16
The adequacy of the ionospheric source in supplying magnetospheric plasma C.R. Chappell a, *, B.L. Giles b , T.E. Moore b , D.C. Delcourt c , P.D. Craven d , M.O. Chandler d a Physics Department, Vanderbilt University, 708 Baker Building, Nashville, TN 37203, USA b NASA/Goddard Space Flight Center, Greenbelt, MD, USA c Centre de Recherches en Physique de l’Environnement/Centre National d’Etudes des Telecommunications, Saint-Maur des Fosses, France d NASA/Marshall Space Flight Center, AL, USA Abstract More than 30 years after the prediction of the polar wind outflow from the high latitude ionosphere, the exact magnitude and ultimate fate of the ionospheric plasma supply remains unknown. Estimates made more than a decade ago suggested that the polar ion outflow might well be of sucient strength to populate the dierent regions of the Earth’s magnetosphere. Direct measurements in the high altitude magnetosphere became possible only with the launch of the Polar spacecraft. The combination of the Thermal Ion Dynamics Experiment and the Plasma Source Instrument has revealed the presence of low energy (<10 eV) ions moving through the polar regions and into the lobes of the magnetotail. These ions would have been invisible to previous un-neutralized satellites because of the high positive spacecraft potentials. Through the use of a recently developed single particle trajectory and energization code, the movement and energy transformation of these measured particles can be estimated. They are found to move into the plasma sheet region and to be energized to typical plasma sheet energies. The magnitude of the flux of the highly variable out-flowing ions mapped to 1000 km altitude is 1 3 10 8 ions/cm 2 s in agreement with the original estimates. Future observations by the TIDE/PSI instruments will be required to determine the extent of the total ionospheric contribution. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction One of the most fundamental issues remaining in the understanding of the Earth’s magnetosphere is the source of its plasma. Early space measurements around the Earth were made with instruments which were sen- sitive only to high energy particles. When these par- ticles were found, it established a trend of thinking related to the Sun and solar wind as the principal source. Since the solar wind plasma has energies of the order of kilo-electron volts and the flare particles from the Sun exhibit energies of millions of electron volts, the earthspace plasmas found in this energy regime were thought of as being particles of solar origin which had gained access across magnetospheric bound- aries. The lack of mass resolution capability on the early instrumentation also made it dicult if not Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421–436 1364-6826/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1364-6826(00)00021-3 * Corresponding author. Tel.: +1-615-343-6794. E-mail address: [email protected] (C.R. Chap- pell).

The adequacy of the ionospheric source in supplying magnetospheric plasma

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Page 1: The adequacy of the ionospheric source in supplying magnetospheric plasma

The adequacy of the ionospheric source in supplyingmagnetospheric plasma

C.R. Chappell a,*, B.L. Gilesb, T.E. Mooreb, D.C. Delcourtc, P.D. Cravend,M.O. Chandlerd

aPhysics Department, Vanderbilt University, 708 Baker Building, Nashville, TN 37203, USAbNASA/Goddard Space Flight Center, Greenbelt, MD, USA

cCentre de Recherches en Physique de l'Environnement/Centre National d'Etudes des Telecommunications, Saint-Maur des Fosses,

FrancedNASA/Marshall Space Flight Center, AL, USA

Abstract

More than 30 years after the prediction of the polar wind out¯ow from the high latitude ionosphere, the exact

magnitude and ultimate fate of the ionospheric plasma supply remains unknown. Estimates made more than adecade ago suggested that the polar ion out¯ow might well be of su�cient strength to populate the di�erent regionsof the Earth's magnetosphere. Direct measurements in the high altitude magnetosphere became possible only with

the launch of the Polar spacecraft. The combination of the Thermal Ion Dynamics Experiment and the PlasmaSource Instrument has revealed the presence of low energy (<10 eV) ions moving through the polar regions andinto the lobes of the magnetotail. These ions would have been invisible to previous un-neutralized satellites becauseof the high positive spacecraft potentials. Through the use of a recently developed single particle trajectory and

energization code, the movement and energy transformation of these measured particles can be estimated. They arefound to move into the plasma sheet region and to be energized to typical plasma sheet energies. The magnitude ofthe ¯ux of the highly variable out-¯owing ions mapped to 1000 km altitude is 1 ÿ 3 � 108 ions/cm2 s in agreement

with the original estimates. Future observations by the TIDE/PSI instruments will be required to determine theextent of the total ionospheric contribution. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

One of the most fundamental issues remaining in the

understanding of the Earth's magnetosphere is the

source of its plasma. Early space measurements around

the Earth were made with instruments which were sen-

sitive only to high energy particles. When these par-ticles were found, it established a trend of thinkingrelated to the Sun and solar wind as the principal

source. Since the solar wind plasma has energies of theorder of kilo-electron volts and the ¯are particles fromthe Sun exhibit energies of millions of electron volts,

the earthspace plasmas found in this energy regimewere thought of as being particles of solar originwhich had gained access across magnetospheric bound-

aries. The lack of mass resolution capability on theearly instrumentation also made it di�cult if not

Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436

1364-6826/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S1364-6826(00 )00021-3

* Corresponding author. Tel.: +1-615-343-6794.

