# Stars/Sun/Heliophysics

"Heliophysics is a fast-developing scientific discipline that integrates studies of the Sun's variability, the surrounding heliosphere, and the environment and climate of the planets."

## Astronomy

Helios is a name recently given to the Sun, but in ancient history it may have referred to Saturn.
With the exception of viewing solar eclipses, the only visible portion of the sun apparently is the outer surface of the photosphere.
Literally, heliophysics is the astrophysics of the solar ball limited by the upper surface of the photosphere.

## Radiation

All of the radiation originating from, at or above the photosphere creates at least the heliosphere ending at the heliopause.

## Heliophysics

"Heliophysics is concerned with laws that give rise to structures and processes that occur in magnetized plasmas and in neutral environments in the local cosmos, both temporal (weather-like) and persistent (climate-like). These laws systematize the results of half a century of exploring space that followed centuries of ground-based observations. During this time spacecraft have imaged the Sun over many wavelengths and resolutions. They have visited every planet, all major satellites and many minor ones, and a selection of comets and asteroids. Beyond this they have traversed the expanse of the heliosphere itself. Out of the vast store of data so accumulated, the laws and principles of heliophysics are emerging to describe structures that are natural to magnetized plasmas and neutrals in cosmic settings and to specify principles that make the heliosphere a realm of numerous, original dynamical modes."
"In the case of heliophysics, probably most of its laws have yet to be discovered, since the project of finding them is young. Moreover, heliophysics is a unique hybrid between meteorology and astrophysics with substantial components of physics and chemistry. Thus, many of the laws of heliophysics that we can list at this time might be subjects for research in meteorology (e.g. the field of aeronomy), astrophysics (e.g. shock waves and cosmic rays), physics (e.g. magnetic reconnection and particle energization), or chemistry (e.g. reaction rates in planetary ionospheres and thermospheres)."

## Theoretical heliophysics

Def. the physics of the heliosphere is called heliophysics.

## Sun

"The Sun is a magnetically variable star and, for planets with intrinsic magnetic fields, planets with atmospheres, or planets like Earth with both, there are profound consequences."

## Solar eclipses

"On 21 August 2017, the first total solar eclipse visible solely from what is now United States territory [...] will occur. This event, which will cross coast-to-coast for the first time in 99 years, will provide an opportunity not only for massive expeditions with state-of-the-art ground-based equipment, but also for observations from aloft in aeroplanes and balloons. This set of eclipse observations will again complement space observations, this time near the minimum of the solar activity cycle."
"Until coronal observations are available from the Moon, or from tandem spacecraft with a distant occulter, eclipse observations remain the only way to get white-light observations of the important regions of the lower and middle corona, in which the solar wind forms, of the lower parts of coronal streamers, and of polar plumes."
"The solar corona itself remains the main focus of scientific research performed during total solar eclipses [...]. There are two main directions for such studies. The first one concerns the time-domain solar corona. In fact, the rhythm of approximately one total solar eclipse every 18 months allows a good sampling of the global-scale changes the solar corona undergoes within the 11-year solar cycle. The second important branch is about the characterization of coronal conditions and its spectrum."
"The intricacies of the solar corona can now be imaged with electronic detectors at a cadence unavailable with film — especially the film sensitivities of a century ago, which necessitated drawing or painting the coronal configurations$^{10}$ — but also at cadences over ten times those available from any current solar spacecraft. At solar minimum, as it was in 2008 during the eclipse observed from Siberia, streamers are concentrated near the solar equator and polar plumes are visible$^{11}$. As solar activity resumes, velocities in streamers become higher, as seen from Easter Island in 2010$^{12}$, and they remained high for the 2012 eclipse (visible from Australia and the Pacific Ocean). For the 2012 eclipse, a coronal mass ejection (CME) appeared in the 40-minute lapse between the observations of the eclipse from inland Australia and from a ship north of New Zealand, allowing an estimation of the CME velocity of over a million km hr$^{–1}$ (refs 13,14). The 2013 total solar eclipse, observed from Gabon, showed two CMEs and an erupting prominence15, which allowed the measurement of CME velocities of the order of 150 km s$^{–1}$ in the lower- and mid-coronal regions that are below the occulting disk of space coronagraphs. Chinese observers using a fibre-optic spectrograph detected coronal dynamics during the Gabon eclipse$^{16}$. Though few papers have yet been published about the 2015 eclipse, whose totality was best studied from Svalbard in the Arctic, my team’s composite images show a hybrid corona, with helmet streamers extending to the north solar pole but with no streamers in the extreme south and with visible south polar plumes$^{17}$. The coronal configuration for the 2016 total solar eclipse, observed from Indonesia, was again transitional, with plumes visible at only one pole$^{18,19}$ [...]."

