P R E L I M I N A R Y O B S E R V A T I O N S O F C O R O N A L M A G N E T I C

F I E L D S B E F O R E A N D A F T E R S O L A R P R O T O N E V E N T S

JESUSA V A L D E Z and M A R T I N D. ALTSCHULER High Altitude Observatory, National Center for Atmospheric Research*, Boulder, Colo., U.S.A.

(Received 16 July, 1970)

Abstract. Maps of the coronal magnetic field before and after several proton flares were examined. There are indications that large scale changes occur in both the photospheric and coronal magnetic field as a consequence of large proton flares.

Recently Altschuler and Newkirk (1969) developed a technique for calculating and displaying the coronal magnetic field configuration. Using the line-of-sight component of the photospheric magnetic field (measured at the Mt. Wilson Observatory), they derive the Legendre polynomial expansion for the potential (current-free) field. Their computer maps show the directions of the magnetic field lines above the photosphere. For the period 27 October to 22 November, 1966 the coronal field maps were com- pared with contemporaneous data from various solar phenomena (Newkirk and Altschuler, 1970). The maps were shown to correlate in many respects with (1) the appearance of the He-spectroheliogram; (2) the coronal features visible in the 12 November, 1966 eclipse; (3) the 9.1 cm radioheliogram; and (4) the X-ray image.

In the present paper we attempt to correlate the configuration of the coronal mag- netic field with solar proton events.

The cautions that must be remembered when using computer maps of the coronal magnetic field were discussed in detail by Altschuler and Newkirk (1969). We summa- rize them here. First, the assumption that magnetic fields are current-free near active regions may not be realistic. Second, there are uncertainties with the imput data: In particular, the measurements of the line-of-sight component of the photospheric magnetic field are representative of the actual field only within about 45 ~ of both the central meridian and the equator. An entire solar rotation is necessary to collect the required data, during which time the magnetograph must maintain the same calibration. Short,lived magnetic features are not always recorded. The surface field is averaged over 648 elements of equal area, thereby limiting the maximum resolution of magnetic features to 13.3 ~ in longitude and 7 ~ (on the average) in latitude. Third, the Mt. Wilson magnetograph saturates at about 80 G. The necessary corrections for saturation (which are significant only near sunspots) are usually satisfactory, but because of their statistical basis can occasionally be erroneous. Finally, there is a decrease in surface field resolution associated with truncating the Legendre poly- nomial expansion at N = 9.

Since 1961, detectors aboard various spacecraft have provided increasing informa-

* The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Solar Physics 15 (1970) 446-452. All Rights Reserved Copyright �9 1970 by D. Reidel Publishing Company, Dordrecht-Holland

PRELIMINARY OBSERVATIONS OF CORONAL MAGNETIC FIELDS 447

tion about proton events. In particular, the period October 1965 to December 1966

has been well observed by several groups, in particular Fan e t al. (1968), McCracken e t al. (1967), Kinsey and McDonald (1968), and Kane e t al. (1968). Moreover, two proton flare projects were organized around the events of 7 to 9 July, 1966 ( A n n a l s

I Q S Y , Vol. 3) and 28 August to 2 September, 1966 (Svestka and Simon, 1969). Because of this, we have chosen to study the period from 5 March 1966 to 1 December, 1966. We obtained the magnetic field maps for nine solar rotations contained in this period with the exception of rotation 1510 (19 July to 16 August, 1966) for which the Mt. Wil- son measurements were not reduced. The solar proton events which occurred during this period are listed in Table I together with the McMath number of the active region associated with it when an identification of the present flare has been made. I t can be noted from this table that matching an observed proton flare with the responsible active region is not always easy or unambiguous. In an attempt to eliminate ambiguous correlations between visual flare events and proton events, we have accepted the iden- tification of an active region with a proton event only if two or more workers agree on it. This reduces the number of active regions that can be studied to six: 8207, 8262, 8348, 8362, 8459-8461, and 8505. Region 8348 is smaller in size than our average area;

hence, its magnetic structure is not apparent in our graphs and we shall eliminate it from our discussion.

