1. Solar wind interaction with the comet Halley and Venus
K. Murawski
University of M. Curie Skłodowska

2. Outline Overview of solar wind interaction with
magnetic and non-magnetic bodies
Numerical simulations of the solar wind
interaction with Venus
interaction with the comet Halley
Summary

3. A global view of the Heliosphere

4. Solar system - Icy Matter ...
Jan Oort ( ), a Dutch astronomer, speculated that a great reservoir of icy/gritty debris orbits our sun at enormous distances compared even to the orbits of remote worlds like Uranus and Pluto. Occasional perturbations or collisions within this cloud send pieces of loosely bound ice and debris into interstellar space or hurtling toward the sun. These sunward travelers appear in our night skies as comets.
Jan Oort’s Cometary Cloud

5. Outskirts of the Solar system - Comets

6. Properties of the solar wind
highly conducting plasma
electrons, protons + alpha-particles
radial expansion
magnetic field “frozen” in the plasma
SW = super-sonic + super-alfvénic
Interplanetary
Magnetic
Field
Planetary
Obstacle
Radial Plasma
Outflow
Solar
Rotation
1 AU
ne ≈ 5 cm-3
T ≈ 105 K
|BIMF| ≈ nT
vsw ≈ km/s
vA ≈ km/s
cS ≈ km/s
The sun emits the highly conducting plasma into the interplanetary space;
this expansion is a radial propagation of the SW plasma fluid away from the sun
during which it must become diluted and at the same time cool down.
The SW consists mainly of electrons and protons,
with an admixture of 5% Helium ions.
Because of the high conductivity, the solar magnetic field is frozen in the plasma
and drawn outward by the expanding SW.
The sound and alfven velocities are the 2 important velocities in the SW,
and since both are significantly smaller than the SW bulk velocity,
the SW is considered as a supersonic and superalfvenic flow.

7. TYPES OF INTERACTION WITH THE SOLAR WIND
INTERNAL
MAGNETIC FIELD
ATMOSPHERE
VENUS
MARS
EARTH
EARTH'S MOON
MERCURY
COMETS
SATURN
JUPITER
URANUS
NEPTUN
The 2 important properties in defining the type of interaction are the atmosphere and the magnetic field.
The question is whether the object has a substantial atmosphere and / or an internal magnetic field.
Saturn, Jupiter and Earth have dynamo driven magnetic fields and a significant atmosphere.
Venus, Mars and comets have just a substantial atmosphere, but no global magnetic fields.
Mercury has only a strong magnetic field and
Earth’s moon has neither a MF nor an atmosphere.
GANIMEDE

8. Simplest case: Earth's MOON NO magnetic field NO atmosphere
The simplest case to start with
is the Earth’s moon,
which has neither a MF nor
a significant atmosphere.

9. MOON – type no magnetic field negligibly thin atmosphere
insulating material
submerged in a flowing plasma
absorption of particles
no bow shock upstream
plasma – absorption wake
magnetic field parallel to the upstream flow → no effect
magnetic field perpendicular to the flow → minimal effect
A body like our moon, composed of insulating material and submerged in a flowing plasma simply absorbs the particles of the plasma that are incident on the body.
A bow shock will not form upstream because there is no obstacle as far as the oncoming flow is concerned.
The magnetic field diffuses into the very weak conducting outer layers at a rapid rate so that it is barely perturbed from its upstream orientation.
The most notable features are associated with the plasma-absorption wake left in the plasma behind the body.
If the body’s magnetic field is zero and the flow speed is high compared with the thermal velocity the wake will persist to large distances, but if the flow is slow relative to the thermal speed, thermal motions perpendicular to the flow direction can refill the empty space within a short distance downstream of the body.
The introduction of a magnetic field can either inhibit the refilling of the wake, if the field is nearly parallel to the upstream flow, or have a minimal effect if it is perpendicular.
Illustration of the interplanetary plasma flow and magnetic-field perturbation by the nonconducting moon.
The wake created by solar-wind absorption closes more quickly when the magnetic field is not aligned with the undisturbed flow.

10. SW interactions with magnetized bodies and an atmosphere
Obstacle = magnetosphere
Now let’s have a short look at the SW interaction with a magnetized planet
which has a substantial atmosphere.

