Fast and Delayed Penetration of the Interplanetary Electric Field to the Earth's Magnetosphere

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Revista Brasileira de Física, Vol. 7, No 2, 1977

Fast and Delayed Penetration of the Interplanetary Electric Field to the Earth's Magnetosphere

W. D. GONZALES, A. L. C. GONZALES, W. A. ALMEIDA

Instituto de Pesquisas Espaciais*, Conselho Nacional de Desenvolvimento Cient/fico e Tecno- lbgico, São José dos Campos SP

and

F. S. MOZER

Space Science Laboratory, University of California, Berkeley

Recebido em 13 de Novembro de 1976

Measured i o n o s p h e r i c e l e c t r i c f i e l d s a t a u r o r a l l a t i t u d e s through 18 b a l l o o n f l i g h t s , and computed e l e c t r i c f i e l d s from an open mgnetosphe- r i c model, show t h a t t h e i n t e r p l a n e t a r y e l e c t r i c f i e l d p e n e t r a t e s t o t h e magnetosphere v i a a f a s t and a delayed mode. These modes a r e i n t e r - p r e t e d as' r a r e f a c t i o n and c o n v e c t i o n waves, r e s p e c t i v e l y . Some i m p l i - c a t i o n s o f these r e s u l t s about t h e understanding o f c o n v e c t i o n i n t h e magnetosphere a r e presented f o r magnet i c a l 1 y d i s t u r b e d and n o t disturbed p e r i o d s .

Campos e l é t r i c o s medidos na i o n o s f e r a de l a t i t u d e s a u r o r a i s a bordo de 18 balões, e campos e l é t r i c o s computados de um modelo a b e r t o da magne- t o s f e r a , mostram que o campo e l é t r i c o i n t e r p l a n e t ã r i o p e n e t r a na magne- t o s f e r a p o r um modo r á p i d o ou p o r um modo l e n t o . Esses modos são i n - t e r p r e t a d o s como ondas de r a r e f a ç ã o e convecção, respectivamente. São também apresentadas algumas das implicações dos r e s u l t a d o s para o en- tend imento de convecçáo na magnetosfera para períodos ma g n e t i camen t e perturbados e não perturbados.

The purpose o f t h i s work i s t o show some r e s u l t s o b t a i n e d on t h e l o c a l c h a r a c t e r o f t h e i n t e r p l a n e t a r y e l e c t r i c f i e l d p e n e t r a t i o n t o a u r o r a l

l a t i t u d e s f o r an open rnagnetospheric model.

*

P o s t a l address: C.P. 515, 11200

-

São José dos Campos SP.

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More than 300 hours o f e l e c t r i c f i e l d d a t a c o l l e c t e d w i t h balloon f l i g h t s between L = 5.4 and L = 8.2 ( ~ o z e r and Ser1 i n , 1969; Mozer a n d Manka, 1971) has been used t o o b t a i n h o u r l y values o f t h e i o n o s p h e r i c e l e c t r i c f i e l d f o r each l o c a l t i m e o f t h e a u r o r a l r e g i o n . The open magnetosphe- r i c model prepared by Gonzelez and Mozer (1974) has been used t o compu- t e h o u r l y values o f t h e i n t e r p l a n e t a r y e l e c t r i c f i e l d a t t h e rnagneto- pause f o r t h e p e r i o d s o f b a l l o o n o b s e r v a t i o n . F i g u r e 1 shows the geometry o f t h e i n t e r p l a n e t a r y e l e c t r i c f i e l d a t t h e magnetopause due t o merging between t h e i n t e r p l a n e t a r y and t e r r e s t r i a l magnetic f i e l d s . F i - gure 2 shows t h e p o t e n t i a l drop computed by t h e model f o r measured values o f t h e i n t e r p l a n e t a r y magnetic f i e l d . For the p r e s e n t w o r k , we have used measured values o f t h e i n t e r p l a n e t a r y magnetic f i e l d d u r i n g the p e r i o d s o f b a l l o o n o b s e r v a t i o n s ( ~ a i r f i e l d , p r i v a t e communication).

Therefore, f o r each l o c a l time, we had a c o l l e c t i o n o f h o u r l y values f o r t h e measured and computed e l e c t r i c f i e l d s w i t h which we c o u l d do a TIME LAG a n a l y s i s t o f i n d o u t p r o p e r t i e s o f t h e i n t e r p l a n e t a r y e l e c t r i c f i e l d p e n e t r a t i o n i n t o t h e a u r o r a l r e g i o n .

From t h e b a l l o o n f l i g h t s , we s e l e c t e d those (18 f l i g h t s ) which d i d n o t have d i s c o n t i n u o u s d a t a and f o r which we had i n t e r p l a n e t a r y m a g n e t i c f i e l d data t o compute t h e model e l e c t r i c f i e l d . From these f l i g h t s , 10 belong t o m a g n e t i c a l l y d i s t u r b e d p e r i o d s and 8 t o n o t d i s t u r b e d periods, a c c o r d i n g t o average

K -

i n d i c e s computed f o r the p e r i o d s o f f l i g h t .

