Moreton waves and coronal waves
Science Nugget: February 8, 2002
The phenomenon of Moreton waves was first reported in 1960 from
the legendary
Lockheed Observatory. Y. Uchida in his
1968-1974 papers nicely explained Moreton waves as
the intersection of coronal shock waves (due to a flare)
with the chromosphere. But since then there had been surprisingly few
follow-up studies, until coronal waves were detected by
SOHO/EIT (hence called "EIT waves") and got lots of
attention especially in conjunction with CMEs. Interestingly,
many people came to identify EIT waves with the coronal waves that
are responsible for Moreton waves, without actually looking
at H
images
in which the latter waves are observed. Only recently, say after 2000,
did H
Moreton waves come to be a research topic again.
At the Yohkoh meeting, K. Shibata gave an impressive
talk to discuss two Moreton wave events, as observed by a
small telescope at
Hida observatory. The real
data analysis was undertaken by his students, of course.
His message was that (1) EIT waves are different entities from
Moreton waves, and that (2) X-ray waves as detected
by Yohkoh SXT, instead, are a coronal counterpart of
Moreton waves, therefore signifying fast mode MHD waves as
predicted by Uchida. These conclusions are based on the
speeds and locations of the three waves.
Moreton waves and EIT waves
The first event was observed on November 4, 1997, at the time of
an intense flare (X2.1 in the NOAA/GOES standard). The reference
for the analysis is S. Eto et al. (submitted to PASJ in January 2002).
This event was
associated with a halo CME, which is considered to be the origin
of a geomagnetic storm that occurred three days later. A movie of
the H
off-band (+0.8 Å) images is given
here.
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Unfortunately, the flare coincided with Yohkoh night, but
EIT waves were observed. The figure on the left (click to enlarge)
shows EIT waves, and compares the locations and speeds of EIT and
Moreton waves.
Here we note that EIT waves cover a wider angular span.
They were also observed further way from the flare site than
Moreton waves, due to the lack of high cadence. Plotting
the locations of the wave fronts (along four representative
lines OA-OD) with time, those for EIT waves do not come on
the extrapolations of Moreton waves as shown in the figure
on the right.
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One could say that this is due to possible slow down of Moreton waves
before they were observed by EIT. But this can be precluded by
the oscillations of a remote filament (see the encircled area in
the figure on the left), for which the authors think Moreton waves were
responsible. Including the time and location of the oscillating
filament, we see that Moreton waves did not decelerate. Therefore,
EIT waves are not a coronal counterpart of Moreton waves.
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Moreton waves and X-ray waves
The second event occurred a day before. The flare, in the same
active region, was much less intense (only C8.6), but was again
associated with an extended CME, although very diffuse.
This is one of the extremely rare cases where waves were observed in
H
and soft X-rays at the same time. The reference for this work
is N. Narukage et al., submitted to ApJ in February 2002.
Click here for
a movie of H
off-band (+0.8 Å) and SXT (saturated, quarter-resolution) images.
The fronts of Moreton and X-ray waves were traced and their distances from
the flare were plotted with time, as shown in the figure on the right.
Unlike EIT waves, X-ray waves are well correlated with Moreton waves.
Moreover, the observed speed of X-ray waves (630 km/s) is consistent
with the speed predicted from MHD shock theory. For a reasonable
range of parameters for temperature and magnetic field,
the inferred shock speed is 400-740 km/s, according to the authors.
Therefore, they concluded that X-ray waves signify MHD fast shocks
responsible for Moreton waves.
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Incidentally, these SXT science nuggets have discussed possible
X-ray waves on several occasions. See, for example,
(1998Dec25),
(1999Aug27),
(2000Jul28)
and
(2001Sep07).
Why are these works important?
In this age of space weather, people often talk about
"CME shocks" as a main agent for solar energetic particle
(SEP) events. Type II bursts in the dekametric and hectometric
wavelengths (observed by
Wind/WAVES, which still sends useful data, to my great relief)
are a familiar signature of CME shocks. They are
often called interplanetary type II bursts because the
frequencies correspond to densities characteristic of the
outer corona, say
2-10 R .
In contrast, metric type II bursts are
usually tied with flare shocks. Although Moreton waves may
intrinsically represent shocks from flares, the two examples dealt
with in this nugget are associated with both metric and hectometric
type II bursts. Are the shocks responsible for these type II
bursts driven by different mechanisms? Please look at the figures
on the right that combine spectra in the two wavelength regimes
(credit to the ISTP Workshop in April 1999), and make your own judgment as
to whether they are not connected.
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Observationally, it seems well established that EIT waves are
correlated with CMEs. But Uchida, whose model of Moreton waves
is still valid, proposed theory at the Yohkoh meeting on the
difference of Moreton and EIT waves in "bubble-type" CMEs that are
distinct from "loop-type" CMEs. We are still not sure how much
of CME launch/acceleration depends on flare explosions
not included in scenarios based on filament-eruption-induced processes.
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Another question may be if there is a characteristic magnetic topology
that results in Moreton/X-ray waves, since the number of events is
still very limited. Interestingly, the two events shown in this nugget
and other two X-ray wave events on
May 6 1998 and
November 3 1997 (but 4.5 hours after
the one included in this nugget) all involve a CME from trans-equatorial
loops. Clearly, studying more events is strongly encouraged.
An encouraging thing is that the Moreton wave events reported here
came from a tiny H
telescope not blessed
with good seeing. Therefore, adding an
H
telescope (including off-band
capability) in future missions
such as the Solar
Dynamics Observatory (SDO) may not be a bad idea.
February 8, 2002
N. Nitta (NVN)
(nitta@lmsal.com), with thanks to K. Shibata, S. Eto and N. Narukage.