1. The nature of impulsive solar energetic particle events N. V. Nitta a, H. S. Hudson b, M. L. Derosa a a Lockheed Martin Solar and Astrophysics Laboratory b Space Sciences Laboratory, University of California Berkeley Impulsive solar energetic particle (SEP) events, which are characterized by enhancement of 3 He and other ions, are thought to come from impulsive solar flares that are well- connected 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 R , 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 http://cdaw.gsfc.nasa.gov/LWS/data/event_list.html). The PFSS model used here is described by Schrijver and DeRosa (Solar Phys. 212, 165, 2003). 1-May-2000 event This event was studied by Kahler, Reames, and Sheeley (ApJ, 562, 558, 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. They are a subset of about 1800 field lines that reach the uniform grid at the source surface. 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. 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 well- connected. 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 well- connected. 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 well- connected field lines, with the other foot-point fixed. Based on observations of other regions, this may represent small magnetic flux emergence around a sunspot, and the 27-Dec-2000 23:45 UT brightening may be also in this category. 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, maybe 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.