associated with antisunward convecting polar ionospheric patches. The gradient drift instability (GDI) has been considered a primary candidate for the generation of these irregularities, or at least the long‐wavelength energy source of the irregularity‐wave cascade process. The Utah State University time‐dependent ionospheric model (TDIM) enables the polar cap ionosphere and its patches to be modeled on a large scale in a time‐evolving manner. Hence, at each point in space and time, the TDIM has adequate information to compute the growth rate of the gradient drift instability. The spatial gradients are based on a 92×92 km TDIM grid resolution, while the time resolution can be as short as 30 s. In this study, we present results of the GDI linear growth rate calculations for the F region. For a first time, snapshots of the instantaneous GDI linear growth rates over the polar ionosphere are shown. These show, in part, the correlation of the GDI with polar cap patches. Then we present the time evolution of the linear growth rate as plasma flux tubes convect. It is shown that a flux tube of plasma can, in fact, change between being unstable and stable to the GDI as the plasma convection pattern switches in response to changing IMF conditions. These convective results are essential information if an assessment of whether or not a plasma flux tube is stable to the gradient drift mechanism is to be made.

Electron temperatures during rapid subauroral ion drift events

Abstract. Examples of data from DE-2 satellite instruments are presented. These illustrate the behaviour of plasma parameters in the F-region and adjacent topside ionosphere during rapid sub-auroral ion drift (SAID) events. In particular, a variety of behaviours of the electron temperature (Te) is demonstrated, both within and equatorward of the SAID region. The Sheffield University plasmasphere-ionosphere model (SUPIM) is used to perform calculations in which a model SAID is applied to a plasma flux tube. The model results indicate that strongly elevated ion temperature (a recognised signature of SAID events) is on occasion sufficient to raise Te to observed values by ion-electron heat transfer. On other occasions, an additional heat source is required. It is suggested that such a source for the electron gas may be due to interaction between the ring current and the plasmasphere at high altitudes. The magnitude of the downward heat flux is consistent with that necessary to produce sub-auroral red arcs. The resulting strongly heated electron gas causes vibrational excitation of molecular nitrogen in the thermosphere. Key words. Electron temperature · Subauroral ion drift · Plasmasphere-ionosphere SUPIM

Trapped particles in the polar wind

The flow of plasma along open field lines at high latitudes is highly variable and depends both on conditions in the underlying ionosphere and thermosphere and on the transport of particles and energy from the magnetosphere. Past attempts to model this time variability have, for the most part, examined the response of the plasma on a stationary field line to certain prespecified boundary conditions and heat sources. While such prespecified conditions may bear some resemblance to what occurs naturally, they are artificial and cannot be expected to yield a truly quantitative understanding of the various physical processes that interact to produce the dynamic polar wind. The present study is one in a series of studies that attempts to eliminate this artificiality by coupling the mathematical description of the polar wind to a three‐dimensional time‐dependent model of the high‐latitude ionosphere. In this study, an individual flux tube of plasma is followed as it moves under the influence of combined corotation and convection electric fields. Boundary conditions at the lower end of the flux tube are obtained from the ionosphere model, which takes into account all significant particle species, chemical reactions, and heat sources that contribute to the state of the ionosphere. A multi‐ion macroscopic particle‐in‐cell code is used to model the plasma in the flux tube. A description of the behavior of H+ and O+ for the altitude range from 2000 km to about 8 Earth radii is obtained as the flux tube moves along the trajectory, which traverses regions of the subauroral ionosphere, dayside and nightside ovals, and polar cap. The goal of the study is to determine the extent to which ion trapping can occur in the polar wind and the effects that collisions, wave‐particle interactions, centrifugal acceleration, and varying ionospheric conditions have on the trapped ions. The main conclusion of the study is that O+ trapping is important and it acts to increase the O+ density at high altitudes.

