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NASA NTRS2003

Investigation of a Light Gas Helicon Plasma Source for the VASIMR Space Propulsion System

An efficient plasma source producing a high-density (approx.10(exp 19/cu m) light gas (e.g. H, D, or He) flowing plasma with a high degree of ionization is a critical component of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) concept. We are developing an antenna to apply ICRF power near the fundamental ion cyclotron resonance to further accelerate the plasma ions to velocities appropriate for space propulsion applications. The high degree of ionization and a low vacuum background pressure are important to eliminate the problem of radial losses due to charge exchange. We have performed parametric (e.g. gas flow, power (0.5 - 3 kW), magnetic field , frequency (25 and 50 MHz)) studies of a helicon operating with gas (H2 D2, He, N2 and Ar) injected at one end with a high magnetic mirror downstream of the antenna. We have explored operation with a cusp and a mirror field upstream. Plasma flows into a low background vacuum (<10(exp -4) torr) at velocities higher than the ion sound speed. High densities (approx. 10(exp 19/cu m) have been achieved at the location where ICRF will be applied, just downstream of the magnetic mirror.

Squire, J. P.↗
NASA NTRS2001

The UAH Spinning Terrella Experiment: A Laboratory Analog for the Earth's Magnetosphere

The UAH Spinning Terrella Experiment has been modified to include the effect of a second magnet. This is a simple laboratory demonstration of the well-known double-dipole approximation to the Earth's magnetosphere. In addition, the magnet has been biassed $\sim$-400V which generates a DC glow discharge and traps it in a ring current around the magnet. This ring current is easily imaged with a digital camera and illustrates several significant topological properties of a dipole field. In particular, when the two dipoles are aligned, and therefore repel, they emulate a northward IMF Bz magnetosphere. Such a geometry traps plasma in the high latitude cusps as can be clearly seen in the movies. Likewise, when the two magnets are anti-aligned, they emulate a southward IMF Bz magnetosphere with direct feeding of plasma through the x-line. We present evidence for trapping and heating of the plasma, comparing the dipole-trapped ring current to the cusp-trapped population. We also present a peculiar asymmetric ring current produced in by the plasma at low plasma densities. We discuss the similarities and dissimilarities of the laboratory analog to the collisionless Earth plasma, and implications for the interpretation of IMAGE data.

Sheldon, R. B.↗
NASA NTRS1984

Fourier analysis of polar cap electric field and current distributions

A theoretical study of high-latitude electric fields and currents, using analytic Fourier analysis methods, is conducted. A two-dimensional planar model of the ionosphere with an enhanced conductivity auroral belt and field-aligned currents at the edges is employed. Two separate topics are treated. A field-aligned current element near the cusp region of the polar cap is included to investigate the modifications to the convection pattern by the east-west component of the interplanetary magnetic field. It is shown that a sizable one-cell structure is induced near the cusp which diverts equipotential contours to the dawnside or duskside, depending on the sign of the cusp current. This produces characteristic dawn-dusk asymmetries to the electric field that have been previously observed over the polar cap. The second topic is concerned with the electric field configuration obtained in the limit of perfect shielding, where the field is totally excluded equatorward of the auroral oval. When realistic field-aligned current distributions are used, the result is to produce severely distorted, crescent-shaped equipotential contours over the cap. Exact, analytic formulae applicable to this case are also provided.

Barbosa, D. D.↗
NASA NTRS1994

Low-latitude boundary layer near noon: An open field line model

We propose that many features of the cusp and low-latitude boundary layer (LLBL) observed near noon MLT can be explained by interpreting the LLBL as being on open lines with an inner boundary at the separatrix between open and closed magnetic field lines. This interpretation places the poleward boundary of the LLBL and equatorward boundary of the cusp along the field line that bifurcates at the cusp neutral point. The interpretation accounts for the abrupt boundary of magnetosheath particles at the inner edge of the LLBL, a feature that is inconsistent with LLBL formation by diffusion onto closed field lines, and for the distribution of magnetosheath particles appearing more as one continuous region than as two distinct regions across the noon cusp/LLBL boundary. Furthermore, we can explain the existence of energetic radiation belt electrons and protons with differing pitch angle distributions within the LLBL and their abrupt cutoff at the poleward boundary of the LLBL. By modeling the LLBL and cusp region quantitatively, we can account for a hemispherical difference in the location of the equatorial boundary of the cusp that is observed to be dependent on the dipole tilt angle but not on the interplanetary magnetic field (IMF) x component. We also find important variations and hemispherical differences in that the size of the LLBL that should depend strongly upon the x component of the IMF. This prediction is observationally testable. Finally, we find that when the IMF is strongly northward, the LLBL may include a narrow region adjacent to the magnetopause where field lines are detached (i.e., have both ends connected to the IMF).

