摘要:

In order to accurately predict geomagnetic storms, we exploit Elman recurrent neural networks to predict theDstindex one hour in advance only from solar wind data. The input parameters are the interplanetary magnetic fieldz‐componentBz(GSM), the solar wind plasma number densitynand the solar wind velocityV. The solar wind data and the geomagnetic indexDstare selected from observations during the period 1963 to 1987, covering 8620h and containing 97 storms and 10 quiet periods. These data are grouped into three data sets; a training set 4877h, a validation set 1978h and a test set 1765h. It is found that different strengths of the geomagnetic storms are accurately predicted, and so are all phases of the storms. As an average for the out‐of‐sample performance, the correlation coefficient between the predicted and the observedDstis 0.91. The predicted average relative variance is 0.17, i.e. 83 percent of the observedDstvariance is predictable by the solar wind. The predicted root‐mean‐square error

ISSN:0094-8276    

DOI:10.1029/96GL00259

年代:1996

数据来源: WILEY

摘要:

The accuracy of two resistive MHD codes based on very different methods is studied by simulating the same problem of plasmoid formation and ejection in the geomagnetic tail initiated by resistive diffusion, using identical initial and boundary conditions and a 2‐D approximation. One of the codes is a modified explicit leapfrog scheme, which has been used extensively in the past for similar problems. The other scheme is an implicit particle‐in‐cell MHD code (FLIP‐MHD). The results from both schemes agree within a few percent in all major characteristics: onset and growth rate of the instability, formation and tailward motion of a plasmoid, scale size of the perturbation and details of the mode structures. FLIP, however, predicts a slightly slower tailward motion of the plasmoid. Both simulations exhibit highly variable earthward flow from the reconnection site with similar averages and similar fluctuation levels, resembling observed bursty bulk flows. Due to differences in the phase of these fluctuations, however, the instantaneous flow may differ in the two simu

ISSN:0094-8276    

DOI:10.1029/95GL03857

年代:1996

数据来源: WILEY

摘要:

The ponderomotive acceleration of multi‐species ions due to circularly polarized electromagnetic waves propagating parallel to the magnetic field is calculated. When applied to a low frequency left‐hand circularly polarized wave, the results show differential accelerations among the different ion species. We demonstrate that differential acceleration of ion species by the ponderomotive force can occur even in the absence of a parallel electric field component. Our work extends the results previously obtained byLi and Temerin(1993) who considered the ponderomotive acceleration due to obliquely propagating Alfvén w

ISSN:0094-8276    

DOI:10.1029/96GL00157

年代:1996

数据来源: WILEY

摘要:

A previously‐developed colissional, interhemispheric flux tube model for photoelectrons (PE) [Khazanov et al.,1994] has been extended to three dimensions by including transport due to and magnetic gradient‐curvature drifts. Using this model, initial calculations of the high‐energy (≥50 eV) PE distribution as a function of time, energy, pitch angle, and spatial location in the equatorial plane, are reported for conditions of low geomagnetic activity. To explore both the dynamic and steady behaviors of the model, the simulation starts with the abrupt onset of photoelectron excitation, and is followed to steady state conditions. The results illustrate several features of the interaction of photoelectrons with typical magnetospheric plasmas and fields, including collisional diffusion of photoelectons in pitch angle with flux tube filling, diurnal intensity and pitch angle asymmetries introduced by directional sunlight, and energization of the photoelectron distribution in the evening sector. Cross‐field drift is shown to have a long time scale, taking 12 to 24 hours to reach a steady state distribution. Future applications of the model are briefly

ISSN:0094-8276    

DOI:10.1029/96GL00148

年代:1996

数据来源: WILEY

摘要:

In the summer polar mesopause region, rocket and radar observations have revealed layers of deep electron depletions for which electron scavenging by ice particles with sizes less than 10 nm appears to be the most plausible explanation. The lack of significant ion depletions and the absence of any relationship between the occurrence of polar mesospheric clouds and auroral activity suggest that the ice particles are generated by heterogeneous nucleation on meteoric dust. Then the Kelvin effect, predicting an increase of the equilibrium water vapor pressure with decreasing particle radius, strongly affects the formation of electron depletions. In particular, weak perturbations of the temperature and water vapor concentration associated with small‐scale internal gravity waves can produce time‐dependent multiple layer structures with steep gradients characterized by length scales of a few hundred meters, in good agreement with observational resu

ISSN:0094-8276    

DOI:10.1029/96GL00256

年代:1996

数据来源: WILEY

摘要:

