S O L A R A C T I V I T Y AT ) .=15 .8 mm (19 G H z ) D U R I N G T H E

P E R I O D 27 O C T O B E R - 4 N O V E M B E R 1968 A N D I T S

R E L A T I O N T O P R O T O N E V E N T S

D . L . C R O O M * and R . J . P O W E L L

S. R.C., Radio and Space Research Station, Slough, Bucks.

(Rcccived 27 March, 1970)

Abstract. Four 15.8 lrll-rl solar bursts are discussed and the results are compared with data on re- cordcd proton events, in relation to the prediction of such events f rom radio observat ions.

1. Introduction

The pcriod from 27 October to 4 November 1968 was one of considerable activity in the southern hemisphere McMath plage No. 9740. "l-he purpose of this note is to discuss thc solar radio data obtained during this period at a wavelength of 15.8 mm at the Slough Solar Radio Observatory.

2. The 15.8 mm Events

The principal events at 15.8 mm were recorded on 29 October, I November and 2 November, and are shown in Figures 1 -3. Two of these arc particularly outstanding

events. The 1 November burst is unusual bccause of its combination of relatively' high intensity (1400 solar llux units peal< increase; 1 sfu = 10 -z2 Wm -z Hz-~), extremely long duration (2--3 h), and because at 15.8 mm it is extremely rare for a burst of this intensity to be non-impulsive, the time from start to maximum being about 1 h.

On the other hand the much more intense burst of 2 November (7400 sfu peak increase) reached its maximum within 7 rain, a more usual value for large 15.8 mm bursts. Dttring the 2~- year period from July 1967 (when observations were started) ttntil mid-1969, only once, on 6 July 1968, (Croom and Powell, 1969), has a more intense 15.8 mm burst been recorded at Slough.

3. Relation of the 15.8 mm Bursts to Solar Protons

From the preliminary analysis already carried out it is already apparent that 15.8 mm bursts can be used as reasonably reliable indicators of the occurrence of solar proton events provided that (a) thc peak flux increase is > 50% of the pre-burst solar flux level (the pre-burst level is usually in the range 600-700 sfu), and (b) the burst en- hancement is /> 10~0 of the pre-burst level for > 5 rain.

* Current ly on leave o f abscnce as 1969-70 N R C - O A R Senior Post-Doctoral Research Associate, A F C R L , t-tanscom Field, Bedford, Mass. 01730.

Solar Physics 14 (1970) 221-225. All Rights Reserved Copyright (~:: 1970 by D. Reidel Publishing Company, Dordrecht-Holland

222 D.L.CROOM AND R.J.POWELL

I ~ 29 Oct . 196B [ / SLOUGH

466sfu I ~ 15.Smm

. I , , I ~ ~ I 12OO 1230 13OO

U.T.

Fig. 1.

29 Oct. 1968 SLOUGH

j~ 15.8ram 262 sfu I \ Sunset

I "15OO 1530 16OO

U.T.

15.8 mm solar radio bursts recorded at Slough on 29 October 1969.

1 Nov. 1968 J~ SLOUGH

\ 15.8 mm

14OO sfu / L~ (2o i%)

, I , , I , , I , i I I I I O8OO O830 0 9 0 0 0930 1OOO

U.T.

Fig. 2. The large 15.8 m m solar radio burst of I November 1968.

Table 1 lists the main cm-wavelength events for the period under discussion including all those which meet the above criteria, and all those which meet the roll owing multi- wavelength U-shaped spectra criteria developed by CasteUi and Aarons (1969):

(a) The burst flux increase has a value exceeding 1000 flux units (10-22 Win- 2 Hz- 1)

in the 9 GHz range. (b) A flux minimum is observed in the 600-2000 MHz range. (c) The flux density rises in the low frequency direction.

SOLAR ACTIVITY AT ,~ --15.8 MM (I 9 GHZ) I)URING 27 O(:T.-4 NOV. 1968 223

In order to get as complete a coverage as possible it has been necessary in some cases

to estimate the 15.8 mm data from 20 mm records made at the Sagamore Hill Observ-

atory of the U.S. Air Force Cambridge Research I_.aboratories (ESSA, 1968), and

from the 18 mm data of' Tokyo Astronomical Observatories (1968).

Whether or not these radio criteria call be regarded as accurate indicators of proton

events or false alarms depends very much on the more detailed analysis of this very

complex sequence of solar events, since once one large proton event has begun it is

7400 sfu (1073%)

Fig. 3.

I 0 9 3 0

l i [5.Bmm

J I I , I I___ IOOO IO30 U.T.

The large 15.8 nan] solar radio burst of 2 November 1968.

often dill'lcult to tell whether or not there were subsequent smaller proton events which

were masked by the earlier one. However, in the linal column an indication has been

given of the main proton events as determined lu the hourly-averaged particle data from the Explorer 34 satellite (I 967--51 J~,) as published in tile U.S. Environmental

Science Services Monthly Solar-Geophysical Data (ESSA, 1968).

It can be seen from Table I that there is only one discrepancy between the 'predic-

tions' based on the two sets of radio criteria, namely the event of 29 October starting

at about 1217. According to the multi-wavelength U-shaped spectra criteria this should not be a proton event, whereas according to the single wavelength 15.8 mm criteria

it should be one. The Explorer 34 data shows that there was in fact a weak proton event starting at

about 16-17 h (.IT. The flux in tile 10-30 MeV range increased by a Ii~ctor of about 6, but there was no detectable increase in the > 30 MeV energy range.

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SOLAR ACTIVITY AT 2 - " 15.8 MM (19 GIiZ) DURING 27 OCT.--4 NOV. 1968 225

4. Observations at 4.2 mm (71 GHz)

N o events were recorded at 4.2 mm wavelength at Slough dur ing this per iod. However

the r ad iomete r klystron failed a few minutes before the largest 15.8 m m burst on

2 November .

Acknowledgements

The work described in this paper was carr ied out at the Rad io and Space Research

Stat ion o f the Science Research Council and is publ ished with the permission o r the

Director .

References

Croom, D. L. and Powell, R. J.: 1969, Nature 221, 945. Castelli, J. P. and Aarons, J.: 1969, 'Radio Burst Spectra and the Short Term prediction of Solar

Proton Events', Proceedings XV Electromagnetic Wave Propagation Meeting (if" AGARD, C.P. No. 49, 1970.

ESSA: 1968, Solar Geophysical Data Nos. 291, 292, Essa Research Laboratories, Boulder, Colo., U.S.A.

Tokyo Astronomical Observatory: 1968, i~[onthly Solar Radio Emission B, lletin October-November 1968, Mitaka, Tokyo, Japan.