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Transcript
CME-associated
dimming regions
Alysha Reinard12, Doug Biesecker1, Tim Howard3
1NOAA/SEC
2University
of Colorado
3Montana State University
SHINE 2006
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

Data reduction


A pre-event image
is subtracted from
each frame
Objective:
determine the
extent of the
dimming in area
and intensity
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
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
mean=1.1±0.9hrs
median=1.1hrs
Dimming durations
range: 0.6-22 hrs
mean=7.4±4.0
median=7.2
 Average dimming
lasts ~3-12 hours
CME widths vs. dimming widths

Dimming widths
are equal to or
smaller than CME
widths


CME expansion
Regions beyond
dimming
participating in
event
Depletion and recovery
depletion


recovery
Depletion time: field lines are open, material escapes
Recovery time: corona replenishes the lost material
Recovery slopes have a two-part
structure ~40% of the time


Two mechanisms operating in coronal recovery?
More study is needed
In situ magnetic field magnitude vs.
distance of dimming from center of
solar disk
r= -0.38
sig=99.3%


CMEs originating near
disk center will
directly intersect the
ACE spacecraft.
Central regions of an
ICME have stronger
magnetic fields
In-situ magnetic field magnitude
vs. dimming area

CMES with larger
dimmings tend to
have stronger
magnetic fields
r= 0.35
sig=97.1%
In-situ composition vs. dimming
area
r= 0.41
sig=98.7%

CMEs with larger dimmings
tend to come from hotter
source regions with higher
elemental abundances?
r= 0.55
sig=99.9%
Total magnetic flux may correlate
with dimming area
r=0.65
sig=98.4%
r=0.75
sig=99.8%
Relatively strong correlation,
but few data points. More
magnetic cloud events are
being added to the analysis
Magnetic cloud size
r=0.65
sig=98.8%
correlation
driven by
Size of magnetic cloud
does not correlate
with area or length of
dimming
r=0.09
Size of magnetic cloud at 1AU is
expected to correlate with dimming
duration if the dimming is a direct
effect of the CME
Magnetic flux
Poloidal flux
With dimming
(36)
37.2±7.7 ↑
Without
dimming (11)
20.1±3.6
Toroidal flux
41.2±15.5 ↑
11.7±3.9
Total flux
78.4±22.7 ↑
31.8±7.4
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)
O+7/O+6
0.53±0.05
0.54±0.11
QFe
12.1±0.3
11.9±0.5
He/H
0.056±0.005
0.039±0.004
Mg/O
0.21±0.02
0.25±0.03
Density
8.1±1.0
7.9±1.0
Temp
57000±7000
53000±7000
Vp
472±16
493±25
B
10.7±0.9
9.2±1.4
Summary

Dimming time evolution



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


Dimming duration averages 7±4 hours
Recovery is slower than depletion; has two slopes 40% of the time
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