auroral particle precipitation and birkeland...

VOL. 12, NO. 2 REVIEWS OF GEOPHYSICS AND SPACE PHYSICS MAY 1974

Auroral Particle Precipitation and Birkeland Currents

R. L. ARNOLDY x

Space Science Center, University of Minnesota Minneapolis, Minnesota 55455

Correlated measurements that provide information on Birkeland (field aligned) currents are reviewed. Because of the obvious importance of field-aligned electrio fields with regard to Birkeland currents, the last sectiov of the paper is devot. ed to a presentation of recent data on the observations of field-aligned auroral electron fluxes and monoenergetic peaks in the spectrum. Experimental data cannot yet define the overall magnetospheric pattern of the Birkeland current system. The largest body of data regarding these currents comes from polar-orbiting satellites. The. se data indicate that current sheets are associated with auroral displays and that field-aligned electron fluxes on the order of 1-keV energy are significant charge carriers for the conventional current out of the atmosphere. Rocket observations verify that electrons responsible for the production of auroral forms carry a significant fraction of the current necessary to produce local magnetic effects that require Birkeland currents for their explanations. There are a few measurements that suggest that the return current might be carried by low-energy electrons (less than a few hundred electron volts) streaming out of the atmosphere.

In devising a horizontal distribution of ionospheric currents to reproduce the observed high-latitude magnetic perturbations that he labeled 'Polar elementary storms' (now called substorms), Birkeland, in 1908, recognized that the actual system might well be three-dimensional, that is, a system in which the horizontal currents are fed by vertical or magnetically aligned currents from above the atmosphere. The intent of this paper is not to review theoretical models that have been proposed for this three-dimensional current system but rather to present experimental evidence, preferably correlated measurements that have been made to date on field-aligned currents (henceforth referred to as Birkeland currents). There are in general two ways of ob- taining experimental information on the existence of and the pattern of Birkeland currents. The first would be a direct measurement of the charge carriers responsible for the current. A second means is an indirect measurement by observ- ing the effects of the current, such as magnetic perturbations and ionospheric heating. Finally, statistical studies provide information as to the large-scale configuration of the current system for the magnetosphere. This latter technique necessarily involves a study of a direct measurement of the charged carriers or of the indirect effects. The indirect measurement of Birkeland currents has perhaps to date given us the most information insofar as the actual particle measurements require sensors to be immersed in the current system.

With regard to particle measurements I would like to clarify at this point the use of the terminology 'field-aligned.' Field-aligned currents are defined as the net transport of charge per unit time through an area perpendicular to the local magnetic field. This means that a particle differential flux measurement would have to be integrated over all energies and over the pitch angle distribution to obtain the net flow of charge through an area perpendicular to the local magnetic field. The term field-aligned fluxes refers to the pitch angle distribution of the particles and is a distribution favor- ing the small pitch angles. Field-aligned fluxes expressed in

•On leave from the Space Science Center, University of New Hampshire, Durham, New Hampshire 03824.

Copyright ̧ 1974 by the American Geophysical Union.

units of particles (cm: s sr keV) -• do not necessarily make the largest contribution to field-aligned currents, since the solid angle represented by small pitch angles is only a minor fraction of a steradian. Since field-aligned electric fields are a possible driving mechanism for Birkeland currents, I would like to spend some time in this paper discussing particle measurements that suggest the existence of such fields, i.e., monoenergetic peaks in electron spectra and field-aligned precipitated fluxes.

EXPERIMENTAL EVIDENCE FOR BIRKELAND CURRENTS

One of the first observations supporting the concept of Birkeland currents was made by the magnetometer aboard the geostationary spacecraft ATS 1 [Cummings et al., 1968]. The similarity of substorm signatures seen at 6.6 RE and below the satellite on the surface of the earth necessitated at-

tributing the equatorial observation not in terms of a return ionospheric current but rather a current in the magnetosphere at a radial distance beyond the ATS spacecraft. Cummings et al. [1968] modeled the current system in terms of a partial ring current coupled by means of field-aligned currents to the auroral electrojet. Earlier, Cummings and Desder [1967] had suggested that the transverse magnetic fluctuations of up to several hundred gammas observed by Zmuda et al. [1966] at 1100-km altitude over the auroral oval by the polar-orbiting satellite 1963-38C could not be explained in terms of hydromagnetic waves because of their restricted location but rather interpreted them in terms of Birkeland currents. The characteristics of such disturbances have been further dis-

cussed and also interpreted in terms of such currents by Armstrong and Zmuda [1970].

