Simulation of the Evolution of the IMF During Four Event Periods

Event Period 1 | Event Period 2 | Event Period 3 | Event Period 4

Description:

We have simulated the evolution of the equatorial-plane IMF during four event periods. The simulations, or "movies," were produced using the Hakamada-Akasofu-Fry "modified kinematic" solar wind model (HAF code). The model boundary conditions are derived in a standardized way from specific solar observation types (Hakamada and Akasofu, 1982; Akasofu and Fry, 1986).  References.

The first simulation represents a period during which a large geomagnetic storm occurred that produced widespread aurora extending down to mid-latitudes. This period was interesting because there was no clear Type II radio burst that could be associated with an event source. The second simulation provides an animation of a previous study by Akasofu and Lee (1988) for a period of three solar events in late March and early April, 1979. The third and fourth simulations are based upon reports for events that occurred during September and November 1998. These events are included in a study of Shock Arrival Time (SAT) predictions, to be presented at the Fall 1999 AGU meeting (Deehr, Sun, Fry and Dryer, EOS Trans., 1999).

Event Period 1: The first movie sequence shows our effort to provide real-time predictions of the IMF configuration following a solar event on Oct. 17, 1999. No Type II radio burst data were available in real-time for this event, so we ran the HAF code using inputs provided by Z. Smith and M. Dryer for event number 96 in their ongoing ISPM/STOA validation study. A variety of data sources were used, including some subsequently provided by the Culgoora spectrograph, Wind/Waves Rad 2 experiment, and the SOHO EIT instrument.

We assumed the event began on Oct.17, at 2318 UT, and was located at about heliographic latitude S30 and longitude E30 (estimated from a filament activation and eruption observed by the EIT instrument). A partial halo CME extended for ~210 deg. from the north pole, counterclockwise, to the southwest limb. This movie begins on Oct. 18 when the Earth is at Heliospheric Ecliptic Coordinates (HEC) longitude ~300 deg.*. In this simulation, an apparent shock can be seen moving outward from the sun and reaching the Earth early on October 21. A large geomagnetic storm was initiated at this time.

The Smith/Dryer prediction was for "decay to MHD wave," based upon the subsequent Type II information. Thus, their prediction was classified as a miss. We assumed (ex post facto) the actual shock speed was twice the value they used, and succeeded in the displayed shock arrival time. The movie also shows the western skirt of the shock merging with a co-rotating interaction region (CIR). Subsequently, the background IMF pattern can be seen to reestablish itself at the Earth after the shock passage.

* In the HEC coordinate system, the zero longitude lies along the intersection of the solar equatorial plane and the ecliptic plane. The Earth is at HEC longitude = 0 on about Dec. 6th of each year as it passes from north to south of the solar equator.

Event Period 2: This simulation of three events begins on Mar. 30, 1979. The Earth is at HEC longitude ~ 100 deg. The first event source is west of Central Meridian (CM, or the sun-Earth line). The resulting shock reaches the Earth on Apr. 2. The background flow and IMF suggests formation of one CIR and two developing CIRs. A second shock can be seen emanating from a region slightly east of CM, driven by an event late on Mar. 31. This shock can be seen reaching the Earth on Apr. 3. The model then produces a third shock structure as a result of a mid-day event on Apr. 2. The third shock can be seen following in the footsteps of the second, reaching the Earth on Apr. 5. For additional information on the kinematic simulation of solar wind conditions during this event period, see Akasofu and Lee (1988).

Event Period 3: The third movie begins on Sep. 10, 1998 when the Earth is at HEC longitude ~ 270 deg. This period covered one of the events (number 16) selected for the Smith et al. (1999) study and used in our Shock Arrival Time (SAT) prediction study. In this case, the presumed event source was near the west limb (W64). As the movie progresses, the western edge of the disturbance can be seen merging with a CIR that rotates through HEC longitude ~ 0 deg. The extremely weakened eastern skirt of the disturbance just clips the Earth at about hour=90. The HAF, ISPM and STOA models all predicted there would be no shock at Earth, and none was observed in the Wind and ACE data. Thus all three models predicted a correct null.

Event Period 4: The Earth is at HEC longitude ~330 deg. when the fourth movie begins on Nov. 5, 1998. The candidate source event for this simulation occurred at 1951 UT on the 5th, and also was one of those used in our SAT study (number 22). Earth was located between two closely-spaced CIRs. In this movie, the event (source location at N18 W21) produces a disturbance that travels outward from the sun in a direction somewhat westward of CM. Following the passage of a CIR at Earth on Nov.7, the simulated shock arrives at the Earth after being strongly deformed by its interaction with the two CIRs. The HAF model predicted a shock arrival at 0400 UT on Nov. 9, about 23 hours later than a shock that was observed in the Wind data.

References:

Akasofu, S.-I., and C. D. Fry, "A First-Generation Geomagnetic Storm Prediction Scheme," Planetary Space Sci., 34, 77-92, 1986.

Akasofu, S.-I. and L.-H. Lee, "A Study of the Interplanetary Disturbances on 1-4 April 1979," Planet. Space Sci., 36, 669-675, 1988.

Deehr, Sun, Fry and Dryer, "Calculation of Shock Arrival Time Following Observed Solar Events," AGU 1999 Fall Meeting Abstracts, EOS Transactions, __, __, Nov. __, 1999.