Geomagnetic micropulsation nature related to distribution of Alfven velocity in magnetospher

Vanyan, L. L.

Zybin, K. YU.

English

NASA Technical Reports Server

National Aeronautics and Space Adninistration 
Goddard Space Flight Center 
Contract No.NAS-5-12487 
ST - PF - GM- 10 5 5 7 
ON THE DISTRIBUTION OF A L h N  VELOCITY 
IN THE MAGNETOSPHERE 
L. L .  Van'yan 
k. Yu. Zybin 
(USSR) 
e / 
~ A G E S I  - ICO'DEI 
30 JANLJARY 1967 
https://ntrs.nasa.gov/search.jsp?R=19670009626 2020-03-16T18:54:08+00:00Z
ST-PF-GM-10557 
ON THE DISTRIBUTION OF ALFVEN VELOCITY 
IN THE ?4lGh!TOSPHEFE 
Kosmicheskiye Issledovaniya 
Tom 4 ,  vyp.6, 935-6, 
Izdatel'stvo t'NALJKA'', 1967 
by L. L. Van'yan 
tj K. Yu. Zybin 
From a peculiarity in the distribution of Alfvkn velocity in the magneto- 
sphere the authors derive a series of consequences that are of impotance for 
the understanding of the nature of geomagnetic micropulsations. The reasonings 
are based upon the data relative to the plasma jump, called "knee", of which 
a large number has been lately accumulated. 
it 
Ji * - 
It is now customary to consider that geomagnetic micropulsations with 
periods in tens of seconds are the consequence of propagation in the magneto- 
sphere of hydromagnetic perturbations occurring at its boundary, Obviously, 
these properties of micropulsations depend essentially on the distribution of 
P-lfven velocity in the region of their propagation. 
magnetic pressure, this velocity determines the propagation of Alfven, as well 
as of fast magnetoacoustic waves. 
formula 
It is well known that if we neglect gas pressure by comparison with the 
llsually the Alfven velocity is computed by 
where H is the intensity of the magnetic field, and pis the density of charged 
particles. At first, the computation of VA for the magnetosphere was performed 
by Dessler [l], taking into account the monotonic decrease of plasma density with 
height, and for the intensity of the dipole field (dashed line in Fig.1). 
However, sharp decrease of plasma density at geocentrical distances of 
K = (4 
during the f E ight of Soviet cosmic rockets. A series of data relative to this 
plasma jump have been collected to-date [ 3 ] ;  it was called "knee", near which 
the cold plasma density drops by several tens of times. 
5)R , where RE is the Earth's radius, was already revealed in 1959 [ 2 ]  
* I< VOPROSU 0 RASPREDELENII AL'FVENOVSKOY SKOROSTI V AMGNITOSIXRT: 
.- 
/. 
i 
2 .  
I t  is natural that such a sharp density decrease must lead to a signifi- 
CL~L increase of Alfven velocity. The corresponding graph, computed for the 
dipole field, is shown in Fig.1 by solid line (a similar graph WELS exhibited 
by h g e y  during a lecture at the Institute of Earth's Physics of the USSR Aca- 
demy oi: Sciences in 1965). 
The characteristic departure of this 
graph from the data [11 is the second m a -  
ximum of VA at R = (4 + 5) 
of relatively low values opVA, bounded by 
two maxima. From this peculiarity of dis- 
tribution of Alfven velocity in the magne- 
tosphere emerge a series of consequences 
which are important for the understanding 
of the nature of geomagnetic micropulsa- 
t ions . 
and the region 
1) The region of decreased velocities 
may serve as an additional resonator for IbJ z.;oJ J . h  vA km/sec the magne toacoustic waves, propagating iso-  
tropically in the exosphere. (So far it was 
considered [4] that the magnetoacoustiL re- 
sonance encompasses the entire cavity of 
Fig. 1 
the magnetosphere) . 
nighttime side of the magnetosphere. 
plaining the type-Pi2 night oscillations, which involved specific difficulties 
for the theories juxtzposing the resonator wall with the magnetosphere boundary 
2) The resonator referred to exists not only on the daytime, but also on the 
Thus, there appears the possibility of ex- 
(see ~ 5 1 1 .  
3) -2s may be seen from Fig.1, three regions may be pinpointed i n  die nragne- 
a) the region bounded by the "knee" and the velocity jump at magnetosphere 
tosphere, in which magnetoacoustic resonances are possible: 
boundary (note that for specific conditions this jump may be fairly small, which 
impairs the quality of the outer resonator) ; 
b) the region between the "knee" and the AlfvGn velocity maximum at R = 1.5R~; 
c) the region, of which the ionosphere and the velocity maximum region serve 
as boundaries for R ss 1. RE. 
In the first two regions the fundamental periods of natural oscillations are 
estimated by the doubled time of wave path length and constitute 20 to 40 seconds 
in each resonator, while in the lower region they are estimated by the qundsuple 
run t i i r e  and constitute 15 to 20 seconds [ 6 ] .  
4 .  In case of interconnection between the resonators there may emerge beat- 
type oscillations with difference frequencies, by which one may apparently ex- 
plain the broad spectrum of simultaneously existing micropulsations of various 
pcriods . 
A** *  T I{ E I: N D * A * *  Manuscript received 
on 11 July 1966 
- *  L 
/' ST-PF-GM-10557 
1 
R E F E R E N C E S  
5 .  
1. 
2. 
A. J. DESSLER. 
V. V. BEZRUXIKH, K. I. GRINGAUZ. Sb."Issledov.kosmicheskogo prostranstva" 
D. L. CARPENTER. J. Geophys. Res., 71, No.3, 693, 1966. 
J. Geophys. Res., 63, No.2, 405, 1958. - 
( S p a c e  Investigations) . p. 177, 1965. 
3. -
4. Y. KATO. Sci. Rept. Tohoku Univ. Ser. 5, Geophysics, 2, 1, 1959. - 
5. 
6. 
V. A. TROITSKAYA ET AL. Geomagnetizm i Aeronomiya, 6, No.3, 533, 1966. - 
C. E. PRINCE, F. X. BOSTICK. J. Geophys. Res., 69, No.15, 3213, 1964. -
Contract N0.NA.S-5-12487 
VOLT TECHNICAL CORPORATION 
1145, 19th St.NW, 
WASHINGTON D. C. 20036 
Tel: 223-6700; 223-4930. 
Translated by ANDRE L. BRICHANT 
on 30 January 1967 
DISTRIBUTION 
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On the distribution of Alfven velocity in the magnetosphere

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