Lyons, L. R.↗
NASA NTRS2003

Structure of the Outer Cusp and Sources of the Cusp Precipitation during Intervals of a Horizontal IMF

The cusp represents a place where the magnetosheath plasma can directly penetrate into the magnetosphere. Since the main transport processes are connected with merging of the interplanetary and magnetospheric field lines, the interplanetary magnetic field (IMF) Orientation plays a decisive role in the formation of the high-altitude cusp. The importance of the sign of the IMF Bz component for this process was suggested about 40 years ago and later it was documented by many experimental investigations. However, situations when IMF Bz is the major IMF component are rather rare. The structure of the cusp during periods of a small IMF BZ is generally unknown, probably due to the fully 3-D nature of the interaction. The present case study reveals the importance of horizontal IMF components on the global magnetospheric configuration as well as on small-scale processes at the cusp-magnetosheath interface. We have used simultaneous measurements of several spacecraft (ISTP program) operating in different regions of interplanetary space and two closely spaced satellites (INTERBALL-1/MAGION- 4) crossing the cusp-magnetosheath boundary to show the connection between the short- and large-scale phenomena. In the northern hemisphere, observations suggest a presence of two spots of cusp-like precipitation supplied by reconnection occurring simultaneously in both hemispheres. A source of this bifurcation is the positive IMF By component further enhanced by the field draping in the magnetosheath. This magnetic field component shifts the entry point far away from the local noon but in opposite sense in either hemisphere.

Nemecek, Z.↗
NASA NTRS1985

Definition of the cusp

The history of the dayside or polar cusp is reviewed briefly, beginning with the work of Chapman and Ferraro, Axford and Hines, and Dungey. Solar wind plasma comes down the cusp to low altitudes; the remarkably broad area over which this penetration occurs prompted the use of the term cleft, apparently at the foot of the entry layer inside the magnetopause. The consensus of discussion at the workshop was that cusp should refer mainly to the narrow feature in the magnetic profile. Cleft would refer to the broader region defined by the solar wind plasma (perhaps somewhat modified by acceleration or retardation processes); distinctive features observable in the ionosphere or on the ground are photo-emission, enhanced plasma density and temperature, plasma convection, electromagnetic waves, and turbulence.

Heikkila, W. J.↗
NASA NTRS1993

Dynamics explorer data analysis

Work in the following areas is discussed: plasma physics of the auroral acceleration region; electrodynamic coupling as a function of substorm phase and interplanetary magnetic field (IMF) direction; and particle injection in the magnetospheric cusp.

Reiff, Patricia H.↗
NASA NTRS1975

Interplanetary magnetic field direction and the configuration of the day side magnetosphere

The direction of the interplanetary magnetic field appears to have a significant effect on the equilibrium size of the day side magnetosphere. The qualitative features of this effect are illustrated here by means of a simple vacuum model that ignores solar wind and ionospheric plasma densities as a first approximation. This idealized model produces earthward displacements of the subsolar magnetopause and equatorward displacements of the polar cusp associated with southward-turning interplanetary fields that are about a factor of 2 greater than observed displacements. The nonzero solar wind plasma pressure and ionospheric conductivity produce large deviations from the simple vacuum configurations but do not change the qualitative conclusions regarding magnetopause and cusp displacements associated with the north-south component of the interplanetary magnetic field.

Hill, T. W.↗
NASA NTRS1992

A theory for narrow-banded radio bursts at Uranus - MHD surface waves as an energy driver

A possible scenario for the generation of the narrow-banded radio bursts detected at Uranus by the Voyager 2 planetary radio astronomy experiment is described. In order to account for the emission burstiness which occurs on time scales of hundreds of milliseconds, it is proposed that ULF magnetic surface turbulence generated at the frontside magnetopause propagates down the open/closed field line boundary and mode-converts to kinetic Alfven waves (KAW) deep within the polar cusp. The oscillating KAW potentials then drive a transient electron stream that creates the bursty radio emission. To substantiate these ideas, Voyager 2 magnetometer measurements of enhanced ULF magnetic activity at the frontside magnetopause are shown. It is demonstrated analytically that such magnetic turbulence should mode-convert deep in the cusp at a radial distance of 3 RU.

