E. J. Lund

Energy Dependence of the Ionic Charge State Distribution During the November 1997 Solar Energetic Particle Event

3 Abstract. Ionic charge state distributions for a variety of species, such as C, O, Ne, Mg, Si and Fe were obtained with the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) on ACE for the strongest of a series of energetic particle events after the November 4 and 7, 1997, flares. The capabilities of SEPICA allow a much more detailed analysis of the charge dis- tributions than previous instrumentation. Over the energy range from ≈ 0.2 to 1 MeV/Nuc a trend is observed that shows charge states increasing with energy, in particular for Mg, Si and Fe. In addition, for Fe a mixed charge state distribution with a distinct peak at lower charge states (10 - 14) is ob- served simultaneously with a tail reaching to charge states up to ≈ 20. This may be an indication of a mixture of different energetic particle populations. 1

FAST observations of preferentially accelerated He+ in association with auroral electromagnetic ion cyclotron waves

The TEAMS instrument on the FAST satellite has detected events in which He+ ions are resonantly accelerated perpendicular to the magnetic field to energies of several keV. The events occur in association with electromagnetic ion cyclotron (EMIC) waves and conic distributions of up to a few hundred eV in H+ and a few keV in O+. Concentrations of He+ can be significantly elevated during the events. Our interpretation is that the He+ ions are accelerated through a cyclotron resonance with the waves. This acceleration is similar to a proposed mechanism for selective ion acceleration in impulsive solar flares.

Transverse ion acceleration mechanisms in the aurora at solar minimum: occurrence distributions

Abstract We present a statistical study of 714 ion conic events detected by the Fast Auroral Snapshot Explorer (FAST) from September 1996 to February 1997. Ninety-nine percent of the events found are associated with either broad-band extremely low frequency (BBELF) emissions or electromagnetic ion cyclotron (EMIC) waves. Lower hybrid waves are much less important in transverse ion acceleration above 2000 km. The BBELF events are more numerous, comprising some 84% of ion conic events identified, and occur at all local times, with a peak near noon and a minimum near dusk. The EMIC events are concentrated in the dusk to midnight sector and are more likely to occur at lower latitudes compared to the BBELF events. The occurrence rate of EMIC conics has an apparent local minimum at 2000–2500 km, while the BBELF conic occurrence rate varies only slowly between 2000 km and the FAST apogee of 4200 km. The occurrence rate of EMIC conics is more strongly correlated with K p than the BBELF conic occurrence rate. These results are consistent with previous studies of ion conics at lower altitudes. The correlation of both BBELF and EMIC ion conics with phenomena that are associated with parallel electric fields suggests that parallel electric fields play a significant role in transverse ion heating in the aurora.

Species dependent energies in upward directed ion beams over auroral arcs as observed with FAST TEAMS

Upward flowing field-aligned ion beams over auroral arcs have been observed with the 3-dimensional ion mass spectrograph TEAMS on FAST. We have performed a statistical study on a sample of 77 ion beams from the auroral campaign in early 1997. All observed beams contain substantial amounts of H+, He+ and O+. A clear ordering of the total energies according to mass is found, with H+ having the lowest and O+ the highest energy. The composition varies significantly from beam to beam, with O+/H+ ratios ranging from ≈ 0.1 to 10. No variation of the energy ratio between species is observed as a function of relative abundance. These results are discussed in the light of earlier observations of higher energies for O+ in statistical studies of beams during solar minimum and attempts to explain this behavior in terms of beam instabilities.

Observation of electromagnetic oxygen cyclotron waves in a flickering aurora

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field-aligned Poynting fluxes of up to ∼10−5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground-based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.