## Spectrum of the solar corona

"Among the dozen or so coronal emission lines in the visible, the strongest are the coronal green line at 530.3 nm, from [Fe xiv], and the coronal red line at 637.4 nm, from [Fe x], The ratio of the coronal red line and the coronal green line vary over the solar-activity cycle, revealing that the overall corona is hotter at solar maximum than at minimum$^{48,49}$. The 530.3 nm line is especially diagnostic in the middle corona$^{50}$. Because of the temperature difference, loops appear in different places when viewed in the coronal red line and the coronal green line$^{51}$."
"Using slitless spectroscopy from Siberia in 2008 and French Polynesia in 2010, during and after solar minimum, 3 Mm helium shells were detected in ionized helium at 468.6 nm and neutral helium at 471.3 nm$^{52}$. The flash spectra obtained during the 2010 eclipse were compared with images taken with the Sun Watcher using Active Pixel System Detector and Image Processing (SWAP) on ESA’s PROBA2 mission, finding many low-excitation emission lines in addition to the helium lines just mentioned, with analysis of prominence cavities in the corona as one result$^{53}$."

## Polarimetry

"Imagine polarimetry and spectro-polarimetry techniques were used in all recent eclipses: [...] spectro-imaging polarimetry results from the 2013 eclipse$^{46}$ highlighted a diverse set of mechanisms in the coronal green line, with polarization up to 3.2% above the continuum polarization on a spatial scale of 1,500 km. Polarization structure within a 7,500 km region led to the conclusion that coronal polarization is highly structured and variable even on such a small scale."

## Inner corona

"The inner solar corona, in addition to the emission lines, shows electron scattering, which is highly polarizing (and which obliterates the Fraunhofer lines$^{38,39}$, except potentially the broad and strong H and K lines). [Polarization] studies [are used] to explore velocities in the corona$^{40}$ and study the structure of the lower corona in preparation for space observations$^{41}$, providing two-dimensional distributions of the polarization angle and of the relative colour index. [Eclipse] observations are an efficient method to measure the electron-scattering corona (the ‘K-corona’). [The] Zeeman and Hanle effects [can be used] to detect polarization in prominences$^{42}$. [The] low fraction of neutral hydrogen that had previously been discovered from rocket observations$^{43}$ [has been detected]."

## Coronal brightness

"The brightness of the corona varies with the solar-activity cycle$^{34}$. It has long been known$^{35}$ that the corona is fainter at solar minimum. Helmet streamer distributions over the sunspot cycle have been compared with solar polar magnetic fields$^{36}$."

## Coronal flattening

"The flattening index — the deviation of the lines of equal brightness from circular — have been shown to match the phase rather than the magnitude of sunspot cycles, though there seems to be no such correlation with the sunspot number$^{37}$."

