摘要:

To determine the source position of the broad‐band bursty emission, we traced rays of X‐mode emission from source positions along magnetic field lines with footprints that form a large grid centered approximately on the south magnetic pole of Uranus. We assumed the emission is just above the RX cutoff frequency, and we included sources only if the ratio of plasma to gyrofrequency was less than about 0.25. Contrary to the results of others who have assumed straight‐line propagation, we found, for large wave normal angles, source regions different from those producing b‐smooth emission, and along field lines with footprints in a relatively large area associated with the south magnetic pole. The emission which was observed prior to closest approach has a source along field lines that are distinct from those which generate emissions observed after closest approach. In order to understand these findings and the bursty nature of the emission we speculate the plasma distribution in the source region and/or the source mechanism is distinct from that which generates the b‐smooth

ISSN:0094-8276    

DOI:10.1029/GL017i002p00109

年代:1990

数据来源: WILEY

摘要:

It is shown that, the intrinsic magnetic field of early Mars was strong enough to deflect the solar wind and form a magnetosphere. The implications of such a magnetosphere earlier in Mars' history are discussed qualitatively from the point of view of atmospheric evolution. Escape of atmospheric constituents through the polar regions could contribute significantly to the atmospheric loss processes inferred from observations.

ISSN:0094-8276    

DOI:10.1029/GL017i002p00113

年代:1990

数据来源: WILEY

摘要:

Analysis of abnormal quiet days (or AQDs) in Sq(H) near years of sunspot minimum have successfully been used to predict the maximum annual mean sunspot number Ra‐max at the following sunspot maximum. In this paper, the method is used for cycle 22 and predicts a value of Ra‐max of 187 ± 36 and a value of the maximum monthly value of R of 254

ISSN:0094-8276    

DOI:10.1029/GL017i002p00117

年代:1990

数据来源: WILEY

摘要:

Large scale one‐dimensional hybrid simulations have been performed of a quasi‐parallel (ΘBn= 20°) high Mach number collisionless shock. It is found that backstreaming reflected ions, i.e., upstream ions with velocities exceeding the shock ram velocity, originate from the outer part (v≳ 1.7vth) of the velocity space of the incident distribution. The backstreaming ions produce very low‐frequency magnetosonic waves which propagate upstream with about 1.3VA(Alfvén speed). As the wave crests convect toward the shock, they steepen up and the shock reforms itself. During shock reformation a large part of the incident ions are reflected. This, in turn, slows the incident ions down. The slowed down incident particle distribution and the reflected particle distribution merge and constitute the new thermalized downstream distribution. In the interval of a relatively stationary shock low‐frequency whistler waves stand at the shock front. During these time intervals the whistler waves are probably responsible for dissipation by nonadiabatic compression of the incident ions. The whistler waves are destroyed by the incoming large amplitude wave crest and reemerge at the new shock front. The reapparance seems to be due to the nonlinear steepening of the incoming wave crest at the u

ISSN:0094-8276    

DOI:10.1029/GL017i002p00119

年代:1990

数据来源: WILEY

摘要:

The collisionless electrical conductivity is calculated for the geomagnetic tail from the stochastic orbits in the plasma sheet. The new conductivity formula is important for the stochastic heating of the plasma sheet and modifies the Galeev‐Zelenyi formulas for the collisionless reconnection growth rate for geomagnetic substorm

ISSN:0094-8276    

DOI:10.1029/GL017i002p00123

年代:1990

数据来源: WILEY

摘要:

We describe the effect of a continuous ion ‐ acceleration in the Earth's magnetotail due to chaotic particle scattering caused by separatrix traversals in the velocity space. This effect operates almost everywhere in the plasma sheet, but outside and independent of neutral lines. Ae a distributed inner source it supports fast ion streams at the boundary of the plasma shee

ISSN:0094-8276    

DOI:10.1029/GL017i002p00127

年代:1990

数据来源: WILEY

摘要:

