Superposed epoch analysis of ion temperatures during CME

Superposed epoch analysis of ion temperatures during CME

Superposed epoch analysis of ion temperatures during CME- and CIR/HSS-driven storms
SM11D-2319
Amy M. Keesee and Earl E. Scime, Department of Physics, West Virginia University
Introduction
Scientific data collection from both satellites of the TWINS mission began in June 2008 [McComas et al.,
2009], enabling the observation of a number of storms driven by either coronal mass ejections (CMEs) or
corotating interaction regions associated with high speed solar wind streams (CIR/HSSs). The ion
temperature and rate of ion heating in the plasma sheet are important elements of understanding how the
dynamics of the ring current and the magnetosphere vary between the two types of storms. Denton et al.
[2006] demonstrated that plasma sheet ion temperatures at geosynchronous orbit exhibit a greater average
increase for CIR/HSS driven storms than for CME driven storms. For CIR/HSS storms, this increase occurs
sharply at convection onset, and the plasma sheet ion temperatures remain elevated for the duration of the
storm, which tends to last longer than those driven by CMEs [Denton and Borovsky, 2008]. Ion temperatures
during the main phase of the CIR/HSS storm on 22 July 2012 calculated using TWINS energetic neutral
atom (ENA) data agreed well with the features observed in the superposed epoch analysis.
ENA Flux Projection and Ion Temperature Calculation
Storm Selection and
Superposed Epoch Analysis
Superposed Epoch Analysis Snapshots
The Sun is to the right and dusk is up.
Three time steps during recovery phase
Geosynchronous
orbit
Date
15‐Jun‐2008
4‐Sep‐2008
11‐Oct‐2008
22‐Jul‐2009
23‐Oct‐2009
15‐Feb‐2010
6‐Apr‐2010
12‐Apr‐2010
2‐May‐2010
29‐May‐2010
4‐Jun‐2010
4‐Aug‐2010
11‐Oct‐2010
12‐Nov‐2010
28‐Dec‐2010
7‐Jan‐2011
1‐Mar‐2011
2‐Apr‐2011
9‐Apr‐2011
12‐Apr‐2011
1‐Jul‐2011
17‐Sep‐2011
26‐Sep‐2011
25‐Oct‐2011
23‐Jan‐2012
25‐Jan‐2012
15‐Feb‐2012
19‐Feb‐2012
27‐Feb‐2012
7‐Mar‐2012
9‐Mar‐2012
12‐Mar‐2012
15‐Mar‐2012
28‐Mar‐2012
13‐Apr‐2012
24‐Apr‐2012
15 Moderate
CME Storms
Min. Dst
>-78 nT
ENA flux in each imaging pixel in a given time interval was mapped along a calculated line of site (LOS) and
assigned to a spatial bin in a 0.5 x 0.5 RE grid in the xy-plane (GSM coordinates) using a fractional field of
view algorithm described by Keesee et al, [2011; 2012]. The effective ion temperature in each spatial bin is
calculated by assuming the spectrum is dominated by protons from the hottest point along the LOS and a
Maxwellian distribution for the protons. Thus,
 nn  z * ni  z *
 E 
jENA

