Photographs of global-scale auroral forms taken by scanning radiometers onboard U.S. Air Force weather satellites in 1972 show that auroral bands exhibit well-organized wave motion with typical zonal wave number of 5 or so. The scale size of these waves is in agreement with that of well-organized neutral wind fields measured by the 1967-50B satellite in the 150- to 220-km region during the geomagnetic storm of May 27, 1967. The horizontal scale size revealed by these observations is in agreement with that of high-altitude traveling ionospheric disturbances. It is conjectured that the geomagnetic storm is a source of planetary and synoptic scale neutral atmospheric waves in the middle atmosphere. The possible existence of a source of waves of the proper scale size to trigger instabilities in middle atmospheric circulation systems may be significant in the study of lower atmospheric response to geomagnetic activity

Chiu, Y. T.

Straus, J. M.

English

NASA Technical Reports Server

14538
The Aurora as a Source of Planetary-Scale
Waves in the Middle Atmosphere
Y. T. Cmu AND J. M. STRAUS
The Aerospace Corp.
Photographs of global-scale auroral forms taken by scanning radiometers onboard U.S.
Air Force weather satellites in 1972 show that auroral bands exhibit well-organized wave
motion with typical zonal wave number of 5 or so. The scale size of these waves is in agree-,
ment with that of well-organized neutral wind fields measured by the 1967-50B satellite in
the 150- to 220-km region during the geomagnetic storm of May 27, 1967. Further, the
horizontal scale size revealed by these observations is in agreement with that of high-altitude
traveling ionospheric disturbances.. It is conjectured that the geomagnetic storm is a source
of planetary'and synoptic scale neutral atmospheric waves in the middle atmosphere. Although
there is, at present, no observation of substorm-related waves of this scale size at meso-
spheric and stratospheric altitudes, the possible existence of a new source of waves of the
proper scale size to trigger instabilities in middle atmospheric circulation systems may be
significant in the study of lower atmospheric response to geomagnetic activity. • " •
The dynamics of the upper stratosphere, and
perhaps the lower thermosphere as well, have
been shown to be strongly affected by the inter-
action of mean zonal winds with planetary Rossby
waves (Charney and Drazin, 1961; Dickinson,
1968; Finger et al., 1966; Matsuno, 1970; Newell
and Dickinson, 1967). Clearly, a source of Rossby
waves in the stratosphere would be associated with
large-scale tropospheric weather systems. 'How-
ever, if a second source of such planetary or
synoptic scale waves were to exist, then it would
be of considerable interest to workers concerned
with upper atmospheric dynamics. In particular,
if such a second source of neutral atmospheric
waves were related to geomagnetic activity, and
if such waves were of the proper dimensions to
interact with the upper atmospheric circulation,
then they may act as the initiating perturbations
to trigger latent aerodynamic instabilities in the
upper atmosphere.
In this paper, we would like to suggest, by
invoking recent satellite observations of planetary-
scale variations of auroral forms (Morse et al.,
1973), as well as direct satellite observations of
polar upper atmospheric winds during magnetic
storms (Feess, 1968; Chiu, 1972), that auroral
substorms may be a source of planetary waves in
the 100-km altitude region. It is understood that
numerous observations of ionospheric and atmos-
pheric disturbances associated with geomagnetic
activity have been reported from time to time;
however, upon examination, most of these are
either in the high-altitude regions (—350 km) or
of such local nature that the lateral extent of the
disturbance cannot be ascertained.
Traveling ionospheric disturbances occurring in
the 200- to 800-km altitude region have been
observed for many years (Davis and daRosa,
1969; Thome, 1968). Well-correlated ionospheric
disturbances of ~ 2000-km horizontal scale and
of ~1- to 2-hr periods have been observed to
propagate from the auroral zone at speeds of
~500 m/s. These disturbances have been inter-
preted generically as due to the passage of gravity
waves. Because the horizontal scale and wave
speed are so large, being reminiscent of longwaves
in the ocean, at least two intriguing questions
must be raised. First, because auroras occur at
83
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84 RELATIONSHIPS BETWEEN SOLAR ACTIVITY AND METEOROLOGICAL PHENOMENA
