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Aggson, T. L.

Publications and source records attributed to Aggson, T. L..

At least 37 records · Page 2
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NASA NTRS1983

Observations of large magnetospheric electric fields during the onset phase of a substorm

An auroral substorm is a highly transient process which is triggered on auroral L shells and leads to an abrupt change of state in the nightside magnetosphere. Direct measurements of electric fields in the outer magnetosphere are of critical importance to understand the processes involved in sudden substorm enhancements. Shepard et al. (1980) reported direct measurements of electric fields from the GEOS 2 satellite at equatorial latitudes which showed 15 mV/m westward inductive electric fields in association with tail collapses. Similar direct electric field measurements of the westward inductive electric field at low latitudes from the ISEE 1 satellite during the onset phase of an auroral substorm are reported in the present investigation. The observations show a rapid inward magnetic field (westward electric field) motion near midnight at low or equatorial latitudes simultaneous with the sudden onset of auroral substorm enhancements near the foot of the satellite field line.

Aggson, T. L.↗
NASA NTRS1983

The plasmaspheric electric field as measured by ISEE 1

The electrodynamics of the plasmasphere has been a topic of considerable interest. Models predict a space charge buildup, or Alfven layer, at the inner edge of the ring current which opposes the dawn-dusk convection electric field in the magnetosphere and thus shields the plasmasphere from the convection electric field. The current study has the objective to present data from the ISEE 1 double cylindrical probe instrument. All measurements reported were made in the plasmasphere with electron densities of the order of 30-50 or greater per cu cm. The average electric field pattern for quiet conditions is found to be qualitatively consistent with previous average results from whistler measurements and radar backscattering measurements. The magnitudes and gross patterns are in qualitative agreement with representative ionospheric dynamo models. The basic convective flow vectors from the penetration of the magnetospheric electric field tend to follow contours which are parallel to those of the average plasmapause boundary on the nightside.

Maynard, N. C.↗
NASA NTRS1982

Turbulent electric fields in the nightside magnetosphere

Electric field measurements from the long-wire double-probe instrument (baseline of 179 m) on ISEE 1 have shown the magnetospheric electric field on auroral L shells to be extremely turbulent during periods of magnetic activity. During intense activity these turbulent electric fields can penetrate to very low L values. The variational component of the electric field is typically larger than the DC value. Measurements are presented at frequencies up to 14 Hz. Magnitudes of over 40 m V/m (zero to peak) have been observed with spectral power levels in the 1-10 Hz range greater than m squareV/sq m Hz. The spectral shape of the most intense events was generally flatter than that predicted by two-dimensional hydromagnetic cascading of energy, which argues that the source of this turbulence must be driving the plasma near these frequencies. This in turn suggests that the instability is in the low-energy plasma.

Maynard, N. C.↗
NASA NTRS1980

Magnetospheric observation of large sub-auroral electric fields

An example of large subauroral poleward electric fields similar to those observed on OGO-6, S3-2 and AE-C (SAID) has been found in the magnetosphere near L = 4 and 2300 MLT using ISEE-1 electric field data. The event is located adjacent to and outside the plasmapause and occurs 1 1/2 hours into a substorm. The event is accompaned by a significant penetration of the convection electric field inside the plasmasphere. Data from similar regions on the next orbit occurring near the beginning of a substorm did not exhibit these effects. Recent theoretical models predict SAID to occur in the trough regions, where substorm dynamics force currents to flow in regions of low conductivity. These models provide a first-order interpretation of this phenomena; however, the overal picture is more complex.

Maynard, N. C.↗
NASA NTRS1978

Early results from ISEE-1 electric field measurements

Typical ISEE 1 electric-field measurements with double-probe floating-potential instruments are illustrated in raw-data form. Attention is given to coordinates, solar-wind measurements, bow-shock fields, magnetosheath fields, and midnight near-tail fields. The existence of field phenomena occurring over dimensions comparable to the probe separation and no greater than the 'characteristic' electron cyclotron radius is suggested.

Heppner, J. P.↗
NASA NTRS1978

Instrumentation for dc and low-frequency electric-field measurements on ISEE-A

It has been found that systematic measurements of the various properties of electric fields existing in space are required for an understanding of the plasma environment. Progress has, however, been limited by a lack of opportunities to fly the long antenna-probes required for double-probe measurements. The improved capabilities of ISEE-A should make it possible to conduct systematic studies of the spatial extent and variability of the zone of strong E-fields, or fast convection, in adjacent magnetospheric regions. The objectives of the ISEE-A measurements are discussed along with the symmetric double-probe floating-potential technique and the antenna-probe design. A description of the electronic design is also provided, giving attention to dc-field measurements, spin coupled dc-field measurements, and calibration and plasma impedance measurements.

Heppner, J. P.↗
NASA NTRS1978

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.

