New Research Enables Space Physicists To Gain Insights Into Heliosphere

For some time  space physicists and solar physicists have pondered the hypothetical properties of the heliosphere - the protective bubble created by the solar wind and depicted in the image above.  Understanding the physics at the bubble's edge- called the heliosheath -  is not easy.  This is given it's in constant flux and pushes out against the broader interstellar magnetic field that permeates our corner of the Milky Way.

For reference, the heliosheath occurs at the far edge of the heliosphere. More technically, it occurs between the "termination shock" and the "heliopause".    The heliopause  is the interface where the solar wind is stopped by the interstellar medium. One can therefore think of it as a three dimensional region or surrounding “envelope” at which the solar wind's strength is no longer sufficient to overcome the stellar winds of the external stars. In technical terms, this absence of counter-pressure signals the end of the solar system.   Thanks to data from the Voyager 1 and Voyager 2 spacecraft,  we now know the outer boundary of the heliosheath  is located roughly 18 billion kilometers from the Sun. Or 119 times the distance from the Earth to the Sun -  right where Voyager 2 found it in November, 2018.

Now,  Dialynas et al.  have combined Voyager data with observations from NASA's Cassini mission - which orbited Saturn from 2004 to 2017- to  gain much more insight.  Basically, the researchers recognized that the missions, although launched 20 years apart, had collected complementary data. Voyager 1 and 2 had instruments that measured energetic ions as the craft crossed the heliosheath and exited the solar system. Cassini, meanwhile, was able to remotely observe energetic neutral atoms  arriving in all directions from the heliosheath.


The energetic neutral atoms come from the heliosheath, where fast solar wind protons collide with neutral hydrogen atoms from interstellar space and “steal” an electron from the interlopers. The Voyager probes took in situ measurements of the parent heliosheath proton distributions as they passed through this region. Meanwhile, the protons with newly added electrons become energetic neutral atoms and shoot off in all directions.

The synergy among the spacecrafts’ observations allowed the researchers to use Voyager data from the heliosheath to ground transmissions and thereby calibrate energetic neutral atom data from Cassini, which was more sensitive to lower energetic particles than Voyager. Together, the spacecraft extended data on the intensity of both energetic neutral atoms and ions to include a broader range of energies, which gave the team a window into the physics in the heliosheath as the solar wind and interstellar medium press against each other.

The researchers found that in the energy range considered in their study (>5 kiloelectron volts), lower-energy ions with energies between about 5 and 24 kiloelectron volts played the largest role in maintaining the pressure balance inside the heliosheath. This allowed the team to calculate the strength of the magnetic field and the density of neutral hydrogen atoms in interstellar space—about 0.5 nanotesla and 0.12 per cubic centimeter, respectively.

Their finding that the lower-energy ions dominate the pressure balance in the heliosheath means that space physicists will have to rethink their assumptions about the energy distribution of such particles in the heliosheath..  See e.g. Geophysical Research Letters, https://doi.org/10.1029/2019GL083924, 2019.

Interestingly, pressure balance considerations also entered in early models and computations to do with another part of the heliosphere:  the solar corona .   In the past century an interesting question was whether the corona was static or not. In a static case its boundary would be more or less fixed, so there'd be no expansion even in times of high solar activity.  For this to occur there would need to be a consistent pressure balance, i.e. between the outward coronal  (& solar wind) pressure and inner directed pressure, from the interstellar medium.

 A static corona  superficially appeared  to be quite reasonable.  And so it was that the father of space physics,  Sydney Chapman,  first assumed a condition for hydrostatic equilibrium applied:

dp/ dr = - r {GM_s/ r²}

Where G is the usual Newtonian gravitational constant, and r defines the plasma density for the corona, with n the number density for protons, e.g. 

r = n(m_p) 

while M_s is the mass of the Sun, and r the distance from the solar center:

The coronal pressure (p) is given by:

p = 2 n T

Provided both protons and electrons are assumed to have the same temperature.

Ultimately, detailed computations showed the static corona model could not be accurate.   If the static model were accurate, the pressure at infinity, i.e.

p(¥) = p(R_o) [exp – 7k/5 * 1/ T(R_o) R_o] 

 Should be zero  (p(¥)   = 0) , not a small finite pressure that’s effectively equal to the coronal base pressure. This finding led to the further investigations that disclosed a solar “wind” had to flow outwards from the corona. 

