I. Roth

FAST satellite observations of large‐amplitude solitary structures

We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC

The authors model the rapid ([approximately]1 min) formation of a new electron radiation belt at L [approx equal] 2.5 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the CRRES satellite. Guided by the observed electric and magnetic fields, the authors represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earths magnetic field. The authors follow the electrons using a relativistic guiding center code. The test-particle simulations show that electrons with energies of a few MeV at L > 6 were energized up to 40 MeV and transported to L [approx equal] 2.5 during a fraction of their drift period. The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field. Their simulation results, with an initial W[sup [minus]8] energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT. 121 refs., 5 figs.

Modulated electron-acoustic waves in auroral density cavities : FAST observations

We report on FAST observations of large amplitude (up to 500 mV m−1) envelope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot (∼keV) component and a minority cold (<60 eV) component. The nonlinear waves are recorded when the spacecraft passes the base of the parallel auroral acceleration region. They form intense packets of electron acoustic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 100 km s−1. They may play a crucial role in the acceleration processes taking place in these regions.

Simulations of radiation belt formation during storm sudden commencements

MHD fields from a global three-dimensional simulation of the great March 24, 1991, storm sudden commencement (SSC) are used to follow the trajectories of particles in a guiding center test particle simulation of radiation belt formation during this event. Modeling of less intense events during the lifetime of the CRRES satellite, with similar morphology but less radial transport and energization, is also presented. In all cases analyzed, a solar proton event was followed by an SSC, leading to the formation of a new proton belt earthward of solar proton penetration. The effect on particle energization of varying solar wind and model pulse parameters is investigated. Both a seed population of solar protons and the SSC shock-induced compression of the magnetosphere are necessary conditions for the formation of a new proton belt. The outer boundary of the inner zone protons can be affected by an SSC and a newly formed belt can be affected by the ensuing or a subsequent storm, which may occur in rapid succession, as was the case in June and July 1991. The acceleration process is effective for both northward and southward IMF, with more energization and inward radial transport for the southward case for otherwise comparable solar wind parameters, because of the initially more compressed magnetopause in the southward case. The inner boundary and stability of the newly formed belt depends on the magnitude of radial transport at the time of formation and subsequent ring current perturbation of adiabatic trapping.

Phase-space electron holes along magnetic field lines

Recent observations from satellites crossing active magnetic field lines have revealed solitary potential structures that move at speeds substantially greater than the ion thermal velocity. The structures appear as positive potential pulses rapidly drifting along the magnetic field. We interpret them as BGK electron holes supported by a population of trapped and passing electrons. Using Laplace transform techniques, we analyse the behavior of one phase-space electron hole. The resulting potential shapes and electron distribution functions are self-consistent and compatible with the field and particle data associated with the observed pulses. In particular, the spatial width increases with increasing amplitude. The stability of the analytic solution is tested by means of a two-dimensional particle-in-cell simulation code with open boundaries. We also use our code to briefly investigate the influence of the ions. The nonlinear structure appears to be remarkably resilient.

Simulation of proton radiation belt formation during the March 24, 1991 SSC

The rapid formation of a new proton radiation belt at L ≃ 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole magnetic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20–80 MeV range, primary contribution is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic conservation of the first adiabatic invariant leads to greater energization of protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations.

First detection of a terrestrial MeV X-ray burst

We report the first detection of a terrestrial X-ray burst extending up to MeV energies, made by a liquid-nitrogen-cooled germanium detector (∼2 keV FWHM resolution) on a high-altitude balloon at 65.5° magnetic latitude (L=5.7) in the late afternoon (1815 MLT) during low geomagnetic activity. The burst occurred at 1532–1554 UT on August 20, 1996, and consisted of seven peaks of ∼60–90 s duration, spaced by ∼100–200 s, with quasi-periodic (∼10–20 s) modulation of the peak count rates. The very hard X-ray spectrum extends to the instrumental limit of 1.4 MeV, and is consistent with bremsstrahlung emission from monoenergetic, ∼1.7 MeV, precipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the precipitation mechanism appears to be highly energy selective. The modulation frequencies suggest scattering of the MeV electrons due to gyro-resonance with Doppler-shifted electromagnetic ion cyclotron waves, but either equatorial proton densities a factor of ∼10² higher than typical for the plasmasphere or significant O+ densities would be required.

