A C T I V I T Y I N T H E Q U I E T S U N

I. Observations o f Macrospicules in H a and D3

BARRY J. LABONTE

Big Bear Solar Observatory, Hale Observatories, Carnegie Institution of Washington, California Institute of Technology, Pasadena, Calif. 91101, U.S.A.

(ReceNed 25 August, 1978)

Abstract. Macrospicules have been observed in Ha and He l 03, on the disk and above the limb. In 1975, a rate of 1400 (A~ day) -1 is inferred, and the ratio of equatorial to polar rates ~<2. D3 intensities are a few x 10 -3 of the disk center, and do not decrease in coronal holes. The ratio of Ha to D3 intensities is ~ 10. The integral number of macrospicules with D3 intensity ->I0 is proportional to Io ~.

1. Introduction

Traditionally, the surface of the Sun has been considered to be divided between

active and quiet regions. Active regions, generally sunspot groups and their asso-

ciated plages, were the sites of explosive phenomena: filament eruptions, surges,

flares. The much smaller material and magnetic field motions in quiet regions were

regarded as the steady-state condition of the solar atmosphere.

Recent observations have shown that in fact spot regions are only the largest of a

continuum of bipolar magnetic eruptions, and that the spectrum is dominated, in

terms of numbers and total magnetic flux, by the smallest scale regions, the

ephemeral regions (Harvey and Martin, 1973; Harvey et al., 1975). These smaller

magnetic regions also have explosive events, and being more widely distributed over the Sun's surface than spot regions, lead to the presence of activity in the quiet Sun.

Interest in this small scale activity was increased by the Skylab observations at

short wavelengths (Golub etaI. , 1974; Bohlin etal . , 1975; Withbroe etal . , 1976). In

this series of papers we report on optical observations of activity in the quiet Sun.

This first paper covers new observations of macrospicules. Subsequent papers cover

the analysis of the macrospicule data, the discovery of a different type of activity, and

analysis of the new events.

The most obvious form of small scale activity in the quiet Sun, the H a polar limb surge, was first reported as a distinct phenomenon by Godoli and Mazzucconi (1967).

This type of event was observed earlier (Roberts, 1945), but not distinguished from

ordinary spicules. Limb surges occur in quiet regions at all latitudes, not only at the

poles, and are called H a macrospicules. Ramsey and Mosher (1976) have described

the structural appearance of Ha macrospicules and their frequency of occurrence as a function of latitude and solar cycle phase.

Recently, Moore et al. (1977) have shown that H a macrospicules correspond to

extreme-ultraviolet (EUV) macrospicules (Bohlin et al., 1975; Withbroe et al.,

1976), and to flares in soft X-ray bright points (Golub et al., 1974). The work of

Solar Physics 61 (1979~ 283-296. 0038-0938/79/0612-0283 $02.10. Copyright �9 1979 by D. Reidel Publishing Co., Dordreeht, Holland, and Boston, U~S.A.

284 BARRY J. LABONTE

Golub et al. (1974) and Harvey et al. (1975) has previously indicated that the X-ray

bright points are the coronal manifestations of magnetic ephemeral regions. We may thus identify the sites of macrospicules as ephemeral regions near the limb. The ability of ephemeral regions to produce small H a flares and surges was recognized when they were first discovered (Harvey and Martin, 1973). It has seemed that the

small scale of activity in ephemeral regions is commensura te with their small magnetic fluxes, short lifetimes, and simple, bipolar structure (Harvey et aL,.1975).

Macrospicules have also been observed in the He I D3 line (Ramsey et al. , 1975). We revort here on further observations in Ho~ and D3.

2. Observations

2.1. INSTRUMENTATION

For this study, filtergram movies were made at the Big Bear Solar Observatory, in H a

and D3, with the 25 cm vacuum refractor. Filter bandwidths were 0.25 A at Ha , and 0.19 or 0 . 3 8 A at D3. The field covered was 4.2 by 5 . 6 a r c m i n at Ha , 5.5 by

7.3 arc min at D3, with the long dimension oriented east-west. Da ta from three observing runs were used: May 20 to June 20, 1975, with 122 hr

of D3 film from 20 days, covering the disk and limb, with a 0.19 ,& filter; September 19 to October 2, 1975, with 99 hr of D3 film from 12 days, deeply exposed at the limb, with a 0.38 A filter; October 28 to November 12, 1975, with 54 hr of D3 film from 15 days, covering the disk and limb, with a 0.38 A filter.

