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Adv. S&we Ret. Vol. 17, No. 2,pi (2)113-&)ll&~ RhedimOtWBrbin. 0273-l 177(95W520-X 027~1177/96s9.50+0.00 THE HELIOLONGITUDINAL DISTRIMJTION OF SOL&R FLARES ASSOCIATED WITH SOLAR PROTON EVENTS D. F. Smart and M. A. Shea Space Physics Division, Geophysics Directorate, Phillips Laboratory, 29 Randolph Road, Hanscom AFB, Be4ford, MA 01731-3010, U.S.A. ABSTRACT We find that the heliolongitudinal distribution of solar flares associated with earth-observed solar pro- ton events is a function of the particle measurement energy. For solar proton events contain@ fluxes with energies exceeding 1 GeV, we find a Gaussian distribution about the probable root of the . Archmdm spiral favorable propagation path leading from the earth to the sun. This distriion is mod&d as the detection threshold is lowered. For > 100 MeV solar proton events with fluxes 210 protons (cm2-sec-ster)-1 we find the distribution becomes wider with a secondary peak near the solar central meridian. When the threshold is lowered to 10 MeV the distribution f&her evolves. For > 10 MeV solar proton events having a flux threshold at 10 protons (cm&c-s&r)-1 the distribution can be considered to be a composite of two Gaussians. One distribution is centered about the probable root of the Arch&dean spiral favorable propagation path leading from the earth to the sun, aud the other is centered about the solar central meridian. For large flux solar proton events, those with flux thresh- old of 1000 (cn?-sec-ster)-l at energies > 10 MeV, we find the diitribution is rather flat for about 40 degrees either side of central meridian. INTRODUCTION One of the best documented early references to a western hemisphere distribution for solar-flare-asso- ciated higb energy (> 430 MeV) proton events is McCracken 111. However, researchers such as War- wick and Haurwitz /2/ noted that when the polar-cap absorption events (ionospheric absorption of me- ter radio waves in the polar region induced by protons with energies in the 5 to 50 MeV energy range) of the 19th solar cycle were associated with possible solar flare activity, the extreme western hemi- sphere bias was not obviously apparent. In the 131,the association of solar proton events at all observed energies was divided into categories ranging from “certain” to “possible” to “no association”. Smart et al., /4/ examined the distribution of significant polar cap absorption (FCA) events for the 19th and part of the 20th solar cycle that could be “confidently associated” with solar flares and concluded that these events formed a Gaussian distribu- tion about 3S” heliographic longitude. More recent analysis of the extremely sensitive low w charged particle detectors operating on modem spacecraft have suggested that proton events are de- tectable from solar flare associated activity occurt@ at any portion of the visible solar disk /5,6/, In thii paper we examine the heliolongitudinal distribution of solar flares time-associatedwith earth-ob- served solar proton events as a function of the observation energy. DATA We used solar proton event data and solar flare associations from a variety of published data sources. For the high energy “ground-level events” (GLE) we used the published event list of Shea and Smart /7/. For lower energy proton events, whenever possible, we examined the initial data source and checked against actual spacecraft data. The solar proton event list of Shea and Smart /8/ was checked against the hourly averaged IMP-8 data file (1973 to present) assembled by Krimigis and Armstrong /9/ and the GGES 6 and 7 synchronous orbit spacecraft data /lo/, and extended to the present. The flare asscsziatiinswere made with the assistance of previously published lists such as /3, 11, 12/, and published proton event catalogues /13, 14/. The NOAA preliminary proton event list /15/ was used as (2)113

The heliolongitudinal distribution of solar flares associated with solar proton events

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Adv. S&we Ret. Vol. 17, No. 2, pi (2)113-&)ll&~

RhedimOtWBrbin. 0273-l 177(95W520-X 027~1177/96s9.50+0.00

THE HELIOLONGITUDINAL DISTRIMJTION OF SOL&R FLARES ASSOCIATED WITH SOLAR PROTON EVENTS

D. F. Smart and M. A. Shea

Space Physics Division, Geophysics Directorate, Phillips Laboratory, 29 Randolph Road, Hanscom AFB, Be4ford, MA 01731-3010, U.S.A.

