Coronal holes, 146 MHz emissions and geomagnetic storms

Chinese Astronomy

4 (1980) 194-201

Pergamon Press. Printed in Great Britain 0146-6364/80/0601-0194-$07.50/0

Acta Astr. Sinica 2~ (1979) 191-198

CORONAL HOLES,

Zhao

1 4 6 MHz EMISSIONS AND GEOMAGNETIC STOP,MS

Department of Geophysics, Beijing University Beijing Astronomical Observatory

Xue-pu

Liu X u - z h a o

Received 1978 April 14

ABSTRACT. Our a n a l y s i s o f t h e r e l a t i o n s h i p storms lends s u p p o r t to the idea t h a t with large areas.

For £wo t y p i c a l

inversely

correlated

The t i m e l a g b e t w e e n t h e o n s e t o f t h e

f a n beam i n t e r f e r o m e t e r In g e n e r a l ,

o f o u r Miyun S t a t i o n

at the

t h e s e r e c o r d s show a r e g u l a r

i m p l y i n g a d i m i n i s h e d e m i s s i o n from t h e c o r o n a l h o l e s and t h a t

holes follow the solar rotation. of the "central

strip

The p o s s i b i l i t y

the

R e s u l t s from a " s u p e r p o s e d d i a g r a m a n a l y s i s "

flux" demonstrate these effects

even more c l e a r l y .

o f u s i n g r a d i o and s u n s p o t d a t a t o d e t e r m i n e t h e C~4P o f

coronal holes is investigated high-velocity

i.

equatorial

with the area of the latter.

Records o f t h e 164 ~ z

recurrent

M-regions are coronal holes

meridian passage of the coronal hole is

times of coronal h o l e s are examined. variation,

the so-called

coronal h o l e s examined, t h e i r

e x t e n s i o n s a r e f o u n d t o be 25 - 50 ° . geomagnetic storm:and the central

between coronal h o l e s a n d geomagnetic

and an a t t e m p t i s made t o e v a l u a t e t h e s p e e d o f t h e

s o l a r s t r e a m and t h e s i z e o f t h e c o r o n a l h o l e from d a t a on

geomagnetic storms.

INTRODUCTION

It is generally believed that recurrent

g e o m a g n e t i c s t o r m s (RGS) a r e c a u s e d by h i g h - v e l o c i t y

s t r e a m s i n t h e s o l a r wind and t h e s e s t r e a m s o r i g i n a t e M - r e g i o n s on t h e s o l a r s u r f a c e .

i n what B a r t e l s h y p o t h e s i z e d a s t h e

But d e c a d e s o f o b s e r v a t i o n s h a v e f a i l e d

H - r e g i o n s , and t h e q u e s t i o n o f t h e o r i g i n

o f t h e RGS had r e m a i n e d o p e n .

we h a v e n o t b e e n a b l e t o make good p r e d i c t i o n s

o f t h e RGS, e s p e c i a l l y

to i d e n t i f y

the

As a r e s u l t ,

of their

times of

b e g i n n i n g and e n d i n g . R e c e n t l y , N o t l e and o t h e r s X-ray c o r o n a l h o l e s t h e CH and RGS.

[1] showed t h a t

the high-velocity

(CH), and S h e e l e y and o t h e r s

streams originate

in soft

[2] showed a g e n e r a l c o r r e s p o n d e n c e b e t w e e n

T h u s , t h e q u e s t i o n o f t h e M - r e g i o n s p u r s u e d f o r o v e r 40 y e a r s c a n be

s a i d to have been s o l v e d . However, f o r t h e p u r p o s e o f p r e d i c t i n g c o n n e c t i o n b e t w e e e n them and t h e CH; a l s o , CH n e e d s t o be e x a m i n e d .

