摘要:

Heliospheric current sheet tilt evolves systematically over the solar cycle. Here we show that this evolution is different than the sunspot cycle and that tilt for the period 1992 – 1996 can be predicted using persistence. That is, the tilt over the coming cycle will be the same as for the past cycle. The Ulysses spacecraft has passed Jupiter and is moving out of the plane of the ecliptic, so we use the prediction of the changing heliospheric current sheet tilt to predict that Ulysses will pass beyond the envelope, or maximum latitude, of the heliospheric current sheet in November 199

ISSN:0094-8276    

DOI:10.1029/93GL00078

年代:1993

数据来源: WILEY

摘要:

Significant magnetic field perturbations in the solar wind were observed during the Phobos‐2 elliptical orbits around Mars when the spacecraft crossed the Phobos orbit. As remarkable feature, these so‐called Phobos events were detected in all cases only on the upstream side of the Phobos orbit. A simple one‐dimensional fluid model is developed which shows that a thin charged dust cloud with an extension of no more than 100 km is able to produce phase‐standing, right‐hand polarized precursor waves with a spatial periodicity of about the proton skin length. This is the same type of upstream waves which is generated by oblique, subcritical shocks. The measured period of the magnetic variations (≈40 s) may be explained as Doppler shift owing to the spacecraft motion through the standing wave structure. The fluid results are confirmed by hybrid code simulations. In conclusion, we suggest the Phobos events to be precursor waves resulting from the interaction of the solar wind with charged dust rings along the P

ISSN:0094-8276    

DOI:10.1029/93GL00236

年代:1993

数据来源: WILEY

摘要:

We have computed frequency spectra of power density and polarization parameters of magnetohydrodynamic waves from observations on board the ICE spacecraft as it flew past comet Giacobini‐Zinner on September 11, 1985. Since the spectral parameters are frequency dependent, we find that the analysis is best carried out in a ‘wave’ reference frame where one of the major axes is along the wave normal direction for each frequency component. The power density along the wave normal direction shows a systematic peak structure which we identify as belonging to cyclotron wave harmonics of pick‐up ions (H+,H2+, andO+) near the comet. The fundamental harmonics of the cyclotron waves are also consistent with the gyrofrequencies calculated from the magnetic fie

ISSN:0094-8276    

DOI:10.1029/93GL00069

年代:1993

数据来源: WILEY

摘要:

A comparison between one‐ and two‐dimensional hybrid simulations of a collisionless quasi‐parallel shock has been done in order to investigate the influence of the spatial dimensionality on the upstream and downstream spectra of the diffuse ions. The simulations are done for a shock Mach number ofMA= 4.6 and a magnetic field ‐ shock normal angle of ΘBn= 10°. Within the same time period (t= 70Ωci−1) the diffuse ions reach in the one‐dimensional simulation, on average, higher energies than in the two‐dimensional simulation. This manifests itself in a considerably flatter upstream spectrum at the high energy end. At lower energies the intensity in the 2‐D case is an order of magnitude higher than in the 1‐D case. The spectral shape exhibits in the 2‐D case a more gradual transition from the solar wind thermal distribution to the diffuse distribution. In both simulations the particles drift about the same distance along the shock. The difference in spectra is due to a smaller motional electric field at the shock (in the shock frame) in the two‐dimensional case as compared to

ISSN:0094-8276    

DOI:10.1029/93GL00076

年代:1993

数据来源: WILEY

摘要:

The convection surge is a mechanism whereby a temporary intensification and subsequent relaxation of the east‐west electric field coexists with an earthward displacement of the nightside curved magnetospheric field lines. Calculations of single particle trajectories for H+and O+during convection surges have found that the first adiabatic invariantµ= E⟂/B (where E⟂is the energy contributed by the motion of the particle perpendicular to the magnetic field and B is the magnitude of the ambient magnetic field) may break down, depending on the particle's species and location. Applying this property to a collection of single particle trajectories, it has been suggested that the boundary between the adiabatic and non‐adiabatic regimes could be interpreted as the injection boundary formed in the middle magnetosphere during substorm expansions. Under this interpretation, the non‐adiabatic conditions establish the character of the populations tailward of the injection boundary. To investigate the behavior of the full particle distributions, we have modified the convection surge numerical code of Mauk [1986] to approximate the breakdown of the adiabaticity. In this report we present simulated results for various parameter conditions. Contrary to the adiabatic results, the non‐adiabatic conditions do not yield the magnetic‐field‐aligned distributions that are commonly observed tailward of the in

