I

NASA-CR-191809

(NASA-CR-191809) A CORRELATIVE

INVESTIGATION OF THE PROPAGATION OF

ULF WAVE POWER THROUGH THE DAYSIDE

MAGNETOSPHERE Status Report, I Jan.

- 30 Jun. 1992 (Augsburg Co11.)

17 p

N93-16679

Unc i as

G3/32 014O849

STATUS REPORT

NASA KESEARCH GP.AI_ NAGW-1567

"A Correlative Investigation of the Propagation of ULF Wave Power Through

the Dayside Magnetosphere"

January 1, 1992 - June 30, 1992

Dr. Mark J. Engebretson

Principal Investigator

Augsburg College

Minneapolis, Minnesota 55454
L Overview

NASA grant NAGW-1567 was awarded to Augsburg College to support Mark

Engebretson, Kenneth Erickson, and a student in a multisatellite study of the

propagation of ULF wave power through the dayside magnetosphere. Project

funding began in January 1989, and was extended through December 1992.

During the period covered by this report, January I, 1992 through June 30,

1992, we completed work on one paper, continued work on two additional studies,

and began work on another. Some results of the first two of these studies were

presented at an invited review on dayside ULF pulsations at the 1992 GEM meeting

in Snowmass, Colorado June 28-30, 1992.

IL Scientific Accomplishments

A_ Radial Pulsation Study

A multisatellite study of radial pulsations, "_'he spatial extent of radial

magnetic pulsation events observed in the dayside near synchronous orbit," by

Mark Engebretson, David Murr (STUDENT), and Kenneth Erickson of Augsburg, Robert

Strangeway of UCLA, David Klumpar and Stephen Fuselier of Lockheed Palo Alto

Research Laboratory, and Larry Zanetti and Tom PotenLra of JHU/APL, submitted to

the Journal of Geophysical Research October 10, 1991, was revised and

resubmitted April 13, 1992 and accepted for publication April 15. Typesetting

of this paper was completed at Augsburg in early July, 1992.
Page 2

In this paper on dayside radially polarized Pc 4 pulsations, we proposed a

new model for wave onset. Although we agree that bounce resonant wave-particle

interactions involving particles of ring current energies probably supply the

free energy for the observed waves, we have found strong evidence that the

influx of cold / thermal plasma as a result of plasmaspheric refilling/expansion

during quiet times triggers the instability of these more energetic particle&

During the paper's revision we added a time-of-flight calculation of

standing wave periods to estimate near-equatorial particle densities, and found

them to be in good agreement with the limited density estimation data available

on board AMPTE CCE from the Plasma Wave Experiment (PWE) instrument [Scarf et

al_ 1985_

B. Wave Polarization Study

One of the goals of this project was to study the propagation of midperiod ULF wave energy through the dayside magnetosphere. Recent case stmiies

and statistical surveys have provided a large body of evidence that upstream

waves related to a quasi-parallel bow shock geometry are a primary source of

dayside Pc 3-4 pulsations. These waves are most prominent in the azimuthal

component, where multiple harmonic resonances are often observed, but more

diffuse power is also often enhanced in compressional and radial component&

Color Fourier spectrograms of magnetometer data from the AMPTE CCE satellite

were produced routinely during the entire satellite lifetime of over 4 year_ A

feature which was noted early in the satellite lifetime was that although the

azimuthally polarized harmonic pulsations appeared to have roughly uniform

amplitude in many cases, the compressionalIy polarized pulsations, which exhibit

power in a characteristically more broadband distribution in this same frequency

range, had their Iargest amplitude near local noon and at the largest L values.
Page 3

In this study, presented initially at the Fall 1991 AGU meeting

[Engebretson et aL, 1991b] we compared the integrated amplitude of Pc 3-4 wave

power in these components as a function of L shell and magnetic local time,

using data obtained by the equatorially orbiting AMPTE CCE satellite (in a

highly elliptical orbit with apogee at 8.8 RE) during Fall 1984. We have found

that statistically, as well as in example events, compressional and radial power

fall off away from local noon and from the subsolar point, while azimuthal power

has a much more uniform distribution in local time and distance from the

subsolar point.

