I N S T I. T U T D ' A E R O N . O M I E S P A T I A L E D E B E L G I Q U E 3 - Avenue Circulaire
B - 1180 B R U X E L L E S
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A E R O N O M I C A A C T A
A - N° 207 - 1979
Impulsive penetration of solar wind plasma and its effects on the upper atmosphere
by
J. LEMAIRE
/
B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M , I E 3 - Ringlaan
B • 1180 BRUSSEL
V
F O R E W O R D
The article "Impulsive penetration of solar wind plasma and, its effects on the upper atmosphere" has been presented as a topical paper at the Chapman Conference "Magnetospheric B o u n d a r y L a y e r s "
which, has been held 11-15 June in Alpbach ( A u s t r i a ) . The article has been published in the Proceedings of this conference.
A V A N T - P R O P O S
Le texte "Impulsive penetration of solar wind plasma and its effects on the upper atmosphere" a été présenté à la Chapman Conférence "Magnetospheric B o u n d a r y L a y e r s " qui s'est tenue du 11 au 15 Juin 1979 à Alpbach ( A u t r i c h e ) . Le texte est publié dans Jes Comptes Rendus de cette conférence.
V O O R W O O R D
Het artikel "Impulsive penetration of solar wind plasma and its effects on the upper atmosphere" werd voorgedragen tijdens de Chapman v e r g a d e r i n g "Magnetospheric B o u n d a r y L a y e r s " welke plaats had te Alpbach (Oostenrijk) van 11 tot en met 15 juni 1979. Dit artikel werd opgenomen in de verslagen van de v e r g a d e r i n g .
. V O R W O R T
Dieser Text "Impulsive penetration of solar wind plasma and its effects on the upper atmosphere" ist während Chapman Konferenz
"Magnetospheric B o u n d a r y L a y e r s " die in Alpbach gehatten wurde, vorgetragen worden.
IMPULSIVE PENETRATION OF SOLAR WIND PLASMA AND ITS EFFECTS ON THE UPPER ATMOSPHERE
b y
J. LEMAIRE
A b s t r a c t
Impulsive Penetration of solar w i n d plasma i r r e g u l a r i t i e s t r a n s f e r s e n e r g y , momentum and p a r t i c l e s i n t o t h e magnetosphere. T h e e f f e c t s on t h e u p p e r atmosphere are examined and d i s c u s s e d . T h e heat deposition in t h e E - r e g i o n ( a n d above) due to Joule d i s s i p a t i o n of d e p o l a r i z a t i o n c u r r e n t s is evaluated and compared to t h e e n e r g y p r o d u c - t i o n due to t h e a b s o r p t i o n of solar r a d i a t i o n .
T h e w i n d speed imparted to ionospheric plasma in t h e t h r o a t r e g i o n and o v e r t h e polar cap is compared to t h e o b s e r v e d c o n v e c t i o n v e l o c i t i e s . T h e impulsive injections of m a g n e t o s h e a t h - l i k e plasma o b s e r v e d at low a l t i t u d e s in t h e d a y s i d e cusps are i n t e r p r e t e d as con- sequences of f i e l d aligned expansions ( s p r e a d i n g s ) of e n g u l f e d solar w i n d filaments ( i n t r u s i o n s ) i n t o the magnetosphere.
Résumé
La pénétration impulsive d'irrégularités' de plasma du vent solaire transfère de l'énergie, une quantité d'impulsion, et des particules vers l'intérieur de la magnétosphère. Les effets sur l'atmo- sphère supérieure sont examinés et discutés. La chaleur déposée dans la région-E (et au-dessus de la région-E) par suite de la dissipation des courants électriques de dépolarisation est évaluée et comparée à la production d'énergie d,ue à l'absorption de la radiation solaire.
La vitesse communiquée au plasma ionosphérique dans la
région du cornet magnétique et au-dessus des calottes polaires est
comparée aux vitesses de convection observées. Les injections
impulsives de plasma de la magnétogaine dans les cornets. magnétiques
sont interprétées comme la conséquence de l'expansion et la dispersion
de filaments du vent solaire enlisés dans la magnétosphère.
Samenvatting
Het impulsief binnentreden van onregelmatigheden van het zonnewindplasma in het inwendige van de magnetosfeer heeft een energieoverdracht en een overdracht van de hoeveelheid van beweging tot gevolg. De invloed op de hogere atmosfeer wordt onderzocht en besproken. De warmteoverdracht in het E-gebied (eh boven dit gebied) ten gevolge van de dissipatie van de elektrische depolarisatipstromen wordt bepaald en vergeleken met de energie toename ten gevolge van de absorptie van de zonnestraling.
De snelheid overgedragen aan het ionosferisch plasma in het
"Cusp"- gebied en boven de poolkappen wordt vergeleken met de waar- genomen convectiesnelheden. De impulsieve injectie van het pjasma uit de "magnetosheath" in de "Cusp" wordt verklaard als zijnde het gevolg van de expansie en de dispersie van zonnewindfilamenten die in de magnetosfeer zijn binnengedrongen.
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Zusammenfassung
Die Eindringen Massenelemente des Sonnen Windes überführt Energie, Momentum, und Teilchen in der Magnetosphäre der Erden herein. Die Effekten auf der oberen Atmosphäre sind besprochen worden.
t
Z . B . die Heizung der E-Schichte, die Geschwindigkeit des iono-
spherischen Plasma und die Teilchen Niederschlagung sind berechnet
worden.
