:Product: Weekly Highlights and Forecasts :Issued: 1999 Oct 26 2112 UT HIGHLIGHTS OF SOLAR AND GEOMAGNETIC ACTIVITY 18 - 24 OCTOBER 1999 SOLAR ACTIVITY RANGED FROM LOW TO MODERATE LEVELS. ACTIVITY WAS AT LOW LEVELS THROUGH 19 OCTOBER WITH ISOLATED C-CLASS SUBFLARES. ACTIVITY INCREASED TO MODERATE LEVELS ON 20 OCTOBER WITH AN M1/1F FLARE FROM REGION 8731 (N12, L = 292, CLASS/AREA FKC/1220 ON 17 OCTOBER) AT 20/0622UT WITH AN ASSOCIATED TYPE II RADIO SWEEP. THIS LARGE REGION DISPLAYED A MIXED POLARITY STRUCTURE AND WAS SLOWLY DECAYING WHEN THE M-FLARE OCCURRED. ACTIVITY DECLINED TO LOW LEVELS ON 21 OCTOBER AND REMAINED SO FOR THE BALANCE OF THE PERIOD. HOWEVER, REGION 8739 (S12, L = 168, CLASS/AREA EKI/440 ON 23 OCTOBER) GREW DURING THE LATTER HALF OF THE PERIOD AND BECAME MODERATE IN SIZE AND MAGNETIC COMPLEXITY INCLUDING A MAGNETIC DELTA CONFIGURATION NEAR REGION CENTER. REAL-TIME SOLAR WIND DATA WERE AVAILABLE FROM THE ADVANCED COMPOSITION EXPLORER (ACE) SPACECRAFT FOR MOST OF THE PERIOD. A CME WAS DETECTED DURING 21 - 24 OCTOBER BEGINNING WITH A SHOCK FRONT PASSAGE AT 21/0138UT. THE CME PASSAGE CAUSED A MAJOR GEOMAGNETIC DISTURBANCE (SEE THE DISCUSSION BELOW). THE FOLLOWING EFFECTS WERE NOTED DURING THE PASSAGE: VELOCITIES INCREASED TO AROUND 720 KM/SEC EARLY ON 22 OCTOBER, THEN GRADUALLY DECREASED; DENSITIES PEAKED AT 41 P/CC IMMEDIATELY FOLLOWING THE SHOCK FRONT PASSAGE, THEN DECREASED A BIT, BUT REMAINED ELEVATED THROUGH 22 OCTOBER; IMF BZ TURNED STRONGLY SOUTHWARD LATE ON 21 OCTOBER INTO THE FIRST HALF OF 22 OCTOBER WITH A MAXIMUM SOUTHWARD DEFLECTION OF 31 NT (GSM); AND THE SOLAR SECTOR SHIFTED FROM AN AWAY (POSITIVE POLARITY) TO A TOWARD SECTOR. NO PROTON EVENTS WERE DETECTED AT GEOSYNCHRONOUS ORBIT DURING THE PERIOD. THE GREATER THAN 2 MEV ELECTRON FLUX DIPPED TO NORMAL TO MODERATE LEVELS ON 22 OCTOBER, BUT WAS AT MODERATE TO HIGH LEVELS DURING THE REST OF THE PERIOD. A MAJOR GEOMAGNETIC STORM OCCURRED ON 22 OCTOBER. THE DISTURBANCE BEGAN WITH A SUDDEN STORM COMMENCEMENT (SSC) AT 21/0226UT. THE SSC WAS IMMEDIATELY FOLLOWED BY A BRIEF PERIOD OF ACTIVE LEVELS WHICH SUBSIDED TO QUIET TO UNSETTLED LEVELS UNTIL LATE 21 OCTOBER. THE DISTURBANCE INTENSIFIED LATE ON 21 OCTOBER AND CONTINUED THROUGH 24 OCTOBER WITH THE MOST INTENSE ACTIVITY OBSERVED DURING 22/0000 - 1200UT. MAJOR TO SEVERE STORM CONDITIONS WERE DETECTED GLOBALLY DURING THIS INTERVAL. THE DISTURBANCE DECREASED IN INTENSITY AFTER 22/1200UT WITH ACTIVE TO MINOR STORM LEVELS DETECTED AT MIDDLE LATITUDES AND ACTIVE TO (BRIEF) SEVERE STORM LEVELS AT HIGH LATITUDES. THE STORM ENDED LATE ON 24 OCTOBER. FORECAST OF SOLAR AND GEOMAGNETIC ACTIVITY 27 OCTOBER - 22 NOVEMBER 1999 SOLAR ACTIVITY IS EXPECTED TO BE AT LOW TO MODERATE LEVELS. OCCASIONAL C-CLASS FLARES ARE LIKELY. ISOLATED M-CLASS FLARES ARE ALSO POSSIBLE THROUGHOUT THE PERIOD. REGION 8739 ALSO PROVIDES A SLIGHT CHANCE FOR A MAJOR FLARE UNTIL IT DEPARTS THE DISK ON 02 NOVEMBER. NO SIGNIFICANT PROTON EVENTS ARE EXPECTED AT GEOSYNCHRONOUS ORBIT. THE GREATER THAN 2 MEV ELECTRON FLUX AT GEOSYNCHRONOUS ALTITUDE IS EXPECTED TO BE AT MODERATE TO HIGH LEVELS THROUGH 31 OCTOBER AND AGAIN DURING 07 - 15 AND 20 - 22 OCTOBER. NORMAL TO MODERATE FLUXES ARE EXPECTED DURING THE INTERVENING PERIODS. THE GEOMAGNETIC FIELD IS EXPECTED TO BE MILDLY DISTURBED THROUGH 30 OCTOBER WITH UNSETTLED TO ACTIVE LEVELS. ACTIVE TO MINOR STORM LEVELS ARE EXPECTED DURING 06 - 13 NOVEMBER DUE TO RECURRENT CORONAL HOLE EFFECTS. UNSETTLED TO ACTIVE LEVELS ARE EXPECTED DURING 18 - 21 NOVEMBER, ALSO DUE TO RECURRENT CORONAL HOLE EFFECTS. .