2.2. A P P E A R A N C E A T T H E L I M B

2.2.1. H a

H a macrospicules are easily observed with our instrument. Figure 1 shows one

example, which illustrates many of the commonly observed features. Events in H a have multiple knots, spikes, loops, and other fine structure, and are more similar in appearance to filament eruptions than to simple spike surges. In Figure 5 are shown the histograms of the maximum projected height (h), lifetime (t) and diameter (2r) for the 32 H a macrospicules seen during the September to October 1975 observing run. Mean values are/ '~= 12 000 km, t" = 11 min and 2~ = 1800 km. For loop events, the diameter is that of the arch, not the distance between ends. There is no significant correlation between height and lifetime. All these propert ies are dependent on the intensity threshold of the data and the subjective selection criteria; the data in Figure 5 should be considered as measures of the events studied here, not of a complete sample of macrospicules.

Differential motion of fine structures relative to each other is normal. In many events, motion of the macrospicule along the limb is seen before the vertical eruption occurs. Apparen t velocities, both horizontal and vertical, can be tens of km s -1 in the early stages. For example, the event of Figure 1 moved horizontally at 40 km s 1 and vertically at 60 km s 1, and that of Figure 2, 40 km s -1 vertically, at some times.

ACTIVITY IN THE QUIET SUN, I

S E P T E M B E R 2 7 , 1 9 7 5

285

2 0 3 6 : 4 6 2 0 4 0 : 5 8

2 0 3 7 : 5 9 2 0 4 1 : 5 2

2 0 3 8 : 5 3 2 0 4 3 : 2 0

i I

2 0 !1

Fig. 1. Macrospicule in H a line center. Event was located at 31 ~ north latitude. The apparent limb is the top of the spicules, about 5000 km above the continuum limb.

The arrow marks the initial appearance of the event; substantial motion along the limb occurred before the vertical extention. Fine structure down to 0.5 arc see is visible. The vertical structures within the macrospicule do not fan out with height, as in spike surges, but lean toward each other. At 20:41 UT, the structure looks like a dosed loop. By 20:43, diffuse material has been spread over a large volume. He ~ D3

emission was detected in this event.

286 B A R R Y J. LABONTE

9 - 2 0 - 7 5

a

b

C

d

e

H a - 1 . 0 D 3

I I ww

2O

Fig. 2. Macrospicule in H a - 1 .~ and D3. Event was located at 80 ~ south latitude: the polar coronal hole boundary this day was at 65 ~ Times (UT) are (a) 18:59:58; (b) 19:02:12; (c) 19:03:22; (d) 19:04:51; (e) 19:08:19.

D3 emission peaks in the lowest, brightest part of the H a structure, not in a sheath surrounding the Ha. The H a object shows much fine structure, and at (b) appears as a faint loop. Simultaneous film at H a + 0.5 A. shows similar gross evolution, but different fine structure, implying Doppler lineshifts which

are consistent with the apparent transverse velocities.

A C T I V I T Y IN T H E Q U I E T SUN, I 287

The complex appearance and evolution of macrospicules determines a definite

time direction; it is easy to tell if the film is being viewed forwards or backwards. Only in a minority of events is a simple up-and-down flow along a tube suggested.

During the September 19 to October 2, 1975 run, in 49 hr at the north and south poles, 14 prominent (in terms of size, intensity, and structure) H a macrospicules were counted. All these events occurred within the polar coronal holes, as deter- mined by scans of the D3 limb band. In 50 hr at the east limb, 18 events were seen. Using the known field size, one may convert these counts into an expected rate for the entire 360 ~ of limb. This yields rates of 4.9 + 1.4 events (hr 360~ -1 at the pole, and 8.3 +2 .0 events (hr 360 ~ 1 at the equator. A higher rate is expected at the equator since the surface density of ephemeral regions is larger in the active latitudes

(Martin and Harvey, 1976). The relatively small difference in the rates at the pole

and equator indicates that the distribution of ephemeral regions in 1975 was nearly uniform up to the pole, a fact which is somewhat masked in the magnetic obser- vations by a steep limb visibility function (Harvey et al., 1975).

