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 WorkshopBoulder, 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 simultaneousaperture synthesis in multiple directions limited solely by the available computing power. Tracking of sources from many locations coupled with sophisticated real-time 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) Propagation2) Sources3) Quiet Inner Heliosphere4) Propagating Transients5) FR in the Context of LWS

Conclusions

Digital Low-Frequency Array

Concept of single receiver station in Western Australia

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).

ASM Properties

2x2 arcmin pixels

0.5 sec integrations

4 MHz bandwidth

15 kHz resolution

Solar & Heliospheric Science

Faraday Rotation (FR) Focus of this poster Magnetic field structure of the inner heliosphere Magnetic topology of transients (CMEs, Flux Ropes) Evolution of turbulence throughout inner heliosphere

Interplanetary Scintillations (IPS) 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

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?

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)The observed RM encodes information about the magnetic field and total electron number

density in the intervening material

Principle of Faraday Rotation

rad 2 RM

]m [rad 1063.2 -213 sdBNRM e

Observing Heliospheric RM

Method 1: Monitor telemetry at one or two frequencies from a spacecraft: Pioneer, Helios, Ulysses

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Sp

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Method 2: Follow Galaxies with, e.g., VLA

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

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

Rotation of Helios 1.4 GHz telemetry signal during single solar crossing. Red curve indicates corresponding effect at 140 MHz

200 Rs

Results of Study

1) Propagation effects: Depolarization, angular broadening

2) Number of sources: Intensity and fractional polarization as functions of frequency and integration time

3) Quiet inner heliosphere: Compare models, tomographic reconstruction

4) 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

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).

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.Could measurements of the heliospheric RM allow us to

remotely establish the validity of these models?

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

(a)

(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.

Simple Model Heliospheres

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.

(a) (b)

Rotation[Degrees]

Rotation[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

Calculations for Each Model

FR Over CR 1751Field of view

The following page shows the expected RMs in a 50ox50o FOV (2500 sources) over Carrington Rotation 1751

Sun

Earth

Negative RMs

Positive RMs

4) Transients: October 2003

o Could we forecast the geoeffectiveness of Coronal Mass Ejections (CMEs) in advance of their arrival at Earth?

o As a case study, we will consider the October 2003 CMEs

o Two large CMEs reached Earth on October 29 and 30, 2003

o While both CMEs triggered geomagnetic storms, the storms would have been stronger if the flux ropes had had different magnetic configurations

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 measurements of the RM due to these flux ropes

have allowed us to predict the magnetic field at Earth in

advance?

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.

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

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) (d)

(e) (f)

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

• 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

• 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

Analysis of Rope Observations

Science Strategy

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)

H

ard

er!

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