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"-' NASA-CR-175629 

19850015098 

PERIODIC SYBSTORM ACTIVITY 

IN THE GEOHAGNETIC TAIL 

by 

. . 

C. Y. Huang l , T. E. Eastman l , L. A. Frankl 

and 

D. J. Wilhams 2 

"_ l"A=SA~({(-/75, b~C; 

U. of Iowa 83·6 

1.

________ ~\VER511YO~ ________ __ 

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oUNDED IStoc 

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"Reproduction in whole or in part is permitted for any purpose of the United 

States Governm~nt." 

Department of Physics and Astronomy 

THE UNIVERSITY OF I~Wifty COpy 

Iowa City, Iowa 52242 

-~nlll1flln IIII nlll nlll lUll \1111 IIII IUI~\ NF00726 : 

J\JH ! 01983 

-lANGLEY RESCAR('H CENTER 

LIBRARY, NASA 

HAMPTON, VIRGINIA 
U. of Iowa 83-6 

PERIODIC SUB STORM ACTIVITY 

IN THE GEOHAGNETIC TAIL 

by 

C. Y. Huang 1 , T. E. Eastman l , L. A. Frankl 

and 

D. J. Williams2 

lDepartment of Physics and Astronomy, University of Iowa, 

Iowa City, Iowa 52242 

2Applied Physics Laboratory, Johns Hopkins University, 

Laurel, MD 20707 

Submitted for publication in Journal of Geophysical Research 

January 1983 

Department of Physics and Astronomy 

The University of Iowa 

Iowa City, Iowa 52242 

Nf'5 - J.3'{P 'i.:#=--
Abstract 

On 19 May 1978 an unusual series of events is observed with the Quadrispherical LEPEDEA on board the ISEE-l satellite in the earth's geomagnetic 

tail. For 13 hours periodic bursts of both ions and electrons are seen in all 

the particle detectors on the spacecraft. On this day periodic activity is 

also seen on the ground, where multiple intensifications of the electrojets 

are observed. At the same time the latitudinal component of the interplanetary magnetic field shows a number of strong southward deflections. We conclude that an extended period of substorm activity is occurring, which causes 

repeated thinnings and recoveries of the plasma sheet. These are detected by 

ISEE, which is situated in the plasma sheet boundary layer, as periodic dropouts and reappearances of the plasma. Comparisons of the observations at ISEE 

with those at IMP-8, which for a time is engulfed by the plasma sheet, indicate that the activity is relatively localized in spatial extent. For this 

series of events it is clear that a global approach to magnetospheric dynamics, e.g., reconnection, is inappropriate. 

i 
1 

1. Introduction 

In most studies of substorm effects in the geomagnetic tail, emphas~s has 

been placed on plasma flows (Bieber and Stone, 1980; Belian et al., 1981), and 

magnetic field topology (McPherron et al., 1973; Hones et al., 1982). Such an 

approach has led to development of the near-earth neutral line model of substorms. In this model a neutral line is supposedly formed by reconnection of 

the earth's magnetic field lines with those of the solar wind, forming an xtype neutral line extending from dawn to dusk (Hones, 1972). In the vicinity 

of the neutral line energetic streaming should be observed (Schindler, 1974; 

Birn, 1980), as the neutral line moves tailwards. Magnetic field rotation 

occurs in conjunction with streaming and plasma dropout. 

In the present work we have examined a period of unusual activity in the 

tail, using primarily the LEPEDEA instruments on ISEE-l and IMP-8, focussing 

directly on the actual plasma d~stribution. On 19 May, 1978 both ISEE-l and 

IMP-8 are in the tail, with ISEE-l approximately 10-15 RE from the earth and 

IMP-8 10 RE further downstream. At ISEE the ion and electron distributions 

show periodic dropouts and reappearances over 13 hours. From ground-based 

observations of the magnetic field we are able to determine that weak substorm 

activity occurs on the same day. The H component shows quasi-periodic fluctuations at a number of stations and the AE index rises during this period. 

The magnetic field measured in the tail magnetosheath by IMP-8 also shows surprising variations in Bz , with several periods of strong southward field observed. Thus it seems that the plasma sheet is undergoing periodic thinnings 

and recoveries for an extended period due to substorm activity. Our examination of the plasma distributions both at ISEE-l and IMP-8 reveals a number of 

interesting facts concerning the tail response associated with geomagnetic 

activity. 
2 

2. Instrumentation 

The observations presented here are from the University of Iowa LEPEDEA 

instruments on ISEE-1 and IMP-8, and the Goddard Space Flight Center magnetometer on IMP-8. Additional information is provided by the medium energy particle detector on ISEE-1. The LEPEDEA plasma instrument on ISEE-1 is described 

by Frank et a1. (1978). For this day the instrument samples electron and positive ion intensities in 32 energy pass-bands from 215 eV to 45 keV, with an 

energy resolution of ~E/E = 0.16. At each energy level 16 azimuthal sectors 

are covered in 16 seconds, while 7 polar angles are simultaneously sampled in 

each azimuthal sector. Thus a full three-dimensional distribution consisting 

of 112 angular samples at each energy level, giving 3584 sections of velocity 

space, is obtained in approximately 8 minutes. 

