norh visit february, 2005angelos vourlidas, nrl on deriving mass & energetics of coronal mass...

NoRH Visit February, 2005 Angelos Vourlidas, NRL

On Deriving Mass & Energetics of Coronal Mass Ejections

Angelos VourlidasNRL

A Tutorial


Overview

• The following questions will be addressed:

– How can we derive information about CME mass/energetics?• What assumptions enter in the calculations?• What are the data analysis steps to extract quantitative CME

information from white light images?

– How good are the numbers?• Can we estimate the errors? How?

– What can we do with this information?• What statistics tell us?• What correlations can we find?


Preliminaries

• Height-time plots, online movies are constructed from UNCALIBRATED LASCO images. Calibrated images are rarely shown.

• All necessary calibration tools exist in the LASCO Solarsoft distribution.

• This talk is relevant to CME measurements ONLY. Coronal background densities, streamers and plumes must be treated differently.

• Remember, a white light CME is defined as an brightness increase relative to the background


Our Objective

Raw C3 Image Calibrated C3 Image (Diff.)

?


CME Mass/Energy Derivation Flow

C3_massimg.pro

cme_

mas

sim

g2to

tal.p

ro


Mass Calculations Primer

Assumptions:• Emission is due to Thompson scattering of photospheric light

from coronal electrons.• All mass is on the sky plane.• Plasma composition is 10% He, 90% H.

Restrictions:• The 3D distribution of the background and CME electrons, Ne,

is unknown.• The temperature of the ejected material is unknown (coronal

should dominate).• Emission is optically thin.


Method:A coronagraph measures the total brightness along the line of sight. We can only measure excess brightness (ICME - IPREEVENT).

Error Sources:exposure time (~0.15%) vignetting (~1%) photon noise (<1.4%)

Phot. Calibration (0.73%) composition (6%) stars (cancel out)

Cosmic rays (few pixels) solar rotation (not important for fast events)Streamer deflections (difficult to estimate) 3D structure (more on that later)

Mass Calculations Primer

Excess DN calibration Btotal Be No. of e- composition Mass


Mass Calculation Methods

• Several ways to obtain a “mass” for an event.

• The choice depends on the objectives:

– After the whole event?– After specific features (i.e., core)?– Flow measurements?

“Typical” C3 Mass Image

SECTOR

Best for automated calculations:

Extent & Upper boundary from CME lists/ht

measurements

TORUS

Best for flow calculations:Position at fixed distance

ROI

Most common:Avoid streamers, planets,

other CMEs


Example Results — Single Event

EM

EK

EP

Etotal Mass

vesc

vCM

Emag

Epot

Ekin

Etotal

vesc

vCM or vfront

mass

More examples in Vourlidas et al (2000), Subramanian & Vourlidas (2004)


How Good Are CME Mass Estimates?

Real mass could be x2 larger


PA Corrected

Sky-Plane

Effect of CME-SkyPlane Distance on Mass Estimates?

CME mass could be 5x larger

CME mass could be 3x less

Sky-Plane

PA Corrected


CME Mass Database (Jan 1996 – Dec 2003)

1. Date/time

2. Width

3. Position Angle

4. Height of CME Front

5. Sector Area

6. Mass

7. Mass density

8. Kinetic Energy

9. Potential Energy

10. Velocity (H-t)

11. Acceleration

12. Escape Velocity.

Thanks to the hard work of Ed Esfandiari an up-to-date CME database has been created:

• The CME information is taken from the CUA/NRL list.• The database includes full-frame mass images for every h-t data

point in the CUA list (6385 events so far).• The mass is derived with the same method (sector) for all frames.• Energy and other calculations are also provided.

The following information is provided for every CME frame:


Results

The analysis of the mass database is based on :• Measurements at the point of maximum mass. (Need for a single “representative” number for each event).

• Does not include events with:• < 5 h-t measurements (frames).• Width > 120°.• Negative mass.• Zero pixels in sector.


Results – Distributions

Parameter LASCO Solwind

<Ekin> (ergs) 4.3 1030 3.5 1030

<Mass> (gr) 1.7 1015 4.1 1015

Total Mass (gr) 4.1 1018 3.9 1018

Mass Flux (gr/day) 3.6 1015 7.5 1015

Duty cycle 81.7% 66.5%


Results – Average Mass

The constant mass density suggests that:1. Only the CME width is needed to derive

the mass2. The bulk of the CME material originates at

high altitudes where the corona is more uniform.

31010 gr/pix

or 1.3104 e/cm3/Rs


Results – Bimodal Distribution?

Do we have “failed” and “successful” CME

populations?


Results – Yearly Variations


Review

• It is easy to calculate CME mass and energetics from the LASCO images (calibration/routines available since 1996).

• The accuracy of the mass values is difficult to estimate without 3D information. Simple simulations suggest that masses could be underestimated by x2 (on average, well-behaved (aka non-halo) events).

• Thousands of measurements of several dynamical parameters for almost all CMEs are now available.

• Mass images for almost all CMEs are also available (for DIYers).

• Preliminary analysis of the mass/energy data yielded a couple of very interesting results:

• CME mass density = constant!• There may be 2 classes of CMEs; “failed” and “successful”.• CME mass/energy distributions are power-laws (like flares!).


BACKUPS


Results – Mass Distribution

Solwind Exponential Fit (Jackson & Howard 1993)

LASCO

Power-law Fit, =-1.8 (Vourlidas & Patsourakos 2004)


LASCO C3 Photometric Performance

Courtesy of A. Thiernisien


Magnetic Energy Estimates

• Problem:Direct measurement is not (currently) possible except

• Radio gyrosynchrontron emission from energetic electrons within the CME (Bastian et al. 2001). Only a handful cases so far.

• Another Approach:1. Select fluxrope-like CMEs.2. Assume the fluxrope feature becomes the IP Magnetic Cloud.3. Assume magnetic flux, Φ is conserved (in the fluxrope).4. Use in-situ measurements of Φ to normalize the magnetic energy, EM.5. Use the coronagraph measurements of the fluxrope area, A and “length”, l to derive the evolution of EM.


Magnetic Energy Estimates

• Relevant Equations:

22

8

1

8

1AB

A

ldVBE

fluxrope

M

2

8

1

A

lEM

Assume fluxrope is cylindrical,B & A are measured/derived from in-situ observations Φ.

A is given by the no. of pixels in the LASCO imagesl is assumed equal to the height of the CM, l rCM.

tfEM

norh visit february, 2005angelos vourlidas, nrl on deriving mass & energetics of coronal mass...

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