RGS, we m u s t i n v e s t i g a t e

more d e e p l y t h e

the procedure for regular

ground o b s e r v a t i o n s o f

On t h e o t h e r h a n d , t h e s e s t u d i e s may i n t u r n l e a d t o b e t t e r

Coronal Holes

195

u n d e r s t a n d i n g o f t h e p a s t r e c o r d s o f g e o m a g n e t i c and s o l a r o b s e r v a t i o n s , to identify

t h e CH i n t h e p a s t and f i n d t h e i r

significance

in the study of solar activity.

2.

law o f v a r i a t i o n .

e n a b l i n g us p e r h a p s

These a r e o b v i o u s l y o f g r e a t

CORONAL HOLES AND GEOMAGNETIC DISTURBANCES

Papers published so far [3] on the connection between CH and geomagnetic disturbances deal with average statistical properties of the global geomagnetic activity (as represented by the A

index or the C index) before and after the central meridian passage (CMP) of a CH. P P In this paper, on the other hand, we shall concentrate on some individual events. For the

geomagnetic disturbance besides using the geomagnetic index

~,Kv

we also used our Peking

i=l

data on geomagnetic storms and equatorial Dst storms, so as to make a more refined analysis and comparison possible. As regards CH, observations on various wavelengths have not been perfectly consistent, and there has been no unified method of characterizing their edges, with the result

that the

CMP times obtained by different people (even from the same data) are ofter very

different.

(See Fig. 4 in [4]). For definiteness, we have chosen two CH with long

observational runs, which were observed simultaneously in the white-light K-corona and soft X-ray ranges (2-32 A, 44-54 A) or at the ultraviolet line of 284 A, and for which the various CMP times given are in fair agreement with one another. Hansen and others [4], these two CH are known as #4 and #5.

In the designation of

#4 extends from the South pole

to the equator, while #5 (which is designated as CHI in [i]) extends from the North pole towards the equator.

Their CMP dates are:

# 4 : 1 9 7 3 Jan 24 Feb 20 Mar 18 Apt 14 May ii* Jun 8 # 5 : 1 9 7 3 May 31 Jun 27 Jul 25 Aug 21 Sep 16 Oct 14 * Neupert and Pizzo judged the date to be May 15; after considering the 27-day period, we preferred r'ay II (cf. Pig.l). i.

Coronal Holes #4, #S and GeomagneticL Storms

In Fig. I, we have marked the CMP dates of CH #4 and #5, and the times of beginning and end of geomagnetic storms observed at Peking, as well as the maximum value of the K-index during the storm.

The figure includes data on other coronal holes and geomagnetic storms

observed at Peking. CH #5 showed clear boundaries in the soft X-rays, hence it was possible to estimate its extension in the equatorial zone, and to determinefairly accurately its CMP dates. fine analysis can be made in this case of the connection

A

between CH and RGS, and the

results are given in TABLE i. From TABLE 1 and Pig. i, we can note the following: a.

One or two days after each CMP of CH #5, which has a large equatorial area, we

find the start of a geomagnetic storm, (GS). b.

The time lag between the onset of the GS and the CMP of the CH varies with the

area of the CH.

In general, the larger the area, the smaller is the time lag.

(The event

of Jul 25 is an exception, it may be due to the flare of Calss 2f occurring at the same

196

Coronal Holes

time). c.

Looking a t F i g . 1 a s a w h o l e , we f i n d t h a t n o t e v e r y CbW o f e v e r y CH i s a c c o m p a n i e d

by a GS, t h i s

is especially

t r u e f o r t h o s e CH w i t h s m a l l e q u a t o r i a l

6

1910

.~ ~

~

•

"zh

1909

:9

4

, , ( Z4 o I * " / . . ,~/ 1 o

_1

1911 1912 1913 1914 1915

16

21

.3]x \ z~:,i~,,,r \

1908

~

11

n\

1907

1,"./ / I

zi

2

17

~

,

2

1

6

,11

{ [~ffN

•

21

(61

,, 2 6 20

4

.17

/

. 171

26 27

,~ (5) ." L/A~,~ ° ~A,q

"'4

~

1,

areas.

t.

