H. J. Fahr - K. Scherer
Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
Received 29 March 2004 / Accepted 12 May 2004
Abstract
For a long time it has been suggested that at larger
solar distances the interplanetary magnetic field may not be
behaving as predicted by Parker`s Archimedian spiral field model.
This phenomenon, partly identified as the "magnetic flux deficit''
paradox, appears as deficits in the azimuthal magnetic field
components connected with "underwound'' magnetic fields, though the
pick-up ion decelerated solar wind should rather lead to excesses.
Up to now no satisfactory explanation for this phenomenon has been
presented. In this paper we study for the first time the
diamagnetic effect of pick-up ions which systematically load the
solar wind with suprathermal ions at its expansion to larger
distances and diamagnetize the plasma. We shall demonstrate that
such ions mainly reduce the azimuthal magnetic field component by
their diamagnetic action. As we can show the field deficits can
easily theoretically be explained by this effect. Also some new
light is now shed on the problem of the magnetic field jump to be
expected at the termination shock.
Key words: magnetic fields - plasma
In the paper by Winterhalter et al. (1990) it was shown
that, on the basis of PIONEER-11 magnetic field data a
systematic flux deficit appears in the outer heliosphere with respect
to field predictions made by the Parker model (Parker 1958). The
resulting deficits amount to about 20 percent at distances of 24 AU.
This phenomenon was confirmed by ongoing measurements as
documented by Smith et al. (1997,2004) based on
ULYSSES magnetic field measurements. However, other authors
came to the conclusion, that no such deficit exists (e.g. Burlaga et al. 2002) and there might be still no sufficient
observational data to decide on this phenomena. Here we argue that,
on a purely theoretical basis, the flux deficit reported by
(Smith et al. 1997) should be expected. The latter authors
clearly revealed that the magnetic flux invariant
behaves normal and interestingly enough,
is revealed as latitude-independent. In contrast to the
azimuthal component
clearly violates Parker's expected
field invariant
,
showing that especially at latitudes with
(i.e. near the ecliptic) an azimuthal field deficit
by about 10 to 15 percent is recognizable. As these authors state,
this latitudinally dependent deficit can hardly be explained by
unknown latitudinal variations of the solar rotation period
,
i.e. effects of
differential solar rotation. Also, earlier explanations of an
out-of-ecliptic transport of magnetic fields due to meridional solar
wind flow components as offered by Pizzo & Goldstein (1987) or
Suess et al. (1985) in view of the latitude-independent invariant
are obviously ruled out. Hence it still remains to explain
why azimuthal field components in the outer heliosphere appear to be
reduced instead of enhanced due to the pile-up in the pick-up ion
decelerated solar wind when compared with the expectations from
Parker`s field model. It is interesting to notice that the well-known
pick-up ion effect to decelerate the solar wind at larger distances
(see Fahr & Rucinski 2001; Isenberg 1997; Fahr & Rucinski 1999)
inducing a magnetic field pile-up should tend to produce flux excesses
rather than deficits. Here, we will restrict ourselves only to
the derivation of the diamagnetic effect of pick-up ions and its
possible implications.
It is well known vacuum electric and magnetic fields,
and
,
are converted into different fields,
and
,
in
dielectric and magnetically susceptible media like solid, liquid or gaseous
materials or plasmas. In such media not only free electric charges and
currents may be present, but also "bound'' charges in form of electrically
polarizable materials or in form of "bound'' electrical currents which are
connected with motions of "bound'' electric charges. In a plasma for instance
charged particles may be magnetically bound to gyro-orbits and thus cause
microscopic electric circle currents
.
One can ask the
question what relations may exist between
and
as well as
and
in a space plasma like the solar wind. In a fully
ionized plasma the polarizability vanishes for stationary cases (i.e. for the
frequency
)
and the dielectric coefficient
is
equal to 1, meaning that
.
The
effective magnetic induction
,
measurable in a plasma background is
nevertheless different from the vacuum field
since the plasma matter
develops bound electric currents
connected with gyrating
electric charges which by themselves produce magnetic fields.