1. INTRODUCTION
An overwhelming amount of in situ observations made for more than 15 years near the ecliptic plane have indicated that the coronal expansion blows uninterruptedly ionised matter out of the Sun with a velocity ( V ) ranging from 320 km s~
1to 710 km s"
1at 1 A . U . The number density (N) of the solar wind protons varies between 3 cm -3 and 20 cm . The electron temperature ( T ) fluctuates between 9
4 4 , n .
x 10 K and 20 x 10 K. The proton temperature ( T p )
1Ssometimes smaller by a factor 5, sometimes larger by a factor of 3 than the electron temperature. The total flux of particles (NV) ranges between
O p _ 1 O
1.5 x 10 cm s and 7.8 x 10 cm s . The flux of momentum
2 ~8 - 8 - 1
density (mNV ) varies even more : from 1 x 10 to 5.8 x 10 g cm s"
2(Ref. 1).
For more than a decade the solar wind plasma has been supposed to be almost uniform and steady over distances comparable to the diameter of the magnetosphere. The interaction between this uniform and supersonic plasma flow and the geomagnetic field leads then to the formation of a smooth and symmetrical magnetopause surface where the normal component of the gas bulk velocity is zero. In front of this ideal surface is the Bow Shock where the solar wind speed jumps from its supersonic value to a subsonic regime. The outcome of steady state theories of the interaction between the solar wind and the geomagnetic field was an impressive series of interesting but controversial magneto- spheric models : "closed" steady state models, and "open" magneto- spheric models have had their advocates at all periods. It is not our purpose to repeat here neither the merits nor the limitations of both these models, indeed this has been explained in several reviews (refs.
2, 3 and 4). Let us just say that both of these alternative approaches
have their own difficulties resulting from the oversimplification on which
these steady state mathematical models are based. At first glance, it
appears often convenient or easier to describe natural phenomena in the
frame of static or stationnary models, but it happens often also that such over-simplifications must be abandoned later on. Indeed sometimes the physical processes involved in Nature are, by essence, non sta- tionary. It is now more and more accepted that this must be the case for the interaction between the corpuscular radiation emitted by the S u n an'd our geomagnetic field environment. Recent high resolution magnetic field and plasma measurements confirm the older radioscintillations observations ( R e f s . 5 and 6) and show that the solar wind is formed of filamentary plasma irregularities. The density fluctuations convected passed a spacecraft or across a radioastronomical source can amount to AN/N = 5 - 10% over distances ranging from 300 km up to scales exceeding the dimension of the magnetosphere. Considering the solar wind as a bundle of interwinded' small-scale irregularities with inhomogeneous momentum densities, it has recently been proposed that the penetration of solar wind plasma into the magnetosphere proceeds impulsively instead of beeing a steady state interaction process ( R e f s . 7 and 8). It is this impulsive entry mechanism that will be recalled briefly in the next section. The consequences for the magnetosphere * atmosphere system are finally discussed in section 3.
2. T H E I M P U L S I V E I N J E C T I O N
A s for a g u s t y wind blowing at the surface or against the ocean, engulfment of air bubbles in the boundary layer depends on the spectrum of the momentum irregularities carried by the impinging wind.
The larger the excess of momentum, the deeper the solar wind plasma irregularity will be able to penetrate into the magnetosphere. T h i s has been schematically illustrated in Fig. 1. The hatched areas represent c r o s s sections of filamentary solar wind plasma elements at different distances from the magnetopause. Consider one of these elements with a 5% excess density compared to the b a c k g r o u n d plasma; when the bulk speed of this irregularity is equal to average solar wind speed its excess
N O N - U N I F O R M & UNSTEADY SOLAR W I N O
I i } l I | | |
! • ' i i 1 i ! ! !
F i g . 1 . - Sequence of events r e p r e s e n t i n g the positions of a solar w i n d plasma i r r e g u l a r i t y p e n e t r a t i n g t h r o u g h t h e Bow Shock and Magnetopause. Jd is t h e sum of m a g n e t i s a t i o n , g r a d - B , and c u r v a t u r e c u r r e n t s ; Jr F is
SW MS M
t h e C h a p m a n - F e r r a r o c u r r e n t d e n s i t y ; B j , B , B , B are t h e magnetic f i e l d i n t e n s i t i e s inside t h e plasma elements, in t h e solar w i n d , in t h e magnetosheath and in the magnetosphere r e s p e c t i v e l y ; E is t h e p o l a r i s a t i o n e l e c t r i c f i e l d ( - V x B . ) induced in t h e magnetosphere b y the plasma elements moving w i t h the v e l o c i t y V ; as soon as t h e plasma element is e n g u l f e d in a region w i t h f i n i t e i n t e g r a t e d Pedersen c o n d u c t i v i t y , t h e excess of k i n e t i c e n e r g y of t h e i n t r u d i n g i r r e g u l a r i t y can be d i s s i p a t e d b y Joule h e a t i n g ; t h e excess of momentum is t r a n s f e r r e d to the ionospheric plasma in t h e t h r o a t r e g i o n .
- 7 -
momentum is also 5% higher than average momentum density. Once behind the bow shock the density inhomogeneity moves at a subsonic velocity with a higher velocity than the b a c k g r o u n d magnetosheath plasma. T h i s is a consequence of the conservation of momentum across the bow shock transition. T h e densities ( N ) and temperatures ( T ) of the plasma have increased both inside and outside the moving volume element in a manner approximately described by the Rankine-Hugoniot equations. The magnetic field intensity ( B j ) and ( S ) , the normal c r o s s section of the filament, v a r y in order to conserve magnetic flux ( B j S ) . The element is compressed in the direction of the shock normal. At the magnetopause where the average solar wind has a zero normal velocity component, the density inhomogeneity still has an excess of momentum.
