White-Light Flares: TRACE and RHESSI Observations

H. Hudson (UCB),

T. Metcalf & J. Wolfson (LMSAL),

L. Fletcher & J. Khan (Glasgow)

The First Observation of a Solar Flare

• Carrington 1859

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Terrestrial response to 1859 flare

White-Light Flares

• “White light” is formed deep in the solar atmosphere and is therefore energetically important (also note “white-light prominences”)

• White-light (and UV) continuum emission associates well, at least in part, with the hard X-ray impulsive phase

• There are some observations from space prior to TRACE (Yohkoh; Matthews et al., 2003), but TRACE and RHESSI are better

• There are now also infrared observations of solar flares (1.56, the “opacity minimum”; Xu et al. 2004)

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The Wilson depression and tilted fields

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Contribution functions

• A given feature in the spectrum forms over a range of [heights], following the transfer of radiation in a given model (the contribution function)

• Work reported by H. Uitenbroek at SPD (May 2005) generalizes this concept to response functions dependent on temperature or magnetic changes in a dynamical model

• One-D radiation hydrodynamics modeling (e.g., Allred and Hawley) should account for this subtlety, and it may be less important for the continuum

Observational questions to be asked for WLFs

• What are the observational characteristics of the new TRACE capability?

• Is the white-light/hard X-ray association always there?

• Do the TRACE broad-band observations differ morphologically from earlier observations?

• Do white light and UV sources match?

• Is white-light emission present in every flare in a proportional manner?

Initial database

• TRACE catalog => events with <10 s cadence in white light and full resolution, GOES C and above

• Total event list consists of 33 events during RHESSI operations through 2004

• 11 well-covered events (X: 0; M: 7; C: 4)

• All 11 events have TRACE WL response

• All 11 events have RHESSI hard X-ray response

Initial database

• TRACE catalog => events with <10 s cadence in white light and full resolution, GOES C and above

• Total event list consists of 33 events during RHESSI operations through 2004

• 11 well-covered events (X: 0; M: 7; C: 4)

• All 11 events have TRACE WL response

• All 11 events have RHESSI hard X-ray response of course!

TRACE

• The TRACE white light channel has significant contributions from 1700 Å to 1 m

• The TRACE pixel size is 0.5”

• TRACE WL images show solar granulation, sunspots and faculae similar to other imaging at short wavelengths

• UV contributions to the white light channel can be reduced by subtracting the TRACE 1700 channel.

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TRACE spectralResponse

Solar-cycle modulation(Lean, 1997)

RHESSI-TRACE WLF event list

25-JUL-02 03:55:00 03:58:00 04:03:00 C2.7 TRACE RHESSI 0039 S13E46 3-sigma 26-JUL-02 18:57:00 19:03:00 19:06:00 M1.0 TRACE RHESSI 0044 S21E21 4-OCT-02 05:34:00 05:38:00 05:41:00 M4.0 TRACE RHESSI 0137 S19W09 5-OCT-02 10:39:00 10:46:00 10:48:00 M1.2 TRACE RHESSI ?? ?? 12-NOV-02 17:58:00 18:18:00 18:24:00 C9.9 TRACE RHESSI 0180 S11W75 12-JUN-03 01:04:00 01:30:00 01:52:00 M7.3 TRACE RHESSI 0375 N13W65 23-OCT-03 02:35:00 02:41:00 02:44:00 M2.4 TRACE RHESSI 0484 N03E15 9-JAN-04 01:33:00 01:44:00 01:54:00 M3.2 TRACE RHESSI 0537 N02E49 22-JUL-04 00:14:00 00:32:00 00:43:00 M9.1 TRACE RHESSI 0652 N03E17 24-JUL-04 00:34:00 00:39:00 00:41:00 C1.6 TRACE RHESSI N12W03 24-JUL-04 13:31:00 13:37:00 13:45:00 C4.8 TRACE RHESSI 0652 N04W16

• C7.8 13-Feb-02• WL contrast ~20%• noise @ 1.8% per pixel• WL observed during RHESSI 25-50 keV• UV contours relatively extended• WL sources are patches unresolved by TRACE

WL+1700A WL difference

Six events from survey, 128 arc s TRACE images

WLF is impulsive: M4.9

GOES

TRACE WL

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TRACE WL TRACE 1700 A

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A gradual white-light event: WL occursin the impulsive phase

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A gradual white-light event: RHESSI overlay

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TRACE WL emission from loop tops

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Loop-top WL emission

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World’s record white-light flare: a C1.3!!

GOES 1-8Aderivative

Raw WL Difference

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Conclusions

• WL emission, as seen by TRACE, shows rapid small-scale features and closely resembles the UV morphology

• The data confirm the impulsive-phase association with hard X-rays, also found for extended non-thermal phase

• Sources may be extended in area, but include many tiny (arc-s scale) emission patches

• The data are consistent with the idea that white light correlates with total flare energy

• Extended events show similar morphology and don’t require further classification

• Contrasts can exceed 100%

Observational questions half-answered

• What are the observational characteristics of the new TRACE capability? Great

• Is the white-light/hard X-ray association always there? Yes

• Do the TRACE broad-band observations differ morphologically from earlier observations? Somewhat

• Do white light and UV sources match? TBD

• Is white-light emission present in every flare in a proportional manner? Yes

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Tentative findings

• WLF contrasts in the very broad-band TRACE response do not differ much from Carrington’s => low temperatures

• All flares are WLFs

• TRACE WL emission can be seen in loops as well as footpoints

• WLF differs morphologically from UV emissions: more concentrated, shorter durations (nb must be careful about the “island in the tide” effect)

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Important open questions

• What can we learn about WLF spectra from the new data (not only TRACE, but MDI, GONG, IVM, etc…)?

• How can we get around the photometric limit of granulation/p-mode noise, plus image jitter?

• What will the improved knowledge of flare energetics tell us?