1. Exploring Large-scale Coronal Magnetic Field Over Extended Longitudes With EUVI
EUVI B
EIT
EUVI A
23-Mar UT
Nariaki Nitta, Marc DeRosa, Jean-Pierre Wülser, James Lemen, Markus Aschwanden
Lockheed Martin Solar & Astrophysics Laboratory
23-24 April 2008
SECCHI Consortium Meeting

2. Potential Field Source Surface (PFSS) Model
No dynamics involved.
Assumes current-free corona (therefore not representing flare-productive active regions).
Once arriving at the magic surface, called the source surface, they are deemed to be radial.
Field lines not reaching the source surface are closed.
The source surface is usually set as a sphere at 2.5 Rsun from Sun center.
Use the photospheric synoptic map as the lower boundary condition.
The PFSS model is the only operational extrapolation that can handle magnetic field open to the heliosphere.
From Longcope: Topological methods for the analysis of solar magnetic fields, 2005, Living Rev. Sol. Phys., 2, 7
23-24 April 2008
SECCHI Consortium Meeting

3. Potential Field Source Surface (PFSS) Model
It works remarkably well, given its simplicity, and serves as a useful tool. Its use includes locating coronal hole boundaries and predicting solar wind speed.
However, we need to understand what it can do and it cannot do.
Possible reasons for its mal functioning include: the assumption itself, compromised spatial resolution, poor polar field correction, lack of simultaneous full-surface magnetograms.
Most points apply also to other sophisticated heliospheric models.
How local is coronal magnetic field?
23-24 April 2008
SECCHI Consortium Meeting

4. SECCHI Consortium Meeting
PFSS model at LMSAL
Based on the evolving surface flux model of Schrijver (2001), as implemented by Schrijver and DeRosa (2003)
Update synoptic maps (1 resolution) of photospheric field every 6 hours.
MDI data inserted to the area <60 degrees from disk center.
The model does not work for a couple of weeks following long MDI outage.
Convenient analysis tools incorporated in SolarSoft, which can be used for any extrapolations.
23-24 April 2008
SECCHI Consortium Meeting

5. Where Is Open Magnetic Field Anchored?
Coronal holes and….
How stable are they?
23-24 April 2008
SECCHI Consortium Meeting

6. Where Is Open Magnetic Field Anchored?
Blue: east limb, White: central meridian, Yellow: west limb
From Nitta and DeRosa 2008
23-24 April 2008
SECCHI Consortium Meeting

7. Where Is Open Magnetic Field Anchored?
Blue: east limb, White: central meridian, Yellow: west limb
A hypothetical bipole far behind the limb can affect the distribution of open field lines on the visible side. This depends on where the bipole is inserted.
Adapted from Nitta and DeRosa 2008
23-24 April 2008
SECCHI Consortium Meeting

8. SECCHI Consortium Meeting
Late March 2008
The presence/emergence of three active regions was captured by EUVI-B before Earth-based observations. One of them produced a M-class flare on 25 March.
The offset from the Sun-Earth line was ~22.
How does the addition of bipoles corresponding to these regions affect the computed coronal field on the visible side?
25-Mar EUVI: 18:30, EIT: 18:35
23-24 April 2008
SECCHI Consortium Meeting

9. SECCHI Consortium Meeting
Late March 2008
The synoptic magnetograms were not updated on the three regions until 28 March (upper: MDI daily update, lower: LMSAL model)
Blue: east limb, White: central meridian, Red: west limb
23-24 April 2008
SECCHI Consortium Meeting

10. SECCHI Consortium Meeting
Late March 2008
Solar wind speed and magnetic field polarity. Note that the high speed stream during March was not well predicted.
23-24 April 2008
SECCHI Consortium Meeting

11. Locating Active Regions
23-24 April 2008
SECCHI Consortium Meeting

12. “Updated” Synoptic Maps
Synoptic map of 23-Mar UT
Approximate active regions with Gaussian poles
Synoptic map of 30-Mar UT
23-24 April 2008
SECCHI Consortium Meeting

13. Comparison Of The Distribution Of Open Flux
Synoptic map of 23-Mar UT
Synoptic map of 30-Mar UT
Approximate active regions with Gaussian poles
23-24 April 2008
SECCHI Consortium Meeting

14. Comparison Of Field Lines
Synoptic map of 23-Mar UT
Approximate active regions with Gaussian poles
23-24 April 2008
SECCHI Consortium Meeting

15. SECCHI Consortium Meeting
Solar Wind Speed
23-24 April 2008
SECCHI Consortium Meeting

16. SECCHI Consortium Meeting
Summary
The bipoles observed in late March 2008 did not produce drastic differences in the computation of global coronal magnetic field.
EUV images are no substitutes to magnetograms.
The attempt of using EUVI data to reconstruct photospheric field over wide longitudes can be useful and should be elaborated.
Coronal holes in EUV images need to be modeled into synoptic maps.
We will investigate the possible relation between the magnetic field strength and EUVI outputs at different wavelengths.
Adjustment of synoptic maps may be iterated until closed loops and coronal holes are well represented.
23-24 April 2008
SECCHI Consortium Meeting