T h i s implies a penetration velocity ( V ) of 20 km s "1 if the original solar _ 1
wind velocity of this element was 400 km s . A s a consequence of conservation of mass and momentum the element dents in the magneto- pause and moves across the geomagnetic field lines which are rooted into the conducting ionosphere of the Earth. The polarization current induced by the inertial force of the moving mass element carries polariza- tion charges toward the east and west sides of the element as indicated in Fig. 1. A n electric potential difference and an electric field
E = " V x ft. (1)
builds up across the plasma element as shown in Fig. 1. The magnetic field lines c r o s s i n g the edges of the plasma element link the polarization charges down into the dayside c u s p s ionosphere. The convection electric field E (measured in a fixed frame of reference !) maps also down to the low altitude E-region near 80° invariant latitude and 12 hours local time.
The Pedersen conductivity is high in this part of the upper atmosphere because of the large collision frequency of the ambiant ions with neutral particles. A t r a n s v e r s e (horizontal) electric current flows
in the " direction of the applied ionospheric electric field. A s a c o n - sequence of this P e d e r s e n c u r r e n t , the polarization c h a r g e s a p p e a r i n g at the s i d e s of the plasma element are partially neutralized a n d the electric field £ is r e d u c e d ( s h o r t c i r c u i t e d ) . A decrease of E ( i . e . / E / < 0 ) implies a d e c r e a s i n g b u l k velocity ( i . e . V = E/B < 0 ) . In other w o r d s the b u l k speed of the plasma element does not remain constant as in the case when the integrated P e d e r s e n c o n d u c t i v i t y ( 2 p ) is zero or v e r y small. B e c a u s e of the finite value of ionospheric c o n d u c t i v i t y ( I = 0 . 2 - 1 S i e m e n s ) the element is slowed down in a characteristic
p -X
time ( x 2 a of the o r d e r of 10 minutes for the example c o n s i d e r e d above a n d taken from Ref. 7. In this time lapse a plasma element with a diameter of 10,000 km can move a distance of one E a r t h r a d i u s b e y o n d the a v e r a g e magnetopause s u r f a c e . T h e e x c e s s of kinetic e n e r g y c a r r i e d b y the i n t r u d i n g filament or plasma slab is d i s s i p a t e d into Joule heating in the ionosphere which is c o n s i d e r e d as the ohmic r e s i s t a n c e in the equivalent electric circuit illustrated in F i g . 2.
B e s i d e s t h i s electro-mechanical interaction there is an electromagnetic interaction between the magnetized plasma element a n d the geomagnetic field. Indeed Lemaire et aj. ( R e f . 9) h a v e s h o w n that penetration and engulfment r e q u i r e s certain orientations for the magnetization vector M associated with a c u r r e n t c a r r y i n g plasma element, t h e magnetization is determined b y the local c u r r e n t d e n s i t y J = curl M, while t h e - e x t e r n a l magnetic field H = B/(Jq - M is determined b y the d i s t a n t ( n o n - l o c a l ) c u r r e n t s y s t e m s . T h e condition for easy penetration a n d even acceleration of the plasma element towards the inside of the m a g n e t o s p h e r e is
M . ( BM - BM S) > O ( 2 )
where B M S is the magnetosheath field and BM is the m a g n e t o s p h e r i c field M
oriented N o r t h w a r d s near the equatorial plane; B is oriented in the s u n w a r d s direction for the n o r t h e r n magnetotail lobe and in the a n t i -
F i g . 2 . - Equivalent electric c u r r e n t induced b y a plasma element moving in the magnetosphere with a velocity V which is p e r p e n d i c u l a r to a magnetic field. T h e hatched area c o r r e s p o n d s to the plasma element; E is the polarization electric field and Jp is the polarization c u r r e n t moving in a direction antiparallel to E; the plasma element is a D . C . electric g e n - erator; the ionosphere is the load (ohmic r e s i s t a n c e ) where P e d e r s e n c u r r e n t s ( J ^ close the c u r r e n t loop formed b y B i r k e l a n d field aligned c u r r e n t s ( J ); Rj is the internal resistance of the M H D g e n e r a t o r r e s u l t i n g from local ( i n s i t u ) wave-particle interactions.
sunwards direction for the southern lobe of the magnetosphere. A similar magnetic interaction and accélération force is experienced by, a small dipole magnet when it is approached close to another magnetic dipole whose magnetic moment is anti-parallel to the former one. If on the contrary the two magnetic moments are parallel to each other the equation (2) is not satisfied and the magnets repel each other. So will also do the geomagnetic field at the magnetopause for a plasma element with an unfavourably oriented magnetization M.
An alternative way of viewing this interaction is via the J x B force which balances the pressure gradient force when plasmas and fields are in equilibrium. Considering the simplest case in the, equatorial plane, the Chapman-Ferraro magnetopause currents ( Jc p) are from dawn to dusk (see Fig. 1). The diamagnetic currents ( Jd) circulating around a piasma pressure and density enhancement are counterclockwise when the magnetosheath field is Southwards ( BM S < O ) ; indeed in this case the magnetic field produced by the current system Jd must have a northward component in order to reduce the magnetic pressure inside the filament to keep the total pressure constant. When these diamagnetic currents at the leading plasma edge combine with the oppositely directed Chapman-Ferraro current, the net (j_, + j _ _ ) x B force is suddenly reduced at the place where the
-d -CF
filament impacts on the magnetopause surface. On the other hand, the plasma pressure gradient force at the leading edge of the plasma element is enhanced. As a consequence of the imbalance of forces acting on the boundary, the plasma irregularity is accelerated towards the inside of the magnetosphere. However, when the IMF or the field in the magnetosheath is northwards the Jd x B and J ^ p x B forces add to each other to oppose the enhanced plasma pressure gradient force (Ref.