Using an area-weighted average rate of -=-7 (360 ~ hr) -1 and a typical height of

10 000 km, a whole Sun rate of = 1400 events (AG day) 1 is inferred. Mosher (private communication) has shown that the number of H a macrospicules detected can vary

by orders of magnitude, depending upon the observing technique; therefore these rates should only be considered as measures of the detection threshold. As an example, Moore et al. (1977), using the 22 cm vacuum refractor at Big Bear, which gives a full disk H a image, found 45 prominent events in 185 hr on film exposed for

the disk, or a rate of 0.24 + 0.04 events (hr 3600) -1.

There was no significant difference in sizes or lifetimes between the polar and equatorial events.

2.2.2. D3

A few events were also detected in D3. Figure 2 shows one example, in both

H a - 1.0 A and D3 line center. In the figure, the H a limb is the top of the spicules, approximately 3300 km (at this wavelength) above the photospheric limb seen in D3. At the time of maximum intensity, H a emission can be traced to 20 000 km above the photosphere, and D3 to 10 000 km. The macrospicule diameter is 2500 km in D3, and is similar in Ha , although finer structure can be seen in Ha .

D3 emission peaks in the lowest, brightest part of the H a structure, as Ramsey et

al. (1975) have noted. It does not come from a sheath surrounding the Ha . Soft X-ray

and E U V emission from similar events prove the existence of hotter material around the H a structure; we infer that the D3 emission is therefore apparently produced not at high temperature , but at high density. In Figure 2, the D3 evolution parallels that of H a ; this is true for the other events as well.

Of the total 32 H a events seen during the September 19 to October 2, 1975 run, D3 film was obtained for 28. Only 4 macrospicules were detected in D3; two at the south pole, and two at the east limb. Although the sample is small, the fact that equal numbers were seen at pole and equator suggests that D3 macrospicules may not share

2 8 8 B A R R Y J. L A B O N T E

the p o l a r d a r k e n i n g ev iden t in the D3 l imb band (Zir in, 1975; Marsh , 1978a). This

will be d iscussed fu r the r in l a te r sect ions.

T h e 4 events wi th d e t e c t a b l e D3 emiss ion were sys temat ica l ly la rger and longer

l ived than the o t h e r H a macrosp icu les , w i t h / ~ = 16 000 km, f = 16 min and 2~ =

2600 km. A l l these va lues a re a b o u t one s t a n d a r d dev ia t ion a b o v e the means for the

to ta l s a m p l e of 32 events .

2.3. EMISSION INTENSITIES

W e have m e a s u r e d the H a and D3 in tens i t ies of the four events wi th de t ec t ab l e D3.

A p h o t o m e t e r was used in a p r o j e c t e d image. Its c i rcular a p e r t u r e had a d i a m e t e r

c o r r e s p o n d i n g to 0.5 arc sec at H a , 0.7 arc sec at D3.

Resu l t s a re s u m m a r i z e d in T a b l e I. C o l u m n 3 gives the a l t i tudes above the

p h o t o s p h e r i c l imb at which the m e a s u r e m e n t s were made . It was des i rab le to

m e a s u r e the D3 at a low a l t i tude where it was br ightes t , bu t in H a , the p re sence of

TABLE I

Ha and D3 macrospicule intensities

Date and time (UT) Latitude Height (km) a IDs/IDc b I H J I D c ~ Radius D3(Ha) (kin)

9/20/75 1853 81~ 3000 (3500, 4• -3 8• 1400 5000) 3 • 10 -2 (+0.5 ,~)

9/20/75 1903 80 ~ S 3000 (3500, 6• 10 -3 6• 10 -2 (-1.0 ,~,) 1300 5000) 3 • 10 -2 (+0.5 A)

