The storm on Oct. 19, 1998 (min Dst=-139 nT) was caused by a CME and its associated shock. At ACE, the shock was observed at ~1900 UT on Oct. 18, at which time the density jumped from ~15 cm-3 to ~40 cm-3 and the flow speed jumped from ~320 km/s to ~360 km/s. Within the shocked solar wind ahead of the CME, the density and flow speed continued to rise, reaching ~65 cm-3 and ~420 km/s at the arrival of the CME at ~0400 UT on Oct. 19. At the leading edge of the CME, the density dropped precipitously to ~5 cm-3, while the flow speed stayed roughly constant. The z-component of the IMF within the shocked region oscillated between weakly + and - on time scales of about an hour, and became durably and substantially negative (~-10 nT) only in the last hour before the leading edge of the CME. Within the CME itself, the field was strongly southward and very smooth for ~10 hours, rolling durably northward at ~2000 UT on the 19th. Thus, most of the high-density solar wind (in the shocked region ahead of the CME) was in variable Bz, except for the last hour (when Bz was ~-10 nT), while within the region of strong negative Bz, the density was fairly low.
At the end of October 18, LANL satellites 1991-080, 1994-084, and LANL-97A were all in the pre-to-post midnight plasma sheet. Prior to the shock arrival at the earth, these satellites observed a fairly nominal magnetosphere: evening plasmasphere bulge, nominal earthward edge of the plasma sheet, modest substorm at ~1500 UT. The plasma sheet density began rising after ~1600 UT, probably in response to the general increase in the solar wind density in that time frame, with a discrete jump up at ~2000 UT at the arrival of the shock. Thereafter, in response to the enhanced solar wind dynamic pressure, the plasma sheet density reached sustained values of ~2-3 cm-3 throughout the first few hours of Oct. 19. These densities are substantially higher (by a factor of >2-3) than typically observed under normal circumstances.
At ~0205 UT there was an apparent substorm injection observed at 080 at ~2.5 LT, probably in response to the interval of modest negative Bz observed at ACE between ~0015 and ~0200 UT. Combined with the strongly enhanced dynamic pressure, this southward interval apparently also led to increased magnetopause erosion, which enabled 084 to enter the LLBL at ~9.5 LT between about 0220 and 0300 UT; during this interval there was also a weak hint of LLBL plasma at 97A at ~7.5 LT.
A stronger interval of negative Bz in the hour before the CME arrived also caused magnetopause erosion such that 084 near 11 LT and 97A near 9 LT both crossed the magnetopause into the magnetosheath between ~0330 and 0440 UT.
At 0440 UT, 084 reentered the magnetosphere near local noon
and encountered a brief interval of cold, dense, plasmaspheric
material immediately adjacent to the magnetopause. After a brief
gap in the cold plasma, the satellite was in the drainage plume
for ~3 1/2 hours (12-15.5 LT). 97A reentered the magnetosphere
at the same time as 084 (~0440 UT, just past 9 LT). The reentry
was not into the drainage plume like 084, but rather into a warm
LLBL-like population of parallel-streaming ions exhibiting energy
dispersion suggestive of ionospheric upflowing O+. The plume entry
was at 0715 UT, near local noon. Subsequently, 080 also entered
the plume, at which point it had a much more structured appearance.
The times and locations of the three plume encounters are summarized
below:
084 0500 - 0830 UT 12 - 15.5 LT
97A 0715 - 1000 UT 12 - 14.6 LT
080 1130 - 1400 UT 12 - 14.5 LT
These observations indicate that between 0830 and 1000 UT, the
back edge of the plume was moving sunward, and then the plume
was beginning to break up. After leaving the drainage plume at
0830 UT, 084 observed a warm outer plasmasphere for about an hour,
consisting of multiple ion species and weak plasma-sheet-like
electrons. 97A encountered the same population (but with hotter,
denser electrons) in the same region.
At ~0945 UT, 084 made a rapid entry into the electron plasma
sheet at ~16.6 LT. This is quite early and indicates that convection
was very strong at this time. Coincident with the plasma sheet
entry, there was a very brief injection at the higher energies.
This corresponds to the pseudo-breakup observed by PIXIE at ~0955
UT at ~17 LT. 97A and 080 subsequently entered the electron plasma
sheet at nearly the same location, suggesting that convection
remained strong for the several hours between 084 and 080 entry.
The approximate entry times and locations are as follows:
084 0945 UT 16.6 LT
97A 1200 UT 16.6 LT
080 1650 UT 17.3 LT
From ~1240-1300 UT, 084 observed a partial flux dropout at 19.8 LT, and the electron temperature dropped at 97A (at 16.6 LT), indicating a stretching of the field. This was confirmed by the strong tilt in the derived field direction at both 084 and 97A during this time. At 1330 UT there was a clear energetic electron injection, corresponding to the second PIXIE event on this day. The next clear particle injection (observed at both 084 and 97A) occurred at ~1650 UT, associated with the fifth PIXIE event and the 080 plasma-sheet entry. The PIXIE events at 1509, 1538, and 1742 UT had no obvious geosynchronous signature.
The plasma sheet density was moderate (~1 cm-3) throughout most of the 19th. The plasma sheet electron density was fairly high (~several keV). Near 1930 UT, 084 (at ~2.5 LT) and 97A (near midnight) observed the arrival of a cooler, denser plasma sheet (most prominent at 084). The field stretching at 084 also decreased at this time. After ~2000 UT, the plasma sheet activity quieted at all the satellites, corresponding to the northward turning of the interplanetary field.
Michelle Thomsen
Los Alamos National Laboratory
505-667-1210, phone
505-665-4414, fax
mthomsen@lanl.gov