9).
The presence of a moving plasma element does not only perturb the local electric field distribution, but it changes also the local magnetic field distribution in the magnetosphere and in the ionosphere
- 1 1 -
( R e f . 7 ) . T h e i n t r u s i o n of solar wind filaments a c r o s s the magnetopause modifies the magnetic field lines in the outer m a g n e t o s p h e r e . B e c a u s e of the finite P e d e r s e n c o n d u c t i v i t y a n d via the field aligned ( B i r k e l a n d ) c u r r e n t s , the geomagnetic field lines can interconnect with those of the i n t e r p l a n e t a r y magnetic field. S u c h a p a t c h y " m e r g i n g " p r o c e s s is not n e c e s s a r i l y a c o n s e q u e n c e of i n - s i t u d i s s i p a t i o n p r o c e s s e s ( w a v e - p a r t i c l e interaction as in conventional m e r g i n g t h e o r i e s ) . Indeed, it can also be based on d i f f u s i o n of magnetic fields out of a magnetized plasma element as a result of classical collisions in the d i s t a n t ionospheric r e g i o n s
( R e f . 10).
3. S O M E C O N S E Q U E N C E S FOR T H E M A G N E T O S P H E R E - A T M O S P H E R E S Y S T E M
A s a c o n s e q u e n c e of impul.sjy_e___penetratipn_ solar w i n d particles, momentum, and e n e r g y are t r a n s f e r e d inside the magneto- s p h e r e a n d deposited in the atmosphere. In this section we will d i s c u s s these three points with more details.
3 . 1 . T r a n s f e r _o_f_ e n e r g y
In the p r e v i o u s section we have seen that the e x c e s s of kinetic e n e r g y c a r r i e d b y the impulsively injected plasma elements is partially d i s s i p a t e d into the lower ionosphere b y Joule h e a t i n g . T h e field aligned c u r r e n t s (J ) are closed in the ionosphere b y t r a n s v e r s e P e d e r s e n c u r r e n t s (J ) in the E - r e g i o n near 115 km altitude where the collision f r e q u e n c y of ambiant N O+ ions becomes equal to their g y r o - f r e q u e n c y . T r a n s v e r s e d i f f u s i o n of c h a r g e s and Joule dissipation are expected in this part of atmosphere which is an ohmic load for the equivalent c u r r e n t s y s t e m since J . E( is positive. Note that the polarization c u r r e n t s ( Jp) in the plasma element itself is anti-parallel to E, as a n y electric generator i.e. Jp . E < 0 ( R e f . 11). One can estimate the power d i s s i p a t e d in the atmosphere b y an i n t r u d i n g solar
- 1 2 -
w i n d plasma element w i t h an excess momentum of 5%. As a consequence of c o n s e r v a t i o n of momentum t h e p e n e t r a t i o n v e l o c i t y ( V ) t h r o u g h the magnetopause is a p p r o x i m a t e l y 20 km s i . e . 5% of t h e solar w i n d v e l o c i t y b e f o r e t h e bow s h o c k . A t y p i c a l value f o r t h e magnetic f i e l d inside of t h e filament is Bj = 30 n T . C o n s i d e r the case when Bj has a s o u t h w a r d component, t h e c o n v e c t i o n e l e c t r i c field g i v e n b y eq. ( 1 ) has t h e n a component in t h e y - d i r e c t i o n i . e . f r o m dawn to d u s k . T h e i n t e n - s i t y of E is 0 . 6 m V / m . When t h i s e l e c t r i c f i e l d is mapped down i n t o the d a y s i d e cusp ionosphere its i n t e n s i t y E. is increased b y a geometrical f a c t o r g i v e n b y 2 L cos a, - w h e r e L is the Mcllwain parameter of the geomagnetic f i e l d lines i n t e r c o n n e c t e d w i t h those of t h e e n g u l f e d f i l - ament; a is the angle between E and the equatorial geomagnetic plane : cos a = B. / B . . For L = 10 and a = 30°, E. = 32 m V / m . T h i s e l e c t r i c
I f Z I '
f i e l d d r i v e s Pedersen c u r r e n t s whose d e n s i t y is g i v e n b y J = op E..
? r 1
T h e power d i s s i p a t i o n b y Joule heating is J . E( = a p E ^ ap is the ion Pedersen c o n d u c t i v i t y
n . e 2
a i (3)
^ m. v. l i n
v. is t h e i o n - n e u t r a l collision f r e q u e n c y ; a is maximum w h e r e
in P '.n
becom'es equal to t h e ion g y r o f r e q u e n c y v = 30 s ( R e f . 12). T h i s o c c u r s between 115 km and 140 km w h e r e the NO n u m b e r d e n s i t y is of +
t h e o r d e r of 3 x 104 c m "3. From eq. ( 3 ) it can t h e n be deduced t h a t ( a ) = 5 x 10~4 mho/m. F u r t h e r m o r e , J = ( O m a v Ei = 1-6
v p ' m a x 9 _7 P J "a x 1 -3 -1
x 1 0 "5 A m and J E, = 5.1 x 10 W m = 5.1 x 10 0 e r g cm s . T h i s power d i s s i p a t e d per u n i t volume element compares well w i t h the value calculated in Ref. 13 u n d e r o t h e r c o n d i t i o n s . Note t h a t J .Ej is a f a c t o r 5 l a r g e r t h a n the heat deposited at t h e same a l t i t u d e s b y solar r a d i a t i o n ( R e f . 14). T h e r e f o r e it can be e x p e c t e d t h a t Joule h e a t i n g b y i n t r u d i n g plasma elements increases s i g n i f i c a n t l y t h e t e m p e r a t u r e of the ions and n e u t r a l gas in t h e E - r e g i o n below t h e d a y s i d e c u s p s . The
energy dissipated there diffuses downwards by thermal conduction and is transported to other latitudes and longitudes by the wind systems o»
the upper atmosphere. Note however, that the Joule heating by an intruding solar wind irregularity is only confined in a relatively small volume of the upper atmosphere. Indeed the vertical extend of the region of dissipation is only one or two atmospheric scale height <i.p.
