It is shown how satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopause current layer and the magnetic field component along this normal. Results from 22 Explorer 12 boundary penetrations indicate normal field components of less than 5 gamma in two-thirds of the cases. Measured field variations within the current layer demonstrate the existence of two fundamentally different types of boundary structure, the rotational and the tangential discontinuity. The rotational discontinuity seems to occur predominantly during magnetic storms. Finally, the calculated normal vector is compared with the normal to the surface of the Mead-Beard magnetosphere model

Sonnerup, B. U. O.

English

NASA Technical Reports Server

Radiophysics Laboratory
Thayer School of Engineering
Dartmouth College
Hanover, New Hampshire 03755
MAGNETOPAUSE STRUCTURE AND DYNAMICS OF THE MAGNETOSPHERE
Final Report, Grant NGR 30-001-011
Submitted to
National Aeronautics and Space Administration
Office of Scientific and Technical Inforamtion
Washington, D.C. 20546
(NASA-C-136107) MAGNETOPAUSE STRUCTURE N74-12153
AND DYNAMICS OF THE MAGNETOSPHERE Final
Report, 15, Apr. 1965 - 30 Jun, 1970-
(Dartmouth Coll.) 6 p HC $3o00 CSCL 04A Unclas
03/13 15847 /
Time period: April 15, 1965 - June 30, 1970
Principal Investigator: Bengt U. O. Sonnerup
Professor of Engineering
Date: August 9, 1973 (originally, August 3, 1970)
EndorsBemente: c nneru
Bengt U.UO. Sonnerup
https://ntrs.nasa.gov/search.jsp?R=19740004040 2020-03-23T12:37:25+00:00Z
The final report on Grant NGR 30-001-011 to Dartmouth
College consists of the following papers:
1. "Magnetopause Structure and Attitude from Explorer 12
Observations", B. Sonnerup and L. J. Cahill, Jr., J.
Geophys. Res. 72, 171-183, 1967.
2. "Large-Amplitude-Whistler Waves in a Hot Collision-Free
Plasma", B. Sonnerup and S. Y. Su, The Physics of Fluids,
10, 462-464, 1967.
3. "Explorer 12 Observations of the Magnetopause Current
Layer", B. Sonnerup and L. J. Cahill, J. Geophys. Res.
73, 1757-1770, 1968.
4. "First-Order Orbit Theory of the Rotational Discontinuity",
S. Y. Su and B. Sonnerup, The Physics of Fluids, 11, 851-
857, 1968.
5. "Motion of Charged Particles in Growing or Decaying Circularly
Polarized Waves", B. Sonnerup, The Physics of Fluids, 11,
682-683, 1968.
6. "Nature of the Rotational Discontinuity", G.A. Gerdin and
B. Sonnerup, The Physics of Fluids, 12, 145-148, 1969.
7. "Acceleration of Particles Reflected at a Shock Wave", J.
Geophys. Res., 74, 1301-1304, 1969.
8. "Resonant Vibration of the Magnetosphere Observed from
Explorer 26", B. Sonnerup, L. J. Cahill and L. R. Davis,
J. Geophys. Res., 74, 2276-2288, 1969.
9. "Magnetic-Field Re-connexion in a Highly Conducting
Incompressible Fluid", B. Sonnerup, Journal of Plasma
Physics, 4, 161-174, 1970.
10. "On the Equilibrium of the Magnetopause Current Layer",
S. Y. Su and B. Sonnerup, J. Geophys. Res., 76, 5181, 1971.
11. "Magnetopause Structure during the Maanetic Storm of
Sept. 24, 1961", B. Sonnerup, J. Geophys. Res., 76, 6717,
1971.
Previously NASA has been supplied with 25 reprints of each
of the first nine of these papers. Five reprints of papers
#10 and #11 are enclosed.
Abstracts of the eleven papers are attached.
2Magnetopause Structure and Attitude from
Explorer 12 Observations
It is shown how satellite magnetometer data at a magnetopaUse
penetration can be used to determine the vector normal to the
magnetopause current layer and the magnetic-field component along
this normal. According to theory such a component is a measure
of the amount of field reconnection at magnetopause. Results from
22 Explorer 12 boundary penetrations are presented indicating
normal-field components of less than 5 y in two-thirds of the cases.
Measured field variations within the current layer are presented
to demonstrate the existence of two fundamentally different types
of boundary structure, the rotational and the tangential discon-
tinuity. The former of these permits a nonzero normal field com-
ponent, whereas the latter does not. The rotational discontinuity
seems to occur predominantly during magnetic storms and two of
these cases, involving substantial normal-field components, provide
compelling evidence that field reconnection takes place during the
storm main phase. Finally, the calculated normal vector is com-
