The Potential for Tracking
Transient Events Through the
Heliosphere to Geospace With
Emerging Low-Frequency Radio
Telescopes
Justin C. Kasper*,1,
Divya Oberoi2, Miguel
Morales1, Alan J.
Lazarus1, Joe Salah2,
Colin Lonsdale2
*jck@mit.edu,
617-253-7611
1
Center for Space Research, MIT
2
Haystack Observatory, MIT
2004 LWS Workshop
Boulder, CO
Abstract
We present the results of an ongoing study of the potential for conducting LWS
science and supporting LWS missions with next generation low-frequency digital
aperture synthesis radio interferometers. These new designs for ground-based
radio observations build on the rapid advancement in computing power and
network bandwidth to enhance observational capabilities. The signals from each
antenna may be digitized and sent to a central processing facility for simultaneous
aperture synthesis in multiple directions limited solely by the available computing
power. Tracking of sources from many locations coupled with sophisticated realtime models of the ionosphere permit the extension of measurements to
previously unexplored low-frequencies. It is possible that these telescopes could
be used to remotely measure magnetic field and density structures from the lower
corona out to 1 AU.
Introduction
Solar observatories
monitor the solar
surface and the
region in its vicinity
through imaging,
spectroscopy, and
total intensity
In-situ
measurements of
solar wind plasma
provide information
on velocity, density,
temperature, and
composition at that
point in space.
The key missing measurements are of the 3D density and magnetic field structures between the
inner heliosphere from the corona to 1 AU.
The large volume of interplanetary space can only be sampled using remote sensing techniques:
Heliospheric imagers (e.g. SMEI, STEREO) and Radio remote sensing (e.g.
Wind/WAVES,IPS,Faraday Rotation)
Faraday Rotation provides a unique opportunity to remotely measure the
magnetic field of the inner heliosphere and to link solar and in-situ
observations
Outline



Digital Low-Frequency Radio Array
Faraday Rotation
Studies
1)
2)
3)
4)
5)

Propagation
Sources
Quiet Inner Heliosphere
Propagating Transients
FR in the Context of LWS
Conclusions
Digital Low-Frequency Array
Our study has focused on the baseline design for the Low
Frequency Array (LOFAR), a potential future radio array
which would operate in the frequency range of about 40 to
240 MHz. In the current design LOFAR is a centrally
condensed array with 25% of the collectors within a 2 km
diameter, 50% within 12 km, 70% within 75 km, and the
remainder extending to 400 km. We have identified
promising capabilities in the Wide-Field Correlator (WFC)
design for LOFAR that would permit new observations of
the state of the inner heliosphere.
An All-Sky Monitor (ASM) making use of the WFC
could produce images of the heliosphere at a
cadence of half a second with 2x2 arc-minute
resolution. These images could then be used to
reconstruct the Faraday Rotation (FR) due to the magnetic
field of the inner heliosphere. The output from the WFC
could also be used to simultaneously monitor several
hundred sources for interplanetary scintillations (IPS).
Concept of single receiver
station in Western Australia
ASM Properties
 2x2 arcmin pixels
 0.5 sec integrations
 4 MHz bandwidth
 15 kHz resolution
Solar & Heliospheric Science
What solar/heliospheric science is possible when you can monitor a large field of
view using an extended array with high temporal (0.5 sec), frequency (15 kHz over
4 MHz) , and spatial (2 arcmin) resolution?

Faraday Rotation (FR)  Focus of this poster




Interplanetary Scintillations (IPS)



Magnetic field structure of the inner heliosphere
Magnetic topology of transients (CMEs, Flux Ropes)
Evolution of turbulence throughout inner heliosphere
Increase number of scintillations by tracking many more sources
Go out to much greater distances
Non-thermal emission from shocks in the corona

Rapid characterization of CME shock properties through spatially resolved emission
spectra
Principle of Faraday Rotation
 Linearly polarized electromagnetic
radiation experiences a rotation in
the angle of polarization as it travels
through an ionized and magnetized
medium.
 The amount of rotation is
proportional to the electron density
and the projection of the magnetic
field along the line of sight
 Observe phase as a function of
frequency and determine Rotation
Measure (RM)
  RM  2 rad
RM  2.63 10
13
 