Hence for instance at some heliocentric distance r in interplanetary
space the fields
and
,
taking into account the
effect of these bound currents
,
are thus connected by
(see e.g. Chen 1977):
![]() |
(1) |
![]() |
(2) |
![]() |
(3) |
At larger solar distances the PUI distribution function due to rapid
pitch-angle scattering can reasonably well be approximated by a
distribution which is essentially gyrotropic and isotropic with respect to
pitch angle
or its cosine
(see
e.g. Chalov & Fahr 1998). Thus the above expression transforms to:
![]() |
= | ![]() |
|
= | ![]() |
(4) |
![]() |
(5) |
![]() |
(6) |
A fairly realistic expression for
can be derived from
results obtained for
by Chalov et al. (1995,1997) as solutions of a complete PUI phase-space
transport equation. As was shown by Fahr & Lay (2000) these
numerical results for
can fairly adequately be
represented by the following analytical formula:
Coming back now to the magnetic moment given by Eq. (5) one thus finds:
![]() |
(13) |
![]() |
(14) |
![]() |
(15) |
Since fields are not measured in the solar wind rest frame, but in a
spacecraft frame which is nearly identical with the heliocentric rest frame, we
have to transform Eq. (16) into the heliocentric rest frame. Therefore, we
use the special relativistic transformation of the fields from the solar wind
rest frame to the heliocentric rest frame and, reminding that the Lorentz
factor
,
and in the comoving
frame
,
we then find:
![]() |
(16) |
The Archimedian Parker field is assumed to behave as frozen-in the solar wind
flow. This is derived from the well-known frozen-in field condition:
![]() |
(18) |
![]() |
(19) |
![]() |
(20) |
![]() |
(21) |
Of interest in the ecliptic of the outer heliosphere is only
the -component of the magnetic field. With the help of
Eq. (17), we then find for the magnetic induction
:
![]() |
(22) |
Perhaps in a more detailed consideration it also needs to be studied that
pick-up ions not only represent bound currents
,
but they also
contribute to "free'' electric currents
for the period of time,
,
before they become fully isotropized by pitch angle
scattering, i.e. as long as they are slipping along the local field lines.
These currents can be estimated by
and they are mainly oriented in radial direction and thus cause a latitudinal
gradient of the azimuthal field component by means of the relation:
.
The Rankine-Hugoniot field relation for the MHD shock given by the Poisson
bracket reads (see e.g. Diver 2001; McKenzie & Westphal 1969; Baumjohann & Treumann 1996):
,
where
is the
unit vector of the shock normal.
For the measurable magnetic field
in a magnetized background plasma
the above relation thus translates to:
![]() |
(23) |
Especially for the case of the nose region of the solar wind termination shock
which represents a perpendicular shock, i.e. with
and
,
one hence derives with the above relation:
![]() |
(24) |
>From the magneto-hydrodynamic Rankine-Hugoniot shock relations (see e.g. Diver 2001) one obtains:
![]() |
(27) |
The functional dependence for the ratio RB/S is shown in Fig. 1
for five different values of
.
It can be seen
that the compression ratio S can be smaller or larger than the
magnetic compression ratio RB, while in the non-diamagnetic case
they are identical. Furthermore, for very large compression ratios S the
standard magnetohydrodynamic Rankine-Hugoniot relations are no longer
valid, since
depends on
for
,
leading to negative values of
.
Nevertheless, we can state here that without the knowledge of the Alfvén
Mach number
and the plasma Mach number
the magnetic
compression ratio RB cannot be fixed and not be connected with the standard
compression ratio S.
![]() |
Figure 1:
The ratio RB/S as function of the standard compression
ratio S. The labels at the curves are the ratio of the Alfvén Mach
number to the plasma Mach number
![]() |
Open with DEXTER |
We have shown, that the diamagnetic effect of PUIs has to be taken into
account, because it reduces dramatically the magnetic induction in the solar
wind with increasing distances. In order to get an analytic expression, we had
to make some moderate simplifications to the problem. Therefore, we derived
only upper limits of the diamagnetic effect, but we clearly have shown, that
the decrease in the magnetic induction
due to the diamagnetism of
the pickup ion plasma has to be taken into account.
Furthermore we have shown, that the magnetic compression ratio RB and the standard compression ratio S are related in a complicated way and cannot be determined from each other without the knowledge of the sound speed and Alfvén speed at the shock.
Similar complications should also arise at cometary shocks, however, here the pick-up ion diamagnetism is harder to estimate since the resulting distribution functions are not well known.
Acknowledgements
The authors are grateful for financial support granted to them by the Deutsche Forschungsgemeinschaft in the frame of the project "Heliotrigger'' (Fa 97/28-1).