the 100-km level, it would be of interest to ask if
these high-altitude ionospheric disturbances may
be related to variations of the low-altitude auro-
ras and associated neutral disturbances. Second,
if such large scale disturbances were indeed neu-
tral waves, then it would be of interest to .investi-
gate the effects of sphericity and the latitudinal
variation of the Coriolis force on their propaga-
tion. These questions will be considered in some
detail here in order that the peculiar properties of
these waves in the auroral region .may be - ex-
ploited for observational purposes. In this respect, ,
it is perhaps relevant to note that, whereas merid-
ional propagation of ionospheric disturbances has
been studied thoroughly in the midlatitude region,
observations of the horizontal scale and propa-
gation of such disturbances in the auroral region
do not seem to be available. • .
Figure 1 shows an extensive auroral form
detected by scanning radiometers on board a
U.S. Air Force weather satellite (Morse et al.,
1973). The most important feature revealed by
this unique observation of planetary-scale auroral
forms is that the aurora shows coherent spatial
variations typical of a wave with zonal wave num-
ber of 3 to 6. Because auroral substorms show
typical temporal variations of, say, 1 to 2 hr, these
observations suggest clearly that auroral sub-
storms, as a source of atmospheric heating in the
vicinity of 100 km, must be rich in Fourier com-
ponents of these zonal wave numbers and wave
periods. Indeed, there is theoretical reason to
believe that such spatial and temporal variations
of the aurora are related to waves in the auroral
current (Hasegawa, 1970). Given the existence of
such wavelike variations of auroral heating, it is
reasonable to consider meridional and vertical
propagation of such planetary waves, to lower
latitudes and to higher altitudes, in the interpre-
tation of traveling ionospheric disturbances.
Despite the observation of clearly wavelike
variation of planetary scale auroral heating, direct
observations of the neutral wind field .associated
with such wave motion would be desirable to
substantiate the suggested relation between the
characteristics of auroral forms and traveling iono-
spheric disturbances. In short, are there in situ
satellite observations of upper atmospheric wind
fields in the auroral region directly related to
specific geomagnetic storms? In this regard, we
wish to point out that the .pattern of crosstrack
wind components, deduced from accelerometer
and attitude control activity .onboard the 1967-
5 OB satellite at altitudes between 150 and 220 km
before and after the onset of a very large geo-
magnetic stormy on May ,27, '1967, is of particu-
lar interest (Feess, 1968'). Figure 2 shows data
from selected orbits in which well-organized cross-
track wind variations were encountered. Although
the major stationary structure near the pole may
inyolve,convective overturning of the atmosphere
(Ghiir, 1972), the .coherent wind variations of
smaller magnitude, which, change from orbit to
orbit, are likely to,.be propagating waves of
~ 2000-km horizontal scale. These structures are
particularly evident at or near satellite orbits 51
and 53. r . , .
The next question then, is,how the stratosphere
responds" to 'the same magnetic storm. In this
regard, it is a fortunate coincidence that detailed
radiosonde data exist for Berlin during the same
period (Scherhag, 1967). Figure 3 shows Scher-
hag's data for the period March to June 1967.
The top three curves show the stratospheric
temperature at 30, 35, and 37 km. The bottom
curve shows the thickness between the 5- and
10-millibar levels in decameters. Scherhag noted
that,all four curves show a rapid rise to a peak
during the .'period May 25 to 26, 1967. This
becomes somewhat more evident if we take the
sum of all four curves so that the random signal
•FIGURE 1.—Ah extensive auroral
form observed by scanning ra-
diometers onboard a U.S. Air
Force weather satellite near the
north auroral zone at 13.51 GMT
on August 1, 1972. The origin
of the grid on the photograph is
the 'north geographic pole. It is
seen that, aside from small scale
variations of < 100-km horizon-
tal scale, the auroral form ex-
hibits planetary-scale variations
with zonal wave number of
'approximately 3 to 6. The co-
herent extensiveness of the asso-
ciated auroral heating is particu-
larly significant. (Courtesy of
E. H. Rogers and D. F. Nelson,
The Aerospace Corp.)
THE AURORA AS A SOURCE OF PLANETARY-SCALE WAVES 85
140°W
70° N-
80° N
80° N
70° N
50° E