It has been the conventional wisdom that azimuthal harmonic pulsations are

driven by mode conversion of compressional (fast mode) ULF waves into transverse

(ALfven mode) ULF wave_ The AMPTE CCE spectrograms gave us the first evidence

that other mechanism might need to be invoked to explain the detailed

characteristics of the observed harmonic pulsationL

STATISTICAL RESULTS

We show in Table 1 a statistical summary of the two data sets (harmonic and

more broadband) used in the first portion of this study. We show the total

number of intervals, the maximum and minimum, the logarithmic mean, and the

values of the logarithmic mean +/- one sigma (determined by multiplying or

dividing the logarithmic mean by the logarithmic standard deviation) for the

power in each component (BRffiradial, BE-eastward, or azimuthal, and BN=northward,

or compressional) and for the power ratios BR/BN and BE/BN.
Page 4

TABLE 1: POWER IN THE 20 - S0 mHz FREQUENCY RANGE

I. HARMONIC PULSATION EVENTS N - 459

BE/BN BE BN BR/BN BR

MAX. 43.683 22.999 8.047 23.754 13.419

imn*istdv 5.701 5.688 1.960 1.923 1.992

IogMEAN 2.698 1.782 0.661 0.986 0.652

IogSTDDEV 2.113 3.192 2.965 1.950 3.055

imn/Istdv 1.277 0.558 0.223 0.513 0.213

gIN. 0.342 0.046 0.035 0.178 0.034

2. MORE BROADBAND PULSATION EVENTS N - 419

BE/BN BE BN BR/BN BR

MAX. 16.491 55.079 10.897 9.055 33.539

imn*istdv 3.389 8.321 2.978 1.863 3.019

IogMEAN 1.641 1.390 0.847 0.938 0.796

IogSTDDEV 2.068 3.828 3.S16 1.986 3.793

imn/istdv 0.795 0.363 0.241 0.472 0.210

MIN. 0.194 0.026 0.035 0.149 0.024

Scatter diagrams of harmonic and more broadband event occurrences as

function of local time and L shell were then prepared according to various

criteria in order to look for spatial pattern_ We show a selection of these

scatter diagrams in Figures 1 and 2.

a

The first panel in Figure 1 shows the location of all 459 harmonic ¢ventm

The next five panels show those events with power levels larger than one

standard deviation above the logarithmic mean for the BN, BR, and BE power, and

the BE/BN and BR/BN ratio; the final two panels show those events with power

levels lower than one standard deviation below the logarithmic mean for the

BE/BN and BR/BN ratio.

In Figure 2a we show the location of all 419 more broadband events. The

next two panels show those events with values of the BN power level and the

BE/BN power ratio higher than one standard deviation above the logarithmic mean.
Page 5

In both data sets there is a clear separation of occurrence location

between events with largest BN value and those with largest BE/BN and BR/BN

value& Although the locations are not mutually exclusive, the largest BN

values occur near the subsolar point (large L values, near local noon), while

the largest BE/BN and BR/BN values occur away from these location_ This

separation is strongest for the resonant pulsation category, but is also evident

for the more broadband pulsationL The largest BE values appear to be rather

evenly distributed, consistent with the findings of Anderson et aL [1990]

based on the complete 1984 data set but using a different methodology. The

distribution the largest BR values appears to be intermediate between those of

the BN and BE value&

A comparison of those distributions with the largest BR/BN and BE/BN ratios

(panels • and f of Figure 1) to those with the smallest BR/BN and BE_N ratios

(panels g and h of Figure 1) also shows a clear spatial separation, with larger

ratios predominantly inward of the smaller ratios. As was the case for the BN

and BE_N comparison, the trends were similar but not as clear for the more

broadband population.

A final step in our study, not yet performed, will be to calculate average

power levels and deviations for each category in local time -- L shell bins.

IMPUCATIONS

Theoretical studies of the transport of solar wind generated wave energy

into the magnetosphere have until recently focused on direct entry of

compressional wave energy across the magnetopause, usually near the equator,

followed by mode conversion to generate resonant transverse pulsations.

