Observing Sequences in View of the SECCHI Science Goals From the EUVI Perspective

1. Observing Sequences in View of the SECCHI Science Goals From the EUVI Perspective J-P. Wülser

2. Introduction It is not too early to plan observing strategies: • S/C separation angle and telemetry change  character and focus of STEREO evolves  missed opportunities don’t reoccur  need mature observing strategy at launch • Observing strategy influences flight software design • Telemetry constraints call for innovative observing strategies to achieve science goals

3. Science Goals (1) • Science goals “Physical Evolution of CMEs” (especially through the outer corona), and “CME Interaction with Heliosphere” need long duration, uninterrupted observations at modest image cadences  Synoptic observing program • Science goals “Initiation of CMEs”, and “Physical Evolution” through chromosphere and low corona need high cadence, high spatial resolution data • The CME initiation process is critical for understanding the CME as a whole

4. Science Goals (2) What image cadence is needed? • Phase A report states 60 s cadence at 3” pixels as a requirement for CME initiation studies • SXT observations with 23 s cadence show acceleration of erupting structure • TRACE observations during CME initiation show rapid changes in the low corona that are barely resolved at 20-40 s cadences  Difficult to achieve at 42 kbit/s telemetry rate

5. Synoptic Observing Sequence(modified from Phase A report)

6. Event Driven Observing Sequences (1) • High cadence only required during relatively short CME initiation / early acceleration period • Automatic detection of CME feasible w/C’graph Strategy: • Observe at high cadence into a rotating buffer • Stop overwriting buffer after CME detection (or x minutes after CME detection) S/C has agreed to provide two SECCHI partitions within its Solid State Recorder: one for synoptic data, and one for event data (overwritable)

7. Event Driven Observing Sequences (2) • Rotating buffer downlinked at every DSN contact (contains either event, or most recent data) • At 300 km/s, CME travels 1.5 Rsun in 1 hour, or in 2 hours, in case of constant acceleration • Overwritable rotating buffer should be sized to hold at least 2 hours worth of high cadence data • Examples assume the following partition sizes: 80% synoptic, 20% rotating event buffer

8. Synoptic + Event Observing Sequence Example 1: Nominal Telemetry

9. Synoptic + Event Observing Sequence Example 1: Nominal Telemetry • Table shows • Sum of event buffer telemetry and synoptic telemetry • Synoptic telemetry alone • Synoptic EUVI & Cor1 cadences slightly reduced • Rotating event buffer receives telemetry for • Full resolution 1600x1280 EUVI images every 37.5 s • Double cadence in 3 other EUVI channels • Quadruple cadence (2.5 min) in Cor1 • Capacity of rotating event buffer: 2.2 hours

10. Synoptic + Event Observing Sequence Example 2: High Rate Telemetry

11. Synoptic + Event Observing Sequence Example 2: High Rate Telemetry • Example assumes that twice the nominal telemetry is available early in the mission • Higher synoptic EUVI, Cor1, Cor2 cadences. 1/4 resolution EUVI data replaced with 1/2 resol. • High cadence buffer receives telemetry for • Full resolution (1600x1280) EUVI images every 30 s • Half resolution (800x640) EUVI images every 90 s in additional channel • Capacity of rotating event buffer: 2.4 hours