9/27/75 2038 38 ~ 4000 (6000) 2• 10 -a 5 • 10 -z (0.0) 1300 10/2/75 1702 31~ 5000 (6000) 6• -3 6• 2(0.0) 2600

Height of intensity measurement. b IDC is the specific intensity of the continuum at disk center.

c Number in parentheses is the wavelength relative to line center.

sp icules fo rced the m e a s u r e m e n t s to be m a d e s o m e w h a t higher . C o l u m n 4 gives the

m a x i m u m D3 l ine cen te r in tens i ty in units of IDC, the specific in tens i ty of the

c o n t i n u u m at d isk center . C o l u m n 5 gives the same quan t i ty for H a , at the

wavelength(s ) re la t ive to l ine cen t e r given in pa ren theses . A t D3, / p c =

3.36 • 106 ergs (cm 2 s s r f~x)-i ; at H a / D E is less by the fac tor 1.15 (Labs and Necke l ,

1968). C o l u m n 6 gives the rad ius r at the he ight of the D3 m e a s u r e m e n t ; due to the

low a l t i tude , the average of these va lues is s o m e w h a t l a rger than tha t q u o t e d above .

The obv ious resul t f rom T a b l e I is tha t H a is much b r igh te r than D3, wi th ra t ios of

ene rgy p e r uni t wave leng th f rom 5 to 25. If the ra t io of H a to D3 l inewidths is la rger

than 1 (for example , be ing = 2 in spicules (Beckers , 1972)), the to ta l ene rgy emiss ion is c o r r e s p o n d i n g l y grea te r .

F o r the p o l a r events , film was o b t a i n e d at two wave leng ths in H a , a l though ne i the r

was line center . The genera l evo lu t ion was s imi lar at bo th wave lengths , but the fine

ACTIVITY IN THE QUIET SUN, I 289

structure was completely different. This difference in the fine structure implies that

the linewidth at any point must be less than the separation of the two wavelengths,

1.5 A. Substantial Doppler shifts are expected if the observed transverse velocities

represent actual mass motions, 1 .& at H a corresponding to 46 km s -1. Intensities observed in the line wings may thus be representative of those measured in the other parts of the macrospicule seen at line center.

The two events of September 20, 1975 occurred in the same region. A third event followed at 19:35 UT, with no detectable D3. Multiple events from a single site are not unique, and were noted on the disk by Harvey and Martin (1973). Relative intensities of these events are better determined than their absolute values, the 19:03 event (Figure 2) being brighter in D3, and fainter at H a - 1 A, by about the ratios of

Table I. The September 27, 1975 event (Figure 1) was at the detection limit in D3. Any

fainter D3 event would be missed. The intensities in Table I have been corrected for the effects of the filter parasitic

light, but not for the convolution of the filter and macrospicule line profiles, as the exact line shapes and shifts are unknown in the macrospicules. For an unshifted gaussian line of width 0.4 A, the D3 intensities are changed to 5, 8, 3, and 7 x 10 -3 IDc for the events listed. For an 0.8 A H a linewidth, the values given are unchanged. Intensities on the disk were taken from the limb darkening curves of Pierce and Waddell (1961) for D3, and White (1962) for Ha .

The estimated absolute accuracy of these measurements is +50%. Principal limits on the accuracy were the difficulty of measuring the characteristic curve of the film over a range of nearly 4 in density, the nearness of the D3 intensities to the detection limit, confusion with spicules in Ha , and uncertainties in the filter parasitic light

contribution.

2 . 4 . A P P E A R A N C E O N T H E D I S K

H a macrospicule events are also visible on the disk, and were found in magnetic ephemeral regions by Harvey and Martin (1973). The most obvious form is the filament eruption, illustrated in Figure 3. In the figure, a filament connecting a bipolar ephemeral region to nearby magnetic fields darkens, erupts in loops, and disappears. Both the ephemeral region and filament are within the interior of a supergranule cell. Events of this type are easily detected and are quite distinct I rom the normal action seen in quiet regions. The presence of a clearly bipolar ephemeral region is neither necessary nor usual, as similar events occur in otherwise blank areas

as well. Surge-like macrospicules are also seen, as in Figure 4. These are distinguished

from normal spicule events by their larger length, width, and lifetimes, composite structure, and, as in the event of Figure 4, significant brightening at the base.