/ ~ A~%~61'-

..~1

"lO'.°/J

19

[

1916

t,r.,

1917

1918

/

/I

//I

~,~,sllllJA(6)"

3 & ( 5)1

14

- (7),

"(o,.,

I /

.'.(5)u

t

'I

/

F i g . 1. O b s e r v a t i o n s o f g e o m a g n e t i c s t o r m s a t P e k i n g , and o f c o r o n a l holes in the soft X-ray, far ultraviolet and w h i l e - l i g h t K - c o r o n a i n t h e p e r i o d 1973 J a n 1 - Nov 20. The open. h a l f - f i l l e d and f i l l e d t r i a n g l e s and t h e numbers above them mark t h e s l o w - s t a r t , s l o w - s u d d e n - s t a r t , and s u d d e n - s t a r t o f g e o m a g n e t i c s t o r m s o b s e r v e d a t P e k i n g , and t h e c o r r e s p o n d i n g UT. Symbol [[ marks t h e end o f a s t r o m and t h e number a b o v e , t h e UT. The number i n b r a c k e t s d u r i n g t h e p e r i o d o f a s t o r m i s t h e maximum o b s e r v e d v a l u e o f t h e K - i n d e x . The CEP d a t e s o b s e r v e d a t 284 A, i n t h e r a n g e 2-54 A, and i n t h e w h i t e - l i g h t K - c o r o n a a r e r e s p e c t i v e l y marked w i t h \ , / , and [. T h e s e d a t e s a r e t a k e n from [ 3 ] , [5] and [1]. The c u r v e d o u t l i n e s a r e b a s e d on K - c o r o n a o u t l i n e s .

Table 1 Coronal hole #5

Geomagnetic s t o r m at Peking

Time of CEP

~hy 31 1332 UT

Jun 27 2119 lIT

Jul 25 0254 UT

Aug 21 0050 UT

Sep 16 2323 liT

Oct 14 1337 UT

Area (1010km)

3.3

4.7

3.0

3.2

2.4

l.l

Time of onset

Jun 2 0300 UT

Jun 28 2100 UT

Jul 26 0300 UT

Aug 22 ii00 UT

Oct 16 0520 UT

37

24

24

34

40

5

6

6

5

6

Jun 2 0700 UT

J u n 28 2300 UT

J u l 26 0900 UT

Aug 23 0300 UT

Oct 16 0800 UT

Duration [hrs) g~x. K - i n d e x

Dst storm

Main p h a s e start Duration (hrs) Max. decrease

41

26

30

46

43

-26y

-31y

-28y

-38y

-29y

2.

C o r o n a l H o l e s #4, #5 and EK p In o r d e r t o s u p p r e s s o t h e r d i s t u r b a n c e s

and t o b r i n g o u t t h e e f f e c t s

we made t h e u s u a l " s u p e r p o s e d d i a g r a m " a n a l y s i s , same CH a r e l i n e d up a t t h e CMP and summed. t o #4, and F i g s .

2B and 2C r e f e r

t o 8, w h e r e a s i n 2C, i t t h e CMP. These f i g u r e s t h e CMP.

t o #5.

i n which t h e % - i n d e x

Our r e s u l t s

o f CH on g e o m a g n e t i s ~ pertaining

a r e shown i n F i g .

2.

to the

Fig.

2A r e f e r s

In F i g s . 2A and 2B, t h e s u m m a t i o n e x t e n d s from 1

e x t e n d s from 1 t o S, c o r r e s p o n d i n g t o t h e 3 - h o u r p e r i o d a r o u n d

show t h a t t h e v a l u e o f ZK r e a c h e s a maximum a b o u t t h r e e d a y s a f t e r P I f t h e maximum c o r r e s p o n d s t o t h e h i g h - v e l o c i t y s t r e a m from t h e c e n t r a l p a r t o f

C o r o n a l Holes

197

the CH, then the time lag between the onset of #4

the GS and the CMP will give us the equatorial width of the CH, which we found to be 25-50 ° . Again from the figure, we note that the maximum value of EK reached is 40 for #4, P and 28 for #5. Using the empirical relation

211

between the velocity of the solar wind and

-6

' 24

EK derived by Synder and others [7], P u = 8.4EK + 330 (km/sec) P we find the average wind velocities %4

= 646 km/sec

%5

= 565 km/sec.