-2
N
B

d
s
[rad
m
]
 e
The observed RM encodes
information about the magnetic
field and total electron number
density in the intervening material
Observing Heliospheric RM
Method 2: Follow Galaxies with, e.g., VLA
Method 1: Monitor telemetry at one or
two frequencies from a spacecraft:
Pioneer, Helios, Ulysses
Mancuso and Spangler, 2000
Issues: Requires tracking by DSN, phase
ambiguities unless RM is small or 24-hour
coverage to track winding; Line of sight in a
single direction per spacecraft;
Measurements at one or two fixed
frequencies (e.g., x-band)
Issues: Require dedicated time on instrument
like VLA; Narrow field of views limits number of
sources which may be tracked
Appeal of Array for RM
Rotation of Helios 1.4 GHz
telemetry signal during single
solar crossing. Red curve
indicates corresponding effect at
140 MHz
 Use ASM with 2x2 arcmin resolution to
routinely track polarization from many
sources at high cadence
 Lower frequencies: sensitivity to
smaller RM and greater distances from
the Sun
 Large bandwidth (4 MHz) and
frequency resolution (15 kHz): defeats
overwinding and uncertainties in overall
phase – look at differential rotation
200 Rs
Results of Study
1)
2)
3)
4)
Propagation effects: Depolarization, angular
broadening
Number of sources: Intensity and fractional
polarization as functions of frequency and
integration time
Quiet inner heliosphere: Compare models,
tomographic reconstruction
Transient events: Tracking flux rope evolution
1) Propagation Effects
While lower frequencies allow us to measure smaller values of RM, we need to
study the effects of scattering and depolarization which grow stronger at lower
frequencies.
Results: From modeling angular broadening RM measurements appear possible
in to 1-2 Rs at 240 MHz and 3-4 Rs at 30 MHz
2) Number of Sources




Require extra-galactic sources (for uniform coverage over
the year) in 40-240 MHz
Generally sources become depolarized as wavelength
increases N ~ λ-0.8, but sources also larger and brighter
Encouraged by Canadian Sky Survey, which identified 1
ideal source for FR per square degree at 400 MHz
Exact numbers will depend on instrument, but LOFAR
baseline ASM could measure RM from 1 source per square
degree in 10 minutes integration
3) Quiet Inner Heliosphere
In the absence of transient events such as solar flares and coronal mass ejections, the
coronal magnetic field gradually evolves as magnetic flux emerges through the
photosphere. Often features such as coronal holes persist for multiple Carrington rotations
(27 day intervals). Under the assumption that the corona remains static for a single
rotation, we may construct synoptic maps of measured properties such as the magnitude
of the photospheric field
There are models (e.g. Force-free
source surface and 3D-MHD
simulations) which take these
synoptic maps and predict the
resulting coronal magnetic fields.
Contour plot of the measured photospheric magnetic
field strength based on 27 days of observations from
the Wilcox Solar Observatory (WSO) for Carrington
rotation 1751 (July 1984).
Could measurements of the
heliospheric RM allow us to
remotely establish the validity
of these models?
Simple Model Heliospheres
(a)
The same photospheric field measurements from
WSO in Figure 2, and two model calculations of
the resulting coronal magnetic fields, based on (a)
2.5 and (b) 3.25 RS source surfaces
(b)
For this study we considered two model calculations of the coronal magnetic field for
Carrington rotation 1751 (July 1984). The models are force-free potential field calculations
from Wilcox Solar Observatory that force the coronal field to be radial at source surfaces of
2.5 and 3.25 RS. We combined the predicted coronal field values with a simple model of the
3D speed and density structure of the solar wind and calculated the Faraday Rotation at 60
MHz for each model.
Calculations for Each Model
(a)
(b)
Rotation
[Degrees]
Rotation
[Degrees]
Simulated maps of the observed Faraday Rotation for the two models. The pixel size is chosen to
represent the expected number of background sources suitable for these observations. The pixels are
colored to indicate the absolute value of the measured rotation due to the 2.5 (a) and 3.25 (b) RS source
surface models for sources at 60 MHz. These simulations include the estimated 2-3 degree uncertainty
in the measured phase angle. The solid red line indicates all sources which would have a rotation of
more than five degrees.
There are approximately 1000 sources with measurable rotations of greater than five
degrees at 60 MHz. Models produce different rotations in 80% of the sources
FR Over CR 1751
Sun
Earth
Field of view
Positive RMs
The following page
shows the expected
RMs in a 50ox50o
FOV (2500 sources)
over Carrington
Rotation 1751
Negative RMs
4) Transients: October 2003
o
o
o
o
SOHO EIT 304 Å MDI; EIT 195 Å; LASCO C2 & C3
SOHO observations of the flare and CME
associated with the event on October 28, 2003.
Could we forecast the
geoeffectiveness of Coronal Mass
Ejections (CMEs) in advance of
their arrival at Earth?
As a case study, we will consider
the October 2003 CMEs
Two large CMEs reached Earth on
October 29 and 30, 2003
While both CMEs triggered
geomagnetic storms, the storms
would have been stronger if the
flux ropes had had different
magnetic configurations
Could measurements of the RM
due to these flux ropes have
allowed us to predict the magnetic
field at Earth in advance?
Model Magnetic Cloud
Method
1.
Fit observed flux rope
at 1 AU with simple flux
tube model
2.
Map flux tube back to
Sun at constant speed
3.
Scale flux tube
dimensions linearly
4.
Calculate RM due to
rope every 10 minutes
Measured values of the magnetic field as a function of
distance of spacecraft through the magnetic cloud. The red
lines are the best fit of the simple cylindrically symmetric flux
rope model to the observations.
Observations During Transit
(a)
(b)
(a-f): Images of the rotation from the first flux rope that LOFAR would have observed as a function of time
since the eruption of the CME. The clock indicates the time since the CME erupted. The pixel color
indicates the Faraday Rotation at 60 MHz (Note that the scale changes for each image!). In (a), the rope
has not yet produced an observable change in the rotation, and the ~2 degree uncertainty of the LOFAR
measurements is apparent. In (b) approximately 20 sources have undergone very large rotations. The
rope gradually expands to fill the sky in (c-e). In (f) Earth is inside the flux rope.
(c)
(e)
(d)
(f)
Analysis of Rope Observations
• Calculations of the rotation induced by
this rope as it propagated through the
interplanetary medium suggest that
LOFAR would easily be able to detect it
within a few hours of the eruption
• It appears the quantities such as the
helicity, radius, and orientation of the
rope (and therefore the latitude and
longitude of the field at Earth) could be
determined from the rotation
measurements
The number of polarized background sources
which would experience more than 5 degrees of
rotation at 60 MHz as a function of time from
liftoff of the October 28 2003 CME and flux rope.
Within seven hours of liftoff there is an observable
rotation in more than 100 sources
• There is, however, a degeneracy
between the density and the field
strength (the rotation is proportional to
the product). This method should be
combined with white light
(SOHO,SMEI,STEREO) or IPS
measurements to constrain the density
Science Strategy
 Harder!




