Electric fields in diffuse aurora

ELECTRIC Physics Department,

FIELDS IN DIFFUSE

AURORA

Ii.P. MAHON University of Massachusetts at Boston, Harbor Campus, Dorchester, MA 02125, U.S.A. M SMIDDY a4 It. C SAGALYN Air Force Geophysics Laboratory, Hanscorn AFB, MA 01731, U.S.A. (Receioed in firmI form 2 1 Januury 1977)

Ah&act-The ionospheric electric field has been measured in the E region above the Churchill aurora1 research range under quiet and under disturbed conditions. Results were obtained 14 and 24 hr before local midnight over an altitude range of 115-165 km. The instruments and analysis differ from those used by other workers. An unusually advantageous vehicle motion resulted in dipole measurements along the magnetic field being modulated by the vehicle motion. Under quiet conditions and in the presence of a d&se, east-west 2 kR aurora1 arc, the predominant vector component of the electric field was also quiet and between 35 and 40mV/m perpendicular to the magnetic field, southward. Parallel to the magnetic field, the vector component increased from -17 mV/m at 130 km, reversed direction at 160 km during the latter third of the flight and fluctuated around +6 mV/m between 155 and 135 km on the descent. Under disturbed conditions during the recovery phase of a large magnetic storm, the electric field was also more disturbed; however, there was no significant electric field along B. Analysis of effects caused when parts of the measurement system are connected by a common magnetic field line, and when one of the probes lies in the wake of the vehicle, shows that measurement perturbations produced by those effects are dominated by the magnetic field line connections and that wake effects are relatively unimportant. INYXODUCllON The electric field has been measured with a dipolar sensor in the E region on two rocket flights above the Churchill Aurora1 Research Range in Manitoba, Canada. An unusually advantageous spin and precessional motion of each rocket resulted in the measurement along the magnetic field being modulated by the vehicle motion. This has made it possible to eliminate from these results the contact and d.c. offset potentials which seriously limit the knowledge of this parallel component of the vector electric field (Kelley et al., 1975). The first measurements were carried out 11 hr before local midnight under quiet conditions with a diffuse 2 kR aurora1 arc somewhat to the north of the trajectory. The second set of measurements were made 21 hr before local midnight during the recovery phase of a large magnetospheric storm. The purpose of this paper is to describe these results and to report the observation of an electric field component parallel to the magnetic field. The results of an examination of perturbation effects observed when parts of the measurement system are connected by a common magnetic field line, are also given. The first useful measurements of electric fields were obtained from the motion of a barium cloud, released into the ionosphere by a space vehicle and par-

tially ionized by solar ultraviolet radiation (Haser, 1967; Foppl et al., 1968; Haerendel and Lust, 1970; Rosenberg, 1971). This method is sensitive to small forces and is relatively economical. However, it is restricted to times near local dusk in order to obtain satisfactory illumination by the Sun and to positions such that the relative motion of the ionized and the different colored, non-ionized cloud may be observed by triangulation. Also, the analysis of the electric field acting upon the luminous clouds is handicapped by cloud polarization and neutral winds (Rosenberg, 1971; Doles et al., 1973; Zabusky et al., 1973). In addition, the barium cloud has not been sensitive to the detection of the strong parallel electric fields, even when their presence has been indicated by a dipolar instrument (Kelley et al., 1975; see also Scholer and Haerendel, 197 1). As of this date, the most successful measurements of ionospheric electric fields have been obtained from the electric-potential gradient between two (dipole) sensors. At balloon altitudes, measurements from three orthogonal dipoles have been used to interpret the electric field in the ionospheric-equatorial plane, on the assumption that the magnetic field lines at the point of measurement are also electrostatic equipotential lines in the equatorial plane (Mozer and Manka, 1971). 859

