D. L. Carpenter

Multisatellite observations of rapid subauroral ion drifts (SAID)

We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (∼9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field-aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field-aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field-aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field-aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the right sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.

The earth's plasmasphere

Preface Foreword Introduction 1. Discovery of the plasmasphere and initial studies of its properties 2. Electromagnetic sounding of the plasmasphere 3. Plasmasphere measurements from spacecraft 4. A global description of the plasmasphere 5. Theoretical aspects related to the plasmasphere Epilogue References Index.

The role of the ionosphere in coupling upstream ULF wave power into the dayside magnetosphere

A series of recent studies of Pc 3 magnetic pulsations in the dayside outer magnetosphere has given new insights into the possible mechanisms of entry of ULF wave power into the magnetosphere from a bow shock related upstream source. In this paper we first review many of these new observational results by presenting a comparison of data from two 10-hour intervals on successive days in April 1986 and then present a possible model for transmission of pulsation signals from the magnetosheath into the dayside magnetosphere. Simultaneous multi-instrument observations at South Pole Station, located below the cusp/cleft ionosphere near local noon, magnetic field observations by the AMPTE CCE satellite in the dayside outer magnetosphere, and upstream magnetic field observations by the IMP 8 satellite show clear interplanetary magnetic field field magnitude control of dayside resonant harmonic pulsations and band-limited very high latitude pulsations, as well as pulsation-modulated precipitation of what appear to be magnetosheath/boundary layer electrons. We believe that this modulated precipitation may be responsible for the propagation of upstream wave power in the Pc 3 frequency band into the high-latitude ionosphere, from whence it may be transported throughout the dayside outer magnetosphere by means of an “ionospheric transistor.” In this model, modulations in ionospheric conductivity caused by cusp/cleft precipitation cause varying ionospheric currents with frequency spectra determined by the upstream waves; these modulations will be superimposed on the Birkeland currents, which close via these ionospheric currents. Modulated region 2 Birkeland currents will in turn provide a narrow-band source of wave energy to a wide range of dayside local times in the outer magnetosphere.

Erosion and Recovery of the Plasmasphere in the Plasmapause Region

Understanding the basic plasmasphere erosion/recovery cycle remains a major, as yet largely unmet, challenge to the space science community. We do not yet have a description of the formation of a new plasmapause boundary, nor have we been able to map the evidently complex electric fields that develop at subauroral latitudes during the process of plasmasphere erosion. Density structure regularly observed in the plasmapause region suggests that instabilities play an as yet unassessed role in the erosion/recovery cycle. Electron density interior to a newly formed plasmapause boundary tends to be reduced by factors of up to 3 in association with the erosion process, so that refilling during recovery occurs there as well as in the more deeply depleted plasmatrough region beyond. The number of electrons lost from this interior region, apparently through interchange with the ionosphere, can be of order 50% of the number lost from beyond the new boundary through flow perpendicular to B. Evidence has been found that of order 20% of the plasma removed from the main plasmasphere during an erosion event remains in the outer afternoon-dusk magnetosphere for extended periods. It is not yet known whether eroded plasmas entering the Earths boundary layers make a geophysically important contribution to the plasma sheet. New insights into these and other important questions await both future photon and radio imaging of the plasmasphere from high altitude as well as continued work with certain excellent, as yet only partially exploited, satellite data sets.

First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz to 3 MHz radio sounder, incorporating modern digital processing techniques and long electronically-tuned antennas, that is flown to large radial distances into the high-latitude magnetosphere on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar-cap ionosphere by RPI even when IMAGE is located at geocentric distances up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron-density distributions from the polar-cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attributed to the cusp and the magnetopause have also been identified, those from the cusp have been identified more often and with greater confidence.

CRRES observations of density cavities inside the plasmasphere

Deep density troughs inside the plasmasphere in which electron density was a factor of from ∼2 to 10 below nearby plasmasphere levels were found in ∼13% of 1764 near-equatorial electron density profiles derived from the sweep frequency receiver data acquired in 1990–1991 by the CRRES satellite. These “inner troughs” appeared in the aftermath of plasmasphere erosion episodes and are interpreted as the near-equatorial manifestations of geomagnetic-field-aligned cavities. Inner troughs were found at all local times but were most common in the 1800–2400 magnetic local time (MLT) sector and least common between 0600 and 1200 MLT. Their inner boundaries, plasmapause-like in form, were mostly at L < 3.5 but in ∼30% of the cases were at L < 2.5 under geomagnetic conditions that traditionally have been associated with plasmapause radii in the L = 3–3.5 range or beyond. The trough outer walls were exceptionally steep, in several cases exhibiting a factor of 4 or more density change within less than 100 km along the near-equatorial satellite orbit. The extent of the troughs in L ranged from ΔL ∼ 0.5 to 2, and various forms of evidence, including earlier studies, suggest an extent of more than 20° in longitude. Such evidence includes plasma waves propagating in a free space mode within the inner trough while extending in frequency well above the upper limit of trapped continuum radiation detected beyond the plasmasphere. We suggest, as have previous authors, that the troughs are translated vestiges of plasma configurations established during preceding periods of plasmasphere erosion. In some such cases, dense plasma features lying beyond the troughs were probably connected to the main plasmasphere in a local time sector to the east of the observing longitude. However, in some of the cases of troughs with steep outer walls the dense plasma feature beyond that wall may have been shaped by a mechanism for detaching plasma from an originally larger outer plasmasphere, such as by shear flows in the premidnight sector associated with subauroral ion drifts.