11. EARTH – type (Jupiter, Saturn)
strong magnetic field
substantial atmosphere
Plasma structures of the Earth's magnetosphere
bow shock
magnetosheath
magnetopause
cusp
lobes
neutral sheet
trapping region
plasma sheet
ionosphere
plasmasphere
solar wind
When the SW hits on the Earth’s dipolar MF, it cannot simply penetrate it but rather is slowed down and to a large extent, deflected around it. Due to the supersonic SW, a bow shock wave is generated, where the plasma is slowed down and a substantial fraction of the particles’ kinetic energy is converted
to thermal energy.
The region of thermalized subsonic plasma behind the shock is called magnetosheath.
Its plasma is hotter and denser than the SW plasma and the magnetic field strength has higher values in this region.
The kinetic pressure of the SW plasma compresses the field at the frontside, while the nightside magnetic field is stretched out into a long magnetotail, which reaches far beyond lunar orbits.
The magnetotail consists of 2 lobes of opposite polarity, separated by a neutral sheet.
A plasma or current sheet is located at the interface of the 2 lobes.
Solar radiation ionizes a fraction of the neutral atmosphere, creating an ionosphere, which gradually merges into the plasmasphere.
The plasmasphere is a torus-shaped volume inside the radiation belt, containing cool but dense plasma of ionospheric origin, which co-rotates with the Earth.
At high latitudes plasma sheet electrons can precipitate along MF lines down to ionospheric altitudes, where they collide with and ionize neutral atmosphere particles. As a by-product, photons emitted by this process create the polar light, the aurora.
plasma sheet:
due to the concentration of particles in the equatorial plane, where the field lines concentrated of lowest field strength and thus the plasma sheet is formed;

12. SW interactions with magnetized bodies but without an atmosphere
Obstacle = magnetosphere
Now let’s considered the case of a magnetized planet which has no atmosphere.
The obstacle to the SW is the magnetosphere.

13. Plasma structures of the Mercury's magnetosphere
MERCURY - type
strong magnetic field
no gravitationally bound atmosphere
Similarities and differences with Earth
Magnetosphere
Absence of an atmosphere and ionosphere
Solar wind conditions
Mercury has a larger fractional volume of its magnetosphere
no stable trapping regions
closed magnetic flux tubes
Solar wind – primary source of magnetospheric plasma
Plasma sheet – higher densities
Plasma structures of the Mercury's magnetosphere
magnetosheath
lobes
magnetopause
bow shock
cusp
solar wind
Mercury possesses a small magnetosphere resulting, as in the case of the Earth, from the interaction of the SW with the planetary MF. However, contrary to the case at Earth, the absence of an atmosphere and consequently, of an ionosphere around Mercury, leads to a magnetosphere which must have very different current systems from the terrestrial magnetosphere.
Because Mercury has no magnetosphere-ionosphere coupling, very unlikely there is no similar current system like we saw before in the case of Earth.
Mercury’s MF is sufficiently strong to stand-off the SW above the surface under usual SW conditions, however, at times of increased SW pressure, the interplanetary particles may impinge directly onto Mercury’s surface.
Mercury has a much larger fractional volume of its magnetosphere than Earth and this implies that the stable trapping regions we see in other planetary magnetospheres, the radiation belts, can not from.
The volume threaded by magnetic flux tubes that are closed (i.e., both ends rooted in the planet) is so small that any trapped charged particle population are expected to be transient.
The SW is expected to be the primary source of Mercury’s magnetospheric plasma, although planetary ions released or sputtered from the surface make some contribution.
The plasma densities in the plasma sheet are higher than in the Earth’s magnetosphere by roughly the difference in SW density at Mercury and Earth’s orbits, i.e., by a factor of 10.
0.382 AU
Mercury

14. COMETS NO internal magnetic field but atmosphere
Obstacle = exosphere
Perhaps the most extreme example of an atmosphere interacting with a flowing plasma
occurs in the case of comets,
which have no global MF and the obstacle to the SW is the exosphere,
which is the uppermost layer of the atmosphere.

15. What is Solar Wind?
COMETS

16. Comet Structure Nucleus: main solid core of the comet.
Tail: gas and dust particles released by the comet.
Coma: gases and dust released by the comet when energy from the sun heats the comet and causes the solid materials to turn into a gas.

17. Comet Tails
Comets develop tails only when the get close enough to the Sun.
Comet tails always point away from the Sun—This is how scientists first realized the existence of solar wind.

