Investigation of the Dayside Boundary Region of the Magnetosphere

(NASA-CR-177275 ) INVESTIGATIO N OP THE N86-27834

DAYSIDE BOUNDAR Y REGION OF TEE MAGNET O SPHERE

Final Technical Report, 1 May 1979 - 31 Mar.

1985 {California Dniv.) 7 p HC A02/MF AO1 Unclas

CSCL 04A G3/46 43124

Final Technical Report

NASA Grant NSG 5351

Dates: 5-1-79 to 3-31-85

Total Amount: $120,935

Principal Investigator: Nancy Crooker

Department of Atmospheric Sciences A \\\Vl/lp[/ \

University of California at Los Angeles

June, 1985
INTRODUCTION

The region near the dayside boundary of the magnetosphere is of particular

interest mainly because it is the most likely site for solar wind energy

transfer. Exactly where and how this transfer occurs is one of the basic

problems of magnetospheric physics. Two of the most important discoveries of

the ISEE spacecraft relevant to this problem are flux transfer events [Russell

and Elphic, 1979] and accelerated boundary layer flows CPaschmann et al., 1979].

These small-scale features are interpreted as signatures of the most commonly

proposed energy transfer process, magnetic merging or reconnection.

The research performed under this grant addresses the global nature of the

energy transfer process. In order to test whether energy transfer affects the

large scale plasma flow in the magnetosheath, global properties of the magnetosheath were analyzed and compared to models with no energy transfer. Also,

global models of merging sites on the magnetopause were modeled with the aid

of the magnetosheath models.

Since energy transfer and intrinsic magnetosheath properties depend upon

the orientation of the interplanetary magnetic field (IMF), it was essential

to have an IMF monitor for these studies. In the course of using ISEE 3 in

this role, statistical studies were performed to test the accuracy of using

data from an orbit so far upstream.

The results are summarized below in the abstracts of papers written with

the support of this grant.

RESULTS

Magnetosheath Data Analysis

Global properties of the magnetosheath energetic ion population, magnetic

field strength and orientation, and flow pattern were determined with ISEE

data in the following four papers:

1. Crocker, N. U., T. E. Eastman, L. A. Frank, E. J. Smith, and C. T.

Russell, Energetic magnetosheath ions and the interplanetary magnetic field

orientation, J. Geophvs. Res.. 86, 4455-4460, 1981.

This paper was presented at the ISEE Workshop on Upstream Waves and

Particles, Pasadena, April, 1980.
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Abstract

Times when energetic ions are absent and present in ISEE 1 magnetosheath

plasma spectrograms are correlated with ISEE 3 IMF orientation measurements.

The study Indicates that when the plasma at the spacecraft is traced along a

streamline to the bow shock surface, the angle between the surface normal at

that point and the IMF is greater than 60° when the energetic ions are absent

and less than 60° when they are present. The pattern is consistent with the

ions coming from the same regions of the bow shock where intermediate and

diffuse ions are found on the upstream side. The 60° criterion is used to

draw schematic patterns of the location of energetic ions in the magnetosheath

as a function of IMF orientation. Some orientations result in layers adjacent

to the magnetopause and other orientations give layers adjacent to the bow

shock.

2. Crocker, N. U. , G. L. Siscoe, and P. R. Mullen, Magnetic field compression

at the dayside magnetopause, J. Geophys. Res.. 87, 10407-10412, 1982.

This paper was presented at the Fourth Scientific Assembly of IAGA, Edinburgh,

August, 1981.

Abstract

The degree of compression sustained by the solar wind field as it convects

to the magnetopause has been determined empirically with magnetometer data

from ISEE 3 in the solar wind and ISEE 1 in the magnetosheath. At the

stagnation point, the strength BSJJ of the magnetosheath field may be expressed

as BSH = -/4 BcuBoj' which implies that BgH is equal to the geometric mean of

the stagnation field strength Bgr and the shocked solar wind field, approximated by 4 Bgg. For the usual spiral pattern of the solar wind field, 83^ on

the duskside of the magnetopause is about 20% stronger than it is on the dawnside. No dependence of Bgjj on the sign of the north-south component of the

solar wind field is apparent.