ABSTRACT

We find that the heliolongitudinal distribution of solar flares associated with earth-observed solar pro- ton events is a function of the particle measurement energy. For solar proton events contain@ fluxes with energies exceeding 1 GeV, we find a Gaussian distribution about the probable root of the

. Archmdm spiral favorable propagation path leading from the earth to the sun. This distriion is mod&d as the detection threshold is lowered. For > 100 MeV solar proton events with fluxes 210 protons (cm2-sec-ster)-1 we find the distribution becomes wider with a secondary peak near the solar central meridian. When the threshold is lowered to 10 MeV the distribution f&her evolves. For > 10 MeV solar proton events having a flux threshold at 10 protons (cm&c-s&r)-1 the distribution can be considered to be a composite of two Gaussians. One distribution is centered about the probable root of the Arch&dean spiral favorable propagation path leading from the earth to the sun, aud the other is centered about the solar central meridian. For large flux solar proton events, those with flux thresh- old of 1000 (cn?-sec-ster)-l at energies > 10 MeV, we find the diitribution is rather flat for about 40 degrees either side of central meridian.

INTRODUCTION

One of the best documented early references to a western hemisphere distribution for solar-flare-asso- ciated higb energy (> 430 MeV) proton events is McCracken 111. However, researchers such as War- wick and Haurwitz /2/ noted that when the polar-cap absorption events (ionospheric absorption of me- ter radio waves in the polar region induced by protons with energies in the 5 to 50 MeV energy range) of the 19th solar cycle were associated with possible solar flare activity, the extreme western hemi- sphere bias was not obviously apparent. In the 131, the association of solar proton events at all observed energies was divided into categories ranging from “certain” to “possible” to “no association”. Smart et al., /4/ examined the distribution of significant polar cap absorption (FCA) events for the 19th and part of the 20th solar cycle that could be “confidently associated” with solar flares and concluded that these events formed a Gaussian distribu- tion about 3S” heliographic longitude. More recent analysis of the extremely sensitive low w charged particle detectors operating on modem spacecraft have suggested that proton events are de- tectable from solar flare associated activity occurt@ at any portion of the visible solar disk /5,6/, In thii paper we examine the heliolongitudinal distribution of solar flares time-associated with earth-ob- served solar proton events as a function of the observation energy.

DATA

We used solar proton event data and solar flare associations from a variety of published data sources. For the high energy “ground-level events” (GLE) we used the published event list of Shea and Smart /7/. For lower energy proton events, whenever possible, we examined the initial data source and checked against actual spacecraft data. The solar proton event list of Shea and Smart /8/ was checked against the hourly averaged IMP-8 data file (1973 to present) assembled by Krimigis and Armstrong /9/ and the GGES 6 and 7 synchronous orbit spacecraft data /lo/, and extended to the present. The flare asscsziatiins were made with the assistance of previously published lists such as /3, 11, 12/, and published proton event catalogues /13, 14/. The NOAA preliminary proton event list /15/ was used as

(2)113

(2)114 D. F. Smart and M. A. Shea

a reference, but was allocated a low reliability. For events prior to 1990, there are often large discrep- ancies between the fluxes reported in this list and the directly comparable flux observed by the IMP-8 spacecraft /8/. After 1989, a new correction algorithm was introduced in the data processing, and these corrected fluxes compare favorably with the IMP8 observations. The NOAA preliminary proton event list /15/ is often incomplete as it only records the start of a flux enhancement period and makes no attempt to identify multiple solar particle injections if the observed flux does not fall below their event threshold. We examined the probable solar flare association of every event and accepted or re- jected events and associations using the criteria summarized below.

Proton Event Selection Criteria

The occurrence of a ground-level event (GLE) as observed by a cosmic ray neutron monitor was inter- preted as evidence that protons with energy exceeding 1 GeV were present in the event. The satellite sensed solar proton data were examined in order to identify events where the > 100 MeV flux met or exceeded 10 (cm2-s-ster)-l . The solar proton data were also examined in order to identify events where the > 10 MeV flux met or exceeded 1000 (cm2-s-ster)-1 and 10 (cmz-s-ster)-*.