ISSN:0094-8276    

DOI:10.1029/93GL00237

年代:1993

数据来源: WILEY

摘要:

There appears a drastic potential drop along a collisionless magnetized plasma flow in the presence of localized large‐amplitude electron plasma waves generated externally. The phenomenon is attributed to ponderomotive force on electrons and nonlinear electron acceleration due to enhanced wave dampin

ISSN:0094-8276    

DOI:10.1029/93GL00074

年代:1993

数据来源: WILEY

摘要:

A new algorithm for calculation of energetic electron fluxes in the atmosphere is described. Results are compared with other methods and the implications for auroral spectroscopic analysis are discussed. Good agreement is obtained with the model ofStrickland et al.[1976, 1989].

ISSN:0094-8276    

DOI:10.1029/93GL00081

年代:1993

数据来源: WILEY

摘要:

Generation of ELF and VLF waves via a thermal instability process in a HF‐modulated polar electrojet has been investigated. It is shown that a positive feedback through the electron‐neutral collisional heating process can cause the transient response of the plasma to the modulated HF heater to grow exponentially. The threshold fields of the instability under normal electrojet conditions are found to be about 2.25 V / m and 1.13 V / m for the operation of the o‐mode and x‐mode heaters with a 50% duty cycle, respectively. For a heater wave field of, e.g., 1.5 V/m, the instability can be excited by the x‐mode heater within a few tens of millisecond. The predicted dependence of the ELF/VLF radiation amplitude on the percentage of the duty cycle of the HF modulation is identical to that of the recent observational results ofBarr and Stub

ISSN:0094-8276    

DOI:10.1029/93GL00070

年代:1993

数据来源: WILEY

摘要:

The rate of loss of ClO on Pyrex, water ice and nitric acid trihydrate, NAT, surfaces at 183 K has been measured in a flow reactor sampled by a mass spectrometer. Upper limits for the rates of loss of ClO on Pyrex at 295 K, Cl2on ice at 183 K and O3on Pyrex, ice and NAT surfaces at 183 K have also been determined. In all cases the probability of loss of the species from the gas‐phase upon collision with the surface is less than 10−4. These small values make it probable that heterogeneous reactions involving these species are unimportant in the winter polar stratosph

ISSN:0094-8276    

DOI:10.1029/93GL00238

年代:1993

数据来源: WILEY

摘要:

The rate constants for the reaction of the OH radical with CF3CFHCF3and with CF3CH2Br have been measured as a function of temperature using the discharge flow technique with laser induced fluorescence detection of the OH radicals. The temperature dependent rate coefficients are well described by a simple Arrhenius expression, k(T) = A exp[−E/(RT)]. For the reaction of OH with CF3CFHCF3we find A = 3.7 × 10−13cm³ molecule−1s−1and E/R = 1615 K; for the reaction of OH with CF3CH2Br we report A = 1.4 × 10−12cm³ molecule−1s−1and E/R = 1350 K. These Arrhenius parameters imply rate coefficients at 277 K of 1.09 × 10−15cm³ molecule−1s−1for CF3CFHCF3and 1.06 × 10−14cm³ molecule−1s−1for CF3CH2Br. We find atmospheric lifetimes for CF3CFHCF3and CF3CH2Br of 42 years and 4.1 years, respectively. We also estimate the steady state ozone depletion potential (ODP) of the brominated species relative to CFCl3as

ISSN:0094-8276    

DOI:10.1029/93GL00239

年代:1993

数据来源: WILEY