Space Weather Science from the
Sun-Earth L5
Nat Gopalswamy
Solar Physics Laboratory, NASA/GSFC,
Greenbelt, MD 20771, USA
Expert Meeting on Improving Space Weather Forecasting in the Next Decade Feb 10-11, 2014
CMEs & Space Weather
10,000
Dst (nT)
CMEs
CIR
0.1
UT (h)
On the way
and upon
arrival
Shock-driving
Capability is
Crucial
VCME -VSW > VMS
SEPs
Magnetic storms
Space systems
Airplanes
atmosphere
Space systems
Magnetosphere
Ionosphere
Atmosphere
Ground
0
- 100
- 200
- 300
- 400
- 500
Upon arrival at
Earth
CME’s Magnetic Structure
Is Crucial
(Bz <0)
Corotating Interaction Regions
(CIRs)
25 Streams
•
•
•
CIRs are identified using
plasma signatures such as
proton density (peaks at the
stream interface)
Proton thermal pressure
(peaks at the interface), solar
wind speed (peaks about half a
day after the interface),and
the flow direction, which
changes from west to east at
the interface
Magnetic measurements: CIR
magnetic content and
structure decide the strength
of the geomagnetic storm
E
SIR
W
Superposed epoch analysis of 25 CIRs
Gosling 1996
Comparing CMEs and CIRs
CIRs
Dst>-150
CMEs
Dst>-1500
Why Go to L5?
CIRs can be observed in situ
~4 days ahead of earth arrival
Earthward CMEs can be measured
without projection effects
SUN
SUN
CIR
2009 October
Earth View
Halo CME
L5 View
Limb CME
A CME in Three Views: True Speed
from STEREO
STEREO A
1000 km/s
1000 km/s
Limb CME
Quadrature observations
2010-2012
Gopalswamy et al. 2013
STEREO B
Halo CME ~600 km/s
Limb CME
S
Earthward CMEs
W/O projection effects
STEREO-B
E
STEREO-A
Earth View
CMEs that Matter for
Space Weather
Difficult to measure the
true speed from the
Sun-Earth line (e.g. L1)
L5 View
The storm CMEs come from close to the disk
center (15W) while the SEP ones comes from the
western hemisphere, typically 45W
Most of the storm and SEP CMEs will appear as limb
CMEs from L5. SEP CMEs will appear as halos from L4.
Storm CMEs will be only 45 east of Sun-L4 line, so the
measurements will be subject to projection effects.
Remote-sensing and
in-situ instruments observe
different parts of the CME
Earth-impacting
part of the CME
not imaged from
Sun-Earth line
• Sun-Earth line is not wellsuited to observe earthaffecting CMEs and CIRs
• Need a different vantage
point: Sun-Earth L5
May 2005
Coronagraph Observations Cannot say
whether a CME is frontsided or Backsided
EUV image superposed on the LASCO image identifies the solar source (Earth-directed CME)
Sometimes the surface signatures may be too weak making it difficult to tell the CME origin
From Off the Sun-Earth line
• Front-back ambiguity is resolved if Earthward CMEs viewed from L5
• The section outside of the red lines is visible from L1 (as halo CME)
• An imager from off the Sun-Earth line can view the CME in broadside
and also its direction (towards Earth or behind the Sun).
CME Propagation Affected by
Preceding CMEs
• In addition to the aerodynamic drag, the propagation of CMEs is significantly affected by
large-scale structures such as CMEs and coronal holes
• Preceding CMEs were observed only in the broadside view (by STEREO), not by SOHO (L1).
• Arrival time predictions by both ESA and ENLIL models were much earlier than
actual CME arrival because they did not account for these preceding CMEs
Gopalswamy et al. 2013
CME- Solar Wind Interaction
a = - α (uCME - usw)
• The background solar wind model is critical in
understanding CME propagation
• This model is generally derived from the
photospheric synoptic magnetograms, which
contain information taken over a Carrington
rotation (WSA-ENLIL)
• An L5 magnetograph can update the older half
of the synoptic map, making the background
model more realistic
Synoptic Maps used for Solar Wind
Models
L5 data can update this
Arrival of Solar Sources (Active Regions & Coronal
holes) to Earth View Known Days Ahead from L5
Shock & Flux Rope
SHOCK
Shock feature can be viewed best
from broadside
Useful measurement can be made
on spatial dimensions (e.g.,
standoff distance, flux rope size)
For Bz prediction we still needs
near-surface observations from
the Sun-Earth line
CME
From SOHO & STEREO to
Earth Affecting Solar Causes Observatory
(EASCO)
• EASCO concept was studied at GSFC and
reported to the Decadal survey committee
• SOHO lacked radio instrument and
magnetometer
• STEREO did not have a magnetograph
• The EASCO mission to overcome these deficits
and expand with a Hard X-ray Imager (HXI)
and UV Off-limb Spectrograph (UVOS)
MDL Study: Spacecraft Assembly &
Launch Configuration
For Space Weather
Total payload mass
138 kg
Summary
• The L5 is the next logical location for observing Earth-affecting
solar disturbances such as coronal mass ejections (CMEs) and
corotating interaction regions (CIRs)
• The arrival of both CIRs and CMEs affecting Earth can be
predicted with long lead time
• Kinematics of Earth-affecting CMEs can be obtained with
minimal projection effects
• Proof of concept the transit of STEREO B through L5
• Best way to obtain input data (CME, magnetogram) needed by
models such as ENLIL
• Details:
http://adsabs.harvard.edu/abs/2011SPIE.8148E..30G
http://adsabs.harvard.edu/abs/2011JASTP..73..658G
Deflection by Coronal Hole
S15W11
2014 Jan 07 19:00 UT
• CME Speed: 2100 km/s
• CME moves mostly to the south and west
S15W11
even though the source is near the disk center
• Prediction errors range from -19 h to -31 h
because earthward speed overestimated
many such events in cycle 23: Gopalswamy et al. 2009