0 i~

2

4

2

4

6

#5

Further, according to the empirical relation, concluded by Notle and others

' "2

-6

I

-4

i

-'2

'IK, -30

[i], between

6 C

#5

the equatorial area A of the CH and the stream velocity u, (1010km2),

-10

we find, for these two CH, the average areas --6

A#4 = 2.75 x I0 I0 km 2

--4

" Z2

'

o

Fig. 2 Superposed Diagrams of EK P

A#5 = 1.80 x I0 I0 km 2 If we use the arithmetical mean of the result

from TABLE i, then we find for CH #5, an equatorial area of about 3.0 × I0 I0 km 2.

THUS,

the values derived first tend to be somewhat low.

3.

CORONAL HOLES AND SOLAR EMISSION AT 146 ~ z

Lantos and Avignon [8] compared the solar section at 169 ~ z photographs of CH at 284 A [figs. 3A, 3B). shown in Fig. 3C. CH effect.

on 1973 Oct 8 with

Our observations at 146 MHz on the same day are

Both the Nan~ay and our Hiyun transit observations showed clearly the

The depressions in the radio records mean that radio emission is reduced inside

a CH. To ascertain whether it is possible to identify CH from radio records at 146 ~ z , analyzed our transit observation of #4 and #5 made with our 146 ~ z i.

we

fan beam interferometer.

Features in. the original transit records at 146 b~z before and after the CMP of CH #5*

The records made before and after the CMP of #5 on b~ay 31 1973 are shown in Fig. 4.

Here,

the solid line marks the actual observed curve at 146 MHz, and the dotted curve represents the normal flux of the solar corona when without any structure. dotted curve measures the decrease in emission.

The depression under the

The figures show that the depression

moved daily from left to right, passing the centre on ~ y

31.

This movement is in agreement

* After completing this paper, we saw the recent work by Drago and others observed results at 169 ]~{z and at 408 ~{z on May 31 and Jun 27.

[9] giving the

198

Holes

Coronal

1973 Hay 28

coronal h o l e emission at 169H.z

Nay 30

B

coronal}

2S4 kh;le

Jun 1

coronal hole 146Msz

Jun 2

emission a t

F i g . 3 Radio t r a n s i t o b s e r v a t i o n s during t h e ~,'Po f a coronal h o l e

(1973 Oct

F i g . 4 Radio o b s e r v a t i o n s a t 146 ~Nz on d a y s b e f o r e and a f t e r t h e CMP o f #5 on Fay 31

8)

w i t h t h e w e s t w a r d m o t i o n o f t h e CH f o l l o w i n g t h e S u n ' s r o t a t i o n . F i g , S shows t h e o r i g i n a l The c e n t r a l

portions

(dotted line). days b e f o r e ,

r e c o r d s on 4 s u c c e s s i v e C ~ d a y s b e t w e e n ~ y

of these records all

show d e p r e s s i o n s r e l a t i v e

The r e c o r d s a r o u n d t h e day o f t h e C~P f o l l o w t h i s the right

side of the record is higher than the left

the reverse is the case.

(cf.

e.g.,

Fig. 4).

d a y s o f o t h e r CH and n e v e r f o u n d a n y m e t e r It thus appears that

31 and Aug 21.

t o the normal s t a t e general pattern: side;

We e x a m i n e d many more t r a n s i t

wave b u r s t s

in the central

t h e p r e s e n c e o f a l a r g e CH w i l t a f f e c t

one o r two

one o r two d a y s a f t e r r e c o r d s on CMP

portion.

t h e f l u x a t 146 ~ z .