We present the first diurnally‐resolved measurements of ClO and NO2at 3.1 and 1.4 hPa, approximately 40 and 46 km. Comparisons between the measurements of ClO and a zero‐dimensional box model utilizing standard photochemistry and constrained by Upper Atmosphere Research Satellite (UARS) tracer measurements show that photochemical models overpredict ClO by a factor of two or more at 3.1 hPa and by a factor of 1.5–2.0 at 1.4 hPa. NO2is well simulated by the model at 3.1 hPa, but systematically overestimated at 1.4 hPa. The model overestimate of ClO results in an overestimate of ozone loss in the upper stratosphere of 1

ISSN:0094-8276    

DOI:10.1029/96GL00154

年代:1996

数据来源: WILEY

摘要:

The heterogeneous hydrolysis of BrONO2via sulphate aerosols has been included in 2‐D model calculations of the impact of projected fleets of supersonic aircraft on atmospheric ozone. Calculations have been performed for aerosol levels ranging from zero to 16× the lower limit in WMO [1992]. The results show that the addition of this reaction has a major effect when the heterogeneous hydrolysis of N2O5has reached saturation in regions where the night length is short. At 4× the lower limit of aerosols, the additional calculated change in aircraft impact due to the inclusion of BrONO2hydrolysis is of the same order as the impact calculated when this reaction is not included. Calculations for background atmospheres at high aerosol levels show that the inclusion of this reaction significantly increases the predicted ozone depletion resulting from volcanically‐enhanced aerosol l

ISSN:0094-8276    

DOI:10.1029/96GL00263

年代:1996

数据来源: WILEY

摘要:

A new instrument, the Airborne Chromatograph for Atmospheric Trace Species IV (ACATS‐IV), for measuring long‐lived species in the upper troposphere and lower stratosphere is described. Using an advanced approach to gas chromatography and electron capture detection, the instrument can detect low levels of CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CCl2F‐CClF2), methyl chloroform (CH3CCl3), carbon tetrachloride (CCl4), nitrous oxide (N2O), sulfur hexafluoride (SF6), Halon‐1211 (CBrClF2), hydrogen (H2), and methane (CH4) acquired in ambient samples every 180 or 360 s. The instrument operates fully‐automated onboard the NASA ER‐2 high‐altitude aircraft on flights lasting up to 8 hours or more in duration. Recent measurements include 24 successful flights covering a broad latitude range (70°S–61°N) during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/

ISSN:0094-8276    

DOI:10.1029/96GL00244

年代:1996

数据来源: WILEY

摘要:

We present a new method of retrieving aerosol optical thickness over land that takes advantage of the dual look capability of the ATSR‐2 instrument. Unlike some methods, the approach does not require dense dark vegetation to be located, nor is it dependent upon invariant targets, nor does it assume that the underlying surface reflectance is purely Lambertian. It makes use of a new surface approximation that exploits the property of the surface which is least variable with wavelength to allow atmospheric parameters to be retrieved and, consequently, is applicable over virtually any land surface. The method is tested using simulated ATSR‐2 measurements based upon a radiative transfer model coupled with a surface bidirectional model that includes the hot‐spot effect, and is shown to be robust to the effects of differing underlying canopy types. At high optical depths, the retrieval process achieves a mean error in optical thickness of less than 2.5%, depending on surface type, with a maximum absolute error (for the same data sets) of 5% while at low optical depths errors are of the order of 5%. The effects of systematic error in the simulated radiances are also shown. The method may be readily extended to other multiple look satellite radiom

ISSN:0094-8276    

DOI:10.1029/96GL00153

年代:1996

数据来源: WILEY

摘要:

A spectroradiometer located at Ushuaia, Tierra del Fuego, Argentina obtained a high quality data set on the solar ultraviolet (UV) spectral irradiance for a full annual cycle during 1992. The unique aspect of the ground‐based irradiances is their ability to characterize the effects of cloudiness. Measured irradiances at 340 nm can be used to characterize the attenuation provided by cloudy skies. When irradiances at shorter wavelengths, 302.5–320.0 nm, are adjusted for this attenuation, they show good agreement with radiative transfer calculations which assume clear skies and utilize data from the Total Ozone Mapping Spectrometer (TOMS) as inputs. The differences which exist are qualitatively consistent with the neglect of spherical geometry in the calculated irradian

ISSN:0094-8276    

DOI:10.1029/96GL00253

年代:1996

数据来源: WILEY