On the generation and propagation of auroral electromagnetic ion cyclotron waves

On a recent sounding rocket flight, electromagnetic ELF waves below the proton gyrofrequency were detected in an auroral arc. These waves would be resonant with ƒcH+ or ƒcO+ in the auroral acceleration region; the latter waves are associated with modulated parallel electron fluxes. Following previous papers, we use homogeneous linear theory to calculate temporal and convective growth rates for electron beam-driven electromagnetic ion cyclotron (EMIC) waves in various plasmas with H+, He+, and O+ ions, although the inhomogeneity in the source region makes this approximation questionable. We find that all three EMIC modes are unstable with growth rates inversely proportional to the mass of the ion associated with the mode; maximum convective growth occurs at frequencies of 0.8–1.0Ωi. However, the growth rate of the O+ EMIC mode is too low to account for the observations unless relatively large beam densities are used, implying that nonlinear or inhomogeneous plasma effects must play an important role in the instability. We also assess the propagation of these EMIC waves in a cold plasma. Our ray tracing calculations show that propagation effects can explain both the localization of the O+ waves and the absence of He+ waves at lower altitudes. We also show that the wider latitudinal spread and low power spectral density of the H+ waves can also be explained by propagation effects, but the H+ rays which can reach the ionosphere are not the rays for which the beam-driven instability produces the highest growth rates in a homogeneous plasma.

Quasi-thermal fluctuations in a beam-plasma system

The quasi‐thermal electrostatic field fluctuations of a stable unmagnetized electron beam‐plasma system is considered. Both the beam and the background plasma are modeled as isotropic Maxwellians. Both the normal modes of the plasma and the contributions from other modes are considered. The dependence of these fluctuations on beam velocity, density, and temperature are examined. The implications of these waves for real beam‐plasma systems are also discussed.

Equator‐S observations of He+ energization by EMIC waves in the dawnside equatorial magnetosphere

gyrofrequency (Pc1 frequency range). These events wereobserved during quiet magnetospheric conditions at the inner edgeof the plasmasheet. At this boundary 10 to 40 keV protons, whichconvect on open drift paths, exhibit a pronounced pitch angleanisotropy providing the free energy for the enhancement of thePc1 emissions. I

Cold flowing O+ beams in the lobe/mantle at Geotail: Does FAST observe the source?

The Geotail spacecraft observed high-energy (∼3–10 keV) cold O+ beams (COBs) streaming tail ward together with protons entering from the magnetosheath in the northern dusk lobe/mantle when the IMF (interplanetary magnetic field) By and Bz are steadily negative. He+ beams were also observed intermittently. During the same period the FAST satellite passed across the dayside northern polar regions from dawn to dusk at low altitudes (1200–3400 km) and observed O+ precipitation on both closed and open field lines. There are regions where the magnetosheath and dayside plasma sheet/ring current components coexist and are precipitating together. In the open field line regions the precipitating O+ seem continuous with the precipitation in the closed regions, while the H+ and He++ precipitations are denser and typically have lower energy than O+. The phase space density (PSD) of the precipitating ions is highly isotropic except for the loss cone in the upward direction. Utilizing Liouvilles theorem, we have compared the PSD of locally mirroring O+ at FAST with the PSD of COBs observed at Geotail. This comparison shows that the PSD in the high-energy precipitation region on closed field lines is comparable to that of the COBs. In regions where the magnetosheath and dayside magnetosphere ions coexist, the O+ PSD is typically smaller than that of the COBs, but at low latitudes it sometimes reaches values comparable to that of the COBs. These results suggest that the high-energy O+ ions in the dayside magnetosphere are a promising candidate for the source of COBs in the lobe/mantle. The ion dynamics on reconnected flux tubes needs to be examined further to clarify the possible energization mechanisms and their effect on the O+ ions.

On quasi‐thermal fluctuations near the plasma frequency in the outer plasmasphere: A case study

We present a derivation of the quasi-thermal electrostatic fluctuation power spectrum in a mult-Maxwellian plasma and show sample calculated spectra. We then apply this theory, which has been successfully applied in oter regions of space, to spectra from two Active Magnetospheric Particle Tracer Explorer/Ion Release Module (AMPTER IRM) passes through the duskside plasmasphere. WE show that the plasma line that is often seen in this region is usually quasi-thermal in origin. We obtain a refined estimate of the plasma frequency and infer a cold electron temperature which is consistent within a factor of 2 with both models and previous meausurements by other techniques, but closer investigation reveals that details of the plasma line cannot be explained with the ususal two isotropic Maxwellian model.