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FAI, HOPE, and Clarity

Hugh Hudson

UC Berkeley; Glasgow

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Flare precursor behavior has a ubiquitous property,

which GOES detects, and which we capture with two

acronyms:

​

• HOPE = Hot Onset Precursor Event: base difference

• FAI = Flare Anticipation Index: running difference

​

The key: Mapping GOES [XRSA, XRSB] to [EM, T]

in the time domain.

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Berkeley 12 August 2025

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Is Gordon Hurford in the audience?

If NO

Slide set A

(16 slides)

If YES

Slide set B

(28 slides)

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HOPE

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• SOL2011-08-09 (X6.9) has a

HOPE lasting for minutes

• The GOES T vs. GOES EM

“loopdeloop” plot has an

apparently smooth horizontal

branch.

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FAI

https://www.astro.gla.ac.uk/users/hsh3f/scratch/fai.html

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FAI

https://www.astro.gla.ac.uk/users/hsh3f/scratch/fai.html

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Flare precursor phenomena

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  • Filament activation (the Skylab astronauts)
  • Pre-event microflares (Harrison cartoon, see Archive)
  • Dm spike burstss
  • Preflare EUV dimmings
  • “Turbulent” line broadening (Harra)
  • Faint microwave emission (Van Hoven & Hurford 1984)
  • Global coronal shear flows
  • …
  • Hot onset precursors (Hudson et al. 2021, Silva et al. 2023, Battaglia et al. 2023, Telikicherla et al. 2024, Hudson 2025)… “HOPE”

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Description of the HOPE [EM, T] correlation trajectory

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History of the [EM, T] graphic

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About HOPEs

  • Virtually every flare has a HOPE. It is a flare prerequisite.
  • The initial temperatures (the “horizontal branch”) tend to be at 10-15 MK
  • The HOPE precedes the hard X-ray ”impulsive phase” (Kane) and represents different plasma physics
  • It is not “pre-heating”; we can’t measure dT/dt
  • It is not the Neupert effect
  • It is not a deflagration wave in a continuous volume, but rather a set of independent (?) footpoint excitations

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Questions

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• The HOPE phase is the energy buildup time

- why so brief an interval?

• HOPE represents the actual flare instability

- why not an Alfvénic time scale?

- why does the impulsive phase get all

the energy?

• Is this a Sweet-Parker phase?

- why does it proceed so rapidly?

• What regulates the plasma temperature?

• Is this a deflagration wave, reflecting slow

restructuring and flux transfer? - No

• Are there radio signatures?

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Who made this movie?

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Standard GOES HOPE plots

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• SOL2022-03-30T standard plots

• GOES data are limited by time confusion

• HOPE is normal in this flare but there is

an overlapping microflare

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The “Faint Frontier”

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• Solar observations typically occupy the bright

end of any parameter space

• The HOPE development emphasizes the

faintest detectable variations

• At radio wavelengths, one solar flux unit (SFU)

equals 104 FU (10-26 W/m2Hz)

• In HXR the quiet Sun is very dark (cf. NuSTAR)

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Future observations should take advantage of

this untouched parameter space.

​

​

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A A-class flare

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• GOES-R new technology from 2017

• This flare lists at B1.6 but really it’s A9

• The new GOES/XRS has pluses and minuses

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A9 flare with error bars

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• 1-minute averages in GOES-R data

with empirical errors: there is

HOPE in microflares

SOL2020-12-31T15:00

XRSB / 5

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Conclusions

• HOPE is universal as seen in soft X-rays

• It leads to a reliable flare “nowcast”, FAI

•  HOPE physics is fundamental to flaring

• Generally, there is a broad landscape of

unexplored parameter space in solar

observation of μwave and HXR at the

“faint frontier” : we need FOXSI!

​

​

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Is there HOPE in filament eruptions?

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Su & van Ballegooijen 2012

(polar-crown “stealth” CME)

• Maybe. But one swallow does

not make a summer (Carrington)

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Some “radio HOPE” history

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• Interferometer sensitivity (better than 1 SFU!)

• Circular polarization diagnostics

SOL1980-03-23 (C7)

10.6 GHz

Van Hoven & Hurford (1984)

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Radio HOPE mechanisms

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• Free-free emission: far too bright

• Coherent radiation: mm waves unlikely

• Thermal gyroresonance: ditto

• Gyrosynchrotron: no HXR

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We really have no idea!

​

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Survey of NoRP data: all X-class �flares with Fermi HXR coverage

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1 2 4 9 17 GHz

SOL2011-02-15T01:56 X2.2 01:47:00 01:44:00 - 2 - -3 - SFU

SOL2011-03-09T23:23 X1.5 23:19:00 23:17:00 - - 7 13 2

SOL2011-09-06T22:20 X2.1 22:16:00 22:12:00 1 2 - - -

SOL2011-09-07T22:38 X1.8 22:35:20 22:33:00 - - - 2 -

SOL2012-03-05T04:09 X1.1 chaos in all bands!

SOL2012-10-23T03:17 X1.8 03:15:00 03:10:00 - - - 2 25

SOL2013-05-14T01:11 X3.2 01:00:00 no rHOPE

SOL2013-05-15T01:48 X1.2 01:32:30 01:27:30 25 20 11 16 -

SOL2013-10-28T02:03 X1.0 no rHOPE

SOL2014-02-25T00:49 X4.9 00:41:00 00:36:00 - - 2 - -

SOL2014-10-19T05:03 X1.1 04:18:00 04:10:00 2 4 7 15 15

SOL2015-05-05T22:11 X2.7 22:07:00 no rHOPE

SOL2022-04-17T03:34 X1.1 03:22:30 no rHOPE

SOL2023-01-06T00:57 X1.2 00:55:00 no rHOPE

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Survey Conclusions

  • rHOPE signatures do exist in the NoRP data: in our NoRP X-class sample, 4/13 hits
  • There is little evidence for coronal activity in the rHOPE phase, based on eCallisto

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Another example

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• SOL2014-09-10 IRIS observations

(Gou et al., 2023)

- note precursor dimming

• RSTN possibly to 15.4 GHz

• But this X1.8 flare is still marginal

via RSTN

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HOPEs at radio wavelengths

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HOPE clearly detectable in RSTN data

- Far above free-free level

- High-frequency cutoff?

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Yet another RSTN example

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SOL2002-07-23 with NoRP

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