But a s

C o r o n a l Holes

p o i n t e d o u t b y Dulk and S h e r i d a n [10],

199

the

d e c r e a s e i n e m i s s i o n a s s o c i a t e d w i t h a CH i s o n l y 10 - 50%, and i t may e a s i l y be masked b y other factors,

h e n c e t h e r e h a s b e e n no o n e - t o

one c o r r e s p o n d e n c e b e t w e e n t h e CH o b s e r v e d i n the soft

X-ray or f a r u l t r a v i o l e t

and t h e

d e p r e s s i o n s i n t h e m e t e r wave e m i s s i o n . 2.

The C e n t r a l S t r i p

further

the effect

145 ~ z ,

Flux

To examine

o f CH on r a d i o e m i s s i o n a t

we made a s e p a r a t e a n a l y s i s w i t h

the measured flux in the central strip

at the time of transit

o f t h e Sun.

This strip

North-South

of the centre

has a width of 6',

for

Aug 21

this is t h e resolving power in the East-West direction of our array at Miyun [ii]. The result of a superposed diagram analysis is shown in Fig. 6. (6A for #4, 6B for #5).

We

note clear minima on the day of CMP here: the radio emission at 146 ~ z

Fig. S Radio transit observations at 146 M~z on 4 successive CMP days of #5

is indeed

reduced in a CH. Comparing F i g s . 6 and 2, we s e e t h a t t h e r e i s a good c o r r e s p o n d e n c e b e t w e e n t h e " 1 4 6

~z

coronal holes" and geomagnetic activity, namely, about 3 days after the appearance of

the depression in the central strip flux, the average ZK reaches a maximum, and, in the P main, the greater the radio depressions, the higher will be the ZK maximum. P

4.

DISCUSSION

The above a n a l y s i s

shows t h a t good c o r r e l a t i o n s

e m i s s i o n and r e c u r r e n t

geomagnetic storms.

predictions of the RGS, and to derive (i)

exist

among c o r o n a l h o l e s ,

These c o r r e l a t i o n s

m e t e r wave

c a n h e l p u s t o make

some properties at Ct~P from the usual observations.

From the point of view of RGS prediction, the key is the timely identification of

the CMP of a CH, especially the first passage.

Although, as shown in Section 3, for CH #4

and CH #S which were observable by several means, the effects of CH on the radio emission at 146 ~ z

are detectable and resolvable, we cannot, in general ascertain the time of CMP

from the fan beam interferometer observations with any confidence, because of the great variability of the solar flux in the meter waves and because of the great variety of factors that affect the radio observations.

Saemundsson

[12] believed that the M-regions appear

in locations where previous sunspots have existed.

This, if true, would be helpful in

ascertaining the CMP from ground observations, so we re-examined this possibility with the sunspot distributions observed by various Chinese observatories in the period 1972 Aug 5 1973 Nov 12.

We found the following result:

if, after the disappearance of a large sunspot,

no new spots of any great size have appeared in the same longitudes within the following

200

Coronal Holes

Central strip flux

#4

A

8.~

L i

I

i

I

-4

-6

~"

i

0

-2

i

i

!

2

i

i

4

6

Central strip flux #5

B

[8.50

7.

"7.00

/

i

I

-6

I

--4

i

i

--2

i

i

)

0

Fig. 6 Superposed d i a g r a m s o f t h e c e n t r a l 3 or 4 solar rotations, Thus, by a c a r e f u l

I

2

I

4 strip

I

I

6 f l u x a t 146 ~ z

then a coronal hole will often appear in that analysis

location.

of the morphological changes in the transit

a t 146 ~ t z , and t h e d e c r e a s e i n t h e c e n t r a l

strip

observations

f l u x , w h i l e making u s e o f t h e above

t i m e - l a g b e t w e e n t h e d e a t h o f s u n s p o t s and t h e b i r t h

of coronal holes,

we may p o s s i b l y

arrive at a fairly effective means of determining the time of the first CMP, that is, the beginning of a series of recurrent geomagnetic storms. (2)

Coronal holes are large-scale structures on the Sun's surface.