Electron temperatures during rapid subauroral ion drift events

Examples of data from DE-2 satellite instruments are presented. These illustrate the behaviour of plasma parameters in the F-region and adjacent topside ionosphere during rapid sub-auroral ion drift (SAID) events. In particular, a variety of behaviours of the electron temperature (Te) is demonstrated, both within and equatorward of the SAID region. The Sheffield University plasmasphere-ionosphere model (SUPIM) is used to perform calculations in which a model SAID is applied to a plasma flux tube. The model results indicate that strongly elevated ion temperature (a recognised signature of SAID events) is on occasion sufficient to raise Te to observed values by ion-electron heat transfer. On other occasions, an additional heat source is required. It is suggested that such a source for the electron gas may be due to interaction between the ring current and the plasmasphere at high altitudes. The magnitude of the downward heat flux is consistent with that necessary to produce sub-auroral red arcs. The resulting strongly heated electron gas causes vibrational excitation of molecular nitrogen in the thermosphere. Key words. Electron temperature · Subauroral ion drift · Plasmasphere-ionosphere SUPIM

Tomographic imaging of the polar-cap ionosphere over svalbard

Abstract First results are presented from an experiment to image the electron density in the polar-cap ionosphere above Svalbard using tomographic techniques. The results show localised enhanced densities in the cusp and throat region, the elevated densities of a polar patch, relatively low densities in co-rotating plasma flux tubes in darkness at lower latitudes and the depleted densities of the polar hole. The reconstruction of these features, known to occur at polar latitudes, confirms the potential of tomography for imaging the ionosphere at these very high latitudes.

Predicting plasmaspheric radial density profiles

A principle question concerning storm time convection is, What physical process or measurable parameter controls the location of the equatorward edge of the large‐scale convection zone expansion during magnetic storms and hence the plasmapause location? Experimental data on convection and particle precipitation consistently show that in the evening to midnight local time sector, the inner boundary of the high‐latitude westward ion convection band colocates with the equatorward boundary of soft electron precipitation (SEB). Low‐energy electron precipitation is usually absent, or very weak, above the lower‐latitude band of the disturbed‐time double‐peaked convection pattern. It follows that large‐scale convection streamlines carrying plasma sheet particles do not enter the polarization jet band which lies on the opposite (inner) side of the Alfven layer, which limits the inward expansion of hot plasma and convection by polarizing the edge of the inner magnetosphere plasma population. We conclude that the SEB (measured and/or modeled) can be used in plasmasphere density models as a substitute for the convection boundary. A time‐dependent convection‐driven plasmaspheric density model (CDPDM) is introduced to describe plasmaspheric thermal density profiles. The CDPDM is based on the convection drift and refilling rate prehistory calculated for a particular plasma flux tube, and its most important ingredient is a realistic convection model for disturbed times. Sharp density gradients (plasmapauses) on the radial profiles are indicators of preceding convection boundary locations outside of which the thermal plasma content was lost. We compare the predictions of the model with storm time ionospheric observations with the Millstone Hill radar and conclude that the CDPDM can be used to predict the locations of plasma density radial gradients, including the plasmapause.

RENDERING THREE-DIMENSIONAL SOLAR CORONAL STRUCTURES

An X-ray or EUV image of the corona or chromosphere is a 2D representation of an extended 3D complex for which a general inversion process is impossible. A specific model must be incorporated in order to understand the full 3D structure. We approach this problem by modeling a set of optically-thin 3D plasma flux tubes which we render these as synthetic images. The resulting images allow the interpretation of the X-ray/EUV observations to obtain information on (1) the 3D structure of X-ray images, i.e., the geometric structure of the flux tubes, and on (2) the internal structure using specific plasma characteristics, i.e., the physical structure of the flux tubes. The data-analysis technique uses magnetograms to characterize photospheric magnetic fields and extrapolation techniques to form the field lines. Using a new set of software tools, we have generated 3D flux tube structures around these field lines and integrated the plasma emission along the line of sight to obtain a rendered image. A set of individual flux-tube images is selected by a non-negative least-squares technique to provide a match with an observed X-ray image. The scheme minimizes the squares of the differences between the synthesized image and the observed image with a non-negative constraint on the coefficients of the brightness of the individual flux-tube loops. The derived images are used to determine the specific photospheric foot points and physical data, i.e., scaling laws for densities and loop lengths. The development has led to computer efficient integration and display software that is compatible for comparison with observations (e.g., Yohkoh SXT data, NIXT, or EIT). This analysis is important in determining directly the magnetic field configuration, which provides the structure of coronal loops, and indirectly the electric currents or waves, which provide the energy for the heating of the plasma. We have used very simple assumptions (i.e., potential magnetic fields and isothermal corona) to provide an initial test of the techniques before complex models are introduced. We have separated the physical and geometric contributions of the emission for a set of flux tubes and concentrated, in this initial study, on the geometric contributions by making approximations to the physical contributions. The initial results are consistent with the scaling laws derived from the Yohkoh SXT data.