exp 
.
32
 cx  E  E
2mi  Ti ( z*) 
 Ti ( z*) 
(1)
In Equation 1, z* is the location of the hottest point along the LOS, σcx is the energy-dependent charge
exchange cross-section [Freeman and Jones, 1974], nn is the neutral density, ni is the ion density, and ξ is a
characteristic width [Scime and Hokin, 1992] resulting from the approximation. These assumptions are only
valid in regions of the magnetosphere that are “optically” thin to ENAs. The region within 3 RE of the Earth is
excluded from our analysis for this reason. An exponential fit to the scaled flux versus energy measurements
yields an effective ion temperature for the hottest region along the LOS.
15 CIR/HSS
Storms
Ion Temperature Averages
Full View
Geosynchronous Region
6.5 RE < r < 7.0 RE
Three time steps centered at zero epoch (minimum Sym-H)
All 36
Storms
15 Moderate
CME Storms
Min. Dst
>-78 nT
15 CIR/HSS
Storms
Forty-eight storms were selected from the period from June
2008 to April 2012. Storms were sorted by driver using the
NOAA Space Weather Prediction Center Weekly Reports.
Sorting yielded 21 CME-driven storms and 15 CIR/HSSdriven storms. Superposed epoch analyses were performed
for the following combinations: all 36 storms, 21 CME storms,
6 intense CME storms (minimum Dst < -78 nT), 15 moderate
CME storms (-78 nT < minimum Dst < -41 nT), and 15
CIR/HSS storms. ENA data was analyzed from 12 hours prior
to minimum hourly averaged Dst to 24 hours following
minimum Dst. Data was divided in time steps of 24 minutes
(20 instrument actuator sweeps). The time of minimum SymH (minute-averaged) from OmniWeb was defined as zero
epoch. The temperatures from each storm in a given time
and spatial bin are averaged to obtain the temperature in the
superposed epoch analysis maps. Storm temperatures
greater than 20 keV, generally caused by large errors in the
ENA flux values at higher energies, are considered
unphysical and are discarded prior to averaging. Spatial bins
with less than 5 temperature values to be averaged are
ignored.
Dst values from Kyoto WDC; (*times shown -1
hour from those on Kyoto site to convert to 0-23
hour scale). Sym-H values from OmniWeb.
Re-sampling to address observational bias in measurement statistics
More ion temperature values toward dusk
15 Moderate
CME Storms
Min. Dst
>-78 nT
While the 2D ion temperature images demonstrate a
dawn-dusk asymmetry, it can be seen from the count
plots that the instrument field of view covers the
duskward side of the magnetosphere more often. Thus,
we must be careful to consider whether that dawn-dusk
asymmetry is caused by an observational bias. To do
this, we have performed a re-sampling analysis of the 15
moderate CME storms. The 15 storms were randomly
ordered. Then, each spatial bin was filled until it reached
5 ion temperature values, from which the average ion
temperature was calculated. This was performed for 4
different random storm orderings. The 2D ion
temperature images in two time steps for these 4 resamplings are shown. The top row is at storm peak
(minimum Dst) and the bottom row is during the
recovery phase. The dusk-dawn asymmetry appears
consistently, though it appears more pronounced in the
recovery phase.
2D images on the
right
show the number
of ion temperature
values used to
calculate the
average ion
temperatures
shown in the 2D
images on the left.
21 CME
Storms
6 Intense
CME Storms
Min. Dst
<-78 nT
Min. Time of Min. Time of Dst min Dst Sym‐H Min. Sym‐
(nT) (UT)*
(nT)
H (UT) Storm Driver
‐41
5:00
‐49
5:15HSS
‐51
4:00
‐67
3:05HSS
‐54 11:00
‐65
11:29HSS
‐78
6:00
‐95
5:54HSS
‐45
2:00
‐56
1:00faint halo CME
‐58 22:00
‐64
0:08**CME
‐73 14:00
‐76
14:12halo CME
‐56
1:00
‐81
2:03CME
‐67 17:00
‐77
20:16HSS
‐85 12:00
‐73
12:06halo CME
‐47
1:00
‐58
0:55HSS
‐65
4:00
‐72
4:42CME
‐79 18:00
‐76
18:55partial halo CME
‐41
4:00
‐53
3:21CME
‐42 17:00
‐47
16:31CME
‐41
6:00
‐46
6:30HSS
‐61 14:00
‐71
14:22HSS
‐41
2:00
‐40
2:09HSS
‐41
2:00
‐38
2:13HSS
‐47
9:00
‐57
9:04HSS
‐47
7:00
‐59
7:22HSS
‐63 15:00
‐63
18:29CME
‐103 23:00
‐116
21:19CME
‐123
5:00
‐135
3:47CME
‐69
5:00
‐87
5:01CME
‐73 10:00
‐82
10:45CME
‐58 16:00
‐59
14:09slow CME
‐54
4:00
‐76
3:54HSS
‐48 19:00
‐62
19:18CME
‐75
9:00
‐98
6:52CME
‐133
8:00
‐150
8:13CME w/sus. SWBz
‐50 16:00
‐67
16:55CME
‐74 20:00
‐79
19:53CME
‐56
4:00
‐68
4:46HSS w/sus. SWBz
‐45
5:00
‐56
7:52HSS
‐102
4:00
‐125
3:26CME
15 CIR/HSS
Storms
Discussion
Ion temperatures during the CIR/HSS driven storms were ~2-3 keV lower than those during CME driven storms, on average. For CME driven storms,
the ion temperatures appear to decrease during the storm main phase, then remain steady during storm recovery. This signature was most apparent in
the intense CME storms. In contrast, ion temperatures increase during the recovery phase of CIR/HSS driven storms. Average ion temperatures in the
region of geosynchronous orbit were similar to the average values for the full view shown (within 20 RE of the Earth). There appears to be higher
temperatures toward dawn than dusk, which is the opposite dawn-dusk asymmetry than has been observed in the magnetotail in long-time averages
using in situ measurements [Wang et al., 2006; Guild et al., 2008], in Magnetospheric Specification Model (MSM) simulations [Wang et al., 2003], during
quiet magnetospheric conditions using our technique [Keesee et al., 2011] and in the inner magnetosphere during geomagnetic storms using our
technique [Scime et al., 2002]. An initial re-sampling analysis has been performed that indicates that the observed dawn-dusk asymmetry is not caused
by observational bias. We note that this asymmetry is observed outside of geosynchronous orbit, and that the opposite asymmetry can be observed
within geosynchronous orbit. Specifically, a cool region is observed consistently in the dawn-noon sector, as is consistent with in situ geosynchronous
measurements [Denton et al., 2006]. The MSM simulations by Wang et al., [2003] do show an enhancement in dawn temperatures during active
periods, though not as high as the dusk temperatures. We will compare the ion energy spectra observed in the dawn and dusk sectors during individual
storms to gain a better understanding at what is causing the increased ion temperatures at dawn.
storm
peak
recovery
phase