See also:

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068607/full


And:

New Research Into The 'Slow' Solar Wind Sheds Ligh...

New Research Into The 'Slow' Solar Wind Sheds Light On Its Source.

The solar wind is a  1, 610,000 km/h stream of charged particles constantly emitted by the Sun in all directions. However, solar wind dynamics are maddeningly complex as the original contributors to the Solar-Terrestrial Predictions Workshop held in Meudon, France (1986) learned.

For example, fast solar wind can attain speeds in excess of 500 km/s and emerges from coronal holes. These latter comprise dark holes visible in coronal imagery indicating where the Sun's magnetic field lines open up and extend into space thereby providing an escape channel for hot solar plasma. In the coronal image below these holes, as well as magnetic field arches, can clearly be seen:

This fast solar wind  is differentiated from the much slower moving stream of solar wind that floods the solar system continuously, and defines the "slow wind". It defines what we call the heliosphere which is essentially the solar wind's" bubble of influence.

The heliosheath occurs at the far edge of the heliosphere. More technically, it occurs between the "termination shock" and the "heliopause".

Thus, it marks a boundary at the edge of solar system space. In May 2005, NASA announced that Voyager 1 had crossed the termination shock and entered the heliosheath in December 2004, at a distance of 94 AU. 

While we can tie the fast solar wind to coronal holes, the origin of the slow solar wind has proven far more difficult to untangle.  One way to possibly resolve the issue is to examine data from the slow wind  intercepted by several near Earth spacecraft.

In fact, this has been reported now by Kepko et al (Geophysical Research Letters, 2016)  see e.g.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068607/full


who have used such data to trace the slow wind's origin. This entailed examining its charge state ratios. For example, it has been found  that the ratio of charged oxygen atoms (ions)  O+7/ O+6,   well reflects and tracks the level of entropy and conditions at the solar surface (cf. Wimmer-Schewingruber et al, 1997, J. Geophysical Research)  Thus, the slow solar wind exhibits higher  O+7/ O+6  ratios than the fast wind.  The slow wind also displays high  O+7/ O+6 variability (cf. Zurbuchen et al, 2000, op. cit.).

The point is that these charge state ratios as well as elemental composition can serve as "fingerprints" for the specific conditions that generate the solar wind. The authors used measurements of charge state abundance and composition measurements at 12 minute intervals made by the ACE (Advanced Composition Explorer)  and WIND spacecraft.  The research team found that the plasma measurements fluctuated at roughly 90 minute cycles.  (This agreed with previous research on charge state and composition variability)

Thereby, Kepko et al discovered that the charge state and composition properties of the slow solar wind oscillate in nonrandom patterns. The oscillations occurred between patterns more typical of fast solar wind and more typical of slow wind. Specifically, the proton (H+) density within the plasma was three times higher at the midpoint of the cycle before decreasing. Meanwhile, the helium, carbon and oxygen abundances peaked late in the 90 minute cycle.

This pattern of oscillation indicated the charge state and composition of the slow wind originate in the solar atmosphere itself, not in transit from the surface. How can this be? Well, only magnetic reconnection at the solar surface could cause the solar wind to alternate properties characteristic of both fast and slow aspects. In other words, what we have is strong evidence for a magnetically-driven slow wind source with the potential to overturn previous models. As the authors put it (op. cit.):

"The major conclusion from our study is that the slow solar wind exhibits a quasiperiodic variability with a time scale of ∼90 min, with a repeatable, systematic, elemental abundance signature. Further, on this time scale, the wind speed shows no correlation with the repeatable charge state or elemental abundance signatures.

This result has far-reaching implications for understanding slow wind origin. First, it rules out all quasi-steady models such as the expansion factor. In the quasi-steady models the solar wind plasma properties in any flux tube are set by the geometry of the flux tube in the corona, which determines the heating and momentum deposition along that flux tube."

In the above quote what Kepko et al are referring to are the "expansion factor" wind models that predict the slow wind issues from the tube like stretches of space containing the solar magnetic field as it peels off into space away from the Sun at the edges of coronal holes. Thus, the authors' model offers instead strong evidence for a magnetically driven slow wind source..

Why is it important which model is correct? Because we will then  be able to more accurately forecast solar wind phenomena including the occurrence of magnetic substorms at Earth's magnetophere.  In effect, we would be better able to protect infrastructure, including power grids on Earth, from the vagaries of space weather.