Verzariu [1973] and Wolfe and Kaufmann [1975] showed theoretically that

compressional wave power could enter, but with severe attenuation, directly
, Page6

across tangential discontinuities at the equatorial, subsolar magnetopause`

Refraction effects were expected to spread the wave energy somewhat, and the

waves were expected to attenuate gradually as they passed into the

magnetosphere, coupling some of their energy into transverse mode_ One-fluid

NIHD calculations nearly a decade later by Kwok and Lee [1984] suggested that ULF

waves of all polarizations could propagate much more efficiently across

rotational discontinuities, in association with the reconnection procceL

Compressional waves are launched in all directions into the magnetosphere, and

transverse waves are guided along cusp and low latitude boundary layer (LLBL)

field lines toward low altitudes. Compressional waves were assumed to couple

and attenuate as they traveled inward, as in the earlier modolL

The observation that compressional wave power is strongest at the highest L

shells and near local noon for both the harmonic and more broadband categories

supports the idea that a considerable amount of compressional wave power enters

the dayside magnetosphere near the subsolar point.

The observation that the average BE/BN ratio is larger for harmonic

pulsation events than for the more broadband pulsation events is consistent with

resonant excitation of the harmonics, as a repeated driving of a resonant system

should increase the amplitude of its resonant oscillation. The average BR/BN

ratio is lower, but roughly the same for harmonic and broadband events,

consistent with the lack of observed resonant behavior in the BR component in

the Pc 3-4 frequency range.

The observations, however, that azimuthal harmonic wave power (and to a

lesser extent broadband wave power) has a roughly uniform power distribution on

the dayside, and that the ratio of both azimuthal and radial power to

compressional wave power is in fact stronger away from the mlbsolar point,
Page 7

suggest that the coupling between compressional and transverse pulsations is

surprisingly weak, and that some other process is (also) involved in creating

and sustaining the transverse pulsations.

DISCUSSION

As noted above, our studies of AMPTE CCE pulsations have found that the

amplitudes of compressional and transverse wave power in the Pc 3-4 range

exhibit different spatial variations. These results cast doubt on the idea that

mode coupling from compressional wave power transmitted earthward from the

subsolar, equatorial magnetopaus¢ is the driving source for these waveL

The statistical results of Anderson et al. [1990], based on all the AMPTE

CCE data for its first 15 months of operation, also provide evidence regarding

entry for Pc 3-4 wave power, while Pc 5 pulsations (their _fundamental events _)

displayed a prominent and consistent trend of increasing occurrence with

increasing L, with occurrence frequency for L - 8 to 9 approximately double that

for L - 6 to 7, the most intense harmonic resonance events (Pc 3-4) were

observed with nearly uniform occurrence at all L values from L - 5 to 9. The

observations cited above thus imply that while propagating compressional waves

may well power Pc 5 pulsations, they do not couple significant power into

dayside transverse Pc 3-4 pulsations in the outer magnetosphere. These

observations also suggest that mode conversion from equatorially transmitted

compressional waves is a surprisingly inefficient process in the outer

magnetosphere_

As an alternative model, Engebretson et aL [1991a] proposed an

_ionospheric transistor" entry model, involving involving particle and current

modulations and cusp/cleft precipitatiom Transverse Pc 3-4 pulsations in the

outer magnetosphere can in this way be driven by high latitude perturbations in
D

Page 8

a manner analogous to the bowing of a violin string, which occurs not near the

center but near the end. A schematic diagram of this high latitude model is

shown in Figure 3. According to this model no wave mode coupling is

requir_ Rather, momentum or pressure fluctuations in a high-beta

magnetosheath (associated with upstream waves) impinge on the magnetopause and

cause 1) modulated precipitation of otherwise trapped electrons, and/or 2) the

launching of quasi-periodic transient field-aligned currents (arrows) along

cusp/clefWTX.BL field line_ The modulated precipitation of electrons and/or

time-varying currents convey wave information to the near-cusp ionosphere.

Either indirectly (via modulated Pedersen conductivity) or directly (as

modulated Region 1 currents) they cause ac variations in ionospheric Pedersen

currents and Region 2 field-aligned currents at lower latitudea These varying

currents cause transverse distortions of magnetospheric field lines, and launch

waves with frequency content matching those upstream.

