摘要:

The circulation of polar wind ions from the high‐latitude ionosphere to the plasma sheet is examined by means of three‐dimensional particle codes. The simulations reveal the possible creation of high‐altitude mirror points for particles traveling in the equatorial magnetotail. It is shown that this feature follows from large centrifugal decelerations in the course of the fastE×Btransport, and not from nonadiabatic motion in the local field reversal. Such decelerations yield the confinement (here termed “centrifugal trapping”) of populations with relatively small parallel speed near the tail midplane. It is also demonstrated that the net polar wind plasma supply to the plasma sheet critically depends upon the dynamics of ions which originate from the highest latitudes of the dayside ionosphere. During quiet times, these particles gain access to the distant tail where they experience durable nonadiabatic trapping. Accordingly, it is found that the polar wind forms a prominent source of plasma for the central nightside sector, yielding appreciable density levels (of the order of several tenths of ions cm−3) in the characteristic plasma sheet energy range (from hundreds of electron volts up to a few kiloelectron volts). A lesser contribution is obtained during active times, as the low‐energy ions are confined to lowLshells where they essentially travel in an adiabatic manner. Thus, in addition to emphasizing the role of ionospheric ion dynamics inside the neutral sheet, the trajectory simulations suggest that the polar wind efficiency in feeding the plasma sheet depends upon the time evolution of the particles, rather than from an increasing plasma supply from

ISSN:0148-0227    

DOI:10.1029/93JA00301

年代:1993

数据来源: WILEY

摘要:

Enhanced transverse magnetic fluctuations observed below the proton cyclotron frequency in the terrestrial magnetosheath have been identified as due to the proton cyclotron and helium cyclotron instabilities driven by theT⊥>T∥condition of the sheath ions. One‐dimensional hybrid computer simulations are used here to examine the nonlinear properties of these two growing modes at relatively weak fluctuation energies and for wave vectors parallel to the background magnetic field. Second‐order theory predicts fluctuating magnetic field energies at saturation of the proton cyclotron anisotropy instability in semiquantitative agreement with the simulation results. In particular, introduction of the helium component enhances the wave‐particle exchange rate for proton anisotropy reduction by that instability, thereby reducing the saturation energy of that mode. The simulations demonstrate that wave‐particle interactions by the proton cyclotron and helium cyclotron instabilities lead to the anticorrelation ofT⊥p/T∥pand β∥p, a result observed by Anderson a

ISSN:0148-0227    

DOI:10.1029/93JA00272

年代:1993

数据来源: WILEY

摘要:

The mirror instability is prevalent in planetary and cometary magnetosheaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived from magnetohydrodynamic fluid theory, later work showed that there were significant differences between the fluid theory and a more rigorous kinetic approach. Here we point out that the instability mechanism hinges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with other uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other instabilities driven by resonant particles. We comment on the implications of these results for our understanding of the observations of mirror instability‐generated signals in spac

ISSN:0148-0227    

DOI:10.1029/92JA02837

年代:1993

数据来源: WILEY

摘要:

We present the results of an innovative and powerful new technique to be used in the interpretation of Geotail plasma wave data when the spacecraft is in the deep magnetotail during potential reconnection events. To provide the best possible magnetotail structure of magnetic fields and plasma densities, an MHD simulation is performed. At precisely the moment of reconnection, during the simulation, the magnetotail structure is used in the ray tracing calculations of various plasma waves expected to be generated in the X point region. Several wave modes are studied in detailed (Lmode ion cyclotron,Rmode ion cyclotron, lower hybrid waves, whistler mode waves, andL‐OandR‐Xmode waves). Ray tracing calculations are performed in these modes as generated from a point source at all wave normal angles in the reconnection region. When we assume each ray is generated at the same intensity, a ray density distribution is derived for the entire spatial reconnection region for several frequencies that each of the plasma wave modes can support in the MHD magnetotail model. As a hypothetical spacecraft transverses the tail reconnection structure along a typical trajectory, a pseudofrequency time spectrogram can then be constructed. It is clear that each of these wave modes has a distinctive pattern of propagation, along the magnetic field, transverse to the magnetic field, or trapped within the resulting plasmoid or reconnection region and should be easily distinguished in Geotail measureme

ISSN:0148-0227    

DOI:10.1029/92JA02901

年代:1993

数据来源: WILEY

摘要:

