The distortion of the geomagnetic field is a key signature of the response of the magnetosphere to the solar wind input. A number of empirical models have been devised to estimate the magnetic field direction and magnitude at any point within the magnetosphere under a variety of conditions. We describe a technique whereby the field-line mapping predicted by such models is tested by matching measurements of magnetospheric plasma energy spectra obtained by Los Alamos instruments at geosynchronous orbit with spectra obtained by instruments on the polar-orbiting DMSP satellites (at an altitude of about 800 km) at times when the two satellites are in approximate magnetic conjugacy. With up to three geosynchronous satellites and as many as four DMSP satellites in operation at any given time, there are a very large number of such two-satellite conjunctions, allowing the model mappings to be tested under a wide range of local times and geomagnetic activity. Preliminary results from the application of this technique are presented for one week of data from March, 1991.
Flowing, cold magnetospheric ions have been observed in conjunction with geosynchronous orbit magnetopause crossings since the earliest ATS and OGO missions. The authors have reported on the occurrence and convection of low-energy (10-100 eV) ions seen by multiple satellites in association with geosynchronous orbit magnetopause and low-latitude boundary layer (LLBL) encounters. More generally, Los Alamos 3-D plasma instruments observe these ions following storm sudden commencements (SSCs), when activity levels are high. The ions appear to be convecting radially outward and usually westward at speeds of a few to several tens of kilometers per second. Often the energy spectra reveal peaks at energies appropriate for cold convecting H{sup +}, He{sup +} and O{sup +}. The occurrence frequency distribution of these dense cold ions is peaked near 1400 LT, with an overall range from 1000 to beyond 1800 LT. This local time distribution is greatly skewed from the overall plasmaspheric distribution, which peaks closer to 1800 LT. Multisatellite observations show that the ions are seen first at late afternoon local times and then at progressively earlier and earlier local times (though usually no earlier than 1000 LT). This apparent evolution in local time suggests that the late-afternoon plasmaspheric plasma moves out andmore » dawnward during times of increased magnetospheric activity. The three-satellite observations also allow the authors to track cold plasma convection at multiple points in the magnetosphere, and potentially provide a glimpse of the large-scale convection pattern.« less
From August 1978 to September 1982.58 intervals with {vert bar}B{sub z}{vert bar} {ge} 10 nT and duration of 3 hours or greater were detected. Using detected shocks, magnetic clouds and bidirectional solar wind electron heat flux events (BDEs) to identify the interplanetary counterparts of coronal mass ejections (ICMEs), it is found that more than half of the 59 strong B{sub z} events are ICME-associated: i.e. their strong B{sub z} occurred at least partially within an ICME, where the field orientation is most likely determined in the corona. Magnetic clouds form a small subset of ICMEs and are responsible for one third of the ICME-associated B{sub z} events. Analysis of 12 major geomagnetic storms with maximum K{sub p} of 8{sub {minus}} or greater in the same period shows that 7 of 9 storms for which the data was conclusive took place when the Earth encountered both an ICME having strong southward component and a shock sheath in which the ambient magnetic field polarity was expected to produce a southward B{sub z} component.
The occurrence of multiple directional discontinuities in the coronal streamer belt at sector boundary crossings in the heliosphere, often ascribed to waves or kinks in the heliospheric current sheet, may alternatively be attributed to a network of extended current sheets from multiple helmet streamers with a hierarchy of sizes at the base of the corona. Frequent transient outflows from these helmets can account for a variety of signatures observed at sector boundaries, including ordered field rotations, planar magnetic structure, and sandwichlike plasma structure.
