The flare-CME relationship – determining factors (if any!)

Sarah Matthews, Lucie Green, Hilary Magee, Louise Harra & Len Culhane MSSL, University College London

The flare-CME relationship – determining factors(if any!)

Sarah Matthews, Lucie Green, Hilary Magee,Louise Harra and Len Culhane

MSSL, University College London

Many surveys have been conducted in an attempt to understand the relationship between flares and CMEs-if indeed there is any.

•Skylab gave the impression that X-ray events fell into2 classes: - long duration events with CMEs - short duration events without CMEs

•Sheeley et al. (1975), Kahler et al. (1977), Sheeley etal. (1983), Burkepile et al. (1994), Munro et al. (1979)& Green et al. (2001) (amongst others) have concludedgenerally that there is an increased CME probabilityfor increasing X-ray intensity and duration.

But clearly there is not a one-to-one relationship,e.g. 6 January 1997 was associated with very weakcoronal activity…

….but relatively long duration

And there have been a number of X-class flareswith no associated CME as reported in e.g.Feynman & Hundhausen (1994), Gaizauskas etal. (1998) and Green at al. (2001)

The question:

What are the factors that actually determine whether a flare will be accompanied by (or accompany!) a CME? And can they help in prediction of solar eventsand extrapolation to the stellar case?

A confined X-class flare

•Magnetic topology clearly plays an important rolesince the field must be opened to produce a CME

•Moore et al. (2001) suggest that a simple model can explain both confined and eruptive flares in sigmoidalbipolar regions with closed field

–> when no eruption occurs it is as the result of the constraints of the overlying field. Only flare heatingoccurs and flare durations are shorter.

Green et al. 2001 have studied an X1.2 event whichoccurred on 30 September 2000 at 23:13 UT onthe West limb. There was no CME detected withthis event.

LASCO difference images show some streamerbrightening, but no front moving out spanning the flare location.

EIT 195 Å images show the evolution of the flareregion and also indicate no opening of the field.Localised dimming is seen only in the flare regiondue to temperature change.

Morphology of the region before and after the flare asseen in SXT. Loop A is the main flare loop.

Expansion is observed above the main flare loop towardsthe south west, the loop is decelerating with v=411->58 km/s.Expansion occurs during the impulsive phase but shows no evidence that any loops open.

From Melrose (1997)

The most probable flare scenario seems to be the interaction of two pre-existing loops in similar scenarioto that proposed by Melrose (1997).

In this case reconnection produces 2 new closed loopsand appears to have no effect on the overlying field,prohibiting eruption.

What about the loop expansion?

->the reconnection produces a magnetic loop which is not fully extended and the magnetic restoring force causes the loop to move to its equilibrium positionproducing the apparent expansion seen in SXT.

Despite being energetically large this flare doesn’t produce the associated eruption we have come to expect,so clearly while large X-ray intensity may be a necessarycondition it is not a sufficient condition for associatederuption.

But is the constraining factor the overlying field or the flare topology? Are all loop-loop interaction flares confined?

While CMEs clearly can occur in association withflares of any duration, there seems to be a tendencyfor them to occur in relation to longer events

- is this a determining factor?

Since there have been a number of very small X-rayintensity events which have been associated with quite large CMEs this could be a useful relationship.

Also, if such a relationship exists can it be used toinfer the occurrence of stellar CMEs in association withstellar flares?

Using data from the period following the launch ofSOHO we are revisiting the flare intensity/durationrelationship to CME occurrence.

•Total of 104 flares of GOES class A to X

•67 events with associated CMEs identified in LASCO and EIT

•12 stellar flares observed by ROSAT & EXOSAT

•Flare durations measured from GOES in 1-8Åchannel for the solar case

•Durations from the published light curves for thestellar case.

•Intensity vs duration for all solar flares - * represent flares with associated CMEs - represent flares without CMEs•Greater scatter in duration at high energy•All flares lasting > 300 mins have associated CMEs

•Peak intensity – duration plot for flares with CMEs•Little evidence of correlation•Wide range of flare durations, especially at highenergies.

•Intensity – duration plot for flares without CMEs•Clear correlation between intensity and duration•Power-law relation with best fit index = 1.50.2

•Stellar flare data from ROSAT and EXOSAT•Peak luminosities from the 0.04 to 2 keV range•Similar power-law relationship to flares without CMEs•Steeper power-law index = 2.1 ± 0.2

Preliminary conclusions…

•No clear relationship between flare intensity andduration for flares associated with CMEs

•Clear correlation for non-eruptive events with higher energy events tending to be longer duration

•Absence of this relationship for flares with massejection suggests the mass ejection affects energy release in the decay of the flare

Future…

•Relationship between duration, peak temperature, EM and magnetic field strength in both solar & stellar flares.

Summary

•The relationship between flares and CMEs isclearly complex – which we already knew!

•The topology of the flare and of the overlying field must play a defining role in determining whether mass is ejected or not, but how are theyrelated?

•Moore et al. (2001) suggest that it is the overlyingfield which determines both the eruptive natureand duration of the flare – is this the only factor?

•STEREO and magnetograms should help us to better understand the topology of both the flare and the overlying field