Survey energy-time (E-t) spectrograms of the plasma measurements are used 

in our analysis. An example is shown in Plate lA for the period 0000 to 2400 

UT on 19 }lay 1978. This spectrogram displays detector response on a colorcoded scale as shown to the right. The upper four panels display the detector responses to ion intensities in four solid angle segments near the solar 

ecliptic plane. From top to bottom these are for ion ve1oc~ties directed 

antisunwards, dawnwards, sunwards and duskwards, respectively. The f~fth 

panel shows the electron intensities averaged over all azimuthal sectors, in a 

plane near the solar ecliptic. 

Color-coded E~ spectrograms provide samples of the velocity distribution 

in terms of energy versus azimuth for each of the seven polar angles. Plate 2 

shows examples of such spectrograms. Displayed are responses of each of the 

seven ~on and electron detectors and of the Geiger-Hueller tube. The 
3 

detectors are numbered 1P through 7P for ions, where the look angles range 

from 13° from the spacecraft spin axis (lP) through 90° (4P) to 167° (7P). A 

similar polar range is displayed for the electron channels numbered IE through 

7E. The GM tube measures particles above a fixed energy as a function of 

azimuthal angle. This detector responds primarily to electrons with ~ 45 

keV. Starting from the lowest energy step each detector sweeps through all 

azimuthal angles (~) before stepping up to the next energy level. 

The LEPEDEA on the IMP-8 spacecraft measures directional, differential 

intensities for ions and electrons in the range 50 eV ( E/Q ( 45 keV. This 

range is divided into 15 energy passbands, each of which is subdivided into 16 

angular sectors during each spacecraft spin. The field of view of the 

analyzer is centered nearly on the ecliptic plane. For further details the 

reader is referred to Frank (1967). The top four panels and the lowest panel 

on the E-t spectrogram for IMP-8 data correspond respectively to the five 

panels of the ISEE spectrogram. An example is shown in Plate lB. 

The Goddard Space Flight magnetometer on IMP-8 provided the threedimensional magnetic field vectors. In this analysis 15.36 second averages 

have been used. Further information can be found in Ness (1970) and Mish and 

Lepping (1976). 

The ISEE-1 medium-energy particles experiment (Williams et al., 1978) 

is used to provide higher time resolut~on information concerning the intermediate-energy plasma flow, and details of gradients in the plasma. 
4 

3. ~ and Analysis 

On Hay 19, 1978 both ISEE satellites and IMP-8 were in the magnetotail. 

The orbits for ISEE-l and IMP-8 are shown in Figure 1 for this day. The 

striking feature in the ISEE spectrogram is the periodic dropout and reappearance of the plasma, starting at 0930 UT and continuing until 2300 UTe In 

addition both ions and electrons are more energetic during the extended periodic interval than during the immediately preceding period (0400-0900 UT). 