In each of the figures two magnetic field graphs are shown. The graph on the right, which we call the weak field map, includes magnetic lines emanating from all photo- spheric regions with greater than 0.16 G field averaged over a surface element; one field line is plotted for each surface element with B~>0.16 G. A 0.16 G field slightly exceeds the Mt. Wilson measuring errors. The graph on the left includes only field lines which emanate from relatively strong photospheric field regions; if the maximum field strength of the 648 surface elements is Bin, then field lines from surface elements with B ~> 0.1 Bm are plotted. The value of Bm may vary from rotation to rotation.

TABLE I Solar proton events for March 5, 1966-July 19, 1966 and August 16, 1966-December 1, 1966

Date of proton event Observed by McMath plage region Flare importance

Marcia 19, 20, 20 McCracken et al. (1967) 8207 21, 22, 23, 24, 25, 28 March 19, 20, 20 Fan et aL (1968) 8207 21, 23, 24, 28 March 24 Kinsey and McDonald (1968) 8207

April 16 McCracken et aL (1967) 8262 Fan et al. (1968) 8262

April 26 McCracken et al. (1967) Not identified

April 29 McCracken et al. (1967) Not identified Kinsey and McDonald (1968) Not identified

1B, 2B, 3B, 2B, 2N, 2N, 3B, 1N, 2B 1, 2, 2, 2, 1,2,2 3B

3B

448

Table I (continued)

JESUSA VALDEZ AND MARTIN D. ALTSCHULER

Date of proton event Observed by McMath plage region Flare importance

May 2

May 6

May 28

June 25

July 4

July 7

July 7-9 July 9

July 13

July 16

August 28, Sept. 2

August 29

September 12

September 14

September 17

September 19

September 20

September 25

September 27

McCracken et al. (1967) Kinsey and McDonald (1968)

McCracken et al. (1967)

McCracken et aI. (1967)

McCracken et al. (1967) Kinsey and McDonald (1968)

Kinsey and McDonald (1968)

Kinsey and McDonald (1968) Fan et aI. (1968) Linet al. (1968) McCracken et al. (1967)

Kinsey and McDonald (1968)

Kinsey and McDonald (1968)

McCracken et al. (1967) Kinsey and McDonald (1968) Fan et al. (1968) Kane et al. (1968)

McCracken et al. (1967) Kinsey and McDonald (1968)

Kane et al. (1968)

Kane et al. (1968)

McCracken et al. (1967) Kinsey and McDonald (1968) Kane et al. (1968)

Kane et al. (1968)

Kane et al. (1968)

Kane et al. (1968) McCracken et al. (1967) Kinsey and McDonald (1968)

Kane et al. (1968)

McCracken et al. (1967) Kinsey and McDonald (1968) Kane et al. (1968)

Not identified 8285 2N

Not identified

8310 2B

8348 2N 8348 1B

Not identified

8362 2B 8362 8362 2B 8362 3B

8397 IN

Not identified

8461 4B, 3B 8461 3B, 2B 8459-8461 8461 3B, 3B

8461 2N NO7/W71 at 2114 UT 1N Not identified

8505 2B

Not identified Not identified 8484 2B

8496 4N

8509 2B

8505 2B 8505 2B 8505 2B

Not identified

Not identified Not identified 8509 F, 1

PRELIMINARY OBSERVATIONS OF CORONAL MAGNETIC FIELDS 449

Figure 1 shows the strong and weak field maps for region 8207. The Mt. Wilson measurements of this active region were taken on 21 and 22 March. By that time, three small proton flares had already occurred. Region 8207 can be classified as a series of low magnetic arches, although it is near several diverging field lines. The region continued to eject protons until it disappeared from view. During the next solar rotation this same region, now numbered 8262, produced its last proton flare on 16 April at 0115 UT (McCracken et al., 1967). Two days later, 8262 was measured by Mt. Wilson and the resulting magnetic configuration is shown in Figure 2. The field lines are now diverging and quite radial and the arches have disappeared.

Region 8362 is responsible for the proton event of 7 to 9 July, 1966, which was the subject of the first proton flare project of 1966. Figure 3 shows this active region. Measurements of the line-of-sight fields were made on 2 and 3 July, several days before the proton flares. Again, this active region is located in a series of low magnetic arches. Unfortunately, we were unable to reduce the Mt. Wilson data for the following solar rotation so that the 'after flare' magnetic configuration is not known.

