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\lefthead{FREY ET AL.}
\righthead{MAGNETIC CLOUD AND AURORA}
\received{October 14,~1997}
\revised{December 24,~1997}
\accepted{January 23,~1998}
\paperid{}
\cpright{AGU}{1998}
%\ccc{0148-0227/97/97GL-00000\$05.00}

\authoraddr{J.~H.  Doolittle, Lockheed Martin Palo Alto Research Laboratory,
3251 Hanover St., Palo Alto, CA 94304-1191.}

\authoraddr{H.~U.  Frey, S.~B.  Mende, H.~B.  Vo, Space Sciences Laboratory,
University of California, Berkeley, CA 94720-7450 (e-mail:
hfrey@ssl.berkeley.edu).}

\authoraddr{T.~J. Rosenberg, A.~T. Weatherwax, Institute for Physical Science 
and Technology, 
University of Maryland, College Park, MD 20742-2431.}

\slugcomment{To appear in the {\it Geophysical Research  
Letters}, 1998.}

\setcounter{secnumdepth}{4}

\begin{document}
\title{The Aurora During the Passage of the May 27, 1996 Magnetic Cloud}
%\title{The Aurora During the May 27, 1996 Magnetic Cloud}
\author{H.~U. Frey, S.~B. Mende, H.~B. Vo} 
\affil{Space Sciences Laboratory, University of California, Berkeley}
\author{J.~H. Doolittle} 
\affil{Lockheed Martin Palo Alto Research Laboratory}
\author{A.~T. Weatherwax, T.~J. Rosenberg} 
\affil{University of Maryland, College Park}

\begin{abstract} On May 27, 1996 two all-sky cameras in Antarctica observed the
aurora during the passage of a magnetic cloud.  Compression of the magnetosphere
by the solar wind dynamic pressure pulse in front of the magnetic cloud caused a
short, strong, and localized blue auroral emission which moved towards noon in
the late morning sector.  The emission was caused by electrons of 8 keV energy.
The high dynamic pressure caused a diffuse large-scale (at least 3 h MLT and
5$^\circ$ latitude) blue emission on closed field lines.  The high energy
electron precipitation disappeared after the enhanced solar wind dynamic
pressure region had passed the sub-solar magnetopause.  \end{abstract}

\begin{article}
\section{Introduction}\label{sec:intro}

Magnetic clouds are ejections of solar plasma characterized by strong magnetic
fields, a smooth rotation of the magnetic field direction, a low plasma $\beta$
and a low plasma temperature [{\it Burlaga,} 1988].  Magnetospheric phenomena
can be studied under stable interplanetary conditions because the duration of
the cloud passage (20-30 hours) is long when compared to the relaxation time of
the magnetosphere ($\approx$ 30 minutes).  The interaction of a significant and
persistent southward interplanetary magnetic field (IMF) with the magnetosphere
generally triggers geomagnetic activity and may cause geomagnetic storms.  It
was shown that most of the large geomagnetic storms were caused by either
compound streams or/and magnetic clouds [{\it Burlaga et al.,} 1987; {\it
Lepping et al.,} 1991].  Periods of high and suddenly changing solar wind
dynamic pressure, southward, and northward IMF are characterized by the
occurrence of dayside auroral phenomena like the dayside auroral breakup and
flux transfer events [{\it Sandholt et al.,} 1990; {\it Lockwood et al.,} 1990].

One recent magnetic cloud passed the Earth between October 18 and 20, 1995 and
another between May 27 and May 29, 1996.  The October 1995 cloud got much
attention due to the strong IMF and geomagnetic activity [{\it Lepping et al.,}
1997].  Unusual auroral emission was described as the rare type A global red
aurora [{\it Sivjee and Shen,} 1997].  However, optical observations could only
be performed during 10 hours of southward IMF.

In May 1996 the Antarctic Automatic Geophysical Observatories (AGO) and South
Pole station were in darkness during the whole magnetic cloud passage.  This
paper presents optical data taken with all-sky cameras at South Pole
($18.7^\circ$E magnetic longitude and $73.9^\circ$S latitude) and P3
($40.2^\circ$E and $71.8^\circ$S).  The cameras operate in 1 min time 
resolution.