pared with the normal to the surface of the Mead-Beard magneto-
sphere model.
Large Amplitude Whistler Waves in a
Hot Collision-Free Plasma
A dispersion relation is derived for large-amplitude circularly
polarized plane waves propagating along a uniform magnetic field
in a hot collision-free plasma.
Explorer 12 Observations of the Magnetopause
Current Layer
The variation in magnetic field magnitude at magnetopause is
examined with the objective of separating boundaries of the rota-
tional-discontinuity type from those of the tangential-discontinuity
type. The field magnitude should remain constant throughout the
current layer in the former case but may vary considerably in the
latter. Also, the field component normal to the layer may be non-
zero in the former case but not in the latter. It is estimated that
approximately one-third of the 91 cases examined may have been of
the rotational-discontinuity type. The field component along a
theoretical vector normal to the current layer is compared for the
two groups. The result indicates that the normal field component,
on the average, may be less than 5 y for the rotational-discontinuity
group also. For the tangential discontinuities, a normal vector,
calculated from the data under the assumption of a vanishing normal-
field component, is compared with a corresponding vector obtained
from the Mead-Beard theoretical magnetosphere model. Substantial
individual, as well as systematic, deviations are found. The latter
3are related to the angle between the solar-wind and the earth's
dipole axis. Finally, the sense of rotation of the tangential
magnetic-field component at magnetopause is examined. For rota-
tional discontinuities, the sense of rotation is found to be
opposite above and below the solar-magnetospheric equatorial plane.
A tentative explanation of this observation, in terms of the
electrical structure of the current layer, is proposed.
First-Order Orbit Theory of the Rotational
Discontinuity
A study is made of the propagation properties and internal
equilibrium structure of a large-amplitude Alfven wave which con-
serves the magnitude of the magnetic-field component tangential
to the wave front but changes its direction. The propagation
properties are found to be identical with those of small-amplitude
sinusoidal waves. The equilibrium wave structure is shown to
involve an electric field along the wave normal. The sign and
magnitude of this field are related to the rate and sense of ro-
tation of the tangential magnetic field component as well as to
the particle distributions. In certain cases the electric field
may be sufficiently large to cause nonnegligible variations in
particle and magnetic pressures within the wave. Observed polar-
izations of the magnetopause current layer may be explained by
the theory.
Motion of Charged Particles in Growing
or Decaying Circularly Polarized Waves
A constant of motion is derived for a charged particle in
the magnetic and electric fields of a circularly polarized wave
with time-variable amplitude. The wave is assumed to propagate
along a uniform external magnetic field.
Nature of Rotational Discontinuity-
The behavior of a large-amplitude intermediate magnetohydro-
dynamic wave in the presence of reflected particles is examined.
For reflections off its front side, the wave becomes a spreading
expansion wave; for reflections.off the back side it steepens to
a shock wave, i.e., an entropy increase is produced across the
wave.
4Acceleration of Particles Reflected
at a Shock Front
In a recent paper Asbridge et al. [1968] report convincing
evidence for particle reflection and acceleration at the magneto-
spheric bow shock, obtained from the particle experiments on
board a series of Vela satellites. Typically, the energy of these
reflected particles, believed to be mainly protons, is observed
to be of the order of four to six times that of an average solar-
wind proton. It is the purpose of the present paper to demonstrate
that particle energization of this magnetude or greater is expected
to occur in the reflection process. The responsible agent is
the interplanetary electric field, and the acceleration takes
place because the particles are displaced in the d'irection of that
field in the reflection process.
Resonant Vibration of the Magnetosphere
Observed from Explorer 26
