C=I have put together a list of times when POLAR CEPPAD/CAMMICE sees C=energetic particles at very high latitudes. In so doing, I noticed that C=the 5/29 and 6/20 events were part of a series of events that occur C=roughly every five days! This of course is the beat frequency between C=the POLAR orbit and the Earth rotation (roughly) so this may C=mean that we are periodically re-encountering a region owing to this C=recurrence. The strength of the feature may be driven by the strength C=of the IMF Bz for instance. Furthermore, these features have become C=more evident as POLAR has rotated toward the dusk plane. For your C=information, here is a short list of periods of "polar cap" events C=(these being where the entire or bulk of the high latitudes are filled C=with energetic particles): C= C=5/29 6/5 6/11 6/16? 6/20 6/25 7/4 7/9 7/14 7/20 7/22 7/26 7/28 7/31 C= C=Note that the frequency increases toward the end. July 17 was the point C=when POLAR crossed the dawn-dusk plane so we may be seeing a LT effect. C=Any thoughts? --- Harlan ---------------------------------------------------------------------- D=+Harlan E. Spence+spence@bu.edu+ips.bu.edu++Thu Aug 29 10:40:41 MDT 1996 C=Dear All, C= This high-latitude, energetic particle, CCR chat session has fallen silent C=over the past several months so I thought I would try to reinvigorate C=the discussion. The silence in e-mail does not reflect inactivity. I have C=been trying to understand the appearance of the energetic particles on C=lobe (or at least very high latitude) field lines and have reached a few C=conclusions that I wish to submit as a short paper to the special ISTP C=GRL issue (deadline is end of next month). Here is a synopsis of where C=I am and would welcome your thoughts and comments: C=
C=(1) I have surveyed all occurrences of >18 keV ions in high L regions from C=POLAR turn-on (2/28/96) through the end of July. Over this five month C=period I have compiled a list of >200 intervals. Many days have no C=events while others have several. Extended events are counted as one so C=long as they were continuous. C=
C=(2) For each occurrence of Polar Energetic Particles (PEPs) I have C=ascribed a "magnitude" (weak/moderate/strong) based on maximum energy and C=flux. In addition, I characterized the duration (short/intermediate/long) C=corresponding to <15 minutes, 15 minutes - 1 hour, > 1 hour. Finally, using C=data from the CAMMICE MICS data I have attempted to establish some sense\ C=of composition (I hope to get some guidance especially from C=Joe and Manuel on this aspect). I keyed on the He++ and high charge C=state oxygen to differentiate between and ionospheric and solar wind source. C=As far as location goes, each interval was tagged with the SPOF-generated C=region identifier which includes cusp, cleft, auroral oval, magnetospere, C=plasmasphere, etc.. Finally, when available, I estimated the average C=IMF components measured by Wind and propagated to the subsolar magnetopause C=for the interval. C=
C=(3) This database has the following gross characteristics: C=
C= a. The median IMF Bz component is 0 and the mean is ~+1 for all events. C=
C= b. About 1/3 of the events occurred squarely in the lobe. The C=remaining events have a predominant "oval" identification - but I am somewhat C=skeptical of these designators so I'm not sure what significance this C=has other than they occur at high latitude. C=
C= c. When the rates are high enough, these particles are most C=clearly of solar wind origin. The alphas and high charge state oxygen C=predominate the heavy ions. This still doesn't differentiate between C=a fresh entry (i.e., mantle plasma on open field lines) versus "once- C=processed" solar wind plasma (i.e., plasma that has entered the deep C=tail plasma sheet through the distant x-line). C=
C= c. The lobe occurrences show a more prominent IMF +Bz C=component both on an average and by median value. C=
C= d. As POLAR has moved from apogee at midnight (4/17) to apogee C=at dusk (7/17), the occurrence rate of PEPS has increased dramatically. C=The 5/29 event that is being talked about is just one of a series of C=PEPs which recurs with great regularity every five days. Five days is C=the beat frequency between the Earth rotation and the POLAR orbit so C=this may mean that we are probing a region prone to produce PEPs and have C=access to that region only when the configuration is optimal. C=
C=I have most recently focussed on the pitch angle distributions of the C=ions seen by using CEPPAD/IPS. Just last night I had a break through. C=There is a characteristic time-series signature that we see in the C=high time resolution data with a chevron shape when a particular pitch C=angle is plotted versus spin phase. This pattern is reproduced C=beautifully with a very simple model of a finite-width plasma structure C=sweeping past the spacecraft. The patter is produced owing to a finite C=Larmor radius effect. I see the correct energy dependence as well as C=Pitch angle dependence. I find that these large scale PEPs are made C=up of fairly small-scale filamentary pieces. From the shape of the C=feature I can actually deduce scale-size and speed (at least their C=projection in a plane at this point). I find that these PEPs reside C=in a structure that is on the order of ~1/4-Re and move past the C=s/c at a few kilometers/sec (dawn-to-dusk in the one case that I have C=firm numbers on). I have identified for three clear cases, a strong C=transpolar arc from the UVI data so we might be seeing a current tube C=filled with hot plasma (i.e., high latitude flux rope?) produced by C=merging poleward of the cusp during northward IMF. The distribution seems C=to peak near 0 and 180 degrees pitch angle (VERY tentative) with a C=possible front to back asymmetry (VERY VERY VERY tentative!) suggestive C=of more ions going down to the ionosphere than out. C=
C=I will try posting some of the illuminating plots so that you can see C=what I am talking about. I should add that not all the PEPs look the same. C=Some are very much more isotropic and I suspect that these may be still C=occurring on closed field lines from a very much expanded plasma sheet. C=At this stage, I am focussing on that class of PEPs which seem to be C=connected more directly with merging poleward of the cusp. C=
C= I welcome any thoughts you might have on my ruminations. As I said, I C=plan to write up a short paper for GRL that will describe the phenomenon C=and to the extent possible, present a plausible if not correct C=interpretation! Your inputs are very much appreciated!! C=
C=Happy days! , - Harlan ---------------------------------------------------------------------- D=+Harlan Spence+spence@bu.edu+ips.bu.edu++Wed May 1 16:57:32 MDT 1996 C=Sorry to be so chatty today, but today is the last day of classes and I C=am feeling particularly "free"! Geoff, the god's must be angry!! It just C=so happens that the s/c flip occurred on the 17th and 18th of April and C=the data during that period are seriously difficult to interpret as the C=spin rate is changing along with the orientation of the s/c. May I propose C=that we look at 4/20 as a candidate? The vitals of that day are: C= o POLAR perigee at ~5UT C= o abrupt entry into midnight sector plasma sheet at just after 1UT C= at "L" ~15 (not a real L of course but it gives on th