E-mail address: [email protected] (C.R. Chap-

pell).

Page 2: The adequacy of the ionospheric source in supplying magnetospheric plasma

impossible to di�erentiate the solar from the terrestrial

components.With the development of the next generation of

instrumentation which could distinguish the mass of

the ions, it was found that the terrestrial source was abona ®de contributor. The oxygen ion measurements

of Shelley et al. (1972) established the Earth's iono-sphere as a viable contributing source for the plasmasheet and ring current regions and forced a fundamen-

tal reconsideration of the origin of magnetosphericplasmas. Early measurements connected the energeticoxygen ions with the aurora and led to initial iono-

spheric source models that were related to auroralenergization processes.

In the years immediately preceding the energeticoxygen measurements, Axford (1968) and Banks andHolzer (1968) theorized that conditions in the top of

the ionosphere should be favorable to stimulate anupward ¯ow of ions and electrons into the magneto-sphere above. The ion component was predicted to be

mainly H� with energies of less than 1 eV thatincreased with altitude because of acceleration by the

ambipolar electric ®eld. The ¯uxes of these particleswere expected to be quite large, 3 � 108 ions/cm2 s,and were expected to be found throughout the iono-

sphere from the polar regions down to the inner mag-netosphere. The out¯ow was named the polar windand its future measurement was predicted by space-

craft that would carry mass spectrometers sensitive toplasma energies of one electron volt or lower.

The idea of an up-¯owing plasma with energies of 1eV matched the whistler measurements of the plasma-sphere Ð a region of plasma of the same energy that

was found to be present in the inner parts of the mag-netosphere out to L values of 4±6 (Park, 1974). Per-

haps the polar wind phenomenon would be themechanism by which the ionosphere could ®ll thisroughly corotating portion of the magnetosphere.

Because of the similarity of energy and composition ofthe plasmasphere and the ionosphere, the two regionswere typically studied by ionospheric scientists, while

the more energetic processes of the aurora, plasmasheet, ring current and radiation belts were studied by

the vast majority of magnetospheric physicists.Because of the obvious connections between the

magnetosphere, the ionosphere, and the atmosphere, a

mission was proposed to measure the three regionssimultaneously. The Dynamics Explorer missionbrought the ionospheric and magnetospheric commu-

nities together in mission design, development and op-eration and stimulated broader thinking about the

coupling of the ionosphere and magnetosphere. Theoverlapping nature of the phenomena under studyforced the development of new trans-regional instru-

ments. These instruments could operate both in theionosphere where plasmas had energies of less than 1

eV and spacecraft potentials were typically slightly

negative to the magnetosphere where particle energiesexceeded 1 MeV and spacecraft potentials reached tensof volts positive. The Retarding Ion Mass Spec-

trometer (Chappell et al., 1981) was one such instru-ment which combined the qualities of a retardingpotential analyzer designed for the ionosphere with the

ionosphere/magnetosphere capabilities of an ion massspectrometer. By using three separate sensor heads on

a spacecraft spinning in a cartwheel mode, the RIMSinstrument could sample all of the positive ions thatcould overcome spacecraft potential and reach the ana-

lyzer heads which, together, covered the complete unitsphere of look directions in one spacecraft spin period.

The RIMS instrument was designed to study thedetails of the plasmasphere and its connection to theionosphere. Its operational regime was to be in the

inner magnetosphere and ionosphere where the plas-mas were predominantly dense and cold and wherespacecraft potential remained negative or only slightly

positive. The apogee of the DE-1 high altitude space-craft was 4.5 earth radii. Because of a limitation on

the downlink telemetry, the data from the DE-1 satel-lite instruments could not be received at all times anddecisions had to be made by the investigator team

regarding the portions of the orbit in which the di�er-ent instruments would be operated. There were two

schools of interest divided along the lines of the iono-spheric and magnetospheric scientists with the formerexpressing more interest in the lower latitudes than the

latter. Indeed, since the goals of the RIMS instrumenthad been to study the plasmasphere/ionosphere inter-action, the portions of the orbits which passed through

the plasmasphere were the optimum ones and thepolar portions of the orbits seemed far less desirable.

Long discussion and compromise led to an operationalapproach which covered each region of interest duringdi�erent times.

To the great surprise of the RIMS investigator team,the polar portion of the orbit revealed a substantial

¯ow of ionospheric ions upward into the magneto-sphere including the direct measurement of the theor-etically-predicted polar wind, (Nagai et al., 1984).