A model of the Birkeland current system that fits the observations was put forth by Armstrong and Zmuda and is reproduced in Figure 1. The Birkeland currents are east-west sheet currents with the current directed out of the ionosphere on the lower-latitude sheet and into the ionosphere on the higher-latitude sheet. The authors suggested that the current was carried by electrons, since they have higher mobility along the field line than ions, and that the lower-latitude sheet terminates in the ionosphere in an auroral display at the location of the westward electrojet. Armstrong and Zmuda pointed out that the current density needed, of about 9 X 10 -5

217

218 ARNOLDY: AURORAL PARTICLE PRECIPITATION AND BIRKELAND CURRENTS

A m -•', could be produced by a reasonable flux of energetic auroral electrons. The higher-latitude return current was speculated to be due to the flow of low-energy electrons out of the ionosphere.

Direct measurement of Birkeland currents by means of particle detectors is a far more difficult observation. Particle

detectors measure only the flow of charge at their location; moreover, to ascertain a net flow of charge, the energy spectrum and pitch angle distribution over 180 ø must be measured. The observation of geomagnetically aligned electron fluxes with rocket detectors [Lampton et al., 1967; Ar- noldy,' 1970; O'Brien and Reasoner, 1971; Bosqued et al., 1971' Whalen and McDiarmid, 1972; Evans et al., 1972; Cloutier et

al., 1973] and satellite detectors [Hoffman and Evans, 196•; Holmgren et al. • 1970; Paschmann et al., 1972] suggests that these particles play an important role as Bitkeland current carriers.

The observations of the field alignment of 0.7- and 2.3-ke V electrons by the Ogo 4 auroral particles experiment first reported by Hoffman and Evans [1968] and further studied by Berko [1973] prove to be most interesting. The field alignment

of 2.3-keV electron fluxes was measured by the Ogo 4 detector to be present in approximately 2% of all precipitation events observed. The field-aligned fluxes, or bursts as they have been called, are concentrated in the nighttime hours in an oval- shaped region coincident with and extending poleward of the hard or band precipitation region and the optically defined auroral oval. This spatial relationship is consistent with a sounding rocket observation by Whalen and McDiarmid [1972] in which auroral electron precipitation was detected to be field-aligned as the rocket pay load passed through the northern edge of a visible display. Berko has statistically compared the location in invariant latitude and magnetic local time of the occurrence of 10 or more field-aligned particle events with the region in which transverse magnetic disturbances were statistically observed at high latitudes by the magne.tometer aboard the polar-orbiting satellite 1963-38C [Zmuda et al., 1970]. This comparison is reproduced in Figure 21 ,in which the latitudinally widest region Of transverse magnetic disturbances is plotted superimposed on the regions of more tha n 10 Ogo 4 field-aligned precipitation events indicated by the cross hatching [Berko, 1973]. The regions of

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Fig. l. Field-aligned current model [from Armstrong and Zmuda, 1970].

ARNOLDY: AURORAL PARTICLE PRECIPITATION AND BIRKELAND CURRENTS 219

90

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Fig. 2. Statistical correlation of Ogo 4 electron bursts, Ogo 4 search coil fluctuations, and transverse magnetic disturbance detected by 1963-38C [from Berko, 1973].

24

occurrence of Ogo 4 search coil fluctuations are also indicated in the figure. General agreement in the location of magnetic field fluctuations and field-aligned precipitation is good ex- cept between 0800 and 1400 MLT when very few particle events were detected, presumably because the precipitation at these hours is below the energy threshold of the experiment.

Recent measurements of the charged particle precipitation during auroral events made aboard sounding rockets with detectors simultaneously looking up and down the field lines have been reported by Choy et al. [1971] and Cloutier et al. [1973]. The Birkeland current density calculated from the charged particle measurements made by Choy et al. for one of the flights is shown in Figure 3 along with the location of the measurements relative to the auroral display. The rocket location projected down magnetic field lines to the 100-km level is given in the top panel of all-sky photographs by black or white dots. The middle panel gives the current expressed in amperes per square meter as a function of flight time, and the bottom three panels indicate where in the electron spectrum the major contribution to the current originated for three flight times. For the first 190 s of flight time the rocket was magnetically connected to auroral luminosity. From 190 to approximately 230 s, when the rocket was beyond the northern edge of the arc, the current increased by nearly an order of magnitude and was carried predominately by electrons of energy less than a few hundred electron volts. This enhanced current carried by a large flux of low-energy electrons precipitating into the atmosphere was labeled a current

sheet by Choy et al. Beyond the current sheet and further north of the auroral display there were several periods of time when the current went to extremely low values or actually reversed direction. During times of reverse flow, there was a net streaming of electrons of a few hundred electron volts out of the atmosphere. Figure 4 shows sample spectra sorted ac- cording to pitch angle at three times during the flight as seen by the up and down viewing' detectors. In the panel on the left are spectra when the rocket was magnetically connected to the auroral luminosity; the 'monoenergetic, peak' in the spectrum is evident at this time. In the center panel, spectra sorted ac- ccrding to pitch angle are shown at a time when the rocket was passing through the current sheet. Note the intensity and softness of the spectrum. Finally, the spectra in the right-hand panel are typical of times when the conventional current reversed direction and was flowing into the atmosphere owing to the streaming of low-energy electrons out of the atmosphere below energy of a few hundred electron volts. Figure 5 is a schematic diagram of the current measured during the flight with respect to the auroral arc. Conventional current out of the atmosphere was carried by the auroral electrons producing the luminosity, and the intense current sheet was on the order of 10 km thick, if one can assume that the observations were a spatial variation rather than a temporal one. The return current north of the arc is speculative, being based on several observations when such a current was

measured for short durations of time; thus it is suggested that the rocket might have been about to enter a region of steady