18. Comet – type Bow Shock Ionosphere Contact Surface Solar Wind Nucleus
no internal magnetic field
substantial atmosphere
Solar Wind
Contact Surface
Nucleus
Cometo-pause
Bow Shock
Ionosphere
When comets are near the Sun they have huge atmospheres and only small solid bodies.
In discussing comets, it is convenient to introduce the term mass loading, which means that the background flowing plasma becomes laden with heavy ions of atmospheric origin and slows down as a result. The gravitationally unbound sublimated neutral atmosphere of a comet flows outward from the very small (a few km/s in diameter) icy nucleus at speeds of about 1 km/s. The huge ionosphere, which is the ionized part of an atmosphere due to solar radiation, that is produced adopts the velocity of the expanding neutrals and creates a planet-sized cavity or obstacle.
This obstacle is created by the dynamic pressure of the outgassing plasma and the boundary of this cavity is called contact surface. However, there is so much neutral atmosphere outside of this pressure- balance boundary that ionization there produces a region of mass-loaded SW that can extend hundreds of thousands kilometers from the nucleus.
Mass-loading by itself substantially slows down the plasma flow long before the obstacle of the contact surface is encountered.
The cometopause marks the location of a composition transition region, where the plasma composition changes from mostly SW in origin to mostly cometary ions.
(Due to the supersonic SW a cometary bow shock is formed as a direct result of mass loading, rather than by deflection of the SW by a solid object.)
A bow shock is a discontinuity in the flow marked by an abrupt increase in magnetic field strength, turbulence, electron fluxes and plasma temperatures and a decrease in flow velocity.
For a comet, the extended mass-loaded region plays the major role in defining the features of the SW interaction.

19. Numerical model - MHD

20. Numerical results

21. Numerical results

22. Numerical results

23. Numerical results

24. Numerical results

25. Numerical results

26. SW interactions with unmagnetized bodies with a substantial atmosphere
Obstacle = ionosphere
One type is left, namely the SW interaction with an unmagnetized body,
which has a substantial atmosphere.
In this case, the ionosphere represents the obstacle to the SW.

27. Venus

28. VENUS – type Ionosphere (photoions)
weak magnetic field or non at all
substantial atmosphere
Illustration of the steps that lead to the formation of an ionospheric planetary obstacle in a flowing plasma like the solar wind. Ionization by solar radiation, for example, is followed by diversion of the external plasma flow only if that flow is magnetized.
Planetary Atmosphere (neutral atoms and molecules)
Ionosphere (photoions)
Solar radiation
Solar Wind
Wake
Bow Shock
Interplanetary Magnetic Field
Magnetosheath
induced Magnetotail
Ionosphere
Magnetic Barrier
This figure shows the interaction of the SW with a neutral atmosphere of an unmagnetized planet.
The neutral atmosphere is ionized by solar EUV and UV radiation.
In the absence of a MF in the SW plasma, the flow would be absorbed by the planet leaving a wake.
A magnetized SW cannot penetrate the highly conducting ionosphere and is diverted around the planet forming a magnetic barrier, bow shock and an induced magnetotail.
(Because the ionosphere is a conductor, the convecting MF generates currents in it that at least initially keep the field from penetrating through the body by generating a canceling field. This situation persists as long as the MF keeps changing its orientation, as it does in the SW, otherwise the field would eventually diffuse into the body on a time scale, that would depend on the ionospheric conductivity.)
This basic picture is appropriate for the SW interaction with planets Venus, Mars and perhaps for Saturn’s satellite Titan, when it is outside of Saturn’s magnetosphere.
This scenario is not appropriate for a body inside of a magnetosphere,
where the MF is relatively steady.

29. Structure of the Ionosphere
Brace and Kliore, 1991

30. Location of the obstacle boundary
ionospheric pressure
ni k Ti
Bow Shock
Magnetic Field Lines
Magnetic Barrier
Ionosphere
Streamlines of Solar Wind Plasma Flow
Ionopause
external pressure
nswkTsw + ρv2 + B2 / 2μ0
thermal pressure
solar wind dynamic pressure
magnetic pressure of the interplanetary magnetic field
It is fair to assume that if the SW flow is stopped at the subsolar point of the ionospheric obstacle,
there must exist a transformation there of upstream pressure, mostly dynamic pressure, to internal pressure.
This figure shows this pressure-balance situation.
When the SW dyn. pressure is LOW, the magnetosheath effectively stops at the altitude, where the ionospheric plasma pressure is equal to the external pressure. This obstacle is called the ionopause.
When the SW pressure exceeds the thermal pressure of the ionosphere, the magnetosheath MF is found to penetrate the ionosphere and contribute to the
total obstacle pressure perceived by the SW.
Of course, the extent to which the field can contribute to the pressure will be limited by the rate of diffusion of field through the ionosphere.
At the bottom of the ionosphere are the insulating neutral atmosphere and solid planet.
When the ionosphere by itself can no longer form the obstacle, the IMF will temporarily hang up in the ionosphere, then make its way through the solid mantle of the planet until it is diverted around a conducting core, if there is one, and finally pulled into the wake.

31. Induced magnetotail Magnetotail Bow Shock Z_vso X_vso Y_vso
This is a 3-dimensional representation of the convection of magnetic flux past Venus.
The field lines closest to the planet move most slowly, become the most highly mass-loaded and
are stretched most into a tail-like configuration.
Magnetic pressure (and curvature forces) accelerate the plasma up to and beyond the SW plasma
down the tail and the field lines gradually regain their original interplanetary directions.
Similar to the plasma sheet in the geomagnetic tail, the 2 lobes in the induced magnetotail,
are separated by a sheet of plasma.
Ultimately far from Venus, the picked up ions that are predominately oxygen,
are the only remaining signature of that SW interaction.