The polar wind

The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision‐dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross‐field transport, which modify the large‐scale polar wind outflow.

Transport in a toroidally confined pure electron plasma

O’Neil and Smith [T.M. O’Neil and R.A. Smith, Phys. Plasmas 1, 8 (1994)] have argued that a pure electron plasma can be confined stably in a toroidal magnetic field configuration. This paper shows that the toroidal curvature of the magnetic field of necessity causes slow cross-field transport. The transport mechanism is similar to magnetic pumping and may be understood by considering a single flux tube of plasma. As the flux tube of plasma undergoes poloidal E×B drift rotation about the center of the plasma, the length of the flux tube and the magnetic field strength within the flux tube oscillate, and this produces corresponding oscillations in T∥ and T⊥. The collisional relaxation of T∥ toward T⊥ produces a slow dissipation of electrostatic energy into heat and a consequent expansion (cross-field transport) of the plasma. In the limit where the cross section of the plasma is nearly circular the radial particle flux is given by Γr=1/2ν⊥,∥T(r/ρ0)2n/(−e∂Φ/∂r), where ν⊥,∥ is the collisional equipartition rate, ρ0 is the major radius at the center of the plasma, and r is the minor radius measured from the center of the plasma. The transport flux is first calculated using this simple physical picture and then is calculated by solving the drift-kinetic Boltzmann equation. This latter calculation is not limited to a plasma with a circular cross section.

Shocks in the polar wind

The occurrence of shock waves in the terrestrial polar wind was predicted many years ago by a time‐dependent three‐dimensional model based on hydrodynamic equations. These shocks were seen to occur for counterstreaming ion populations and for cases when a convecting flux tube entered a region of sharply increasing electron temperature, such as the dayside cusp. Other studies conducted at about the same time showed that the shocks induced by counterstreaming ion populations may simply be artifacts of the adopted hydrodynamic model. The validity of shocks induced by electron temperature enhancements has remained an open question. Using a macroscopic particle‐in‐cell (PIC) code, we have verified the hydrodynamic prediction that sudden electron temperature enhancements can launch shock waves in a convecting flux tube of plasma. Our simulation follows a flux tube as it convects antisunward across the dayside auroral oval, the polar cap, and the nightside auroral oval. The electron temperature at 2000 km altitude is assumed to be relatively low (3000 K) in the subauroral ionosphere and in the polar cap but much higher (7000 K) in the dayside and nightside auroral oval. As the flux tube enters the auroral oval, either on the dayside or the nightside, forward and reverse shock pairs in the H+ component of the plasma are created at the bottom of the flux tube and propagate upward until they exit the simulation region at the top. The forward and reverse shock fronts propagate at speeds greater than and less than the drift speed of the H+ gas, respectively.

On the Magnetospheric Effects of Experimental Ground Explosions Observed from AUREOL-3.

A new analysis is described for the published data from the MASSA active experiments held in 1981-1983 in the USSR. These tests were aimed to simulate earthquake effects in the magnetosphere and ionosphere using surface chemical explosions. Measurements in the magnetosphere above the explosions were made from the AUREOL-3 satellite (Galperin et al., 1985a, 1985b). ELF/VLF noises were excited within an altitude range from 410 km to ≈ 2000 km along the Explosion plasma Flux Tube (EFT), based on the E-region above the explosion point (L ≈ 1.5). During the MASSA-1 experiment, at L ≈ 1.3, before the EFT crossing, 5 minutes after the explosion at L = 1.5, a strong upward moving MHD-impulse was registered with a sharp front. The front was accompanied by excited ion sound waves. Indirect arguments are presented for the origin of the impulse within the EFT, its bouncing and accompanied cross-L drift to, and then trapping at L = 1.3. The effects observed suggest the existence of a natural MHD-impulse generator and/or its power amplifier located in the middle atmosphere not higher than 70 km. It generates/amplifies the electromagnetic response in the magnetosphere and ionosphere. It is suggested that the form of electric discharges in the middle atmosphere known as “red sprites” or “blue jets” can serve the role of the natural generator/power amplifier for the effects in the ionosphere and magnetosphere observed in the MASSA experiments. If real, it can be speculated, that similar “amplification” effects through atmospheric electrical discharges could play a role in the generation of some earthquake precursor effects observed in the magnetosphere and ionosphere.