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Fig. 3. Field-aligned current measured by sounding rocket detectors [from C•oy el al., 1921]. (Top) All-sky photographs of the auroral display. The rocket location projected down magnetic field lines to the 100-kin level is given by black or white dots. (Middle) Current expressed in amperes per square meter as a function of flight tiae. (Bottom) Loca- tion in the electron spectrum of where the major contribution to the current originated for three flight times.

return flow. The rocket did not pass south of the auroral arc; hence there is no information from this flight as to whether return currents might be found at this location with respect to the auroral display. The suggestion here that a return current existed north of the arc is consistent with the model put forth by Armstrong and Zmuda '[1970] to explain their satellite magnetic observations. Magnetometer measurements were made aboard this flight by the University of Minnesota group headed by L. J. Cahill, Jr.; however, the sensitivity of the instruments was not adequate to sense a signature for current on the order of 5/•A/m :. This magnitude of Birkeland current would amount to a total angular shift of the earth's field on the order of 0.24ø.

With a sensitive magnetometer system flown aboard sounding rockets the Rice University group has compared the magnetic signature of Birkeland currents with auroral particle measurements [Cloutier et al., 1970, 1973; Vondrak et al., 1971; Park and Cloutier, 1971]. The vector magnetometer system consisted of a cesium-vapor sensor and a single-axis bias coil system aligned perpendicular to the vehicle spin axis.

This magnetometer allowed reconstruction of the ambient field vector with a resolution of roughly 1-• magnitude and +0.05 ø in angle. The particle detectors were orientated such that the precession of the rocket pay load provided pitch angle scans. For the flight of February 1969 the detectors were sensitive to electrons having energies greater than 2 keV. The magnetometer data obtained from the 1969 flight over a homogeneous and stable arc during a small substorm of approximately 50-• magnitude were modeled with the current system shown in Figure 6. The return current, i.e., the conventional current into the atmosphere, flows south of the auroral arc and electrojet. Cloutier et al. [1970] point out that this is a fit to the data and certainly by no means unique. A similar model has been fit to data obtained by the Rice group from a sounding rocket flight in February 1971 [Cloutier et al., 1973]. Conclusions drawn from these measurements are as follows:

first, no purely horizontal current configurations could be found that reproduced the magnetic data; second, the maximum flux of precipitated electrons coincided with the visible arc and the horizontal electrojet; third, the upward


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geomagnetically aligned sheet current as inferred from the magnetometer data was approximately coincident with the maximum precipitated electron flux; and finally, precipitated electrons in the energy range from 2 to 18 keV carried a significant fraction (one third) of the total upward current. No significant energetic fluxes were observed in the region of the downward current inferred from the magnetometer data.

I wish to turn now to satellite observations, which present evidence of a return current carried by electrons of energy less than a few hundred electron volts. For one pass, A ckerson and Frank [1972] established that the precipitation of inverted V

electrons just north of the 45-keV trapping boundary observed by the polar-orbiting satellite Injun 5 was closely associated with a visible aurora. Figure 7 shows the correlation that they have made using all-sky camera data recorded below the satellite and the measurements of electrons greater than 45 keV and between 50-1500 eV made by the Injun 5 detectors. The inverted V event was detected between 01h

52m 30s and 01h 53m 00s UT. During this time, as indicated in the upper schematic sketches of all-sky camera data, the satellite was magnetically conjugate to and passed through the eastern edge of an auroral display. The auroral precipita-

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tion is called an inverted V event because of the characteristic

shape on an energy-time spectrograph in which the electron spectrum is soft at the edges and hard in the center of the precipitation band. These data also show that at the northern edge of the inverted V the precipitated flux exceeds by almost 2 orders of magnitude the trapped flux; hence there is field alignment of particles during this time. It is during times of the inverted V events, specifically, times of soft electron precipitation, that there is good correlation between the low- energy electrons and the observation of VLF with sensors aboard the lnjun 5 spacecraft [Gurnett and Frank, 1972]. The VLF produced above the spacecraft is observed as well as events that Gurnett and Frank have termed saucers

propagating up from the atmosphere below the spacecraft. When the spacecraft is over the southern hemisphere and the In jun 5 detectors are looking downward into the atmosphere, 100-eV electrons have been detected in coincidence with VLF

observations. Hence Gurnett and Frank speculate that it is these electrons streaming upward from the atmosphere that are responsible for the production of saucers. Figure 8 shows the correlation of low-energy electrons at pitch angles of 90 ø streaming out of the atmosphere (a = 0 ø) when Injun 5 was over the southern hemisphere, VLF being observed at 7.35 and 70 kHz. The 45-keV trapping boundaries are indicated by the dashed vertical lines. These data support the rocket measurement by Choy et al. [1971] in which low-energy electrons were observed to be streaming from the atmosphere north of the auroral display. One might speculate here that perhaps the atmospheric low-energy electrons observed by both Injun 5 and the sounding rocket are the high-energy tail of the return Birkeland current.

