C.A.T. Scans of the Solar Wind

Guest article by Dr. Mario M. Bisi (Research Focus)

The Computer Assisted Tomography (C.A.T.) technique has been used for many years now and is well known for use on people where certain health conditions need more thorough, detailed, and deeper scans into the human body and the need for three-dimensional (3D) reconstructed imaging. However, similar such scans can also be used on the solar wind to discover the shapes and sizes of structures near Earth and throughout the inner heliosphere in three dimensions. These scans have been carried out for some time, pioneered in the most part by those at the Center for Astrophysics and Space Sciences (CASS), University of California, San Diego (UCSD) in La Jolla, CA, U.S.A. in close-collaboration with the Solar-Terrestrial Environment Laboratory (STELab), Nagoya University, Toyokawa, Japan…

The solar wind is an extension of the Sun’s corona and is the final part of the Sun’s atmosphere. It is best described as a supersonic outflow of coronal plasma (hot charged particles) into interplanetary space and varies considerably in density, velocity, and composition. The Sun’s magnetic field is “frozen-in” to the plasma as it travels out into the solar system, thus causing the solar wind to also have a magnetic field, and in addition, carries the explosive events of the Sun out into interplanetary space. These resulting features travelling out in the solar wind are known as transient events (in the broad sense); the most popularly defined being coronal mass ejections (CMEs) which can have various consequences at the Earth and on other planets such as by creating magnetic storms (at Earth, these are known as geomagnetic storms) and thus often resulting in magnificent displays of aurorae.

The solar wind carries away about 10^-13 of the Sun’s mass every year. However, this is nothing to be concerned about since if the Sun were to live long enough for all of it’s mass to be turned into solar wind outflow, it would need to survive for another 10,000,000,000,000 (10¹³) years. The Sun has an expected life of just 10,000,000,000 (10¹⁰) years and presently we’re at around the ½-way point in our star’s life.

The existence of the solar wind was first thought about back in the early nineteenth century in an effort to explain the phenomenon of the aurora that appeared at mainly high latitudes in the Earth’s atmosphere and the photo (above) is an example of the Aurora Borealis taken from the European Incoherent SCATter radar (EISCAT) Tromsø site on mainland Norway in October 2003. However, there are several possibilities of aurora being documented as far back as 2000 B.C. by the Chinese, around 600 B.C. by the Xenophanes (who mentioned moving accumulations of burning clouds), and also in some Greek literature. Thus, at least the colourful effects of the solar wind on the Earth have been known about for quite some time. Eventually, it was from comet-tail observations that led to the realisation of such a flow of particles from the Sun out into the solar system through the interplanetary medium. This flow was later termed “The Solar Wind”.

C.A.T. scans of the solar wind are routinely undertaken at CASS-UCSD using interplanetary scintillation (IPS) observations from Earth-based radio arrays/telescopes or white-light (visible-light) observations from the low Earth orbit (LEO) Solar Mass Ejection Imager (SMEI). The inner heliosphere can be reconstructed in three dimensions through the use of the CASS-UCSD 3D C.A.T. reconstruction programs incorporating a time-dependent (or co-rotating) kinematic solar wind model which conserves mass and mass flux of the solar wind. This program uses the IPS density and velocity determinations (or SMEI density determinations) to reconstruct the 3D density and velocity distributions in the inner heliosphere out to 3 AU. All the examples in this article are computed using the 3D time-dependent model which uses perspective views of outward-flowing material crossing the observing lines of sight from Earth to radio source (for IPS) or from Earth to sunlight scattered off of electrons in the heliosphere (for SMEI).

IPS observations of astronomical radio sources to enable measurements of solar wind density and velocity have been carried out for over 45 years. IPS is the amplitude fluctuation of radio waves from distant compact radio sources caused by density inhomogeneities (variations) moving outward in the solar wind. Such observations can be carried out using the EISCAT/ESR systems, but 3D C.A.T. of the inner heliosphere has most-often been carried out using IPS observations made using the radio arrays operated by STELab. In addition, successful tests have been carried out using the Ootacamund (Ooty) Radio Telescope (ORT) located in the Nilgiri Hills of southern India for both density and velocity 3D reconstructions of the inner heliosphere.

These C.A.T. 3D reconstructions allow solar/heliospheric scientists to view (in the form of various images, some examples are given here in this article) the 3D inner heliosphere and to determine the extent of structures moving out in the heliosphere. In addition, much physics can be gained from the reconstructions such as the determination of masses of CMEs, solar wind “ram” pressure, velocity, density, and magnetic field at the four inner planets (Mercury, Venus, Earth, and Mars), and also the reconstructions can be very useful for Space Weather Forecasting.

For the SMEI reconstruction example (top), the scale is the left and is the number of protons per cubic centimetre. Near the Earth at this time, this reconstruction shows two interplanetary CMEs (ICMEs) which formed from a combination of 4-5 CMEs and resulted in two strong geomagnetic storms at Earth during 04-12 November 2004.

For the STELab reconstruction (above), all the features that were not related to a CME which launched from the Sun on 13 May 2005 have been removed for clarity of viewing (another very useful tool when using this technique). The X and Y axes in this example define the Earth’s orbital plane and the Z axis is perpendicular to this into the northern heliospheric hemisphere. Density is shown from 8 electrons cm^-3 and upward. The view is from East of the Sun-Earth line. The dense material heading out from the Sun to the Earth is that of an ICME resulting from the 13 May 2005 CME. The ICME later hit the Earth on 15 May 2005 and again resulted in a geomagnetic storm.

For the EISCAT example, the spiral-type features seen here are known as co-rotating regions (as opposed to the precious transient examples) and are a result of the Sun’s rotation plus the various speeds of solar wind which can interact as they travel out from the Sun through the heliosphere. These co-rotating regions often result in regions of strong interaction between slower and faster solar wind speeds, and as such, these too can often cause geomagnetic storms provided the interaction is suitably intense.

This article just briefly touches on what 3D C.A.T. scans of the inner heliosphere are, and how they can be used; but there are many detailed abstracts from conference presentations (along with links to some of these presentations) and papers which describe and use this technique for solar, space, and heliospheric scientific research.

Much of this work is sponsored in the U.S.A. by the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the U.S. Air Force (AFOSR-AFRL).