Rotational pumping and damping of the m=1 diocotron mode

An effect called rotational pumping by the authors (by analogy with magnetic pumping) causes a slow damping of the m=1 diocotron mode in non-neutral plasmas. In a frame centered on the plasma and rotating at the diocotron mode frequency, the end confinement potentials are nonaxisymmetric. As a flux tube of plasma undergoes E×B drift rotation about the center of the column, the length of the tube oscillates about some mean value, and this produces a corresponding oscillation in T∥. In turn, the collisional relaxation of T∥ toward T⊥ produces a slow dissipation of electrostatic energy into heat and a consequent radial expansion (cross-field transport) of the plasma. Since the canonical angular momentum is conserved, the displacement of the column off axis must decrease as the plasma expands. In the limit where the axial bounce frequency of an electron is large compared to its E×B drift rotation frequency theory predicts the damping rate γ=−2κ2ν⊥,∥ (r2p/R2w)(λ2D/L20)/(1−r2p R2w), where κ is a numerical constant, λD is the Debye length, Rw is the radius of the cylindrical conducting wall, rp is the effective plasma radius, L0 is the mean length of the plasma, and ν⊥,∥ is the equipartition rate. A novel aspect of this theory is that the magnetic field strength enters only through ν⊥,∥. As the field strength is increased, the damping rate is nearly independent of the field strength until the regime of strong magnetization is reached [i.e., Ωc≳v̄/b=(kT)3/2/√me2], and then the damping rate drops off dramatically. This signature has been observed in recent experiments. For completeness, the theory is extended to the regime where the bounce frequency is comparable to the rotation frequency, and bounce-rotation resonances are included.

A three‐dimensional time‐dependent model of the polar wind

A time‐dependent global model of the polar wind was used to study transient polar wind perturbations during changing magnetospheric conditions. The model calculates three‐dimensional distributions for the NO+, O2+, N2+, N+ and O+ densities and the ion and electron temperatures from diffusion and heat conduction equations at altitudes between 120 and 800 km. At altitudes above 500 km, the time‐dependent nonlinear hydrodynamic equations for O+ and H+ are solved self‐consistently with the ionospheric equations. The model takes account of supersonic ion outflow, shock formation, and ion energization during a plasma expansion event. During the simulation, the magnetic activity level changed from quiet to active and back to quiet again over a 4.5‐hour period. The study indicates the following: (1) Plasma pressure changes due to Te, Ti or electron density variations produce perturbations in the polar wind. In particular, plasma flux tube motion through the auroral oval and high electric field regions produces transient large‐scale ion upflows and downflows. At certain times and in certain regions both inside and outside the auroral oval, H+‐O+ counterstreaming can occur, (2) The density structure in the polar wind is considerably more complicated than in the ionosphere because of both horizontal plasma convection and changing vertical propagation speeds due to spatially varying ionospheric temperatures, (3) During increasing magnetic activity, there is an overall increase in Te, Ti and the electron density in the F‐region, but there is a time delay in the buildup of the electron density that is as long as five hours at high altitudes, (4) During increasing magnetic activity, there is an overall increase in the polar wind outflow from the ionosphere, while the reverse is true for declining activity, (5) In certain regions, however, localized ionospheric holes can develop during increasing magnetic activity, and in these regions the polar wind outflow rate is reduced, (6) During changing magnetic activity, the temporal evolution of the ion density morphology at high altitudes can be different from, and even opposite to, that at low altitudes.