CME-associated dimming regions

1
CME-associated dimming regions

• Alysha Reinard12, Doug Biesecker1, Tim Howard3
• 1NOAA/SEC
• 2University of Colorado
• 3Montana State University
• SHINE 2006

2
Data

• SOHO/EIT 195Å bandpass
• Search for dimmings
• CME associated dimmings from 1997 to 2005
• 100 dimming events
• 55 events with known
• ICME counterparts
• 14 include magnetic
• cloud parameters (more
• are being added!)
• ICME parameters obtained
• from the ACE spacecraft

3
Data reduction

• A pre-event image is subtracted from each frame
• Objective determine the extent of the dimming
  in area and intensity

4
Data reduction

• Dim pixels pixels which are at least one
  standard deviation below the image mean
• Region of interest selected by hand
• area dim pixels
• intensity sum of dim pixels

5
CME/dimming time difference

• Most CMEs are seen in C2 0-2 hours after
  associated dimming
• Consistent with similar initiation time

range -0.9-3.1hrs mean1.10.9hrs median1.1hrs
6
Dimming durations
range 0.6-22 hrs mean7.44.0 median7.2

• Average dimming lasts 3-12 hours

7
CME widths vs. dimming widths

• Dimming widths are equal to or smaller than CME
  widths
• CME expansion
• Regions beyond dimming participating in event

8
Depletion and recovery
depletion
recovery

• Depletion time field lines are open, material
  escapes
• Recovery time corona replenishes the lost
  material

9
Recovery slopes have a two-part structure 40 of
the time

• Two mechanisms operating in coronal recovery?
• More study is needed

10
In situ magnetic field magnitude vs. distance of
dimming from center of solar disk

• CMEs originating near disk center will directly
  intersect the ACE spacecraft.
• Central regions of an ICME have stronger magnetic
  fields

r -0.38 sig99.3
11
In-situ magnetic field magnitude vs. dimming area

• CMES with larger dimmings tend to have stronger
  magnetic fields

r 0.35 sig97.1
12
In-situ composition vs. dimming area
r 0.55 sig99.9
r 0.41 sig98.7

• CMEs with larger dimmings tend to come from
  hotter source regions with higher elemental
  abundances?

13
Total magnetic flux may correlate with dimming
area
r0.65 sig98.4
r0.75 sig99.8
Relatively strong correlation, but few data
points. More magnetic cloud events are being
added to the analysis
14
Magnetic cloud size
r0.65 sig98.8
Size of magnetic cloud does not correlate with
area or length of dimming
correlation driven by
r0.09
Size of magnetic cloud at 1AU is expected to
correlate with dimming duration if the dimming is
a direct effect of the CME
15
Magnetic flux
With dimming (36) Without dimming (11)
Poloidal flux 37.27.7 ? 20.13.6
Toroidal flux 41.215.5 ? 11.73.9
Total flux 78.422.7 ? 31.87.4
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ICMEs with associated dimmings are very similar
to those without, except Helium which is more
enhanced in dimming-related events
With dimming (36) No dimming (11)
O7/O6 0.530.05 0.540.11
QFe 12.10.3 11.90.5
He/H 0.0560.005 0.0390.004
Mg/O 0.210.02 0.250.03
Density 8.11.0 7.91.0
Temp 570007000 530007000
Vp 47216 49325
B 10.70.9 9.21.4
17
Summary

• Dimming time evolution
• Dimming duration averages 74 hours
• Recovery is slower than depletion has two slopes
  40 of the time
• CME/dimming
• Dimmings usually begin 0-2 hours before CME is
  visible in C2
• CME widths are larger, on average than dimming
  widths
• Weak correlation between dimming area and
  magnetic cloud flux/composition parameters
• Larger dimmings are associated with larger, more
  energetic CMEs that have more magnetic energy,
  are more strongly heated and contain higher heavy
  ion abundances
• Dimming-related events have more magnetic flux
  and more helium at ACE
• Larger CME events are more likely to produce
  dimmings
• Helium measurements provide some evidence that
  dimming material directly enters the body of the
  CME