POST CME EVENTS: COOL JETS AND CURRENT SHEET EVOLUTION

POST CME EVENTS: COOL JETS AND CURRENT SHEET POST CME EVENTS: COOL JETS AND CURRENT SHEET EVOLUTIONEVOLUTION A. Bemporad, G. Poletto, S. T. Suess

IAU Symposium 226 Coronal and Stellar Mass EjectionsSeptember 13-17, 2004 – Beijing, China

POST CME EVENTS: COOL JETS AND POST CME EVENTS: COOL JETS AND

CURRENT SHEET EVOLUTIONCURRENT SHEET EVOLUTIONA. BemporadA. Bemporad11, G. Poletto, G. Poletto22, S. T. Suess, S. T. Suess33

1Astronomy & Space Science Dept., University of Firenze, L.go E. Fermi 2, 50125 Firenze, Italy

2INAF - Arcetri Astrophysical Observatory, L.go E. Fermi 5, 50125 Firenze, Italy3NASA Marshall Space Flight Center, Mailstop SD50, Huntsville, AL, USA



Summary:

We present here an analysis of the SOHO/UVCS, LASCO, EIT and Ulysses/SWOOPS, SWICS observations of a CME on November 26-27,

2002. The talk will be organized as follows:

• The coronal morphology evolution during and after the CME as seen by LASCO.• SOHO/UVCS observations.• UVCS data analysis and interpretation.• Ulysses/SWOOP & SWICS data.• Conclusions.



The CME as seen by LASCO:

LASCO/C2(2002/11/26, 00:06 11/29, 23:06 UT)

LASCO/C3(2002/11/26, 00:18 11/29, 22:18 UT)

Beginning of UVCS observations



SOHO/UVCS observations:

Slit position:Latitude: 27°NHeight: 1.7 Rsun

Time interval:11/26, 18:39 11/29, 02:56

Main spectral linesobserved:• Fe XVIII 974.9, Fe XV 481.4,Si XII 499.4 - 520.7, Ca XIV 943.6• O VI 1031.9 – 1037.6Ly 1025.7Ly972.5, C III 977.0



SOHO - Ulysses quadrature observations:

• A SOHO – Ulyssesquadrature occurs twice a year (simbols) when the angle between the SOHO Line of Sight (LOS) and the radial from the Sun to Ulysses is ~ 90°.

• In this geometry, thesame plasma studied withremote sensing techniqueby UVCS is observed laterin situ by Ulysses.



SOHO: remote sensing observations



UVCS data: the “cool” plasma jets

SOHO/EIT He II 304 difference images

EIT images show recursiveejection of plasma from theChromosphere at latitudesbetween ~ 35°N and 55°N.

These jets, expected to havea plasma temperature lower than the ~106 K Corona, showemission in UVCS data from neutral H (Lyand Ly and the C III ion (Tmax= 8 104 K).Doppler line shifts indicate aplasma vLOS up to 250 km/s.

The same jets are also visible in the LASCO/C2 images as an increase in the white lightintensity.



UVCS data: emission from high temperature plasma

Between the latitude of ~ 15°N and 30°N, UVCS data show emission from the Fe XVIII ion (Tmax ~ 6 106 K);the main emission is centered at ~ 23°N (bin #35). Such tempera-tures are unusually high even for Active Regions.

At this latitude the Fe XVIII line intensity increases reaching itsmaximum on November 27, at~ 14:00 UT (i.e. about 21h afterthe initiation of the CME), thendecreases, while Fe XV intensity increases continuously.



UVCS data analysis: plasma temperature determination

In coronal conditions, the electron density Ne is low enough that the lines form by electron collisional excitation followed by spontaneous emission. The line intensity Iline is thus:

where nel /nH is the elemental abundance relative to H and ne is the local electron density. G(Te) is the contribution function defined as:

where nion /nel is the ionic fraction which is a function of theelectron temperature Te , Bline is the branching ratio forthe line transition and qline is the electron excitation rate.The emissivity can be defined as Fe X, Fe XV and Fe XVIII Emissivities from the CHIANTI

spectral code (v. 4.01, computed with the ionization equilibria of Mazzotta et al., 1998).

Assuming the ratio between intensities of spectral lines from different ions of the same element to be equal to the ratio between their emissivities; this ratio depends only on the electron temperature Te , hence the observed ratio gives an estimate for Te .