H. P. MAHON,M. SMIDDYand R. C. SAGALYN

860

Single dipoles, and multipolar sensor arrays, have also been used on rockets and satellites (Fahleson, 1967; Mozer and Fahleson, 1970; Mozer, 1969, 1973; Maynard and Heppner, 1970; Kelley et al., 1971; Cauffman and Gurnett, 1971). This scheme is founded upon the definition of the electric field (EF) component along the dipole axis, E = A V/d, and the assumption that the only factor aIIecting the potential difference AV between the two sensors separated by a distance d, is the ionospheric electric field in the dipole frame of reference. We report on electric field measurements obtained on two Nike Iroquois rockets launched from Ft. Churchill An improved dipole electric field instrument as well as the method used to determine the average value of the three components of the electric field from data obtained with a single dipole are described. This method does not have to assume the value of the electric field parallel to B which is required to reduce the precession irregularities to zero. In addition, this method is not dependent upon an unknown shift in the plasma-contact potential of the sensors. ELECTRIC

FIELD

EXPERIMENT

The electric field experiment consisted of a sensitive differential electrometer with dipole sensors. The system was capable of resolving fields of 1 mV/m without loss of linearity, in the presence of ambient and motional fields as large as 300 mV/m. Data were presented through three channels using a range expander to avoid loss of resolution through the telemetry (TM) link. The electrometer input resistance was 10” R, which was much higher than night-time plasma impedances above 90 km. Time- and temperature-dependent drifts and other electronic noise from the electrometer introduced negligible error in the results. Payload space limitations permitted inclusion of only one pair of sensors in a dipole configuration. Spherical sensors (61 cm dia.), coated with graphite (aerodag G) were mounted at each end of the 2.34-m dipole. The deployed dipole was located 83 cm from the tip of the payload and 3.92 m from the exhaust of the Iroquois motor. Although magnetic shadowing of the sensors by the long Iroquois tail was frequently observed and will be discussed later, no interference from the TM radiation, nor from motor outgassing was detected and on both llights relativeIy small interference from wake effects was observed. The validity of the dipolar measurement of the electric fields requires high symmetry with respect to the plasma in order to ensure equal current flow

to both sensors, and requires negligible loading by the electrometers (Fahleson, 1967). The latter was implemented by placing the electrometers adjacent to the sensors in an enlarged portion of the supporting boom to minimize the capacitance between the sensor and the electrometer input. Symmetry with respect to charged particle flow in the Earth’s magnetic field was implemented by continuing a section of the same diameter out through the other side of the sensor for a distance greater than the plasma shielding length. The electric field, E’ derived from the potential difference data represents the component parallel to the dipole as measured in the moving frame of the space vehicle. The ambient EF in the Earth fixed frame is given by the transformation E=E’-VxB, where V is the spacecraft velocity relative to the Earth and B is the Earth’s magnetic field. V X B is of the order of 60 mV/m for the yehicle trajectories used in this investigation. Since V x B is of the same order of magnitude as the ambient EF, accurate knowledge of both V and B is necessary. The vector orientation R of the dipole was obtained using data from a Whittaker miniature attitude reference system, MARS, and with ancillary data from two Schoenstedt flux gate magnetometers. The gyroscope functioned extremely well on these two flights, Because of the large precessional motion during night-time conditions this aspect data was indispensible to our determination of the dipole vector orientation. The position and velocity were obtained from radar trajectory data with one of the two systems used for tracking locked to a Vega 2065 beacon transponder in the payload. Uncertainty in determining vector orientations are of the order of 1” initially and increase to a little less than 2” at the end of the gight. Total timing errors are less than 4 m sec. Maximum uncertainty in the magnitude of the electric field is of the order of 2mV/m at the end of flight, and approximately half that value at apogee. For the first flight this uncertainty is a constituent predominantly of E,, the northward field component. The eastward electric component is measured with a maximum error of
10 December ~~g~l

rocket was The first launched at 04:31:00.2 GMT, 10 December 1969 at an elevation