The inductive interaction between a conducting body and a magnetized plasma in relative uniform motion generates a system of stationary waves in the frame of the body. This wave system is composed of both Alfvénic and magnetoacoustic perturbations associated with each of which there are corresponding electric potentials and currents. Here we develop the Green's function for each of the modes. The well‐known Alfvén “wings” are represented by delta functions which propagate the parallel components of vorticity and current along the Alfvén lines. The magnetoacoustic modes are characterized by total pressure (plasma plus magnetic) and dilatation perturbations which are propagated along the envelopes of the fast and slow mode characteristics. The concomitant electric potentials are then obtainable from a component of the momentum equation which can be written in the form of a wave equation for the potential with an Alfvénic wave operator and the magnetoacoustic pressure gradient acting as the driving term. The important consequence is that the potential associated with the compressive modes is hybrid in nature in that it is singular both on the Alfvén lines and on the magnetoacoustic characteristics so that the properties of both modes are interwoven in a complicated fashion. On the other hand, the slow mode potential and current perturbations exhibit singularities on the slow mode wings and the Alfvén lines away from both of which they decay rather gently in a two‐dimensional dipolelike fashion. By using the method of stationary phase we elucidate the detailed fine structure of the slow mode wave crests which consist of two closed, hollow wings, whose cross section reflects the topology of the slow mode group velocity surface and which emanate from the conducting body and extend out parallel and antiparallel to the background magnetic field. As an example of how Green's functions may be used to construct more general solutions and in an attempt to tackle the problem of the self‐consistent source current distribution inside the conducting body we formulate an integral equation which determines the current along a thin wire of finite length. We demonstrate that including the effect of induced fields radiated in the magnetoacoustic modes enhances the effective wave impedance of the plasma environment relative to the results of conventional treatments which only take account of

ISSN:0148-0227    

DOI:10.1029/92JA01549

年代:1993

数据来源: WILEY

摘要:

The monthly and daily samples of the Ap geomagnetic index for 51 years, 1932‐1982, were investigated by means of the power spectrum technique. In general, the results confirm previous findings about possible periodicities in the geomagnetic activity. However, in our opinion the following aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the double peak structure observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the 6‐month period and other peaks as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are consider to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. The study of the seasonal variation was complemented by a superposed epoch analysis. The profiles resulting from this analysis seem to show a multiple origin of the 6‐month periodicity, so that it does not seem realistic to search for a unique cause for this well‐known seasonal variation. This conclusion is also supported by the histograms of the occurrence of storms above a given intensity level, taken over short duration intervals (i.e., 8 days). According to these histograms, for large data samples the dates with largest number of storms are spread out around those predicted by the different theoretical models, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the randomly generated source of storms. It was also found that an additional seasonal peak seems to exist in July, with an amplitude comparable to those of the equinoctial peaks, for the range of the most intense storms (Ap ≥ 150 nT). A weak periodicity around 158 days, well correlated to that of about 155 days observed in the solar activity, has also been detected for some years during solar

ISSN:0148-0227    

DOI:10.1029/92JA02200

年代:1993

数据来源: WILEY

摘要:

The propagation, growth and absorption of electromagnetic ion cyclotron waves in the Pc 1 frequency range is investigated using the HOTRAY ray tracing program for a realistic distribution of thermal plasma (H+, He+and O+) that is assumed to be in diffusive equilibrium inside the plasmasphere and collisionless in the low‐density region outside the plasmapause. Free energy for L‐ mode wave growth is provided by a bi‐ Maxwellian distribution of energetic H+and O+with a temperature and density modelled on satellite observations. Solutions to the hot plasma dispersion relation show that inside the plasmasphere the spatial growth rates are small whereas they increase outside the plasmapause with increasingLshell. Ray tracing shows that inside the plasmasphere guided L‐ mode waves only grow during one crossing of the magnetic equator and only achieve small path‐integrated wave gain (≤ 2e‐ foldings). At the plasmapause the density gradient enables guided mode waves to grow during several equatorial crossings and the net path‐integrated gain is much larger (≃ 8.7e‐foldings). For the largest observed ring current densities of 4 × 106m−3atL= 4 the gain is above the critical level (10e‐foldings) for amplification to observable levels. Just outside the plasmapause the waves only grow during the first equatorial crossing and the gain is smaller. In the absence of nonconvective instabilities the path‐ integrated amplification of the guided mode tends to increase withLshell and reaches the critical level for observable waves only in the outer magnetosphere (L≥ 7). Unguided L‐ mode waves have very small wave gain. ForL≥ 7 the plasma beta becomes large (β⊥>1) and should lead to the onset of nonconvective instabilities. However, we suggest that inhomogeneities in the medium and quasi‐linear scattering will prevent absolute instabilities from occurring and that in reality the waves are propagating with very low group velocities. We suggest that the waves observed by Anderson et al. (1990, 1992a, b) beyondL= 7 near local noon are influenced by the enhanced wave gain due t

