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


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