This paper examines in detail a set of large-amplitude pulsations
in magnetic-field magnitude and proton fluxes (E>98 kev) observed
by Explorer 26 in connection with a bay event during the magnetic
storm of April 18, 1965. The peak-to-peak amplitude-was about
40 y and the period about 300 sec. These pulsations were such that
the field magnitude was high when the proton fluxes were low and
vice versa. They were accompanied by field-direction changes cor-
responding to an elliptical polarization. The observations are
compared with four models involving (1) drifting plasma clumps,
(2) ripples traveling in longitude on the surface of a particle
belt, (3) a particle belt in periodic radial motion, and (4) a
particle population sloshing back and forth along the field lines
in the acoustic mode. The last of these models is found to be the
most attractive one, and its consequences in terms of periodic
modulation of the particle energy are examined. The study also
indicates that periodic field-aligned currents accompanied the
event.
Magnetic-field Re-connexion in a Highly
Conducting Incompressible Fluid
A theoretical model is proposed for the process of re-connexion
of frozen-in magnetic-field lines at an X-type null point in the
field. The model involves a diffusion region, immediately adjacent
to the null point and an outer wave-dominated region. For the
latter an exact solution to the magneto-hydrodynamic equations is
obtained; for the former an approximate relationship is derived be-
tween the field-re-connexion rate., measured by the Alfvn number
of the incident flow, and magnetic Reynolds number, based on the
width of the diffusion region. The maximum field-re-connexion rate
is determined entirely by conditions near the null point and may
under suitable conditions become large. A condition for maximum
conversion of magnetic energy is derived.
5On the Equilibrium of the Magnetopause
Current Layer
A numerical study was made of the confinement of a uniform
magnetic field by a warm plasma with a net velocity parallel to
the confined field. Parker's assumptions of a vanishing electric
field and vanishing trapped-particle currents and pressures were
adopted. For a given proton temperature.equilibriums were found
whenever the electron temperature exceeded a certain critical
value. This result is in qualitative agreement with an estimate-
presented by Parker, but it is in direct conflict with Lerche's
nonexistence proof. A crucial error in that proof is identified.
The properties of the equilibrium solutions are discussed. Finally,
the equilibrium. confinement with the incident streaming plasma
tangential to the magnetopause but perpendicular to the confined
field is treated. It is concluded that in the geomagnetic
equatorial plane equilibriums of Parker's type are possible on
the morning side but not on the afternoon side of the magnetosphere.
Magnetopause Structure during the Magnetic Storm of
September 24, 1961
Explorer-12 magnetic-field observations of the magnetopause
current layer were made on the inbound pass of September 24, 1961,
which occurred during the main phase of a moderately strong magnetic
storm.. The satellite was located near the subsolar point. The
accuracy of the Explorer-12 data reported here has been increased
by use of an improved filtering procedure. Five magnetopause
penetrations were observed in a time period of about 20 min and
for each of these,data were used to calculate the vector normal
to the current layer, the magnetic-field component along that
vector, and.the polarization of the current layer. Normal
magnetic-field components significantly different from zero were
obtained, and it is suggested that the observations during this
pass in general are consistent with the so-called open magnetosphere
model. In fact, it appears that the first and last of the boundary
penetrations may have occurred on opposite sides of, but very
near, the X-type magnetic null line, which is located on the front
side of the open magnetosphere model. The direction of the normal
magnetic-field component as well as the sense of rotation of the
tangential magnetic-field component in the current layer were
found to be opposite for these two boundaries.


Magnetopause structure and dynamics of the magnetosphere

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