Weekly Notes from the Yohkoh Soft X-Ray Telescope
(Week 24, 2002)
Science Nugget: June 14, 2002
Properties of big flares in this cycle
Introduction
In
a previous nugget (2001 August 17), we reported on
the Yohkoh observations of X-class flares.
Since then an additional 15 X-class flares have occurred (two in 2002),
and in the meantime Yohkoh operations were closed
in December 2001. Therefore, it is time to
give an update to the previous nugget. Unfortunately, the number
of the flares observed by Yohkoh in the early phase has increased
by only three (to 26), as compared with the total number of X-class flares
being 58 now. But we have at least some information on pre- and post-flare
morphology (from SXT full-disk images) for most of the X-class flares.
Durations
As already noted in the previous nugget, not many of these X-class flares
are of long duration, often referred to as LDEs (long decay events or
long duration events). This may be seen in the following plots, which show
the distribution of decay times in a 30 min bin and the correlation between
the rise and decay times. These times refer to the GOES 1-8 Å flux,
and depend, of course, on how to define the start and end times. Here
we take the start time from the NOAA event list, but define the end time
as the time when the GOES flux drops to 5% (as opposed to 50% as used at NOAA)
of the peak. There are only five LDEs with decay time longer than 200
minutes, but in the following
discussion let us include those with rise time longer than 20 minutes in
the LDE category.
Overall morphology
As we see below, most of these X-class flares are associated with CMEs.
But the two soft X-ray morphologies often connected to CMEs seem to avoid
X-class flares. The first is the sigmoid (see
1,
2,
3).
The definition of a sigmoid is less straightforward than that of an LDE,
but even using a favorable eye only a couple (out of the 35 X-class
flares in the longitudes range E60-W60) seem to be sigmoids. For example,
the five homologous X-class flares in late November 2000 typically lacked
this morphology. Another CME-related morphology is trans-equatorial loops
(see
4,
5). But again, we could not find the phenomenon of the disappearance
of trans-equatorial loops in the X-class flares, except for one
(1998 May 6), which has been taken up in these nuggets several times.
High-energy behavior
Out of the 58 X-class flares since July 1996, 27 were observed by HXT and
WBS spectrometers on Yohkoh. Seven of them seem to have a
bremsstrahlung spectrum hard enough to extend to the MeV range. Although
in
the previous nugget
we reported the lack of the soft-hard-harder spectral behavior (which was
characteristically associated with X-class flares in solar cycle 21),
we probably have to correct it, since we seem to have find one example.
The following plot shows the light curves from HXT (L: 14-23 keV,
M1: 23-33 keV, H: 53-93 keV) and the power-law index from the ratio
of the H and M2 (33-53 keV) channels. Interestingly, the 33-53 keV
images show a compact source at what seems to be a loop footpoint
unlike the coronal emission in the 14-23 keV images, as shown in
this figure, where the
two contours correspond to the two channels.
The 1981 May 13 flare,
which may be a typical example of the soft-hard-harder pattern, indicated
high-energy emission from 20000 km or above. The
soft-hard-harder pattern should be confirmed in HXS/GRS data.
CME association
If we pick up as LDEs those flares with
decay time longer than 200 minutes or rise time longer than 20
minutes, the
break-down is as follows. Here, the "halo CME" includes partial halos.
The CME information is taken from
the official LASCO site
at CUA.
LDEs are all associated with halo CMEs, but
note that 85% (33/39) of short-duration X-class flares are associated
with at least some kind of CMEs.
| Association of X-class flares with CMEs |
| LDE | non-LDE |
| halo CME | 12 | 25 |
| CME (non-halo) | 0 | 8 |
| No CME | 0 | 6 |
| No LASCO obs | 0 | 7 |
An interesting thing is that when an X-class flare is preceded by another
X-class or weaker flare, the earlier one is usually not associated with
a CME.
Association with type II bursts
Distinction or identification of metric and dekametric/hectometric (DH)
type II bursts is important for understanding the origin of shock(s).
The following table shows that no type II burst in either frequency
range is seen (with one exception) when there is no CME. The association
with type II bursts in either metric or DH regime is not 100% for halo
CMEs, meaning that not all halo CMEs produce shocks. The correlation between
metric and DH type II bursts is not high, suggestive of different origins
in a larger num,ber of cases.
The information is taken from
the NOAA ftp site and
the Wind/WAVES
Web site.
| Association of X-class flares with metric
and dekametric/hectometric type II bursts |
| metric, DH | metric, no DH |
no metric, DH | no metric, no DH |
| halo CME (LDE) | 7 | 3 | 2 | 0 |
| halo CME (non-LDE) | 20 | 4 | 1 | 0 |
| CME (non-halo) | 2 | 4 | 0 | 2 |
| no CME | 0 | 1 | 0 | 5 |
Association with SEP events
Based on
the information available at NOAA,
we have learned which of the flares are likely to be associated with
interplanetary protons. Correlating this with other observations, we find the
following.
- Out of the 19 X-class flares associated with SEPs that are > 10 pfu
at > 10 MeV, 17 are associated with DH type II bursts and 18 with
metric type II bursts.
- Their longitude ranges from behind the east limb to the west limb.
- X-class flares in the western hemisphere without SEP association (11)
typically lack halo CMEs.
- SXT images for X-class flares with SEPs (9) always show ejecta
(but how can we tell them from "waves"?).
Concluding remarks
Study of solar flares has gone into a new dimension in recent years,
through the recognition that larger-scale disturbances, i.e., CMEs,
could result in heating and therefore electromagnetic radiation.
"Flares" resulting from CMEs may last long, hence an excellent correlation
between LDEs and CMEs. But we should recall that there is a whole spectrum
of other flares that are really confined and non-eruptive. These flares
may represent "pure" flare processes as opposed to the aftermath of CME
eruptions. What if these processes are intense? Can't they launch a CME?
The flare origin of CMEs may be an outdated idea, but we wonder if we can
generalize this to all CMEs, especially since CMEs are intimately associated
with big flares including short ones.
Part of the reason we need to study intense flares in the space
weather context is to identify processes that have to do with the
initiation of CMEs apart from the assumed
(i.e., not yet clearly observed) destabilization of large-scale
magnetic field. The numbers we have given above suggest that the
flare duration may be not an important factor for understanding
the possibly different relations between flares and CMEs. The
involvement of filament
eruptions (at different temperatures) and the way the filament erupts
may give us more useful information.
[Topical
index] -o- [Chronological
index]
June 14, 2002
N. Nitta (NVN) (nitta@lmsal.com)