Download Detecting and Tracking Solar Ejecta with Next

Detecting and Tracking Solar Ejecta with Next-Generation Heliospheric Imaging Systems
Janet C. Johnston
AFRL/RVBXS, Space Weather Center for Excellence, Air Force Research Laboratory, Hanscom
AFB, MA 01731-3010
David F. Webb
ISR, Boston College, Chestnut Hill, MA 02467 USA
1. Background and Need
Most significant space weather is initiated at the Sun. Coronal mass ejections (CMEs), huge
blobs of plasma with embedded magnetic structure, are the primary cause of severe space
weather at Earth because they drive shocks and trigger geomagnetic storms that can damage
spacecraft and ground-based systems. Magnetic storms can damage both military and civilian,
space and ground assets. These hazardous storms are difficult to forecast and there are many
false alarms.
What makes solar and heliospheric disturbances geoeffective, in terms of causing storms, is
primarily southward interplanetary magnetic field (IMF) and compression. Southward IMF is
important because it allows merging of the IMF and Earth’s magnetic field and transfer of solar
wind energy and mass into the magnetosphere. Compression is important because it strengthens
existing southward IMF and, to a lesser extent, increases density. CMEs usually contain longduration flows of southward IMF and fast CMEs compress any southward field ahead.
Sometimes the geoeffective compressed, southward field can be entirely within the shock
“sheath”. In addition, CMEs themselves can carry high-density structures, such as solar
filaments. High-speed streams are geoeffective when they compress any southward IMF in
corotating interactive regions (CIRs), and also because they are associated with the acceleration
of electrons in the magnetosphere to hazardous levels.
While coronagraphs observe the launch of a CME close to the Sun, the wide variety of CME
speeds and morphologies make it necessary to image and track them through interplanetary
space to predict their arrival, duration, strength and effectiveness at Earth. Space imaging is
required because of the difficulty to detect the dim CME light against brighter background
sources such as the zodiacal light and starlight.
Tracking and specification of significant solar ejecta in the next decade will rely on a data
assimilative approach, incorporating imagers, interplanetary scintillation, wide field radar arrays,
and other assets not yet in operation. As DoD systems (communications, navigation,
surveillance) become more complex, space weather effects will play a more significant role in
the fidelity of Earth and space situational awareness. Anomaly assessments are needed both
reliably and within a useful timeframe. Forecasting can also be useful both to postpone critical
space operations and take mitigating actions to protect sensors or to take advantage of adverse
conditions. “All clear” forecasts are also valuable to the operational community. Heliospheric
imagery can lead to improvements to solar wind forecast models. The accuracy of these models
is crucial should a failure of surveillance assets occur.
2. Prior Remote Sensing Missions
Since the Helios mission it has been known that electrons from CMEs can be viewed in white
light from space through Thomson-scattering (Richter et al., 1982). Beginning early 2003, a new
era in heliospheric imaging started with the operation of the Air Force Space Test Program
demonstration experiment, the Solar Mass Ejection Imager (SMEI – Eyles et al., 2003). SMEI,
launched into a circular, Sun-synchronous orbit, captures a full sky image (minus a 20° exclusion
zone around the Sun and a small patch at the anti-solar direction) every orbit (102 minutes). A
photometric limit of at least 0.01% of the background sky must be attained to detect and track
CMEs (Howard et al., 2006; Webb et al., 2006; 2009).
Since late 2006 SMEI’s data have been augmented by imagery from the SECCHI suite of staring
instruments (Howard et al., 2009) on the twin STEREO spacecraft in ~1 AU orbits around the
Sun. Each STEREO has two heliospheric imagers (HI-1 and HI-2; Harrison et al., 2008; Eyles et
al., 2009) which together have 70o x 90° views of the heliosphere centered east and west of the
ecliptic. STEREO-A is ahead of the Earth and B lags behind, and thus provides an even longer
lead time forecast for some phenomena. The STEREO craft diverge from each other at the rate
of 22° each per year and will soon be 180° apart. The SMEI and STEREO HI data are
complementary. HI-1 fills the gap between a coronagraph and the SMEI inner camera and HI-2
overlaps with SMEI over tens of degrees and has better spatial resolution. SMEI suffers from
area obscurations due to particles in Earth orbit, but despite these gaps, SMEI observes nearly
the entire sky. Both HI fields have about the same temporal resolution, about 2 hours. The
improved spatial resolution of the HIs allows detection of plasma, or density waves that may be
associated with stream interaction regions. Although their fields of view are more limited than
SMEI’s, HI data are free of the contamination suffered by SMEI from particle hits from
traversing the South Atlantic Anomaly, the horns of the auroral zones and the noise introduced
by high altitude auroras.