A third type of disk macrospicule is the flare brightening, sometimes accompanied by a small surge. Examples are given by Glackin (1975). In these events, the brightening dominates the appearance, rather than the surge. Again, the presence of

290 B A R R Y J . L A B O N T E

N o v e m b e r 7, 1 9 7 5

2O

Fig. 3. Macrospicule on the disk in Ha line center. A filament connecting a bipolar magnetic ephemeral region to nearby fields darkens, erupts into loops, and disappears. The ephemeral region had a total lifetime of 25 hr, but lasted only 2 hr after this event. No D3 absorption or emission was detected in this event. The location was 4 ~ S, 12 ~ E. Times (UT) are: top (left to right) 15:55, 16:17, 16:22; bottom (left to

right) 16:25, 16:28, 16:36.

a distinct ephemera l region is unnecessary , as br ightenings have been observed in

previously b lank locations. There are no examples of such events with s imul taneous

D3 film, however .

In 176 hr of film f rom the May to June 1975 and Oc tober to N o v e m b e r 1975

observ ing runs, which covered the disk in the equ iva len t of a raster pa t te rn , 10 H a

events were seen. Al lowing for a �89 reduc t ion of the effective area covered due to the

inclus ion of plages, fi laments, and sky in the field, a rate of 2.8 + 0.9 events per hour

on the visible disk is inferred. If we take 10 000 km as a typical height, the area

covered by observa t ions above the l imb corresponds to ~ 0 . 2 4 of the area of the

visible disk. The disk rate may then be conver ted to a l imb rate of 0 . 7 +

0.2 events (hr 360~ -1. This is lower than the total rates ob ta ined at the l imb, as

expected, since the disk contras t of the smallest events seen off the l imb is too low to

permi t detect ion. Marsh (1978b) has found a rate of 25 + 6 events per hour on the

ACTIVITY IN THE QUIET SUN, I

M A Y 1 5 , 1 9 7 5

291

Fig. 4. Surge-like macrospicule on the disk in Ha. Arch fibrils (A) connect the site of the macrospicule (B) to nearby magnetic areas, demonst ra t ing the mixture of magnetic polarities. No D3 was detected in this event. Event location was 0 ~ N, 25 ~ E. Times (UT) are: top (left to right) 23:01, 23:06, 23:08; bottom

(left to right) 23:10, 23:12.

visible disk in Big Bear data from 1977, but as both our selection criteria are subjective, the rate difference cannot be ascribed to solar cycle variation.

Of the 10 H a events seen on the disk, D3 film was available for 6, including those of Figures 3 and 4. No D3 was detected in any part of any of these events, with a contrast threshold of 2 to 3%. The invisibility of macrospicules on the disk in D3 is consistent with their low intensities observed at the limb.

2.5. H c e / D 3 INTENSITY R A T I O

To obtain statistical data on the appearance of macrospicules in D3 relative to Hc~, film from Lockheed Solar Observatory twin eclipsing telescope (Ramsey etal., 1975)

was studied. This instrument records the entire limb in both lines simultaneously, through wide filters ( ~ 1 0 A F W H M ) ; the H a beam is further filtered so that quiescent prominences have the same film density in both images. The characteristic curve of the film is sufficiently similar at these two wavelengths to make relative intensity measurements meaningful. Stellmacher 's (11972) data on 10 quiescent prominences give R o = I (Ha ) / I (D3) = 11-4- 3.

292 B A R R Y J. L A B O N T E

Fig. 5.

MACROSPICULES

15-

N U M B E R

IO-

5-

o I 4

t I

I

I i i i

8 12 16 HEIGHT (103 km)

I 2'0

t 24

1 5 -

N U io- M B E 5- R

0 0

I I

I , I

D I A M E T E R (10 3km)

unit M B I

i i i

4 8 12 16

LIFETIME (rain)

' I i

20 24

Histograms of the projected height (h), diameter (2r) and lifetime tt) for a sample of 32 Ha macrospicules. Mean values are /7= 12 000 km, 2F = 1800 km, r = 11 min.