Another correlation of satellite measurements of

precipitated particles and auroral luminosity has been made by Morse et al. [1973] with detectors aboard an Air Force polar-orbiting satellite. Figure 9 shows the count rate in the 6.5-keV channel of the low-energy electron detector plotted to the same time scale as that indicated in the lower photograph along the trajectory of the satellite. The photograph is that of the visible auroral emission measured by a sensitive scanning

radiometer aboard the spacecraft having a resolution of 3.7 km. Geographic north is centered on the lower edge of the photograph, and the local time at the center of the photograph is approximately 1930. The discrete and diffuse parts of the aurora extend for this particular event more than halfway around the oval. The correlation of particle precipitation with the auroral light is in general quite good, although there is a great deal more structure in the particle precipitation that can be resolved in the photograph. Particle spectral measurements made aboard the spacecraft indicate that north and south of the aurora the electron spectrum might be ap- proximated as a power law, whereas over visible forms there is a large number of particles in the 1- to 10-keV range and the spectrum might be best fit with a power law plus a Max- wellian distribution. Frank and Ackerson [1971] have fit auroral electron precipitation detected by Injun 5 in a similar manner.

Haerendel et al. [1971] have inferred the existence of field- aligned currents as a result of an analysis of ground level magnetometer data at the time of an electric field measurement at 12.5 Re by a barium cloud experiment released from Heos 1. Magnetic perturbations measured aboard the Heos 1 satellite were also compared with the E field and ground data. The data suggest the existence of Birkeland currents flowing from the near tail along the horns of the plasma sheet into the morning side of the auroral oval. This would be a current system consistent with the picture of A kasofu and Meng [1969] and originally suggested by Birkeland [1908] in which Birkeland currents feed the end of the auroral electrojet. Evidence for the Birkeland currents was derived from (1) an analysis of the ground magnetograms, in particular, fid at the mid-latitudes, (2) the inability of the electric field measured in a certain region of the polar cap and mapped into the ionosphere to drive an ionospheric current that would give rise to the observed magnetic perturbations at the nearest observatory, and (3) an analysis of the transverse magnetic perturbations observed on Heos 1.

A correlation of data from particle detectors has been made by Berko et al. [1973]. Field-aligned fluxes of electrons of 0.7,


2.3, and 7.3 keV were correlated with the UCLA Ogo 4 search coil magnetometer. Pitch angle flux anisotropies for the 0.7- and 7.3-keV detectors were inferred from the pitch angle measurements made by the 2.3-keV detector. An example of such a correlation is given in Figure 10. The degree•of field alignment of the electron fluxes is indicated by the curve marked ratio. For example, during the segment of data marked A, the 0 ø pitch angle 2.3-keV detector measured a flux that was 10 times larger than the one measured by the 60 ø pitch angle 2.3-keV detector. It is immediately apparen.t from Figure 10 that during times when particle fluxes were field aligned, the search coil magnetometers showed the largest fluctuations. Berko et al. have attempted to qualitatively and quantitatively compare the particle data and the search coil fluctuations. It is, of course, impossible to separate space and time variations; hence the authors have interpreted the search

coil fluctuations in terms of three models. The first was a tem-

poral variation model, the second, a spatial variation model with the satellite passing through or near the center of a cylin- drical current configuration, and, finally, a spatial variation model with the satellite passing through a current sheet. The authors interpret the search coil data for time periods A and B in terms of the spatial variation model with the satellite passing through the center of a circular field-aligned current system. The idealized wave form expected for these cases in the X search coil output is indicated in Figure 10 by the smooth curves above the X component data. Assuming that the pitch angle distribution as measured by the 2.3-keV detectors is applicable for 0.7- and 7.3-keV detectors and fitting a power law spectrum to the particle data, Berko et al. calculated for event B in Figure 10 the Birkeland current carried by the particles under the assumption that the current

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was of a circular cross section and that the satellite moved

through the center of this configuration. The authors con- cluded that the majority of the current carriers were actually measured by the particle detectors because the amplitude of the disturbance field calculated from the modeled Birkeland

current and that measured by the search coil agreed. The time marked C in Figure 10, coincident with the high-latitude cutoff of the 7.3-keV electron precipitation, has been iden-

titled by the authors as a temporal variation in the current consistent with the ,g_ and Y search coil data. Although a large quantity of Ogo 4 data was collected, only a limited number of successful simultaneous particle and field measurements were made. This is expected, since the search coil magnetometer responds to changes in B without actually passing into or through the current system, whereas the particle detectors respond only to particles actually reaching the

ARNOLDY.' AURORAL PARTICLE PRECIPITATION AND BIRKELAND CURRENTS 225

instrument and then only if their energies lie within the range of the instrument. As a result of the good quantitative comparison in the few cases observed, however, Berko et al. con- cluded that in the night sector the field-aligned currents are carried predominately by electrons of energy 0.7 keV and greater and occur when electrons of these energies become field aligned. This result is consistent with that obtained by Cloutlet et al. [1973] that a significant fraction of the upward Birkeland current was carried by electrons of energy greater than a few keV.