Their relationship

with solar active longitudes, the Hale boundaries etc. is not clear at present.

Therefore,

it will be of great interest if we can derive the CMP dates of coronal holes in the past from historical records and correlate these with other relevant quantities.

In doing this,

in addition to the sunspot and meter wave observations, data on geomagnetic activity are of great value also.

As demonstrated in Section 2, a knowledge of the beginning of a RGS

series is helpful in determining the appearance of a coronal hole and its longitude on the Sun, and a superposed diagram analysis can lead to the relative sizes of the coronal hole. Also, we should remember Allen's analysis of the ~*-regions [13]: then from the life-spans

Coronal Holes

of single coronal

geomagnetic

hole,

storms,

are

far

of a series,

occur,

Although the correlations relations

we s h o u l d b e a b l e t o e s t i m a t e

from the duration

t i m e o f y e a r when t h e s t o r m s

its

the

latitude

among c o r o n a l

from being

one-to-one.

life-span

holes,

m e t e r wave e m i s s i o n

clearer

understanding

of the physical

of the coronal

This is obviously

of the space between the Earth and the Sun.

streams,

the equatorial

state

a n d how t h e l a t t e r

width of the hole,

and from the

on t h e S u n .

effects

the high-velocity

201

Our n e x t

of coronal propagates

holes

a n d RGS a r e g o o d ,

due to the neglect task

is

therefore

o f how t h i s

state

to gain a fashions

i n s p a c e f r o m t h e Sun t o u s .

REFERENCES J. T. Notle et al., Coronal holes as sources of solar wind, Solar Phys., 46 (1976), 303. N. R. Sheoley Jr., et al., Coronal holes, Solar wind Streams, and recurrent geomagnetic disturbanees, 1973---1976, So/at Phys., 49 (1976), 271. [ 3 ] W. 1M. Net, pert and V. A. Pizzo, Solar Coronal holes as sources o! recurrent geomagnetie distmrbances, J. Geophys. Res., 79 (1974), 3701. [ 4 ] R. T. Hansen, et alq Longlivea coronal structures and recurrent geomagnetic patterns in 1974, planet. Space 8oi., 24 (1976), 381. [ 5 ] B. Bell and G. Noel, Intensity of the Fe XV emission line corona, the level of geomagnetic activity and the velocity of the Solar wind, J. Geophys. ties., 81 (1976), 4508. [ 6 ] J. T. Notle et aL, An atlas of coronal hole boundary positions May 28 to November 21, 1973, Solar Phys., 46 (1976), 291. [ 7 ] C. W'~ Snyder and Marcia Neugehauer, The Solar wind velocity and its eorrelation with eosmie ray variations and with solar and geomagnetic activity, J. Geophys. Res., 68 (1963), 6361. [ 8 ] P. Lantos and Y. Avignon, The metric quiet sun during two cycles of activity and the nature of coronal holes, Astro. As~ophys., 41 (1975), 137. [ 9 ] France C%iuderi Drago, et al., Structure of coronal holes from UV and radio observations, Solar Phys. 51 (1977), 143. [10] G. A. Dulk and K. V. Sheridan, The structure of the middle corona from observations at 80 and 160 MHz, Solar Phys., 36 (1974), 191.

[1] [2]

[ii] [12] [13]

the

of the

Liu Xu-zhao and He Xiang-tao, Acta Astronomica Sinica 15 (1974) 61-72. Saemundseon, Th., Statistics of geomagnetic storms and Solar activity, Men. Not. Roy. AsfrO~t. So~., 123 (1962), 299. C. ~V. Allen, M--reglons, Pla~tet. Space Sci., 12 (1964), 487.