We present five magnetic impulse events observed at South Pole and in the magnetically conjugate area at Iqaluit. The magnetic signatures recorded during these events appear to be similar to those discussed by several authors as possible ionospheric signatures of sporadic dayside reconnection via the flux transfer process. Coordinated ground‐based image‐intensified all sky camera data were acquired at South Pole during the events. The optical data show that prior to the onset of each of the events the dayside cusp aurora is several degrees poleward of the station, signifying event initiation on closed field lines. Overhead aurora appears at the onset of the magnetic event and precipitation seems to expand over a large region equatorward of the quiescent cusp aurora. As the event decays the aurora dies away and the quiescent dayside cusp aurora, in four out of five cases, returns poleward of the station. This morphology is inconsistent with the model in which the ground based signature of an FTE is caused by a field aligned current configuration moving over a ground station in the antisunward direction. The observations show temporal events which begin at or inside the boundary of closed field lines. These are consistent with sudden precipitation activity on flux tubes which were previously equatorward of the cusp/cleft boundary and which became temporarily disconnected (non‐dip

ISSN:0094-8276    

DOI:10.1029/GL017i002p00131

年代:1990

数据来源: WILEY

摘要:

The three‐dimensional computer codes NASCAP/LEO and POLAR have been used to calculate current collection by the mother payload of the CHARGE‐2 rocket under conditions of positive and negative potential up to several hundred volts. For negative bias (ion collection) the calculations lie about 25 percent above the data, indicating that the ions were less dense, colder, or heavier than the input parameters. For positive bias (electron collection) NASCAP/LEO and POLAR calculations show similar agreement with the measurements at the highest altitudes. This agreement indicates that the current is classically magnetically limited, even during electron beam emission. However, the calculated values fall well below the data at lower altitudes. We suggest that beam‐plasma‐neutral interactions are responsible for the high values of collected current at altitudes below

ISSN:0094-8276    

DOI:10.1029/GL017i002p00135

年代:1990

数据来源: WILEY

摘要:

Groundbased and onboard video images of a sunlit Shuttle Orbiter water dump are interpreted as showing that the continuous ∼1 mm diameter liquid stream quickly breaks up in near‐vacuum to form ice/snow particles of two characteristic sizes. Discrete large droplets (shown from laboratory measurements, to have diameters comparable with that of the initial coherent quasicylinder) are most evident in the close‐in photographs, and unresolved submicron “fog” from recondensation of overexpanded evaporated water appears to dominate the ground‐telescope photographs of the ∼2½ km long optically detectable trail. We estimated the mean diameter of the smaller particles from the spatial distribution of visible radiance using a model of their energy balance, (small) surface roughening as they sublime, and Mie scattering of pre‐dawn sunlight. The results are consistent with those from recent space

ISSN:0094-8276    

DOI:10.1029/GL017i002p00139

年代:1990

数据来源: WILEY

摘要:

Lidar measurements of mesospheric Fe were conducted at Urbana, IL (40°N, 88°W) during 4 nights in late October 1989. The average Fe abundances, layer centroid heights and rms widths varied between 1.0 – 2.0×1010cm−2, 89.0 – 90.5 km and 3.2 – 4.1 km, respectively. The peak densities of the layer near 90 km varied between 15 – 25×103cm−3. Considerable gravity wave and tidal activity was observed in the Fe profiles. On 3 nights the formation and dissipation of 3 large sporadic Fe (Fes) layers was observed. These are the first observations of this phenomenon. The 2 largest Feslayers formed at 102.4 km and 91.4 km over a period of about 20 min and their widths were approximately 2.3 km FWHM. The peak densities of the Feslayers were as high as 31×103cm−3and the abundances were as high as 5.8×109cm−2. The observations are compared with previous measurements of mesospheric Fe and with recent observations of spo

ISSN:0094-8276    

DOI:10.1029/GL017i002p00143

年代:1990

数据来源: WILEY