SWPC CME Imaging Requirements for the post-SOHO era

Douglas Biesecker3/25/2014

CME General RequirementsRequirement Threshold GoalData Continuity 67% duty cycle in any 45 minute window

Contamination All requirements shall be met during the following conditions:• >10 MeV integral proton fluxes <=

1x105 pfu (S5 Radiation Storm)

Spectral Response Any white-light bandpass, that is, lie within 5000-8000 Angstroms

Polarized Brightness The use of polarization techniques to separate the K-corona signal from other signals may be provided

Pointing Knowledge Line of sight 25 arcsecDirection of solar north 1 deg

Flux resolution ≤1x10-12 B/Bsun/pixel at outer radius ≤2x10-11 B/Bsun/pixel at inner radius

Straylight Measured at the midpoint of the FOV shall not exceed 5x10-11

CME Measurement RequirementsRequirement Threshold GoalSampling Frequency* 1 total brightness image per 15 minutes 1 total brightness image per 10 minutes

Inner/Outer FOV cutoff* 3-17 solar radii

Maximum CME speed 3400 km/s†

# of CME images 3 4

Measurement Range 1x10-11 – 1x10-8 B/Bsun/pixel 1x10-11 – 5x10-8 B/Bsun/pixel

Flux Measurement Accuracy 10%

Data Latency 15 minutes 10 minutes

*note, there is a direct correlation of sampling frequency and FOV. Should one be made larger, the other must be made larger as well. The driver’s are how many CME imagesyou need and what’s the fastest plane of sky speed you can expect.

†CDAW LASCO CME catalog for 10 Nov 2004

ΔFOV(in solar radii) = (# of CME images)(CME speed)(sampling frequency)(60)/(695980km)e.g. ΔFOV = (3*3400km/s*15min*60s/min)/695980km = 13.2 solar radii

Data Continuity

67% duty cycle in any 45 minute window• The worst case scenario means we need at least 2

images of every CME.• The proposed requirement is specific to the

current set of requirements– A duty cycle of 67% means getting 2 of 3 images in

any 45 minute period.• I realize there’s a 1 image ambiguity (intentional) It really

only takes 30 minutes to get 3 images

• Can this be generalized?

Contamination

No more than 10% of pixels in an image shall have a count rate more than 5 sigma above the background during a solar radiation storm reaching the S5 level.• The need is to be able to identify the CME edges

– A rough study by Biesecker (also Thernisien) showed that if 20% of an image is significantly contaminated, then resolving the edges becomes difficult

– Since the normal mode of viewing is difference images, this limits the contamination per image to 10%.

– We use the threshold of an S5 event, which has never been observed• 5 min avg, ≥10 MeV ions, reaching 105 particles/sec/sr/cm2

– Alternatively, Llebaria et al 1998 showed that if fewer than 30% of pixels are contaminated, then the image can be corrected

Spectral Response

The bandpass selected shall be ‘white-light,’ that is, lie within 5000-8000 angstroms• Based on the fact that all forecasting to date,

and really the definition of a CME itself, is in the visible light portion of the spectrum

Polarized brightness

The ability to use polarization techniques to separate the K-corona signal from other signals may be provided• The benefits

– Automatic rejection of most of the F-corona (in the FOV in question)

– Narrower scattering function– Total brightness can still be computed and the difference

between B and pB is effectively an additional view (well, maybe 2.5d instead of 2)• But is anybody out there really using it to great effect?• It adds to the cost of the instrument, so I need a good reason to

have it.

Pointing Knowledge

The line of sight pointing knowledge shall be 24 arc-sec.The direction of solar north shall be known to within 1 degree• In order to accurately characterize a CME for

input to Enlil, I need to know where the Sun is pretty accurately. That’s trivial to do, as we get plenty of stars to calibrate against. So, this is overspecified, but not a big deal.

Flux Resolution

The measurement precision (or flux resolution) shall be ≤1x10-12 B/Bsun/pixel in the outer part of the FOV. In the inner part of the FOV the flux resolution shall be ≤2x10-11 B/Bsun/pixel.• Need to have sufficient resolution to identify

CME leading edge• Differs across FOV as the background (and

CME brightness) both vary with radial distance

Straylight

The straylight measured at the focal plane at the midpoint of the FOV shall not exceed 5x10-11 B/Bsun• Need to keep straylight manageable. If it’s

constant, it’s easy to remove. Requirement assumes it’s not constant, so we have to beat it down.

Sampling Frequency

A cadence of at least one full FOV image in 15 minutes• As noted, cadence is a strong function of FOV.• Higher cadences, with same FOV, offer advantages– Easier to separate overlapping CMEs– More images to pick ‘best’ ones for analysis– Provides margin against even faster CMEs

A cadence of at least one full FOV image in 10 minutes

Inner/Outer FOV Cutoff

The field of view (FOV) shall be an annulus, centered on the Sun.The inner radius of the annulus shall be 3.0 solar radii.The outer radius of the annulus shall be at least 17 solar radii.• The inner FOV is set ‘high’ because

– CME’s are still evolving (self-similar expansion assumption has not yet set in)

– Makes straylight rejection easier– Not too high, though, as overlapping CME’s still need to be

distinguished as individuals• The outer FOV needs to be far enough out to allow enough

images of the fastest CME.– We bias towards larger fields of view as we want the CME asymptotic

speed. Acceleration in the low corona is not important.

Maximum CME Speed

3400 km/s• A ‘derived’ requirement• This is the fastest CME in the LASCO era– Linear speed for 10 November 2004 West Limb

CME

# of CME Images

3• The minimum number of CME images needed

is 2, to derive direction, width, and speed• Setting it to 3 gives us some margin• You’d really like to have more– That drives cost

4

Measurement Range

The measurement range shall be ≤1x10-11 B/Bsun at the outer radius of the FOV. The measurement range shall be ≥2.6x10-9 B/Bsun at the inner radius of the FOV. • Driven by K+F+CME• Depends on the inner/outer FOV cutoff

Flux Measurement Accuracy

The data shall be calibrated to an absolute accuracy of 10% at the midpoint of the FOV.• Need isn’t there today, as all inputs are

morphology driven• Mass inputs to models are clearly needed and

we need calibration accuracy sufficient to meet those needs

Data Latency

Data latency from the time an exposure finishes to the time of delivery to NOAA/SWPC shall not exceed 15 minutes• This matches the image cadence, meaning we don’t

require on-board storage. • Operations assumes constant telemetry• This plays into lead time for extreme events.Data latency from the time an exposure finishes to the time of delivery to NOAA/SWPC shall not exceed 10 minutes