UVCS data analysis: temperature at ~ 23°N of latitude

In this work Te has been estimated from the observed ratio between the Fe XV and Fe XVIII

spectral lines.

From this ratio we estimate that, at the beginning of our observations and at a

latitude of ~ 23°N, Te was higher than 8.7 106

cooling



UVCS data: interpretation

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

• CME models predict that, after the eruption of the flux rope (top panel) 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 converting magnetic energy into kinetic and thermal energies.• The heated plasma in the closed loops is expected to cool down via radiative and conductive cooling.

Are we looking at the top of the loopsor at the Current Sheet?



The rising of reconnecting loops:

(EIT Fe XII images)

vNorth ≃ 2.3 km/s ; vSouth ≃ 1.3 km/s

If we were looking at the top of a newly reconnectingloop system, we would expect a vrise of ~ 100 km/s.



The cooling of reconnecting loops:

Order of magnitude estimates of theconductive and radiative cooling time for a semi – circular loop of height of 0.7 Rsun

shows that the loops cool mainly by conduction over times on the order of 1h.

cooling

We concluded that we are observinga CURRENT SHEET.



• Over the past few years UVCS has found evidence of current sheet (see for instance Ko et al. 2003): long lasting, narrow, hot regions were prominent in the Fe XVIII line.• A long lasting current sheet in the aftermath of fast CMEs (v0 flux rope = 1000 km/s) has also been predicted on the basis of theoretical arguments by J. Lin 2002.

Our Current Sheet last more than 2 days and the CME front speed is about 480 km/s.

The Current Sheet and the rising neutral point

The rising of the neutral point height p vs time (from J. Lin 2002)



Ulysses: in situ observations



Ulysses data: the identification for the same hot plasmaobserved in situ at ~ 4.3 AU.

• In order to identify the in situ observation interval including the 26 November CME, we made a first extrapolation back to the Sun using the average solar wind speed of 431 km/s.

The solar wind speed measured by Ulysses/SWOOPS (Solar Wind Observations Over the Poles of the Sun)

• Hence, because in general CMEs will slow asthey propagate into the interplanetary medium,we assumed that the ICME propagated at half thesum of the average solar wind speed plus theinitial speed of the CME from the LASCO/CME list.

The 26 November CME is identified as the plasmaobserved at Ulysses on 14 – 15 December 2002

(DoY 348 – 349).

(From G. Poletto, S. T. Suess, A. Bemporad et al., ApJL 2004)



The Fe charge state percentage vs day of year in 2002 from Ulysses/SWICS (Solar Wind Ionization state and Composition Spectrometer)

In between DoY 348 – 349 (bottom black bar) SWICS observed the highest charge state (Fe16+) reported in the data (top red bars): this could be a benchmark case to relate the in situ freeze-in temperature to

temperature of the coronal source wich must have been about 9 106 K.

Ulysses data: the identification for the same hot plasmaobserved in situ at ~ 4.3 AU.




Conclusions:

• Many cool jets are observed after the CME, not directly correlated with the post – CME evolution.• We identified the high temperature plasma as the current sheet following a CME.• Lifetime of the current sheet is longer than 2 days.• Temperature of the current sheet is higher than of 8.7 106 K and decreases in about 2 days down to 3.3 106 K. • We presented for the first time evidence for the same hot plasma after CME events, in both remote and in situ observations.



“Hot” and “cool” plasma distribution

• In the following evolution the peak of the Fe XVIII emission is located between the two maximain the O VI emission.

• At the beginning of the UVCSobservations there is very littleemission in Fe XVIII and a diffuseemission in O VI.

The hot plasma is located inbetween branches of cool plasma.




• Over the past few years UVCS has found evidence of current sheet in the aftermath of CMEs (Ciaravella et al. 2002; Raymond et al. 2003; Ko et al. 2003): long lasting, narrow, hot regions were prominent in the Fe XVIII line.• Rising post CME loops was also observed by SOHO/EIT and Yohkoh/SXT.

(not in scale)

tcooling ~ 2 hours (T ~ 1.5 106 K → 6.0 104 K)

POST CME EVENTS: COOL JETS AND CURRENT SHEET EVOLUTION

Documents

Transcript of POST CME EVENTS: COOL JETS AND CURRENT SHEET EVOLUTION