Physical Characteristics and Recent Observations of Magnetic Cloud Events

Ron Lepping and Davin Larson gave a tutorial on magnetic clouds for the members of the ISTP community who are not familiar with the phenomenon. By an impressive coincidence, the LASCO coronagraph on SOHO observed a halo CME the same day as the talks, while a magnetic cloud was observed in SOHO and WIND in-situ data a few days later. A description of the event is provided on a separate page. The magnetic cloud of October 18, 1995 (6 weeks before SOHO's launch) was observed as part of an IACG campaign - a Goddard Space Flight Center report on the event contains a thorough description of magnetic clouds and the campaign. (Also check "the WIND Magnetic Cloud and Events of October 18-20, 1995; Interplanetary Properties and as Triggers for Geomagnetic Activity," Lepping et al., in press, Journal of Geophysical Research). There are currently investigations underway to determine whether these events were correlated.

The organizers would like to thank Drs. Lepping and Larson to their contribution to the workshop.

Ron Lepping's talk:

Magnetic clouds behave like a "magnetosphere moving through the solar wind". They have very low plasma beta and a high magnetic field magnitude; as a result, the plasma in the cloud remains relatively isolated as it propagates. It expands and the density decreases accordingly.

^^^ Figure 1 (above): Diagram of a magnetic cloud, from Lepping et al. (J. Geophys. Res., 1990).

^^^ Figure 2 (above): October 18, 1995 magnetic cloud, indicating shock and shock-like regions preceding and following the event. The shock following it is interpreted as a "corotating stream" interacting with the magnetic cloud and possibly the heliospheric current sheet as well.

The event shows a relatively constant magnetic field - a jump from about 5 to 20 nanotesla, with a slow but steady rotation of the magnetic field from the negative to the positive z direction. The density is significantly depleted and relatively constant throughout the passing of the cloud, and the solar wind speed jumps to over 400 km/sec.

^^^ Figure 3: Plot of Alfvén mach number, plasma beta, pressure, solar wind pressure, and location of the magnetopause. The pressure is divided up into magnetic and plasma pressure - the magnetic pressure clearly dominates.

^^^ Figure 4: Constant alpha force-free fit to the October 18 1995 data. The model assumes a force-free magnetic configuration in the cloud and that a constant factor () relates the magnetic field and current.

^^^ Figure 5: Interpretation of magnetic cloud orientation from model in Figure 4.

^^^ Figure 6: Interpretation of spacecraft trajectory from model in Figure 4.

^^^ Figure 7: Approximate spacecraft trajectory through the cloud. WIND appears to have been hit "head on" by the cloud.

^^^ Figure 8: Force-free fit to previous magnetic cloud observations of November 17, 1975 and September 18, 1980, from Lepping et al., (J. Geophys. Res., 1990).

^^^ Figures 9: Force-free fits to previous magnetic cloud observation of December 19, 1980, from Lepping et al., (J. Geophys. Res., 1990).

Since SOHO launch, there are five possible magnetic cloud events identified by the WIND MFI and SWE:

Calendar Day           Date 
148.7-150.5 (1996)     May 27-29 
183.5-185.0            July 1-3
220.5-222.5            August 7-9
357                    December 24 
10.0 (1997)            January 10, 1997 

^^^ Figure 10: Magnetic cloud data from May 27-29, 1996 event.

^^^ Figure 11: Magnetic cloud data from July 1-3, 1996 event.

^^^ Figure 12: Magnetic cloud data from August 7-9, 1996 event.

Davin Larson's talk:

^^^ Figure 1: Data from the October 18 1995 magnetic cloud event, indicating approximate CME ejection time, radio storm, and Type III events.

^^^ Figure 2: Data from October 18 event, including electron data from the WIND 3D Plasma and Energetic Particle Experiment. The presence of the time-of-flight characteristics of the electrons (high energy arriving first, followed by lower energy) are assumed to correspond to closed connected field lines within the cloud, while the absence of this behavior is attributed to crossing a disconnected field line (see data just below the electron plots). Judging from the relative times of flight, he determines the approximate field line length, or starting point, of these groups of electrons.

^^^ Figure 3: Cartoon showing magnetic field lines corresponding to closed flux tubes and "disconnected" field lines, identified in Figure 2.

^^^ Figure 4: WIND 3D Plasma and Energetic Particle Experiment electrons (from Bob Lin): it appears that this distribution contains at least three Maxwellian populations (indicated by dot-dashed lines) designated the "core," "halo" and "superhalo." Origins of these populations are unclear.

Return to ISTP SOHO Workshop Page


EVENT LISTS - Possible Targets for Correlative Study

POINTS OF CONTACT - Participants in coordinated investigations

DATA LINKS - Data sites and starting points for various instruments and data facilities


This page was created 6 January 1997.
This page was revised on 12 January 1997.
Barbara J. Thompson