3. Crocker, N. U. , J. G. Luhmann, C. T. Russell, E. J. Smith, J. R.

Spreiter, and S. S. Stahara, Magnetic field draping against the dayside

magnetopause, J. Geophys. Res.. 90, 3S05-3S10, 1985.

This paper was presented in combination with Paper 8 at the Spring AGU

Meeting, Cincinnati, May, 1984.

Abstract

Interplanetary magnetic fields observed upstream of Earth's magnetosphere

at ISEE 3 form input for a gas dynamic model of magnetic field draping in the

dayside magnetosheath. Model results near the magnetopause are compared with

appropriately lagged observations at ISEE 1. In 16 of 24 cases the angle

between the transverse component of the model and observed fields is less

than 20°. The agreement is surprisingly good in view of the uncertainty

introduced by the large distances between ISEE 1 and ISEE 3. The results

indicate that magnetohydrodynamic and energy transfer processes at the magnetopause do not cause large distortions of the magnetosheath magnetic field. In

addition, a comparison between observed and model field magnitudes indicates

that immediately outside the magnetopause the observed field behaves like the

model field at a distance of ~0.5 Rg from the magnetopause, outside the region
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where magnetohydrodynamic effects make the gas dynamic model inapplicable.

Patterns of model magnetic field orientation at the magnetopause are presented

for practical application.

4. Crocker, N. U., G. L. Siscoe, T. E. Eastman, L. A. Frank, and R. D.

Zwickl, Large-scale flow in the dayside magnetosheath, J. Geophys. Res.. 89.

9711-9719, 1984.

This paper was presented at the Fall AGU Meeting, San Francisco, December,

1983.

Abstract

The degree of control over plasma flow direction exerted by the compressed

magnetic field in the dayside magnetosheath is examined by comparing 1SEE 1

LEPEDEA data with hydrodynamlc and magnetohydrodynamic predictions. Measured

flow directions projected toward the subsolar region pass within ~1 R£ of the

aberrated theoretical hydrodynamic stagnation point in 11 of 20 cases analyzed.

The remaining nine cases pass within ~2-3 Rg of the stagnation point. One case

with large deflection has been studied in detail with high-time-resolution

plasma and magnetic field data both from ISEE 1 and from ISEE 3, in the role of

a solar wind monitor. The deflected flow Is persistent over a period of 1 1/2

hours, and its direction is consistent with a stagnation point displacement

resulting from increased, asymmetric magnetic field pressure contributions

during periods of low Alfven Mach number, as predicted by Russell et al. Of

the other eight cases with large deflections, four are associated with flux

transfer events identified independently by Berchem and Russell. The observed

deflections in these cases are consistent with either the subsolar merging line

or the antiparallel merging hypothesis, but not exclusively with one or the

other. The results relating to the formation of a stagnation line rather than a

stagnation point are inconclusive.

The prediction referenced above, in the abstract of Paper 4, was reported

in the following paper, to which this grant contributed peripheral support:

5. Russell, C. T., H.-C. Zhuang and R. J. Walker, A note on the location

of the stagnation point in the magnetosheath flow, Geophvs. Res. Lett., 8, 984-

986, 1981.