\section{Observations}

On May 27, 1996, a shock wave traveling at 425 km/s hit the WIND spacecraft at
1210 UT followed by a magnetic cloud which arrived at the spacecraft at 1442 UT.
Within the shock the solar wind dynamic pressure increased rapidly to 4 nPa
(Figure 1).  Behind it, the IMF $B_z$ component jumped from $\approx$0 nT to -8
nT.  During the following 33 hours, the magnetic field remained relatively
constant at 8-10 nT but the direction rotated steadily from southward to
northward.

The shock and the cloud passed WIND at 150 $R_E$ upstream and passed IMP8 at
36.5 $R_E$ 28.5 min later (not shown).  Extrapolating to the bow shock and
taking into account the magnetosheath velocity and the time it takes for the IMF
information to travel to the ionosphere [{\it Jacobsen et al.,} 1995],
ionospheric responses would be expected after 1250 and 1522 UT.

On May 27, 1996, the all sky camera at South Pole observed the usual diurnal
movement of the red 630.0 nm aurora during negative to zero IMF $B_z$ conditions
(Figure 2).  The movement is due to the rotation of the Earth under the
asymmetric auroral oval producing an apparent poleward motion to -76 to
-80$^\circ$ magnetic latitude prior to local magnetic noon (1530 UT) and
equatorward motion to -70 to -72$^\circ$ magnetic latitude around 1700 UT (1400
MLT).

At 1016 UT, the intensity of the red and blue emission suddenly increased due to
a substorm at magnetic midnight (Figure 3) identified from the ISTP key
parameters of POLAR and magnetometer recordings in Canada and Scandinavia (not
shown).  During the following decrease of the blue and red emissions, the
pressure pulse hit the magnetosphere and caused a short (5 min) increase of the
red at 1250 UT and a long lasting (140 min) diffuse blue emission.  Figure 3
shows the mean intensity of the emissions in the central 90$^\circ$ field of
view of the all sky camera.  A similar behavior could be determined with the P3
all sky images, however due to a malfunction of the camera only 40 images were
recorded between 1000 and 2000 UT (not shown).  Therefore, the diffuse blue
emission appeared in an area of at least 3 hours MLT and 5$^\circ$ magnetic
latitude.

A 110 km long and 20 km wide region of strong blue emission
suddenly appeared in the 1250 UT image and moved towards noon at 5 km/s (Figure
4 A-C).  It reached the edge of the field of view after one minute and was not
visible in the next image.  This motion produced a trail in the red
emission from the original position at zenith to the position at 1251 UT and
this is evidence that the blue emissions in both images (1 min apart) are
correlated to and caused by the motion of a single feature.  At 1255 when there
were simultaneous images from South Pole and P3 (Figure 4 D) the feature had
already left the field of view of both stations.

Another short lived region of blue emission appeared at 1628 UT.  It may be
correlated to the short pressure and $B_z$ increase observed by WIND at 1538 UT.
It disappeared after 1630 UT, but this time it moved at only 250 m/s sunward.

\section{Discussion}

The structure of the magnetic cloud very much resembles that from October 18-20,
1995 [{\it Lepping et al.,} 1997].  Both were south-north turning clouds  
preceded by a solar wind pressure enhancement.  However, the magnetic field
strength in October 1995 was much higher (20 nT) as well as the pressure in the
front shock (8-20 nPa).  The October 1995 cloud caused a major magnetic storm
and the May 1996 cloud did not.  The maximum negative excursion of the Dst index
was -127 nT in October, while in May 1996 it was only -31 nT.

When a pressure pulse hits the magnetosphere, an inward motion is expected at
the sub-solar point [{\it Roelof and Sibeck,} 1993].  South Pole was at 0920 MLT
and the localized strong blue emission must be related to the compression of the
magnetosphere and the energization of plasma.  Except at 1628 UT no other such
structure was observed during this and the two days before and after.  The
brightness within the short-lived bright spot was 0.2 kR and 3.0 kR in the red
and blue, respectively.  An electron energy of 8 keV can be estimated [{\it Rees 
and Luckey,} 1974].  Simultaneous imaging riometer observations
from South Pole (not shown) confirm the motion of an absorption structure from
west to east above the observation site.  The riometer absorption up to 0.7 db
is consistent with precipitation of $\approx$ 8 keV electrons.