And, characteristic of the serendipity of science, thisout¯ow phenomenon became arguably the most signi®-cant discovery of the investigation along with the orig-

inally-sought information about the ionosphere/plasmasphere connections. When combined with the

more energetic measurements of the Energetic IonComposition Spectrometer (Yau et al., 1985), the totalupward ¯owing plasma with energies above the posi-

tive spacecraft potential could be measured and it wassigni®cant. In fact it was of such a magnitude that thestrength of the ionospheric source moved into the cat-

egory of a contender as a supplier of magnetosphericplasmas. It appeared that these out¯ow measurements

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436422

Page 3: The adequacy of the ionospheric source in supplying magnetospheric plasma

could be combined with crude trajectory calculationsto estimate the density of magnetospheric plasmas that

could be supplied by the ionosphere (Chappell et al.,1987).During the completion of the RIMS instrument and

the Dynamics Explorer satellites, the magnetosphericcommunity developed the concept of a set of space-craft to make multi-point measurements of the solar

wind and magnetosphere. Originally called the Originof Plasmas in the Earth's Neighborhood (OPEN), theset of four and later three spacecraft were made avail-

able for proposal. Still interested in the interconnectionof the plasmasphere and ionosphere, a broadenedRIMS team submitted a proposal for the Thermal IonDynamics Experiment (TIDE) and an accompanying

Plasma Source Instrument (PSI) for spacecraft poten-tial control (Moore et al., 1995). The TIDE instrumentpermitted the achievement of better angular resolution

and higher sensitivity to low ion ¯uxes while the PSIinstrument o�ered control of spacecraft potential atthe higher altitudes. It was proposed for the Polar

Plasma Laboratory and Equatorial Magnetotail Lab-oratory of the OPEN mission. This mission withoutthe Equatorial Magnetotail Laboratory was to ¯y

more than 15 years after the original proposal andwould be in an ideal position to make the next gener-ation of measurements on the strength of the iono-spheric source. At the time of the proposal this source

had not even been measured for the ®rst time byDynamics Explorer.As a result of the TIDE/PSI instruments on what is

now called the International Solar Terrestrial Physics(ISTP) program, we are in a position to review theionospheric source strength as measured by Dynamics

Explorer and to give a second generation look at thesource strength now being measured by a satellitewhich was proposed but not developed prior to thetime of the ®rst generation measurements. This paper

reviews the original estimates of the contribution ofthe ionospheric source to the magnetosphere and re-examines it with the new and more capable TIDE/PSI

measurements. In addition, the development of a codeby Delcourt et al. (1992, 1994), which calculates singleparticle trajectories and energization, permits the more

accurate prediction of the ultimate magnetospheric lo-cation of the up¯owing ionospheric plasma and itsenergization to become the higher energy plasma that

was initially thought to be entirely of solar origin. Thecode uses only the fundamental energization mechan-isms linked to the large scale convection electric ®eldand gravity.

2. Review of the original ionosphere source estimates

Stimulated by the surprising strength of the ion

out¯ow measured by Dynamics Explorer, Chappell

et al. (1987) estimated the total ¯ux of ions movingout of the ionosphere and used this out¯ow to esti-mate the magnetospheric plasma densities that

would result in the di�erent regions such as theplasmasphere, plasma trough and plasma sheet.

Table 1, which is taken from the Chappell et al.(1987) paper, shows the strength of the di�erentionospheric sources in units of 1025 ions/s. These

data combined theoretical estimates of the polar windwith limited measurements and added the measuredsource strength of the cleft ion fountain, the auroral

zone, and the polar cap from the RIMS instrumentand the Energetic Ion Composition Spectrometer

reported by Yau et al. (1985). Note that the estimatedpolar wind ¯ux is the dominant one, ranging from 10to 46� 1025 ions/s. These estimates utilize the theoreti-

cal polar wind ¯uxes of 1±3 � 108 ions/cm2 s andassume that there is a polar wind up-¯ow for all invar-iant latitudes greater than 518, the boundary of the

inner/outer plasmasphere regions. Flux tubes throughL-shells above this boundary are thought to be not

®lled to di�usive equilibrium and hence available toreceive a strong upward polar wind ¯ux. This largearea of the up-¯ow strongly in¯uences calculations of

the total upward ¯ux.The out-¯owing ions could move into many

di�erent regions of the magnetosphere depending on

their L-shell and magnetic local time location aswell as their subsequent corotation and convective

drifts as they move up along the magnetic ®eldline. Fig. 1 shows a schematic of the out-¯owingions as envisioned in the Chappell et al. (1987)

paper. Note that the particles can move directlyupward to populate the plasmasphere and plasmatrough above or they can convectively drift across

the polar cap, through the lobes of the tail andinto the plasma sheet where they are available to

be energized by neutral sheet processes.This entry approach across the polar cap and

through the lobes of the magnetotail had not been

strongly considered in the past because out-¯owingplasma in the polar caps and tail lobes had not

been observed and because the background densitieswere low. The low densities were misleading in twoways. First, because of the high parallel velocity of

the polar wind streams, it is possible to have sub-stantial parallel ¯uxes of plasma ions even in a lowdensity background which had caused this region of

substantial ¯ux to be overlooked. Second, the space-craft charging in these low density regions can be

high (10s of volts positive) and this would prohibitthe measurement of all ions with energies less thanspacecraft potential. This low energy population of

ions would hence have been invisible on all previousspacecraft (Chappell, 1988). Only the addition of a

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436 423

Page 4: The adequacy of the ionospheric source in supplying magnetospheric plasma

spacecraft potential control device like the PSI

would enable the observation of these out-¯owing

ions.