Abstract

Observational and theoretical investigations indicate that the shape of

the magnetosphere is nearly symmetric about the plane defined by the aberrated

solar wind velocity and the earth's magnetic dipole axis. Nevertheless, many

phenomena such as magnetic pulsations and geomagnetic activity seem to indicate

that the effective solar wind arrival direction is perhaps 15° to the dawn side

of noon. This apparent paradox may have a simple resolution since it can be

shown that the location of the stagnation point . in the magnetosheath flow is

shifted towards dawn by MHD effects not included in the simple gasdynamic model

of the solar wind interaction. The shape of the magnetosphere is little

affected by the inclusion of these MHD effects. However, the size of the stagnation point shift is very sensitive to the Alfven Mach number and can only

account for the largest reported shifts (~1S°> at low Mach numbers (5.3.S).
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Magnetopause Merging Model

Research on modeling of dayslde merging sites was based on the earlier

antiparallel merging model [Crocker, 19791. A follow-on paper to the earlier

work was completed with support from this grant:

6. Crocker, N. U, The half-wave rectifier response of the magnetosphere

and antiparallel merging, J. Geophys. Res.. 85, 575-578, 1980.

Abstract

In some ways the magnetosphere behaves as if merging occurs only when the

interplanetary magnetic field (IMF) is southward, and in other ways it

behaves as if merging occurs for all IMF orientations. An explanation of this

duality is offered in terms of a geometrical antiparallel merging model which

predicts merging for all IMF orientations but magnetic flux transfer to the

tail only for southward IMF. This is in contrast to previous models of

component merging, where merging and flux transfer occur together for nearly

all IMF orientations. That the problematic duality can be explained by the

model is compelling evidence that antiparallel merging should be seriously

considered in constructing theories of the merging process.

A realistic computer version of the model was developed by other

researchers, with peripheral support from this grant:

7. Luhmann, J. G., R. J. Walker, C. T. Russell, N. U. Crocker, J. R.

Spreiter, and S. S. Stahara, Patterns of potential magnetic field merging sites

on the dayside magnetopause, J. Geophys. Res.. 89, 1739-1742, 1984.

Abstract

Models of the magnetospheric and magnetosheath magnetic fields are used to

determine the relative orientations of these fields at the dayside magnetopause

in order to locate potential merging sites. Areas on the magnetopause with different fractional antiparallel components are displayed by contour diagrams for

a variety of interplanetary field orientations. For interplanetary fields

oriented perpendicular to the solar wind velocity the areas of nearly antiparallel field agree with those obtained by Crocker using simplified representations for the magnetic field geometry. Here, the application of more realistic

models gives the locations of areas where any antiparallel component occurs.

Potential merging sites for interplanetary fields with radial components are

also illustrated. The results suggest that the topology of the magnetosheath

and magnetospheric fields provides antiparallel components over a substantial

fraction of the magnetopause for most interplanetary field orientations.

The computer model was then used to explore asymmetries in merging

patterns:

8. Crocker, N. U., J. G. Luhmann, J. R. Spreiter, and S. S. Stahara,

Magnetopause merging site asymmetries, J. Geophys. Res.. 90. 341-346, 1985.
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Abstract

Regions where a draped model magnetosheath magnetic field is nearly

antiparallel to a model geomagnetic field are shown to be asymmetric for an

interplanetary magnetic field (IMF) at the garden hose angle, as suggested by

Heelis. When the IMF has a southward component, the asymmetry favors the dawn

region for both IMF polarities. The dusk region is favored when the IMF has a

northward component. If the regions of antiparallel fields are assumed to be

sites of maximum magnetic merging, then the asymmetry is consistent with

observed seasonal variations of geomagnetic activity and with dawn-displaced

magnetospheric phenomena. In the alternate merging geometry of a merging line

passing through the subsolar region, the asymmetry is predominantly northsouth rather than dawn-dusk. The subsolar merglng-line geometry is consistent

with the seasonal variations but not with the dawn-displaced phenomena.

Statistical IMF Studies

Two papers address the question of how well the IMF near Earth can be

predicted from upstream measurements at the sunward libration point:

9. Crocker, N. U., G. L. Siscoe, C. T. Russell, and E. J. Smith, Factors

controlling degree of correlation between ISEE 1 and ISEE 3 interplanetary

magnetic field measurements, J. Geophys. Res.. 87. 2224-2230, 1982.