The feasibility of radio sounding in the magnetosphere

A radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmasphere, as well as other important density features elsewhere in this region. Using digital integration and tuned reception at frequencies from a few kilohertz to a few megahertz with 400-m to 500-m tip-to-tip dipole antennas and 10 W transmitter power, such a sounder would be capable of 10% density resolution and 500 to 1300 km spatial resolution in only a few minutes at distances of up to 4 RE. By providing such detailed observations of its principal density structures, such a sounder would then clearly revolutionize magnetospheric research.

A case study of plasma structure in the dusk sector associated with enhanced magnetospheric convection

In a case study from June 8–9, 1982, data from ground whistler stations Siple and Halley, Antarctica, located at L ∼4.3 and spaced by ∼2 hours in MLT, and from satellites DE 1 and GEOS 2, have provided confirming evidence that the bulge region of the magnetosphere can exhibit an abrupt westward “edge,” as reported earlier from whistlers. The present data and previous MHD modeling work suggest that this distinctive feature develops during periods of steady or declining substorm activity, when dense plasma previously carried sunward under the influence of enhanced convection activity begins to rotate with the Earth at angular velocities that decrease with increasing L value and becomes spirallike in form. For the first time, whistler data have been used to identify a narrow dense plasma feature, separated from the main plasmasphere and extending sunward into the late afternoon sector at L values near the outer observed limits of the main plasmasphere bulge. The westward edge of the main bulge, found by both whistler stations to be at ∼1800 MLT, appeared to be quasi-stationary in Sun-Earth coordinates during the prevailing conditions of gradually declining geomagnetic agitation. It is possible that outlying dense plasma features such as the one observed develop as part of the process leading to the occurrence of the more readily detectable abrupt westward edge of the bulge. It was not possible in this case to determine the extent to which the outlying feature was smoothly attached to or isolated from the main bulge region.

In situ and ground‐based intercalibration measurements of plasma density at L = 2.5

[1] Two independent ground-based experiments and two satellite-borne experiments are used to interpret the changes in plasmaspheric composition at the same point in space during moderate geomagnetic activity on 22 January and 14 February 2001. Mass density at L = 2.5 was determined from an array of magnetometers on the Antarctic Peninsula, while the electron number density along the same flux tube was determined from analysis of the group delay of man-made VLF transmissions from north-east America. The IMAGE satellite RPI experiment provided in situ measurements of the electron number density in passing the equatorial region of the same field line, while the EUV Imager experiment was able to resolve the He+ abundance by looking back toward the same place a few hours later. On 22 January 2001 all measurements were consistent with a moderately disturbed plasmasphere. On 14 February 2001 there appeared to be a significant response of the plasmasphere to the moderate (Kp = 5) activity levels. Both the electron number density and the mass density determined from the ground-based experiments were markedly higher than on 22 January 2001. Also, the IMAGE RPI gave a markedly lower electron number density than did the ground-based data; this is explained by differences in the longitude at which the measurements were made and the presence of localized plasmaspheric structures. At Antarctic Peninsula longitudes a He+ column abundance value of 6 × 1010 cm-2 is found to be equivalent to plasmaspheric electron density levels of 3000 cm-3 at L = 2.5. For these conditions the He+ mass abundance was about 12–16% compared with H+. Both decreases and increases in the He+ column abundance measured by the EUV Imager appear to be linearly correlated to changes in the percentage occurrence of He+ as determined from a combination of ground-based VLF and ULF observations.

Active Wave Experiments in Space Plasmas: The Z Mode

The term Z mode is space physics notation for the low-frequency branch of the extraordinary (X) mode. It is an internal, or trapped, mode of the plasma confined in frequency between the cutoff frequency fz and the upper-hybrid fre- quency fuh which is related to the electron plasma frequency fpe and the electron cyclotron frequency fce by the expression f 2 uh = f 2 pe + f 2 ce; fz is a function of fpe and fce. These characteristic frequencies are directly related to the electron number density Ne and the magnetic field strength |B|, i.e., fpe(kHz) 2 ≈ 80.6Ne(cm −3 )a nd fce(kHz) 2 ≈ 0.028|B|(nT). The Z mode is further classified as slow or fast depending on whether the phase velocity is lower or higher than the speed of light in vacuum. The Z mode provides a link between the short wavelength λ (large wave number k =2 π/λ ) electrostatic (es) domain and the long λ (small k) electromagnetic (em) domain. An understanding of the generation, propagation and reception of Z-mode waves in space plasma leads to fundamental information on wave/particle interac- tions, Ne, and field-aligned Ne irregularities (FAI) in both active and passive wave experiments. Here we review Z-mode observations and their interpretations from both radio sounders on rockets and satellites and from plasma-wave receivers on satellites. The emphasis will be on the scattering and ducting of sounder-generated Z-mode waves by FAI and on the passive reception of Z-mode waves generated by natural processes such as Cherenkov and cyclotron emission. The diagnostic applica- tions of the observations to understanding ionospheric and magnetospheric plasma processes and structures benefit from the complementary nature of passive and ac- tive plasma-wave experiments.