**SOLAR SOURCES OF GEOACTIVITY DURING SEPTEMBER-OCTOBER 1999**

David Webb

Invited talk for Workshop 3 on the Sept.- Oct. 1999 Space Weather Month
at

First S-RAMP Conference; Sapporo, Japan; Oct. 2-6, 2000

**I. INTRO**

A. [Summary of Magnetic Storm Activity]{.underline}

Figure is a summary of all magnetic storm events (numbered 1-5) in Sept.
1999 with min. Dsts less than -50 nT (moderate to major storm events)
within the core campaign interval (September 6 - 25). Within this Sept.
interval, 4 moderate and 1 major storms occurred. Major Oct. event added
later.

**II. GEOEFFECTIVE STORM EVENTS**

There were roughly three periods of shocks, ejecta and southward
interplanetary magnetic fields that triggered magnetic storms during the
core campaign interval in Sept. 1999. First two were moderate storms.
Third period in Sept. and added one in Oct. were major storms with Dst
-164 and -231, resp. I will spend most time on these last 2.

A. [General IP Signatures of CMEs]{.underline}

IP Shocks, MCs, BDEs, low T, Enhanced He.

B. [12-14 September 1999 Minor Storms]{.underline}

1\. Solar Wind:

A shock (Sept 12, 0315 UT) arrived prior to a Sector Boundary (Sept 12,
0800 UT) with strong density and pressure enhancements. No obvious
ejecta signatures are seen in the data during this time interval. IMF
enhanced but highly fluctuating with long southward intervals. These
intervals of intermittent Bs extend into early Sept. 14, triggering the
magnetic storm activity on Sept. 12-14 (Storms 1 and 2).

2\. Solar Sources:

No obvious solar events assoc. with these geoeffective solar wind
signatures. Both the SB and following (ff.) high-speed stream drive the
magnetic storms. The presence of the shock and enhanced IMF indicates
solar ejecta may also be present.

C. [15-16 September 1999 Minor Storms]{.underline}

1\. Solar Wind:

The second geoeffective interval contains 2 IP shocks on Sept. 15, at
0710 & 19:40 UT. 1^st^ shock ff. closely by sm. \'probable\' magnetic
cloud (MC) from \~0900 - 1730 UT, with S-N field rotation and Bmax \~ 15
nT. Strong Bs field early in MC triggers a moderate storm (No. 3; Dst
\~-53 nT). 2nd shock at 1940 UT not ff. by obvious solar ejecta. But a
period of sustained Bs triggers storm Sept. 16 (No. 4; Dst \~-67 nT).

2\. Solar:

There were 2 partial halo CMEs observed by LASCO on Sept. 13 that are
the possible sources for the 2 shock events on Sept. 15. However,
required travel times to Earth are a bit shorter than would be
considered usual.

1st halo on Sept. 13 at 0930 UT with speed of 730 km/s. This halo assoc.
with \>C5 flare erupting from AR #8699 near SC. Est. travel time (TT) to
shock onset is 49 hours or a speed of 850 km/s; TT to storm peak is 50
hrs.

2nd halo CME on Sept. 13 at 1731 UT with speed of 410 km/s. This event
assoc. with C3 flare from AR #8693. TT from CME onset to 2nd shock is 51
hours with or a speed of 817 km/s; between the CME and storm peak is 64
hours or a speed of 650 km/s. The later speed is more reasonable.

D. [22-23 September 1999 Major Storm]{.underline}

1\. Solar Wind:

Forward shock on Sept. 22, 1130 UT ff. by period of strong IMF with
prolonged Bs field which causes storm. **Burlaga CE Figure**

The strong IMF period is from the shock to a reverse shock or wave on
Sept 23, 0920 UT. with Bmax=28 nT. Reverse shocks inside of 1 AU used to
be rare, but are now obs. more frequently by sensitive WIND MFI. The
strong IMF period does not have all signatures of a MC. But is part of a
long complex event (CE)/ICME extending for 4 days, identified by hi He,
counterstreaming electrons, and low T.

SB crossing on Sept 22 at 1300 UT at front of strong IMF and ahead of a
high-speed stream. So strong Bs early in the ICME assoc. w/ejecta at SB.
It drives a major storm (No. 5) with Dst \~-164 nT on Sept 23, 0000UT.
ICME prob. "carries" entire toward sector. So high speed is from ICME;
Corotating Interaction region (CIR) in ff. away sector wind stream
compresses rear of ICME.

2\. Solar:

Most likely solar source of event was a faint, but complete halo CME on
Sept 20, 0606 with projected speed of 450 km/s. The TT between onset of
CME and of ejecta at 1 AU is 63 hrs (2.62 days) or a speed of 660 km/s.
Compares with avg. wind speed in ICME of \~500 km/s; but 600 at front.