This is particularly evident in the sudden change in electron energy at 1030 

UTe The energization of the plasma appears to occur coherently over a number 

of the events, i.e. the energy increases in each successive burst, up to a 

maximum at 1900 UT, and decreases after 2000 UTe 

A plot of the H component of several ground-based magnetograms, obtained 

from the World Data Center (A), is shown in Figure 2. Identification of the 

symbols used for the various stations, together with their positions, is given 

in Table 1. From these signatures it is clear that there is sustained magnet-

~c activity over approximately 10 hours. The enhancements of the H component 

(or X component in the case of Fort Churchill and Great Whale River) are also 

periodic, with a period of 1-2 hours. This behaviour is also evident in the 

Alberta chain of stations over the same interval of time (G. Rostoker, private 

communicat~on, 1982). The electrojet activity appears to be centered within a 

narrow latitudinal range from 65° to 69°, with the largest fluctuations seen 

at Fort Churchill. Weak activity begins at 0800 UT near local midnight at the 

northernmost stations in this range (Fort Churchill and Great Whale River), as 

seen in the intensification of the westward electrojet. At the same time an 

increase in eastward electro jet activity is seen at College and Barrow, and by 

1000 UT auroral activity has expanded southwards past these two stations. 

Dixon and Sodankyla are both on the days ide and show the same periodic 
5 

increases in electro jet activity but with time delays in the onsets. The AE 

index (not shown) begins to rise shortly after 0800 UT and reaches a peak of 

400( at 0940 UTe 

It is clear that periodic substorm-associated activity occurs on this 

day. However the rapid sequence of events makes it difficult to assign an 

onset time to each event, or even to decide whether each negative H bay should 

be considered as separate from the others. The length of time between separ-

-I 

able substorms has variously been estimated as approximately an hour (Rostoker 

et al., 1982) to 2-3 hours (McPherron, 1978). In addition there does not 

appear to be an agreement on the minimum electro jet intensification required 

to constitute a substorm (Rostoker et al., 1980). It is possible that the 

entire active interval on May 19, 1978 consists of a single storm with multipIe surges, or that a series of weak substorms occurs. 

Figure 3 shows the plasma and magnetic pressure observed on ISEE-1, where 

the rises and dropouts can be clearly seen in both quantitites. In addition 

in the lower panel is shown the magnetic field data from IMP-8. This shows 

sharp southward turnings a number of times during the interval 1100-1500 UTe 

(Between 1600 and 1700 UT the satellite was engulfed by the plasma sheet which 

provides for some two-satellite comparisons.) As IMP-8 is in the magnetosheath 

during the intervals of negative Bz it is clear that the interplanetary field 

rotates southwards a number of times during this period (Fairfield, 1976). 

Such sharp southward turnings are usually correlated with increases in 

geomagnetic activity in most substorm studies (Rostoker et al., 1980). 

The appearance and disappearance of the plasma can thus be attributed to 

thinnings and subsequent recovery of the plasma sheet due to substorm activity 

(Hones et al., 1967; Hones et a1., 1971; Lui et a1., 1975). From Figure 1 it 
6 

can be seen that for most of this period ISEE is situated near the northern 

boundary of the plasma sheet, towards dusk. This boundary region is thought 

to respond most rapidly and dramatically to substorm expansions (DeCoster and 

Frank, 1979; Williams, 1981). The spacecraft is also appropriately positioned 

to observe the effects of neutral line formation, if any, since its XGSM position varies from 10 to 15 RE from the earth. 

The first sign of activity in the tail is seen at 0900 UTe The distribution prior to this time is shown in Plate 2A. The ions, particularly as seen 

in detectors 3P, 4P and 5P, show a distinct sunward-directed anisotropy. Flow 

values at this time (in GSM coordinates) in the ecliptic plane are Vx = 44 km 

sec-I, Vy = 9 km sec-I. The electron intensities also exhibit field-aligned 

anisotropy in detectors 3E and 4E. The ion temperature is low, around 1 keV. 

Such distributions are not uncommon near the flanks of the plasma sheet, and 

may indicate the penetration of cool boundary layer plasma (Eastman et al., 

1976) into the plasma sheet where return flow and/or isotropic distributions 

may be seen. 

The distribution at 0906 UT is shown in Plate 2B. The anisotropy in the 

ions has reversed - the distribution is now peaked in the tailward direct10n, 

with Vx = -24 km sec-I, Vy = 3 km sec-i. The bulk of the electron distribution is below the lower energy cutoff of 215 eV for this mode. At 0923 UT the 

distribution of ions has again reversed, with the peak intensity directed sunwards. During this time an expansion is observed at Fort Churchill, beginning 

at 0840 UTe Thus the plasma sheet thinning associated with this onset would 

place the satellite further out along the upper flank of the plasma sheet 

where the antisunward boundary layer distribution is sampled. Recovery begins 
7 

at Fort Churchill soon after 0900 UTe No energization of the plasma is apparent at the satellite position, and there is little evidence of high-speed 

streaming. 

The first plasma dropout occurs at 0920 UT with subsequent recovery at 

1030 UTe Thereafter the thinnings and recoveries take place seven times in 

approximately 12.5 hours. From Figure 3 it can be seen that the energy density during the plasma bursts increases to a peak value of 1.1 keV cm-3 at 1920 

UTe During the following two bursts this decreases sharply. On the ground 

there is also a gradual increase in westward e1ectrojet intensity at various 

stations, followed by gradual recovery. After 1800 UT very little auroral 

activity is seen (see Figure 2). 

An E~ spectrogram at 1629 UT, one of the times when p1~sma dropout occurs, is displayed in Plate 2C. The electron intensities appear to be isotropic, while the ions exhibit a weak anisotropy. The bulk of the plasma may 

be present below 215 eV and could show the anisotropy seen earlier at 0830-

0930 UTe Both density and temperature are low. Such cold, weakly anisotropic 

distributions recur throughout the 12.5 hour period whenever the plasma sheet 

thins below the satellite. 