P

Most o f t h e f l i g h t s s t a r t e d a t 22 hours l o c a l time and 'lasted f o r appro- x i m a t e l y 12 hours. For t h e t i m e l a g a n a l y s i s , we s e l e c t e d a l o c a l t i m e range between 0 2 and 09 hours l o c a l time, s

the a n a l y s i s showed good s t a t i s t i c s , namely f l i g h t s were i n v o l v e d i n t h e a n a l y s i s o f a1 cases, r e s p e c t i v e l y .

The time r e and a

2 2 6

l a g a n a l y s i s t t h e equator

ince f o r t h i s range o f hours t h a t a t l e a s t 15, 8 and 6 1, d i s t u r b e d n o t d i s t u r b e d

has been c a r r i e d o u t both a t the a u r o r a l ionosphe- i a 1 p l a n e i n a n o n - r o t a t í n g f rarne o f r e f e r e n c e

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Fig.1

-

I n t e r p l a n e t a r y e l e c t r i c f i e l d and rnagnetic merging geometry a t t h e magnetopause.

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-10 -8 - 6 -4

-2

O

2

4 6 8 10

8, , GAMMAS

Fig.2

-

P o t e n t i a l drop model a t the magnetopause due t o magnetic

mer-

ging as a function o f the i n t e r p l a n e t a r y magnetic f i e l d .

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between the computed e l e c t r i c f i e l d and the measured e l e c t r i c f i e l d (ionosphere) o r mapped e l e c t r i c f i e l d ( e q u a t o r i a l plane)

.

The mappi ng procedure used f o r t h i s a n a l y s i s was described by Mozer (1970). For the ionospheric e l e c t r i c f i e l d , we choose t h e southward component (Es) and f o r the e q u a t o r i a l f i e l d the dawn t o dusk component ( ~ ~ ~ 1 . The o t h e r components, ionospheric westward and e q u a t o r i a l t a i l t o sun, showed poor c o r r e l a t i o n s i n the a n a l y s i s and t h e r e f o r e were neglected.

Since the r e s u l t s obtained i n t h e a n a l y s i s o f t h e e q u a t o r i a l plane a r e very s i m i l a r t o those o f t h e ionosphere, we s h a l l discuss o n t l y t h e i o - nospheric case and s i m i l a r conclusions can be extended t o t h e equato- r i a l plane. Figure 3 shows t h e r e s u l t s o f t h e a n a l y s i s f o r each l o c a l time as a c o r r e l a t i o n c o e f f i c i e n t a g a i n s t time l a g and f o r a11 t h e se- l e c t e d f l i g h t s . Since f o r each l o c a l time there are peaks o f best time l a g (wi t h h i g h e r c o r r e l a t i o n c o e f f i c i e n t s ) , one f o r s h o r t delay times and the o t h e r f o r longer delay times, we s h a l l consider them as due t o

two d i f f e r e n t processes which are goíng t o be discussed l a t e r . I n Figu- r e 3, these two peaks are j o i n e d by d o t t e d tines. From Figure 3,wese- l e c t e d t h e best time lags (BTL), defined as the hours o f time l a g w i t h best c o r r e l a t i o n c o e f f i c i e n t s , and t h e i r associated values o f c o r r e l a - t i o n c o e f f i c i e n t (CCBTL). Therefore, we had two groups o f BTL and associated CCBTL as mentioned above. These values are p l o t t e d i n Figure

4

a g a i n s t l o c a l time. The e r r o r bars are p r o p o r t i o n a l t o .the uncertain- t y i n o b t a i n i n g the BTL values from Figure

3.

The l ines are best f i t s t o those values, and are labeled CW and RW f o r reasons explained l a t e r . The best f i t t o the s h o r t delay-group o f p o i n t s i s a l i n e w i t h approximately zero slope and w i t h a uniform delay time o f approximately one hour f o r a11 l o c a l times. The o t h e r group o f p o i n t s w i t h a longer time delay has a best f i t l i n e w i t h p o s i t i v e slope, i n t e r c e p t i n g midnight a t approximately 6 hours o f time delay, and shows f u r t h e r a time delay i n - crease from midnight towards noon. Figures

5

and 6 are s i m i l a r p l o t s f o r t h e magnetically d i s t u r b e d and not d i s t u r b e d cases, r e s p e c t i v e l y , and o b t a i ned f rom graphs s i m i l a r t o Figure 3.

Thus, the main r e s u l t o f t h i s a n a l y s i s seems t o be t h a t t h e i n t e r p l a n e - t a r y e l e c t r i c f i e l d penetrates t o t h e aurora1 region v i a two e q u a l l y

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O 1 2 3 4 5 6 7 8 9 10 l l I 2 L A G T I M E ALL FLIGHTS(d0ta from s 16 tllghtsi

AURORAL IONOSPHERE E,- COMPONENT

Fig.3

- C o r r e l a t i o n c o e f f i c i e n t versus time l a g f o r each l o c a l time i n v o l v e d i n t h e a n a l y s i s .