Using measured convection electric ®eld values for

quiet and active times, the trajectories of the up-¯ow-

ing ions were estimated and their destinations in the

di�erent regions of the magnetosphere determined. It

is indeed the case that these crude trajectories pre-

dicted a dominant contribution not only to the plas-

masphere and plasma trough of the inner

magnetosphere as expected, but to the plasma sheet as

well. The calculated densities, using the ionospheric

out¯ow, an estimated volume of the region and a

characteristic residence time in the region, matched the

range of observed densities in all of these principal

magnetosphere regions. Of particular signi®cance were

the predicted densities of 0.4 to 6 ions/cm3 for the

plasma sheet and 0.03 to 0.08 ions/cm3 for the magne-

Table 1

Total ionospheric source strengtha

Quiet Acive

H� He� O� H� He� O�

Polar wind

Solar maximum 15.0(1� 108)b 1.1(7.05� 106) 10.0(6.5� 107) 0.40(2.61� 106)

Solar maximum 46.0(3� 108) 0.59(3.8� 106) 31.0(2� 108) 0.28(1.82� 106)

Cleft ion fountion

Solar maximum 0.33 1.6 0.43 4.8

Solar minimumc 0.63 0.73 0.43 1.9

Auroral zone

Solar maximum 2.1 1.6 2.6 7.7

Solar minimumc 1.7 1.0 3.3 3.2

Polar cap

Solar maximum 0.24 0.24 0.61 2.5

Solar minimumc 0.43 0.39 1.0 1.5

a Each entry is to be multiplied by 1025 ions sÿ1.b The numbers in parentheses represent the polar wind ¯uxes in ions per square centimeter per second that were used for the

di�erent solar and magnetic conditions.c The Yau et al. (1985) DE data were taken in two sets; one near solar maximum and one about half way between solar maxi-

mum and minimum (see Fig. 1).

Fig. 1. A schematic representation of ionospheric ¯ow into the magnetosphere from Chappell et al., (1987) showing `invisible'

polar out-¯ow through the magnetotail lobes to the plasma sheet.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436424

Page 5: The adequacy of the ionospheric source in supplying magnetospheric plasma

totail lobes. The former is more than adequate to

account for the measured plasma sheet ion densitiesand the latter con®rms the very low density of the taillobes which would cause substantial positive charging

of any un-neutralized spacecraft in that region. Thispositive charging would make the low energy ions in-

visible.The spacecraft potential in the high altitude polar

cap and entry interface to the tail lobes has beenmeasured by the Electric Field Instrument on thePolar spacecraft (Harvey et al. 1995) and is shown in

the upper panel of Fig. 2. Note that it reaches 40 Vpositive with respect to the surrounding plasma. The

lower panel of Fig. 2 shows the e�ect of the PSI oper-ation which clamps the Polar spacecraft potential to

just over 2 V positive, thereby permitting the obser-vation of the low energy out-¯owing ions with energiesgreater than 2 eV.

It was the above-mentioned calculations of the den-sities which led to the general conclusion of the orig-

inal Chappell et al. (1987) paper that the ionosphere isa fully adequate source of plasma for the magneto-

sphere. But what pieces were missing in this earlier

work? First, the out¯ow measurements were made at

lower altitudes below the 4.5 RE apogee of the DE-1spacecraft. There were no direct measurements of theparticles at higher altitudes in the polar cap entering

the lobes of the magnetotail for transport to theplasma sheet region. Second, the trajectories calculated

for the up-¯owing ions were very crude based onboundary estimates of the di�erent regions and not the

exact single particle trajectory calculations. Third,there was no calculation of the energization of the par-ticles as they moved upward into the magnetosphere.

This omission is not as important in the plasmasphereand plasma trough where the cold particle energies are

similar to their ionospheric source. It is important,however, in the plasma sheet where energy enhance-

ments of a factor of 1000 would be required to boostthe ionospheric particles up to the observed plasmasheet energies. Fourth, the theoretical ion out¯ow con-

tribution of the polar wind used only the classicalpolar wind models which did not include the variety of

energization processes such as ion heating in the polarcleft which can dramatically change the polar wind

energy and composition. Finally, there was no direct

0

10

20

30

40

50

15:00:00 16:00:00 17:00:00 18:00:00 19:00:00 20:00:00

1997 August 6 15:00-20:00 UT

PO

LAR

EF

IS

pace

craf

t Pot

entia

l (V

)

Time (UT)

0

1

2

3

4

11:00:00 13:00:00 15:00:00 17:00:00 19:00:00 21:00:00

1997 August 15 11:00-22:00 UT

PO

LA

R E

FI

Spa

cecr

aft P

ote

ntia

l (V

)

