1. Lessons learnt from the STEREO Heliospheric. Imagers: Tracking
Lessons learnt from the STEREO Heliospheric Imagers: Tracking & modelling CMEs from Sun to Earth Richard Harrison Jackie Davies Chris Davis Chris Eyles Steve Crothers

2. The heritage Coronagraphs (OSO, Skylab, SMM, P78-1, SOHO, STEREO)
Helios photometers
SMEI (Coriolis)
Coronal Mass Ejections (CMEs) – Discrete solar ejecta, ‘typically’ 1012 to 1013 kg of matter expelled at a few 100 kms-1.
The challenge: Identify and predict the impact of Earth-directed CMEs.

3. STEREO Concept

4. Heliospheric Imager
Concept: Wide-angle imager with occultation & baffle system; light rejection of ~ of the solar brightness.
Consortium: RAL, University of Birmingham, CSL and NRL
Principal elements of the design: CFRP structure and baffles, CCD wide-angle camera system, passive cooling, one-shot door, mounted on side of spacecraft
Harrison et al., 2008, Solar Phys. 247, 171
Eyles et al., 2009, Solar Phys. 254, 387

5. Door
Radiator
HI-1
Side & rear baffles
Door latch mechanism
HI-2
Inner Baffles
Forward Baffles
Direction of Sunlight

6. < 0.1 CME intensity at inner edge of FOV
Launch – Oct ; Doors opened – Dec (HI-A) & Jan (HI-B); Scientific Operations – from Apr 2007
HI-1
HI-2
Direction of Centre of FOV
14 degrees
53.7 degrees
Angular Field of View
20 degrees
70 degrees
Angular Range
4-24 degrees
degrees
Image Array (2x2 binning)
1024x1024
Image Pixel Size
70 arcsec
4 arcmin
Spectral Bandpass nm nm
Nominal Image Cadence
40 min
120 min
Brightness Sensitivity (Bo = solar disk)
3 x Bo
3 x Bo
Straylight Rejection
3 x Bo
10-14 Bo
< 0.1 CME intensity at inner edge of FOV
~ magnitude 12; governed by anticipated CME intensity at outer edge
[Adapted from Socker et al.]

7. You are here!
Smile – we are photographing you every 2 hours!

9. The events of 7 March 2012

10. The view from STEREO A

12. Apparent acceleration at large elongations – the ‘Fixed Phi’ approximation
HI pixels
Elongation angle φ
Sun A
Ho~1AU
Time
(Sheeley et al., JGR,1999)
Assuming the CME propagates radially (φ constant) and at a constant speed;
(φ)
(φ)

13. Geometrical models:
(a) Fixed Phi (Alexis Rouillard et al.) – open ‘dots’
(a) Harmonic Mean (Noe Lugaz, Christian Möstl et al.) – filled ‘dots’
(b) Self Similar Expansion (Jackie Davies et al.) [λ: 0 = FP; 90 = HM]

14. (Davies et al., 2012)

15. Triangulation: Self Similar Expansion model using two viewpoints
(Davies et al. in prep)

18. Points to note:
Only scratched the surface! (Forward modelling, Variable
speeds, hybrid codes, drag, deflection, CME-CME interaction, also: CIRs, comets...)
No other instrument has ever imaged CMEs approaching & running over Earth in this way
Note! STEREO is a science mission: the spacecraft continue to drift
Given the importance of understanding & predicting CMEs, this demonstration is ripe for using in an operational scenario (one spacecraft or two; L4/L5; <60o etc...)

19. Spare slides....