Heppner, J. P.↗
NASA NTRS1977

Observations of large transient magnetospheric electric fields

Transient electric field events were studied by means of the long double-probe instrumentation carried by the Imp 6 satellite, and nine clearly defined exceptionally large amplitude events are described. The events were observed in the midnight sector at geocentric distances of 3.5 to 5.5 earth radii at middle latitudes within a magnetic L shell range of 4.8 to 7.5; the duration is from one to several minutes with peak power spectra amplitudes occurring at a frequency of about 0.3 Hz. The events occur under magnetically disturbed conditions and are often associated with negative dH/dt excursions. The magnetospheric motions calculated for these electric fields indicate a quasi-stochastical diffusive process rather than the general inward magnetospheric collapsing motion expected during the expansive phases of auroral substorm activity. It is likely that the transient electric fields are responsible for the impulsive acceleration and injection of plasma to populate the outer radiation belt.

Aggson, T. L.↗
NASA NTRS1977

Observations of large transient magnetospheric electric fields

Transient electric field events were observed with the long, double probe instrumentation carried by the IMP-6 satellite. Nine, clearly defined, exceptionally large amplitude events are presented here. The events are observed in the midnight sector at geocentric distances 3.5 to .5.5 R sub e at middle latitudes within a magnetic L-shell range of 4.8 to 7.5. They usually have a total duration of one to several minutes, with peak power spectra amplitudes occurring at a frequency of about 0.3 Hz. The events occur under magnetically disturbed conditions, and in most cases they can be associated with negative dH/dt excursions at magnetic observatories located near the foot of the magnetic field line intersecting IMP-6. The magnetospheric motions calculated for these electric fields indicated a quasi-stochastical diffusive process rather than the general inward magnetospheric collapsing motion expected during the expansive phases of auroral substorm activity.

Aggson, T. L.↗
NASA NTRS1976

Grad E x B effects on heavy ions in the magnetosphere

Following a theory briefing on the currents in a plasma of mixed species caused by the motion of charged particles, the paper presents a report on the observations of regions of the magnetosphere in which significant values of grad E (E being the electric field) can be inferred from measurements of E made on board the IMP 6 satellite using two pairs of long (91.5 m and 45.7 m) unfurlable antenna sensors. Electric fields were measured by applying double floating probe techniques to the antenna sensors, and grad E values of up to 1.4/1000 emu were inferred from the fluctuations in E along the orbit. The critical values of grad E for various ions, in a range of magnetic fields are compared, and the difference in the behavior of Ba+ ions which cause Ba+ currents in the subcritical and supercritical regions is discussed. The results show that the electric currents flowing in the E x B direction may cause secondary polarization fields on the barium cloud. The possible significance of these effects to natural geophysical phenomena is also pointed out.

Cole, K. D.↗
NASA NTRS1973

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.

Aggson, T. L.↗
NASA NTRS1973

Investigation of mid-latitude ionospheric currents by combined rocket techniques.

A comprehensive range of neutral atmospheric and simultaneous ionospheric structure measurements were obtained from a Skylark rocket launched at Woomera during evening twilight on a quiet magnetic day. Above 150 km, good agreement was found between the results obtained from three different methods of electric field measurement despite the low field strength (about 1.5 mV/m). In the immediate vicinity of an intense sporadic E layer, the probe measurements indicated large excursions of the electrostatic field amounting to 6.5 mV/m at 105.3 km and 2.7 mV/m at 104.8 km. The calculated ionospheric current system in the vicinity of the rocket trajectory was of similar magnitude to that indicated by local ground-based magnetometers, but was diametrically opposed in direction.

Rees, D.↗
NASA NTRS1972

Satellite and ground-based observations of a red arc.

The results of simultaneous satellite and ground-based observations carried out during the red-arc period of August 8-9, 1970, are discussed in the light of present day theory. The formation of the arc at the electron temperature peak and density trough seems to support the thermal-conduction theory of red-arc formation.

Nagy, A. F.↗
NASA NTRS1972

Satellite and ground-based observations of a red arc.

Simultaneous observations carried out during the red-arc period of Aug. 8-9, 1970 at Richland, Washington and by the Ogo 6 satellite are examined in the light of the present-day theoretical knowledge. The observed formation of the arc at the electron temperature peak and density trough supports the thermal-conduction theory of red-arc formation. The lack of dc electric fields in the red-arc region rules out these fields as an excitation mechanism. Particle precipitation as a direct source of red-line emission can also be ruled out by the observations presented. The formation of the arc in the plasmapause region and the detection of low-frequency ac fields are consistent with a theory that predicts wave-particle interaction at the plasmapause as a mechanism for supplying thermal energy to the plasmasphere electrons.

Nagy, A. F.↗
NASA NTRS1972

Recent probe measurements of dc electric fields from IMP 1

The measurement of dc electric fields in the magnetosphere by IMP 1 are presented. Electric fields on the order of 1 mV/m in the solar wind, with about a 40% jump at the bow shock were observed. Observational results are graphed.

Aggson, T. L.↗