Some transient structures in the dayside aurora were described as midday auroral
breakup [{\it Sandholt et al.,} 1990] and our short lived blue emission is
somewhat similar in spatial scale, appearance of red and blue emission, and
longitudinal motion at 5 km/s.  However, the keogram data confirm that the main
red aurora was poleward of South Pole and the blue spot overhead was equatorward
of the dayside soft aurora, most probably on closed field lines.  Furthermore,
this structure did not move poleward across the cusp/cleft aurora, no periodic
occurrence was observed, and the average precipitation energy was much higher
than 0.3-2 keV.  The ground signature of a dynamic pressure pulse is an
anti-sunward moving convection vortex associated with north/south magnetic field
perturbations and increased particle precipitation [{\it Sibeck,} 1990].
However, the magnetic recording from South Pole (Figure 5) does not show the
typical bipolar magnetic field variation.  The motion of the bright blue spot
towards noon is also inconsistent with this interpretation.  The sunward
convection direction is consistent with the local closed field line
configuration.  But the motion at 5 km/s can not be related to the motion of
plasma in the ionosphere because these motions generally are an order of
magnitude slower.

Until 1500 UT the red intensity decreased to less than 50 R and the mean
intensity of the blue remained quite stable around 0.6 kR.  Thus, the average
electron energy was about 8-10 keV.  This long period of enhanced hard
precipitation is caused by the high solar wind dynamic pressure.  The continuous
compression of the magnetosphere creates an increased dayside magnetic field
strength and this changed magnetic topology leads to increased pitch angle
scattering and created the enhanced precipitation in the dayside region from 900
to 1300 MLT.

The disappearance of the blue emission at 1514 UT (Figure 3) can be explained by
the expansion of the magnetosphere after the solar wind dynamic pressure
decreased and the IMF B$_z$ component jumped to -8 nT.  The lack of enhanced
precipitation after that time shows that compression by the dynamic pressure is
more likely to cause precipitation than the erosion caused by the southward IMF.
The ratio of the red and blue emissions allows for an estimate of an average
electron energy of less than 1 keV after 1530 UT.  This is
consistent with the 500 eV $\pm$ 100 eV reported for the October 18, 1995
magnetic cloud during the southward IMF period [{\it Sivjee and Shen,} 1997].
However, observations in only two wave length bands do not allow for an
identification if the aurora of May 27, 1996 was type A global red aurora as
well.

The apparent motion of the red aurora from poleward to equatorward between 1600 
and 1800 UT on May 27 is a common feature during southward IMF
conditions.  On May 27 the IMF $B_z$ was - 8 nT and $B \approx 8-9$ nT and the
motion from -78$^\circ$ to -72$^\circ$ magnetic latitude took only 1.5 h.  The
velocity of this motion, however, was considerably larger than the velocity on
other days e.g.  May 16, 22, and 25, when it always took about 2.5 h with an IMF 
$B_z$ of about - 4 nT and $B \approx 5$ nT.

\section{Conclusions}\label{sec:con}

This paper presents optical and riometer observations of an increased,
long-lasting, diffuse blue auroral emission in the pre-noon and post-noon sector
during the 140 min compression of the magnetosphere by an enhanced solar wind
dynamic pressure in front of the magnetic cloud.  This large area (at least 3 h
MLT and 5$^\circ$ latitude) high energy precipitation (8-10 keV) is an important
energy supply into the dayside ionosphere.  Earlier investigations [{\it
Sibeck,} 1990 and references therein] concentrated on transient and sudden
events.  The disappearance of the diffuse emission exactly matches the time when
the dynamic pressure decreased again.  The $B_z$ negative phase of the cloud
event did not cause such enhanced energetic precipitation.

The pressure pulse caused a localized enhanced blue auroral emission.  A
connection to local reconnection at the magnetopause is unlikely because the
feature was located in the area of closed field lines and because the IMF $B_z$
component was close to 0 nT.  However, this transient phenomenon does not show
the main characteristics of dayside auroral breakup and traveling convection
vortices.  

\acknowledgements The U.S.  Automatic Geophysical Observatory program in
Antarctica is supported by National Science Foundation grant OPP9529177 to the
University of Maryland.  The South Pole riometer and magnetometer data were
acquired with the support of NSF grant OPP9505823 to the University of Maryland.
We thank J.  Etter for assistance with the riometer data processing and display
and L.  J.  Lanzerotti for providing us with the South Pole ground magnetometer
recordings.  The authors are indebted to R.~P.  Lepping for the permission to
use the MFI data and to R.~P.  Lin for the 3DP WIND data.

\newpage
\begin{thebibliography}{}
\bibitem[{\it Burlaga}]{bur1}
\reference
Burlaga, L.~F., Magnetic clouds and force-free fields with constant alpha, {\it 
J. Geophys. Res., 93,} 7217--7224, 1988.

