Solar/Heliospheric Physics
If we consider Physics to be the study of the scientifically determined rules of the universe, Solar Physics is the scientific study of the “rules” associated with the Sun. However, data has been collected throughout the Solar System, and the Sun’s effects extend well beyond our outermost planet, throughout what is referred to as the heliosphere. The heliosphere is the area in space that contains our solar system, solar wind, and the entire solar magnetic field. It extends well beyond the orbit of the outermost planet of our Solar System, out to the heliopause, thought to extend 100,000,000,000 miles from the Sun . (See image above)
From the diagram, you can see that the effects of the Sun extend past the yellow disk we see in the sky, extending far into space.
Additional areas seen on the diagram:
Interstellar Wind - Particles from a variety of sources (other stars, etc) travelling throughout space between the stars.
Bow Shock - The shock wave caused by the blast of the interstellar wind particles against the Sun’s heliosphere. Interstellar wind is slowed and heated very rapidly as it arrives at the bow shock.
To study any complex system, such as the Sun and its heliosphere, enormous amounts of data must be collected to test our theories (what we think it might be like) about the Sun with observation (what our data reveals). Several factors have made this type of study more feasible.
The first was the dawning of the “Space Age” which produced spacecraft that have collected this kind of data since before 1963. These spacecraft collect valuable data about the speed of the solar wind, and magnetic field information. The solar wind is the plasma or ionized gas of charged particles coming out of the Sun in all directions. (For examples and locations of some present near Earth, Sun observing spacecraft, see http://nssdc.gsfc.nasa.gov/cohoweb/form/cw_ds.html and enter “4 Day Combined Orbit @200 RE” into the plot field. RE = Earth radius)
The explosion of the internet is a second factor that has made studying solar/heloispheric phenomena more practical. Near real time (within 12 hr) images and data can be easily accessed at sites like http://www.maj.com/sun/index.html . Additionally, the ability to archive and manipulate data has allowed for the creation of data visualizers, where huge amounts of data can be graphically accessed by both amateur and professional scientists. Enter OMNIWeb!!!
What is OMNIWeb and what does it do?
OMNIWeb is a WWW-based data retrieval and analysis interface to NSSDC's OMNI data which consist of 1-hour-resolution "near-Earth" solar wind magnetic field and plasma data, energetic proton data, and geomagnetic and solar activity indices. It allows you to select a subset from the available OMNI data to view or retrieve. It also provides a graphical browsing capability to analyze and preview the data as time series plots. OMNIWeb data has been collected since 1963 and has involved 18 spacecraft.
Visiting OMNIWeb at http://nssdc.gsfc.nasa.gov allows you to study the Sun and heliosphere by selecting variables of interest and display that data as graphs or tables. For example, when asking “Does the magnetic field change prior to increases in solar activity at solar max?”, a plot like the one shown above is useful. Note the spike in magnetic activity prior to the solar max in 1979-80.
A sampling of data that is available on OMNIWeb
Sunspot Data
Sunspots are large scale storms on the sun which can be many times larger than the entire earth itself. Why are sunspots of any concern to us here on earth? We now realize that sunspots are indicators of amazing events occurring every second, on the surface of
the sun. Radio emissions, for example, follow an 11 year cycle just as sunspots, reaching a maximum intensity during the same time as sunspot maximums. Radio waves are not blocked by the atmosphere so it is easy to study and record these with ground based receivers.
It is also thought that the sunspot cycle has an affect on the Earth's climate. Examples include the period of time from 1645 to 1715, called the Maunder Minimum, when almost no sunspots were detected. This is believed by some, to have caused a simultaneous mini-ice age in Europe.
R - Number of sunspots observed on the Sun.
Magnetic Field Data
Since the solar wind is made up of charged particles that have been discharged from the Sun, these particles interact with magnetic fields that they experience along the way, and are affected by these magnetic fields. This is why magnetic field measurements in the heliosphere are so important!
A good measure of the magnitude of the magnetic field at different points is found in OMNIWeb data, gathered from magnetometers on board many spacecraft. Field magnitude average provides data on the strength of the magnetic field at the location of the measuring spacecraft. (Why not simply make these measurements on the Earth’s surface? - see a later discussion of this issue.) The answer
Other OMNIWeb variables give specific information about vector components and direction of the magnetic field, and its average location.
Plasma Data
Plasma is an ionized gas. At temperatures similar to those found on the Sun, hydrogen atoms are ionized, and become protons. Thus, the majority of plasma coming from the Sun is composed of free particles like protons or electrons. For scientists who study this data, it is important to note changes in plasma temperature, how many particles or ions are in a region of space (ion density), and how fast the flow is (flow speed).
Proton Flux Data
Particles that are part of the solar wind have a wide range of speeds and energies. Especially important are variables that express how energetic the particles and ions are that are flowing through a given amount of space per unit time, referred to as flux. With energy measured in Mega electron volts (MeV), energetic proton flux data is readily available as well. This data can be used to determine how many particles of higher and lower energies are flowing through a region of space.
Important Indices
In general, each index expresses that effect felt at or near the Earth
Kp - The planetary three-hour-range Kp index was introduced by J. Bartels in 1949 and is derived from the standardized K index (Ks) of 13 magnetic observatories. It is
designed to measure solar particle radiation by its magnetic effects as seen on Earth.
C9 - The C9 index is an index devised to express geomagnetic activity on the basis of the Cp index which is a qualitative estimate of overall level of magnetic activity for the day.
Dst - (Disturbance Storm Time) The Dst index is an index of magnetic activity derived from a network of near-equatorial geomagnetic observatories that measures the intensity of the globally symmetrical equatorial electrojet (the "ring current").
How is this data used by scientists?
1. To infer correlations (relationships or patterns) between two or more variables.
As discussed previously, different data sets can be observed and easily analyzed to determine if and where patterns and relationships may exist.
2. To study the effects of a particular event and “track” or follow that event as it moves through the heliosphere.
As a CME or other events move through the Solar System, the collected data provides a cross section of what is happening at what location. In some ways, this is analogous to studying the historical track of a hurricane, or looking at a weather map for a specific, prior day.
Answer to previous question
Why not simply make these measurements on the Earth’s surface?
Examine the diagram below (clearly not to scale). Note the existence of a protective, magnetic sheath around the Earth, the Earth’s magnetosphere. Any data taken in this region would not be indicative of unmodified solar wind phenomena, but instead would reflect small changes due to the solar wind. Therefore, although the data is valuable for monitoring the interactions between the solar wind and the Earth’s surface level magnetic field, no valid data directly related to the solar wind can be taken from those locations.
Please explore OMNIWeb and its relative COHOWeb (at http://nssdc.gsfc.nasa.gov/cohoweb/form/cw_ds.html) in fun and informational investigations. To learn more, see other educational briefs on the topics.
LINKS TO ED BRIEFS AND INVESTIGATIONS TO BE INCLUDED HERE.
CREDITS:
Pat Keeney
GESSEP Program
Linda McClelland
GESSEP Program
Dr. Joseph King
Director NSSDC, Goddard Space Flight Center
Dr. Farzad Mahootian
Education Specialist Sun-Earth Connection
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