Case Studies of Interplanetary Coronal Mass Ejections
A.B. Galvin*1,2, K.D.C. Simunac1 and C.J. Farrugia1,2
1.Space Science Center, Institute for the Study of Earth, Oceans and Space, 2. Department of Physics, University of New Hampshire
Abstract We contrast the solar wind characteristics for interplanetary coronal mass ejections (ICME) cases at different phases of the solar
cycle using STEREO observations. The Plasma and Suprathermal Ion Composition (PLASTIC) instruments on the twin spacecraft STEREO A
(STA) and STEREO B (STB) were commissioned in January 2007 and have been operating continuously (orbit, Fig. 1). The mission to date
encompasses the decline into the solar minimum of December 2008 and predicted maximum of cycle 24 (Fig. 2). The PLASTIC investigation
measures the solar wind protons, alphas, and selected minor ions. This past solar minimum was characterized by weak transients. In contrast the
rising solar cycle included extremely fast ICMEs, with one such ICME observed in situ by STEREO A exceeding 1500 km/s at 1 AU. We will
compare specific cases of slow and fast ICME solar wind, as observed in situ by STEREO, to general solar wind parameters, particularly for iron
ions.
Introduction – The Current Cycle
Solar wind data near Earth are routinely available from the mid 1960’s
onwards thanks to NASA’s OMNI-2 data compilation (King and
Papitashvilli), providing a tremendous resource for both event-oriented and
long-term studies. With the STEREO data set, we also acquire, for the first
time, continuous longitudinal data at 1 AU (Fig. 1,4).
General solar wind trends for the current solar cycle indicate that there are
differences in this cycle from earlier cycles.
Figure 1
Coronal holes, dominant during solar minimum, are associated with higher
speeds and higher kinetic temperatures, creating a modest correlation of
these parameters with the minimum cycle phase. Note however the
significant low speeds (Fig 3-blue oval, Fig. 4-5) and kinetic temperatures
(Fig. 3-blue oval) observed in the 2009 solar wind during the sustained
recent solar minimum. A reduced mass flux has also been observed (Fig. 3),
which has implications for magnetospheric and heliospheric boundaries
(McComas et al., 2013). With STEREO, we confirm that these trends are
seen at all longitudes when long term averages are used (e.g., Fig. 4, 27-day
running averages).
Figure 2
*Contact:
[email protected]
Interplanetary Coronal Mass Ejections – Different Phases of the Cycle
Of particular interest to the mission objectives of the STEREO mission are the identification of the passage of interplanetary coronal mass
ejections (ICMEs). For ICME identification, PLASTIC provides the proton and alpha bulk parameters and ionic charge states of selected minor
species, such as iron.
Figure 6
Shown in Fig. 6 are the daily average helium relative
abundance and iron ionic charge state Fe<Q>, derived
from hourly accumulations, observed by STA
PLASTIC from commissioning through 2013. As
expected, the occurrence frequency of higher Fe<Q>
and He/H events is increasing with the approach to
solar maximum. These instances are sporadic, lasting
several minutes to days, and have been identified as a
robust signature of an ICME passage.
However, although high ionic charge states are a
reliable indicator of an ICME (e.g., Reinard, 2008), not
all ICMEs exhibit extreme composition. The ICMEs
observed during this past solar minimum were
typically characterized by very modest Fe charge
states, as reported by Galvin et al. (2009, 2011). Jian et
al. (2011) report that the ICMEs in the recent solar
minimum were smaller, slower and weaker than those
observed during the 1996 minimum.
Low Charge State ICMEs – Small Transients. Using STEREO and near Earth assets, this recent solar minimum
revealed a number of small transients (ST), as first reported by Kilpua et al. (2009, 2012), and later by Yu et al. (2013). In some cases, the
origins have been tracked. Foullon et al. (2011) detected a detached plasmoid in the heliospheric plasma sheet (HPS) associated with the
heliospheric current sheet (HCS). Rouillard et al. (2011) linked plasmoid release observed remotely to in-situ observations.
STEREO
Charles Farrugia and Wenyuan Yu have used STEREO A, STEREO B, and Wind to identify small transients during the recent solar minimum
period (2007-2009). Their selection criteria included: (1) Duration less than 12 hrs, (2) Low Tkin and/or low beta, (3) Enhanced magnetic field
strength relative to 3 year average, and one or more of the following: (4) Decreased B variability, (5) Large, coherent rotation of the field vector,
(6) Low Alfven Mach number, and/or (7) Te/Tp higher than 3 year average. They found 126 samples: Average duration was 4.3 hours, with 75%
less than 6 hours; 81% of which were in the slow solar wind (Vp < 450 km/s). Many start or end with sharp field discontinuities. Year 2009 was
the bumper crop. They found one main difference from longer duration ICMEs during the same interval was that the Tkinetic was not “cold”
compared to expectations. (ICMEs often have low Tkin’s). In addition, as shown here (Fig 7), the composition was indistinguishable from the
ambient solar wind.
Figure 3
Omni-2
Figure 4
Speed
Minimum
late 2009
Figure 7. Composition for three small transient
events, identified by Yu et al (2013). Shown from
top to bottom for each day of an event period: the
iron charge state distribution (top), average and
peak Fe charge state (middle), and the iron solar
wind speed (bottom). The start and stop times of
the transient interval are shown below their
respective figure. All 45 events for STEREO A used
in Yu et al (2013) were analyzed. They all looked
like this.
High Charge State ICMEs. The highest Fe<Q> observed to date on STA is during the July 2012 magnetic cloud event. This
event is considered a “Carrington Class” due to its high speeds and magnetic fields (Russell et al., 2013).
Figure 8
STEREO A PLASTIC
Transient Solar Wind
Charge States
Figure 5
(Note: Fe Q ≥ 22)
Ambient Solar Wind
Charge States
Acknowledgments
References
Foullon, C., et al, doi 10.1088/0004-637X/737/1/16, 2011.
Galvin, A.B., et al., Ann. Geophys., 27, 3909-3922, 2009.
Galvin, A.B., doi 10.1007/978-90-481-9787-3_11, 2011.
NASA STEREO Grant NNX13AP52G at UNH
Jian, L.K., C.T. Russell, and J.G. Luhmann, Solar Phys., 274:321-344, 2011.
Kilpua, EKJ, et al, doi 10.1007/s11207-009-9366-1, 2009.
Kilpua, EKJ, et al, doi: 10.1007/s11207-012-9957-0, 2012.
J. H. King and N. Papitashvilli at NASA GSFC and CDAWeb for the ONMI data.
McComas, D., et al., doi 10.1088/0004-637X/779/1/2, 2013.
Rouillard, AP., et al., doi 10.1088/0004-637X/734/1/7, 2011.
Reinard, A.A., Astrophysical Journal, 682:1289-1305, 2008.
Russell, CT., et al., doi 10.1088/0004-637X/770/1/38, 2013.
Yu, W., et al., doi 10.1002/2013JA019115, 2013.