EIT shows filament eruption with bright arcade loops and dimming at
\~S30W10, and a possible coronal wave on Sept. 20, \~0500; also sm. C2.5
flare. Figure shows sequence of EIT 195Å images of bright arcade loops
and dimming regions associated with fil. eruption (arrows) in South
polar crown area crossing Central Meridian. Assoc. full halo CME was
faint. Best seen in movie.

E. [22 October 1999 Major Storm]{.underline}

1\. Solar Wind:

Figure: Forward shock on Oct. 21, 0220 UT ff. by switch in field
polarity for \~26 hours. Field then reverses back to original direction,
so not a normal SB.

Probably strong MC (Bmax=38nT) w/N-S rotation. Like Sept., ICME prob.
"carries" entire toward sector, but here only 1 day long. ICME speed not
as high as Sept., but strong, prolonged Bs at rear caused by compression
by ff. away sector wind stream. ICME is recurrent w/Sept. event and one
in Nov. 16-17. Could be magnetic merging at 1^st^ HCS crossing (B drops
sharply at change in N). Strong density and He peak at end Oct. 21 at
start of rapid B2 rot. to S could be evidence of fil. near rear of MC.
Some BDE here too.

So strong Bs at rear of MC interacting with CIR causes major storm with
Dst=-231nT on Oct. 22, 0700.

2\. Solar:

Most likely solar source of event a partial 210 deg. wide halo CME on
Oct. 18, 0006. Showed acceleration to final speed of 450 km/s. Brightest
in NE. The TT bet. onset of CME and of ejecta at 1 AU is 77 hrs (3.21
d.) or a speed of 540 km/s. Compares w/wind speed in ICME of \~470 km/s.

EIT shows fil. eruption with wide arcade loops, activity over a wide
area in the SE (\~S35E20), including brightenings to the E and NE, and
dimming, and a coronal wave starting on Oct. 17, 2312; no reported Ha
flare. Figure shows EIT image of area in SE assoc. with fil. eruption.
Figure shows assoc. halo CME.

III\. **Summary of Sources of Sept.-Oct. 1999 Storms**

1\. Both storms driven by strong Bs fields within or ahead of ICMEs from

Earthward CMEs.

Both had leading IP shocks.

2\. Storms were recurrent: Same polarity IMF came from toward-sector

region on Sun. Thus, 27 days apart.

Figure Source Surface fields at 2.5 Rs. Explain. Note

evolution from 2 to 4-sector structure during rise to maximum.

Figures show Source Surface fields for Sept. and Oct. solar

rotations.

Bar indicates source fil. eruption/arcade on surface. 2-3 day TT of

ICME from Sun to Earth. Both ICMEs carry entire SB.

Note evolution of sectors; toward sector shrinks in Oct. Leads to

squeezing by strong away sec. at back of ICME.

3\. Source of high speeds & strong Bs fields.

Sept. event: High speed & strong Bs at front of ejecta. Some

interaction with slower leading away sector.

Oct. event: Likely MC w/ N-S rotation. Maybe Right-Handed

helicity; not typical for this cycle. Strong B, Bs and increasing speed

at back of MC. Due to Bs-ff. field in flux rope and interaction with ff.

High Speed wind in ff. away sector.

**IV. SUMMARY**

Two Major Storms during Campaign Interval.

Both IP Structures & Storms Recurrent:

\- Arise from same Southern Sector/Region on Sun.

\- Part of 4-Sectors Typical of this Phase of Cycle.

\- ICME carries Entire Sector with It.

Sources:

> \- Halo CMEs from Fil. Eruptions & Coronal Arcades.

Also Dimmings & Waves; *All Typical of Halo CMEs*.

\- NO Major Flares! *\[Remember Jan. 1997 Event!\]*

Neither Event Energetic but Both cause Significant Storms. 1st Event
Faster but 2^nd^ causes Larger Storm. *Why?*

\- Due to Both Solar Conditions & IP Interactions.

Both near Sun Center & carry Strong Bs Fields.

*Sept. Event Long-lived (4 days).*

*Oct. Event from Wide Area on Surface.*

\- Causes of Bs Fields:

Sept. Event Y Hi Speed & Strong Field at ICME front.

Oct. Event Y Stronger Field causes Larger Storm.

*Bs Field at rear of Flux Rope Y Compression by*

*Hi Speed Stream at CIR.*

Thus, Strong, Prolonged Bs Field Most Geoeffective.

\- Speed Less so, except via Compression Effects.