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TitlePhysical Signatures of Magnetospheric Boundary Layer Processes
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Physical Signatures of
Magnetospheric Boundary Layer Processes

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NATO ASI Series
Advanced Science Institutes Series

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and Physical Sciences Dordrecht, Boston and London

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1 Global Environmental Change

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Series C: Mathematical and Physical Sciences - Vol. 425

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Figure 6. MSP grayscale intensity plot for the 23 December 1992, 0900 UT - 1000
UT.

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Figure 7. ASTV images for the 23 December 1992 in 1 minute intervals.

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A number of observations of PMAFs that rebrighten have been shown. These
examples show a number of cases where the rebrightenings occur at ::::: 2 minute in-
tervals. The PMAFs can also move poleward, slow down and stop while maintaining
their luminosity for some time or rebrighten. An example (event c on the 16 Jan-
uary 1985) was shown where it was necessary to use both the MSP and ASTV data
to correctly interpret the motion of the PMAF. This event also separated into two
parts during its poleward drift. During times in which a PMAF rebrightened another
PMAF was observed moving away from the dayside auroral oval. From these obser-
vations the PMAFs can be classified into three categories: (1) PMAF1: move into
the polar cap and fade from view [no examples shown of this type, refer to Sandholt
et al., 1986; Lockwood, 1991], (2) PMAF2: rebrighten as they move into the polar
cap, (3) PMAF3: same as PMAF2, but slow down and stop while maintaining their
luminosity for some time or rebrighten.

The next section presents some first results from a statistical study of the exami-
nation the PMAFs.

3. Statistical Analysis

A statistical study of twelve years, 1981-1992, of the dayside aurora has been under-
taken. This study is based on ground-based optical data obtained from Longyearbyen,
Svalbard (74.9 N, 114.6 E, geomagnetic coordinates). Both the ASTV and MSP were
used to obtain the optical data used for this study. The data was collected during the
months of January and December and also the last two weeks in November. During
this time period the sun is far enough below the horizon so that the aurora can be
seen during the daytime. The time of interest was between 0630 UT and 1030 UT,
with local magnetic noon falling around 0830 UT. In between this time interval we
should be able to observe aurora generated by the interaction of the solar wind and
the earths magnetosphere occurring around the subsolar point.

First, twelve years of ASTV data were viewed in order to determine the clear days
with auroral activity. A total of 476 days were analyzed from 1981-1992. Next the
MSP was stripped off for the clear days between 0630 UT and 1030 UT for the
channels which contained the green [01] 557.7 nm and red [01] 630.0 nm emissions.
The data was then displayed as MSP intensity grayscale plots. During the intervals
where PMAFs were located, smaller one hour time segments were taken off and then
plotted in the above manner.

For each identified PMAF the beginning and end times were recorded off of the one
hour MSP grayscale intensity plots. A total of 476 PMAFs were identified, including
PMAF1, PMAF2, and PMAF3 events. Only the clear distinct PMAFs, as displayed
in Figure 1, were included in this survey.

Two graphs were obtained from this data set. The first graph, Figure 8, shows
the time distribution of the PMAFs. The beginning times (in UT) for each event
were plotted against the number of occurrences in fifteen minute bins. The PMAFs
appear to be evenly distributed around local noon, ::::: 0830 UT. A small dip occurs
between 0745 UT and 0830 UT, but the statistics are not good enough to say this is
significant.

The second graph shown in Figure 9, displays the characteristic life span of the
PMAFs. The beginning time was subtracted from the end time for each individual
event and plotted against the number of occurrences in one minute bins. The average