Enrichment of 3He and Heavy Ions in Impulsive Solar Flares

The acceleration of 3He and heavy ions by electromagnetic hydrogen cyclotron waves in a direct single-stage process in impulsive solar flares is investigated analytically and with the help of test particle simulations. We illustrate in detail the mechanism by which a single monochromatic wave can accelerate such ions to MeV and even GeV energies. While somewhat idealized, a monochromatic wave well illustrates the importance of the background magnetic field gradient in the acceleration process. An interesting result of our analysis shows that the acceleration rate is proportional to the magnitude of the magnetic field gradient and is independent of the wave amplitude, while the maximum energy gained increases with decreasing magnetic field gradient and increasing wave amplitude. Heavy ions can also be accelerated by these electromagnetic hydrogen cyclotron waves in a single-stage process by the second or higher harmonic resonance. The acceleration of heavier ions by the same mechanism raises the question of their low enrichment in comparison to much higher enrichment of 3He. The solution is related to the initial small acceleration of the thermal heavy ions at the higher harmonic resonances. The enrichment of the heavy ions increases with the amplitude of the electromagnetic waves and decreases with the plasma density due to Coulomb collisions and absorption of wave energy. Comparison between the rate of cooling of thermal heavy ions due to collisions and heating by waves gives wave intensity and heavy ion ratios which are consistent with observations. The relation between the accelerated heavy ion abundances and their gyrofrequencies in the corona is used to estimate the temperature in the acceleration region. The existence of electromagnetic hydrogen cyclotron waves in flare plasmas is supported by observations in auroral plasmas and by solution of the dispersion relation, which shows that such waves can propagate over long distances along coronal magnetic fields.

Observation of relativistic electron microbursts in conjunction with intense radiation belt whistler-mode waves

We present multi-satellite observations indicating a strong correlation between large amplitude radiation belt whistler-mode waves and relativistic electron precipitation. On separate occasions during the Wind petal orbits and STEREO phasing orbits, Wind and STEREO recorded intense whistler-mode waves in the outer nightside equatorial radiation belt with peak-to-peak amplitudes exceeding 300 mV/m. During these intervals of intense wave activity, SAMPEX recorded relativistic electron microbursts in near magnetic conjunction with Wind and STEREO. The microburst precipitation exhibits a bursty temporal structure similar to that of the observed large amplitude wave packets, suggesting a connection between the two phenomena. Simulation studies corroborate this idea, showing that nonlinear wave--particle interactions may result in rapid energization and scattering on timescales comparable to those of the impulsive relativistic electron precipitation.

Loss of ring current O(+) ions due to interaction with Pc 5 waves

The behavior of ring current ions in low-frequency geomagnetic pulsations is investigated analytically and numerically. We focus primarily on ring current O+ ions, whose flux increases dramatically during geomagnetic storms and decays at a rate which is not fully explained by collisional processes. This paper presents a new loss mechanism for the O+ ions due to the combined effects of convection and corotation electric fields and interaction with Pc 5 waves (wave period: 150–600 s) via a magnetic drift-bounce resonance. A test particle code has been developed to calculate the motion of the ring current O+ ions in a time-independent dipole magnetic field, and convection and corotation electric fields, plus Pc 5 wave fields, for which a simple analytical model has been formulated based on spacecraft observations. For given fields, whether a particle gains or loses energy depends on its initial kinetic energy, pitch angle at the equatorial plane, and the position of its guiding center with respect to the azimuthal phase of the wave. The ring current O+ ions show a dispersion in energies and L values with decreasing local time across the day side, and a bulk shift to lower energies and higher L values. The former is due to the wave-particle interaction causing the ion to gain or lose energy, while the latter is due to the convection electric field. Our simulations show that, due to interaction with the Pc 5 waves, the particles kinetic energy can drop below that required to overcome the convection potential and the particle will be lost to the dayside magnetopause by a sunward E × B drift. This may contribute to the loss of O+ ions at intermediate energies (tens of keV) observed during the recovery phase of geomagnetic storms.