ORIGINAL PAGE IS
OF POOR QUALITY
180° E
90° E
86 RELATIONSHIPS BETWEEN SOLAR ACTIVITY AND METEOROLOGICAL PHENOMENA
53
SCALE:
270°
FIGURE 2.—Lower thermospheric winds deduced from
accelerometer and attitude control activity onboard
the satellite 1967-50B on May 27, 1967, near the
north geographic pole, the origin of the figure. The
satellite paths are labeled by the orbit numbers (49-
61), and the dashed curve indicates the locus of points
for which the satellite altitude is 150 km. The polar
plot shows the measurements for the northern hemi-
sphere. The magnetic storm onset was at the 50th
orbit. It is seen that well-organized wind components
with a horizontal scale of ~ 2000 km seem to be
associated with an extremely disturbed but stationary
structure at the pole. These features are particularly
well illustrated on orbits 51, 53, and 59. It should be
noted that both features are coherent and planetary in
scale. (After Feess, 1968; for summary see also Chiu,
1972.)
is reduced. Indeed, the sum shows three clear
events (April 24, May 3, and May 26) which
interestingly occurred during the most magneti-
cally disturbed days of the period (SKP = 32, 47,
and 51, respectively; Kp is the geomagnetic activity
index).
In conclusion, there seems to be some in situ
evidence that the auroral substorm is a source of
planetary waves in the 100-km region neutral
atmosphere. These neutral wind disturbances may
have caused some stratospheric response although
data from a wider area would be required to con-
firm it. In any event, we emphasize that detailed
testing of any theoretical mechanism reduces, in
the final analysis, to an in situ layer by layer cor-
relation study of the responses from thermospheric
levels to the stratospheric levels.
1967
FIGURE 3.—Stratospheric temperatures at 30, 35, and 37
km and the 5- to 10-millibar thickness for the period
March to June 1967 (Scherhag, 1967).
ACKNOWLEDGMENT
This work was conducted under U.S. Air Force
Space and Missile Systems Organization (SAMSO)
contract no. F04701-73-C-0074.
REFERENCES
Charney, J. G., and P. G. Drazin, 1961, "Propagation of
Planetary Scale Disturbances From the Lower Into the
Upper Atmosphere," J. Geophys. Res., 66, pp. 83-109.
Chiu, Y. T., 1972, "Thermospheric Convective Instabil-
ity," Space Res. XII, Akademie-Verlag, Berlin, pp.
1025-1028.
Davis, M. J., and A. V. daRosa, 1969, "Traveling Iono-
spheric Disturbances in The Auroral Oval During
Polar Substorms," J. Geophys. Res., 74, pp. 5721-5735.
Dickinson, R. E., 1968, "Planetary Rossby Waves Propa-
gating Vertically Through Weak Westerly Wind Wave-
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Feess, W. A., 1968, "LOGACS Wind Analysis," TOR-
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Calif. [Also paper presented at the COSPAR 14th
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Finger, F. G., H. M. Woolf, and C. E. Anderson, 1966,
THE AURORA AS A SOURCE OF PLANETARY-SCALE WAVES 87
"Synoptic Analyses of the 5, 2 and 0.4 Millibar Sur-
faces for the IQSY Period," Man. Weather Rev., 94,
pp. 651-661.
Hasegawa, A., 1970, "Theory of Aurora Bands," Phys.
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Matsuno, T., 1970, "Vertical Propagation of Stationary
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Morse, F. A., D. F. Nelson, E. H. Rogers, and R. C.
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Newell, R. E., and R. E. Dickinson, 1967, "On the Ap-
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Scherhag, R., 1967, "Stratospheric Warming and Polar
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DISCUSSION
AKASOFU: I do not think you can associate that
type of picture of the aurora with Rossby waves because
in a matter of 10 min, the pattern of the aurora might
change drastically. I understand that Rossby waves are a
much more stable phenomenon. These are very high-
latitude phenomena at geographic latitudes above ap-
proximately 70; I am sure that Rossby waves are at
something like latitude 50.
CfflU: I agree that the phenomenon is not a Rossby
wave. The point, however, is that the auroral heating
would have a spatial structure of 2000 km, even though
it changes in a few minutes. If you consider the aurora,
or the particle deposition associated with it, as a heating
source that produces waves, then it would be rich in the
Fourier components in spatial structure of 2000 km. I
am not trying to associate Rossby waves with the auro-
ral waves.


The aurora as a source of planetary-scale waves in the middle atmosphere

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