861

Electric fields in diffuse aurora angle of 86” and reached an altitude of 167 km. The payload spin rate was approximately Z$rev/see; it precessed once each 27 set with a very wide angle. This spinningtumbling performance, in contrast to the measurements previously reported (Mozer, 1973, p. 293), enables all three components of the electric field to be determined from an a.c. signal, thus removing the d.c. errors that have been a problem in previous measurements of parallel B fields. This motion illustrated in Fig. 1 resuited in the dipole being frequently almost parallel to 8. Twice during the flight the dipole passed to within one degree of parallel o~entation with B. Therefore, the vehicle performance, which might be described as “irregular,” was in fact quite advantageous for the objective of measuring a threedimensional electric field from the projection of a single component at any one time along the dipole axis, and ideal for determining the field along B. Approximately 200 set of data were obtained above 115 km. Typical data obtained from a portion of the flight near apogee are shown in Fig. 2. The potential difference between the sensors multiplied by - 5 is represented by Curve A. The prominent features of the data are the moduiation of the voltage measured due to the spin of the vehicle, and the small double-peaked prominences (e.g. at 210 set), at times when a probe is connected by a magnetic field line (a relatively high conductivity path) to the rocket. Data multiplied by 100 are represented by Curve B, and the offset voltages to the TM channel monitoring the step voltage is shown by Curve C. For example, at 205 set, the result of switching between two steps

FIG.

to keep on scale the positive going signal represented by Curve B can be seen. Throughout the remaining discussion, the motional electric field, VXB, has been subtracted so that the results represent EF components in the Earth fixed coordinate system. Results for the portion of the flight above 115 km are shown in Figs. 3-5. In Fig. 3, the north-south, east-west and down pointing components Ex, E, and E,, respectively, are given vs time. These results are obtained by assuming that the ambient field E remains constant in magnitude and direction throughout a 20-set calculated period (200 data points). Each sample of V,,,, the potential difference between sensors, is given by: V,,,=E’.R+V,=(E+V

x B)*R+V,.

V and B are known. R, the dipole position vector, which rotates with a combination of two sinusoidal motions with the spin period (9 set) and precession period (27 set) is also known. The voltage V, in the above equation is the sum of the contact potential difference between each of the sensors and the plasma and the zero drift of the two input electrometers. By trying different sampling periods, it was found that a 20-set period best displayed the ambient d.c. electric field values obtainable from the data. The method of analysis was to assume V, remained constant and E varied linearly with time through the sampling period. A least squares fit to the data gives values of V, and E. The sampling window is moved forward 2 set and the fitting process repeated, obtaining with each repeat a measure of E and V,. The curves of Figs. 3-5 show

~.TYPYCALR~~TMO~ON~~~~TO~GN~C~ELD DECEMBERFLIGHT.

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FIO. 2. ELJXIIUC FIELDEXPERIMJZNT OUTPUIS AS A FUNCTION OF TIMJZ. and B represent electrometer gains of -5 and +lOO, respectively. C represents the amplifier offset for output B. 10 December 1969.

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10 December 1969.

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864

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M. GIDDY and R. C. SAGALYN

the components of the average magnitude of E obtained from this process. The fluctuation observed over times shorter than 20 set in Figs. 3 and 5 are merely an indication that E is not, as might be expected, constant over the 20-set period. The significant results are obtained from the mean over longer time periods. The straight line results for each of the data components obtained using a 200set linear fit to the entire data set are also shown in Fig. 3. Figure 4 is a vector representation of the electric field at various points along the rocket trajectory. The magnitude of the electric field varied between 32 and 43 mVfm. As can be seen in Fig. 4 the direction of the resultant field changed from 11” east of south early in the Aight to 30“ west of south (and dipping to 15” below the horizon) before re-entry. The largest vector component of the electric field, E,, was essentially constant in direction and magnitude throughout the flight at approximately -35 to -40 mV/m (pointing south). The east vector component E,,, decreased from about lOmV/m at 115 km, to between - 10 and -20 mV/m (directed west) on the downward leg. The z-component which early in Bight, was - 12 to -5 mV/m (pointing upwards) increased steadily to lOmV/m (downward) as the rocket dropped below 155 km. A south-pointing direction for the night-time E component of the electric field measured in the aurora1 zone before magnetic midnight, is a feature Kelley et al. (1971) have found and is in agreement with the measurements by other workers. Heppner (1972) has noted the association of southerly directed electric fields with negative bays and of opposite polarity electric fields with positive bays during magnetically active conditions. Figure 5 shows the components of the etectric field in a coordinate system tilted 6$” such that it is aligned with the Earth’s magnetic field and with the positive x-axis taken in the northerly direction. The variation during this flight, of the electric field component parallel to the Earth’s magnetic field is given by ErB in this figure. The existence of electric fields parallel to the Earth’s magnetic field has been a subject of considerable controversy. The important result supported by these measurements is that substantial electric fields parallel to the Earth’s B field do exist. A maximum magnitude of 17 mV/m directed upward, antiparallel to B was measured at 130 km. The magnitudes of the fields measured are well above the possible experimental errors and differ markedly from the results using the same techniques, obtained during subsequent flights.

