Lessons Learnt from SOHO: CME Onsets

Lessons Learnt from SOHO: CME Onsets

CME Properties:

1011 to 1013 kg

50-2000 km/s

Average span 45o

Significance:

- Coronal evolution

- Space weather

“We should be able to sort this one out within 5 years” B.C.Low in 1985.

What Have We Done? - JOPs 3 & 67

Basic Aim: EUV spectroscopic information of CME source region

EIT

CDS

LASCO

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?

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…)

A Typical Event - EUV Dimming

Harrison & Lyons, 2000, A&A, 358, 1097 Harrison, 1997, ESA SP-415, 121

Other Events

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)

Coronal Dimming and CME Onsets - Harrison, Bryans and Lyons, 2001

Dimming: Mass Loss

Mass (kg) calculated from Si X ratio or DEM method:

Date Si X Mass DEM Mass LASCO Mass

Jul 16 1997 1.1x1011 4.0x1010

5x1010

May 8/9 1999 1.8x1012 1.0x1011

3x1011

Jul 4, 1999 1.1x1012 6.2x1011

1x1011

Jul 25, 1999 2.4x1012 4.7x1011

3.5x1012

Feb 8, 2000 3.3x1012 2.9x1011

-

Feb 19, 2000 8.6x1013 3.4x1013

1.1x1012

Aug 19, 2000 8.4x1011 3.9x1011

4.7x1011

Coronal Dimming and CME Onsets - Harrison, Bryans and Lyons, 2000

Dimming Location (PA)/Onset (UT)

Date CDS LASCO

Jul 16 1997 256-284 at 10:18 259-275 at 10:50

May 8/9 1999 65-80 at 23:09 58-78 at 23:31

Jul 4 1999 <277-295 at 07:29 323-350 at 06:15

Jul 25 1999 300-310 at 12:55 232-360 at 13:20

Feb 8 2000 90-110 at 15:48 -

Feb 19 2000 250-260 at 07:51 260-292 at 07:40

Aug 19 2000 221-231 at 07:07 237-245 at 07:56

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.]

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.

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.