14 research outputs found
Magnetospheric electric field measurements during sudden commencements
Direction measurements of electric fields were made in the outer magnetosphere during two sudden commencements in 1972. These measurements were observed with the double floating probe experiment carried aboard the IMP 6 satellite. The initial variations of the measured electric field consisted of an increase from a background of about 1 mv/meter to some 10 mv/meter at about 7 rE (earth radi) and to some 4 mv/meter at 3 rE. These initial electric field disturbances were longitudinal, oriented counter clockwise about an axis pointed north. A solution of Maxwell's third equation is derived for these measurements using a quasi-static version of Mead's model of the magnetosphere B (t). This solution seems to describe well the magnitude and direction of the initial perturbation of the electric field vectors observed during these two sudden commencements. After the initial increase, the measured electric field rings several times with periods of the order of minutes. This observed oscillatory behavior correlates with magnetic observatory records taken near the foot of the magetic field line passing through the satellite
Early results from ISEE-A electric field measurements
In the solar wind and in middle latitude regions of the magnetosphere, spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: (1) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock, (2) that scale lengths for large changes in E at the subsolar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause, and (3) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels. Spike-like, non-oscillatory, fields lasting less than 0.2 seconds are occasionally seen at the bow shock and at the magnetopause and also intermittently appear in magnetosheath and plasma sheet regions under highly variable field conditions
The spherical probe electric field and wave experiment
The experiment is designed to measure the electric field and density fluctuations with sampling rates up to 40,000 samples/sec. The description includes Langmuir sweeps that can be made to determine the electron density and temperature, the study of nonlinear processes that result in acceleration of plasma, and the analysis of large scale phenomena where all four spacecraft are needed
Plasma gradient effects on double-probe measurements in the magnetosphere
The effects on double-probe electric field
measurements induced by electron density and temperature gradients are
investigated. We show that on some occasions such gradients may lead to marked
spurious electric fields if the probes are assumed to lie at the same probe
potential with respect to the plasma. The use of a proper bias current will
decrease the magnitude of such an error. When the probes are near the plasma
potential, the magnitude of these error signals, ∆E, can vary
as ∆E ~ Te(∆ne/ne)+0.5∆Te,
where Te is the electron temperature, ∆ne/ne
the relative electron density variation between the two sensors, and
∆Te the electron temperature difference
between the two sensors. This not only implies that the error signals will
increase linearly with the density variations but also that such signatures grow
with Te, i.e., such effects are 10 times larger in a
10-eV plasma than in a 1-eV plasma. This type of error is independent of the
probe separation distance provided the gradient scale length is much larger than
this distance. The largest errors occur when the probes are near to the plasma
potential. At larger positive probe potentials with respect to the plasma
potential, the error becomes smaller if the probes are biased, as is usually the
case with spherical double-probe experiments in the tenuous magnetospheric
plasmas. The crossing of a plasma boundary (like the plasmapause or
magnetopause) yields an error signal of a single peak. During the crossing of a
small structure (e.g., a double layer) the error signal appears as a bipolar
signature. Our analysis shows that errors in double-probe measurements caused by
plasma gradients are not significant at large scale (»1 km) plasma boundaries,
and may only be important in cases where small-scale (<1 km), internal
gradient structures exist. Bias currents tailored for each plasma parameter
regime (i.e., variable bias current) would o1q1improve the double-probe response
to gradient effects considerably