With a two-component flux-gate magnetometer aboard the Azur satellite, Theile and Praetorius [1973] had observed transverse magnetic disturbances with amplitudes ranging from 5 to 300 3' nearly always measured at auroral and polar latitudes. During periods of enhanced magnetic activity the region in which the disturbances were measured moved both poleward and equatorward. Simultaneous measurements of the transverse magnetic field variations and auroral emissions with photometers aboard the same satellite suggest that the field variations are due to Birkeland currents feeding the auroral event. Figure 11 is an example of the correlation between the variations of the magnetic field and the 3914- and 2972-• auroral emissions; the intensities of each are plotted as a function of invariant latitude. A similar type of study has been undertaken by Armstrong e! al. [1973] using magnetic data from the polar-orbiting satellite Triad and the

Alaskan chain of all-sky camera stations. This correlation shows that at the time of magnetic conjugacy with an auroral arc the magnetic north-south and east-west components of the field increase abruptly and then decrease below the pre- event level with a slow recovery. These two correlations with auroral emissions again suggest that particles responsible for the production of the auroral display are the major charge carriers for Birkeland currents directed out of the at-

mosphere. Wescott et al. [1970] and Haerendel and Liist [1970] have

pointed out that the polar cap horizontal magnetic disturbance vector AH cannot be explained in terms of ionospheric currents driven by polar cap electric fields as measured by rocket barium cloud experiments. Heœœner et al. [1971] have confirmed this finding by citing the lack of correlation between time variations in E and AH as well as the lack of

evidence that polar cap conductivities can support Hall currents of the required magnitude. Heppner et al. have shown that plasma density discontinuities in the ionosphere result in Hall current discontinuities that either polarize auroral forms or produce field-aligned currents. Since strong electric fields indicative of polarization at the boundaries of auroral forms are not experimentally verified, Birkeland currents are needed. On the basis of a distribution of dec-

trojets to satisfy the worldwide substorm signature, Heppner et al. modeled field-aligned sheet currents to flow into the

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ionosphere between 0600 and 1200 and out between 1800 and 2400. This current flo TM is consistent with the model proposed by Schield et al. [1969]. This field-aligned current system is also one that produces a significant fraction of and satisfies the polar cap AH measurements.

RECENT ROCKET MEASUREMENTS OF FIELD-ALIGNED

AURORAL PARTICLE FLuxES

A.s was already pointed out in this paper, the actual measurement of the charged carriers responsible for Birkeland currents is the most difficult one to make and the area in which we have the least information at the present time. The Ogo 4 satellite measurements that I have discussed imply that it is the field-aligned particles that are primarily responsible for the carrying of the current. Moreover, field

alignment of fluxes is suggestive of electric fields that might be of importance in understanding Birkeland currents. It is for these reasons that I would like to discuss here recent

measurements that have been made of precipitating electrons by means of sounding rocket detectors in which the pitch angle distributions of the electrons have been observed to be field aligned.

Whalen and McDiarmid [1972] have reported sounding rocket measurements of intense fluxes of low-energy electrons (10 •ø el cm -•' s -• sr -• keV -• at 0.5-keV energy) at the northern edge of an auroral arc. The electrons were high field aligned and carried a current on the order of 2 X 10 -4 A m-:. This electron current sheet had a north-south thickness of 10 km if

we assume that the measurements reflected a spatial variation. For energies greater than 25 keV, no field alignment was

ARNOLDY: AURORAL PARTICLE PRECIPITATION AND [•IRKELAND CURRENTS 227

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Fig. 11. Simultaneous magnetometer data and auroral photometer observations of 3914-,• (circles) and 2972-,• (triangles) emissions made by detectors aboard the Azur satellite [from Theile and Praetorius, 1973].

observed over the range of pitch angles measured. At lower energies there was a cutoff pitch angle below which the electrons were field aligned and above which the fluxes were isotropic. The cutoff pitch angle was smaller for higher-energy particles. Whalen and McDiarmid argued that these results were consistent with a local (confined to a distance less than 1 Rx from the ionosphere) electric field with a potential drop greater than 7 k V. The cutoff in the pitch angle could be explained if electrons were assumed to be injected isotropically at all potentials between 0 and 7 kV and if the typical injection energy was independent of the injection potential (or altitude). The source of low-energy electrons injected into the potential region at all altitudes was suggested to be secondary electrons produced by collisions of energetic particles with the ambient atmospheric constituents. A significant result of this observation is that large field-aligned currents can be p•'oduced as a result of field alignment of electron fluxes and that the current was most intense at the northern edge of the auroral band. A similar observation of the intensification of

Birkeland current at the northern edge of an auroral band by Choy et al. [1971] has already been discussed.