Farrell, W. M.↗
NASA NTRS1994

Statistical survey of pitch angle distributions in core (0-50 eV) ions from Dynamics Explorer 1: Outflow in the auroral zone, polar cap, and cusp

Core (0-50 eV) ion pitch angle measurements from the retarding ion mass spectrometer on Dynamics Explorer 1 are examined with respect to magnetic disturbance, invariant latitude, magnetic local time, and altitude for ions H(+), He(+), O(+), M/Z = 2 (D(+) or He(++)), and O(++). Included are outflow events in the auroral zone, polar cap, and cusp, separated into altitude regions below and above 3 R(sub E). In addition to the customary division into beam, conic, and upwelling distributions, the high-latitude observations fall into three categories corresponding to ion bulk speeds that are (1) less than, (2) comparable to, or (3) faster than that of the spacecraft. This separation, along with the altitude partition, serves to identify conditions under which ionospheric source ions are gravita- tionally bound and when they are more energetic and able to escape to the outer magnetosphere. Features of the cleft ion fountain inferred from single event studies are clearly identifiable in the statistical results. In addition, it is found that the dayside pre-noon cleft is a dayside afternoon cleft, or auroral zone, becomes an additional source for increased activity. The auroral oval as a whole appears to be a steady source of escape velocity H(+), a steady source of escape velocity He(+) ions for the dusk sector, and a source of escape velocity heavy ions for dusk local times primarily during increased activity. The polar cap above the auroral zone is a consistent source of low-energy ions, although only the lighter mass particles appear to have sufficient velocity, on average, to escape to higher altitudes. The observations support two concepts for outflow: (1) The cleft ion fountain consists of ionospheric plasma of 1-20 eV energy streaming upward into the magnetosphere where high-latitude convection electric fields cause poleward dispersion. (2) The auroral ion fountain involves field-aligned beams which flow out along auroral latitude field lines; and, in addition, for late afternoon local times, they experience additional acceleration such that the ion energy distribution tends to exceed the detection range of the instrument (greater than 50-60 eV).

Giles, B. L.↗
NASA NTRS1975

Magnetospheric physics - Magnetic fields

A report on progress on magnetospheric magnetic fields during the years from 1971 to 1974 is presented. The topology of the magnetosphere is considered along with the bow shock, upstream phenomena, the magnetosheath and magnetopause, the polar cusp, and the tail and the plasma sheet. Attention is given to the outer magnetosphere and magnetic field models, questions of convection, field-aligned currents and electric fields, and aspects of merging.

Russell, C. T.↗
NASA NTRS1982

Initial results on the correlation between the magnetic and electric fields observed from the DE-2 satellite in the field-aligned current regions

Initial results of the electric and magnetic field observations from the DE-2 satellite show a remarkably good correlation between the north-south component of the electric field and the east-west component of the magnetic field in many passes of the field-aligned current regions. For a dayside cusp pass on August 15, 1981 the coefficient of correlation between these components was 0.996. A preliminary inspection of the available data from the first 6 months of the DE operation indicates that the similarity between the electric and magnetic field signatures of the field-aligned currents is a commonly observed feature at all local times. This high correlation is interpreted to be an indication that the closure of the field-aligned current is essentially meridional. When the correlation between these components is not good, the closure current is likely to be flowing along the auroral belt. When the correlation between the electric and magnetic fields is high, it is possible to estimate the height-integrated Pedersen conductivity from the observed field components.

Sugiura, M.↗
NASA NTRS1992

A global magnetohydrodynamic simulation of the magnetosheath and magnetosphere when the interplanetary magnetic field is northward

We have used a new high-resolution global magnetohydrodynamic simulation model to investigate the configuration of the magnetosphere when the interplanetary magnetic field (IMF) is northward. For northward IMF the magnetospheric configuration is dominated by magnetic reconnection at the tail lobe magnetopause tailward of the polar cusp. This results in a local thickening of the plasma sheet equatorward of the region of reconnection and the establishment of a convection system with two cells in each lobe. In the magnetosheath the plasma density and pressure decrease near the subsolar magnetopause, forming a depletion region. Along the flanks of the magnetosphere the magnetosheath flow is accelerated to values larger than the solar wind velocity. The magnetopause shape from the simulations is consistent with the empirically determined shape.

