Why Study the Sun?

One of the major science topics of the OVRO Long Wavelength Array (OVRO-LWA) is the physics of the Sun and its atmosphere.  The Sun is often the brightest radio source in the sky due to its greater than 1 million degree coronal temperatures at quiet times, but also due to its propensity to unleash vast numbers of nonthermal particles during solar flares, which can reach brightness temperatures greater than 1015 K. We have many reasons to study the Sun, ranging from the basic science of how the Sun produces the solar system’s biggest explosions, to the realization that other stars and planetary systems produce the same phenomena, which can only be studied in the minutest detail in our nearest star, to the very practical reason that the Sun affects our daily lives as well as the health of our technological infrastructure on the Earth and in space.

Frequency Corresponds to Electron Density Corresponds to Height

The OVRO-LWA operates in the frequency range 20-88 MHz, the so-called metric radio range because the corresponding wavelengths are about 3-15 meters.  Fundamental physics tells us that the completely ionized atmosphere of the Sun, like any plasma, oscillates at different frequencies according to the plasma frequency of the local medium, which depends only on the electron density  according to

[Sorry folks, I don’t have Word, so please send equations in Google doc or pdf or some other easily readable format]

Any disturbance in this ionized solar atmosphere (the corona) will cause plasma oscillations at this frequency that are then easily converted to electromagnetic waves (radio waves at the same frequency (or in some cases the 2nd harmonic of that frequency).  Because the electron density of the solar corona falls with increasing distance from the Sun, the specific range of 20-88 MHz (electron densities ranging roughly from  to  electrons/cm-3 according to the above equation) corresponds to a height range of 1.3 to 2  (solar radii, measured from the center of the Sun).  This is an important height range to study, because it is over this range that the solar wind is accelerated and the coronal structure at lower heights transitions to the “Parker spiral” magnetic structure that defines the remainder of the heliosphere.

Types of Disturbances

Anything that disturbs the solar atmosphere will start it oscillating, but two types of traveling disturbances are the most common and dramatic.  One type of disturbance is a shock wave, which results from a magnetic instability that drives a coronal mass ejection (CME).  These ejections of up to 1013 kg of mass and their associated magnetic field can accelerate to high speeds (> 1000 km/s) and drive a shock wave at similar speeds ahead of it.  This can result in a radio burst that starts at higher frequencies when the shock wave is still at low heights and shifts to progressively lower frequencies over about ten minutes as it moves to greater heights.  This type of radio burst is called type II.  Another type of traveling disturbance is the generation of an electron beam due to a much more rapid magnetic instability called magnetic reconnection.  This sudden and rapid release of magnetic energy accelerates electrons to a good fraction of the speed of light () and drives a radio burst that again starts at higher frequencies and shifts progressively to lower frequencies as it moves outward, but now the entire burst may take only seconds to traverse that distance.  This type of burst is called type III.  However, there are many other disturbances possible, and many other wave modes in addition to the plasma oscillations discussed earlier.  The myriad disturbances that can occur during solar flares and eruptions, and the interplay among the possible plasma wave modes, give rise to extremely complex radio emission signatures in both frequency and time.

The OVRO-LWA Innovation—Combining Spatial and Spectral Capabilities

Figure 1 shows a group of type III bursts observed at one of the Radio Solar Telescope Network (RSTN) sites that monitor the Sun for the U.S. Air Force and other government agencies.  The frequency range is roughly the same as OVRO-LWA, but the sensitivity is far lower.  The same group of type III bursts was observed with the Long Wavelength Array station LWA-1, with similar frequency and time resolution as OVRO-LWA.  The vast improvement in frequency and time resolution, and sensitivity, reveals many features of the burst that were unseen in the RSTN record, including the occurrence a U burst (near 20:38:35 UT) where the electron beam stopped traveling outward and turned back toward the Sun.  U bursts are thought to be electron beams in large but closed magnetic loops, allowing the particles to return to the Sun rather than escaping.

The illustration in Figure 1 uses data from an older Long Wavelength Array station (LWA-1), but OVRO-LWA will improve on LWA-1 in two important respects.  The first is that OVRO-LWA has a solar-dedicated beam on the Sun whenever the Sun is visible.  That means continuous coverage of solar activity rather than the occasional coverage possible with LWA-1, which ensures that we will not miss interesting events that could lead to a potential breakthrough in understanding the underlying solar processes.  That also represents a great challenge!  The 2-minute period covered by Figure 1, when extended over an entire day, day after day, represents a vast amount of data that must be analyzed and visualized.  We expect to develop machine-learning tools to permit automated identification and classification of events.

The second major improvement is that OVRO-LWA obtains images of the emission at more than 700 frequency channels at 0.1 s time resolution, so that the sources can be spatially located with respect to disturbances imaged in other wavelengths (allows mapping of trajectories of electron beams and the outward-propagating source regions of shock-related emission).  This is illustrated in Figure 2 for one event observed in 2015 with an earlier version of OVRO-LWA.  Note that the data available for the event in Figure 2 was much narrower band (40-70 MHz vs. 20-88 MHz), lower time resolution (9 s vs. 0.1 s), and 2x lower spatial resolution than we have with the upgraded OVRO-LWA.

The great advance of OVRO-LWA for solar science is represented by the spatially resolved spectrum in the Bottom-Right panel of Figure 2.  The technique of isolating the radio spectrum as a function of position in a source is called dynamic imaging spectroscopy, and provides the opportunity to measure physical parameters point-by-point in the source region.  In the example of Figure 2, the source was found to have a magnetic field strength of 1.42 G, and a nonthermal electron density of 5400 electrons/cm-3 distributed as a powerlaw in energy with a powerlaw index of 3.7.  Such specificity is the key to a deep understanding of the processes involved in these events, and events like them that may be occurring on other stars.