Around 245 set, the field aiong B passed through zero and reversed direction as the rocket dropped below 160 km. Sharp field reversals have been associated with polarization boundaries (Kelley er al., 1971). Field reversals would be expected at the boundaries of current systems and might also be due to the consequences of soft particle precipitation. The Churchill range is at geomagnetic latitude 68”48’ (invariant latitude A = 70.03’) corresponding to an L value of 8.57. This is a region where strong temporal and spatial changes exist and the boundaries between precipitation regions are complex. The field reversals observed may correspond to the crossing of these boundaries. The sIow nature of the field reversal measured is consistent with the diffuse aurora1 conditions existing at the time of flight. The specific mechanisms which allow parallel fields of this magnitude to be supported in the E region are not understood at this time. Mozer (1975) has discussed the theoretical basis and experimental evidence for the presence of anomalous resistivity and parallel electric fields at low altitudes. He concludes that there is strong evidence that wave turbulence is a dominant process at low altitudes in the aurora1 zone and thus it is a region where anomalous resistivity effects may be observed. The results reported here strongly support the existence of anomalously high plasma resistivity along the magnetic field. Workers from three other laboratories have reported evidence for large parallel fields (Mozer, 1973; Fahleson, 1972; Kelley et al., 1975; Mozer and Fahleson, 1970). During the flight, a quiet and stationary arc of approximately 4 kR extended between the east and west horizons. It was only barely discernible on the all-sky photographs to be somewhat north of the rocket trajectory. The absence of north-south drift in the behaviour of the aurora1 arc shown by the ail-sky photographs indicates that the perpendicular component of the electric field is a constituent of the ionosphere rather than the result of mapping into the magnetosphere. The northerly component of the electric field, Ed, perpendicular to the magnetic field drives the Hall or east-west current system. The xcomponent of the magnetic field at the Earth is sensitive to variations in the east-west current systern and hence to variations in E,,. In Fig. 6 are shown the ground-based data from the Schoenstedt magnetometers for the north, east and downward magnetic components B,, B,, B,, respectively, and from the riometer which was relatively constant

Electric fields in diffuse aurora r’

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ALTITUDE (km) FIG. 6. GROUND-BASED

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DECEMBER 1969 ROCKET MEASUREMENTS.

and near the base line value of 15.2 dB of absorption. The absence of variations in I?, during this period is consistent with the essentially constant, x-component of the electric field. A quiescent condition for the ionosphere is indicated by these data. Activity throughout the magnetosphere was also low during this period. For the 3 days previous, the Zurich sunspot number had been less than 53. The proton flux measured by Pioneer VI in the solar wind component due to arrive at the Earth’s bow shock a few hours prior to this measurement was 1.8-1.9 ems3 at a velocity of approximately 580 kmlsec. Data from Explorers 33 and 35 indicated that the flux from solar X-rays in the 2-12 8, spectrum was notably quiet the day before launch and that the peak-to-valley ratio for the day of the flight was low at 4-. The Kp index was 0+ during the flight and had been l- during the previous 3 hr interval. Trajectories of AZUR and OGO-6 satellites were positioned in the dawn sector and their data also point to the quiet nature of the magnetosphere and indicate that the precipitatin boundary at that time may have been to the south of the point of launch (Rossberg, 1974). Thus the electric field data obtained on the first flight are consistent with the quiet nature of the magnetosphere. They are interesting in comparison with the electric field data obtained under the more disturbed conditions of the second flight.