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Several comments were provided in answer to these questions. Regarding nomenclature, the
discovery of increasingly smaller-scale phenomena has led to the subdivision of larger-scale
regions, and the mapping of magnetospheric regions to the ionosphere has resulted in some
disagreement and confusion over names and definitions. For example, the cusp itself may have
different meanings if identified with magnetic fields or with particle or optical emissions. There
appears to be some agreement that the ionospheric projection of the LLBL is the "cleft" as depicted
in Figure 1. The ionospheric projection of the cusp is imbedded in the larger cleft (see Fig. 1), and
its position is intimately related to the IMF By component. The ionospheric projection of the plasma
sheet completes a continuous ring with the LLBL projection, which encircles the magnetic pole.
This is shown in Figure 1 as the projection of only the PSBL which, taken alone, is too thin to provide
a sufficiently wide (3°_5° latitude) auroral ring. There is also some question about the local time
extent of the LLBL projection (and where the plasma sheet projection begins). Evidence provided
at this workshop supports the view that the LLBL projection, the cleft, extends from dawn (0600
MLT) through noon to dusk (1800 MLT). The region in the ionosphere poleward of the cleft and
imbedded cusp is connected by geomagnetic field lines to the mantle.

The structure and physics of the boundary layers and the relationship of LLBL, cusp, and mantle
were thoroughly discussed and debated. The physics of these regions and their ultimate relationship
to the solar wind is not well understood. Part ofthe difficulty (and lack of agreement) is in describing
the penetration of solar wind plasma into the magnetosphere and down to the ionosphere. The
concept of a closed magnetosphere, in which no solar wind plasma enters, was not defended in this
workshop except possibly in the form of viscosity in MHD formulations. There was general
agreement that reconnect ion or merging plays an important role in plasma entry, polar convection

........
.... : ': ':'::',
PLASMA
MANTLE

I TERIOR
CUSP (Ell

.:-.:-=-=.-.: ---------= - - ---
LOW-LATITUDE
BOUNDARY

LAYER I
~------------'vr------------~

MAGNETOPAUSE BOU DARY LAYERS

PLASMA
BOUNDARY

LAYER

RI G CURRE T
PLASMA SHEET

Fig. 1. Schematic diagram of various observed magnetospheric boundary layers (left) and their
mapping to the ionosphere (right) from Vasyliunas [1979, his Figure 1]. The interior cusp here is
also referred to as the entry layer (EL).

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patterns, and field-aligned current generation, and some general recognition that reconnection or
merging can occur over a wide range of time and spatial scales from steady-state to transient and
globally to patchy. Considerable debate existed on the exact nature of these processes and their
projection to the ionosphere.

There was a consensus that transient events, defined in terms of magnetic field, particle, and
optical signatures, are a fundamental aspect of dayside phenomenology. The sources of these
phenomena include intermittent reconnection (FfEs), MHD waves related to solar wind perturba-
tions, and a combination of both. There is promise to use ground-based measurements of red and
green emissions to sort out steady from patchy or bursty merging and to identify waves associated
with pressure pulses. Some question remains as to how to relate the microphysics visible from
ground-based observations to the global understanding available from satellite images.

This brings up the issue of mapping, which is a critical problem in space physics. The coupling
of the magnetospllere, ionosphere, and thermosphere will not be complete until reliable techniques
are developed that can be used to connect these distant regions. A dependable model of the
geomagnetic field is a good start, and several such models were discussed in the workshop.
Statistical patterns of precipitating particles and their relationship to patterns of polar convection and
Birkeland currents have also been used to make major advances in mapping, and these are contained
in several papers in this volume. The importance of "time of flight" effects on mapping was brought
forward at the workshop and must be evaluated.

Our understanding of the dayside magnetosphere has been significantly advanced since the last
NATO workshop, but important questions still remain, which is appropriate for a system this large
and complicated. These questions include fundamental aspects of the boundary layers, like how
thick they are and how they vary in time. We face obstacles as well, such as inadequate physics
presently available to describe magnetic merging-MHD formulations cannot do it and the mapping
of phenomena from magnetosphere to ionosphere is still unresolved. The expectation is high that
some of these issues will be settled in the future by the perseverance of the space science community
and the continued support of govemment agencies.

References

Heikkila, W. and Winningham, J. D.,J. Geophys. Res., 76,883,1971.
Vasyliunas, V. M., Proc. Magnetospheric Boundary Layers Conf., ESA Report SP-148, June 1979.
Sandholt, P. E. and Egeland, A. (Eds.), Electromagnetic Coupling in the Polar Clefts and Caps,

Vol. 278, NATO Advanced Science Institute Series, Kluwer Academic Publishers, Dordrecht,
1989.

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