Ogino, Tatsuki↗
NASA NTRS2001

A Co-Investigator Proposal for the Cornell University CAPER Rocket: Cleft Accelerated Plasma Experiment Rocket

The objectives are: 1) To support the Cornell wave instruments in a study of dayside ion outflow. 2) In conjunction with the U. of Alaska ground optical data, a prime objective was to measure the topside Cusp ion spectra responsible for dayside proton aurora. If such a correlative measurement could be made, then monitoring of this cusp precipitation from the ground could be routinely achieved. The nature of Cusp ions has and will continue to provide information about dayside magnetic reconnection. 3) A third objective was to study the dayside microburst electron precipitation from the BPS/CPS population. The Scifer rocket flight showed a bursty electron population at 1000 km altitude correlated closely with pulsation ground aurora on closed field lines. The frequency of this pulsating aurora is about 1 Hz. Ground pulsation measurements have recorded dayside bursts of Pc1 waves which could very well be the source of the electrons responsible for the pulsating aurora. In support of the Caper flight, UNH provided ground induction antennas to measure the equatorial Pc1 waves that might be dumping trapped electrons from the Central Plasma Sheet population. The Caper flight was launched on the last day of the window and all the Cornell and UNH instrumentation worked perfectly. Unfortunately the rocket trajectory flew very far to the west of the ground site at Longyearbyen missing conjugacy by several hundred kilometers. This meant the intended aurora was not crossed and all the ground experiments were far from being near the foot print of the rocket ruling out correlative science. The "miss" was primarily due to a decision during the countdown by the Andoya Rocket Range to move the azimuth of the rocket to the west to avoid Norwegian fishing boats at the splash point of the first two stages, and to make matters worse, the dispersion of the fourth stage of the rocket added entirely in this direction. Although no publications have resulted from the UNH data up to this point, a joint publication with the U. of Alaska group using both the Scifer and Caper data is in preparation. The data will continue to be worked on. There is interest in the Caper data as evidence by two recent papers presented at the "Low Latitude Boundary Layer Chapman Conference" held in New Orleans, Louisiana in April 2001.

Arnoldy, Roger L.↗
NASA NTRS1989

Plasma observations near Neptune - Initial results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Belcher, J. W.↗
NASA NTRS2007

Recent Plasma Observations Related to Magnetic Merging and the Low-Latitude Boundary Layer. Case Study by Polar, March 18, 2006

We have begun an investigation of the nature of the low-latitude boundary layer in the mid-altitude cusp region using data from the Polar spacecraft. Magnetosheath-like plasma is frequently observed deep (in terms of distance from the magnetopause and in invariant latitude) in the magnetosphere. One such case, taken during a long period of northward interplanetary magnetic field (IMP) on March 18, 2006, shows injected magnetosheath ions within the magnetosphere with velocity distributions resulting from two separate merging sites along the same field lines. Cold ionospheric ions were also observed counterstreaming along the field lines, evidence that these field lines were closed. Our results support the idea of double reconnection under northward IMP on the same group of field lines can provide a source for the LLBL. However, the flow direction of the accelerated magnetosheath ions antiparallel to the local magnetic field and given location of the spacecraft suggest that these two injection sites are located northward of the spacecraft position. Observed convection velocities of the magnetic field lines are inconsistent with those expected for double post-cusp reconnection in both hemispheres. These observations favor a scenario in which a group of newly closed field lines was created by a combination of high shear merging at high latitudes in the northern hemisphere and low shear merging at lower latitudes at the dayside magnetopause.

Chandler, M.↗
NASA NTRS1972

Imp 5 magnetic-field measurements in the high-latitude outer magnetosphere near the noon meridian.

Results of Imp 5 magnetic-field measurements at geomagnetic latitudes up to 75 deg and at distances beyond six earth radii, revealing the permanent existence of a broad depressed field region centered on the polar or dayside cusp. Field strengths at seven earth radii on cusp field lines that connect to the earth are typically only 50 to 70% of that of an undistorted dipole field. The transition region between the magnetosheath and the point where the fields are clearly of dipolar origin is characterized by large-amplitude fluctuations and the lack of a clear magnetopause boundary. Magnetic-field perturbations are observed in the cusp region with magnitudes up to 45 gamma and in directions that are approximately perpendicular to the average field. These perturbations are suggestive of field-aligned currents, and their magnitudes are consistent with the low-altitude measurements obtained on polar-orbiting spacecraft.

Fairfield, D. H.↗
NASA NTRS1972

Review of magnetic field observations.

Recent observations in previously unexplored regions of the magnetosphere, particularly in the polar-cusp region, compliment and reinforce emphasis on particle access to the plasma sheet via the polar neutral points. Significant distortions of the geomagnetic field in the polar-cusp region suggest field-aligned currents at large geocentric distances which can be related to low-altitude polar-cap phenomena. Studies of the microstructure of the field reversal region of the plasma sheet embedded in the geomagnetic tail suggest a periodic structure of more complexity than earlier assumed simplified single neutral-line models.

Ness, N. F.↗