A typical example of one of the plasma bursts is shown in Plate 2D. At 

1928 UT the ion density and temperature appear to be enhanced. A lower cutoff 

in the ion distribution is seen at a few hundred eVe Again the ions are only 

weakly anisotropic and remain so throughout the interval. At higher energies, 

however, there is clear indication of field-aligned anisotropies. Counterstreaming 35 keV ions are observed flowing along magnetic field lines. The 

counter-streaming is dynamic, showing considerable variation within the interval when the burst occurs. These observations are in agreement with those of 
8 

Williams (1981), who suggested that they were the result of a deep-tail accelerat~on process, with subsequent mirror~ng near the earth to create the return 

beam. The portion of the d~stribution that exhibits such streaming is very 

small, and it is clear that the bulk of the ion population is stably trapped 

even during the enhanced ion bursts. Temporal variations in the bulk plasma 

which occur on a scale shorter than the instrument cycle time of 8 minutes, 

are in general small. The major characteristics of the velocity distribution 

as seen at 1928 liT and during other plasma bursts persist over several cycles. 

The electron intensities show distinct field-aligned anisotropies at 1928 

liT and during the other periods of enhanced plasma pressure. Such signatures 

are often seen in the plasma sheet boundary layer where field-aligned current 

sheets are frequently present (Frank et al., 1981). A direct measurement of 

the current carried by the electrons is possible. By using the simultaneously 

measured magnetic field deflection the current sheet intensity can be 

obtained, and thus an estimate of the thickness of the current sheet can be 

made. Evaluation of the thickness of the current layer using the plasma 

velocity and the time interval during which the currents are detected lead to 

large uncertainties as the time needed to obtain a statistically significant 

velocity measurement in this tenuous plasma is long (several minutes) relative 

to the sampling time (S 2 minutes) for the currents. 

+ Thus the value of V used 

in any calculation may not accurately reflect the instantaneous velocity at 

the time the currents are observed. 

The electron distribution function for the instrument cycle at 1928 liT ~s 

+ shown in velocity space in Figure 4. Contours of constant F(V) are plotted as 

a function of Vn and Vl, the velocities parallel and perpendicular to the magnetic field, respectively. The units of Fare sec-3 cm6• The bulk of the current 
9 

comes from lower energy electrons, at about 1 keV (see Plate 2D). This cor-

+ responds to the innermost contour, where a clear anisotropy directed along -B 

is seen. The net integrated current density is 2.54 X 10-8 A m-2, directed 

along +B. This is well above the estimated threshold of 2 X 10-9 A m-2 for 

the instrument, and is in general agreement with previous observations (Frank 

et al., 1981). In calculating the current density we have assumed that the 

charge-carrying species is electrons and have ignored ion contributions. As 

the ions are quasi-isotropic throughout with low bulk flow velocities (less 

than 50 km sec-I) this assumption is well justified. 

Table 2 shows the values of JII at a number of different times during the 

day, with the magnetic field deflection and estimated current sheet thickness 

at those times. The current sheet is noticeably thinner than the plasma sheet 

boundary layer itself, which has been estimated as < 1-2 RE using two IMP 

spacecraft (DeCoster and Frank, 1979). These current signatures appear during 

the plasma bursts for several instrument cycles, although their intensity 

varies during the approximately hour-long burst. This suggests that the current sheets may be filamentary in structure with some temporal variation 

seen within each filament. 

We can also make an approximate evaluation of the diamagnetic current due 

to the pressure gradient in the following way: 

We have determined that the motion of the plasma sheet is essent1ally in the 

ZGSM direction (see later section on expansion velocities), so that the pressure gradient term can be replaced by dP/dz~. Knowing the value of (Vz )GSll 

+ and the temporal rate of change of the pressure term we can estimate Jl by 
10 

+ - 1 dP z X B 

Vz dt B2 

At 1521 UT the pressure grad~ent is -4.5 X 10-18 erg cm-4 , which gives r~se to 

a diamagnet~c current of 

i.e., the perpendicular current is approximately an order of magnitude smaller 

than the field-aligned current evaluated at this time. Part of the magnetic 

deflection is due to this perpendicular current which is relatively large at 

each of the expansions and contractions of the central plasma sheet. 

During the cycles when we have evaluated the field-aligned current densities the currents are all directed into the ionosphere. This agrees with 

the observations of Iijima and Potemra (1976) who found that in the premidnight secto~ region 2 currents, which are directed into the ionosphere, are 

dominant. We do not see any currents of opposite sign. However, as noted 

earlier the main electro jet activity is restricted to a narrow latitudinal 

range, between 65° and 69° so that the region sampled maps to region 2 (Iij~ma 

and Potemra, 1976; Saflekos et al., 1982). 