Radio techniques have also been used to remotely detect and track and coarsely image
heliospheric disturbances related to CMEs. These use km-wavelength radio observations from
space and interplanetary scintillation (IPS) observations from the ground. The km-wavelength
observations can track the type II emission typically from strong shocks traveling ahead of fast
CMEs. Such instruments have been flown on the ISEE-3, Wind, Ulysses and STEREO
spacecraft. The IPS technique relies on measurements of the fluctuating intensity level of signals
from strong, point-like distant radio sources from one or more ground arrays operating in the
MHz range. IPS arrays detect changes in density inhomogeneities in the (local) IP medium
moving across the line of sight to the source. Disturbances are detected by either an enhancement
of the scintillation level and/or an increase in velocity.
3. Space Weather Requirements and Tradeoffs
Extensive studies have shown that a SMEI–like instrument can detect and track most earthbound
CMEs capable of generating moderate to severe geomagnetic storms (e.g., Webb et al., 2009).
However, “halo” CMEs, those CMEs which have an earthward component when viewed from a
detector along the Sun-Earth line, require aggressive processing for determining the distance to
Earth of the incoming material. With such detectors, only the angular distance (elongation
angle) from the Sun is measured and for halo events, there can be an ambiguity between a wide
CME near the Sun and a narrow CME near the Earth. And the geometric shape of the leading
edge is in question. White light imagers detect light produced by Thomson scattering, so the
brightest part of an image is a convolution of the amount of material and its angle from the
illumination source.
We are currently testing several methods and models that use HI data for improving forecasts.
One method is to assimilate SMEI data into a forecast model. Currently the HAFv2 model is
AFWA’s operational solar wind model. SMEI data can enable a mid-course correction to the
HAF-only forecast up to about a day before CME arrival. AFRL developed and transitioned a
simple point-&-click tool/operator interface that permits a forecaster to fit the leading edge of a
CME using SMEI data to estimate the arrival time at 1 AU. This tool also yields estimates of the
CME speed and its direction angle with respect to the Sun-Earth line.
The Tappin-Howard (2009) model is a 3-D interplanetary CME reconstruction model based on
leading edge measurements from heliospheric images and deriving the CME geometry using
Thomson-scattering physics. As input, the model currently uses SMEI and/or STEREO HI data.
It outputs (1) the basic structure, speed, direction, size, height- and speed-time profile, and (2) an
estimation of time of arrival and likelihood of impact with the Earth. The systems impact for the
AF/DoD is that it has the potential to provide a fast and accurate prediction of the arrival time of
any observed CME at the Earth, or any other point in space.
In the near future we should be able to incorporate not only enhanced plasma density data but
also velocity and IMF information in assimilative 3-D interplanetary reconstruction models.
Such assimilative processes are increasingly being used operationally by AFWA and other
government agencies in geospace weather models such as the Global Assimilation of Ionospheric
Measurements (GAIM) model (e.g., Schunk et al., 2004). The velocity data are available now
from the ground-based IPS arrays, but these are scattered in time and space coverage.
International studies are underway to collect, intercalibrate and make accessible all the IPS data
(Oberoi and Benkevitch, 2010).
An important parameter of CMEs is the orientation of the embedded magnetic field. A structure
having a southward-pointing magnetic field is more likely to cause significant geomagnetic
storming at Earth because it reconnects with the predominantly north-pointing geomagnetic field
allowing solar wind plasma and energy to flow into geospace. During the next decade radio
facilities will be developed to image and track the magnetic structures within CMEs. The MWA
sited in western Australia is currently being constructed and will be capable of obtaining both
greatly enhanced IPS data and unique IMF imaging data via the Faraday rotation technique
(Jensen et al., 2010).
Relevant requirements for operational space weather support include: time of arrival (ToA)
accuracy 1 -3 hours; lead or warning times > 12 hours; a 15 min. cadence, high precision
measurements of CMEs to establish initial speed, geoeffectiveness degree; 30 arc-second
sampling resolution; the ability to detect and track corotating interaction regions (CIRs); and an
optimum CME viewing perspective. This latter involves the orbits into which an HI system
could be placed. We discuss three possibilities (Table 1 and Figure 1):
- Earth orbit: Any spacecraft/instrument in Earth orbit suffers from radiation damage and
penetrating trapped particles. Also low earth orbit (LEO) and others that pass through the South
Atlantic Anomaly (SAA) or auroral zones suffer from particle effects and/or bright auroral
emission. In a LEO, near-equatorial orbit, the particle and auroral contamination is reduced, and
other Earth orbits such as medium Earth orbit (MEO) are possible.
- Along the Sun-Earth line: For deep space, an L1 halo orbit is probably the easiest and cheapest
to get to and to maintain an orbit. However, coronagraphs and HIs cannot directly observe CMEs
heading towards Earth, which appear as halos around the Sun, because of the necessity to block
the bright sunlight around the Sun.. Thus, that part of the Earth-directed CME that is most
geoeffective is not seen. Therefore, CMEs and CIRs need to be observed from a different
vantage point that provides a full view of the CMEs, a profile en route, and the CIRs well before
they arrive at Earth.