## Coronal heating

Continued analysis of "Fabry–Pérot narrow-band interferograms, including the images from the 2001 total eclipse observed from Zambia$^{29}$ [produced] components in the line profiles, perhaps related to coronal heating via type II spicules, jet-like bright structures that originate in the chromosphere and fade away rapidly in the corona, and [...] the presence of a persisting blue-shift."
"Narrow-band imaging can also be used to search for fast (>2 Hz) oscillations of coronal loops that could help discriminate among coronal heating theories. Such observations have been reported for the eclipses of 2006$^{30}$, 2009$^{31}$ and 2010$^{32}$. Evidence exists in the power spectra for sub-second oscillations, which would be typical of surface Alfvén waves, whereas body Alfvén waves would be associated with oscillations with periods of tens of seconds for coronal loops, which are observed just above the solar limb. Perhaps both Alfvén-wave heating and nanoflare heating coexist$^{33}$."

## Solar winds

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV.
"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."
The solar wind is divided into two components, respectively termed the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 km/s, a temperature of 1.4–1.6×10$^6$ K and a composition that is a close match to the corona. By contrast, the fast solar wind has a typical velocity of 750 km/s, a temperature of 8×10$^5$ K and it nearly matches the composition of the Sun's photosphere. The slow solar wind is twice as dense and more variable in intensity than the fast solar wind. The slow wind also has a more complex structure, with turbulent regions and large-scale structures.
The slow solar wind appears to originate from a region around the Sun's equatorial belt that is known as the "streamer belt". Coronal streamers extend outward from this region, carrying plasma from the interior along closed magnetic loops. Observations of the Sun between 1996 and 2001 showed that emission of the slow solar wind occurred between latitudes of 30–35° around the equator during the solar minimum (the period of lowest solar activity), then expanded toward the poles as the minimum waned. By the time of the solar maximum, the poles were also emitting a slow solar wind.
The fast solar wind is thought to originate from coronal holes, which are funnel-like regions of open field lines in the Sun's magnetic field. Such open lines are particularly prevalent around the Sun's magnetic poles. The plasma source is small magnetic fields created by convection cells in the solar atmosphere. These fields confine the plasma and transport it into the narrow necks of the coronal funnels, which are located only 20,000 kilometers above the photosphere. The plasma is released into the funnel when these magnetic field lines reconnect.
"The development of narrow-band filters with a full width at half maximum of about 0.2 Å in the red and near infrared, using multiple-coat reflection and temperature-controlled systems, allowed imaging of the solar corona with each filter probing a different temperature. [Using] filters not only at the coronal green forbidden [Fe xiv] line at 530.3 nm and the coronal red [Fe x] line at 637.4 nm, but also the 789.2 nm [Fe xi] line and even the 1,074.7 nm [Fe xiii] line [probes temperatures]."
"The [...] electron temperature and iron charge states [have been mapped] for the 2006 and 2008 eclipses, pointing out that only in collisional plasmas do the emission-line intensities give temperatures, linking the solar wind to electron temperatures below 1.2 × 10$^6$ K (ref. 23). [Observations have been connected] to in situ measurements from NASA’s Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) and on Ulysses throughout solar cycle 23 (1998–2009)$^{24}$. With observations of [the] four spectral lines from the same eclipse, prominences [were discovered that] are enshrouded in hot plasmas held in place by twisted magnetic fields$^{25}$. For the 2010 eclipse, from Tatakoto in French Polynesia, [with] filters [added] at Hα, [Fe ix] at 435.9 nm, and [Ni xv] at 670.2 nm, provided a temperature differentiation of 200,000 K, and [allowed them to be] compared with extreme ultraviolet observations$^{26}$."

## Classical planets

"The Golden Age of Re was the age of An [Anu], Yama, or Kronos. One thus finds of interest an Egyptian ostrakon (first century B.C.) cited by Franz Boll: the ostrakon identifies the planet Saturn as the great god Re.$^4$ ... [T]he expression "star of Helios" or "star of Sol" was applied to Saturn.$^5$ Though the Greek Kronos was the Latin Saturn, Nonnus gives Kronos as the Arab name of the "sun." ... the Greek name Helios so closely resembles the Greek transliteration of the Phoenician El ... Plato (or his pupil Phillip of Opus ... gave the name Helios to Saturn."

## Hypotheses

* The mass of the Sun may not directly reflect the amount of matter in the Sun.