Seismic imaging of the Sun's far hemisphere and its applications in space weather forecasting

Abstract

The interior of the Sun is filled acoustic waves with periods of about 5 min. These waves, called "p modes," are understood to be excited by convection in a thin layer beneath the Sun's surface. The p modes cause seismic ripples, which we call "the solar oscillations." Helioseismic observatories use Doppler observations to map these oscillations, both spatially and temporally. The p modes propagate freely throughout the solar interior, reverberating between the near and far hemispheres. They also interact strongly with active regions at the surfaces of both hemispheres, carrying the signatures of said interactions with them. Computational analysis of the solar oscillations mapped in the Sun's near hemisphere, applying basic principles of wave optics to model the implied p modes propagating through the solar interior, gives us seismic maps of large active regions in the Sun's far hemisphere. These seismic maps are useful for space weather forecasting. For the past decade, NASA's twin STEREO spacecraft have given us full coverage of the Sun's far hemisphere in electromagnetic (EUV) radiation from the far side of Earth's orbit about the Sun. We are now approaching a decade during which the STEREO spacecraft will lose their farside vantage. There will occur significant periods from thence during which electromagnetic coverage of the Sun's far hemisphere will be incomplete or nil. Solar seismology will make it possible to continue our monitor of large active regions in the Sun's far hemisphere for the needs of space weather forecasters during these otherwise blind periods.

Keywords: Sun; solar activity; space weather.

Figure 1

Unlike electromagnetic radiation in a vacuum, sound waves in the solar interior are continuously refracted, following curve paths that return them to the surface in distance usually much shorter than that of the straight‐line trajectory tangent of the path at the source, S. Waves with periods around 5 min are specularly reflected at the surface. They continue for multiple skips beneath the surface before they eventually lose their coherence or decay.

Figure 2

Wavefronts, represented here as expanding turquoise rings, emanate from seismic sources embedded in a model of the solar interior. Where these wavefronts break to the surface, they manifest outwardly propagating surface ripples beginning directly above respective sources and propagating outward along the surface therefrom. Curved trajectories terminated by arrow heads represent segments of the ray paths that characterize refraction in the solar medium (see Figure 1). Reproduced from Lindsey et al. [2011], courtesy of InTech (http://www.intechopen.com).

Figure 3

Coherent computational regression of the surface acoustic field into a computationally accessible model of the solar interior. A record, ψ ₀, of the surface acoustic field is applied computationally to the model surface in time reverse to drive a disturbance that propagates backward into the model interior. This coherent acoustic egression, H ₊(r, t) can be sampled anywhere in the model interior. We call the acoustic power, |H ₊(r, t)|², of this regressed acoustic field the “egression power” of ψ ₀. This is averaged over time along a “sampling surface” (red) whose depth is the same as that of the leftward seismic source in Figure 3 and plotted radially beneath it (bottom of frame). Reproduced from Lindsey et al. [2011], courtesy of InTech (http://www.intechopen.com).

Figure 4

Diagram of wavefronts (loci fading from red to turquoise) and ray paths (solid red, solid turquoise, and green and yellow arrows) representing a cross section of waves traveling to and from a “focus” (top) and reflecting from the Sun's surface once during their travel between the far hemisphere (top) and the near hemisphere (bottom). Reproduced from Lindsey et al. [2011], courtesy of InTech (http://www.intechopen.com).

Figure 5

Composite maps of the Sun's far hemisphere (amber) and the line‐of‐sight magnetic field (blue gray) show NOAA AR 11498 at (190°W, 12°S) crossing (top) central meridian in the far hemisphere (amber) approaching the (middle) east limb (amber region moving leftward with respect to [190°W, 12°S]), and (bottom) having passed into the near hemisphere (blue gray), at which time it received the foregoing NOAA designation. The phase correlation signature is rendered in terms of the travel time perturbation, τ, carried by the echo from the magnetic photosphere in the far hemisphere as compared with the quiet Sun.

Figure 6

(top left) Seismic map of NOAA AR 11158 in the Sun's near hemisphere is shown at 15 February 2011 concurrently and cospatially with (bottom left) a visible continuum intensity map, (top right) a line‐of‐sight magnetic map, and (bottom right) a He ii 304 Å intensity map.

Figure 7

Composite seismic (amber) and magnetic (blue gray) map of the far and near hemispheres of the Sun in longitude (horizontal) and latitude (vertical) on 5 November 2014, a time of intense solar activity in the far hemisphere. Signatures of large active regions in the Southern Hemisphere are designated FS‐103, at Carrington longitude 250° and FS‐101 at 345°.

Figure 8

(top left) SDO/HMI visible intensity and (top right) line‐of‐sight magnetic and (bottom left) AIA intensity maps in 1700 Å and (bottom right) He ii 304 Å on 17 November 2014, with both FS‐101 and FS‐103 (see Figure 7) now in direct view from Earth. At this point, NOAA has designated FS‐101 as ARs 12208 (east) and 12207 (west) and, subsequently, FS‐103 as ARs 12214 (east), 12209 (middle), and 12213 (west).

Figure 9

(top) The seismic map shown in longitude‐latitude format in the top frame (amber) is projected (bottom left) onto the Sun as viewed from (bottom right) SDO/HMI in a line‐of‐sight magnetogram 8.5 days later. The seismic signatures are insensitive to the magnetic polarity. However, the Hale polarity law offers a resource whereby this may be guessed with sufficient dependability to give us more realistic models of the coronal magnetic field than are possible without knowledge of a newly emerged seismic signature.

Figure 10

(c) Massive halo CME imaged by the Large Angle Spectrometric Coronagraph (LASCO), aboard the Solar and Heliospheric Observatory (SOHO) at 00:31 UT on 15 August 2001. (a) Fluxes of high‐energy protons, which began to arrive minutes after first appearance of the CME to SOHO. (b) Excess X‐ray flux from the SOHO vantage is essentially nil, because the CME emanated (we think) from the Sun's far hemisphere; hence, any X‐rays from the active region were radiated into the far side of the solar system. (d) A seismic image, presenting the Sun's far hemisphere as viewed directly from SOHO through the near hemisphere, shows the clear signature of an active region at ∼0.3 solar radii south (below) and slightly east (left) of disk center. This active region was a composite destined to be designated NOAA 9557 and 9591 about a week later, after it had rotated into direct view from Earth. Reproduced from Lindsey et al. [2011], courtesy of InTech (http://www.intechopen.com).