FR Monitoring
Track FR from all sources in
FOV and determine RM as a
function of time and location
Predict RM using models and
simulations of inner
heliosphere and compare
with observed values
Fit model structures to
observed transient events
Deconvolve 3D density and
vector magnetic field




Additional Topics
Targeted observations of
galaxies at full temporal and
spatial resolution (small scale
structures)
Synchrotron and non-thermal
emission at shocks in corona
(physics of emission
mechanism, predictive for
energetic particles?)
IPS at very low frequencies
(farther out, see e.g. formation
of CIRs)
Angular broadening of sources
in the corona (track amplitude
and correlation length scales in
corona)
Conclusions






Improvements in computation and network bandwidth will soon
lead to fully digital low frequency radio arrays
The digital nature of these arrays will be exploited to produce
wide field of view images and permit multiple simultaneous
observations
This creates the potential for continual monitoring of the Sun
and inner heliosphere (IPS,Faraday Rotation,Direct imaging)
Measurements such as Faraday Rotation would open a new
window on the inner heliosphere to connect solar and in-situ
observations
Faraday Rotation could be used both to validate models of solar
wind evolution in the inner heliosphere and to track and
characterize potentially geoeffective ejects
Ground-based radio observations can make significant and
unique contributions to LWS
Acknowledgements
 Investigation of the potential for low frequency radio arrays such as LOFAR
to further heliospheric science and space weather forecasting through
coronal and interplanetary observations is supported by a NSF NSWP grant
ATM-0317957
 The synoptic photospheric field map and the two source surface
calculations were taken from the Wilcox Solar Observatory website
(http://wso.stanford.edu/)
 The modeling of the flux rope associated with the October 28, 2003 CME
made use of plasma and magnetic field data from the Wind and ACE
spacecraft
 MIT/Haystack LOFAR web page
 http://web.haystack.mit.edu/lofar/lofar.html
 Summary of workshop at UCSD on Solar/Helio Studies
 http://web.haystack.mit.edu/lofar/solar-meet.html