10 km). The horizontal cross section of the heated volume corresponds to the field aligned projection of the engulfed solar wind filament upon the ionosphere. For instance a plasma element of 10,000 km diameter near the magnetopause dissipates its excess of kinetic energy over an horizontal area of only 150 km latitudinal width. This Joule d i s s i p a t i o n
in the dayside cusps E-regions is a first example of interaction b e t w e e n
the magnetosphere and the terrestrial atmosphere.
At higher altitude where the Pedersen conductivity becomes smaller, dissipation of the field aligned currents can also be envisaged as a consequence of electron-ion Coulomb collisions. The parallel conductivity
n e 2
a = — (4) m v . e ei
is typically of the order of 10 mho/m at 300 km altitude in the F-region where n = 10e e ei _ .4 6 cm'3, T = 1000 K, and v . = 1.5 x 103 s "1. The limiting field-aligned electric current density is J ~ n e v. = 10 A
- 2 e i
m , where v. is the ion thermal speed. Consequently, E S J /o -
- 2 1
10 mV/m. The maximum Joule heating resulting from dissipation ot such Birkeland currents by Coulomb collisions is equal to J . E -
-Q -8 -3 -1 10 W m = 10 erg cm s .
The minimum UV absorption (for K < 1026 A ) is of the same order of magnitude in the 250-300 km range in low temperature atmo-
• spheric models (Ref. 14). To increase the efficiency of dissipation of
-14-
field aligned currents the parallel conductivity should be reduced.
Wave-particles interactions, triggered when the currents approach the limiting value and become instable, reduce a below the value corresponding to Coulomb collisions. The additional heat deposition due to ohmic or anomalous dissipation of electric c u r r e n t s at high altitudes increases the plasma temperature within the magnetic flux tube inter- connected with the solar wind plasma i r r e g u l a r i t y . The formation of field aligned ionospheric irregularities' can result from such a patchy and localized heating of the upper ionosphere. Alternative possibilities for the formation of small scale ionospheric irregularities have been proposed in Refs. 15, 16 and 17.
Observational evidence for heating of the. ionospheric plasma up to 1000 km altitude, is shown in Fig. 3 taken from Titheridge (Ref.
18). The narrow temperature peak seen at high latitude (L = 20) is located below the dayside c u s p s . When the magnetic activity index is increased this temperature peak moves towards lower latitudes (L = 10).
T h i s . corresponds to the equatorward motion of the neutral points when the magnetosphere is more heavily compressed by the solar wind i.e.
when the solar wind plasma irregularities are stopped in the average at smaller radial distances (Ref. 19).
There is also evidence that the thermospheric temperature and densities are enhanced in the range of latitudes and local times corresponding to the dayside c u s p s (Ref. 20). The patchy and localized heating of the dayside c u s p s atmosphere may possibly modify the global wind circulation in the E-region and even deeper into the terrestrial » atmosphere.
3.2. Transfer_qfj^ome_n_t_u_m
i
When a plasma irregularity moves across the geomagnetic field lines as discussed above, the convection electric field E map in the
- 1 5 -
a b
CTi I
40 60 80 GEOMAGNETIC LATITUDES (degrees)
2000 -
¥ o
E O O
UJ i— <
(X. lii Q. 2 UJ
1600 -
1200
40 60 80 GEOMAGNETIC LATITUDES (degrees)
F i g . 3 . - A v e r a g e i o n o s p h e r i c t e m p e r a t u r e s as a f u n c t i o n o f g e o m a g n e t i c l a t - i t u d e s d u r i n g q u i e t g e o m a g n e t i c c o n d i t i o n s ( K = 1 . 5 , s o l i d c u r v e s ) a n d d u r i n g p e r t u r b e d c o n d i t i o n s ( K s 4 . 5 , d a s h e d c u r v e s ) . T h e n a r r o w p e a k s a t L = 10 ( a n d L = 20) a r e l o c a t e d a l o n g t h e f i e l d l i n e s o f t h e d a y s i d e c u s p . T h e s e l o c a l i z e d t e m p e r a t u r e e n h a n c e m e n t s a t
1000 km a n d 400 km a l t i t u d e a r e d u e to d i s s i p a t i o n o f t h e e x c e s s k i n e t i c e n e r g y c a r r i e d b y i n t r u d i n g ' s o l a r w i n d i r r e g u l a r i t i e s ( f r o m R e f . 1 8 ) .
