Walther N. Spjeldvik

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.

The cause of storm after effects in the middle latitude D-region

Abstract Fluxes of outer zone radiation belt electrons are considerably enhanced during the main phase of geomagnetic storms. Throughout the storm recovery the electrons radially diffuse to lower L and decay slowly to reform the characteristic two zone structure. Precipitation loss during the storm recovery is shown to be a major D - region ionization source responsible for storm after effects at middle latitudes. Within the plasmasphere, electron precipitation results primarily from resonant pitch angle scattering with naturally occurring ELF whistler mode turbulence. The pitch angle distribution and energy spectrum of precipitating electrons is obtained by evaluating the overall pitch angle diffusion coefficient due to both wave scattering and atmospheric collisions. Ion production rates from precipitating electrons at L = 4 ( Λ = 60°) are computed to be typically 1–10 cm −3 sec −1 at D - region altitudes. The precipitation is strongly latitude dependent and is insignificant below Λ = 45°. Ion production rates are expected to maximize a few days after the storm main phase due to a combination of delayed inward radial diffusion to lower L and the intensification of scattering turbulence which is strongest in the storm recovery phase. Enhanced D - region ionization should persist for approximately a week following the storm, consistent with electron precipitation lifetimes.

Rapid enhancement of radiation belt electron fluxes due to substorm dipolarization of the geomagnetic field

The classical pure radial diffusion mechanism appears not to fully explain the frequently observed rapid enhancement in the timescales of minutes to hours in the radiation belt electron fluxes in the Earths magnetosphere. We here consider other physical mechanisms, such as energization mechanisms associated with substorm processes, to account for these sudden increases. A three-dimensional electron kinetic model is used to simulate the dynamics of the geomagnetically trapped population of radiation belt electrons during a substorm injection event. In the past this model has been extensively used to study dynamics of energetic ions in the ring current. This work, for the first time, constitutes the development of a combined convection and diffusion model to radiation belt electrons in the 0.04–4 MeV kinetic energy range. The Tsyganenko 89 geomagnetic field model is used to simulate the time-varying terrestrial magnetosphere during the growth phase elongation and the expansion phase contraction. We find that inductive electric field associated with the magnetic reconfiguration process is needed in order to transport substorm electrons into the trapped particle region of the magnetosphere. The maximum enhancement in energetic electron fluxes is found to be located around the geosynchronous orbit location (L = 6.6), with up to 2 orders of magnitude enhancement in the total fluxes (0.04–4 MeV). Although this enhancement in the inner magnetosphere is very sensitive to the temperature and, to a less extent, density of the source population in the plasma sheet, we suggest that the substorm-associated energization in the magnetotail and the subsequent adiabatic acceleration in the earthward region account for the enhanced MeV electrons (killer electrons) seen at the geosynchronous orbit during storms and substorms.

Expected charge states of energetic ions in the magnetosphere

This paper reviews major developments in our understanding of the physics of energetic heavy ions in the Earths plasma environment during the past four years (1974–1977). Emphasis is placed on processes that influence or are influenced by the ion charge states. This has been a period of growing awareness of the important role heavy ions play in space plasmas. Large fluxes of helium ions and even heavier ions have been observed at the geostationary altitude and in the heart of the radiation belts. Such ions have also been observed on low latitude rockets and satellites, and oxygen ion precipitation exceeding that of protons has been reported. In the outer parts of the Earths plasma envelope there is mounting evidence for significant fluxes of heavy ions: in the magnetotail, the magnetosheath and in the polar cusp regions. In the inner magnetosphere there is a limited theoretical understanding of equatorially mirroring ions, but generally only radial diffusion at one pitch angle and pitch angle diffusion at one L- shell have been studied; for ions the coupled equations are yet unsolved even for the simplest case of only one charge state (protons). Theoretical modeling of the charge state structures of geophysical heavy ion populations is in part frustrated by the lack of adequate laboratory measurements of the pertinent charge exchange cross sections. A first attempt has, however, been made to treat the charge state transformation processes in the radiation belts for equatorially mirroring atomic oxygen ions. Wave-particle interactions in the magnetosphere become much more complex in multi component and multi charge state plasmas where hybrid resonances and wave-particle interaction induced non-linear species-species coupling could be important. Heavy ion plasma physics in the Earths magnetosphere and in the magnetospheres of other planets should be a field of fruitful study for both experimentalists and theoreticians in the years ahead.

A simplified D-region model and its application to magnetic storm after-effects

Abstract Energetic electron precipitation from the Earths radiation belts can provide the dominant D-region ionization source at middle latitudes during the recovery phase of geomagnetic storms. To study the ionospheric response, a simplified D-region ionic model is developed with emphasis placed on obtaining a reasonably accurate evaluation of the free electron concentration. The precise distribution amongst the multiply hydrated water cluster ions and high affinity negative ions is not considered. Instead the complicated heavy ion chemistry is replaced by a scheme which simply applies an average rate of clustering and subsequent recombination. The model provides an adequate fit with observed quiet-time electron concentration in the D-region. It is shown that typical poststorm electron precipitation can increase mesospheric electron densities by a factor of three during the day and by more than an order of magnitude at night. Such enhancements are consistent with VLF and LF radio wave disturbances observed during the recovery phase of major geomagnetic storm. The disturbances are expected to exhibit considerable short term variability and should persist for over a week consistent with the observed decay rate for the enhanced geomagnetically trapped electron flux.

