# Stars/Sun/Heliospheres

The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself.

## Theoretical heliosphere

Def. the "region of space where interstellar medium is blown away by solar wind; the boundary, heliopause, is often considered the edge of the Solar System" is called the heliosphere.

## Outer coronas

The "observation of fine-scale structure in the outer corona at solar maximum [has occurred]. The processed images reveal radial structure with high density contrast at all observable scales down to the optical limit of the instrument, giving the corona a "woodgrain" appearance. Inferred density varies by an order of magnitude on spatial scales of 50 Mm and follows an f$^{−1}$ spatial spectrum. The variations belie the notion of a smooth outer corona. They are inconsistent with a well-defined "Alfvén surface," indicating instead a more nuanced "Alfvén zone"—a broad trans-Alfvénic region rather than a simple boundary. Intermittent compact structures are also present at all observable scales, forming a size spectrum with the familiar "Sheeley blobs" at the large-scale end."
"The P78-1 (Solwind) coronagraph (Michels et al. 1980), operating from 1979 to 1985, was a duplicate of the OSO-7 coronagraph, but was modified to record a full 256 × 256 pixel image of the corona out to 10 R$_⊙$ in about 4.4 minutes, instead of 44 minutes. It was operated at a regular cadence and therefore was able to observe many CMEs (Howard et al. 1982; Webb & Howard 1994), including the "halo CME"—the first Earth-directed CME observed in white light (Howard et al. 1982). The Solar Maximum Mission coronagraph (MacQueen et al. 1980) observed the corona in 1980 and 1984–1989 out to 6 R$_⊙$ in a "quadrant mode" that enabled CME detection with higher spatial resolution than previously; accomplishments included the discovery of the three-part CME (Illing & Hundhausen 1985)."
"Then in 1995, the era of the charge-coupled device (CCD) detector began. The Large Angle and Spectrometric Coronagraph (LASCO; Brueckner et al. 1995) was launched on the Solar and Heliospheric Observatory (SOHO; Domingo et al. 1995). The three LASCO coronagraphs each carried a 1024 × 1024 CCD, which had higher spatial and photometric resolution than the previous instruments and together imaged the corona out to 32 R$_⊙$. The sensitivity improvements revealed an unanticipated level of variability along coronal structures, in both spatial and temporal scales, with clearly outflowing plasma mimicking the acceleration postulated for the solar wind (Sheeley et al. 1997)."
"Beginning in 2007, the five telescopes within the SECCHI suite (Howard et al. 2008) carried on the Solar Terrestrial Relations Observatory (STEREO) spacecraft (Kaiser et al. 2008) observed the heliosphere from the surface of the Sun to about 384 R$_⊙$ and, for the first time, imaged the fluctuating solar wind beyond 30 R$_{Sun}$ (Sheeley et al. 2008). In addition to CME imaging (e.g., Thernisien et al. 2009; Liewer et al. 2010; Poomvises et al. 2010; Mishra et al. 2015), the wide-field imagers in SECCHI have yielded important results on the structure of the solar wind itself, including observation of small-scale periodic density enhancements convected out with the solar wind (Viall et al. 2010; Rouillard et al. 2011). More recent analyses include measurements of the outer limits of the corona (DeForest et al. 2016) and observations of the nascent stages of a stream interaction region (SIR; Stenborg & Howard 2017)."

## Termination shocks

The point where the solar wind slows down is the termination shock.
Further out "is the heliosheath area ... As of June 2011, the heliosheath area is thought to be filled with magnetic bubbles (each about 1 AU wide), creating a "foamy zone".

## Electron winds

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."

## Protoplanetary disks

"The German philosopher Kant was the first to conceive the idea that the Sun originated as a condensation from a nebula, and the same idea in a more elaborate and refined form was later put forward by Laplace (1796)."

## Heliopauses

The point where the interstellar medium and solar wind pressures balance is called the heliopause.
The point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock.
The outer edge of the solar system is the boundary between the flow of the solar wind and the interstellar medium. This boundary is known as the heliopause and is believed to be a fairly sharp transition of the order of 110 to 160 astronomical units from the sun. The interplanetary medium thus fills the roughly spherical volume contained within the heliopause.

## Interstellar medium

The interstellar medium begins where the interplanetary medium of the Solar System ends. The solar wind slows to subsonic velocities at the termination shock, 90—100 astronomical units from the Sun. In the region beyond the termination shock, called the heliosheath, interstellar matter interacts with the solar wind. Voyager 1, the farthest human-made object from the Earth (after 1998), crossed the termination shock December 16, 2004 and later entered interstellar space when it crossed the heliopause on August 25, 2012, providing the first direct probe of conditions in the ISM.

## Hypotheses

* Each star has its own version of a heliosphere, perhaps called a stellasphere.