t o p s i d e i o n o s p h e r e . T h i s e l e c t r i c f i e l d ( E | ) imposes a d r i f t motion to the ionospheric electrons and ions t r a p p e d in the magnetic t u b e of f o r c e i n t e r c o n n e c t e d w i t h t h e i n t r u d i n g plasma element. For E. - 32 mV/m and Bj = 0 . 5 10 T t h e ionospheric d r i f t v e l o c i t y , g i v e n b y - 4
h
xh
Bi
is equal to 640 m s . Since E and E( are d i r e c t e d from dwan to d u s k t h e low a l t i t u d e convection v e l o c i t y is d i r e c t e d t o w a r d s the magnetic poles. Momentum is f i r s t t r a n s f e r e d to t h e ionospheric plasma from the middle of t h e t h r o a t s i . e . at the feet of t h e "last closed" f i e l d line t a n g e n t to the magnetopause. When t h e element proceedes i n w a r d s in t h e f r o n t s i d e magnetosphere, momentum is t r a n s f e r e d at the feet of more e q u a t o r w a r d f i e l d l i n e s , i . e . at lower L - v a l u e s in the noon local time s e c t o r . R e p e t i t i v e actions of t h i s k i n d can i m p a r t a general convec- t i o n p a t t e r n from the t h r o a t region over the polar caps as i l l u s t r a t e d in f i g u r e 4 ( R e f . 2 1 ) . T h e ionospheric convection velocities o b s e r v e d at
- 1 - 1
250 km a l t i t u d e in t h e d a y s i d e cusps are 500 ms - 2000 ms , in agreement w i t h t h e value calculated above f o r a t y p i c a l solar w i n d i r r e g u l a r i t y i n t r u d i n g into the f r o n t s i d e magnetosphere.
A t lower a l t i t u d e s where the collision f r e q u e n c y between ionospheric p a r t i c l e s becomes i m p o r t a n t the convected ions t r a n s f e r t h e i r momentum to n e u t r a l atmospheric c o n s t i t u e n t s and d r a g the n e u t r a l atmosphere o v e r t h e polar caps. A t 150 km a l t i t u d e t h e c h a r a c t e r i s t i c time to t r a n s f e r t h e horizontal momentum to the t h e r m o s p h e r e and to
- 1
i m p a r t a t r a n s p o l a r v e l o c i t y of 500 ms is only of the o r d e r of 1 h o u r ( R e f . 13).
3 • 3. _Transfer_of _sol_a_r__win_d _garticjes
Impulsive p e n e t r a t i o n of solar w i n d i r r e g u l a r i t i e s as d e s c r i b e d above is not t h e o n l y mechanism able to t r a n s f e r momentum into t h e magnetosphere. Indeed h y d r o m a g n e t i c waves p r o p a g a t i n g at t h e s u r f a c e
- 1 7 -
Throat
Schematic view of the dayside high-latitude ion flow pattern observed near 250 km altitude. Momentum is Iransfered to ionospheric plasma in the throat region (from Ref. 21).
of the magnetopause can also impart some momentum from the magneto- sheath to the magnetospheric plasma and field (Ref. 22). However, it is difficult to see how such hydromagnetic waves could inject particles across the average magnetopause into the magnetosphere. That magneto- sheath-like particles are present earthward from the magnetopause and at almost all latitudes and longitudes has been clearly demonstrated by in situ observations ( R e f . 23 - 30 and 37).
A n up to date review of observational evidence for the exis- tence of a well defined Piasma__Bqund_a_ry__Layer ( P B L ) earthward of the Magnetopause Layer has recently be given by Eastman and Hones (Ret.
27). Magnetosheath-like electrons and ions are, seen in the PBL on closed geomagnetic field lines at the same time and at the same place as trapped magnetospheric electrons and ions. Figure 5 illustrates how the overall proton density and bulk flow velocity decrease from the magneto- pause layer to the inner surface of the plasma boundary layer (going from right to left in figure 5) and are accompanied by an increase in thermal energy and continued magnetosheath-level low frequency magnetic field fluctuations, given by the standard deviation S D g . The plasma p (ratio of plasma to magnetic field energy density) usually d r o p s to values smaller than unity in the inner portion of the P B L , consistent with the decay of field fluctuations. Within the P B L the plasma flow directions are more variable and usually shift into a direct- ion that is more nearly tangent to the average magnetopause surface than is the nearby magnetosheath flow direction (Ref. 27).
A theory of propagating hydromagnetic waves at the magneto- pause surface as proposed in Ref. 22, nor steady state diffusion processes across the relatively thin magnetopause current layer as discussed in Ref. 31, hardly can account for all the observations made in the P B L (see Ref. 19). Direct evidence of detached magnetosheath- like plasma intrusions into the magnetosphere have recently been obtained from I S E E observations ( R e f . 27). These observations support
-19-
)
20 DENSITY . 5 10
(cm-3) 2 1 .5 log 3
S P E E D 2 ( k m / s )
M- BULK FLOW
VELOCITY
. >
'^^y\iyiy rr
-ANTI-SUNWARD
<t>D (deg) 180
B 90
16^00
MAGNETOSPHERE"
16:30
Boundary Layer
MAGNETOPAUSE LAYER
17:00
MAGNETOSHEATH
Fig. 5.- Basic characteristics of the Low Latitude Boundary Layer äre illustrated. Various plasma regions are identified : U denotes the total energy density (in keV/cm3). S Dß denotes the standard deviation of magnetic field fluctuations (in n T ) ; B, <t>ß/ and Aß are the average magnetic field intensity, latitude and longitude; the bulk flow velocity direction and magnitude are also shown; n and E are the. plasma density (in cm" ) and mean energy (in eV). (from Ref. 27).
-20-
impulsive penetration of solar wind irregularities with an excess of momentum.
The Plasma__BoumdarY__Layer extending sometimes one Earth radius earthward from the magnetopause layer can be viewed as the stopper region where all plasma irregularities with any excess of momen- tum are eventually slowed down ( R e f . 19). The thickness of this PBL should therefore depend on the degree of irregularity of the impinging solar wind momentum density, but it should also be a function of the integrated conductivity along the polar c u s p s field lines. A nearly uniform and steady solar wind would give a v e r y thin P B L . For a very inhomogeneous solar wind flow the penetration distance of the plasma irregularities increases and the thickness of the P B L must be larger.
Furthermore, since the ionospheric conductivity is modulated by seasonal variations one could expect annual variation in the average P B L thickness as well.