We can estimate the distance from the satellite to an absorption boundary 

by using the technique described by Williams (1979). Using the med~um-energy 

particle detector, gradients in the plasma can be sounded and the distance to 

these gradients calculated. For this day the d~stance from the satellite to 

the lobe has been sounded for several ~ntervals. Table 3 lists a ser~es of 

such distance estimates as a function of time in one plasma pulse. The plasma 

sheet boundary layer thickness is thus seen to be < 2400 km during this interval. However the thickness ~s clearly a dynamic quantity related to substorm 
11 

activity. The plasma sheet boundary layer may be regarded as a primary transport region during active times, characterized by time-varying high-speed 

streaming distributions (DeCoster and Frank, 1979) which are created by a 

deep-tail source (Williams, 1981). 

From the electron signatures throughout the 12.5-hour interval it appears 

that both spatial and temporal variations associated with substorm activity 

occur in the plasma sheet boundary layer. The plasma dropouts are due to 

thinning of the plasma sheet below the spacecraft position. The reappearance 

of the plasma is accompanied by filamentary current signatures which show 

temporal intensity variations during the recovery period. The extent of the 

activity is limited, both in spatial extent and intensity. Energization of 

ions and electrons is only apparent at the start of periodic activity at 1030 

UTe After the initial observation of the hotter plasma the period1c dropouts 

and reappearances do not indicate any significant further energization. Instead the ion thermal energy decreases after 1530 UTe At this time the value 

of kT is 5.3 keV, at 2200 UT it has decreased to less than 2 keV. This energy 

pattern may indicate that each plasma burst is not an isolated event but may 

be part of a larger system with small-scale features, e.g. multiple surges 

within a substorm. As we have suggested earlier the periodicity may be linked 

to the periodic switching of the interplanetary field latitudinal component. 

It has also been noted by Akasofu (1977) that if a perturbation occurs after a 

period of southward Bz in the interplanetary field, the magnetosphere can show 

more than one response, as if pumping down from an excited to a ground state 

in stages. 

The plasma sheet response is, as we have pointed out above, limited in 

spatial extent, as ind1cated by ground-based magnetograms. Observations made 
12 

by IMP-8 indicate that between 1619 and 1648 liT the plasma sheet expands past 

this satellite. Before and after this interval the spacecraft is in the magnetosheath or boundary layer. During the same time that the plasma sheet is 

expanding at IMP-8, the LEPEDEA on ISEE-l observes a dropout indicative of 

plasma sheet thinning. From Figure 1 it is clear that the two satellites are 

widely separated throughout the day, particularly in the Y component. The 

disparity in response confirms our earlier conclus~ons that the act~ve region 

is spatially limited, and indicates that a global approach to substorm dynamics via magnetic merging w~th the concept of a cross-tail neutral line is not 

always appropriate. 

In order to estimate the rate at which the plasma sheet recovers, we have 

utilized two-spacecraft measurements made by the li. C. Berkeley energetic 

particle instruments on ISEE-l and -2 (Anderson et al., 1978). The general 

method used is to identify distinctive similar features in the flux observations at ISEE-l and -2, and estimate the time delay between the two. Knowing 

the spacecraft separation we are then able to calculate an expansion velocity. 

Compar~son of these expans~on velocities with the bulk velocity of the plasma 

as measured simultaneously by the LEPEDEA ind~cates that expansion takes place 

primarily in the z d~rection. The results are shown ~n Table 4. These values 

are in good agreement with those measured previously (Hones et al., 1970; 

Parks et al., 1979; DeCoster and Frank, 1979). 

We can also make an estimate of the energy flux dur~ng one of the plasma 

enhancements. At 1728 liT the bulk veloc~ty is 29 km sec-I, ion density is 

0.16 cm-3 and thermal energy 4.4 keV. The energy flux is then 

F 3.3 X 10-3 erg cm-2 sec- 1 
13 

Assuming an average half-thickness of the plasma sheet to be 2 RE, and a w1dth 

of 30 RE gives an input power of energy production of at least 8 X 1016 erg 

sec-i. As we have no observations of the central plasma sheet, where temperatures and densities are likely to be higher at this time, this estimate gives 

a lower limit to the total value expected. The average duration of the plasma 

bursts is 1 hour, giving a total energy input of 2.9 X 1020 ergs. Th1s agrees 

with the value for a weak substorm (Akasofu, 1977, p. 561). 
14 

4. Conclusions 

On 19 May 1978 an unusual 12.5 hour interval of quasi-periodic plasma 

bursts is observed in the tail by ISEE-l. At the same time ground-based magnetograms show periodic intensifications of the electrojets and there are 

strong southward fluctuations in the interplanetary magnetic field. We thus 

conclude that the plasma observations are due to substorm activity which causes 

periodic thinning and expansion of the plasma sheet. The spatial extent of the 

tail activity appears to be limited, as (1) the latitudinal range in which the 

bulk of the electrojet intensification is seen is restricted to about 4°, and 

(2) IMP-8 which is approximately 10 RE tailwards of ISEE observes plasma sheet 

expansion at a time when there is a dropout due to thinning at ISEE. Thus the 

observations at ISEE indicate temporal and spatial variations within a relatively local region. 