Nuclear reactions in the uppermost Earth atmosphere as a source of the magnetospheric positron radiation belt

A physical mechanism for the formation of a natural positron belt in the Earths magnetosphere is considered. It is assumed that a natural source of energetic positrons as well as electrons can be created owing to the decay of charged pions π± → μ± → e±, which have their origin in nuclear collisions between energetic trapped inner zone protons and heavier atoms (He and O) in the upper atmosphere of the Earth. Simulations of these processes demonstrate that there is a predominant production of positive pions over negative pions, and consequently the decays result in a substantial excess of positrons over electrons at energies greater than tens of MeV. This positron excess is found to be energy-dependent and to decrease with increasing incident proton energy; this excess is essentially absent at proton energies corresponding to cosmic ray primaries of ≥8 GeV. Our numerical computations for the resulting e+/e− fluxes provide ratio values of ∼4 at multi-MeV energies and at L = 1.2 ± 0.1. The simulation results presented herein are compared to the existing and recent experimental evidence.

Steady-state observations of geomagnetically trapped energetic heavy ions and their implications for theory

Measurements of energetic heavy ions using the Explorer 45 and ATS-6 satellites are reviewed and the resulting implications for theory are evaluated. The measured ions are basically protons and helium ions in the energy range from 0.1 to 1 MeV/nucleon. The equatorial energetic ion distributions inside L = 4.5 are found to be very stable for extended periods of time. These ions are very closely confined to the equatorial plane and are sharply peaked as a function of L around a value designated as Lmax. Beyond L = 5.0 the fluxes of these ions are more variable with order of magnitude variations being observed at L = 6.6 on the time scales of minutes, hours, or days. The region inside L = 4.5 appears to be well described by radial diffusive transport driven by fluctuations in the geomagnetic field coupled with losses due to charge exchange and Coulomb interactions with ambient hydrogen geocorona and terrestrial plasma environment. From an analysis relating the position in L-value of the maximum intensity, Lmax, observed for a given ion species and energy, it is argued that the influence of fluctuations in the convection electric field as discussed by Cornwall (1972) are not effective in radially diffusing in L ions with energies greater than a few hundred kiloelectron volts per nucleon. The source of these ions remains basically undetermined and its determination must await further measurements.

Transport, charge exchange and loss of energetic heavy ions in the earth's radiation belts - Applicability and limitations of theory

Abstract Ions in the trapping region of the earths magnetosphere are subject to physical and chemical interactions which control their absolute and relative abundances. Charge exchange reactions act to establish a distribution of ionic states that is largely determined by the chemical properties of the individual species. Convection (“drift”) mechanisms and cross-L diffusion cause ions to be distributed over the entire trapping region with flux intensities determined by the nature and strength of the ion source, transport and loss mechanisms which in general are dependent on energy, mass and charge. Current theories describe ion transport through path tracing for individual particles or by radial diffusion for a population as a whole based on stochastic analysis; a comprehensive treatment of the combined convection and diffusion for trapped and non-trapped ions is yet to be developed. Even in studies where diffusion is the sole transport mechanism considered, only equatorially mirroring particles ( α 0 = π 2 ) have been theoretically treated. There are clearly both upper and lower bounds on the ion energy beyond which diffusion theory ceases to be valid: at high energies where the ion gyroradius becomes too large for the adiabatic approximations to be valid and at low energies where convective drift is a dominant process. In spite of the known shortcomings of the diffusion theory and associated modeling, intriguing theoretical predictions of the relative ionic composition of the radiation belts have been made and some of them are now confirmed by direct observation. Among them is the predicted importance of ions heavier than protons at ring current energies of tens of keV which follows from the charge exchange chemistry.

Dynamics of the low‐altitude energetic proton fluxes beneath the main terrestrial radiation belts

At the interface between the upper atmosphere and the radiation belt region there exists a secondary radiation belt consisting mainly of energetic ions that have become neutralized in the ring current and in the main radiation belt and then re-ionized by collisions in the inner exosphere. The time history of the proton fluxes in the 0.64–35 MeV energy range was traced in the equatorial region beneath the main radiation belts during the 3-year period from February 21, 1984, to March 26, 1987, using data obtained with the High-Energy Particle experiment on board the Japanese OHZORA satellite. During most of this period a fairly small proton flux of ∼1.2 cm−2 s−1 sr−1 was detected on geomagnetic field lines in the range 1.05 < L < 1.15. We report a few surprisingly deep and rapid flux decreases (flux reduction by typically 2 orders of magnitude). These flux decreases were also long in duration (lasting up to 3 months). We also registered abrupt flux increases, such that magnitude of the proton flux enhancements could reach 3 orders of magnitude and with an enhancement duration of 1–3 days. Possible reasons for these unexpected phenomena are discussed.

Maintenance of the middle-latitude nocturnal D-layer by energetic electron precipitation

SummaryEnergetic electrons are continually removed from the radiation belts by resonant pitch-angle scattering with ELF turbulence. A realistic simulation of the concomitant precipitation loss of such electrons to the atmosphere shows it to be a significant source for the nocturnal ionospheric D-region. During geomagnetically quiet (non-storm) periods, precipitating electrons are expected to provide the dominant nocturnal ionization source at medium invariant latitudes corresponding to field lines just inside the plasmapause. When the level of scattering turbulence is high the quiet time precipitation can dominate for an extended range of latitudes (Λ∼ 55° to 65°). Observed fluctuations in the level of scattering turbulence should produce modulations in the concentration of nocturnal middle latitude D-region electrons which may be detected using radio probing techniques.