Q March flight The results from a second flight on 9 March 1970 (03~2555.1 GMT) shown in Fig. 7, were considerably different. This flight occurred during the recovery phase of an extensive magnetic storm (see Fig. 8) following a 7007 negative bay. The rocket also had a Q-set spin period and a flat precession with a 27-set period. This tumbling motion made it possible to determine ah three components of the electric field throughout the flight. As on the first Right, the dipole took on ail spatial orientations with the result that (a) it was possible to derive all three components of the electric field from an a.c. electric field signal thereby e1iminating.d.c. contact potential and offset errors from the result; and (b) any effect of the wake on the dipole measurement would be the same on the ascent and descent since the dipole passes through the wake in both flights. Results of the electric field measurements show the components to be irregular in both direction and magnitude (Fig. 7). There is no significant component measured parallel to the magnetic field. At the 115 km level on the ascent a IO mV/m field was measured, directed to the south-west. This quickly died away and up to apogee the total field varied about zero with a very irregular direction. On the descent, the westward component, -Era, grew to about 20mV/m, as the rocket dropped

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below 140 km, and fluctuated about this level for the remainder of the flight. Below 140 km the northward component of the field Eti grew in magnitude and reached about 35 mV/m at 100 km. The descent measurements were made in a region

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of low or non-existent optical activity. Throughout this flight, during considerably more disturbed ionospheric conditions than the flight of 10 December, there was no significant potential drop along the magnetic field. The overall motion of the

Electric 6eld.sin ditfuse aurora A07.907 -2 9 MARCH 1970 To = 03121155.12 I

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The proper operation of the electric field measurements depends upon the existence of a high degree of symmetry of the dipole sensor system with respect to the ionosphere. Two mechanisms can reduce this symmetry: the so-called wake effect (Fahleson, 1972; Knott, 1970; Rawer and Spenner, 1970) where one end of the probe is in the plasma disturbed wake behind the forward velocity of the rocket, and magnetic shadowing. The 10 December flight results have been analyzed to determine the influence of these effects upon the electric field measurements. The geometrical magnitudes for these effects, together with their associated perturbations of the experimental data, which appear in the residual of the analysis, are shown in Fig. 9.

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aurora1 forms during the entire measurement interval was northerly. Neither the optical appearance nor the motion of the aurora1 forms appear to be strongly correlated with tire local electric field components. The results obtained on these two flights indicate the complexity and variability, both spatial and temporal, of the electric field in the high latitude E region. MAGIC

1

A!?lTUDEIS0 (km

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9 MARCH ROCKET

(A) Magnetic shadowing of the sensor by some portion of the rocket occurs frequently, and produces a recognizable artifact in the data during this interval. The anisotropic conductivity is higher along magnetic field lines. The various surfaces of the rocket and payload sections are charged to different, non-equilibrium potentials in the plasma. During the interval when a magnetic field line intersects the spinning rocket and the sensor (s), there is an asymmetric connection by a path of decreased resistivity to an artificial potential. In addition, the rocket represents an obstacle to charged particle flow along field lines during this period, and results in a perturbation to symmetric current collection by probes. The results of this flight show that the asymmetry is reduced when both sensors are connected by paths of approximately equal lengths and areas to an equipotential surface on the payload. The times during which connections occurred between sensor (s) and rocket payload along magnetic field lines are indicated in Fig. 9. The vertical amplitudes during the connection are found to be inversely proportional to the geometrical term of the plasma conductivity, that is, inversely proportional to the shadowing area divided by the path length. It is important to realize that the

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(a) DIFFERENCE (RESIDUAL) BETWEENEXPERIMENTALDATAAND DATA RECONSTRUCTEDFROM ELECl?UCFIELD COM'0NEN-l-S: (B) kfAGNETICSHADOWlNG: (C) WAKEEFFECTS,EACH VSTIMEAND ALTITUDE.

