Jolene S. Pickett

POLAR observations of coherent electric field structures

The POLAR plasma wave instrument often detects coherent electric field structures in the high altitude polar magnetosphere. The structures appear to be positively charged potentials which are found to move both up and down the ambient magnetic field. Typical estimated velocities and parallel scale sizes are the order of 1000 km/s and 100-1000 meters, respectively. We have observed the structures at radial distances of 2.02 to 8.5 Re and L shells of 6 - 12+, although the they are likely to occur over a broader range of space than suggested by this initial study. The structures are responsible for some of the the spectral features of broadband electrostatic noise, and are similar to recent GEOTAIL and FAST observations of solitary waves.

Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere

The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new events showed several unusual features: (1) They were detected in the dayside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individual event could last for hours; (3) the measured helium ion had energies up to and many times in excess of 2.4 MeV; (4) the intensity of 1-200 KeV/e helium was anticorrelated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic energy density; (5) the events were associated with an enhancement of the low-frequency magnetic noise, the spectrum of which typically extends from a few hertz to a few hundreds of hertz as measured by the Plasma Wave Instrument (PWI) on Polar; and (6) a seasonal variation was found for the occurrence rate of the events with a maximum in September. These characterized a new phenomenon which we are calling cusp energetic particle (CEP) events. The observed high charge state of helium and oxygen ions in the CEP events indicates a solar source for these particles. Furthermore, the measured 0.52-1.15 MeV helium flux was proportional to the difference between the maximum and the minimum magnetic field in the event. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated with the high-altitude dayside cusp. These observations represent a potential discovery of a major acceleration region of the magnetosphere.

Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss

We interpret observations of low-altitude electromagnetic ELF hiss observed on the dayside at subauroral latitudes. A divergent propagation pattern has been reported between 50° and 75° of geomagnetic latitude. The waves propagate with downward directed wave vectors which are slightly equatorward inclined at lower magnetic latitudes and slightly poleward inclined at higher latitudes. Reverse ray tracing using different plasma density models indicates a possible source region near the geomagnetic equator at a radial distance between 5 and 7 Earth radii by a mechanism acting on highly oblique wave vectors near the local Gendrin angle. We analyze waveforms received at altitudes of 700–1200 km by the Freja and DEMETER spacecraft and we find that low-altitude ELF hiss contains discrete time-frequency structures resembling wave packets of whistler mode chorus. Emissions of chorus also predominantly occur on the dawnside and dayside and have recently been considered as a possible source of highly accelerated electrons in the outer Van Allen radiation belt. Detailed measurements of the Cluster spacecraft at radial distances of 4–5 Earth radii show chorus propagating downward from the source region localized close to the equator. The time-frequency structure and frequencies of chorus observed by Cluster along the reverse raypaths of ELF hiss are consistent with the hypothesis that the frequently observed dayside ELF hiss is just the low-altitude manifestation of natural magnetospheric emissions of whistler mode chorus.

Isolated electrostatic structures observed throughout the Cluster orbit: relationship to magnetic field strength

Abstract. Isolated electrostatic structures are observed throughout much of the 4RE by 19.6RE Cluster orbit. These structures are observed in the Wideband plasma wave instruments waveform data as bipolar pulses (one positive and one negative peak in the electric field amplitude) and tripolar pulses (two positive and one negative peak, or vice versa). These structures are observed at all of the boundary layers, in the solar wind and magnetosheath, and along auroral field lines at 4.5-6.5RE. Using the Wideband waveform data from the various Cluster spacecraft we have carried out a survey of the amplitudes and time durations of these structures and how these quantities vary with the local magnetic field strength. Such a survey has not been carried out before, and it reveals certain characteristics of solitary structures in a finite magnetic field, a topic still inadequately addressed by theories. We find that there is a broad range of electric field amplitudes at any specific magnetic field strength, and there is a general trend for the electric field amplitudes to increase as the strength of the magnetic field increases over a range of 5 to 500nT. We provide a possible explanation for this trend that relates to the structures being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding dependence of the duration of the structures on the magnetic field strength, although a plot of these two quantities reveals the unexpected result that with the exception of the magnetosheath, all of the time durations for all of the other regions are comparable, whereas the magnetosheath time durations clearly are in a different category of much smaller time duration. We speculate that this implies that the structures are much smaller in size. The distinctly different pulse durations for the magnetosheath pulses indicate the possibility that the pulses are generated by a mechanism which is different from the mechanism operating in other regions.

Structure of the separatrix region close to a magnetic reconnection X-line: Cluster observations

We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away from the X-line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X-line is up to 50 ion inertial lengths away.

Plasma waves in the dayside polar cap boundary layer: Bipolar and monopolar electric pulses and whistler mode waves

We report four different types of plasma waves detected in and near the dayside polar cap boundary layer (PCBL) region at high altitudes (>6 RE). One wave type is narrowband whistler-mode emission at frequencies just below fce (5.5 kHz). These emissions could be locally generated by resonant wave-particle interactions involving an electron beam of ∼100 eV energy. A second type is a low frequency (200–300 Hz) whistler mode wave, which may be locally generated by ∼25 keV electrons or ∼45 keV ions. It is also possible that these latter waves are generated at low altitudes near the ionosphere and then converted from the ion cyclotron mode into whistler-mode during propagation from the generation region to the spacecraft. Two further types of waves are large-amplitude bipolar and monopolar solitary “electrostatic” waves. The bipolar wave structures are possibly generated all along the magnetic field lines in the field-aligned current regions (at all local times). The monopolar structures could be evolved bipolar solitary waves. A one-d schematic is presented to explain the paired monopolar structures as a result of splitting of an electron hole into two parts.