Time (UT)

Spacecraft Potential for typical agogee pass of POLAR

Spacecraft Potential for typical agogee pass of POLAR with PSI in operation

Fig. 2. Two plots of Polar spacecraft potential as the spacecraft moves across the high altitude polar cap. In the upper panel with-

out the plasma neutralizer the potential reaches 40 V positive with respect to the surrounding plasma. The lower panel with the

Plasma Source Instrument operating shows a potential of only 2.5 V.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436 425

Page 6: The adequacy of the ionospheric source in supplying magnetospheric plasma

measurement of the ion out¯ow actually reaching theplasma sheet region of the tail due to the elimination

of the Equator spacecraft from the program. We canhope that the Polar mission will be extended su�-ciently to permit the line of apsides of the orbit to

rotate into the magnetotail region so that the space-craft will be able to cross the tail lobe/plasma sheetinterface.

3. The ISTP era

In the decade since the original ion source estimates,

there have been a number of measurements of iono-spheric out¯ow. See the excellent review articles byYau and Andre (1997) and Andre and Yau (1997). In

addition, there have been two important developmentsÐ the improved instrumentation and the single par-ticle trajectory codes, (see for example, Cladis, 1986;

Delcourt et al., 1994; Ashour-Abdalla et al., 1999).The development of the TIDE instrument with its highsensitivity, and high angular resolution combined withthe PSI for spacecraft potential control has e�ectively

removed the blinders which made the low energy ionout¯ow invisible to earlier high altitude missions. Thesingle particle trajectory code of Delcourt et al. (1994)

has enabled the more accurate calculation not only of

the path of the ion transit into the magnetosphere but

of the energization of the particles as they move in thelarge scale convection ®eld all the way from the iono-sphere to the inner plasma sheet. In addition to the

out¯ow trajectories, the Delcourt code has beenupgraded for this study to permit an integration back-

wards in time using for the ®rst time a tilted dipolemagnetic ®eld model to trace the particles measured

high in the polar cap back to their point of origin inthe ionosphere below.

The importance of the ISTP program to magneto-spheric physics is well known. The ability to make

multi-point coordinated measurements throughout geo-space opens up new avenues of investigation of particleand energy ¯ow. The unique orbit of the Polar space-

craft as shown in Fig. 3 samples the high altitudenorthern polar cap of the Earth and covers the ®eld

lines along which out-¯owing ions would enter thenorthern lobes of the magnetotail. When the TIDE/

PSI combination is operated through this region, theout-¯owing ions are very evident as shown in Fig. 4which is a set of passes across the high altitude polar

cap. The plots display ¯ux levels in a color scaleplotted versus time on the X-axis and spin angle on

the Y-axis. The line across each plot is the ion out¯owmoving close to the magnetic ®eld line direction. The

PSI instrument is turned o� before the start of the

Fig. 3. A typical orbit of the Polar spacecraft shown in solar-magnetospheric coordinates.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436426

Page 7: The adequacy of the ionospheric source in supplying magnetospheric plasma

PSI ONPSI OFF

Fig.4.A

seriesofpasses

ofthePolarsatellite

acro

ssthehighaltitu

depolarcapwhich

show

the¯ux(co

lorsca

le)plotted

versu

s

timeontheabscissa

andspin

angle

ontheordinate

ofeach

plot.These

measurem

ents

were

madebeginningjust

befo

reapogee

at

8.9

REgeocen

tricwith

thePSIneutralizer

opera

tingonallexcep

tthelast

twopanels.

C.R.Chappell

etal./

JournalofAtm

ospheric

andSolar-T

errestrialPhysics

62(2000)421±436

427

Page 8: The adequacy of the ionospheric source in supplying magnetospheric plasma

third panel from the bottom. In the last three panels

only the higher energy ions which can overcome space-craft potential are seen. The TIDE/PSI combination isonly operated for two weeks every two months. In the

course of the total mission, the number of passes isbuilding up and the ion out¯ow statistics are beginningto improve.

Before examining the characteristics of the ion out-¯ow measured by the TIDE instrument, it is instructive

to look at where the ions will travel as they move outinto the magnetosphere. To get an idea of their desti-nation, a point has been selected every hour along the

satellite pass through the high altitude polar cap for allof the passes in which the TIDE/PSI combination has

been operated. The mean energy of each of the ionout¯ows has been determined as well as its location inthe polar cap. These parameters have been used as

input to a three-dimensional test particle simulationcode along with the 3-h Kp that existed at the time ofthe measurement and the appropriate dipole tilt lo-

cation for that period. The code then calculates theparticle trajectory for the ion as it moves out into the

magnetosphere. Of the runs made, about two-thirds ofthe ions move into the plasma sheet region of the mag-netotail; the remainder precipitate in the northern or

southern hemisphere or are lost out of the model mag-netic ®eld 70 RE down the tail.The particle trajectory code is an adaptation of that

