Enhanced Particle Acceleration in Plasmoid-Looptop Collision

1. Enhanced Particle Acceleration During a Plasmoid-Looptop Collision Observed with RHESSI and STEREO • Ryan Milligan (CUA/NASA-GSFC) • Collaborators: R. T. James McAteer (TCD), Brian R. Dennis (NASA/GSFC) & C. Alex Young (Adnet/NASA-GSFC)

2. Outline • What is a plasmoid? • Predictions from numerical simulations • Overview of 25 Jan 2007 event as observed by RHESSI and STEREO • Discussion of X-ray source motions • CME kinematics • Summary

3. Plasmoid formation • In the standard flare model magnetic reconnection occurs in the current sheet between an erupting CME and the underlying flare loop • The energy released during reconnection accelerates the CME, particles and heats the local plasma

4. Plasmoid formation • It is possible that reconnection can occur multiple times along CS • Plasmoids form as magnetic ‘islands’ between X-points • Due to density and magnetic field strength decreasing with height, the majority of plasmoids are observed to rise

5. Plasmoid formation • It is possible that reconnection can occur multiple times along CS • Plasmoids form as magnetic ‘islands’ between X-points • Due to density and magnetic field strength decreasing with height, the majority of plasmoids are observed to rise

6. Simulations of Plasmoid Formation • Barta et al. (2008) developed a 2D numerical simulation of plasmoid formation during CME eruption • Upward (top row), downward (middle row) and coalescing (bottom row) plasmoids form during CS tearing • The merging of the downward-moving plasmoid with the looptop results in additional energy release (red boxes) • This should be visible as chromospheric or HXR emission due to enhanced particle acceleration

7. Plasmoid Motions • The direction of motion of the plasmoid depends on the relative reconnection rates above and below the source • This motion is driven by the magnetic tension in the field lines, which is governed by the rate of reconnection • Upward moving plasmoids have a greater reconnection rate below the source (v1B1<v2B2), while downward moving plasmoids have a greater reconnection rate above (v1B1>v2B2)

8. 25 Jan 2007 • Eruptive Event (CME+C6 Flare) above eastern limb • Observed by STEREO-B and RHESSI • Flare footpoints were occulted • Long Duration Event

9. 25 Jan 2007 • Eruptive Event (CME+C6 Flare) above eastern limb • Observed by STEREO-B and RHESSI • Flare footpoints were occulted • Long Duration Event

10. RHESSI images at flare onset revealed a high-altitude coronal source at 5-10 keV (Plasmoid; 06:29 UT) • An underlying source brightened almost 10 minutes later (Looptop kernel that lay above post-flare arcade) • Plasmoid appeared to merge with looptop kernel between 06:37-06:41 UT

11. During the rise phase of the flare, enhanced emission was detected in the 9-18 keV HXR lightcurves* from RHESSI • A similar feature was observed in the 245 MHz band from Learmonth Radio Telescope • Both are evidence for enhanced particle acceleration in the corona • This enhanced emission was concurrent with the time over which the two X-ray sources appeared to merge • Merging and enhanced emission both occurred around the time of peak CME acceleration *RHESSI photon spectrum during merging also implied that >9 keV emission is nonthermal

12. Only one previous report of plasmoid-looptop merging(Kolomanski & Karlicky 2007) • Using Yohkoh and Ondrejov they found enhanced HXR and radio emission during merging • Velocity of downward moving source was 16 km/s; similar to our value of 12/km/s

13. 12-10 keV 12-14 keV 14-16 keV • Previous RHESSI observations of rising plasmoids (e.g. Sui & Holman 2003) show that higher energy emission appears at lower altitudes (the reverse is true for looptop sources) • Evidence that reconnection occurred between it and the underlying looptop • K&K07 found no energy gradient in their plasmoid

14. Imaging over finer energy bands (2 keV) reveals the energy gradient of the sources • Plasmoid source had higher energy emitting from higher altitudes • Looptop source showed no energy displacement with height Note that RHESSI and EUVI images were taken 15 minutes apart

15. 10-12 keV 12-14 keV 14-16 keV • Rising plasmoids appear with higher energy emitted at lower altitudes • Downward-moving plasmoid shows higher energy at higher altitudes • This supports the idea that the dominant reconnection site generates greater magnetic tension in the field lines, thereby driving the plasmoid motion

16. CME Kinematics • Height of the CME front was tracked as function of time (EUVI, COR1, COR2) • h(t) profile differentiated twice using 3-point Lagrangian method: v(t), a(t) • Downward motion of RHESSI plasmoid occurred during CME acceleration phase implying both were driven by the same reconnection in the CS

17. Evidence for change in the accelerator..? • Early spectral analysis revealed distinct decrease in the low energy cutoff (EC) and increase in total electron flux during merging • This was only seen in one detector (#6) • Unreliable due to poorly known calibration in 2007 • Also possibly due to change in relative intensities of the two sources

18. Summary • Evidence is presented for a downward-moving X-ray plasmoid merging with a looptop kernel during CME eruption - the first to be observed with RHESSI • Enhanced (coronal) HXR (9-18 keV) and radio (245 MHz) emission at the time of the merging suggests that additional particle acceleration took place due to a secondary reconnection process in the current sheet between the two sources • The velocity of the downward moving source is in agreement with a similar event observed with Yohkoh but much lower than that predicted by a 2D numerical simulation • The energy gradient along downward plasmoid is opposite to that observed in rising plasmoids suggesting that the dominant energy release site was above, rather than below, the source. • The merging occurred around the time of peak CME acceleration suggesting that current sheet tearing took place

19. Future Work... • Is the evidence for a change in the low energy cutoff real? • Would like to see the same effect in multiple detectors at a time when the calibration is better known. • Does the inflow velocity greatly affect the magnetic tension force, and therefore, the plasmoid velocity and resulting energy released? • SDO/AIA data may help with this. • Is there evidence for chromospheric rebrightenings? • Models predict a secondary brightening at the footpoints as the plasmoid is tied to the same field lines.

20. Least-squares fit to our downward-moving source yielded velocity of 12 km/s (similar to K&K07: 16 km/s) • Velocity predicted by Barta et al. 2008 is of the order of the Alfven speed (few hundred km/s) • Plasmoid velocity is dependent on the magnetic tension in the field lines, which is determined by the reconnection rate