ISSN:0148-0227    

DOI:10.1029/92JA02972

年代:1993

数据来源: WILEY

摘要:

Using data from 41 substorm events in the near‐Earth magnetotail, we have combined plasma, energetic ion, and magnetic field data from the AMPTE/IRM spacecraft to perform a superposed epoch analysis of changes in the total pressure and in the magnetic field configuration as a function of time relative to substorm onset. We find that unloading is evident in the total pressure profile, with the pressure beginning to decrease at onset time and reaching a minimum about 45 min later. The pressure decreases by ∼20%. The pressure increase associated with the growth phase is not evident in this analysis. This indicates that the pressure changes during the growth phase are not as uniform for the different substorms as the pressure changes during the expansion phase. To study the changes in the magnetic field configuration, we have determined the development of the plasma pressure profiles inzfor an average of data from 15 to 19RE. We combined this information with the known radial gradient in the total pressure to determine the field configuration changes during a substorm. The stretching of the field during the growth phase is clearly evident in this analysis. At substorm onset, the field line dipolarization begins on the innermost field lines and then progresses to the outer field lines. The field lines map the closest to the Earth about 45 min after substorm onset, and then begin to stretch out again during the recovery phase of the subst

ISSN:0148-0227    

DOI:10.1029/93JA00630

年代:1993

数据来源: WILEY

摘要:

X ray images of the Earth's auroral zone give a large‐scale view of the precipitation patterns of energetic electrons through the bremsstrahlung produced by the electrons stopping in the atmosphere. Here we present X ray maps based on observations by the XRIS imager on the S81‐1 polar orbiting satellite; discrete X ray arcs are the most prominent features in these data. The full width half maximum spatial resolution of XRIS was 40 km, but high time resolution measurements of electrons above 16 keV could detect 2 km spatial variations in one dimension along the track of the satellite. The technique was used to study 26 auroral arcs observed in 4‐40 keV X rays by the XRIS and showed that in 2/3 of the cases the full width of the precipitation region was smaller than 40 km. Furthermore, comparisons with electron data lead to the conclusion that significant spectral variations occur on kilometer size s

ISSN:0148-0227    

DOI:10.1029/92JA02669

年代:1993

数据来源: WILEY

摘要:

An examination of stable auroral red (SAR) arc emissions over the last solar cycle (Slater and Kleckner, 1989) indicates that the strongest emissions, during the lifetime of a particular SAR arc, often occur in association with the main phase of the magnetic storm. Previous observations of thermal and energetic particle populations at high and low altitudes on SAR arc field lines by Kozyra et al. (1987a) indicate that the energy source for these emissions is the O+in the ring current. The O+content of the ring current increases with increasing magnetic activity reaching its maximum percentage contribution near minimumDstfor a particular storm. This variation in the O+content of the ring current is inconsistent with an early main phase enhancement of SAR arc emissions. To investigate the source of main phase enhancements in SAR arc emissions, a study of the September 19‐24, 1984, magnetic storm period during which SAR arc emissions were observed by the ground‐based mobile automatic scanning photometer network in both the main and recovery phases is presented. The emissions associated with the main phase (∼ 400 R) were an order of magnitude greater than those associated with the recovery phase (tens of R). Energetic particle measurements from the DE 1 and AMPTH spacecraft, on field lines that map to the SAR arc position at low altitude, were examined to determine if differences in the energy sources during these time periods were evident. In agreement with previous work, ring current O+supplied the bulk of the electron heating during storm recovery phase as a result of Coulomb collisions of O+with the plasmaspheric electrons; contributions by ring current H+were negligible. A new result of the present work is that an enhancement of the 15‐25 keV H+component of the ring current during the main phase of the September 19 magnetic storm was responsible for an approximately one order of magnitude increase in the electron heating rate and SAR arc emissions during the main phase compared to the recovery phase. The increase in the H+flux occurred in association with a ring current “nose event”, a front of ions injected into the inner magnetosphere in response to a discontinuous change in the cross‐tail electric field. The association between nose events and intensifications of SAR arc emissions in the main phase has not previously been explored but is a natural consequence of the injection of significant fluxes of relatively low‐energy ring current ions earthward of the plasmapause during early storm time ring cu

ISSN:0148-0227    

DOI:10.1029/92JA02554

年代:1993

数据来源: WILEY