32. Pick – up and escape processes Energetic neutral atoms (ENAs)
Illustration of planetary pickup – ion trajectories of Venus. The cycloid sizes are approximately scaled for O+ (oxygen is the main constituent of the Venus upper atmosphere).
Photoion
Escape
As a result, photoions that might be produced above the ionopause can be picked up.
Neutral atoms high in the atmosphere are ionized by solar radiation,
and then accelerated into cycloidal paths by the SW electric field.
Although the SW interaction confines the Venus ionosphere to a relatively
thin layer surrounding the planet,
the neutral atmosphere of Venus is not so confined.
The upper atmosphere in fact extends via a hot oxygen exosphere well out
into the region of the SW interaction.
These atoms are subject to ionization by 3 processes:
(photoionization, charge exchange and impact ionization;)
when they become ionized, start to gyrate around the MF lines and they are accelerated in the direction of the electric field of the SW and escape from Venus.
Additionally, ENAs can be produced in collision processes between SW protons and atmospheric neutral atoms, which also escape from the planet.
Energetic neutral atoms (ENAs)
Escape

33. Ionospheric magnetic field
Examples of observed altitude profiles for the ionospheric electron densities (points) and magnetic fields (solid line) at Venus. The ionopause is located where the magnetosheath field decreases and the plasma density increases.
Orbit 186
Orbit 177
Orbit 176
Iono- pause
Ne(cm-3)
200
160
120 80
100 60 20 40
500
400
300
Altitude (km)
102
105
104
103
106
Altitude profiles of MF and ionospheric electron density obtained at Venus during solar maximum by in situ measurements from PVO.
The ionopause cut-off in the density is usually clear and sharp at solar maximum, as in the upper left corner, Orbit 186, for such cases the magnetosheath MF stops abruptly, where the ionospheric density rises.
The MF strength builds up as the flow slows, resulting in the formation of a magnetic barrier, which is located just outside the ionopause.
Orbit 186 is an example of the unmagnetized case of the ionosphere, where the magnetic pressure balances the thermal pressure of the ionosphere at the ionopause, which is located at high altitudes and it is quite thin.
Oribts 176 and 177 represent the magnetized case of the ionosphere of Venus, where the SW dynamic pressure was high.
Notice the broad ionopause at low altitudes and also the characteristic vertical structure of the ionospheric MF with its minimum near 200 km and maximum near 170 km.
When the SW dyn. pressure exceeds or is comparable to the maximum ionospheric pressure, magnetic flux is pushed deep into the ionosphere and is convected downward to the lower ionosphere where it is subject to Ohmic dissipation.

35. Altitude (Km)
Number Density (cm-3)

36. Numerical model - Draping magnetic field lines
Solar wind

37. Physical model – 2 component MHD

38. Parameters of the physical model

39. Pressure distribution
Interaction region
IMF
bow shock
magnetic barrier
ionosphere

40. Pressure profiles in the subsolar regionX

41. Plasma profiles X

42. Magnetic field lines and nightside ionosphereZ
XZ plane
Solar wind X
IMF Y
XY plane

43. Concluding remarks Flowing plasma interactions with various types of
magnetized planets or
unmagnetized / weakly magnetized bodies
Each plasma interaction has distinctive features
Earth: magnetic field and atmosphere
Mercury: magnetic field but NO atmosphere
Moon like bodies: neither a magnetic field nor an atmosphere
Venus and Mars: no internal magnetic field but a substantial atmosphere
Comets: atmospheres with insignificant bodies
We have learned now, how the SW plasma interacts with magnetized or unmagnetized bodies, which have atmospheres or not.
In the case of Earth and Mercury, which possess a strong MF, we saw that the SW interaction with a magnetosphere is a complex process.
Both planets have some similarities but also show a lot of differences mainly due to the fact that Mercury lacks a substantial atmosphere.
Then we have considered the case of moonlike bodies, where the moon absorbs the incident plasma and leaves an empty wake but produces relatively little distortion in the MF.
In the case of Venus and Mars( which have enough of an atmosphere to make a major difference in their plasma interactions,) their ionospheres deflect the incident plasma, thereby forming a bow shock and magnetosheath but also produce some near-planet mass-loading that probably leads to the formation of the induced magnetotail in the wake.
Last but not least, comets, which are (practically) atmospheres with insignificant bodies, show how an interaction involving mass-loading by itself, appears.

44. Thank you!