Interplanetary magnetic fields as cause of comet tail

Beard, D. B.

Nakada, M. P.

English

NASA Technical Reports Server

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Interplanetary Magnetic Fields as a Cause of 
Comet Tails 
IT has been generally recognized recently that the 
historic explanation of comet tails in terms of solar 
radiation pressure by either light or particles is impossible. 
Solar light pressure has been found to be insufficient 
by several orders of magnitude. Coulomb collisions or 
acceleration coupled with charge transfer hy prot’ons i~ 
the solar wind have S!SQ heen she-zii by Eieixlann an& 
Treffitzl, in particular, to be grossly insufficient procosses 
to account for comet tails streaming within a cylinder of 
small diameter and great length R . W I I . ~  fmrr? the SLY. It 
is the purpose of this communication to point out that the 
gases in cometary comas will be efficiently ionized by the 
solar wind of ionized hydrogen embedded in a magnetic 
field2-4 and that an interplanetary magnetic field will 
couple the cometary gas to the solar wind. 
The neutral gases evaporated from comet nuclei will 
be ionized both by thermal electrons4 in the solar wind 
which have energies of  several tens of electron volts and, 
aa Heubner5 has emphasized, by AlfvBnW process of 
ionization of a neutral gas by a fast moving plasma 
embedded in a magnetic field. 
A comet head may be regarded aa a deqsely ionized 
plasma essentially stationary in a high-pressure solar 
wind containing a low-pressure magnetic field moving 
with the wind. Recent satellite measurements4 have 
established that the solar wind consists of plasma the 
protons of which have an isotropic thermal energy of a 
few electron volts and a kinetic energy due to their stroam 
velocity of a kilovolt or so. The interplanetary magnet,ic 
field observed to be embedded in the plasma is about 5y 
( ly  is gauss). Thus the magnetic pressure, H 2 / 8 x ,  is 
about equal to the thermal (isotropic) pressure of the wind 
in the moving co-ordinate frame of the wind. 
The stream pressure of the solar wind, however, is the 
overwhelmingly dominant pressure in space, being two 
orders of magnitude larger than the free space magnetic 
pressure. Hence, the interplanetary ma,gnetic field is 
compressed against any obstacle such as the stationary 
plasma provided by the comet coma or the stationary 
magnetic field provided by the geomagnetic dipole. Just 
as in the case of the example furnished by the geomagnetic 
dipole’, the interplanetary field is compressed against the 
obstacle offered by the comet coma so that the magnetic 
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pressure increases to approximate equality with the stream 
pressure. If the interplanetary field is parallel to the solar 
wind velocity, the compressed field tails off parallel to tho 
solar wind in a cylindrical shape for a large distance down- 
stream from the comet coma confining the comet plasma 
within this volume. If the interplanetary field is perpen- 
dicular to the wind velocity, the compressed field lines on 
the solar side of the comet slip around the edge of the 
comet and then continue moving with the solar wind'. 
The comet plasma will become embedded in the moving 
field -lines and will be carried radially away from the Sun, 
pcquiring the velocity of the magnetic field as Heubner5 
first proposed. 
Quite simply the magnetic field may be regarded as the 
mechanism by which the solar wind is efficiently coupled 
to the comet plasma which Coulomb collisions are not 
Fluctuations in solar wind-pressure, or 
the magnetohydrodynamic waves the fluctuations gener- 
ate, will also create the accelerations in comet tails which 
first led Biermann2 to deduce the presenco of the solar 
wind. 
._ 
'' able to provide. 
DAVID B. BEARD* 
M. PAUL NAKADA 
Goddard Space Flight Center, 
Nnt,ion:LI Aeronautics and Space Administ,r>Ltion, 
Greenbelt, Maryland. 
, 
* National Academy of Sciences-National Research Council Reseprch 
Associate with N.A.S.A. Permanent address: Physics Departincnt, Univcr- 
sity of California, Davis. 
' Bierm;inn, I,., M d m .  Soc. Roll. Sei. Lidye ,  Ser. 4, 13, 201 (1953). 
Biermann, L. ,  and Trcffitz, E., 2. Astrophys., 49, 111 (1960). 
l{ierin:~nii, L., Xumo Cimmlo ,  Supp. 13,189 (1959). 
' Neugeb.ruer, BI., and Snyder, C., Scienre, 138, 1095 (1%:). Coleman, jnn., 
P. J . ,  Davis, jun., L., Smith, E. J., and Soniirtt, ( . P., S c w n w ,  138, 
1099 (1962). 
Heubncr, W. I?., Reus. Mod. Phus., 33, 408 (1961). 
" Alfvfn, H., Rra. Mod .  Phys., 32, 710 (1960). 
' IIRiiriI, 1).  II., J. Geophys. Res. (to IJc p~~hlisIi?il). 
I'rinlerl in Great Rritain by  Fisher. Knight ht Co , Ltd., St. Albaw. 
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Interplanetary magnetic fields as a cause of comet tails

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