R A D I O E V I D E N C E F O R A N E X P A N D I N G M A G N E T I C

A R C H B E Y O N D 20 S O L A R R A D I I

(Research Note)

R. G. STONE and JOSEPH FAINBERG Radio Astronomy Branch, Laboratory for Extraterrestrial Physics, Goddard Space Flight Center,

Greenbelt, Md., U.S.A.

(Received-4 February, 1975)

Abstract. A magnetic loop located beyond 20 R| appears to be the later evolution of an expanding magnetic arch observed at 2 R o. The expansion speed is of the order of 100 km s -a.

A magnetic loop located at a height greater than 20 R o has been deduced from long- wavelength solar radio observations (Stone and Fainberg, 1971). The purpose of this note is to suggest that the loop is a later stage of an expanding magnetic arch observed at 2 R o on the previous day (Wild, 1969). The long-wavelength observations also constitute an example of the interplanetary manifestation of a moving type IV solar radio event.

Spacecraft observations of solar radio bursts in the frequency range from 10 kHz to 10 MHz correspond in general to radio emission from phenomena occurring at distances from the Sun of 5 R o to 1 AU and beyond (Fainberg and Stone, 1974). The overwhelming majority of observed events are type lII bursts produced by super- thermal electrons propagating along open field lines over this distance range. Type III burst dynamic spectra are characterized by a fast drift from high to lower frequencies as the exciter moves outward. There have been a few rare instances in which the dynamic spectrum at low frequencies shows a burst drifting from high to lower fre- quencies, followed by a subsequent drift back to higher frequencies, leading to its classification as a 'U' burst. The analysis of approximately one year of data from the RAE-1 satellite has revealed less than a dozen U bursts in the low-frequency range (Malitson, 1974). These bursts are an indication that the exciter particles are confined to move along a magnetic loop type configuration (e.g. Zheleznyakov, 1964)

The observation of a U burst on 1968 November 23 (Stone and Fainberg, 1971) by the RAE-1 spacecraft between 5 and 0.7 MHz is shown in Figure 1. The coronal levels in the figure were obtained from the emission level scale determined from a statistical study of type III bursts observed from RAE-1 (Fainberg and Stone, 1971, 1974). From an initial analysis of the U burst, Stone and Fainberg (1971) estimated the height of the magnetic loop to be of the order of 35 R o. Further analysis suggests that the height may be more nearly 25 Ro, as suggested from the turning frequency in Figure 1. There is difficulty in selecting the turning point because during the down-frequency phase of the event, both a U burst and a conventional type III burst (drifting to still lower fre- quencies) were observed (see Figure 1; Stone and Fainberg, 1971).

Solar Physics 42 (1975) 179-181. All Rights Reserved Copyright �9 1975 by D. Reidel Publishing Company, Dordrecht-Holland

180 R. G, STONE A N D J. F A I N B E R G

5.0

4.0

~, 3.0

5 2.0

1 .0-

0.7 19h35 m

,03 1 7 I06 09 0 3 - - 10 12 o6<- 15 09 ~-~'N' / /~- ' , ~ / 18 12~"-.~./,_, I\ \

2O

3O 19h40 m

TIME - NOV. 23, 1968

g

> ILl -.A -.A

Z

O (.3

Fig. 1. Dynamic spectrum of a low frequency U burst (Stone and Fainberg, 1971). Numbers on the figure represent intensity in dB relative to cosmic noise background. The right hand scale is the coronal

height based on the RAE-1 emission level scale (Fainberg and Stone, 1974).

One form of moving type IV solar radio burst observed at higher frequencies sug- gests an expanding magnetic arch structure located above an active prominence (Wild, 1969, 1970). An example of such an event observed at 80 MHz with the Culgoora radioheliograph is illustrated schematically in Figure 2a. The evolution and interpreta- tion of this radio burst have been discussed in detail by Wild (1969, 1970). Here we note that the outburst occurred on 1968 November 22 at 01 h. The 80 MHz emission first appeared around the innermost arch. As the arch expanded to the intermediate position, the radio emission remained continuous. At times, the arch became strongly circularly polarized, one half right-handed (RH) and the other half, left-handed (LH). Later in time, the radio emission became concentrated into three discrete sources, SL(LH) and SR(RH) and S0(unpolarized). The projected rate of expansion found by Wild was initially 300 km s -a and slowed to about 100 km s -a when source S O was at a height of about 1 R o above the photosphere. The three sources faded about 45 min after the start of the event, i.e. at 01 h 45 m.

We believe that the same expanding magnetic arch is responsible for both the mov- ing type IV observed at Culgoora and the U burst observed by RAE-1. Figure 2 il- lustrates schematically the locations of the magnetic arch as deduced from the two sets of data. For both sets of observations, the event was located toward the east limb of the Sun. The Culgoora observations place the loop at 2 R o at 01 h 45 m while the RAE-1 observations place it at 25 R o at 19 h 30 m the following day. The expanding arch traveled 23 R o in 41.75< This would correspond to a constant expansion speed of the order of 100 km s -a consistent with the Culgoora observations at 80 MHz.

The results presented in this note constitute the first report of a moving type IV solar radio burst as it occurs in the interplanetary medium. Unfortunately the limited data available for this event precludes an investigation of the further evolution of the dis- turbance.

RADIO EVIDENCE FOR AN EXPANDING MAGNETIC ARCH BEYOND 20 R| 181

SL ." S ~ ~FLARE

k ~ / ~ FI LAMENT So~-~ �9 SR

1968 NOVEMBER 22 AT oIh O0 m

SCALE

I" i R| "I

. - c �9 , e o o 3 ~

.~. SCALE

I o I0 Re

c~j ,,o ,,5 ,d

/

~ SUN

TO EARTH

1968 NOVEMBER 23 AT 19 h 30 m

Fig. 2. Upper panel: Expanding magnetic arch moving type IV solar radio event observed by the Culgoora radioheliograph at 80 MHz. Dashed curves show successive positions of the expanding arch (after Wild, 1970). Lower panel: the same magnetic arch as deduced from the low frequency observa-

tions about 42 hours later. Circular arcs show the RAE-1 emission scale.

R e f e r e n c e s

Fainberg, J. and Stone, R. G.: 1971, Solar Phys. 17, 392. Fainberg, J. and Stone, R. G. : 1974, Space Sei. Rev. 16, 145. Malitson, H. H. : 1974, private communications. Stone, R. G. and Fainberg, J. : 1971, Solar Phys. 20, 106. Wild, J. P. : 1969, Solar Phys. 9, 260. Wild, J. P. : 1970, Proe. ASA, 1, 365. Zheleznyakov, V. V. : 1964, Radio Emission of the Sun andPlanets (English translation, 1970, Perga-

mon Press N.Y.)