current sheets from wl and uv data: open questions alessandro bemporad inaf – arcetri...

Current Sheets from WL and UV data: open questions

Alessandro Bemporad

INAF – Arcetri Astrophysical Observatory

1° ISSI Group Meeting1° ISSI Group MeetingOctober 23-27, 2006, BernOctober 23-27, 2006, Bern

““CS from WL and UV data: open questions”CS from WL and UV data: open questions”A. BemporadA. Bemporad

1° ISSI Group Meeting1° ISSI Group Meeting

October 23-27, 2006, BernOctober 23-27, 2006, Bern


SUMMARY

FIRST PART: introduction• CS & CME flux rope model• CS observations in WL: some open problems• CS observations in UV: some open problems

SECOND PART: our studies• CS evolution• CME early evolution• Conclusions




FIRST PART: introduction




CS IN CME MODEL

(from Lin et al., 2004, ApJ, 602, 422)

• CME flux rope models predict that, after the eruption of the flux rope (top) and the formation of the current sheet below the expanding bubble, magnetic reconnection, starting from the chromospheric level, relaxes the open configuration into a closed.

• Magnetic reconnection heats plasma (bottom) converting magnetic energy into kinetic and thermal energies.

How do observations comparewith this model?




WL OBSERVATIONS OF CSs

(from Webb et al., 2003, JGR, 108, 1440)

Post-CME CSs in white light images appear as thin radial persistent features which are:

• observable only in ~½ of CMEs• brighten ~4 hours after the CME• observed sometimes within multiple ray-like structures (which one is the CS?)• ~ 6x104 km thick, constant with time• ≥ 8 hours long

These observations leave at present some open questions, i.e.:

1) Why CSs are not observable in all CMEs? (only orientation?)2) Why CSs brighten late? (Density increase? Thickness increase?)3) How can we explain the CSs long duration? (at present reproduced using an empirical density model)→ time dependent theory of driven reconnection needed!

SMM




UV OBSERVATIONS OF CSs

Ciaravella et al., 2002, ApJ, 575, 1116

Ko et al., 2003, ApJ, 594, 1068

Post-CME CSs identified also in EUV coronal emission:

• Strong Fe XVIII emission → CS plasma at high temperatures (~ 5 ּ106 K)• CS have long (~days) lifetime• Elemental abundance fractionation similar to that of the ambient corona (FIP effect) → CS material brought in from its sides• In the low corona rising post-CME loops are located at the lower end of the CS

These observations leave at present some open questions, i.e.:

1) Why some CS are observed in UV, not in WL (Ciaravella et al. 2002)?

2) How can we explain such high T? (only magnetic reconnection?)




SECOND PART: our studies




CS EVOLUTION

On November 2002 UVCS observed a post-CME coronal reconfiguration over ~2.3 days → ideal case for the study of the CS evolution

UV images of the low corona show a post-CME rising loop system (in agreement with CME models); these bridge over the solar limb → CS ║ to the plane of the sky

The UVCS slit FOV cuts across the CS; selected spectral ranges include lines from “low temperature” (105.9 K) and high temperature (106.7 K) ions.What is the observed CS line intensity evolution?

(from Bemporad et al., 2006, ApJ, 638, 1110)

LASCO/C2 EIT Fe XII




CS EVOLUTION

At CME latitudes a strong persistent Fe XVIII emission is observed during the whole observation interval

Fe XVIII (Tmax=106.7) line intensity increases reaching its maximum about 21h after the CME initiation, then decreases, while Fe XV (Tmax=106.3) intensity increases continuously

By a comparison of intensities of these two lines from different ionization states of the same element we infer the CS temperature evolution, while with a different technique we estimate the CS density.




CS EVOLUTION: Te and Ne

Results:

1. CS temperature > 8 ּ106K immediately after the CME; then in the following days the CS plasma slowly cools down

2. CS density keeps approximately constant (~7 times larger than the ambient coronal density)

In the computation we assumed a constant CS thickness (as observed in

WL data).