The presence of a well developed P B L should also depend on the orientation of the Interplanetary Magnetic Field direction. Indeed impulsive penetration across different parts of the magnetopause is favored when the IMF assumes certain orientations (Ref. 9). The correlation between the thickness of the Plasma Mantle and the southward component of the IMF shown in Refs. 32 and 33 s u p p o r t s this conclusion.
The impulsive penetration model can also account for Carlson and T o r b e r t ' s observations better than any steady state model of the interaction between the solar wind and the magnetosphere. Indeed d u r i n g two rocket flights in the dayside c u s p s Carlson and Torbert (Ref. 34) observed magnetosheath-like ions and electrons injected impulsively into the magnetosphere and precipitating into the cleft ionosphere. The dynamical ion spectra shown in Fig. 6 have been interpreted as a series of solar wind plasma irregularities injected along
GREENLAND CUSP 18 DEC 74 11=04 UT LAUNCH DISPERSION IN ARRIVAL OF IONS
U N I V E R S A L TIME A L T I T U D E INV. L A T I T U D E
1V.06 11:07 2 7 0 3 8 5 74.8 75.1
1V.12 11-13 500 435
75.8 75.9
F i g . 6 . - D i s p e r s i o n c u r v e s of t h e i n v e r s e v e l o c i t y (labelled b y c o r r e s p o n d i n g e n e r g y per c h a r g e ) v e r s u s a r r i v a l times at low a l t i t u d e c o r r e s p o n d i n g to ion spectra, p e a k s . T h e H+ lines are t h e least s q u a r e s fit to dom- i n a n t p e a k s . T h e H e+ + and He+ are t h e o r e t i c a l d i s p e r s i o n c u r v e s w i t h slopes 1/V2 and 1 A/4 -of t h e c o r r e s p o n d i n g p r o t o n line. The injections times are at t h e i n t e r s e c t i o n of t h e d i s p e r s i o n lines on t h e U n i v e r s a l Time a x i s . T h e slopes c o r r e s p o n d to injection distance of 12 ± ' fca> th radii ( R e f . 3 4 ) .
- 2 2 -
d a y s i d e c u s p s g e o m a g n e t i c f i e l d lines at a d i s t a n c e c o r r e s p o n d i n g to t h e m a g n e t o p a u s e ( 9 - 19 R£) . E x t r a p o l a t i o n of t h e d i s p e r s i o n lines in t h e h i g h e n e r g y l i m i t d e t e r m i n e s t h e i n s t a n t s of t h e s u c c e s s i v e i n j e c t i o n e v e n t s . T h e o b s e r v a t i o n s of i m p u l s i v e i n j e c t i o n s of m a g n e t o s h e a t h - l i k e p a r t i c l e s d o w n to r o c k e t a l t i t u d e s a r e d i r e c t e v i d e n c e s t h a t s o l a r w i n d plasma i r r e g u l a r i t i e s a r e t h r o w n i n t o t h e m a g n e t o s p h e r e i m p u l s i v e l y as s u g g e s t e d in R e f . 8.
F i g . 7 i l l u s t r a t e s a d e t a c h e d m a g n e t o s h e a t h - l i k e plasma i n t r u - sion e n g u l f e d i n t o t h e m a g n e t o s p h e r e . T h e p r e s e n c e of a c u r r e n t c a r r y i n g plasma m o d i f i e s l o c a l l y t h e m a g n e t i c f i e l d d i s t r i b u t i o n . The g e o m a g n e t i c f i e l d is in g e n e r a l i n t e r c o n n e c t e d w i t h t h e m a g n e t i c t i e i d i n s i d e t h e plasma e l e m e n t . M a g n e t i c f i e l d lines t r a v e r s e t h e t h i n c u r r e n t sheet s e p a r a t i n g t h e solar w i n d plasma element f r o m t h e m a g n e t o s p h e r i c plasma. C h a r g e s e p a r a t i o n e l e c t r i c f i e l d s normal to t h e plasma s u r f a c e m a i n t a i n local q u a s i - n e u t r a l i t y . In p r a c t i c e e x t r e m e l y small excesses of p r o t o n s o r e l e c t r o n s d e n s i t i e s are s u f f i c i e n t t o s a t i s f y Poisson's e q u a t i o n . Lemaire a n d S c h e r e r ( R e f . 35) h a v e c a l c u l a t e d an e l e c t r i c p o t e n t i a l d i s t r i b u t i o n a l o n g a m a g n e t i c f i e l d e x t e n d i n g f r o m t h e d a y s i d e i o n o s p h e r e u p i n t o a m a g n e t o s h e a t h - l i k e plasma element (see F i g . 8 ) . T h e s h a r p p o t e n t i a l d r o p o f '25 V o l t s at 10.000 km a l t i t u d e s e p a r a t e s t h e w a r m m a g n e t o s h e a t h plasma ( Tg w = 10 K ,6 6 T p w = 7 x 10 K , n ^ = 10 c m "3) f r o m t h e c o l d i o n o s p h e r i c p o l a r w i n d plasma ( T = 3,500 K . T Q + = 1500 K ; Th+ = 3 , 0 0 0 K ; TH e+ = 3 , 0 0 0 K ; ng c = ' 2 7 , 0 0 0 c m "3, see R e f . 35. T h e e l e c t r o s t a t i c s h o c k t r a n s i t i o n a c c e l e r a t e s c o l d i o n o - s p h e r i c ions u p w a r d s a n d i m p a r t s them a c e r t a i n a m o u n t of p a r a l l e l e n e r g y . N a r r o w h e l i c o i d a l beams of H + , He+ a n d 0+ ions r e s u l t f r o m t h e e l e c t r o s t a t i c a c c e l e r a t i o n . T h e p r e s e n c e of a c c e l e r a t e d i o n o s p h e r i c ions in t h e Plasma B o u n d a r y L a y e r and in d e t a c h e d plasma i n t r u s i o n s is t h e r e f o r e t o be e x p e c t e d . T h e k i n e t i c s o l u t i o n shown in F i g . 8 is not u n i q u e , h o w e v e r . In g e n e r a l t h e e l e c t r o s t a t i c s h o c k f r o n t is p r o p a g a t i n g d o w n w a r d s a n d t h e e l e c t r i c p o t e n t i a l is not n e c e s s a r i l y a monotonic d e c r e a s i n g f u n c t i o n of t h e a l t i t u d e as in t h e case s h o w n i n F i g . 8. T h e
- 2 3 -
0.1 -1 kV
0.5 eV
warm
Magnetosheath plasma
Fig. 7.- Schematic representation of a solar wind plasma element engulfed in the magnetosphere. The leading surface of the density irregularity is an (oblique) electrostatic shock separating the warm magnetosheath-like plasma from the magnetospheric and ionospheric plasma. Polar wind ions can be accelerated upwardly by the potential difference across the thin surface layer and form field aligned ion beams in the Plasma
• Boundary Layer of the magnetosphere. The plasma filament expands and spreads along the geomagnetic field lines interconnected with magnetic field lines inside the intruding volume element.
