ACE News #21
September 9, 1998

Simultaneous Observations of Upstream Events at ACE and WIND

During a 6-day period starting on Day 243, 1997, when both ACE and Wind were upstream of Earth's bow shock, the EPAM instrument on ACE and the STEP instrument on WIND observed 22 and 20 intensity enhancements of ~150 keV protons, respectively. Such "upstream events" are commonly observed when a spacecraft crosses field lines connected to the bow shock and are attributed to particle acceleration at the bow shock or leakage of magnetospheric particles. The positions of ACE and WIND during this period are shown above. Note that they were traveling in nearly opposite directions, with ACE on its way to the L1 libration point and WIND moving toward Earth's magnetosphere. During this period there were four events observed by both spacecraft (on days 243, 244, 247, and 248, as indicated on the trajectory curves). This study differs from previous studies in which one spacecraft was either inside or very close to the bow shock and the other was far upstream at L1.

For each of these events the time profiles, anisotropies, and magnetic field orientation were studied. We define the linear miss-distance as the distance that a straight line extrapolated along the locally measured interplanetary magnetic field vector misses an intersection with the planet. When the miss-distance is within ~10 RE it appears likely that the field line is connected to the bow shock. Three of the simultaneous events had a miss-distance of <10 RE.

It has previously been shown that the time profile of an upstream event may vary dramatically from one spacecraft to another when their separation is large. This study addresses the question of whether both spacecraft really observed the same event or whether they observed two different and distinct events. A simulation using random event start times finds that for only ~2% of the events observed at one spacecraft would another event be seen at the other spacecraft purely by chance. The observations show a far greater percentage of simultaneous events, 4 out of 20 (~20%), clearly suggesting that the spacecraft were observing the same events.

Contributed by Dennis.Haggerty and Rob Gold of the ACE/EPAM team and Mihir Desai, Glenn Mason, and Joe Dwyer of the WIND/STEP team.

See The EPAM Home Page at Johns Hopkins University Applied Physics Laboratory for more information about the EPAM instrument.

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Last modified 10 September 1998, Steve Sears
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