Film from four days (June 25-27, July 3, 1974) was studied, totaling 31 hr, selected for its image quality and freedom from active regions at the limb. The limb was divided into quadrants, giving separate data for the poles and equator. A soft X-ray photograph from June 27, 1974 (D. Rust, private communication), in the middle of the days studied on Lockheed film, shows the polar hole boundaries at 60 to 65 ~ latitude, 2 with the holes occupying about g of each polar quadrant. These values for the boundary latitudes are typical of those found in the Big Bear D3 limb scans in 1974-1976 (currently reported in Solar-Geophysical Data). The X-ray picture also shows a hole near disk center which may have contaminated the we'st limb data of July 3; but that is only �88 of the total equatorial data.

Maerospicules were selected in Ha , then rated on a visual intensity scale in each line (Table fI). Ninety-two macrospicules were found. The rate of appearance in the polar quadrants, 4.0 + 0.5 events (hr 360~ -t , was constant from day to day, within 10%. The rate in the equatorial quadrants varied inversely with the amount of prominence material visible, equaling the polar rate on the best days. Confusion with

ACTIVITY IN THE QUIET SUN, I 293

TABLE II

Macrospicule Ha and D 3 intensities

/H~ /D3

0 l 2 3 Total

3 0(0) 2(0) 3(4) 0111) 5(5) 2 4(1) 15(6) 4(4) 0(1) 23(12) l 24(12) 8(0) 3(0) 0(0) 35(12)

Total: 28(13) 25(6) 10(8) 0(2) 63(29)

Entries are the number of events seen in both polar (equatorial) quadrants. Visual intensity scales. Events with ID3 = 0 were invisible in D3.

H a prominences prevented identification of the fainter macrospicules. This problem did not occur with the Big Bear data, because the larger image scale, lack of occulting, and higher resolution permit ted the separation of macrospicules and moving prominence material.

On average, the D3 intensity was proportional to the H a intensity, but much weaker. The correlation coefficient r = 0.53, and the probability of no correlation P(r = 0Jr, 92) = 2 x 10 6. No events were found with an intensity ratio of Ho~ to D3 in

the macrospicule, RM, less than Ro ; in only 5 % was RM = Ro. In 45 % of the events, D3 was indetectable, RM >> Ro. To calculate the average ratio for all events,/~M, the effects of the equatorial bias toward brighter events must be removed. Values of

RM(II-I,) are determined separately for the pole and equator, and the average ratio is then

R M -- ~ IH~N( IH '~ )RM(IH 'J

E,~ N(IH~) '

where the distribution of number versus intensity, N(IH~), is taken to be the polar distribution. /~M is the same at the poles, 2 .0+0 .3 on our visual scale, and the

equator, 1.8 + 0.4. This confirms, with a larger sample (51 detected D3 events), the suggestion from the Big Bear data that D3 macrospicules are not d immer at the poles.

Considering the film gamma of =3 , even a small systematic intensity difference should have been obvious.

2.5.1. Doppler Brightening

Ramsey et al. (1975) suggest that part of the H a enhancement (i.e., RM >Ro) is caused by Doppler brightening (Hyder and Lites, 1970) in the moving material of the macrospicule. The Doppler mechanism predicts intensity maxima during ascending and descending phases, when the velocity transverse to the line of sight is large, and an intensity minimum at the time of maximum height, when the transverse velocity is zero. It also predicts wavelength shifts of the H a line (Rompolt , 1967), but confusion

294 BARRY J. E.ABONTE

with line-of-sight velocities is unavoidable. Indeed, most events do fade as they rise, and their velocities decrease. However, because a descending phase is rarely observed, the expected brightening as the material falls cannot be confirmed. Rather, most macrospicules fade to invisibility upon reaching their maximum height.

This monotonic intensity decrease must be a sum of any Doppler effects plus a change in the state of the material in the macrospicule. From the morphology, it is clear that the macrospicule undergoes a monotonic expansion, with a corresponding reduction in density. This in turn gives a reduction in the recombination rates. Since the hydrogen ionization rate is primarily radiatively controlled (Pottasch and Thomas, 1959), it is independent of the local density. Therefore, as the macrospicule expands, the ionization fraction of hydrogen increases, reducing the bound level populations, and the H a intensity.