-24-
Fig. 8.- Electrostatic potential distribution along a polar cusps field line. The potential difference .between the ionosphere and the high altitude magnetosheath-like plasma element was arbitrarily taken -equal to 25 Volt. The distribution of the electric potential obtained by an iterative method satisfies the quasi-net,trality condition of any point along the magnetic field line. The sharp pot en fiai drop near 20,000 km altitude separates the predominantly warm magnetosheath plasma from the cold ionospheric plasma (from Ref. 35).
charge separation electric field distribution across the plasma boundary can for instance reverse direction.
The electrostatic field distribution within an electrostatic shock does not prevent the whole structure to propagate downwards along the magnetic field lines. An excess of total pressure in the pla'sma irregularity will drive its edge toward lower altitude. This increases the volume of the element and reduces the excess of density and pressure inside the engulfed filament. A steady state solution like that illustrated in fig. 8 can eventually be reached when the sum of kinetic and magnetic pressures balance each other on both sides of the collisionless shock transition. Time dependent models of propagating electrostatic shocks driven by the field aligned the expansion of engulfed magneto- sheath-like elements seem to be necessary to interpret quantitatively the precipitation events observed by Carlson and Torbert (Ref. 34).
4. R E F E R E N C E S
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\
12. WALBRIDGE, E . , The limiting of magnetospheric convection by dissipation in the ionosphere, J . Geophys. Res., 72, 5213- 5230, 1967.
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21. H E E L I S , R . A . , HANSON, W.B. and BURCH, J . L . , Ion convection velocity reversals in the dayside clefts, J. Geophys. Res., 81, 3803-3809, 1976.
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23. FREEMAN, J.W. J r . , WARREN, C . S . and MAGUIRE, J . J . , Plasma * flow directions at the magnetopause on January 13 and 14, 1967, J . Geophys. Res., 73, 5719-5731, 1968.
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24. A K A S O F U , S . I . , HONES, E.W. J r . , BAME, S . J . , A S B R I D G E , J . R . a n d . L U I , A . T . Y . , Magnetotail and boundary layer plasmas at a geocentric distance of ~ 18 R£ : Vela 5 and.6 observations, J . Geophys. Res., 78, 7257-7274, 1973.
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26. PASCHMANN, G . , H A E R E N D E L , G . , S C K O P K E , N . , R O S E N B A U E R , H. and HEDGECOCK, P . C . , Plasma and magnetic field characteristics of the distant polar cusp near local noon : the entry layer, J . Geophys. Res., 81, 2883-2899, 1976.
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30. H A E R E N D E L , G.,. PASCHMANN, G . , S C K O P K E , N . , R O S E N B A U E R , H. and HEDGEKOCK, P . C . , The boundary layer of the magnetosphere and the problem of reconnection, J . Geophys.
Res., 83, 3195-3216, 1978.
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32. S C K O P K E , N. and PASCHMANN, G . , The plasma mantle. A survey of magnetotail boundary layer observations, J . Atmos. T e r r . P h y s . , 40, 261-278, 1978.
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33/ S C K O P K E , N . . , P A S C H M A N N , G . , R O S E N B A U E R , H. and F A I R F I E L D , D . H . , Influence of the interplanetary magnetic field on the occurence and thickness of the plasma mantle, J. Geophys. R e s . , 81, 2687-2691, 1976.
34. C A R L S O N , C.W. and T O R B E R T , R . B . , Solar wind ion injections in the morning auroral oval, Space Sciences Laboratory, University of California, Berkeley, July 1977.
35. L E M A I R E , J. and S C H E R E R , M . , Field aligned distribution of plasma mantle and ionospheric plasmas, J. Atmos. T e r r . P h y s . , 40, 337-342, 1978.
36. F R E E M A N , J.W., H I L L S , H . K . , H I L L , T . W . , R E I F F , P . H . and H A R D Y , D . A . , Heavy ion circulation in the earth's magneto- sphere, Geophys. Res. Lett., 4, 195-197, 1977.
37. E A S T M A N , T . E . , H O N E S , E.W. J r . , B A M E , S . J . and A S B R I D G E , J . R . , The magnetospheric b o u n d a r y layer : site of plasma, momentum and energy transfer from the magneto- sheath into the magnetosphere, . Geophys. Res. Letters, 3, 685-688, 1976.
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