The LASCO partial-halo CME occurring late on Jan. 6 was used to forecast the arrival at Earth on Jan. 10 of the magnetic cloud which produced a geomagnetic storm. This association was made because the CME was halo-like, there were reports of activity near sun center on the Earthward-facing side, and the travel time to Earth was about right for a CME with a typical speed of 450 km/s. However, until now no one has tried to understand the nature of this activity and the likelihood of its connection to the CME. The importance of this event is that it is the first time that such a halo-type CME has been associated during solar minimum with a significant geomagnetic storm, and never have we had available such an impressive array of instruments with which to study it.
SHINE is a group whose purpose is to provide solar-interplanetary inputs to space weather studies. This event provides an ideal example of how SHINE can help to better understand the physics of geoeffective events through study of their sources at the sun and how they propagate through the interplanetary medium to the Earth. This report is a preliminary step in such a study for the Jan. 6-10 event, which has already been well described at the Earthward end of the chain by the ISTP group.
Details of the CME are described in the LASCO press release and on the ISTP Web page. It was observed first in the C2 coronagraph on Jan. 6 at 17:34 and later in the C3 coronagraph before 19:50. Measurements of the expansion speed of the front on a height/time diagram yield 4 estimates of the onset time of the CME, two times for each coronagraph at the solar limb and at disk center.
Onset at sun center Onset at limb C2 14:02 15:48 C3 15:08 16:24
The expansion speeds are as projected in the plane of the sky and, therefore, are lower limits for the actual CME speed along the sun-Earth line.
We also note that the CME was observed as a partial arc over the SSW part of the sun moving in a southwesterly direction. A similarly positioned partial halo CME on Oct. 5, 1996 was associated with an erupting filament/active region and X-ray arcade south of sun center. Thus, we might expect a similar source location on the disk for this event.
However, at first glance the solar surface appeared relatively quiet on Jan. 6. The GOES whole-sun plot showed little activity above the A1-level background, and Ha and Yohkoh X-ray images early and late in the day showed little change. The only enhanced regions were NOAA Reg. 8009, a small but bright emerging flux region with a few sunspots west of sun center, and Reg. SN84, a large, weak plage area with no sunspots near central meridian at S30. This region was bifurcated by a NW to SE trending polarity inversion line over which filament fragments had come and gone during the disk passage. This region is where the southern polar crown of filaments bends sharply to the north; there is evidence that such locations more frequently produce mass ejections.
The Air Force observers at Ramey AFB in Puerto Rico, one of the stations in the AF SOON network acquiring daily optical image of the sun, provided this report of their observations on the 5th and 6th. At this time of year solar observations at Ramey extend from about 11:00 to 21:00 UT. During the overnight gap observers note whether significant activity, especially disappearing filaments (DSFs), may have occurred. Ramey reported no DSFs overnight on either Jan. 4-5 or Jan. 6-7. They also saw no DSFs on the disk on Jan. 5 during the observing day: 11:17-21:19 UT. However, a small filament did disappear in SN84 between Jan. 5 and 6 confirmed by the SOON station at San Vito, Italy (see next table).
This filament was gone by the start of the observations at Ramey on the 6th at 11:13 (SOONSPOT movie from the Ramey AFB site - Filament Evolution January 6 - 7, 1997). Another filament centered at S24W01 disappeared between 13:01 and 14:53. This filament was not visible on an Ha image at 08:50 from Meudon Observatory in France, so that it may have formed on the 6th between 08:50 and the 11:13 start time at Ramey. Here is a summary of the Ramey report (at 18:00) on this DSF.
A 5-deg. long, normal density filament lying over the northern fringe of reg. SN84 "disappeared in a slightly eruptive fashion" [between the times above]. The filament had been very stable beforehand. It suddenly began dissipating with motion over a curving path. Some material began reappearing within 30 min., but then again dissipated along the reverse direction. Simultaneously with the DSF, an 8-deg. long filament lying over the southern part of the region suddenly displayed strong structural changes. This filament had also been very stable before. In addition, numerous small-scale plage flucuations occurred in the region center (between the filaments) during this period. Two small sunspots appeared at ~S35E05 during and after this period. The northern filament did not reform before the end of the observing day at 20:43.
DSFs on Jan. 5-6, 1997 Station Disp. Time Location Length Jan. 5: None reported Ramey >5, 2119 <6, 1113 S17E05 5 deg. San Vito >5, 1939 <6, 2300 S19E06 S20E10 (9 deg.) Ramey 6, 1301 - 1453 S23W03 ~5 deg. S24W01 S27W00 Jan. 7: None reported
A closer look at radio and X-ray data on Jan. 6 shows that there was at least weak coronal activity associated with this DSF. N. Gopalswamy reports that a "radio filament" consistent the location of the Ha filament, disappeared between 17 GHz images at 06:45 and 23:45 from the Japanese Nobeyama Radio Observatory. Gopalswamy and H. Hudson of ISAS in Japan also examined the Yohkoh SXT images during Jan. 6. They found that a faint loop system above the filament position north of the main plage loops had disappeared between images at 08:30 and 15:11. There is also as yet unconfirmed evidence of motion, but not necessarily expulsion, of the large loop system extending from the region to the south. Finally, an extended plot of one of the GOES X-ray channels on Jan. 6 shows evidence of a weak long-duration event (LDE) from 14:30 - 16:30 UT. Such events have been associated with filament eruptions and CMEs and the timing is consistent with the later phase of the DSF. However, geomagnetic activity can cause such low-level signals in GOES, so the identification of the LDE as a solar signal is uncertain.