Finally, I would like to discuss a series of rocket flights in which I have flown low-energy detectors capable of making simultaneous measurements of electron and proton spectra from a few electron volts to 15 keV in directions looking up and down the magnetic field line. The NASA flight 18:91 has been reported in the literature by Choy et al. [1971], and the

spectrum of electrons below 1 keV for three of the flights, by Arnoldy and Choy [1973]. I would like to concentrate here on three flights in which observations were made of field-aligned electron fluxes. It should be pointed out that the threshold flux for the detectors was on the order of 105 particles (cm '• s sr keV) -•. Never in the five flights during various auroral con- ditions have significant fluxes of protons above this threshold been observed in the energy range from a few electron volts to 15 keV.

Flight 18:91 flew into a bright auroral band and over its northern edge (see Figure 3). For approximately 20 s of flight time the electron spectra had two maximums in the 1- to 10- keV range. Six contiguous spectra from this flight are shown in Figure 12. The pitch angle of the measurements can be obtained from the smooth curve accompanying each spectrum by using the last two decades of the flux scale to cover the range from 1 ø to 100 ø. At energies in the neighborhood of 6 keV there is a monoenergetic peak that is maintained throughout the 6-s period and shows little dependence on pitch angle. A second peak was observed at 139 and 144 s, which becomes evident only when the detectors scanned through the smallest pitch angles. At 141 s there seems to be a filling in of the spectrum, which is apparently a short-lived time variation, since the same energy electrons were detected at similar pitch angles 1 s later at 142 s and the enhanced flux was not present. These data show that there was a monoenergetic peak at high energies that is isotropic in pitch

228 ARNOLDY'. AURORAL PARTICLE PRECIPITATION AND BIRKELAND CURRENTS

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Fig. 12. Sequential electron spectra taken by sounding rocket detectors aboard NASA flight 18:91. Pitch angle of the electrons is given by the smooth curve in the last two decades of the flux scale to cover the range from 1 ø to 100 ø.

angle and a second peak in the neighborhood of 2 keV that is sensed by the detectors only when they scanned the smallest pitch angles, hence a peak that is field aligned. It cannot be ascertained from these data whether the 2-keV peak was in- deed monoenergetic, since it was not long-lived enough for the detector to scan other energies when at low pitch angles. The field-aligned peak occurred only during this flight from approximately 125 to 145 s. As seen in Figure 3, this was the time period when a bright auroral form moved through the rocket position close to the northern edge of the auroral band. There is an increase on the order of a factor of 5 in the Birkeland current calculated from the particle measurements during this time. Such field alignment of electrons was not measured when the rocket passed over the northern edge and observed the current sheet that has already been discussed.

A second observation that I have made of field-aligned electron fluxes was aboard an Air Force Black Bryant rocket,

flight 18:109, launched from Fort Churchill in April 1972. The particular aurora into which this flight was fired was a diffuse postbreakup event. Throughout the flight there were observations of field-aligned electron fluxes; moreover, the field- aligned fluxes were measured to be monoenergetic, as can be seen in Figure 13. Five-second intervals of data were used to obtain pitch angle distributions for electrons of 1, 2, and 3 keV and are shown in the figure. It is seen that 1- and 3-keV electrons were isotropic in pitch angle or even perhaps showed a trapped distribution, whereas there was strong field alignment for the 2-keV fluxes. These data were used to con- struct pitch angle sorted energy spectra presented in Figure 14. The 0ø-10 ø spectrum has a large monoenergetic peak at 2 keV, whereas the other two pitch angle sorted spectra, 30o-40 ø and 50ø-60 ø, show a much smaller peak at a higher energy, 2.8 keV. The 2.8-keV peak is isotropic in pitch angle up to the maximum pitch angle, 60 ø, measured. This is therefore

ARNOLDY: AURORAL PARTICLE PRECIPITATION AND BiRKELAND CURRENTS 229

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a case similar to that observed for flight 18:91, namely, that there were field-aligned fluxes and that the energy of these electrons was less than that of the isotropic monoenergetic peak. This flight has shown further that the field-aligned electrons were also monoenergetic.

The third flight, NASA flight 18:152, during which field- aligned electrons were measured was launched in March 1973 from Poker Flats, Alaska. Six sample electron spectra from this flight are shown in Figure 15, and again the pitch angle of the electrons observed is indicated by the curve in the first two decades of each spectrum. It appears that for a range of energies, the detector measured a maximum flux whenever the smallest pitch angles were scanned. The energy range in which field alignment of fluxes is observed appears to be from about 0.75 to 3 keV. These data are currently being analyzed. The field alignment offluxes for this flight occurred during two time intervals, very briefly when the rocket crossed the southern edge of the auroral display and then for approximately 20 s when the rocket passed over the diffuse northern edge of the display.