In Figure 3b we show the "transistor action" at the ionospheric foot of the

proposed wave entry path in more detail Dayside Region I and Region 2 currents

are coupled by ionospheric currents at cleft/cusp latitude_ Periodic

precipitation of particles will modulate the ionization and conductivity of the

cusp/cleft ionosphere, and thus modulate ionospheric and Region 1 -- 2 current

flow, in a manner analogous to the way in which a small, variable base current

in a transistor modulates a much larger variable flow of current from collector

to emitter. In both cases a small "base current" modulates the electrical

properties of a region of relatively low conductivity connecting two regions of

relatively high conductivity. In this manner magnetosheath turbulence will be

transmitted inward to L shells in the dayside magnetosphere to the extent of the

inner edge of the Region 2 current sheet, Le_ in a region covering most of the

magnetosphere outside of the plasmapause.
Page 9

C_ Radial Boundaries of Pc 3-4 Pulsations in the Dayside Magnetosphere

As documented in a recently published paper by Takahashi and Anderson

[1992], there is a significant gap typically near L - 4 where no transverse Pc 3

pulsations occur. This gap was first presented by George He, who is now a

graduate student at the University of Maryland. He et aL [1991] interpreted

this gap as evidence that the excitation of Pc 3 pulsations outside the gap was

due to field aligned current excitation rather than the conventional excitation

mechanism by mode coupling from compressional wave_u In response to comments

made when the He et aL [1991] paper was presented at the Spring 1991 AGU

meeting, we have this past spring resumed efforts to model the radial profile of

plasma densities in the dayside outer magnetosphere. In this we have been

guided by the observed density profiles reported by Horwitz et aL [1986,

1990], who observed a multiple-plateau structure on the dayside, which during

quiet times which could extend well beyond synchronous orbit. We have now

modeled the resonant frequencies corresponding to such plateaus, using a

simplified density distribution model, and find confirmation for our initial

suggestion_ We intend to present these new results at the Fall 1992 meeting of

the AGU in San Francisco.

D. Source Regions for Correlated ULF-VLF Pulsations

We have just begun work correlating data from AMPTE CCE with simultaneous

ground data from two Antarctic stations, South Pole (at cusp latitudes, L - 13)

and Halley Bay (near L - 4). We have found excellent agreement in occurrence

times between Pc 3 pulsations at all three stations when all are located on the

dayside. As expected, when IMP 8 magnetometer IMF data are available, they show

conditions favorable for the growth of upstream waves (low IMF cone angles) when

the magnetospheric Pc 3 puisations are o's-served. It has been observed for
Page10

several years that dayside high latitude VLF hiss and/or chorus is at times

modulated at Pc 3 frequencies, but the mechanism(s) of this modulation are not

understoocL We have been collaborating with Keith Morrison of the British

Antarctic Survey in this study, and expect to make a first report of our results

at the spring 1993 AGU meeting.

REFERENCES

Anderson, ]3. J_ M. J. Engebretson, S, P. Rounds, L J. Zanetti, and T.

A. Potemra, A statistical study of Pc 3-5 pulsations observed by

the AMPTE/CCE magnetic fields experiment, 1, Occurrence distributions,

J. GeophyL Res, 95, 10495, 1990.

Engebretson, M. J, L J. Cahill, Jr, IL L Arnoldy, B. J. Anderson, T. J.

Rosenberg, D. L Carpenter, U. S. Inan, and R_ H. Father, The role of

the ionosphere in coupling upstream ULF wave power into the dayside

magnetosphere, J. GeophyL Res, 96, 1527, 1991-

Engebretson, M. J, D. L. Murr, and B. J. Anderson, A statistical study of

Pc 3-4 wave polarization in the dayside outer magnetosphere, presented

at the Fall 1991 Meeting of the American Geophysical Union, San

Francisco, CA, December 9-13, 1991b. (Abstract in EOS Transactions, 72,

397, 1991.)

Ho, G, M. J. Engebretson, IL N. Erickson, N. Lin, I_ J. Anderson, L J.

Zanetti, and T. A. Potemra, MultisateUite studies of the spatial

extent and simultaneity of Pc 3-4 harmonic pulsations in the dayside

outer magnetosphere, presented at the Spring Meeting of the American

Geophysical Union, Baltimore, MD, June 1, 1990. (Abstract in EOS

TranL AGU, 71, 605, 1990)
Page 11

Ho, G, D. I. Murr, M. J. Engebretson, K. N. Erickson, IL J. Anderson, I- J.

Zanetti, T. A. Potemra, and R. J. Strangeway, A study of factors

governing the radial extent of Pc 3-4 harmonic pulsations in the

dayside outer magnetosphere, presented at the Spring 1991 Meeting of

the American Geophysical Union, Baltimore, MD, May 28-31, 1991.

Horwitz, J. L., S. Menteer, J. Turnley, J. L Burch, J. D. Winningham, C- R_

ChappeU, J. D. Craven, L. A. Frank, and D. W. Slater, Plasma

boundaries in the inner magnetosphere, J. Geophyr. Res, 91, 8861,

1986.