This paper was presented at the Fall AGU Meeting, San Francisco, December,

1980.

Abstract

The degree of correlation between ISEE 1 and ISEE 3 IMF measurements is

highly variable. Approximately 200 two-hour periods when the correlation was

good and 200 more when the correlation was poor are used to determine the

relative control of several factors over the degree of correlation. Both IMF

variance and spacecraft separation distance in the plane perpendicular to the

earth-sun line exert substantial control. Good correlations are associated

with high variance and distances less than 90 R£. During periods of highest

variance, good correlations occur at distances beyond 90 R£ up to 120 R£, the

maximum range of ISEE 1 - ISEE 3 separation. Thus it appears that the scale

size of magnetic features is larger when the variance is high. Abrupt changes

in the correlation coefficient from poor to good or good to poor in adjacent

two-hour intervals appear to be governed by the sense of change of IMF

variance: changes in correlation from poor to good correspond to increasing

variance and vice versa. The IMF orientation also exerts control over the

degree of correlation. During periods of low variance, good correlations are

most likely to occur when the distance between ISEE 1 and ISEE 3 perpendicular

to the IMF is less than 20 R£. This scale size expands to ~50 Rg during

periods of high variance. Solar wind speed shows little control over the

degree of correlation in the speed range 300-500 km/s.

10. Kelly, T. J., N. U. Crooker, G. L. Siscoe, C. T. Russell, and E. J.

Smith, On the use of a sunward-libration-point orbiting spacecraft as an IMF
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monitor for magnetospheric studies, to be submitted to J. Geophys. Res.. 1985.

This paper was presented at the Fall AGU Meeting, San Francisco, December,

1983.

Abstract

Magnetospheric studies often require knowledge of the orientation of the

IMF. In order to test the accuracy of using magnetometer data from a

spacecraft orbiting the sunward libration point for this purpose, the angle

between the IMF at ISEE 3, when it was positioned around the libration point,

and at ISEE 1, orbiting Earth, has been calculated for a data set of two-hour

periods covering four months. For each period, a ten-minute average of ISEE 1

data is compared with ten-minute averages of ISEE 3 data at successively

lagged intervals. At the lag time equal to the time required for the solar

wind to convect from ISEE 3 to ISEE 1, the median angle between the IMF

orientation at the two spacecraft is 20°, and 80Z of the cases have angles

less than 38°. The results for the angles projected on the y-z plane are

essentially the same. The minimum angle between the IMF orientation at the two

spacecraft has a median of 11°, with 80% less than 19°. These low values

indicate little temporal or small scale variation between the spacecraft. The

minimum angle generally occurs at a lag time different from the convection

time. The sign of the difference depends on IMF orientation in the sense that

magnetic features tend to arrive sooner when the IMF is directed along the

line between the spacecraft. However, the difference between a lag time

appropriate to this corotation geometry and the convection lag time is not

large enough to produce a significant decrease in the angles between the IMF

vectors at the two spacecraft. We conclude that the IMF at a libration-pointorbiting spacecraft, lagged by the time required for the solar wind to

convect to the earth, is a good, convenient predictor of the IMF near the

earth.

Reprints of Papers 1, 2, 3, 4, 6, 8, and 9, and a preprint of Paper 10 are

appended to this report.

References

Crocker, N. U., Dayside merging and cusp geometry, J. Geophys. Res.. 84. 951-

959, 1979.

Paschmann, G., B. U. 6. Sonnerup, I. Papamastorakis, N. Sckopke, G. Haerendel,

S. J. Bame, J. R. Asbridge, J. T. Gosling, C. T. Russell, and R. C. Elphic,

Plasma acceleration at the Earth's magnetopause: Evidence for reconnection,

Kature. 282. 243-246, 1979.

Russell, C. T., and R. C. Elphic, ISEE observations of flux transfer events at

the dayside magnetopause, Geophys. Res. Lett.. 6, 33-36, 1979.