SUMMARY

Perhaps one of the safest statements to make at this time in summary of our knowledge of Birkeland currents and verifications of such by experimental measurements is that more observations are needed. There appears to be little doubt that Birkeland currents exist; however, the identifica- tion of their magnetospheric pattern, i.e., whether they feed the east and westward edges of the auroral electrojet or flow

north and south of the auroral arc or a combination of both, is certainly unclear at the present time. The largest body of data comes from polar-orbiting satellites, which indicate that the current sheets are associated with auroral displays and that field-aligned electron fluxes on the order of 1-keV energy are significant charge carriers for conventional current directed out of the atmosphere. Rocket observations verify that electrons responsible for the production of auroral forms carry a significant fraction of the current necessary to produce local magnetic effects that require Birkeland currents for their explanation. The return current is the illusive part of the overall system. It is uncertain whether it is carried by positive charges or very low energy electrons below the threshold of the instruments that have attempted to measure it. There are a few measurements that indicate that low-energy electrons streaming out of the atmosphere are the high-energy tail of the distribution responsible for the return current.

Field-aligned fluxes, monoenergetic peaks, and field- aligned monoenergetic fluxes all could be important in the Birkeland current system and must be experimentally in- vestigated more fully and considered in models of the mechanism for driving the currents. In attempting to explain some of our observations of field-aligned electrons with electric field models, only limited success has been achieved. Coordinated measurements that are very much needed are those made in the ionosphere with sounding rockets and above the ionosphere by means of polar-orbiting satellites. The role of the ionosphere with regard to monoenergetic peaks, the field alignment of precipitation, the driving of Birkeland currents, and the possible triggering of the substorm itself is uncertain. The ionosphere appears to be, for ex-

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Fig. 14. Pitch angle sorted spectra obtained from rocket detectors aboard AFCRL flight 18'109.


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Fig. 15. Sequential electron spectra taken by detectors aboard NASA flight 18:152. Pitch angle of electrons is given by smooth curves.

ample, most important in the formation of the monoenergetic peaks, since these peaks are much sharper when measured at rocket altitudes than typically observed by polar-orbiting satellites and are nonexistent in the equatorial plane [Sharp et al., 1971].

Acknowledgments. This work was supported under NASA grant NGR 30-002-054. The manuscript was completed for publication while I was a Visiting Professor of Physics at the University of Minnesota. I wish to acknowledge support received while at the University of Minnesota from the School of Physics and Astronomy and from Laurence Cahill, Jr., and John R. Winckler under their NASA grants NGR 24-005-253 and NGL 24-005-008, respectively.

REFERENCES

Ackerson, K. L., and L. A. Frank, Correlated satellite measurements of low-energy electron precipitation and ground level observations of a visual arc, J. Geophys. Res., 77, 1128, 1972.

Akasofu, S.-I., and C.-I. Meng, A study of polar magnetic substorms, J. Geophys. Res., 74, 293, 1969.

Armstrong, J. C., and A. J. Zmuda, Field-aligned current at 1100 km in the auroral region measured by satellite, J. Geophys. Res., 75, 7122, 1970.

Armstrong, J. C., A. J. Zmuda, S.-I. Akasofu, and G. Rostoker, Three axis measurements of magnetic fluctuations at auroral latitudes: Preliminary results (abstract), Eos Trans. A GU, 54, 420, 1973.

Arnoldy, R. L., Rapid fluctuations of energetic auroral particles, J. Geophys. Res., 75, 228, 1970.

Arnoldy, R. L., and L. W. Choy, Auroral electrons of energy less than 1 keV observed at rocket altitudes, J. Geophys. Res., 78, 2187, 1973.

Berko, F. W., Distributions and characteristics of high-latitude field- aligned electron precipitation, J. Geophys. Res., 78, 1615, 1973.

Berko, F. W., R. A. Hoffman, R. K. Burton, and R. E. Holzer, Simultaneous particle and field observations of field-aligned currents, NASA-GSFC Doc. X-646-73-45, 1973.

Birkeland, K., The Norwegian Aurora Polaris Expedition 1902-1903, vol. 1, sect. 1, H. Aschehoug, Christiania, Norway, 1908.


Bosqued, J. M., G. Cardona, H. Reme, P. Souleille, and M. Jean- Francois Denisse, Observations of impulsive precipitation of anisotropic fluxes of low energy electrons in an aurora, C. R. Acad. Sci., Ser. B, 273, 933, 1971.

Choy, L. W., R. L. Arnoldy, W. Potter, P. Kintner, and L. J. Cahill, Jr., Field-aligned particle currents near an auroral arc, J. Geophys. Res., 76, 8279, 1971.

