Transcript Document

The nature of impulsive solar energetic particle events
N. V. Nittaa, H. S. Hudsonb, M. L. Derosaa
a Lockheed
b Space
Martin Solar and Astrophysics Laboratory
Sciences Laboratory, University of California Berkeley
Impulsive solar energetic particle (SEP) events, which are characterized by enhancement
of 3He and other ions, are thought to come from impulsive solar flares that are wellconnected to Earth. This is in contrast with large, gradual SEP events, which are
attributed to shocks driven by fast CMEs. In order to see if this is the case, we compare
the foot-points of the well-connected field lines with the locations of the associated flares.
Here, we use a combination of simple techniques, i.e., the ballistic
approximation that maps the magnetic field observed near Earth to the
source surface, placed at 2.5 Ro, and a potential field - source surface
model (PFSS) to map the field the rest of the way back to the solar
surface. In such a model any non-radial excursion of a field line must
occur below the source surface.
We have selected a couple of events that were studied during the LWS
CDAW from last summer. At this CDAW, a total of 38 impulsive SEP
events were analyzed, and their solar sources were identified (see
1-May-2000 event
http://cdaw.gsfc.nasa.gov/LWS/)
This event was studied by Kahler, Reames, and Sheeley (2001), who reported its
association with a narrow CME. The left panel shows open field lines plotted on an SXT
image during the early phase of the associated M1.1 flare. Tracing lines from the
photosphere out reveals mostly closed field lines (middle panel). The right panel shows that
the calculated open field lines match coronal hole boundaries, and indicates (by an arrow) a
set of field lines from the flare region to the longitude range of W45-W55 and B0 angle +- 2
degrees, I.e., well connected.
The PFSS model used here is described by Schrijver and DeRosa
(Solar Phys. 212, 165, 2003).
14-Apr-2001 event
This event was studied by Tylka et al. (ApJ 581, L119, 2002),
who contrasted it with another major SEP event associated
with a fast CME. The left panel shows that the region
indicated by an arrow has open field lines, but they do not lie
close to the ecliptic plane. The right panel shows a zoom-in
view of open field lines, now plotted on a TRACE image.
Although the location indicated by a white arrow seems to
cause a diffraction pattern due to flare brightening, the related
flare may have occurred somewhere else. The likeliest would
be the region to the northeast, which appears to be wellconnected.
28-Dec-2000 event
The associated solar event was a small B3 flare that occurred
north of the preceding spot. This region seems to be highly
open and some of the open field lines were well connected.
At least two of
these events show
clear solar
association, and
the solar region
seems to be wellconnected. Then,
are there
peculiarities about
the associated
flares?
Some properties of flares associated with impulsive SEP events
The 1-May-2000 flare shows a hard X-ray foot-point
that moves significantly toward the direction of wellconnected field lines, with the other foot-point fixed.
This may represent small magnetic flux emergence
around a sunspot, similar to the 27-Dec-2000 23 UT
flare.
The 14-Apr-2001 event was probably
associated with a short-duration hard
X-ray burst, but its soft X-ray/EUV
counterpart is not clearly identified,
possibly lying high in the corona (see
Cliver and Kahler, ApJ 366, 91, 1991).
Intense impulsive flares in the well-connected
locations but without SEPs
The 27-Nov-1999 flare is one of those intense flares in the
western hemisphere but without SEPs. There were at least
four such X-class flares during 1996-2001.
The flare region (arrow)
seems to be well-connected,
indeed. However, this figure
is problematic, since no open
field lines are reproduced in
the southern coronal hole (!).
Although the flare was
associated with a metric type
II burst,it was radio quiet in
longer wavelength (see the
Wind/WAVES plot),
and had no CME. In
soft X-rays, there
was a confined
ejection.
Conclusions
The simple ballistic approximation and PFSS model indicates, for a couple of events presented here, that the magnetic field connectivity is very important for detecting
impulsive SEP events. The properties of the associated flare may be related to the magnetic field environment. We need to analyze more events, especially those
observed by RHESSI, to understand how flare and escaping particle populations are coupled and also to assess the usefulness of the technique. Eventually we would also
like to use the SEP associations to help improve the magnetic modeling.