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important ways. One i s a s h o r t time p e n e t r a t i o n w i t h 1 hour a v e r a g e time delay which may be the r a r e f a c t i o n wave (RW) described by Coroni t i and Kénner (1973). This wave probably penetrates from the dayside magnetopause (mergi ng region) d i r e c t l y towards midnight t r a v e l i ng i n s i de t h e closed magnetosphere and around t h e plasmapause, sucking plasma and m g n e t i c f i e l d l i n e s and t r y i n g t o get a steady s t a t e s f o r t h e merging process. The I- hour o f average time delay o f t h i s wave i s p r o b a b l y con- s i s t e n t w i t h severa1 A l f v é n t r a v e l times between the clused m a g n e t o s - phere and the ionosphere. The o t h e r way o f p e n e t r a t i o n has a longer time delay, a r r i v e s t o midnight a f t e r approximately 6 hours and then propagates toward noon w i t h an average e x t r a delay time o f approximately 5 hours. This second way o f p e n e t r a t i o n may be a convection wave ( C W ) . The

5

hours o f average e x t r a delay from midnight t o noon i s c o n s i s t e n t w i t h t h e propagation time o f a convecting f l o w around the plasmapause computed w i t h t h e

i!

x

+

B v e l o c i t y associated t o the measured e l e c t r i c f i e l d and t o a magnetic f i e l d model ( F a i r f i e l d , 1968) averaged a t the corresponding L-values o f t h e convection region.

From Figures

4,

5 and 6, one can a l s o make the f o l l o w i n g comnents:

The average convection times between m i dni ght and noon are approximatel y 5,

4

and 6.5 hours f o r a l l , d i s t u r b e d and not d i s t u r b e d cases, r e s p e c t i - vely. These numbers were obtained w i t h t h e help o f t h e dotted extrapo- l a t i o n s of the best f i t l i n e s . This suggests t h a t d u r i n g d i s t u r b e d pe- r i o d s e i t h e r the t r a v e l path f o r the convecting f i o w i s s h o r t e r o r t h a t the convection speed increases o r both.

The convection wave takes approximately 6 hours t o a r r i v e a t midnight, under both q u i e t and di'sturbed c o n d i t i o n s . This conclusion may irnply t h a t the magnetospheric t a i l has s i m i l a r convection p r o p e r t i e s both f o r d i s t u r b e d and not d i s t u r b e d periods.

The c o r r e l a t i o n c o e f f i c i e n t increases from midnight toward noon which i s probably c o n s i s t e n t w i t h the existence o f higher p a r a l l e l p o t e n t i a l drops on f i e l d l i n e s around midnight.

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uam

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CCBTL BTL ( Hours i

-

^N

o O

CCBTL B T L (Hours)

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From F i g u r e 6, i t appears t h a t t h e r a r e f a c t i o n wave i s l o s t on i t s way toward m i d n i g h t when t h e p e r i o d i s n o t d i s t u r b e d .

We may conclude by saying t h a t p r e v i o u s c o r r e l a t i o n s concerning magne- t o s p h e r i c substorms have been p r o b a b l y d e a l i n g o n l y w i t h t h e r a r e f a c t i o n wave w i t h an average d e l a y time o f 1-hour between t h e "southward s h i f t "

o f t h e i n t e r p l a n e t a r y magnetic f i e l d and t h e onset o f a substorm. Howe- ver, from t h i s a n a l y s i s one might as w e l l suggest t h a t e q u a l l y i m p o r t a n t c o r r e l a t i o n s may be found f o r l o n g e r time d e l a y s .

REFERENCES

1. C o r o n i t i , F.V. and Kennel, C.F., C m

t h e i o n o s p h e r e r e g u l a t e magnetospherie convection?,

J

.

^Geophys

.

^Res

.

^78, 2837, 1973.

2, Fa i r f i e1 d, D .H.,

Average magnetic fietd c o n f i g w t i o n o f the outer mgnetosphere,

J. Geophys. Res. 73, 7329, 1968.

3. Gonzalez, W.D. and Mozer F.S., A

qwzntitative modet for thepotentiai!

resutting from reconnection with an arbitrary interpzanetary magne t i c f i e t d ,

J. Geophys. Res. 79, 4186, 1974.

4. Mozer, F. S.

, EZectric fieZd mapping i n the ionosphere a t the equato- r i a l plane,

Planet. Space S c i . 18, 259, 1970.

5. Mozer, F .S. and Manka, R.H

. , kgnetospheric eZectric fieZd properties deduced from simuttaneous batloon fZights,

J. Geophys. Res.

76,

1697,

1971.

6. Mozer, F.S. and Ser1 i n, R.,

Magnetospheric eZectric field measure-

ments with battoons,

J. Geophys. Res.

74,

4739, 1969.