The second proton flare project of 1966 was organized around the events of 28 August and 2 September, both of which were attributed to the active region 8461.

Fig. 1.

8262 Return of 8207

8262 Return of 8207

/

Fig. 2.

8362

450 JESUSA VALDEZ AND MARTIN D. ALTSCI-ILILER

836

�9 ~

8461 8459

Fig, 3.

8461 8459

Fig. 4.

8514 Return of 8461

8514 Return of 8461

Fig. 5.

PRELIMINARY OBSERVATIONS OF CORONAL MAGNETIC FIELDS 4 5 ]

The 2 September proton event has the largest intensity of all the events in the period we examined; according to its PCA effect, it was the largest event since July, 1961 (Masley and Goedeke, 1968). Figure 4 shows the active regions 8459 and 8461. Region 8461 was observed by Mt. Wilson on 30 August, two days after the 28 August proton flare and two days before the unusually large 2 September event. Again, a series of low magnetic arches can be observed over the active region. Following the proton flares, the magnetic fields of 8461 must have decayed quite rapidly since in the next solar rotation, no trace of it is found in the strong field map of Figure 5. However, in the accompanying weak field map, it can be seen that the remaining magnetic field is now composed of diverging field lines.

Figure 6 shows the active region 8454. It was measured on 24 August, almost a full solar rotation before the proton flare on 20 September. The characteristic low magne- tic arches are once more evident. Figure 7 displays the magnetic field configuration taken several hours after the proton flare. Once again, the field appears more divergent, and the arches have disappeared.

8454 8454

Fig. 6.

8505 8505

Fig. 7.

452 JESUSA VALDEZ AND MARTIN D. ALTSCHULER

To summarize, low magnetic arches seem to characterize an active region before

proton events, but have disappeared after the proton event. In addition, except for 8454 (Figure 6), the active region lies just outside the arches in the strong field maps. However, after proton flare activity has ceased, the magnetic field configuration be-

Comes diverging and in one case, quite radial.

For purposes of comparison, we wished to study the development of an active region which does not produce a proton flare. We were unable to accomplish this because it is impossible to say with certainty that any active region has not produced

a proton event. An attempt was also made to examine the 'before' and 'after' magnetic

configurations of active regions that gave Hc~ flares of class ~< 2N. But this also turned

out to be an inconclusive task for various reasons. Some flare regions were too small

to have distinguishable structure, others died after one solar rotation and the rest evolved from or into a region suspected of a proton flare.

I f further investigation shows that the three cases presented here are typical of the magnetic behavior of an active region before and after a proton flare, it would indicate

that proton flares are associated with large-'scale changes of the magnetic field con- figuration of an active region. Obviously these results, based on a mere three cases and a technique with limited resolution in time and space are only preliminary.

Acknowledgements

We are grateful for the assistance of R. Howard (Mt. Wilson Observatory) and Miss

D. Trotter for assembling the data needed in this work. and to G. Newkirk and

U. Anzer for many helpful comments.

References

Altschuler, M. D. and Newkirk, G. : 1969, Solar Phys. 9, 131. Annals IQSY, Vol. 3, MIT Press, Cambridge, Mass. Fan, C. Y., Pick, M., Pyle, R., Simpson, J. A., and Smith, D. R.: 1968, J. Geophys. Res. 73, 1555. Howard, R., Bumba, V., and Smith, S. F. : 1967, Atlas of Solar Magnetic Fields 1959-1966, Carnegie

Institute of Washington, Publication No. 626. Kane, S. R., Winckler, J. R., and Hofman, D. J. : 1968, Planetary Space Sci. 16, 1381. Kinsey, J. H. and McDonald, F. B.: 1968, 1AU Syrup. 35, 536. Lin, R. P., Kahler, S. W., and Roelof, E. C.: 1968, Solar Phys. 4, 338. Masley, A. J. and Goedeke, A. D. : 1968, Can. J. Phys. 46, $766. McCracken, K. G., Rao, U. R., and Bukata, R. P.: 1967, J. Geophys. Res. 72, 4293. Newkirk, G. and Altschuler, M. D. : 1970, Solar Phys. 13, 131. ~vestka, Z. and Simon, P.: 1969, SolarPhys. 10, 3.