An analysis of the plasma observations has been carried out. This reveals 

that throughout the disturbed interval the plasma exhibits weak anisotropy. 

Counter-streaming energetic ions are observed at the same time as the lower 

energy d1stribution, but these flowing ions constitute a small percentage of 

the total population. During the plasma dropout the statistics on the highenergy ions are too poor to determine whether high-speed flows are present. 

The low-energy ions remain quasi-isotropic, although larger anisotropies may be 

present below 215 eVe 

During the plasma bursts the electron intensities show distinct fieldaligned anisotropies. These have been integrated directly to yield current 

densities which range from 1.7 X 10-8 A m-2 to 2.5 X 10-8 A m-2, in agreement 

with previous measurements. Comparisons with current intensities estimated 

from the magnetic field observations on ISEE indicate that the currentcarrying sheets are narrow, generally a few hundred kilometers thick. This 
15 

suggests that the plasma sheet boundary layer, generally thought to be several 

thousand kilometers in thickness, has either a relatively limited current 

sheet structure, or that the current sheet is filamentary in nature. 

The electron anisotropies and ion streaming vary temporally as well as 

spatially. The velocity distributions indicate that the spacecraft is in a 

plasma sheet boundary which undergoes temporal change throughout the interval. 

The substorm activity is accompanied by repeated thinning and recovery of the 

central plasma sheet. Hence ISEE samples different spatial regions of a I 

dynamically varying boundary layer. 

The rapid sequence of events makes it difficult to establish whether each 

plasma burst is an isolated weak substorm. However the pattern of a gradual 

increase in the plasma pressure in successive cycles, followed by a gradual 

decrease suggests that we are observing an active period in which multiple 

surges occur. The periodicity of these surges may be due to the rapid changes 

in the interplanetary field. A possible mechanism for multiple onsets is the 

streaming tearing mode (Sato and Walker, 1982) in which an initial fieldaligned flow enhances the growth rate of the instability. 

This case is difficult to interpret in the framework of the conventional 

near-earth neutral line model. Two-satellite observations indicate that the 

disturbance is local and not global in extent. The magnetic field at ISEE 

shows no sharp rotations, as predicted by the model. In addition plasma dropout and consequent high-speed streaming are absent. The bulk of the populat~on remains stably trapped, while higher energy ions show simultaneous 

counter-streaming. The events on this day indicate that a limited spat~al 

region in the tail can show dramatic response to substorm activity, in the 

form of repeated thinnings and recoveries. Other observational features 
16 

regarded as typical of active periods (magnetic field rotation, plasma dropout, etc.) do not predominate for this case, despite the favorable position of 

ISEE for detection of such features. 
17 

Acknowledgments 

We wish to thank R. P. Lepping of Goddard Space Flight Center for the 

IMP-8 magnetometer data, C. T. Russell of UCLA for ISEE-1 magnetometer data, 

and G. K. Parks of the University of Washington for high-time resolution particle data from ISEE-1 and -2. We also wish to acknowledge valuable assistance offered by G. Rostoker concerning the interpretation of ground-based magnetograms. This research was supported in part by the National Aeronautics 

and Space Administration under contract NAS5-26257 and NAS5-25530 and grant 

NGL-16-001-002 and by the Office of Naval Research under grant N00014-

76-C-0016 and N00024-83-C-5301 under Task 12UOSDO. 
18 

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Hones, E. W., Jr., J. Birn, S. J. Bame, G. Paschmann and C. T. Russell, On the 

three-dimensional structure of the plasmoid created in the magnetotail at 

substorm onset, Geophys. ~. ~., 1, 203, 1982. 

Iijima, T. and T. A. Potemra, Large-scale characteristics of field-aligned 

currents associated with substorms, J. Geophys. Res., 83, 599, 1978. 
20 

Lui, A.T.Y., E. W. Hones, Jr., D. Venkatesan, S.-I. Akasofu and S. J. Bame, 

Thinnings of the plasma sheet observed by Vela satellites and the 

interplanetary magnetic field, ~. Geophys. Res., 80, 4649, 1975. 

McPherron, R. L., The use of ground magnetograms to time the onset of 

magnetospheric substorms, ~. Geomag. Geoelectr., 30, 149, 1978. 

