1. Lessons Learnt from SOHO: CME Onsets CME Properties: 10 11 to 10 13 kg 50-2000 km/s Average span 45 o Significance: - Coronal evolution - Space weather “We should be able to sort this one out within 5 years” B.C.Low in 1985.

2. What Have We Done? - JOPs 3 & 67 Basic Aim: EUV spectroscopic information of CME source region EIT CDS LASCO

3. Why? - Part 1 Basic Information: Where is the source structure of the CME? Can’t do this with an occulting disc! Have not done too well with imagers! When did the eruption start? Can’t do this with an occulting disc! What are the plasma characteristics (density, temperature, flow history; mass change; timing; bright spots or jets?). Can’t do all of this with a broad-band imager?

4. Why? - Part 2 CME Model Discrimination: Flares? Do we see density/temperature signatures of such a driver? Gradual? Loop-loop interactions/reconnection sites? Location of mass removed Do we see other coronal or non-coronal signatures? (jets, hot-spots, etc…)

5. A Typical Event - EUV Dimming Harrison & Lyons, 2000, A&A, 358, 1097 Harrison, 1997, ESA SP-415, 121

6. Other Events

7. Coronal Dimming and CME Onsets - Harrison, Bryans and Lyons, 2001 Dimming detected in all 7 events, but degree of dimming in each line (temperature) varies considerably. No consistency between lines. Average dimming and range given below: He I 584 Å (20,000 K) = -20% (range 0-82%, 4 cases of 0) O V 629 Å (250,000 K) = -23% (range 0-70%, 4 cases of 0) Mg IX 368 Å (1 million K) = -22% (range 10-40%, no cases of 0) Si X 347 Å (1.3 million K) = -22% (range 4 to 71%, no cases of 0) Si X 356 Å (1.3 million K) = -23% (range 9 to 62%, no cases of 0) Fe XVI 360 Å (2 million K) = -30% (range 2-87%, no cases of 0)

8. Coronal Dimming and CME Onsets - Harrison, Bryans and Lyons, 2001 Dimming: Mass Loss Mass (kg) calculated from Si X ratio or DEM method: DateSi X MassDEM MassLASCO Mass Jul 16 19971.1x10 11 4.0x10 10 5x10 10 May 8/9 19991.8x10 12 1.0x10 11 3x10 11 Jul 4, 19991.1x10 12 6.2x10 11 1x10 11 Jul 25, 19992.4x10 12 4.7x10 11 3.5x10 12 Feb 8, 20003.3x10 12 2.9x10 11 - Feb 19, 20008.6x10 13 3.4x10 13 1.1x10 12 Aug 19, 20008.4x10 11 3.9x10 11 4.7x10 11

9. Coronal Dimming and CME Onsets - Harrison, Bryans and Lyons, 2000 Dimming Location (PA)/Onset (UT) DateCDSLASCO Jul 16 1997256-284 at 10:18259-275 at 10:50 May 8/9 199965-80 at 23:0958-78 at 23:31 Jul 4 1999<277-295 at 07:29323-350 at 06:15 Jul 25 1999300-310 at 12:55232-360 at 13:20 Feb 8 200090-110 at 15:48- Feb 19 2000250-260 at 07:51260-292 at 07:40 Aug 19 2000221-231 at 07:07237-245 at 07:56

10. Principal Conclusions 1. EUV dimming is due to mass loss from low corona associated with CME onset. 2. Mass and location of dimming events are consistent with ‘source region of CME’. So what? - Identification of mass/source (have had enough trouble doing this over the years!) - Plasma parameters provide evidence for processes leading to eruption. [See Harrison, Bryans and Lyons, 2001.]

11. But… 1. Really need magnetic data for source region as well, but current campaign is (necessarily) on/near the limb. The geometry of the STEREO mission will allow this. 2. 2-D view limits our ability to understand the magnetic configuration and the true association of events and structures. The twin-platform STEREO view will provide this for the first time. 3. The SOHO cadence is, at best, 7 min for EIT and 45 min for the full CDS spectral scans. STEREO does not have the spectroscopic information (pity!) but the EUVI cadence of down to an OM better plus better spatial resolution provides a good basis for improvement of observation of the spatial and temporal characteristics of the dimming.

12. STEREO CMEO JOP EUVI and COR1/2 at best cadence, covering the Earth-directed limb. Cycle all EUVI bands, if possible out of sync between spacecraft! Coordinate with magnetic data (ground-based, SOHO?, Solar-B) Coordinate with EUV spectral data from Solar-B or even SOHO. Perform image subtraction on board to produce dimming Flag (test this with EIT data) if operational change required (e.g. to have improved cadence on small field) or if to be used for CME prediction.