characteristic duration of the shadowing event is approximately 2 sec. (B) Wake effects have resulted in data artifacts which have been found to be less significant than those caused by magnetic shadowing. The times during which one of the probes is within the geometrical wake of the rocket are indicated in Fig. 9. The vertical amplitudes are inversely proportional to the probe-rocket path length, in a direction parallel to the rocket velocity vector. The polarity symbols indicate which probe is within the geometrical wake. Early in the flight, the velocity vector is more nearly aligned with the general direction of the magnetic field and hence, the events in Fig. 9(b) and (c) occur closer together than, for example, at apogee. (C) The curve of Fig. 9(a) is the residual of the least square analysis. This is the difference between the experimental flight data and a reconstruction of the voltage between the sensors based upon the spin and precession of the dipole and the electric field components shown in Fig. 3. This analysis shows that the residual is composed primarily of effects attributed to magnetic shadowing, atmospheric noise spikes and wake effects. These are listed in order of their importance, magnetic

shadowing having a maximum effect. All of the components of the residual of amplitude >2 mV can be associated with one of these effects. The atmospheric noise spikes are identified as having close to zero width and having no noticeable effect on the curve adjacent to the spike. Noise spikes occur most frequently before 130 sec. Data earlier than 100 set were not used in the analysis because of large noise signals. Figure 9 also shows that wake associated effects do not make a large contribution to the residual. For example, the wake event at 173 set, which is distinctly separate from magnetic shadowing effects, is associated with a residual with an amplitude of the order of only 3 mV. As shown in Fig. 9, the residual associated with magnetic shadowing has a narrow central peak with a width typically 1 set, and a wider shoulder with an approximately 3-set width. Almost all of the residual over 2 mV in amplitude can be associated with a magnetic shadowing of one or both probes. Magnetic shadowing events (Fig. 9b) at 193, 212 and 222 set are clearly separated from other events. The effects are much larger than that due to the wake effects as can be seen at 173 set, for example. The magnetic shadowing with the more

Electric fields in diffuse aurora

distant and insulated tail section of the rocket, produces a residual of opposite sign as illustrated at 245 sec. This is also greater than any wake effect. The extraction of these shadowing artifacts in Fig. 9(a) provide further evidence of the proper analysis of the flight data and in part, provide confirmation of the proper choice of analysis period for the least square fit. If an analysis period is used which is much shorter, say $ set, these artifacts are removed from the residual and the perturbations show up in the electrical field components. Also, it has been seen from the results of 9 March that employment of the same analysis methods on data from a rocket of quite similar flight behaviour under greatly disturbed ionospheric conditions give very different results for the electric field components. This choice of analysis period also means that rapidly varying field components of ionospheric origin will be extracted as indicated by the appearance of the noise pulses in Fig. 9(a) at about 115 sec.

869

effect on the electric field measurements. Electric field measurements obtained on a 9 March rocket flight during the recovery phase of a large magnetic storm show no significant electric field component along B. The results of both flights indicate that all three vector electric field components may be determined from a single dipole when an a.c. signal for each component is obtained as a result of a wide precession angle, tumbling performance of the spinning rocket. Acknowledgements-We would like to thank P. Wildman for a number of useful discussions and L. Rossberg for making his unpublished work available to us. We happily acknowledge the engineering assistance provided by W. Sullivan with technical support from A. Romanelli and J. Borghetti, and manuscript preparation by J. Noonan. The electric field numerical calculations were done by M. Schneeberger and R. Doherty. We thank the staff of the Churchill Research Range for the numerous services and courtesies extended to us. The support of E. Cronin and the Analysis and Simulation Branch is gratefully acknowledged.

SUMMARY

E-region electric field measurements obtained in the vicinity of a diffuse aurora under quiet ionospheric conditions on 10 December have shown the following: 1. The vector electric field was obtained over the altitude range 115-168 km. The magnitude was found to vary between 32 and 43 mV/m throughout the flight. At 134 km on the ascent, the direction of the component perpendicular to the magnetic field was 8” east of south, and on the descent it shifted to 21” west of south. 2. The predominant vector component was southerly, with a magnitude between 35 and 40 mV/m. 3. An electric field component along B was observed throughout the flight. It reached its maximum value of 17 mV/m, antiparallel to B at 130 km on the ascent. This represents a potential drop of the order of l/2 kV over the altitude range 116-167 km. On the descent it reverses sign and the parallel value is about 6 mV/m for a period before re-entry. The result is important for the understanding of high-latitude processes, as it shows that finite resistivity exists along the magnetic field during certain quiet aurora1 conditions. 4. An analysis of vehicle motion induced effects on the electric field measurements shows that sensor-rocket magnetic field line connections are the major perturbing influence on the measurements. 5. Wake effects were found to have a negligible

REFERENCES

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