of Delcourt and co-workers (Delcourt et al., 1992,1993, 1994, 1997). For this application the model uses

the Tsyganenko (1989) c model magnetic ®eld and theVolland (1978) model of the ionosphere potential dis-tribution for the convection electric ®eld. In addition,

to more closely re¯ect the conditions of each hourlyobservation, the model ®elds are matched to the mag-netic activity (Kp) level and magnetic dipole tilt angle

of the observing UT time.It should be noted that single particle calculations

cannot be considered exact. As with magnetosphericmodels of all types, the calculations contain approxi-mations. For example, the Tsyganenko 89c and Vol-

land models do not provide self-consistent magneticand electric ®elds. However, these models do re¯ect by

design the average ¯ow characteristics observed in theplasma sheet (Angelopoulos et al., 1993). We are treat-ing the ions in this study as test particles in the mod-

eled electric and magnetic ®elds. Eventually, one has totreat the e�ects of these particles back on the ®elds,but this is beyond the scope of this study. A recent

®rst look at the e�ects of the ionospheric source onthe plasma sheet can be found in this volume (Winglee,

2000). In addition, although processes such as waveparticle interactions are not included in the model, theresults of particle energization and trajectory desti-

nations are remarkably similar to measured plasmasheet particle characteristics. Thus, the modeled trajec-

tories provide further quantitative grounds on which

to view the ionosphere as a signi®cant plasma sheetsource.Fig. 5 shows one of the trajectory calculations. The

top panel shows the X±Z plane of the magnetosphere,the middle panel the X±Y equatorial plane, the lower

left panel the Y±Z plane looking toward the sun withthe lower right panel showing the energy of the particleas it moves along the trajectory. The initial observed

energy of the particle is 11 eV. In the top panel thepoint of transit is shown for 1.5 and 2 h transit timeÐ before and after entry into the plasma sheet region.

These times may be matched with the horizontal axisof the lower right panel. Notice the dramatic energiza-

tion of the particle as it enters the plasma sheet chan-ging from an energy of less than 100 eV to an energyof 3 keV in less than 10 min. This energization is a

result only of single particle motion in the distendedmagnetic ®eld of the neutral sheet region and thecross-tail convection electric ®eld.

Spatially the particle moves from the high altitudepolar region through the low density lobe of the geo-

tail where its energy remains below a few tens of eVand then into the plasma sheet region where it is ener-gized to plasma sheet energies and moves toward the

Earth eventually ending up in the dusk ring currentregion having acquired an energy in excess of 10 keVtypical of the ring current particles. Hence, we see a

very low energy out-¯owing ion being transformedinto di�erent parts of the energetic magnetospheric

particle population in the precise places where particlesof the calculated energies are found. Note also that ittakes the out-¯owing particles one to two hours to tra-

vel from the ionosphere to the plasma sheet. Thesetimes are somewhat longer than the duration of a typi-cal substorm cycle, but typical of storm development

scales. DE-1/RIMS and recent POLAR/TIDEmeasurements, Moore et al. (1999), indicate that day-

side out¯ows are strongly enhanced by solar windpressure ¯uctuations. Thus it may be that the mainphase of storms is enabled or facilitated by the arrival

of enhanced ionospheric out¯ows at the plasma sheet.Figs. 6±8 show three other examples of calculated

trajectories for out-¯owing ions with initial energies of13.3, 4.8, and 7.4 eV respectively. In each of theseexamples, the ion transits the northern tail lobe at very

low energy (less than 20 eV) and is then rapidly ener-gized to energies of several keV as it enters the plasmasheet. The particle subsequently drifts toward the

Earth where it reaches energies greater than 10 keV atthe inner edge of the inward ¯ow.

If the ions are traced backwards from their obser-vation points in the polar cap where guiding center-based arguments are valid, their foot points of origin

in the ionosphere can be calculated. In the upper twopanels of Fig. 9, an example of these downward trajec-

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436428

Page 9: The adequacy of the ionospheric source in supplying magnetospheric plasma

tories is shown in the X±Z and Y±Z planes. In the

example shown the ion energy was 11.2 eV. If all of

the trajectory footprints from the data are plotted ver-

sus invariant latitude and magnetic local time, the plot

at the bottom of the page is the result. This plot

suggests that the origin of the upward ¯ux of ions is

spread across the polar cap and extends to lower lati-

tudes in the dawn dusk sector. The satellite coverage

of these out¯ows is not uniform and is decreased

within two hours either side of the noon midnight mer-

idian. This non-uniform coverage may well distort the

picture of the ionospheric source and should be ®lled

in with future measurements.