CS EVOLUTION: PLASMA HEATING

eOUT

eCS

1

OUT

CS

T

T

N

N

e

e

Plasma heating in the CS can be provided by ohmic heating, adiabatic compression and other processes (wave heating, shocks, etc…)Given density & temperature inside and outside the CS, we check whether the CS plasma is heated via adiabatic compression. In this case:

→ At a late stage of the event adiabatic compression can provide for the high CS temperatures, while at earlier times other processes should be invoked.




CS EVOLUTION: FURTHERQUESTIONS AND FUTURE WORK

• Which heating processes dominate the CS evolution?• Is there any theoretical prediction on Te and Ne evolution in CS?• Which are the physical CS plasma conditions in its early stages?• How long the CS thickness can be considered really constant?• So far each CS has been observed in UV at a single altitude: are there data that allow us to check on the CS behaviour at different altitudes?• How does density that we found from UV compare with CS densities derived from WL? Is it possible to provide physical CS parameters along it length using, e.g., UVCS, Mauna Loa, LASCO/C2/C3?

• FUTURE WORK: make a comparative UV – WL study of a CS! needed a model describing temporal evolution of CS physical parameters!




EARLY CME EVOLUTION

In CS/CME flux rope models the high temperature plasma heated in the CS is injected in the outer shell of the CME bubble. At the same time, mainly because of adiabatic expansion, internal shells cool down.

As a consequence a temperature decrease from the CME outer shell to the CME core (built up of “cool” chromospheric material) is expected .

However we never observed in in situ data a high-low-high temperature variation when crossing an ICME…why? (geometry?)




EARLY CME EVOLUTION

On January 2000 the early evolution of a slow CME has been observed both in UV and WL. This CME showed at this early stages the typical three-part structure → ideal case for the study of the CME temperatures across the bubble.

Mauna Loa 19:23 – 19:32EIT 19:25 – 18:36

Mauna Loa 19:23 EIT 19:25

(Difference images) (Normal images)

Y point

CME coreUVCS FOV

(fro

m B

em

po

rad

et

al.,

20

07

, A

pJ,

in p

ress

)

LINE INTENSITY EVOLUTION

Ne peaks at the core, while The core moves faster thanLy (Ne) and OVI intensities the front, but Doppler dim-(Ne and Ne

2) peak at the front ming it’s not enough

We need a front → coreTe increase

SiXII intensity (Ne2) does

not increase at the core

106.40

106.45

106.40

106.30

106.40

106.45

106.40

106.30

Te higher than the Si XII emissivity peak (106.3 K)

1° ISSI Group Meeting1° ISSI Group MeetingOctober 23-27, 2006, BernOctober 23-27, 2006, Bern





CME TEMPERATURES

From a comparative WL – UV data analysis we derived that across the CME bubble temperature maximizes at the CME core (106.45 K) and increases between 1.6 and 1.9 R; the front (at 106.30 K) may be heated by adiabatic compression of the ambient coronal plasma (at 106.15 K).

This core behaviour is in agreement with laboratory experiments on free expansion of plasma spheromacs (conservation of magnetic helicity and dissipation of magnetic energy → plasma heating > adiabatic cooling)

High in situ temperatures are really due only to CS?




CONCLUSIONS & QUESTIONS

• We observed the typical three-part structure of a CME both in UV and WL data at early stages of the event

• Temperature increases from the outer shell to the CME core

• Which physical processes can account for the observed temperature behaviour?

• This CS is observed only in UV and not in WL. Why?

• Photospheric fields show this CME to originate in a quadrupolar configuration. CS formations are envisaged both in catastrophe and breakout models: which observable features may allow us to distinguish between these two models?




CS: SWEET & PARKER MODEL

• In a region where B reverses the Lorentz force jxB pushes toghether the oppositely field lines driving reconnection. • Reconnection inflow opposes the CS resistive diffusion.• A stationary state implies a balance between inflow and diffusion that gives vin = /l.• Large stresses in the vicinity of the null-point result in a magnetic catapult that ejects plasma along the CS with velocity ~ vA.

(from Boyd & Sanderson, “The physics of plasmas”, 2003)

current sheets from wl and uv data: open questions alessandro bemporad inaf – arcetri...

Documents

uv data

open problems cs observations

open problemssecond

open configuration

cme models

wl ciaravella

postcme css

postcme loops