\bibitem[{\it Burlaga}]{burb}
\reference
Burlaga, L.~F., K.~W. Behannon and L.~W. Klein, Compound streams, magnetic 
clouds, and major geomagnetic storms, {\it J. Geophys. Res., 92,} 5725--5734, 
1987.

\bibitem[{\it Jaco}]{jaco}
\reference
Jacobsen, B., P.~E. Sandholt, W.~J. Burke, W.~F. Denig, and N.~C. Maynard, 
Optical signatures of prenoon auroral precipitation: Sources and responses to 
solar wind variations, {\it J. Geophys. Res., 100,} 8003--8012, 1995.

\bibitem[{\it Lepping}]{lepp}
\reference
Lepping, R.~P., L.~F. Burlaga, B.~T. Tsurutani, K.~W. Ogilvie, A.~J. Lazarus, 
D.~S. Evans, and L.~W. Klein, The interaction of a very large interplanetary 
magnetic cloud with the magnetosphere and with cosmic rays, {\it J. Geophys. 
Res., 96,} 9425--9438, 1991.

\bibitem[{\it Lepping}]{lep97}
\reference
Lepping, R.~P. et al., The Wind magnetic cloud and events of October 18-20, 
1995: Interplanetary properties and as triggers for geomagnetic activity, {\it 
J. Geophys. Res., 102,} 14049--14063, 1997.

\bibitem[{\it Lockwood}]{lock90}
\reference
Lockwood, M., S.~W.~H. Cowley, P.~E. Sandholt, and R.~P. Lepping, The 
ionospheric 
signatures of flux transfer events and solar wind dynamic pressure changes, 
{\it J. Geophys. Res., 95,} 17113--17135, 1990.

\bibitem[{\it Rees}]{rees}
\reference
Rees, M.~H., and D. Luckey, Auroral electron energy derived from ratio of
spectroscopic emissions 1. Model computations, {\it J. Geophys. Res., 79,} 
5181--5186, 1974.

\bibitem[{\it Roelof}]{roel}
\reference
Roelof, E.~C. and D.~G. Sibeck, Magnetopause shape as a bivariate function of 
interplanetary magnetic field B$_z$ and solar wind dynamic pressure, {\it J. 
Geophys. Res., 98,} 21421--21450, 1993.

\bibitem[{\it Sandholt}]{san90}
\reference
Sandholt, P.~E., M. Lockwood, T. Oguti, S.~W.~H. Cowley, K.~S.~C. Freeman, 
B. Lybekk, A. Egeland, and D.~M. Willis, Midday auroral breakup events and 
related energy and momentum transfer from the magnetosheath, {\it J. Geophys. 
Res., 95,} 1039--1060, 1990.

\bibitem[{\it Sibeck}]{siba}
\reference
Sibeck, D.~G., A model for the transient magnetospheric response to sudden solar 
wind dynamic pressure variations, {\it J. Geophys. Res., 95,} 3755--3771, 1990.

\bibitem[{\it Sivjee}]{sivj}
\reference
Sivjee, G.~G. and D. Shen, Auroral optical emissions during the solar magnetic 
cloud event of October 1995, {\it J. Geophys. Res., 102,} 7431--7437, 1997.

\end{thebibliography}{}


\end{article}

\begin{figure} 
\caption{Solar wind B$_z$ and dynamic pressure measured by WIND on May 27, 
1996.} \end{figure}

\begin{figure} \caption{Keogram of South Pole in 427.8 nm
(upper part) and 630.0 nm.  Horizontal stripes are data gaps.}  \end{figure}

\begin{figure} \caption{Mean intensities in the South Pole all-sky camera field
of view on May 27, 1996.}  \end{figure}

\begin{figure} \caption{Geographic projections of the all sky images conforming
with viewing the aurora from above in the northern hemisphere or looking from
the north pole through the Earth to the southern hemisphere.  The sun is towards
the top, dawn is to the right, and the direction to the magnetic pole is marked
by the solid line.  A) South Pole 1250 UT, 630.0 nm; B) South Pole 1250 UT,
427.8 nm; C) South Pole 1251 UT, 630.0 nm; D) South Pole and P3 in geomagnetic
presentation 1255 UT, 630.0 nm; the average auroral oval is given.}
\end{figure}

\begin{figure} \caption{Magnetometer recording at South Pole.}  \end{figure}

\clearpage

\end{document}