There is, however, an uncertainty in the location of

these foot points which comes from an assumption

used in the trajectory calculation. Since the trajectory

code does not include wave-particle interactions or

energization mechanisms other than those related to

large scale convection and gravity, the backwards tra-

jectory from the point of observation in the high alti-

tude polar cap back to the ionospheric foot point

assumes a particle energy equal to the energy measured

at the satellite. This energy of 11.2 eV for the case

shown in the top of Fig. 9 may be misleading. If the

particle began its journey as a low energy (1 eV) classi-

cal polar wind up-¯owing particle, it could have drifted

farther in an anti-sunward direction as a very low

energy particle before becoming energized to 11.2 eV

through one or a series of acceleration processes as-

sociated with the polar cleft, for example. Hence, the

1.5 hours

2 hoursSunward

Dawn

Dusk

Observation @ 6Jun1997 1345UT8.2 RE, 9.17MLT, 80.51MLAT, Kp=2

Fig. 5. Calculated particle trajectories which trace the path and energization of a measured out-¯owing ion as it moves through the

magnetotail lobe into the plasma sheet. The top two panels show the motion in solar-magnetospheric coordinates. The bottom two

panels show solar-magnetospheric coordinates on the left and energy versus time of travel on the right.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436 429

Page 10: The adequacy of the ionospheric source in supplying magnetospheric plasma

points plotted in the lower part of Fig. 9 probably rep-

resent the most anti-sunward location for the foot

point of ionospheric origin with the more probable lo-

cation being on the dayside equatorward of the polar

cleft region.

To get an estimate of the lower latitude limit of the

point of origin on the dayside, we have calculated the

backward trajectories of several of the sunward points

near noon using an energy of 1 eV and assuming that

the particles moved up the ®eld line with this lower

velocity. Hence, they would have drifted farther in an

anti-sunward direction before receiving the additional

10 eV of energy just before reaching the spacecraft.

This calculation gives initial foot points in the latitude

range of 55±688.Determining the precise point of origin of the out-

¯owing ion is made even more di�cult by the variety

of energization processes that can take place as it

leaves the ionosphere. Initially, perhaps it is a classical

polar wind ion which is subsequently acted upon by

many `non-classical' energization processes. Fig. 10

from Schunk and Sojka (1997) illustrates several of the

processes that cause changes not only in the energy of

the out¯ow but in its composition mixture as well.

Many of these occur when the particles drift through

the auroral regions such as the polar cleft and the

nightside auroral zone. The drift path is often quite

involved as shown in Fig. 11 which also comes from

the work of Schunk and Sojka (1997). In their coupled

model of the ionosphere and the resulting out¯ow, a

large variety of ¯uxes, energies and compositions are

possible. Therefore, a great deal of detailed modeling

must be brought into play before accurate backward

tracing of out-¯owing particles can be accomplished.

Observation @ 17Mar1997 1558UT8.7 RE, 19.8MLT, 79.7MLAT, Kp=1

Fig. 6. Calculated particle trajectories as shown in the previous ®gure.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436430

Page 11: The adequacy of the ionospheric source in supplying magnetospheric plasma

The Schunk and Sojka (1997) model, predicts a peak

out¯ow of 3 � 1025 ions/s out of the total ionosphere

for the magnetic storm which they modeled. This is a

factor of three below the lower end of the range of

out¯ow from the Chappell et al. (1987) predictions.

Fig. 12 shows the out-¯ux of all ions adjusted to

1000 km as measured by the TIDE/PSI instrument

combination in the high altitude polar cap. The ¯ux

magnitude is in the same range as the theoretical polar

wind ¯uxes of 1±3 � 108 ions/cm2 s used in the Chap-

pell et al. (1987) paper. These measured ¯uxes come

from 14808 two minute averages made from apogee

passes between March 1997 and September 1999 in

which the TIDE/PSI instrument combination was

operating. It is clear that the range of upward traveling

ion ¯ux matches or exceeds the original estimates and

can reach 8� 108 ions/cm2 s at high Kp.

Although the individual ¯uxes are comparable to or

greater than the polar wind numbers used in the pre-

vious paper, the total out¯ow that results across the

whole ionosphere remains uncertain because the area

of up-¯ow has not been completely measured. Due to

a limitation in the operational range of the TIDE

instrument, the high altitude polar wind out¯ow for L-

values less than about eight cannot be measured.

Hence the areal breadth of the up-¯ux cannot be

directly determined at high altitudes. It is expected

that information on polar wind up-¯ow from the peri-

gee passes in the southern hemisphere will be import-

ant in addressing this issue.

A second area of future study will be the variation

Observation @ 20Mar1997 0100UT8.4 RE, 2.98MLT, 72.9MLAT, Kp=0+

Fig. 7. Calculated particle trajectories as shown in the previous ®gure.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436 431

Page 12: The adequacy of the ionospheric source in supplying magnetospheric plasma

in ion out¯ow with changing solar wind parameters.Initial studies on this important subject have been

made by Moore et al. (1999) and Giles et al. (1998)and show a surprising apparent control by the solarwind pressure ¯uctuations. Further measurements to

build adequate statistics will be contingent on the con-tinued operation of the TIDE/PSI instruments on thePolar spacecraft.