Cloutier, P. A., H. R. Anderson, R. J. Park, R. R. Vondrak, R. J. Spiger, and B. R. Sandel, Detection of geomagnetically aligned currents associated with an auroral arc, J. Geophys. Res., 75, 2595, 1970.

Cloutier, P. A., B. R. Sandel, H. R. Anderson, P.M. Pazich, and R. J. Spiger, Measurements of auroral Birkeland currents and energetic particle fluxes, J. Geophys. Res., 78, 640, 1973.

Cummings, W. D., and A. J. Dessler, Field-aligned currents in the magnetosphere, J. Geophys. Res., 72, 1007, 1967.

Cummings, W. D., J. N. Barfield, and P. J. Coleman, Jr., Magnetospheric substorms observed at the synchronous orbit, J. Geophys. Res., 73, 6687, 1968.

Evans, O., B. Maehlum, and T. Wedde, High-latitude observations of field-aligned electron beams (abstract), Eos Trans. AGU, 53, 731, 1972.

Frank, L. A., and K. L. Ackerson, Observations of charged particle precipitation into the auroral zone, J. Geophys. Res., 76, 3612, 1971.

Gurnett, D. A., and L. A. Frank, VLF hiss and related plasma observations in the polar magnetosphere, J. Geophys. Res., 77, 172, 1972.

Haerendel, G., and R. Lfist, Electric fields in the ionosphere and magnetosphere, in Particles and Fields in the Magnetosphere, edited by B. M. McCormac, p. 213, D. Reidel, Dordrecht, Netherlands, 1970.

Haerendel, G., P. C. Hedgerock, and S.-I. Akasofu, Evidence for magnetic field aligned currents during the substorms of March 18, 1969, J. Geophys. Res., 76, 2382, 1971.

Heppner, J.P., J. D. Stolarik, and E. M. Wescott, Field aligned con- tinuity of Hall current electrojets and other consequences of density gradients in the auroral ionosphere, in The Radiating Atmosphere, edited by B. M. McCormac, p. 407, D. Reidel, Dordrecht, Netherlands, 1971.

Hoffman, R. A., and D. S. Evans, Field-aligned electron bursts at high latitudes observed by Ogo 4, J. Geophys. Res., 73, 6201, 1968.

Holmgren, L. A., P. Christophersen, and W. Riedler, On the pitch angle dependence of auroral electron fluxes in the keV range, Phys. North., 4, 85, 1970.

Lampton, M., M. R. Albert, K. A. Anderson, and L. M. Chase, Rocket observations of charged particles in the auroral zone, paper presented at Birkeland Symposium, Sandefyord, Norway, 1967.

Morse, F. A., D. F. Nelson, E. H. Rogers, and R. C. Savage, Low- energy electrons and auroral forms (abstract), Eos Trans. A GU, 54, 404, 1973.

O'Brien, B. J., and D. L. Reasoner, Measurements of highly collimated short-duration bursts of auroral electrons and com-

parison with existing models, J. Geophys. Res., 76, 8258, 1971. Park, R. J., and P. A. Cloutier, Rocket-based measurements of

Birkeland currents related to an auroral arc and electrojet, J. Geophys. Res., 76, 7714, 1971.

Paschmann, G., R. G. Johnson, R. D. Sharp, and E.G. Shelley, Angular distributions of auroral electrons in the energy range 0.8 to 16 keV, J. Geophys. Res., 77, 6111, 1972.

Schield, M. A., J. W. Freeman, and A. J. Dessler, A source for field- aligned currents at auroral latitudes, J. Geophys. Res., 74, 247, 1969.

Sharp, R. D., D. L. Carr, R. G. Johnson, and E.G. Shelley, Coor- dinated auroral electron observations for a synchronous and a polar satellite, J. Geophys. Res., 76, 7669, 1971.

Theile, B., and H. M. Praetbrius, Field-aligned currents between 400 and 3000 km in auroral and polar latitudes, Planet. Space Sci., 21, 179, 1973.

Vondrak, R. R., H. R. Anderson, and R. J. Spiger, Rocket-based measurement of particle fluxes and currents in an auroral arc, J. Geophys. Res., 76, 7701, 1971.

Wescott, E. M., J. D. Stolarik, and J.P. Heppner, Auroral and polar cap electric fields from barium releases, in Particles and Fields in the Magnetosphere, edited by B. M. McCormac, p. 229, D. Reidel, Dordrecht, Netherlands, 1970.

Whalen, B. A., and J. B. McDiarmid, Observations of magnetic-field- aligned auroral electron precipitation, J. Geophys. Res., 77, 191, 1972.

Zmuda, A. J., J. A. Martin, and F. T. Heuring, Transverse magnetic disturbances at 1100 kilometers in the auroral regions, J. Geophys. Res., 71, 5033, 1966.

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(Received November 26, 1973.)

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