The GOES data do show that a distinct solar LDE did occur on Jan. 5 from 13:31 - 16:10 with a peak flux of A6. The Yohkoh SXT images indicate that this event came from the same south-central region as the above DSFs; they show some brightening and changes in the S-shaped structure of the region. Initially, we thought that this event was the source of the CME on Jan. 6 and the cloud on the 10th. However, it occurred too early to match the extrapolated onset time of the CME, and also it is not consistent with the timing of the interplanetary disturbance (see below). In addition, we have not found any reports of DSFs on Jan. 5, although Ha images from Culgoora Solar Observatory in Australia show that a moderate-size dark filament just north of Region SN84 gradually dissipated during that day.
Another consistency check on the solar source of the CME and cloud is provided by an assesssment of its transit speed between the source and the Earth in comparison to the solar wind speed observed at 1 AU. On Jan. 10 the WIND spacecraft was located between L1 and the magnetosphere. The front of the magnetic cloud was detected at WIND on Jan. 10 at 04:45 UT, the interplanetary shock arriving at 01:00. If the cloud traveled at a typical speed of 450 km/s from the sun to the Earth, its onset time at the sun would be Jan. 6 at 0900. This is consistent with the extrapolated onset times of the CME and the DSF near sun center after midday on Jan. 6, given the uncertainities of the measurements and of how the material was accelerated.
We can invert this procedure by assuming we know the source time and calculating the transit time to the shock (or cloud) at 1 AU. Assuming that the Jan. 5 LDE event is the solar source of the shock/cloud gives an average transit speed of 385 km/s. If the DSF is the source on Jan. 6, the transit speed is ~500 km/s (assuming a launch of the CME at 14:00, consistent with the C2 onset time.) Cliver, Feynman and Garrett (JGR, 95, 17103, 1990) found a generally linear relationship between the maximum in-situ solar wind speed of disturbances with confidently identified solar sources and the associated shock transit speed. On Jan. 10 the peak hourly-averaged wind speed in the WIND data following the shock was 465 km/s. Although the data points for both of the Jan. 5 and 6 candidate sources for the shock lie above the best-fit line in the Cliver et al. study, this analysis indicates that the Jan. 6 event is the preferred candidate source.
There were two other unusual aspects of the shock/cloud event. First, there were no enhanced fluxes of energetic protons or ions associated with it seen at WIND. This was confirmed by measurements from the Low Energy Telescope of the EPACT experiment, which measures particle fluxes above ~2 MeV/nuc with unprecendented sensitivity (D. Reames, priv. comm.). This lack of energetic particles particles is unusual for a fairly strong driver gas/shock event. Second, the Jan. 6-10 interval is the first time that a traveling type II kilometric radio burst has been detected by the WIND WAVES experiment. The burst can best be seen on the WAVES frequency vs time plot as a persistent bright band of emission which peak intensity moves to lower frequencies with time. This pattern suggests an interplanetary shock was moving outward from the sun. In the past data such kilometric bursts have usually been associated with energetic flares and strong shocks (e,g, Cane, JGR, 90, 191, 1985).
Our preliminary conclusion is that the cloud and storm at Earth on Jan. 10-11 were both associated with the CME near the sun on Jan. 6, which in turn had its source in a DSF and weak coronal activity just south of solar disk center around midday on Jan. 6. Although the solar activity was fairly weak, such weak correlations between classic solar observables and CMEs and interplanetary disturbances are not uncommon. Indeed this is a key reason why the forecasting of geoeffective disturbances is so difficult. A fundamental problem is our lack of near-sun observables of the ejected coronal material itself.
This report is preliminary. Corrections, new information or comments are solicited and should be sent D. Webb at the address below. I would like to thank the following people who provided data and/or analysis efforts for this study: N. Gopalswamy of the Univ. of Maryland for Nobeyama and Yohkoh SXT data, H. Hudson of ISAS and L. Acton of Montana State Univ. for SXT analysis, E. Cliver of PL/GP for wind speed analysis, D. Reames of GSFC and S. Kahler of PL/GP for WIND type II and particle data, J. Steinberg of MIT for WIND plasma and IMF data, Sgt. D. Rose and S. Dahl of Ramey AFB for Ha data, C. St. Cyr of NRL for LASCO CME data, P. McIntosh of Heliosynoptics for Ha data, and S. Keil of PL/GP for Ha data.
Report produced by David F. Webb of Boston College and Phillips Lab Geophysics Dir.