4. Conclusions

The advances in instrumentation and computer

modeling of the past decade have given us an enhancedability to measure and predict the contribution of theionospheric source in supplying magnetospheric plas-

mas. The predictions of Chappell et al. (1987) have

been con®rmed in several areas. First, the idea of an

out-¯owing plasma in the polar regions which moves

`invisibly' toward the lobes of the magnetotail to

supply the plasma sheet has been demonstrated by the

TIDE/PSI measurements of out-¯owing plasma near

the polar cap/tail lobe interface. The single particle tra-

jectory calculations provide insights into the destiny of

this ionospheric plasma in the plasma sheet. Second,

these same calculations show the straight-forward ener-

gization of these invisible out-¯owing ions as they

enter the neutral sheet region Ð an energization that

matches the energy of plasma sheet particles that have

been observed there for decades.

Thirdly, the magnitude of the ion out¯ow measured

by TIDE/PSI at 1±3 � 108 ions/cm2 s matches the

Observation @ 20Mar1997 0240UT7.8 RE, 2.28MLT, 65.9MLAT, Kp=O+

Fig. 8. Calculated particle trajectories as shown in the previous ®gure.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436432

Page 13: The adequacy of the ionospheric source in supplying magnetospheric plasma

¯uxes used in the original paper to predict the strength

of the ionospheric source. This ¯ux can be highly vari-

able in both spatial location and in response to chan-

ging solar wind and ionospheric solar illumination

conditions and requires continued observation with the

TIDE/PSI combination as the Polar mission pro-

gresses. The accurate determination of the total ion

out¯ow requires a subsequent study to characterize the

spatial breadth of the out¯ow across the full range of

ionospheric invariant latitudes. In the original paper

the polar wind out¯ow was assumed to exist in all

un®lled ¯ux tubes at L-values above the inner/outer

plasmasphere boundary at L � 2:5: The TIDE/PSI

high altitude measurements to date have been limited

to L-values greater than eight and hence have left a

gap in measured out¯ow that needs to be investigated

further to facilitate the determination of the total ion

out¯ow. Even with the limited observations currently

in hand, we can state that the regions of out¯ow that

have been observed by POLAR/TIDE are essentially

1997 June 9 01:10 UT

Footpoints of alltrajectories

Typical trajectory tracing

12

18 6

0 MLT

8070

6050

Fig. 9. A calculated particle trajectory moving backward in time to estimate the location of the origin footprint of a measured out-

¯owing ion. The top two panels show the trajectory in solar-magnetospheric coordinates and the lower panel shows the footprints

of all the measured particle traces in an invariant lattitude-magnetic local time plot.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436 433

Page 14: The adequacy of the ionospheric source in supplying magnetospheric plasma

Fig. 10. A schematic drawing of the out-¯ow processes that in¯uence the energy and motion of polar wind up-¯owing ions. From

Schunk and Sojka (1997).

Fig. 11. A calculated convection trajectory of a ¯ux tube of

plasma moving in magnetic local time and magnetic latitude

during changing magnetic activity. The tic marks show the

time in hours. From Schunk and Sojka (1997).

Fig. 12. A plot showing the average magnitude of the parallel

up-ward ¯ux as a function of invariant latitude from 14,808

samples measured by the TIDE/PSI instrument combination

between March 1997 and September 1998. The lines through

each square show the standard deviation for each invariant

latitude summed over all magnetic local times and corrected

to the equivalent ¯ux at 1000 km.

C.R. Chappell et al. / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 421±436434

Page 15: The adequacy of the ionospheric source in supplying magnetospheric plasma

those that feed directly through the lobes to the plasmasheet. The lower latitude regions we have missed are

largely those that are on more dipolar ®eld lines andare therefore within the closed inner magnetosphericcirculation pattern except during enhanced convection

periods during higher Kp.In the future, in addition to continued TIDE/PSI

measurements to understand the ion out¯ow variations

with changing solar and solar wind conditions, theexciting opportunity exists to measure the dramaticchanges in energy of the low energy ions as they move

from the lobe of the magnetotail into the neutral sheetregion. This eagerly anticipated measurement can bemade with TIDE/PSI when the apogee of the Polarspacecraft has precessed su�ciently toward the equator

to give an orbital track which traverses the innerplasma sheet. This will replace some of the measure-ments that were lost with the cancellation of the

Equator spacecraft and can shed further light on thedetails of the dominant ionospheric supply process forthe central plasma sheet that has been predicted

recently by the multi-¯uid magnetospheric simulationsof Winglee (1998).In summary, the predictions of the adequacy of the

ionospheric source strength of a decade ago still lookintriguing and reasonable according to the newmeasurements. More TIDE/PSI observations arerequired and more study in several speci®c areas must

be done. As of this point in time the ionosphericsource appears to be rich in its mixture of ions andimpressive in its strength in a variety of magneto-

spheric conditions.

Acknowledgements

The authors would like to recognize the considerablee�orts of the TIDE/PSI engineering and software

teams at NASA's Marshall Space Flight Center. Thisresearch was supported by the Global GeospaceScience program at NASA's Goddard Space Flight

Center.

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