Plasma Diagnostics Package Initial Assessment of the Shuttle Orbiter Plasma Environment

A primary objective of the Plasma Diagonostics Package (PDP) on the third Space Shuttle flight (STS-3) was to assess aspects of the Orbiters induced gaseous, plasma and electrical environment with respect to the conduct of scientific investigations. Instrumentation temperatures were found to be within predicted limits, payload bay pressure varied from ambient (10 ~7 Torr) up to almost 10 ~3 Torr with thruster firings, electromagnetic interference (EMI) levels were found to be below worst-case estimates but included Orbiter-induced electrostatic noise, and Orbiter potential was consistent to first order with VxB motional potentials varying ±5 V with respect to the plasma. Electrostatic noise, neutral pressure and potential all exhibited orbit-period modulation. Payload bay plasma varied in density and composition from ambient to a rarefied mixture with Orbiterproduced H2O+ . Energetic electrons and ions with energies up to 10s of electron volts were observed occasionally. Primary and vernier thrusters typically induce a momentary perturbation to the electron density, to the pressure, and to the electric field and spacecraft potential with low-energy ions and electrons occasionally observed. With the PDP on the remote manipulator system (RMS), both automode and manual modes were used to seek sources of EMI, to characterize the Orbiters plasma wake, and to measure beam-plasma phenomena.

Extremely intense ELF magnetosonic waves: A survey of polar observations

A Polar magnetosonic wave (MSW) study was conducted using 1 year of 1996–1997 data (during solar minimum). Waves at and inside the plasmasphere were detected at all local times with a slight preference for occurrence in the midnight-postmidnight sector. Wave occurrence (and intensities) peaked within~±5° of the magnetic equator, with half maxima at ~±10°. However, MSWs were also detected as far from the equator as +20° and 60° MLAT but with lower intensities. An extreme MSW intensity event of amplitude Bw = ~± 1 nT and Ew = ~± 25 mV/m was detected. This event occurred near local midnight, at the plasmapause, at the magnetic equator, during an intense substorm event, e.g., a perfect occurrence. These results support the idea of generation by protons injected from the plasma sheet into the midnight sector magnetosphere by substorm electric fields. MSWs were also detected near noon (1259 MLT) during relative geomagnetic quiet (low AE). A possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, in turn leading to ion instability. The wave magnetic field components are aligned along the ambient magnetic field direction, with the wave electric components orthogonal, indicating linear wave polarization. The MSW amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes (hiss) increased. We argue that intense MSWs are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.

Propagation analysis of plasmaspheric hiss using Polar PWI measurements

We have analyzed high-rate waveform data, taken by the POLAR Plasma Wave Instrument at high altitudes in the equatorial plasmasphere, to study plasmaspheric hiss in the range of frequencies between 100 Hz and several kHz. These emissions are found almost everywhere in the plasmasphere, and their origin is still controversial. Our analysis of several cases shows that most of the waves were propagating more or less parallel to the Earths magnetic field, but sometimes a few of them were propagating obliquely with their normals near the Gendrin angle. Evidence of amplification was found near the geomagnetic equator. The results suggest that waves with normals both parallel and anti-parallel to the magnetic field were being amplified by the classical mechanism that involves gyroresonant interaction with energetic electrons.

Auroral zone dayside precipitation during magnetic storm initial phases

Abstract Significant charged-particle precipitation occurs in the dayside auroral zone during and after interplanetary shock impingements on the Earths magnetosphere. The precipitation intensities and spatial and temporal evolution are discussed. Although the post-shock energy flux (10– 20 erg cm −2 s −1 ) is lower than that of substorms, the total energy deposition rate may be considerably greater (∼ an order of magnitude) than nightside energy rates due to the greater area of the dayside portion of the auroral oval (defined as extending from 03 MLT through noon to 21 MLT). This dayside precipitation represents direct solar wind energy input into the magnetosphere/ionosphere system. The exact mechanisms for particle energization and precipitation into the ionosphere are not known at this time. Different mechanisms are probably occurring during different portions of the storm initial phase. Immediately after shock compression of the magnetosphere, possible precipitation-related mechanisms are: (1) betatron compression of preexisting outer zone magnetospheric particles. The anisotropic plasma is unstable to loss-cone instabilities, leading to plasma wave growth, resonant particle pitch-angle scattering and electron and proton losses into the upper ionosphere. (2) The compression of the magnetosphere can also lead to enhanced field-aligned currents and the formation of dayside double-layers. Finally (3) in the latter stages of the storm initial phase, there is evidence for a long-lasting viscous-like interaction occurring on the flanks of the magnetopause. Ground-based observations identifying the types of dayside auroral forms would be extremely useful in identifying the specific solar wind energy transfer mechanisms.