McPherron, R. L., C. T. Russell and M. P. Aubry, Satellite studies of magnetospheric substorms on August 15, 1968. 9. Phenomenological model for 

substorms, ~. Geophys. Res., ~, 3131, 1973. 

Mish, W. H. and R. P. Lepping, Magnetic field experiment data processing systems: Explorers 47 and 50, NASA/GSFC Doc. X-694-76-158, 1976. 

Ness, N. F., Magnetometers for space research, Space Sci. Rev., ll, 459, 1970. 

Parks, G. K., C. S. Lin, K. A. Anderson, R. P. Lin, H. Reme, ISEE 1 and 2 particle observations of outer plasma sheet boundary, J. Geophys. Res., 84, 

6471, 1979. 

Rostoker, G., S.-I. Akasofu, J. Foster, R. A. Greenwald, Y. Kamide, K. Kawasaki, A.T.Y. Lui, R. L. McPherron and C. T. Russell, Magnetospheric 

substorms - definitions and signatures, ~. Geophys. Res., ~, 1663, 1980. 

Rostoker, G., M. Mareschal and J. C. Samson, Response of days ide net downward 

field-aligned current to changes in the interplanetary magnetic field and 

to substorm perturbations, ~o Geophys. ~o, ~, 3489, 1982. 

Saflekos, N. A., R. E. Sheehan and R. L. Carovillano, Global nature of fieldaligned currents and their relation to auroral phenomena, Rev. Geophyso 

Space Phys., 20, 709, 1982. 

Sato, T. and R. J. Walker, Magnetotail dynamics excited by the streaming 

tearing mode, J. Geophys. Reso, ~, 7453, 1982. 
21 

Schindler, K., A theory of the substorm mechanism, J. Geophys. Res., ~, 2803, 

1974. 

Williams, D. J., Magnetopause characteristics inferred from three-dimensional 

energetic particle distributions, !. Geophys. Res., ~, 101, 1979. 

Williams, D. J., Energetic ion beams at the edge of the plasma sheet: ISEE 1 

observations plus a simple explanatory model, !. Geophys. ~., ~, 5507, 

1981. 

Williams, D. J., E. Keppler, T. A. Fritz, B. Wilken and G. Wibberenz, The ISEE 

1 and 2 medium energy particles experiment, IEEE Trans. Geosci. Electron., 

~, 270, 1978. 
Plate 1. 

Plate 2. 

22 

Plate Caption 

(a) ISEE-1 E-t spectrogram for 19 May 1978, showing 

repeated plasma bursts between 0930 liT and 2300 liT. 

(b) IMP-8 E-t spectrogram for the same day, showing 

magnetosheath signatures except during 1600-1700 liT 

when the satellite is in the plasma sheet. 

(a) Distribution at 0840 liT on 19 May 1978, showing 

anisotropic ion responses. (Feature in frame 5E 

should be ignored.) 

(b) E-¢ spectrogram for the following cycle where the 

anisotropy in the ion distribution has reversed 

direction. (Feature in frame 5E should be 

ignored.) 

(c) Typical sample taken during a plasma dropout. 

(d) Response during an enhancement, showing increased 

plasma pressure and field-aligned electron 

anisotropies. 
23 

Figure Captions 

Figure 1. Orbits of ISEE-l and IMP-8 spacecraft on 19 May 

1978. The time in hours (UT) is indicated on 

each graph. 

Figure 2. Ground-based magnetograms from six auroral-zone 

stations, showing periodic intensifications of 

the electrojets. 

Figure 3. The top panel shows the variation in plasma and 

magnetic pressure observed at ISEE-l. The 

diamagnetic effect is clearly seen. The lower 

panel displays the total magnetic field IBI, and 

latitudinal component Be, detected by IMP-8, 

where strong southward rotations are evident. 

Figure 4. Contours of phase space density plotted relative 

to the magnetic field direction for electrons 

detected at 1928 UTe The inner contour shows a 

+ net drift along -B, giving a field-aligned 

current of 2.54 X 10-8 A m-2• 
24 

Table 1: Magnetometer Stations 

Geomagnetic Geomagnetic Local 

Symbol Station Name Latitude Longitude Midnight (UT) 

Dr Dixon 63 0 162 0 1748 

BA Barrow 68 0 241 0 1235 

CO College 65 0 256 0 1133 

FC Fort Churchill 69 0 323 0 0704 

GWR Great Whale River 66 0 347 0 0526 

SO Sodankyla 640 1200 2032 

Table 2: Current Densities 

Time (UT) JII (A m-2) II By(Y ) II Z(km) 