Solar Flare - Proton Event Association Criteria

It was our opinion that a solar flare must have some outstanding energy release characteristic in order to be a candidate for association with a solar proton event. While these choices are subjective and subject to selection bias we generally adhered to the “big flare syndrome” /16/, long duration soft X- ray emission, coronal mass ejection association (or CME proxy association), and time association as a function of helilongitude of the flaring region. In general the time associated flare should conform to the time associations previously observed /17/ and developed for proton prediction /18, 19/.

RESULTS

The results of our investigation are presented in the following figures. Figure 1 illustrates the western hemisphere asymmetry typically found for high energy solar proton events. Figure 2 illustrates the distribution of solar flares associated with solar proton events having energies > 100 MeV. Figure 3 illustrates the changing distribution of solar flares associated with large flux solar proton events con- taining energies > 10 MeV exceeding a flux threshold of 1000 (cm&+ster)-l. Figure 4 illustrates the changing distribution of all solar flares confidently associated with solar proton events having energies > 10 MeV. Note the changing distribution as the detection energy threshold is lowered.

GLE P >I 000 MeV

-180 -140 -100 -80 -20 20 60 100 140 180

HELIOLONGITUDE Fig. 1. Histogram of the distribution of solar flares confidently associated with solar cosmic ray ground-level events (assumed to indicate the presence of pro- tons with energies ,1&M Mev). The heavy line is a Gaussian fit to the data.

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PROTONS A 00 MeV FLUX >I 0 (CM2 S STER)_l

v) II

!g 14 12

ILi 10

t5 2 iI5 6

g 4

g 2

0

-180 140 -100 HELIOLONGITUiiE -20 -20 20 100 140 180

Fig. 2. Histogram of the distribut?on of solar flares confidently associated with > 100 iUeV solar proton events meeting a flux threshold of 10 (cm%ster)-I. The dotted line (triangles) indicates a Gaussian in the western hemisphere and the dashed line (squares) indicate a Gaussian about a near central meridian shock source. Zhe heavy line is a composite of the two Gaussians.

PROTONS A 0 MeV FLUX >l 000 (cm* s sterkl

-180 -140 -100 60 100 140 180

Fig. 3. Histogram of the distribution of solarmres confidently associated with soiar proton events having energies > 10 iUeV and flux threshold of loo0 (cm&ster)-1.

PROTONS A 0 MeV Flux >I 0 (cm2 s sted-1

Fig. 4. Histogram of the distribution of solar flares confidently associated with > 10 MeV solar proton events meeting a flux threshold of 10 (cm2-s-ster)-1. lhe dotted fine (triangles) indicates a Gaussian in the western hemisphere and the dashed line (squares) indicate a Gaussian about a near central meria%an shock source. lhe heavy line is a composite of the two Gaussians.

(2)116 II. F. Smart ad Id. A. Shea

DISCUSSION

These results show that the heliolongitudinal distribution of solar flares associated with earth-observed solar proton events is a function of the particle measurement energy. However, these results are sta- tistically limited; the stringent association requirements imposed greatly reduced the total number of solar proton events meeting our flux criteria that can confidently be associated with solar flares. The largest data set, the > 10 MeV flux, only contains 170 flare-associated solar proton events. The high energy (> 1 GeV) solar proton source location distribution can be described by a Gaussian distribution about the probable root of the Archimedean spiral favorable propagation path leading from the earth to the sun. This distribution is modified as the detection threshold is lowered. There is also a flux level effect; for the large flux events, those with fhrxes > 1000 (cm2-see-ster)-l at energies exceeding 10 MeV, we find an almost flat distribution extending about 1ooO. When we consider all > 10 MeV events with a flux 210 (cm2-sec-ster)-1 the distribution further evolves into a dual distribution. One distribution is centered about the west side of the sun, and the other is centered slightly east of the so- lar central meridian.

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