2.6. INTENSITY D I S T R I B U T I O N

Observed H a macrospicule rates at the poles were equal within the errors, in the Big Bear and Lockheed data. If the actual rate of macrospicule production was the same during both observing runs, then the detection level in H a was also the same. Mosher (private communication) finds no detectable solar cycle variation in the polar macrospicule rate; although solar cycle variations of ephemeral regions (Harvey et al., 1975) and coronal bright points (Davis et al., 1977) have been observed, the expected variation between the observing runs is less than the counting errors.

In the Lockheed data, a ratio of total energy emission in H a to D3 of R o = 11 is required to produce equal film densities. In the Big Bear data, assuming a ratio of H a to D3 linewidths of 2, the required energy ratio is -~4. The faintest D3 events detected on the Lockheed film are thus 2.8 times less intense than those on the Big Bear film. With this higher sensitivity, the fraction of macrospicules detected in D3 rose from 17% to 56%, a factor of 3.3. The integral number of macrospicules brighter than a given D3 intensity I0 is thus nM(I > I o )~ Io l'2•176

2 . 7 . C O M P A R I S O N WITH EUV M A C R O S P I C U L E S

A peculiar property of EUV macrospicules is that they are detected only in coronal holes (Bohlin et al., 1975). Explanations given for this effect (Moore et al., 1977) have been: that there is a true enhancement in the rate of macrospicules in holes over that in normal quiet regions; that macrospicule material rises higher in the open field lines of coronal holes than in the closed field of normal quiet regions; and that coronal holes are free of obscuring material (prominences). Our Big Bear data refute the first idea, as the rate is actually larger at the equator than in the polar holes. They also show no evidence of the second, as optical macrospicules look identical at all latitudes. Since EUV macrospicules appear taller, however, this effect may occur above the height of our events. The Lockheed data do illustrate the third idea. On days with many equatorial prominences, the ratio of polar to equatorial macrospicule rates appeared enhanced by a factor of 4. This effect may thus be important in the EUV as well.

A C T I V I T Y IN T H E Q U I E T SUN, I 295

3. Summary

New obse rva t ions of macrosp icu les have been made in Hc~ and D3. The pr inc ipa l

results are:

(1) In 1975 the to ta l ra te of macrosp icu les de t ec t ab l e in Hc~ was

>~ 1400 (AG day) -1.

(2) In 1975 the ra t io of equa to r i a l to po l a r macrosp icu le ra tes pe r unit a r ea was

~<2.

(3) The line cen te r in tensi t ies of H a and D3 in the br igh tes t macrosp icu les are

~-5 x 10 -2 and -~5 x 10 -3 IDC, respect ively . This ra t io of Hce/D3 also holds for fa in te r

events .

(4) Macrosp icu le D3 shows no intensi ty reduc t ion in po la r corona l holes, unl ike

the rest of the ch romosphe re .

(5) The n u m b e r of events with [D3>--lo is p r o p o r t i o n a l to I o 1"2.

Acknowledgements

W e thank the fo l lowing peop l e for the i r he lp dur ing this inves t iga t ion: R o n M o o r e ,

for ex tens ive discussions, and Ken Marsh and J im M o s h e r for o the r useful dis-

cussions; A l a n Ti t le for the use of the L o c k h e e d Solar O b s e r v a t o r y da ta , D a v e Rus t

for p rov id ing A m e r i c a n Science and Eng inee r ing soft X - r a y da ta ; the obse rvers at

Big Bea r So la r Obse rva to ry , G e n e Longb rake , Jack K l e m r o t h , Pe te r K u p f e r m a n ,

G a r y Phillis, and s tuden ts John B a r n a r d and Dave K o d a m a ; and Jeff N e n o w and

Juan Sanchez for thei r work on the figures.

This work was s u p p o r t e d by N A S A under grant N G R - 0 5 - 0 0 2 - 0 3 4 and N S F unde r

g ran t A T M 7 6 - 2 1 1 3 2 .

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