Horwitz, J. L. R. IL Comfort, and _ IL Chappell, A statistical

characterization of plasmasphere density structure and boundary

locations, J. Geophys. Res, 95, 7937, 1990.

Kwok, Y. C, and L C- Lee, Transmission of magnetohydrodynamic waves

through the rotational discontinuity at the earth's magnetopause,

J. GeophyL Re_, 89, 10697, 1984.

Scarf, F. I_ The AMPTE/CCE plasma wave investigation, IEEE Tramu GeoscL

Remote Sen_, GE-23, 250, 1985.

Verzariu, P, Reflection and refraction of hydromagnetic waves at the

magnetopause, Planet. Space ScL, 21, 2213, 1973.

Wolfe, A_ and R. L Kaufman, MHD wave transmission and production near the

magnetopause, J. Geophy_ Re_, 80, 1764, 1975.

Yumoto, K, T. Salto, S.-L Akasofu, B. T. Tsurutant, and E. J. Smith,

Propagation mechanism of daytime Pc 3-4 pulsations observed at

synchronous orbit and multiple ground-based stations, J. Geophys.

Re_, 90, 6439, 1985.
Page 12

HGURE CAPTIONS

Figure 1. Scatter diagrams of harmonic pulsation event occurrences as a

function of local time and L shell Panel (a) shows the distribution

of all 459 harmonic even_ The remaining panels show the

distribution of events with power level higher than one standard

deviation above the logarithmic mean for BN (b), BR (c), BE (d),

BE/BN (e), and BR/BN (f). Panels (g) and (h) show the distribution

of events with power level lower than one standard deviation below

the logarithmic mean for BE/BN and BR/BN.

.Figure 2. Scatter diagrams of more broadband pulsation event occurrences,

as in Figure |. Panel (a) shows the distribution of all 419 more

broadband events. Panels (b) and (c) show the distribution of

events with power level higher than one standard deviation above the

logarithmic mean for the BN power and BE_N power ratio.

Figure 3. A sketch of the proposed AC current model of entry of upstream wave

signal_ Left: Magnetosheath turbulence impinging on the magnetopause

causes 1) modulated precipitation of otherwise trapped electrons,

and/or 2) modulated flow of field-aligned currents (arrows) along

boundary layer/cleft/cusp field line_ The view is from the dusk meridian

Right: Ionospheric detail of the model. Modulated precipitation varies

the conductivity of the ionosphere, which controls the amplitude of

Pedersen currents and thus the connection between region | and region

2 Birkeland currents. These region 2 currents drive the Pc 3-4 pulsations.
0

v-,

g

N

mIo

qD

i..UU0

e_ om

n_u0Ei

_3
0 0

• I

_ ._ _ .i _._ I "_-. "-. t ",

• ._,.x/"/I-..------_,--_.. "-,i ,

t/iii \\t\ _t i<i I x/ ' _ _ : _ _ _dx i I.. _______JI....... LL_L.L_L

i".,o \\),.\.b,._.._ _ A/.// , "..=_ """

"...,k:2..._',.. ",>".,<"...... i -'</; :_ / / /I /

_ Xx_,_,,,,... ---._. /./,,,,. / / _® v" _x "x" ""-- ".--._l.J / ".:, / /

",,;"., '.,..."--.... --" i" .>,/ _o \, >:.',.,,,,:,__J__t",//1

_ _,._"-.,"---....__,/" ./ o_ <..,," .... ,.."_--_l..__--i"i./"--'>"

o_. - -.-. --- .-- ....----_l-_i .>-"

, "_ -.-_._____ .. _i ........__4__Ji

o= _ o= ,,.C

8

-8
2

II

I

Q

II

¢1

o

II

0

il

II

I1,,

i-

,,Ill

41

0

i_

I1

•/'x / _..f- .... ,,._-,..-,_,'-\

' . . ' i

,,"

.......................

_ \\\\\

,_i-'i-i'-_-

i!_

i_ ',.><", \_',, t it l S

_,l l%!

,, ,, ,-, , z//!t :_, _ ,,,,",,,,

_,:k ",. "." "--..,x i" "_.,/ I i

_.,, W''.__ "----.J .." X//

• ,_.:<,,,,,,___...-- ,t >,I

,_l li,