1339 1.66 X 10-8 6 289 

1521 2.27 X 10-8 5 201 

1555 2.03 X 10-8 12 473 

1911 1.92 X 10-8 4 167 

1928 2.54 X 10-8 5 157 
25 

Table 3: Distance from ISEE-l to Lobe 

Time (HHMM:SS)(UT) d(km) 

1337:47 48 + 24 

1338:23 516 + 168 

1339:00 1980 + 32 

1339:36 )2400 

1340: 12 2232 + 96 

Table 4: Expansion Velocities 

Time Estimated Vz (km sec-I) (spacecraft coordinates) 

1212 15 

1424 25 

1500 15 

1640 16 

1947 15 

2117 12 
26 

CD 
27 

o 

o 
28 

B-G82-662 

ISEE-I ORBIT IMP-6 ORBIT 

-20 

-22 -20 -16 

XGSM 

Figure 1 
01 

SA 

CO 

FC 

(X COMPONENT) 

GWR 

(X COM PONE NT) 

SO 

8 10 

DAY 139, 1978 

H COMPONENT 

12 14 

Figure 2 

29 

B-G82-342-2 

16 18 UT 
eV CM-3 

sty) 

1600 

1400 

1200 

1000 

800 

600 

C-G82-370-1 

19 MAY 1978 (DAY 139) 

PLASMA AND MAGNETIC FIELD DATA FROM ISEE -I AND IMP-J 

, 

, " \ 

, , , I \ , \ 

I 

I 

I 

, 

, 

, 

, 

r 

, 

\ 

\ 

\ 

I 

I 

I 

I ,'" , I 

j'-J I 

I I I I 

I I 

I 

I 

I 

, 

, 

, 

I 

I 

, 

I 

I ,.., __ J 

I ( I , 

I I ~ , 1'..1 

" \ , 

\1 

30 

o~~~~--~--~--~--~--~~~~--~--~--~--~~ 

16~~---.--~~~-.--.---r--.---r--.--.~-r--.-~ 

12 

8 

4 - 40 

o 8 (GSM) 

-40 

~~--~--~--~--~--~--~~--~--~--~--~--~~ 

-80 

1000 1200 1400 1600 1800 2000 2200 UT 

Figure 3 
3 

2 

D-G82-418-1 

1SEE-1 

MAY 19, 1978 (DAY 139) 

1928 UT 

ELECTRON DISTRIBUTION 

FUNCTION 

CONTOURS OF CONSTANT 

PHASE SPACE DENSITY 

f(VII , Vl.) [CM-6 S3] 

o +_'::;=~+-~~-l-~~r--+--+--* ____ -lVl. 

4XI09 

-I 

-2 --<=0...---

-3 

-4X109 

CM (SEC)' 

Figure 4 

3 

CM (SEC)-I 

--- REFERENCE, 

ISOTROPY WITH 

NO BULK FLOW 

FIELD -ALIGNED CURRENT DENSITY 

= 2.54 X 10-8 AMP M-2 

31 
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PERIODIC SUBSTORM ACTIVITY IN THE GEOMAGNETIC 

TAIL Scientific - May 1978 

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To be published in Journal of Geophysical Research, 1983. 

19 KEY WORDS (Contlnu. on r." ..... Id. /I n.~ ••• &IT and Id.ntlty by block n ..... b.r) 

Substorm 

Geomagnetic Tail 

Boundary Layer 

~o ABSTRACT (Conllnu. on r.".r ••• ,d. If n.~ ••• .".. _d Id.ntlty by block n ..... b.') 

(see page following) 

DD Foq ... 

1 JAPoo 73 14i3 EDITION OF 1 HOV 6S IS OBSOL.ETE 

SIN 0102·LF.OI4-6601 UNCLASSIFIED 

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'. 

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On 19 May 1978 an unusual series of events is observed with the Quadrispherical LEPEDEA on board the ISEE-1 satellite in the earth's geomagnetic 

tail. For 13 hours periodic bursts of both ions and electrons are seen in all 

the particle detectors on the spacecraft. On this day periodic activity is 

also seen on the ground, where multiple intensifications of the electro jets 

are observed. At the same time the latitudinal component of the interplanetary magnetic field shows a number of strong southward deflections. We conclude that an extended period of substorm activity is occurring, which causes 

repeated thinnings and recoveries of the plasma sheet. These are detected by 

ISEE, which is situated in the plasma sheet boundary layer, as periodic dropouts and reappearances of the plasma. Comparisons of the observations at ISEE 

with those at IMP-8, which for a time is engulfed by the plasma sheet, indicate that the activity is relatively localized in spatial extent. For this 

series of events it is clear that a global approach to ~agnetospheric dynamics, e.g., reconnect ion , is inappropriate. 

UNCLASSIFIED 

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