- L5: Coronagraphs and an HI at the fifth Sun-Earth Lagrange point (L5) orbit position (60o
trailing the Earth at 1 AU) will be able to detect CME speed and direction for a range of solar
origin locations, especially those expected to be most geoeffective. In addition, L5 has the
advantage that it views beyond the eastern limb of the Sun as viewed from Earth. Thus, active
regions and coronal holes can be viewed before they arrive on the Earth-facing disk, and their
location, size and activity assessed. Finally, the geoeffective space weather resulting from high
speed solar wind streams, such as radiation belt relativistic electron enhancements, can be
forecast days in advance. Observations at or near this location using STEREO-B as a pathfinder
for such a mission are discussed by Webb et al. (2010).
4. Summary
Interplanetary space is a dynamic environment, driven to first order by the Sun. The DoD has
invested in the capability to specify conditions and phenomena in the heliosphere. An L5 imager,
and/or an L1 or LEO Sun-Earth line imager can be accomplished with low risk, banking on
knowledge gained from SMEI and STEREO. The ideal system would incorporate both and lead
to a system producing stereo viewing of a CME, unambiguous profiling of its leading edge,
advanced viewing of solar activity, and an inexpensive Sun/Earth line instrument giving full sky
images. Interplanetary scintillation and wide-field radio arrays promise to add details of
embedded magnetic fields to complete the CME specification for use in research and space
weather forecasting.
References
Eyles, C.J., G.M. Simnett, M.P. Cooke, B.V. Jackson, A. Buffington, P.P. Hick, N.R. Waltham,
J.M. King, P.A. Anderson, and P.E. Holladay, Solar Phys., 217, p. 319-347 (2003)
Eyles, C.J. et al., Solar Phys., 254: 387– 445 (2009)
Harrison, R.A. et al., Solar Phys., 247, 171 (2008)
Howard, R.A. et al., Space Sci. Rev., 136, 67 (2008)
Howard, T.A., D.F. Webb, S.J. Tappin, D.R. Mizuno and J.C. Johnston, J. Geophys. Res., 111
(2006)
Jensen, E.A., Hick, P.P., Bisi, M.M., Jackson, B.V., Clover, J., Mulligan, T.: Solar Phys., 265:
31–48 (2010).
Oberoi, D. and L. Benkevitch, Solar Phys., 265: 293–307 (2010)
Richter, I., C. Leinert, and B. Planck, Astron. Astrophys., 110, 115 (1982)
Schunk, R. W., et al., Radio Sci., 39, RS1S02 (2004)
Tappin, S.J. and T.A. Howard, “Interplanetary Coronal Mass Ejections Observed in the
Heliosphere: 2. Model and Data Comparison”, Space Sci. Rev., 147, 55 (2009)
Webb, D.F. et al., J. Geophys. Res., 111 (2006)
Webb, D.F., T. A. Howard, C. D. Fry, T. A. Kuchar, D. R. Mizuno, J. C. Johnston and B. V.
Jackson, Space Weather, 7, S05002 (2009)
Webb, D.F., D. A. Biesecker, N. Gopalswamy, O. C. St. Cyr, J. M. Davila, C. J. Eyles, B. J.
Thompson, K. D. C. Simunac and J. C. Johnston, Space Research Today, 178, 10 (2010)
Table 1. Options for HI Orbits/Locations
Location
1. Earth orbit
Polar LEO
Near-equatorial LEO
MEO, etc.
2. Sun-Earth Line
L1 orbit
Other
3. L5 orbit
L5-L1 Constellation
Advantages
Lower cost; more launch
options.
Disadvantages
Must be >1000 km. height;
Bright auroral emission;
Solar & auroral zone
particles; SAA particles &
radiation dose.
Lower cost; more launch
Must be >1000 km. height;
options; Few solar particles; Still some auroral emission
Reduced auroral emission
and solar & auroral zone
& radiation damage.
particles.
Minimize auroral emission. Still some auroral emission
and solar & auroral zone
particles.
Lowest cost & ease of
More expensive than LEO;
maintaining orbit; avoids
HIs cannot observe
Earth orbit problems;
nose/core of CME; Hard- &
Detectors (CCDs) easily
software: rad hard & deep
cooled
space conditions
L2 has Earth in Sun view;
no other desirable orbits.
With a coronagraph can
Higher cost with weight &
view geo- eff. CMEs from
power restrictions; Hard- &
side & from Sun to Earth;
software: rad hard & deep
Early views of solar
space conditions.
features east of Earth view;
Early detection of geoeff.
high speed streams.
Combines two views to
Higher cost to launch &
reconstruct 3D kinematics
support two missions; Hard& geometry of CMEs &
& software: rad hard &
CIRs.
deep space conditions.
Figure1: Proposed orbits for a heliospheric imager with a CME (not to scale).